Presentation on theme: "Tracer Summit User’s Group 5/22/2012"— Presentation transcript:
1 Tracer Summit User’s Group 5/22/2012 [LEE] Okay, Don. Thanks for that physics review. We’ll refer to it more than once to make sure we’re not breaking these laws as we discuss drive applications.The first application of a drive we will examine will be for a “free discharge” fan. The example we’ll use for this type of fan is a draw through cooling tower. Another example of a system with free discharge fans is fan coil unitsLater Don will come back and discuss different types of fans and their control characteristics.VSDs and their effect on HVAC system components
5 Darcy-Weisbach Equation path diameterpath lengthvelocityfluid densityL r V²D 2 gcDp = ffriction factorgravitational constant[SLIDE] So how do we measure this resistance? Sometimes engineering is the feat of making the simple seem complex. This is the Darcy-Weisbach equation that quantifies the resistance to flow measured in units of pressure. It looks complicated, but that’s only because the versatility of this equation. It works for any “well behaved” fluid, including air and water.[CLICK] It also works any place. For example as soon as we know what planet we’re on we know the gravitational constant gc.[CLICK] When we know the roughness of the pipe and the viscosity of the fluid we can determine the friction factor.[CLICK] Knowing the fluid, we know the density of what we wish to move.[CLICK] And to make the fluid go where we want it to go, we need a pathway, duct work for air and pipe for water. Once the air distribution system or water distribution system is defined we know length and diameter of the conduit.[CLICK] The only variable left is velocity.
6 Darcy-Weisbach Equation resistance a velocity²[SLIDE] Resistance is proportional to velocity squared.
7 System Resistance return duct dampers supply duct [DON] Now that the resistance, or static-pressure loss, we must overcome to move a given quantity of air through the air distribution system can be determined, [SLIDE] Let’s turn our attention to the device that moves the air … the fan.diffusers and grillescoil
8 cfm a rpm Dp a rpm² a cfm² hp a rpm³ Fan Laws Fans obey a list of rules we call “Fan Laws.”[CLICK] First rule. The faster we shovel, the more stuff we move. The volume of air moved is proportional to rotational fan speed, or revolutions per minute.[CLICK] Second rule. Pump head or fan static is proportional to rpm squared.[CLICK] If cfm and rpm are proportional, this means static pressure produced by the fan is proportional to the velocity of air moved by the fan squared. Note how this matches the resistance to velocity predicted by the Darcy-Weisbach equation which reminds us resistance is also proportional to velocity squared. How fortuitous, we should find a way to exploit this.[CLICK] And it gets even better. Third rule. The energy required to move air or pump water is proportional to rpm cubed.[DON] Everybody wants to save energy. The fan laws also apply to pumps. Fan laws, pump laws, or affinity laws as we sometimes call them; are different names for the same set of relationships.
9 resistance a velocity² Chiller Laws?resistance a velocity²[DON] How come we don’t have chiller laws? Chillers do have to obey the same rules of physics.[SLIDE] However, in a refrigeration system resistance to mass flow is not a function of refrigerant velocity. The resistance a compressor must work against is the pressure difference between condenser pressure and evaporator pressure. [CLICK] We call this pressure difference lift. Ryan will will tell us more about lift and compressor work later in the broadcast.[DON, LEE] Lee, how about applying these fan laws to cooling tower fans?resistance a “lift”
10 Practical Application: Free Discharge Fans [LEE] Okay, Don. Thanks for that physics review. We’ll refer to it more than once to make sure we’re not breaking these laws as we discuss drive applications.The first application of a drive we will examine will be for a “free discharge” fan. The example we’ll use for this type of fan is a draw through cooling tower. Another example of a system with free discharge fans is fan coil unitsLater Don will come back and discuss different types of fans and their control characteristics.VSDs and their effect on system components
11 Free Discharge System Draw-through cooling tower Propeller fan louversI’m sure most of you are familiar with a draw through cooling tower.[SLIDE] The one shown here uses a propeller type fan mounted on the top which pulls air through the tower louvers, fill and falling water. The tower cooling effect is caused by the evaporation of some fraction of the water as it falls from the distribution pans on the top to the collection sump at the bottom.[LEE] For the purpose of exploring the performance of a free discharge fan system we are going to ignore the impact of changes in air density at different tower loads.So, what do the fan laws tell us about the performance of this system?outdoor airfillsump
12 General Fan Performance cfm a rpmDp a rpm²hp a rpm³[SLIDE] As Don discussed, the total pressure the fan must work against at various loads is related to approximately the square of the airflow rate. But this is really only true for free discharge fan systems.
13 system performance Static Pressure 100system resistance8060static pressure, %40friction pressure[SLIDE] Many fan system applications require a fixed static pressure component, as shown on this system/pressure flow diagram. [CLICK] This actually pushes the system curve up and causes a deviation from the fan law prediction.[LEE] But we’ll let Don worry about the effect of this.20fixed pressure20406080100airflow, %
14 system performance Free Discharge System 1008060system resistancestatic pressure, %40The good news with a cooling tower is that there is no fixed static pressure component. So the fan law equation describes the system performance pretty well.[SLIDE] For our purposes this will be the definition of a Free Discharge System—one in which the static pressure component is equal to zero. The result is that the system curve intersects the chart’s origin, and the pressure rises per the square of the flow to the system design point.Now that we understand the system curve, we need to look at the fan curve for a propeller-type fan and see how it interacts with the system curve.2020406080100airflow, %
15 fan performance curves Fan Speed (N) 10080N260N1static pressure, %40[SLIDE] The basic fan curve at a specific speed might look similar to this. It rises somewhat from the blocked tight, no flow condition, then at higher flows falls to the final design volume and pressure.[CLICK] When you slow the fan speed, the whole curve shifts downward. At a given static pressure—at a lower speed—the fan moves less volume.[LEE] What about the fan’s mechanical efficiency at different loads and speeds?As fan speed varies …20so does airflow volume20406080100airflow, %
16 fan performance curves Speed N vs. Efficiency h 100h280h1N260h2'N1static pressure, %h1'40If you were to draw lines of equal fan mechanical efficiency between the different speed curves, they would lay in something like this.[SLIDE] The exact efficiency curves are dependent on the fan design. The important question we must answer is how does all of this interact with the system curve?Let’s add the system curve back and see what observations we can make.2020406080100airflow, %
17 performance curves Fan and System 10080h1N260N1static pressure, %h1'40[SLIDE] When we add the free-discharge system curve, we see one characteristic that makes it a great application for variable speed capacity control. The system curve typically tracks relatively closely to the fan constant efficiency lines.[LEE] So if an efficient fan is chosen at design conditions, it will maintain its mechanical efficiency as the capacity is reduced via speed reduction.2020406080100airflow, %
18 fan performance curves Cooling Tower 1008060fan power, %40Let’s look at some fan energy curves using a few different capacity control methods.[SLIDE] If the towers capacity is modulated by varying the fan speed so that the fan curve tracks the system curve, the fan’s energy use would follow this orange curve. The fan energy use is proportional to approximately the “cube of the speed.”How else might the airflow be modulated?2020406080100airflow, %
19 fan performance curves Cooling Tower 1001-speed motor8060fan and motor power, %40potential energy savings[SLIDE] Simple on/off control could be used to achieve an AVERAGE fan volume based on the time weighted average of full on and full off airflow. In this case the theoretical fan energy use would follow a straight line between the full on and off energy values. It’s obvious that at any fan airflow other than zero and 100 percent this control strategy’s energy use is significantly greater than that for variable speed.[CLICK] The area between the orange curve and the black line represents the potential energy savings between the two forms of control.2020406080100airflow, %
