Presentation on theme: "The Cause and Effect of Various Design Concepts"— Presentation transcript:
1 The Cause and Effect of Various Design Concepts Process Heat TransferThe Cause and Effect of Various Design Concepts
2 Exchanger Variables Fouled surface area Non-condensible gases Flooded surface areaVariable process inlet and outlet temperaturesVariable process flow ratesAll of these change the BTU demand on the heater, changing the pressure and temperature of the heat transfer media
3 Fouled Surface AreaFouled surface area decreases the heat transfer efficiency of the tube bundleThis inherently causes adjustments in the pressure and/or temperature of the heat transfer media being supplied to the exchanger
4 Fouled Surface AreaResulting in more surface exposed to the transfer media in a level control system. This will increase the BTU transfer rate.Higher delivery pressure from the inlet control valve decreases the efficiency of the heat exchanger. Higher pressure lacks the same latent heat content of lower pressure. Energy consumption will increase, while production levels remain unchanged.
5 Non-Condensible Gases Presence of non-condensibles’ occupies valuable steam spaceA reduction of viable heat transfer area can result due to the insulating propertiesPromotion of carbonic acid formation is inherentExcessive amounts can inhibit drainage
6 Flooded Surface Area Promotes corrosion and fouling Can develop into water hammerControls process temperature by decreasing available surface area for heat transfer (Level Control)Typically causes process outlet variations
7 Variable Process Inlet & Outlet Temperatures Changes the BTU exchange rate required or (Delta T)These variable temperatures can increase or decrease exiting pressure based on condensing rate of the heaterWill promote flooding on low exchange rate demand
8 Variable Process Flow Rates Variable flows will change BTU demand on the exchangerHigher flow rates will increase the surface area needed, raising or lowering the outlet pressure based on available surface areaLower flow rates will decrease surface area needed, raising or lowering the outlet pressure based on available surface area
10 Level ControlLevel control systems flood exchangers to reduce the amount of useable surface area for BTU transferExchangers run flooded due to the control valve on the condensate outlet, modulating to maintain the desired process outlet temperature
11 Steam ControlAllows the exchanger to run at the lowest possible steam pressure, which maximizes energy efficiency due to latent heat contentLess energy consumed for the same amount of product produced
12 Process Design Summary Utilize all of the surface areaEliminate corrosion and fouling by keeping the exchanger dryEliminate non-condensiblesOptimize the design by using the lowest pressure steam, to gain more latent heat content per pound
14 FillingSteam/Air In - ClosedSteam/Air Out - OpenOpen CheckValveClosed Check ValveStep 1. During filling, the steam or air inlet and check valve on pumping trap outlet are closed. The vent and check valve on the inlet are open.
15 Begin PumpingSteam/Air In - OpenSteam/Air Out - ClosedCheck ValveClosedOpen Check ValveStep 2. Float Rises with level of condensate until it passes trip point, and then snap action reverses the positions shown in step one.
16 End PumpingSteam/Air - InSteam/Air - ClosedClosed CheckValveOpen Check ValveStep 3. Float is lowered as level of condensate falls until snap action again reverses positions.
17 Repeat FillingSteam/Air In - ClosedSteam/Air Out - OpenOpen Check ValveClosed Check ValveStep 4. Steam or air inlet and trap outlet are again closed while vent and condensate inlet are open. Cycle begins anew.
26 Understanding and Benefiting from Equipment Stall
27 Q = U · A · D T Q = Design Load (BTU/Hr) U = Manufacturer’s Heat Transfer Value (BTU/ft2/°F/Hr)A = Heat Transfer Surface Area (ft2)DT = (Ts – T2) Approaching Temperature (°F)Ts = Operating Steam Temperature (°F)T2 = Product Outlet Temperature (°F)
28 What is wrong with this application? Modulating heat exchanger with overhead lift.Flooding problemsWater hammer problemsWhat is wrong with this application?
29 Effects of “Stall” Inadequate condensate drainage Water hammer Frozen coilsCorrosion due to Carbonic Acid formationPoor temperature controlControl valve hunting (system cycling)Reduction of heat transfer capacity
30 Factors Contributing to “Stall” Oversized equipmentConservative fouling factorsExcessive safety factorsLarge operating rangesBack pressure at steam trap dischargeChanges in system parameters
31 Finding “Stall” Where does Stall occur?? Modulating Control Air heating coilsShell & tube heat exchangersPlate & frame heat exchangersAbsorption chillersKettlesAny type of heat transfer equipment that hasModulating Control
33 What is the “Stall” Solution? Use a bigger steam trap?Use a vacuum breaker?Implement a safety drain?Install a Posi-Pressure system?Use an electric pump?
34 Keys to OperationHow quick it can fill: This is dictated by head pressure & inlet pipe and check valve sizeVent/Equalization: Vent connection must always be in vapor spacePump Out: Motive vs. back pressure and gas used
35 VocabularyFilling Head: Distance between the top of the pump and the bottom of the receiver or reservoir pipe
36 Vocabulary (Continued) Receiver/Reservoir Pipe: This is a temporary holding place to store condensate while the pump is in the pump down cycle. The receiver/reservoir pipe is designed and sized to prevent condensate from backing up into the system.
