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 Temperature
DDC Control Calculations
Signal to Valve Actuator
Position of Valve Stem
Water flow through valve / coil

3. Temperature Control Loop
Cooling Coil
Air Flow
Off-coil Air
Temperature
Sensor
Valve
Actuator
CHWS
CHWR
Setpoint and other
Tuning Parameters
DDC Controller

4. Temperature Control Loop
Control Loop Setpoint

5. Temperature Control Loop
Control Loop Setpoint
Setpoint of control loop is fixed by building engineer. Choosing too high of a setpoint will result in:
lower chiller energy
higher fan energy
reduced system capacity
higher RH in space

6. Temperature Control Loop
Control Loop Setpoint
Too 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.

7. Temperature Control Loop
Chilled Water Valves

8. Temperature Control Loop
Chilled Water Valves
The 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 Body
Packing nut
Packing guide
Packings
Bonnet (Centerpiece)
Stem
Disk (Integral w/Plug)
Plug
Cage (Integral Seat)
Body

10. Temperature Control Loop
Normally Open Flanged Globe Valve Body
Stem
Packing nut
Packing guide
Packings
Packing box
Centerpiece
Disk holder
Disk
Plug
Seat (screwed in)
Plug guide
Body

11. Temperature Control Loop
Resilient Seat Butterfly Valve Body
Stem Bushing,
Stem Seal
Stem Retaining Ring
Valve System
Neck
Body
Valve Seat
Disc

12. Temperature Control Loop
Valve
Plug
Design
% Flow
100
Quick Opening 75
Linear 50 25
Equal Percentage 25 50 75
100
Quick Opening
% Valve Stem Lift
Linear
Equal 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 Capacity
100
100 75 75 50 50 25 25 25 50 75
100 25 50 75
100
% Valve Stem Lift
% Design Flowrate

15. Temperature Control Loop
A roughly linear system characteristic
100
% Design Capacity
% Valve Stem Lift 25 50 75

16. Temperature Control Loop
All valves experience uncontrollable flow when almost closed.
Uncontrollable flow

17. Temperature Control Loop
Rangeability
A high rangeability means good control under low load conditions.
There are different standards to define
“Minimum Controllable Flow”

18. Temperature Control Loop
Cavitation
As 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
Cavitation
Plug to Seat Clearance * * *
Flow
Implosions
Velocity
Liquid Vapour Pressure
Pressure
Pressure
Velocity

20. Temperature Control Loop
Valve Authority
Cooling Coil
Pv
Pt

21. Temperature Control Loop
Valve Sizing
Undersized 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 Sizing
For 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 Sizing
Flow 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 Sizing
Cv = Valve flow coefficient
Q = Flow in USGPM
Pv = Pressure difference between
inlet and outlet in psi

25. Temperature Control Loop
Valve Sizing
Example
Q = 90 USGPM
Pv = 5 psi (typical)
Cv = 90/2.24 = 40

26. Temperature Control Loop
Valve Sizing
Valves 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 Body
Normally
Closed
Common
Normally Open

28. Temperature Control Loop
Three Way Valves
Pump
Cooling
Coil C S
Three way valve in mixing
application piped N.C. to coil
N.C.
N.O. R
N.C.
Cooling
Coil R
Three way valve in bypass
application piped N.C. to coil C
N.O. S

29. Temperature Control Loop
Three Way Valves
Three way valves are used for constant flow systems. Constant flow systems are not popular because they waste pump energy.
Constant Flow to Building
Variable Flow to Building

30. Temperature Control Loop
Valve Actuator
There are two critical issues when selecting the valve actuator:
Close off pressure
Spring return
End switch

31. Temperature Control Loop
Valve Actuator
For 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.

32. Temperature Control Loop
Temperature Sensors

33. Temperature Control Loop
Temperature Sensors
An averaging temperature sensor is mounted in ductwork in front of cooling coil to measure off-coil temperature.

34. Temperature Control Loop
Temperature Sensors
Resistance 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 Sensors
Different 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 1003to 1006as the temperature varies from 23°C to 24°C.

36. Temperature Control Loop
Temperature Sensors
Two common metals used for RTD are Nickel and Platinum.
Both type of sensors are available with a base resistance of 1000Platinum sensors are also available with a base resistance of 100
Base resistance is not as important as sensitivity and linearity.

37. Temperature Control Loop
Temperature Sensors
Nickel 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 Sensors
Platinum 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 Sensors
Point 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 Sensors
Calibrate 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.