Presentation on theme: "Temperature Control Loop"— Presentation transcript:
1 Temperature Control Loop The purpose of the temperature control loop is to maintain a constant off-coil temperature.The control loop senses the actual off-coil temperature and compares it to a setpoint. The controller sends an output signal to a valve actuator which adjusts the valve stem position which affects the waterflow through the valve and the coil which impacts the off-coil air temperature.
2 Temperature Control Loop Off-Coil TemperatureDDC Control CalculationsSignal to Valve ActuatorPosition of Valve StemWater flow through valve / coil
3 Temperature Control Loop Cooling CoilAir FlowOff-coil AirTemperatureSensorValveActuatorCHWSCHWRSetpoint and otherTuning ParametersDDC Controller
5 Temperature Control Loop Control Loop SetpointSetpoint of control loop is fixed by building engineer. Choosing too high of a setpoint will result in:lower chiller energyhigher fan energyreduced system capacityhigher RH in space
6 Temperature Control Loop Control Loop SetpointToo low a setpoint may cause the loop to saturate meaning no control.Common practice is to fix off-coil temperature setpoint between 10°C and 14°C.
8 Temperature Control Loop Chilled Water ValvesThe chilled water valve should be mounted in the chilled water supply pipe.If the chilled water valve is in the return valve, then when the valve is closed the coil is full of cold water and condensation will result increasing maintenance problems.
9 Temperature Control Loop Normally Open Cage Trim Globe Valve BodyPacking nutPacking guidePackingsBonnet (Centerpiece)StemDisk (Integral w/Plug)PlugCage (Integral Seat)Body
10 Temperature Control Loop Normally Open Flanged Globe Valve BodyStemPacking nutPacking guidePackingsPacking boxCenterpieceDisk holderDiskPlugSeat (screwed in)Plug guideBody
11 Temperature Control Loop Resilient Seat Butterfly Valve BodyStem Bushing,Stem SealStem Retaining RingValve SystemNeckBodyValve SeatDisc
12 Temperature Control Loop ValvePlugDesign% Flow100Quick Opening75Linear5025Equal Percentage255075100Quick Opening% Valve Stem LiftLinearEqual Percentage
13 Temperature Control Loop Use quick opening plug for two position applications.Linear plug is typically used for industrial applications.Equal percentage plug typically used for HVAC applications. Why?
14 Temperature Control Loop Valve Characteristic + Coil Characteristic =% Flow% Design Capacity100100757550502525255075100255075100% Valve Stem Lift% Design Flowrate
15 Temperature Control Loop A roughly linear system characteristic100% Design Capacity% Valve Stem Lift255075
16 Temperature Control Loop All valves experience uncontrollable flow when almost closed.Uncontrollable flow
17 Temperature Control Loop RangeabilityA high rangeability means good control under low load conditions.There are different standards to define“Minimum Controllable Flow”
18 Temperature Control Loop CavitationAs water goes through the restricted area between the plug and the seat, the velocity increases and static pressure decreases. If the static pressure drops below the vapour pressure, air bubbles form. After passing the restriction, velocity decreases and static pressure increases. As the static pressure rises, the bubbles implode causing shock waves which result in pitting on the inside of the valve.
19 Temperature Control Loop CavitationPlug to Seat Clearance***FlowImplosionsVelocityLiquid Vapour PressurePressurePressureVelocity
20 Temperature Control Loop Valve AuthorityCooling CoilPvPt
21 Temperature Control Loop Valve SizingUndersized valves cannot deliver sufficient flow for maximum load conditions.Oversized valves must operate at the end of their stroke making tuning more difficult.
22 Temperature Control Loop Valve SizingFor 2 position valves, select the valve according to line size.For modulating valves, use the design pressure drop and flow rate to calculate the flow coefficient.
23 Temperature Control Loop Valve SizingFlow Coefficient (Cv) is the USGPM of 60°F water that will flow through a fully open valve with a 1 psi drop across it.
24 Temperature Control Loop Valve SizingCv = Valve flow coefficientQ = Flow in USGPMPv = Pressure difference betweeninlet and outlet in psi
25 Temperature Control Loop Valve SizingExampleQ = 90 USGPMPv = 5 psi (typical)Cv = 90/2.24 = 40
26 Temperature Control Loop Valve SizingValves are only manufactured with specific flow coefficients.After calculating the desired flow coefficient, select the valve with the next lowest coefficient.Most systems are oversized so slightly undersizing the valve will not compromize system performance, but it does improve system controllability.
27 Temperature Control Loop Three Way Mixing Valve BodyNormallyClosedCommonNormally Open
28 Temperature Control Loop Three Way ValvesPumpCoolingCoilCSThree way valve in mixingapplication piped N.C. to coilN.C.N.O.RN.C.CoolingCoilRThree way valve in bypassapplication piped N.C. to coilCN.O.S
29 Temperature Control Loop Three Way ValvesThree way valves are used for constant flow systems. Constant flow systems are not popular because they waste pump energy.Constant Flow to BuildingVariable Flow to Building
30 Temperature Control Loop Valve ActuatorThere are two critical issues when selecting the valve actuator:Close off pressureSpring returnEnd switch
31 Temperature Control Loop Valve ActuatorFor a two way valve, the close off pressure must be sufficient to overcome the pump head.For a three way valve, the close off pressure is twice the pressure drop across the valve plus the pressure drop across the coil.If spring return in provided, the valve will return to a known position if the control signal is removed or during a power failure.
33 Temperature Control Loop Temperature SensorsAn averaging temperature sensor is mounted in ductwork in front of cooling coil to measure off-coil temperature.
34 Temperature Control Loop Temperature SensorsResistance Temperature Detectors (RTD) measure temperature using the resistance property of metals such as nickel or platinum.The resistance of a thin wire varies linearly over a limited temperature range. Low current is used to avoid self-heating.
35 Temperature Control Loop Temperature SensorsDifferent metals have different sensitivity to heat. Thermal sensitivity is measured in ohms/ohm/Deg C.For example, a sensor with a sensitivity of ohms/ohm/Deg C and a base resistance of 22°C will change from 1003to 1006as the temperature varies from 23°C to 24°C.
36 Temperature Control Loop Temperature SensorsTwo common metals used for RTD are Nickel and Platinum.Both type of sensors are available with a base resistance of 1000Platinum sensors are also available with a base resistance of 100Base resistance is not as important as sensitivity and linearity.
37 Temperature Control Loop Temperature SensorsNickel sensors have a high sensitivity ( C).Nickel sensors have a slight non-linearity at extreme temperatures and software correction may be required under these conditions.Nickel sensors are low cost and can be used for point or averaging elements.
38 Temperature Control Loop Temperature SensorsPlatinum sensors have a low sensitivity ( C).Platinum sensors are very linear for the range of temperatures found in HVAC applications.Platinum sensors are higher cost and are not available as averaging elements.
39 Temperature Control Loop Temperature SensorsPoint Element : Appropriate for Return Air Temperature or Room Temperature (well mixed air)Averaging Element : Used for Off-Coil Temperature or Supply Air Temperature (turbulent/stratified air)
40 Temperature Control Loop Temperature SensorsCalibrate a temperate sensor by:Disconnect RTD element.Replace RTD element with a precision 1000 resistor.Adjust offset in DDC Controller to read base temperature (this compensates for resistance of wiring).Replace RTD element.