20 fan performance curves Cooling Tower 100801-speed motor60fan and motor power, %402- speed motor[SLIDE] The other historically common method for varying cooling tower fan capacity is the application of a two-speed motor, or two different motors, a large motor for design conditions and a smaller pony motor that operates at a lower speed.In this application, the fan cycles between high, medium and off. The energy used in achieving a specific AVERAGE fan volume is based on the time weighted average of the run time at these three speeds.[LEE] It’s really interesting that just by using a two-speed fan, you can come VERY close to achieving the fan energy of variable speed control. However this comes at the cost and complexity of a two-speed motor or two motors and the accompanying electrical switch gear. In the past the cost of the motor and switch gear was less than the cost of a VFD. But with the continued drop in the cost of VFDs, we understand that in a number of cases VFD cost is the same or even less than a two-speed motor and switch gear.2020406080100airflow, %
21 fan performance curves Cooling Tower 10080VSD60fan and motor power, %40So let’s talk about the VFD application. The FAN energy drops with the cube of the speed. But what does the customer’s utility meter see?[SLIDE] If we plot the electrical energy use of the tower fan MOTOR we see that it tracks above the fan energy curve. The motor and VFD efficiency ratings must applied to get the actual electrical energy used. Depending on the motor and VFD selected this would be between 3-6% at full load and greater at part load.[LEE] What does this all mean in terms of actual energy savings?2020406080100airflow, %
22 cooling tower application Fan Energy Comparison Control strategyEnergy use factor1-speed fan cycling (base)100% kWh2-speed fan cycling39% kWhHere’s an example that tells it all. [SLIDE] When controlling a cooling tower to a fixed setpoint, a two speed motor draws less than half of a single-speed application. A VFD application could save close to 80 percent. Both very impressive percentages.variable-speed control19% kWhsource: Marley Technical Report H-001A
23 fan/tower performance curves Free Cooling at Low Load 100100tower capacity80806060fan and motor power, %tower capacity, %4040[SLIDE] There is some other good news when it comes to draw-thru cooling tower operation. It has nothing to do with fan operation and results in significant “free cooling.”Because of its open construction, a draw-thru cooling tower can provide between 5 and 15 percent of its rated capacity with its fan off. Therefore, at low system loads the tower can provide true free cooling![LEE] This is good news no matter what type of fan modulation you use. It’s one of the few times you get something for nothing—as long as you ignore the condenser water pump energy, anyway.2020“free” cooling20406080100airflow, %
24 free discharge fans Summary Performance approximates the “cube of the speed”Variable-speed drives (VSDs) are a great option for modulating capacityWhen considering VSDs for chilled water plants, start at the cooling tower[LEE] As we’ve seen in this discussion free discharge fan applications benefit greatly from variable speed control.[SLIDE] With little or no fixed static pressure they can take full advantage of the fan law relationships. We’re often asked if variable speed drives should be applied in building HVAC systems. When it comes to cooling tower control I would suggest the answer is a resounding - YES - almost always.[LEE & DON] Don, I got the easy fan type. How about you do some hard work and discuss the operation of other fan applications?
25 Practical Application: Ducted Indoor Fans [HIDDEN SLIDE]VSDs and their effect on system components
26 System Resistance 3,500 cfm 2.0 in. wg static pressure airflow [DON] Let’s turn our attention to an indoor fan and associated duct work to deliver the air.[SLIDE] Assume that a system is designed to deliver 3,500 cfm and that to overcome the system pressure losses, the fan must generate 2.0 in. of water static pressure.airflow
27 L r V² Dp = f D 2 gc System Resistance [SLIDE] The Darcy-Weisbach equation tells us the static-pressure loss varies with the square of airflow.
28 system resistance curve 3,500 cfm2.0 in. wgsystem resistance curvestatic pressureThis system resistance curve represents the static pressure that the fan must generate, at various airflows, to overcome the resistance—or static-pressure loss—within this particular system. This curve is true only if all elements of the duct system obey the Darcy- Weisbach equation.[DON] We have found devices in the air distribution system that don’t follow Darcy- Weisbach.2,000 cfm0.65 in. wgairflow
29 some devices don’t obey the rules for System Resistance Valves. Actually valves seems to do just the opposite of Darcy-Weisbach.[SLIDE] Air valves and water valves appear to produce the highest pressure loss when flow is low and produce a lower pressure drop when mass flow is high. Valves are the devices we employ to regulate mass flow.[DON] Valves regulate mass flow consuming energy … energy produced by the fan or pump.
30 system resistance curve valvesclosedvalvesopensystem resistance curvestatic pressure[SLIDE] Air valves in a VAV system and water valves in a variable primary system alter the system resistance curve by creating more or less resistance to mass flow.airflow
31 VAV System system resistance actual design surge region static pressuremodulationrange[SLIDE] This modulation causes the actual system resistance curve to shift. In a VAV system, therefore, the fan no longer operates at a single point on its performance curve but must operate over a range of such points.airflow
32 Fan Performance 1,100 rpm blocked-tight static pressure [DON] Fans create the pressure necessary to overcome resistance to flow imposed by ductwork. The geometry of the fan will influence fan efficiency as well as the relationship between airflow, static pressure, and power.[SLIDE] For example, many of you in the audience recognize this curve as typical of a forward curved fan. This curve graphically illustrates the performance of this fan when it is operated at a constant speed. The curve extends from blocked-tight static pressure, with a corresponding zero airflow, to wide-open airflow, with a corresponding zero static pressure.wide-openairflowairflow
33 Fan Speed static pressure 1100 rpm 900 rpm 700 rpm 500 rpm airflow [DON] The fan laws predict performance characteristics of the fan at other rotational speeds. Air flow is proportional to rpm and static pressure is proportional to rpm squared.[SLIDE] The result is a family of curves that represent airflow capacity and static pressure at various fan speeds.700 rpm500 rpmairflow
34 VAV System system resistance static pressure 1100 rpm 900 rpm 700 rpm [SLIDE] The fan is compelled to obey the fan laws. Resistance imposed by ductwork is predicted by the system resistance curve. Fan and ductwork both operate at the same point: That point where the system curve and fan curve intersect.[DON] But variable air volume is a dynamic system, with an ever changing system resistance curve, and a large family of fan curves at various speeds.700 rpm500 rpmfan speedairflow
35 VAV System System resistance changes as valves modulate … T T In a VAV system, the quantity of air being delivered to each space is controlled by an air valve.[SLIDE] This device is controlled by a thermostat to provide only the quantity of conditioned air needed to balance the space load. [CLICK] As the air valves modulate, the overall system resistance changes as well as the desired air flow.[DON] We need some type control system to alter the cfm moved by the fan as well as changing the static pressure produced by the fan.air valves900 cfm500 cfmTT
36 comparison of methods Fan Modulation 1001BI fan with discharge dampers8012AF fan with inlet vanes6023FC fan with discharge dampers404FC fan with inlet vanesdesign power, %3[SLIDE] These curves describe the performance characteristics of various methods of fan capacity control. Many methods of fan control have been employed. Fan speed control can be applied to any fan type and practically all fan applications.[DON] What is it we want the fan modulation system to do? Is it more than just save energy?45fan-speed control205vaneaxial fan with variable-pitch blades6620406080100design airflow, %
37 fan modulation Objectives Produce adequate static pressureEliminate excess static pressureExploit diversityMaximize energy savings at fanProvide stable controlKeep everyone comfortable[DON] There are a number of assignments given to a capable fan capacity control system. [SLIDE] First, the capacity control system must keep fan static pressure within the desired range; but we also want the control to be stable, we want to save a lot of energy, and of course we want everyone to be comfortable.
