At-A-Glance
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NA7.10.3.1 Evaporative Condensers and Condenser Fan Motor Variable Speed Controls |
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Use Document NRCA-PRC-05-A |
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Purpose of the Test |
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This test ensures that the condensing temperature of the condenser is reset in response to ambient wet-bulb temperature, per §120.6(a)4E. This test ensures that the condenser fan speed is continuously variable, and the condenser fans are controlled in unison per §120.6(a)4C. This test ensures that the minimum condensing temperature control setpoint is 70°F or lower, per §120.6(a)4C. |
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Instrumentation |
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Performance of this test will require measuring the ambient wet-bulb temperature, relative humidity, and condenser operating pressure. The instrumentation needed to perform the task may include, but is not limited to: |
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A temperature sensor calibrated to +/- 0.7°F between -30°F and 200°F A relative humidity (RH) sensor calibrated to +/- 1 percent between 5 percent and 90 percent RH A pressure sensor shall be calibrated to +/- 2.5 psi between 0 and 500 psig |
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Test Conditions |
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The test will be performed by varying the control parameters used by the condenser control system. Therefore, the condenser control system must be installed and operating, including completion of all start-up procedures per manufacturer’s or designer’s recommendations, to perform the test. |
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Document the value of the initial control parameters before starting the test. |
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Estimated Time to Complete |
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Construction inspection: 1 hour (for one evaporative condenser) |
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Functional testing: 3 hour (for one evaporative condenser) |
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Acceptance Criteria |
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The evaporative condenser minimum condensing temperature control setpoint is 70°F or lower. |
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The target condensing temperature is reset in response to ambient wet-bulb temperature, by using a temperature difference (TD) between the condensing temperature and the ambient wet-bulb temperature. |
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The condenser fan speed is continuously variable and the condenser fans are controlled in unison – varying the speed of all fans serving a common high-side at the same time. |
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Potential Issues and Cautions |
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Coordinate test procedures with the refrigeration or controls contractor, or the facility supervisor since they may be needed to assist with the manipulation of the control system. To ensure proper overall system operation, make sure that the system pressure is not held at excessively low or high values for an extended period of time when varying the saturated condensing temperature (SCT) control setpoint. Avoid abrupt changes in pressure. Coordinate with facility operator or refrigeration contractor. |
A. Test Application
Newly Constructed Buildings: Applies to functional testing and verification of fan motor variable speed control for evaporative condensers.
B. Construction Inspection
The field technician should check the following:
•The minimum saturated condensing temperature (SCT) control setpoint is at or below 70°F.
•The SCT value used by the control system is the temperature equivalent reading of the condenser pressure sensor.
•All drain leg pressure regulator valves (if used) are set below the minimum condensing temperature/pressure setpoint and all receiver pressurization valves, such as the outlet pressure regulator (OPR), are set lower than the drain leg pressure regulator valve setting. This ensures that the pressure regulator valve and receiver pressurization valve settings do not force the actual condensing temperature to be higher than the minimum condensing temperature setpoint. (Note: These regulators are only used on small systems and rarely with evaporative condensers.)
•All pressure, temperatures and humidity sensors have been calibrated and read accurately.
•Temperature and humidity sensors are mounted in a location away from direct sunlight.
•All sensor readings used by the condenser controller convert or calculate to the correct conversion units at the controller (e.g., saturated pressure reading is correctly converted to appropriate saturated temperature; dry-bulb and relative humidity sensor readings are correctly converted to wetbulb temperature, etc.).
•All condenser motors are operational and rotate in the correct direction.
•All condenser fan speed controls are operational and connected to condenser fan motors, and not in bypass.
•All speed controls are in “auto” mode.
•Records showing calibration was performed, what offsets or control system calibration values were used, and documentation of the instrumentation used for calibration.
C. Functional Testing
The system cooling load must be sufficiently high to run the test, i.e. with a condensing temperature above the minimum SCT setpoint. The loads can often be increased somewhat as required to perform the Functional Testing. For example, the cooling loads can be temporarily increased by lowering the zone temperature setpoint or allowing more infiltration into the space by opening doors.
If there is insufficient load or the weather is too cold to operate the condensers above the minimum SCT setpoints, there are several options: The test could be scheduled for a warmer day, additional load could be arranged, or a portion of the condenser capacity could be reduced. Methods for reducing condenser capacity include turning off part of the spray pumps, or covering part of the condenser surface (e.g. with cardboard) or fans (taking care not to overload motors).
Step 1: Override any possible conflicting controls. This may include, but is not limited to heat reclaim, hot gas defrost, or defrost head pressure override before performing functional tests.
Work with refrigeration contractor or facility staff to disable controls that may interfere with the Functional Testing.
Step 2: Document current conditions
•Ambient dry-bulb temperature (DBT), wet-bulb temperature (WBT), and relative humidity (RH).
•Current condenser control temperature difference (Control TD) parameter in the control system. Some control systems may use a pressure equivalent.
•Refrigeration system condensing temperature (SCT) or condensing pressure in psig.
•Calculate the actual condenser temperature difference (Actual TD) which is the temperature difference between the current SCT and the current WBT. This value may be the same as the Control TD.
•Current head pressure control setpoint in °F SCT or psig.
Step 3: Program into the control system a condensing temperature/pressure setpoint equal to the reading or calculation obtained in Step 2. This is typically accomplished by setting the condenser Control TD parameter to the Actual TD from Step 2. The resulting SCT or psig setpoint will be referred to as the “Test Setpoint.” Allow 5 minutes for condenser fan speed to normalize.
Step 4: Using the control system, raise the Test Setpoint in 1 degree (or 3 psi) increments until the condenser fan control modulates to minimum fan motor speed. Raising the test setpoint can be accomplished by increasing the Control TD parameter. The fans may cycle off completely if the control range limit is met so it is important to increase the Test Setpoint in small increments to produce a slow control response.
Verify:
•Condenser fan motor speed decreases.
•All condenser fan motors serving common condenser loop decrease speed in unison in response to controller output; observed at the control system and at the condensers(s).
Document:
•Minimum fan motor control speed (rpm, percent of full speed, or Hz) as observed in the control system and VFD readouts, as available.
Step 5: Using the control system, lower the Test Setpoint in 1 degree (or 3 psi) increments until the condenser fan control modulates to increase fan motor speed. Lowering the Test Setpoint can be accomplished by decreasing the Control TD parameter.
Verify:
•Condenser fan motor speed increases.
•All condenser fan motors serving common condenser loop increase speed in unison in response to controller output observed at the control system and at the condensers(s).
Step 6: Document the current minimum saturated condensing temperature (Min. SCT) setpoint, which should be set to 70°F SCT or lower.
Document:
•Current minimum SCT setpoint in the control system
Using the control system, change the Min. SCT setpoint to a value greater than the current system SCT.
Depending on system load or weather condition:
1. Reduce the Control TD and/or reduce system load to reduce the operating SCT until actual operation is observed at the Min. SCT value. Verify that fan speed modulates to maintain the Min SCT Value.
2. If weather conditions are too warm, and on load is too high to accomplish the previous test from part A, the Min. SCT setpoint can be increased (above the 70°F value) to observe control at the higher value. Verify that fan speed modulates to maintain this temporary Min. SCT value.
Step 7: Using the control system, restore the system head pressure controls to original settings documented in Steps #2 and 6 (Control TD, Min SCT).
Verify that the control system is restored back to correct control setpoints.
Step 8: Restore any controls that were disabled in Step 1.
Verify that the control system is restored back to original conditions.