37 Open System Configuration Closed System Configuration
38 Open System Cover the basic points of operation in a vented system. Be sure to point out the need for:Vacuum breakerAir ventFill head
39 Open System Advantages: Disadvantages: Drain multiple pieces of equipmentCan use Air or Steam for pump trap operationEasiest to understandDisadvantages:Lose valuable flash steamMust run a potentially expensive atmospheric vent lineSize the pump trap based total design loadMust compete with electric pumps
40 Closed System One heat exchanger per pump in closed-loop. F&T is placed downstream of the pump trap with check valve afterwardsAlternate vent line configurationsIf less than ½ psi pressure drop in HX, then vent line can be tied back to HX. Then, there is no need for a vacuum breakerIf more than ½ psi pressure drop, usually not known, then tie vent line from pump back to reservoir as shown.
41 Closed System Advantages: Disadvantages: No flash steam loss No need to run long expensive vent linesUse a smaller pump than in a open system*Return condensate hotterDisadvantages:Dedicated pump for a single piece of equipmentMore complexCannot use air as motive force
42 Pump Sizing / Receiver Sizing Determine head available from equipment (distance from equipment outlet to grade)Select either closed loop or vented design (Note: If multiple sources of condensate, vented system must be used to prevent short circuiting)
43 Pump Sizing / Receiver Sizing Determine maximum pumping loadCalculate maximum back pressure (including lift)Determine motive pressure and gas to be used (use capacity correction factor if using a medium other than steam)
44 Pump Sizing / Receiver Sizing Check and specify head pressure (distance from bottom of receiver/reservoir to top of selected pump)Make sure to use capacity correction if more or less head is available than standard catalog dimension
45 Pump Sizing / Receiver Sizing Calculate maximum flash rate & needed vent size – if vented systemDetermine and size reservoir – if closed loop systemSize downstream F&T trap if needed for closed loop system
46 Vented Receiver Sizing Note: When draining from a single or multiple pieces of equipment in an “open” system, a vented receiver should be installed horizontally above and ahead of the pump trap. In addition to sufficient holding volume of the condensate above the fill head of the pump trap to hold the condensate during the pump trap cycle, the receiver must also be sized to allow enough area for flash steam and condensate separation.
47 Closed Loop Receiver Sizing Note: When draining from a single piece of equipment in a closed loop system, to achieve maximum energy efficiency a reservoir should be installed horizontally above and ahead of the pump trap. Sufficient reservoir volume is required above the filling head level to hold condensate during the pump trap discharge cycle. The chart above shows the minimum reservoir sizing, based on the condensate load, to prevent equipment flooding during the pump trap discharge cycle.
48 Critical Design Criteria Summary Maximum condensate flow from exchangers and reboilersMaximum differential pressure across the systemMinimum differential pressure across the system (specifically when clean)Minimum tower height needed to achieve maximum condensate flow rate at minimum differentialMaximum motive pressure (steam, air, nitrogen, etc.) available to power pumps
49 Critical Design Criteria Summary Maximum instantaneous discharge rate for downstream pipe sizing & trap sizingTemperature differential of condensate source vs. condensate header designPiping layout to prevent hydraulic shockTotal installed cost savings, including construction, on turnkey jobsIntegrity of mechanical design due to the critical nature of the serviceMinimize potential problems with proper designs
50 Maximum Differential Pressure Across the System Maximum pressure from control valve, including minimal dropMinimum drop across exchangerMaximum pressure – should tube leak occurElimination of back pressure (bypass to grade)Consider fouled surface area
51 Minimum Differential Pressure Across the System Consider maximum percentage of turndown on process flow vs. design flow (plus factor)Consider over-surfaced heat transfer areaEvaluate downstream relief valve settings on condensate side as traps (etc.) fail and pressurize the return systemUndersized return lines are common in facility expansions. Verify effects of additional flow on pipe velocities and back pressures.
52 Minimum Head PressureSkirt height on reboilers can be minimized by evaluating discharge capacity needed and setting height accordingly. This should be done early in the job scope as it effects tower construction.Additional pump capacity can be achieved by increasing head pressure
53 Maximum Motive Pressure Steam, Air, Nitrogen Ensure stable source with negligible variationsInstall drip station to insure dry gas is always present at motive steam valve (pipers often do not realize it is a dead-end steam line)
54 Maximum Design Pressure Utilization of 2/3 Rule can eliminate relief valves on low pressure side needed for tube rupture casesUse of liquid drain traps can eliminate gas discharge into return header
55 Maximum Instantaneous Discharge Rate Pump discharge rate must be used when sizing condensate return leads (use bi-phase flow)Pump discharge rate also critical to downstream traps in Pump / Trap combinations
56 Temperature Differential of Source vs. Header Minimize thermal shock by maintaining DT of 150°F or lessWhen feasible, run separate headers for vacuum temperature condensateVacuum condensate headers can be sized on single phase flow if dedicated solely for vacuum temperature condensate
57 Piping Layout to Prevent Hydraulic Shock Discharge lead from pumps should be piped into top of return headerFlow patterns should be continual – no opposing flowsCheck valves should be installed at major elevation changes to disperse hydraulic shock
58 Pipe SizingDischarge piping should be based on 2-3 times the normal condensing rate due to instantaneous discharge rate of the pumpMinimize elevation changes to prevent hydraulic shockUtilize check valves at main header to minimize backflow
59 Pipe SizingRun separate lines for vacuum temperature condensate to minimize thermal shock potentialAlways calculate the maximum flash rate in return linesInsure adequate pipe and nozzle diameters to facilitate bidirectional two-phase flow
60 “Expect many enjoyable experiences!” “Expect many enjoyable experiences!”David M. Armstrong