38 Static Pressure Control Insufficient static pressure?[DON] When the static pressure upstream of the air valve, or damper, is too low the VAV unit is unable to delivered the desired airflow.[SLIDE] Comfortable conditions cannot be maintained no matter how low we set the thermostat. We better give the maintenance department a call.VAV box delivers too little airflow
39 Static Pressure Control Excessive static pressure?Excessively high upstream static pressure can cause more problems than just wasting energy. High duct pressures make air flow control difficult and can increase damper generated noise.[DON]“Why is MY desk located under noisy duct work?”Wasted energyPoor comfort controlPoor acoustics
40 Fan Control Loop static pressure sensor supply fan S controller In order to ensure adequate static pressure at the VAV terminal units, a simple control loop is used.[SLIDE] First, the static pressure is sensed from a location in the system. Next, a controller compares this static-pressure reading to the system’s set point. Finally, the fan capacity is varied to deliver the required airflow at a static pressure that maintains this set point at the location of the system’s sensor.[DON] That seems simple. “Where do I locate the sensor?”controller
41 VAV System Modulation system resistance actual design static pressure VAV modulation curveA better understanding of how fan capacity is controlled will help answer that question.[SLIDE] Assume that the air conditioning load decreases, causing all or some of the VAV dampers to modulate closed. [CLICK] This causes the system resistance curve to shift upwards. In response, the fan begins to “ride up” its performance curve. As a result, the fan delivers a lower airflow at a higher static pressure. The system static-pressure controller senses this higher static pressure and sends a signal to the supply fan controller to reduce fan speed. [CLICK] A lower fan speed results in a new fan-system balance point, bringing the system static pressure down to the sensor’s set point.[CLICK] This action results in the fan unloading along a curve called the VAV system modulation curve. This curve represents the fan modulation needed to balance the static pressure produced by the fan with the static pressure required by the system. The fan operating point will always be on the VAV system modulation curve because fan speed is control by the static pressure sensor, not the velocity squared value predicted by Darcy- Weisbach.[DON] The sensor needs some non zero measurable setpoint. Substantial energy is saved. However, some of the dampers will be throttling flow and consuming energy. It is the need for fan control that causes energy saved to be less than rpm cubed.Notice we control system static pressure, not system cfm.1100 rpm800 rpmairflow
42 static pressure control Sensor at Fan Outlet supply fanS[SLIDE] One possible location for the static pressure sensor is near the outlet of the supply fan. The controller is set to maintain the static pressure required at design flow.The appeal of this method is that the sensor can be factory-installed and tested, resulting in greater reliability and no field installation cost. If fire dampers are included in the supply duct, this method ensures that the sensor is on the fan side of the damper so that the duct is protected from high pressures. Also, depending on the layout of the duct system, this method may eliminate the need for multiple duct-mounted sensors. It is not, however, as energy efficient as the other methods.[DON] This concept requires all the dampers to throttle back increasing fan static to a level above design. Sensing this over pressure, the fan controller will reduce fan rpm. However the savings presented by the form of speed control are limited because all the dampers are throttled to an energy wasting position.VAV boxescontroller
43 static pressure control Sensor Down 2/3 of Duct supply fanS[SLIDE] In the most common method for sensing and controlling system static pressure, the static-pressure sensor is located in the supply duct system. Determining the best sensor location for all load conditions can be difficult—often determined by trial and error or by using multiple sensors. [CLICK] A typical starting point is two-thirds of the distance between the supply fan and the end of the supply duct.[DON]“Why two thirds?”Notice that dampers located downstream of the static pressure sensor have no vote. [SLIDE-CLICK] Because the controller maintains a fixed static pressure upstream of these zones, these dampers must close, wasting fan energy, in order to deliver the desired airflow in those zones.VAV boxescontroller
44 static pressure control Sensor Down 3/4 of Duct supply fanS[SLIDE] Fan energy savings can be increased by moving the sensor further down the air distribution system. This will increase the amount of load diversity sensed above the static pressure sensor.[DON] In essence more dampers are allowed to move to a more open position. Less fan energy is wasted by dampers throttled closed.VAV boxescontroller
45 static pressure control Sensor Down 3/4 of Duct supply fanS[SLIDE] There is a possibility that this far location in not sensitive to all upstream damper conditions. [CLICK] Some dampers upstream of the sensor may be starved for air. The simple solution is a higher setpoint at the static sensor. This defeats the the gain of moving the sensor downstream.[DON] In the end static pressure sensor setpoint may be more critical the actual sensor location. ASHRAE Standard 90.1 requires static pressure setpoint to be no greater than one-third the total design fan static pressure, if this control method is used.VAV boxescontroller
46 optimized static pressure control Sensor at Fan Outlet VAV damper positionssupply fan speed or inlet vane positionstatic pressureSstatic pressure setpointAn optimized static-pressure control method combines the location-related benefits of fan outlet control with operating cost savings that exceed those of supply duct static-pressure control.[SLIDE] A single static-pressure sensor is located at the fan outlet, and the controller dynamically adjusts the static-pressure set point based on damper position in the VAV units.The DDC/VAV controllers know damper position and, because they are pressure independent, they modulate damper position to maintain required airflow. The building automation system continually polls the VAV units looking for the most-open damper. The controller resets the static-pressure set point so that at least one damper, the one requiring the highest inlet pressure, is nearly wide open. The result is that the supply fan generates only enough static pressure to get the required flow through this “critical” vav damper.[DON] This method allows the sensor to be factory-installed and tested. If the vav units use DDC control, the system-level communications are already in place, making this the lowest-cost, highest energy savings strategy. ASHRAE Standard 90.1 requires VAV systems with direct digital control of individual zone boxes reporting to the central control panel, to reset static pressure setpoint based on the zone requiring the most pressure.communicating BAS
47 static pressure control methods Performance Comparison Fan static pressure2.7 in. wg2.1 in. wg1.9 in. wg1.5 in. wgFan input power13 hp22 hp12 hp9.5 hpFull-load power60%100%55%43%Airflow24,000 cfm (full load)18,000 cfmFan outlet[SLIDE] A comparison of these static-pressure control methods demonstrates the energy savings potential. At this representative part-load condition, using the optimized static- pressure control method allows the supply fan to use only 43% of its full-load power versus 55% for the supply duct static-pressure control method.[DON] In addition to the supply fan energy savings, because the optimized static-pressure control method allows the system to operate as if the static pressure sensor was at each individual terminal unit, it ensures that no spaces are “starved” for air. There are also acoustical benefits at part load by operating the supply fan and VAV terminal units at the lowest possible duct static pressure.Supply ductOptimized
48 VSDs and their effect on system components System Demonstration[HIDDEN SLIDE][LEE & DON] Wow - so much for the thought that “a fan is a fan is a fan.” Don, that’s a great reminder that the application and control really does impact the benefit of the modulation technology applied.Let's now turn to pumps, variable flow and variable speed.VSDs and their effect on system components
49 Practical Application: Pumping Water a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006Practical Application: Pumping Water[HIDDEN SLIDE][LEE & DON] Wow - so much for the thought that “a fan is a fan is a fan.” Don, that’s a great reminder that the application and control really does impact the benefit of the modulation technology applied.Let's now turn to pumps, variable flow and variable speed.VSDs and their effect on system components
50 why care about Pump Energy a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006why care about Pump EnergyAccording to the DOE ...Pumps represent 5% of industrial energy consumptionTotal cost of owning a pump is 90% energy consumptionPump energy consumption generally can be reduced by as much as 20%Of course we’re interested in pumps because they consume a significant amount of global energy.[SLIDE] According to the DOE, better pumping system design and control could save up to one fifth of the total pumping energy used world wide.Also the use of variable water flow in HVAC systems is an exploding trend. One reason is because of its energy saving potential and a second is that many codes require it.
