Prior to functional testing, verify and document the following:

1. Verify the control system minimum saturated condensing temperature (SCT) setpoint is at or below 60°F. If the design saturated suction temperature (SST) of the intermediate suction group is greater than or equal to 30°F, verify the control system SCT setpoint is at or below 70°F.
2. Verify accuracy of refrigerant pressure-temperature conversions and consistent use of either temperature or pressure for the controlled variable setpoint in the control system.
   1. The condensing temperature has an equivalent pressure during subcritical operation.
   2. Either pressure or temperature may be used in the control system as the controlled variable to maintain gas cooler pressure (condensing temperature) during subcritical operation, as long as the setpoint value is similarly expressed in pressure or temperature.
3. Verify the gas cooler outlet temperature sensor reads accurately, using a NIST traceable instrument, including verification of at least two different gas cooler outlet readings. Calibrate if needed. Replace if outside manufacturer’s recommended calibration range. If multiple gas coolers are installed in parallel, ensure sensor is installed on the common header.
4. Verify the discharge pressure sensor (or gas cooler pressure if used) reads accurately, using a NIST traceable reference pressure gauge or meter, and with pressure checked for at least two pressures within the typical operating range. Calibrate if needed. Replace if outside manufacturers recommended calibration range.
5. Verify the ambient dry bulb temperature using a NIST traceable instrument, including verification of at least two different ambient readings. Calibrate if needed. Replace if outside manufacturer’s recommended calibration range. If the ambient dry bulb temperature sensor is installed between the adiabatic pad and the gas cooler coil for adiabatic gas coolers, verification must be performed when operating in “dry” mode.
6. Verify the ambient dry bulb temperature is not mounted in direct sunlight or is provided with a suitable solar shield. The ambient dry bulb temperature sensor may be installed between the adiabatic pad and the gas cooler coil for adiabatic gas coolers and is referred to as the precool air temperature sensor.
7. Verify that all sensor readings used by the gas cooler controller display correct values at the controller, as well as derived values (e.g., observed pressure is correctly converted saturation temperature for CO2)
8. Verify that all fan motors are operational and rotating in the correct direction.
9. Verify that gas cooler fan speed controls are operational and controlling all gas cooler fan motors in unison.
10. Verify that all speed controls operate automatically in response to changes in pressure, gas cooler outlet temperature, and ambient dry bulb or precool air temperature.
11. Verify the installation of the gas cooler holdback valve, which may be located near the inlet of the intermediate pressure vessel or near the outlet of the gas cooler.

Planning: The system cooling load must be sufficiently high, and ambient conditions sufficiently below the critical point, to operate subcritically with all gas cooler fans in operation and observe controls in average conditions. Account for weather conditions in scheduling testing by, if necessary, artificially increasing or decreasing evaporator loads in order to perform the Functional Testing at typical system conditions.

Step 1: Verify mechanical controls and other strategies will not affect tests

1. Turn off any heat reclaim controls and any intermittent defrost pressure offset strategies that would affect gas cooler setpoint control.
2. If testing an adiabatic gas cooler, adjust setpoints to ensure that the gas cooler stays in “dry” mode or “precool” mode consistently throughout the test.

Step 2: Operate in control range and verify stable control

1. Verify the gas cooler control value is operating in the variable setpoint control range, i.e. above the minimum SCT setpoint and below the maximum SCT setpoint.
   • If necessary, increase or decrease the system load.
   • If necessary, during low load or low ambient conditions with system observed at the minimum SCT, temporarily adjust the minimum SCT to a lower value, if the refrigeration system design will allow, or increase the control TD to result in a higher control value.
2. Observe control operation for at least 30 minutes to confirm stable control operation, as shown by gas cooler fan speed varying as compressor capacity changes, and not ranging from maximum to minimum fan speed or constant “hunting”. If required, adjust control response setpoints to achieve stable operation.
   Note: Since gas cooler control settings require fine-tuning over time, this is often accomplished using control system history or visual trends, showing one hourly and daily operation.

Step 3: Identify control TD

1. Record the current outdoor ambient air dry bulb or precool air temperature and refrigeration system condensing temperature/condensing pressure readings from the control system. Note whether discharge pressure or a dedicated gas cooler pressure sensor is used for gas cooler pressure control.
2. Document current head pressure control setpoints, including the TD setpoint.
3. Calculate and record the actual observed temperature difference (TD), defined as the difference between the ambient dry bulb temperature or precool air temperature and the refrigeration system saturated condensing temperature (SCT).
4. Confirm agreement between the current control system TD setpoint and the observed TD. If values are different, address and correct control system methods.

Step 4: Test adjusted control TD

1. Enter a smaller TD value into the control system, sufficient to cause an observable response, such as 1-2 degrees smaller, but not small enough to cause system to operate continuously at 100% fan speed. Record this value as TD Test Setpoint 1.
2. Observe change in control system operation which should include an increase in fan speed and a decrease in condensing temperature.
3. Allow time for the control system to achieve stable operation.
4. Document current head pressure control setpoints, including the TD setpoint.
5. Calculate and record the actual observed temperature difference (TD), defined as the difference between the ambient dry bulb or precool air temperature and the refrigeration system saturated condensing temperature (SCT).
6. Confirm agreement between the current control system TD setpoint and the observed TD. If values are different, address and correct control system methods.
7. Perform the above test sequence with a second TD value, recorded as TD Test Setpoint 2, and record the same values above to confirm agreement between the current control system TD setpoint and the observed TD. If needed perform corrective actions and repeat testing until variable setpoint control can be confirmed and documented.

Step 5: Verify and document all fans operate in unison down to minimum SCT

1. Document that all fans are in operation, fan speed, actual SCT and control system minimum SCT setpoint, by recording control system screens or trends along with observations.
   1. In cool weather and/or light loads, this may be the observed operation during testing without need to manipulate system setpoints.
   2. In warmer weather and/or higher loads, the control system minimum SCT value can be increased slowly to a value equal to, and then above, the current operating condition, in order to observe the fans operating in unison and fan speeds dropping as the minimum SCT setpoint is achieved.

Step 6: Restore setpoints

1. Restore any heat reclaim or defrost functionality that was turned off to allow testing.
2. Reset the minimum condensing temperature setpoint if it was adjusted during Step 5.
3. Reset adiabatic mode controls to original values.

Planning: Ambient conditions must be sufficiently above the critical point to operate supercritically. Account for weather conditions in scheduling testing by, if necessary, artificially increasing or decreasing evaporator loads in order to perform the Functional Testing at typical system conditions.

Step 1: Verify mechanical controls and other strategies will not affect tests

1. Turn off any heat reclaim controls and any intermittent defrost pressure offset strategies that would affect gas cooler setpoint control.
2. If testing an adiabatic gas cooler, adjust setpoints to ensure that the gas cooler stays in “dry” mode or “precool” mode consistently throughout the test.

Step 2: Operate in supercritical mode and verify pressure control

1. Observe operation for at least 30 minutes or reference control system history or visual trends to verify the gas cooler holdback valve modulates its opening in response to changes in ambient dry bulb or precool air temperature resulting in a change in gas cooler pressure. Fan speeds are allowed to operate fixed at 100% to maximize the temperature reduction of the outlet gas or modulate to maintain a temperature difference between the ambient dry bulb or precool air temperature and the gas cooler outlet temperature. Reference the original equipment manufacturer operating manual or sequence of operation descriptions to confirm the observed variation in the pressure setpoint is consistent with the design control strategy.

Step 3: Restore setpoints

1. Restore any heat reclaim or defrost functionality that was turned off to allow testing.
2. Reset adiabatic mode controls to original values.