51 pumping chilled water ASHRAE 90.1-2001 a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pumping chilled water ASHRAERequires variable chilled water flow if:Total pump power exceeds 75 hpANDSystem includes > 3 control valvesASHRAE 90.1 for example requires that any heating or cooling water system that has a total pump power that exceeds 75 horsepower and has more than three control valves be variable flow.
52 pumping chilled water ASHRAE 90.1-2001 a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pumping chilled water ASHRAERequires 30% design wattage at 50% flow if:Any variable-flow pump motor > 50 hpANDDesign head pressure > 100 ft[LEE] In addition, once any system pump that must overcome 100 feet of pressure difference has at least a 50 horsepower motor,[SLIDE] it must have the capability to greatly reduce its energy consumption at system part load conditions.[CLICK] This is almost always achieved by using a variable speed drive on the pump motor.Typical solution: Variable-speed drive
53 chilled water system Variable Primary Flow a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006chilled water system Variable Primary Flow12750 gpm[LEE] Okay, so we’ll assume we have a variable flow chilled water system. Let’s look at an example system and see how it reacts to variable flow and how different pump control options impact pump energy use.[SLIDE] Shown here is a flow diagram for a simple two chiller variable primary flow system. Lets take this system and examine its pressure drop profile at full and part load flows using different pump control techniques.
56 pumping system Pump & System Curves a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pumping system Pump & System Curves100pump80design point60pump head, %system40Let’s plot the system and pump curves on a flow verses pressure chart. [SLIDE]As the system flow increases the pressure drop rises up [CLICK] in accordance with the pump affinity laws.[CLICK] The pump pressure falls following that particular pump’s flow verses pressure curve.[CLICK] The intersection of the curves represents the design flow and pressure drop operating point for the system.2020406080100water flow, %
57 pump characteristics Power a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pump characteristics Power100pump80design point60pump head, %system40[SLIDE] If we add the pumping power to this chart we can find the corresponding pump power for the system. We also see that the power drops per the pump affinity laws.[LEE] Its drop is proportional to the change in flow and pressure drop.20100pump power50power20406080100water flow, %
58 variable-flow water system How Does It Unload? a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable-flow water system How Does It Unload?Depends on:Chilled-water system curvePump curvePump control methodRide the curveDifferent pump sizesVary the speed[SLIDE] The key question that we want to answer is how does the system unload with various load control techniques? [CLICK] We’ll start by examining a system that varies its capacity simply by throttling the control valves and RIDING the pump curve.[LEE] Let’s see what this looks like on the system and pump curve and then on a system diagram.
59 pump characteristics Ride the Curve a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pump characteristics Ride the Curve100pump80part-load pointdesign point60pump head, %system40[SLIDE] The pump in this case is operating at a constant speed and its curve is fixed. If we want to reduce the system flow and therefore capacity we must add resistance to the system to push the system curve back up the pump curve. This is what is known as riding the pump curve. The way resistance is added to the system is simply by modulating the control valves more closed. Note that the lower the flow the higher the pump pressure and the more pressure the control valves must close off against.[LEE] The pump energy saved is PROPORTIONAL to the reduction in flow and INVERSELY proportional to the increase in pump pressure. Therefore the actual energy saved is a function of the shape of the pump curve. If the pump curve is flat then the savings will be significant. If the curve is steep and the pressure rises quickly then the savings will be smaller. That is why on this type of system, pumps with a flat curve should always be selected.20100pump power50power20406080100water flow, %
61 variable-flow water system How Does It Unload? a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable-flow water system How Does It Unload?safe zoneDepends on:Chilled water system curvePump curvePump control methodRide the curveDifferent pump sizesVary the speed[LEE] Rather than ride a single pump curve lets try applying multiple pumps of different sizes.
62 Pump Characteristics pump pump head, % design point system pump power a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006Pump Characteristics100pump80design point60pump head, %system40pump power[SLIDE] Here again we see our design-condition system/pump curve.2010050power20406080100water flow, %
63 pump characteristics Different Pump Sizes a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pump characteristics Different Pump Sizes100systempump80design point60pump head, %part-load point40pump power[SLIDE] If we have available a second smaller pump selected for a lower flow at a lower pressure drop for part load not only will the system pressure go down, rather than up, but that pump’s power curve will be lower also.2010050power20406080100water flow, %
64 pump characteristics Different Pump Sizes a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pump characteristics Different Pump Sizes100systempump8060pump head, %40pump power[SLIDE] Add a third even smaller pump for even lower loads and it gets even better. MAYBE we’re on to something here?[LEE] But the complexity of selecting and operating all these different pumps would be daunting. Well, this is really just a bait and switch on my part.2010050power20406080100water flow, %
65 pump characteristics VFD = Different Pumps a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006pump characteristics VFD = Different Pumps100system801750 rpm1488 rpm60pump head, %1225 rpm40[SLIDE] Having a variable speed pump is really just like having an infinite number of different size pumps.Excellent! We can have the perfectly sized pump at every load point IF we can control the PUMP speed to slide down the SYSTEM curve rather than riding up the PUMP curve.[LEE] Now this is a neat concept but the actual energy use can vary greatly depending on how the pump speed is actually controlled. If we don’t control the pump speed to ride the system curve we will not achieve the potential energy savings. Again, lets examine a few examples and see the impact of different control methods.2010050power20406080100water flow, %
66 variable-speed pump Control Methods a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable-speed pump Control MethodsPressure control (DP) at pumpPressure control (DP) at end of systemCritical-valve pressure reset[SLIDE] We’ll look at these three pump control methods and graphically evaluate their impact on system energy use.
68 variable-speed pump Control Methods a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable-speed pump Control MethodsPressure control (DP) at pumpPressure control (DP) at end of systemCritical-valve pressure reset[LEE] But if we’ve gone as far as putting in a variable speed pump and DDC control can’t we do better with our controls strategy? The answer is absolutely! Let’s try controlling the pump speed to a pressure that we measure at the end of the system ...
70 variable-speed pump Control Methods a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable-speed pump Control MethodsPressure control at pumpPressure control at end of systemCritical valve pressure reset[LEE] [Hidden Slide]Let’s examine one more strategy to see if we can squeeze just a little more energy savings out of our system. Maybe it won’t be so little.
76 variable-flow pumping Summary a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable-flow pumping SummaryEnergy savings depends on:Pump selectionsFixed vs. frictional pressure componentsControl strategyEnergy savings can approach “cube of speed”Great application for variable-speed drives[LEE] The potential amount of energy saved with variable flow is a function of a number of system design criteria - control strategy being a key one.[SLIDE] Depending on the design, the energy savings can approach that predicted by the pump affinity laws.And finally, over all, this is a great application for VFD-based speed control. This is born out by the overwhelming use we see for VFDs in this application.[LEE] Well, that’s my time. Mick, your turn. Take us through variable flow on condenser water systems.
77 variable-flow condenser water Pump Speed a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable-flow condenser water Pump SpeedDetermining minimum speedHow variable flow affects:PumpCooling towerChillerControlling flow to improve system performance[MICK] Thanks for covering the chilled water pump portion of the system, Lee.Now let’s move over to the condenser water side.[SLIDE] There are three topics we’ll cover:Determining minimum pump speedThe effect of variable condenser water flow on the pump, cooling tower and chiller, andControlling the condenser water pump to give better system performance.
78 condenser water pump Minimum Speed a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006condenser water pump Minimum SpeedDeterminants:Minimum condenser flowTower static liftMinimum tower flowNozzle selectionPerformanceCompare curve with cubic[SLIDE] The minimum pump speed depends on a number of variables.The first is the minimum flow rate allowed through the chiller’s condenser. This information can be gotten directly from the chiller manufacturer.Next, when we consider the condenser water loop, a portion of that loop is closed and a portion of the loop is open.
79 cooling tower Static Lift a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006cooling tower Static Liftstatic lift[SLIDE] Within the closed portion of the loop the water elevation difference doesn’t add pressure the pump must push against. However, the pump must always overcome any elevation difference in the open portion of the loop.This open portion is at the cooling tower. The elevation difference between the cooling tower’s sump and the top of the cooling tower is known as “static lift” and must always be overcome.So in this case, the minimum for the pump is not a flow rate, but a speed at which the pump can still produce the pressure required to overcome the system pressure drop plus the tower static lift.
80 cooling tower Water Distribution a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006cooling tower Water DistributionThe third possible limit for minimum condenser water flow is the minimum flow rate the tower manufacturer will allow.In order to maintain effective heat transfer the tower fill must remain wetted. If the flow rate drops too far portions of the tower can become dry – obviously affecting heat transfer.So, if you’re considering variable condenser water flow, make sure you work closely with the cooling tower provider. That way, they can provide the tower that can maintain good heat transfer as flow is reduced.
81 a Trane Engineers Newsletter Live satellite broadcast VSDs and Their Effect on System Components • 1 Feb 2006Example1500 gpm system, 1770 rpmMinimum flows: Chiller 658 gpm Tower 750 gpmTower static lift 12.2 ftPump: Speed 974 rpm Pump flow 875 gpm[SLIDE] Here’s an example system that has 1500 gpm as its design and the minimum flow rates on the slide. The limiting factor in this case is the tower static lift. To meet that lift requirement we need to maintain a flow rate of at least 875 gpm.
83 operating dependencies Full Flow a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006operating dependencies Full Flow[SLIDE] The chiller and tower work “together” in that they are “connected” by condenser water temperature and flow. As we’ll see in Ryan’s portion this sets the “lift” or pressure difference the chiller must work against.The chiller power is also dependent on its design and the load its producing at this point in time.The tower operation is dependent on its design as well as the ambient conditions and heat rejection load,[CLICK] Both the chiller and the tower are affected when the condenser water flow rate is reduced.Let’s look at an example to see how much.Tower designCondenser water temperature & flowHeat rejectionWet bulbChiller designCondenser water temperature & flowLoad
84 variable condenser water flow Effect on Tower a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable condenser water flow Effect on Tower110100100% fan90tower entering water, °F80[SLIDE] Cooling towers can reject heat at many conditions.Shown here is the performance of the cooling tower, with a chiller load of 70% and an ambient wet bulb temperature of 70 F. We’ve charted the temperature of water entering the cooling tower.On the right side of the chart we see that the water temperature entering the tower is about 84 degrees.If we reduce the water flow rate, the temperature entering the tower goes up. Since we are rejecting the same amount of heat with lower water flow, the water temperature entering the tower rises. For example at 50% flow it’s 96 degrees.Now, since the tower entering water temperature is the same as the chiller leaving condenser water temperature, we can look at the chiller power at each of these conditions.70605060708090100condenser water flow, %
85 variable condenser water flow Effect on Chiller a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable condenser water flow Effect on Chiller30096°F LCWT25020084°F LCWTchiller power, kW150[SLIDE] At the same chiller load, as the water temperature leaving its condenser rises, so does chiller kW.Ryan will talk about the chiller change in a few minutes.If we just looked at chiller kW, we’d never reduce the condenser water flow rate.But we need to add the cooling tower fan kW, the pump kW and the chiller kW so we can see how the system operates.Remember, this is a snapshot in time – 70% load and 70 degree wet bulb.100Conditions:70% load70°F WBFull-speed tower fan505060708090100condenser water flow, %
86 variable condenser water flow Effect on System a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable condenser water flow Effect on System300system250chiller200component/system power, kW150[SLIDE] The minimum chiller plus condenser water pump plus cooling tower kW occurs between 80 and 90% of the condenser water flow rate.However, it’s not that easy. So far we’ve assumed that the cooling tower fans are operating at full speed all the time.Since ASHRAE 90.1 requires speed control on cooling tower fans in many applications, a lot of people put VFD’s on their cooling tower fans.10050towerpump5060708090100condenser water flow, %
87 reducing flow & fan speed Effect on Tower a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006reducing flow & fan speed Effect on Towerfan speed11050%60%10080%100%90tower entering water, °F80conditions:70% load70°F WB[SLIDE] So, we need to examine how the entering tower water temperature changes when we reduce the flow rate AND tower fan speed.Since reducing tower fan speed reduces airflow – and heat rejection capability, the tower entering water temperature rises at reduced tower airflows as shown in this chart.So, let’s look at all of these flow rates and tower fan speeds and see how the system kW compares.70605060708090100condenser water flow, %
88 reducing flow & fan speed Effect on System a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006reducing flow & fan speed Effect on System30080%60%50%fan speed250100%200system power, kW150conditions:70% load70°F WB[SLIDE] When we look at all the possible operating points we see that the system kW is at its minimum when the fans and pumps are at about 90% flow rate.But this graphic also shows another important fact.Take the top, red line where the cooling tower fan is operated at 50% speed. Once the water flow rate drops below 80%, the temperature of water produced by the cooling tower rises so much that it sends the chiller’s compressor into a surge region – again Ryan will cover this.If you’re going to reduce flow rates and tower fan speeds, do so in a manner that keeps the chiller from surging.Okay, what we just looked at was the chiller operating at 70% load when the wet bulb is 70 degrees.100505060708090100condenser water flow, %
89 reducing flow & fan speed Effect on System a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006reducing flow & fan speed Effect on Systemfan speed30025050%60%80%200100%system power, kW150conditions:70% load50°F WB[SLIDE] What if the chiller is at 70% load, but the ambient wet bulb is 50 degrees?This might occur in a multiple chiller plant when only one chiller is operating.Here, a tower fan speed of about 80% and a water flow rate of 80-90% is the best.Note that if the water flow gets turned down too far, the total system kW rises significantly.A piece good news is that since the ambient wet bulb is low enough, the chiller doesn’t surge.100505060708090100condenser water flow, %
90 reducing flow & fan speed Effect on System a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006reducing flow & fan speed Effect on Systemfan speed300250conditions:30% load50°F WB200system power, kW150100%80%[SLIDE] Our final example examines the same system at 30% load, and a wet bulb of 50 degrees.The system power consumption is fairly flat.The one thing I’ll note is that reducing the tower fan speed makes sense at all water flow rates. Said another way, while chillers can operate with entering condenser water temperatures of 55 degrees, don’t try to drive the tower water temperature as cold as possible. Back off on tower fan speed and save some system energy consumption.1005060%50%5060708090100condenser water flow, %
91 variable condenser water flow Summary a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable condenser water flow SummaryDetermine what savings, if any, are possibleAre pumps already low power?Can reducing tower-fan speed achieve most of the savings?[MICK] Remember that the charts you saw here were for one specific application of a chiller, pump and cooling tower. Depending on the efficiency and power draw of each, the results can vary significantly.Nevertheless, sometimes we’re asked for guidance when people are considering variable condenser water flow.[SLIDE] First, you need to determine whether or not reducing condenser water flow rates should even be considered. If you’ve already minimized the condenser water pump kW, for example by its reducing design flow rate to 1.5 or 2 gpm/ton, it may not even make sense to take the time to look further.[MICK] Also, reducing just the tower fan speed may give you most of the energy savings, and it’s simpler.
92 variable condenser water flow Summary a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable condenser water flow SummaryIf you decide to reduce flow:Find minimum condenser- water flow rateExamine system at various loads and wet-bulbs … keep chiller out of surgeDocument the sequence of operationHelp commission the system[SLIDE] If you do decide to reduce condenser water flow rates during operation, [CLICK] know the minimum condenser water flow rate for your system.Then, to find efficient operating points at various loads and ambient conditions, [CLICK] take the time to examine the system at all those loads and ambient wet bulb temperatures. What occurs in real life is quite different than what the ARI chiller rating standard assumes.One reason to study the system is to keep the chiller out of its surge region and allow it to meet the cooling load. This is especially important at wet bulb temperatures closer to design.Once you’ve done this work, [CLICK] write a detailed sequence of operation.Finally, there is no replacement for being out at the job site with the system operator and seeing what happens during operation. [CLICK] Consider being part of the commissioning team.
93 variable condenser water flow Guidance a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006variable condenser water flow GuidanceCan provide savings …Finding proper operating points requires more time, more fine-tuningTwo-step process:1 Reduce design pump power2 Is variable condenser-water flow still warranted?The bottom line is that variable condenser water flow allows some systems savings, but it takes more design examination and on-site fine tuning to achieve those savings.We suggest that you first reduce condenser water pump power at design – you can easily do this by reducing the design flow rate. Then determine if variable flow is warranted.
94 Practical Application: How VSDs Affect Chillers a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006Practical Application: How VSDs Affect Chillers[HIDDEN SLIDE][MICK] Okay, I touched on how a chiller is impacted by external variables in a system. For example condenser water entering and leaving temperatures, and condenser water flow rate.Ryan is now going to take us inside the chiller so we can understand their impact, and more specifically how a variable speed drive affects chiller performance as these external parameters change. We’ll see that it’s quite different than the pump or fan effects we’ve seen so far.VSDs and their effect on system components
95 VSDs and Chiller Laws resistance a velocity² resistance a “lift” a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSDs and Chiller LawsVariable-speed drives benefit centrifugal compressors in water chillersReview “chiller laws”Explore scientific cause-and-effect relationshipsMaximize benefits[RYAN] Thanks, Mick.Clearly, drive technology offers tangible energy-saving opportunities … and, the same CAN hold true for chillers.[SLIDE] [CLICK] As Don pointed out earlier, the centrifugal chiller laws remind us that “resistance” to mass flow is not a function of refrigerant velocity. Rather, the “resistance” correlates to the pressure difference, or “lift,” the compressor must generate between the condenser and the evaporator. Therefore, resistance is not proportional to velocity squared; instead, it is proportional to “lift.”[RYAN] I will explore these ideas in detail, but before I do, I’d like to share an often-told story on this topic, as many believe the following analogy formulates a better understanding.resistance a velocity²resistance a “lift”
96 VSD and centrifugal chillers A Simple Analogy a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSD and centrifugal chillers A Simple Analogybrake(inlet guide vanes for unloading)accelerator(speed control of chiller motor)[SLIDE] It goes like this … [CLICK] Think of your vehicle's accelerator as the chiller’s electric motor speed control. [CLICK] You can also think of the vehicle's brake as the inlet guide vanes, or the unloading device.
97 a simple analogy Constant-Speed Chiller a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006a simple analogy Constant-Speed ChillerMotor runs at constant speed, regardless of load[RYAN] If you have a constant-speed chiller, the electric motor runs at a constant speed regardless of load. If the chiller needs to operate at off-design conditions, inlet guide vanes restrict the amount of refrigerant allowed in the compressor.[SLIDE] Applying this concept to the story, then, [CLICK] one might think of the accelerator being pegged to the floor, and [CLICK] to slow down, you press the brake.Inlet guide vanes restrict refrigerant at off-design conditions
98 a simple analogy Variable-Speed Chiller a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006a simple analogy Variable-Speed ChillerBased on load, motor speeds up or slows down[SLIDE] If you have a variable speed chiller, the analogy implies the electric motor slows down or speeds up depending on the load, just as the accelerator on an automobile.
99 VSDs and centrifugal chillers An Analogy a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSDs and centrifugal chillers An AnalogyIn each case:Energy is wastedMechanical wear-and-tear is increased[RYAN] Common sense tells you that pegging the accelerator to the floor and using the brake to slow down is a terrible waste of fuel and can lead to added wear and tear. The analogy implies the same conclusions, that is without a VSD on the chiller, electricity is wasted and mechanical wear and tear increases.Simple, and easily understood. Let’s “test” this accuracy by looking into the science.
109 Lessons Learned To reduce lift: a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006Lessons Learnedlvg cond waterTo reduce lift:Decrease condenser pressure by reducing leaving-tower water temperatureIncrease evaporator pressure by raising chilled water setpointVSDs optimize chiller lift efficiency99°F75°Fcompressor work[SLIDE] Applying these concepts to the compressor work rectangle will graphically demonstrate that LIFT is reduced only through 2 means:1. [CLICK] Lowering the condensing temperature by delivering colder condenser water than design.Or,2. [CLICK] Increasing the evaporator temperature by resetting the chilled water setpoint warmer.Understanding the differences between load and lift is critical to ensure the benefits of drive technology are attainable and balance the higher first cost with potential energy savings.[RYAN] As discussed, centrifugal compressors are variable volume; therefore, either inlet guide vanes alone or the combination of inlet guide vanes and a variable-speed drive will respond to a change in load and result in reduced power consumption. All types of centrifugal chillers benefit from lower condensing temperatures. Variable-speed drives simply allow more benefit by improving the part lift efficiency of chillers.[MICK] Ryan, let me take a shot at showing how chiller savings compares to fan and pump savings we’ve already looked at.[RYAN] Okay.45°F41°Flvg evap water
110 various system components Energy Use a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006various system components Energy Usefree discharge fan100chilled water pump (DP at end of loop)80condenser water pump (stopped at 875 gpm)60chiller w/low liftenergy use, %40[MICK] I think of the “static lift” of the cooling tower as comparable to the compressor “lift.” A condenser pump must always overcome the water elevation difference between the cooling tower’s sump and the top of the cooling tower, and the chiller’s compressor must overcome the difference in pressures between the evaporator and the condenser.[RYAN] I like the comparison.[SLIDE][MICK] So, from a comparison standpoint we can look at the energy consumption of:[CLICK] A free discharge cooling tower fan[CLICK] A chilled water pump using a DP sensor at the end of the loop,[CLICK] A condenser water pump that had to stop at 875 gpm, and[CLICK] A chiller running with reduced condenser water temperature – and thus lower lift.[RYAN] So this shows that chillers do not benefit from the “cube of the savings” rule of thumb.[MICK] Exactly. The chiller evaporator and condenser water temperatures make what might be thought of as a “DP setpoint” for the chiller.Okay, with that said, let’s look back at that analogy you began with … it seemed pretty simple. Maybe too simple?static lift2020406080100load, %
111 VSDs and centrifugal chillers A Simple Analogy a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSDs and centrifugal chillers A Simple Analogybrake(inlet guide vanes for unloading)But misleading and technically incorrectaccelerator(speed control of chiller motor)[SLIDE] [RYAN] Yes. I think of it as simply misleading and technically wrong. As we’ve seen, it misrepresents how a VSD integrates with the fundamentals of centrifugal compression leading people to incorrect conclusions and misapplication of drives.[RYAN] Simply stated, variable-speed-drive chillers do not follow the “cubic savings” rule of thumb.Now let me take a few minutes to discuss how chillers are rated.
112 chiller efficiency at part load IPLV and NPLV Conditions a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006chiller efficiency at part load IPLV and NPLV Conditions100%85°F ECWT75%75°F ECWT50%65°F ECWTlift[RYAN] Because there is a clear need to establish minimum efficiency standards, ARI, the Air-conditioning & Refrigeration Institute, created Standard 550/590 as an attempt to identify a single variable or unit of measure to represent a chiller’s performance rating. IPLV, or integrated part-load value is defined as a weighted average of compressor work by combining both part load and associated part lift into four arbitrary, predefined conditions.Now, remember, part load and part lift are different, so the assumptions made are critical. Unrealistic assumptions will lead to incorrect performance conclusions.NPLV, or non-standard part-load value was also established to allow the chiller’s design operating conditions to vary while re-using the same IPLV formulas and associated load and lift predefined buckets.[SLIDE] Symbolically, applying the compressor work rectangle to the IPLV/NPLV variable might look something like this … [CLICK] the assumption is all hours of operation would fit into these 4 fixed conditions. That is to say, all the hours operating at certain part load conditions would simultaneously be operating at reduced lift conditions. This model might provide a means by which to compare one chiller’s performance to another, but it will not, nor was it intended to, be representative of how a chiller will operate within a system.[RYAN] The ARI standard simply cannot accurately represent a chiller’s energy use in a system, nor can the IPLV/NPLV value predict the savings that can be associated with the additional investment of a drive. As the standard suggests in its Appendix D, careful analysis is the only real method to reach fiscally responsible decisions.25%load
113 VSDs and centrifugal chillers A Closer Look at IPLV a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSDs and centrifugal chillers A Closer Look at IPLVLoadWeightingECWTkW/Ton100%0.0185°F0.57275%0.4275°F0.42950%0.4565°F0.324[SLIDE] Looking closer, and combining representative chiller performance values with the four arbitrarily chosen conditions for IPLV/NPLV highlights another controversial industry issue. That is, the recent claims that running two chillers with variable-speed drives at part load is more efficient than one chiller fulfilling the same load… Of course, if attention is focused on these numbers, the assertion seems authentic. Here the 50% load value of kW/ton has a commanding efficiency advantage over the full load value of[CLICK] But, the 50% load number has condenser water that is 20 degrees colder than the full load number! Of course it looks more efficient – its lift has been reduced! Again, variable-speed-drive chillers are part-lift devices.[RYAN] We need to compare the chillers at the same condenser water temperature. Let’s do that.50%0.4565°F0.393VSDs improve part-lift performance, so running two chillers with VSDs at part load seems more efficient than one chiller at double the same load, but …
114 VSDs and centrifugal chillers Performance at 90% Load a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSDs and centrifugal chillers Performance at 90% LoadECWT85°F80°F75°F70°F65°F2 Chillers*306.4268.0230.8195.2160.31 Chiller268.0238.0210.6185.7164.3Difference–38.4–30.0–20.2–9.5+4.3[SLIDE] The results, for a system load of 90% and with the condenser water temperatures shown, are represented here in the table.Unmistakably, in all but the most reduced “LIFT” condition, 1 variable-speed chiller operating at 90% load consumes less power than 2 equally loaded variable-speed chillers.[RYAN] It’s worth emphasizing that this comparison does what IPLV cannot – compare “like chillers" in a “like condition.” … Apples to apples, if you will.Note: Data shows only chiller power.*Load equally divided
115 VSDs and centrifugal chillers Performance at 90% Load a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSDs and centrifugal chillers Performance at 90% Load3502 chillers: 45% load each1 chiller: 90% load300250chiller power, kW200150[SLIDE] Representing the same data in a graphical format, punctuates that running 1 chiller consumes less power than running 2 at all but the coldest of condenser water temperatures.[RYAN] Remember, as Mick discussed, these cold condenser water conditions may not be economically attained. Also, pump and tower energy are not included in these numbers; only the chiller energy is shown here. When a chiller is brought on line, the associated ancillary equipment consumes power, too.If we leave a chiller off, there is no ancillary consumption. After all, there is no substitute for “OFF” when trying to maximize energy savings!100506570758085entering condenser water, °FConclusion: 1 chiller uses less power than 2 chillers
116 Analyze the System Model: Building use Local weather Economizers a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006Analyze the SystemModel:Building useLocal weatherEconomizersUtility ratesSystem designUse programs like TRACE™, DOE 2.x, Chiller Plant Analyzer, HAP[RYAN] We’ve found that VSD chillers can often be justified when:There are lots of run hours at low lift conditions, and with high energy rates during those run hours.One example where a VSD chiller often pays back is a chiller that operates during the winter. It has a lot of hours at low lift conditions.But, it’s imperative to analyze the system to make proper life-cycle decisions.[SLIDE] Only a system study that incorporates all aspects of location dependant weather, building diversity, and chiller performance characteristics will be able to determine if applying variable-speed drives to chillers is economical, such as a comprehensive hour-by- hour analysis tool. Bin methods can’t do this. Programs like TRACE, DOE, Chiller Plant Analyzer or HAP help you give the owner the information to make life-cycle cost decisions.
117 VSDs and centrifugal chillers Summary a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VSDs and centrifugal chillers SummaryVSDs improve chiller part-lift performanceLots of operational hoursReduced condenser water temperaturesHigher costs of electricityIPLV is not an economic tool[RYAN] The science of variable speed technology and its application to centrifugal chillers is similar, but discernibly different from pumps and fans. I leave you with these thoughts:First, there are many analogies and misconceptions surrounding this topic, and hopefully the discussion exposes the need for concern and a word of caution. [SLIDE] Perhaps the use of the compressor work rectangle may better serve the industry to best represent drives on chillers.Also, remember that variable-speed drives impact centrifugal chillers part-lift efficiency, not part load. As such, the combination does not follow the often referenced “cubic savings” rule of thumb.And, only an analysis, not a single number like IPLV/NPLV, can adequately represent how drives will perform.[RYAN] Bottom line, variable-speed drives are viable and effective, and should be applied to centrifugal chillers to maximize energy savings … in the right application.
118 Answers to Your Questions a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006Answers to Your Questions[HIDDEN SLIDE][MICK] You know, it always comes back to using the proper technology in the proper application.Now, it’s time to move to our question and answer session. Thanks for faxing yours in today. If we can’t get to yours while we’re on the air, we’ll get back to you within 4 weeks.VSDs and their effect on system components
119 wrap-up VSD Effect Differs a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006wrap-up VSD Effect DiffersCubic relationship to speed only occurs in “free discharge” systemsControl parameters affect savingsIn chillers, external parameters define lift (pressure difference)[MICK] To summarize what the team covered today, all drive effects are not created equal.While we know what the fan laws say about the cubic relationship, this very rarely occurs in HVAC systems.[SLIDE] Fan and pump savings are affected extensively by the control method chosen. And in chillers, the pressure difference the compressor must overcome is defined by external parameters – most often chilled and condenser water temperature.
120 wrap-up VSD Effect Differs a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006wrap-up VSD Effect DiffersCooling towers: Nearly cubicHVAC fans: Not cubicDepends on control strategyFan pressure optimization is bestChilled water pumps: Not cubicAffected by valves and control methodConsider pump pressure optimization based on critical valve[RYAN] Lee showed us that the closest we come to the cubic relationship is at the cooling tower –[SLIDE] [CLICK] and that reducing cooling tower fan speed can increase system efficiency – even though the chiller uses more power.[CLICK] Don covered how VFD’s on fans work within the system – and showed us that the savings are predicated on the control strategy. Fan pressure optimization is the best – and remember that ASHRAE 90.1 requires this on DDC/VAV systems.[CLICK] Chilled water pumps work in a similar manner to HVAC supply fans in that the control method is also critical. Pump pressure optimization is being used as a more common system control method.
121 wrap-up VSD Effect Differs a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006wrap-up VSD Effect DiffersCondenser water pumps: Not cubicMust meet minimum flow or pressureTower static liftMinimum condenser water flowMinimum tower flowReduced flow affects chiller and tower performanceBefore applying a VSD, reduce pump design power (CW flow rate)[SLIDE] [CLICK] On the condenser water side you have to keep the pump speed high enough to meet the minimum flow rate or pressure – and you really need to understand how the reduced condenser water flow rate affects the chiller and tower performance.[MICK] We suggest that before taking the time to investigate that relationship for your particular job, that you reduce the condenser water pump design power, by selecting it at lower flow rate.
122 wrap-up VSD Effect Differs a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006wrap-up VSD Effect DiffersPower for any chiller is reduced at part load and liftChiller savings? Not even close to cubicVSD helps more at part-lift conditionsMUST reduce lift for VSD to slow down and give benefitUse same condenser water temperature to compare constant- and variable-speed chillers[SLIDE] Next, no matter how a chiller is controlled, power is reduced at part load and part lift.The present rating standard makes inaccurate assumptions about lift. A better analysis is warranted, and the tools are available.Having a VFD on a chiller can work very well – as long as it’s in the proper application – that is, reduced lift conditions.In order for the drive to save energy it must be able to slow down.For the drive to slow down the pressure difference or lift must be reduced.[MICK] So, remember that a VFD on the chiller helps part lift operation – and this needs to be analyzed properly.
123 VFDs and Gensets Trane Engineers Newsletter volume 35-1 a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006VFDs and GensetsTrane Engineers Newsletter volume 35-1“How VFDs Affect Genset Sizing” by Court Nebudalocation.aspx?item=5[MICK] Another aspect of drives we didn’t have time to cover today is that engine generators – or gensets – are being used in more facilities today to provide backup for critical loads, such as hospitals or data processing.A myth sometimes heard today is that the genset can be downsized when variable-speed drives are used.[SLIDE] This is not true across the board – in fact there are times when the genset actually has to be oversized! In February of 2005 the national ASHRAE meetings held a very clear discussion on this topic.Please take a look at the newsletter written by Court Nebuda to help you understand genset issues if that application arises.
124 references for this broadcast Where to Learn More a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006references for this broadcast Where to Learn More2005 ENL “Cooling Towers and Condenser Water Systems”Bibliography[MICK] There are a number of publications available so you can investigate today’s topic further.[SLIDE] We’ve put together a bibliography detailing a number of third-party articles and web links that you might find helpful. It’s available from your local site coordinator.
125 mark your calendar 2006 ENL Broadcasts a Trane Engineers Newsletter Live satellite broadcastVSDs and Their Effect on System Components • 1 Feb 2006mark your calendar 2006 ENL BroadcastsMay 3 HVAC systems and airside economizersSep 13 HVAC design for places of assemblyNov 8 Energy-saving designs for rooftop systems[MICK] We thank you in advance for filling out an evaluation before you leave.[SLIDE] The rest of the 2006 broadcasts are shown on the screen, we hope you’ll join us in May, September and November. We’re planning something special for September, since it will be our 25th ENL broadcast.See you in May.
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