Functional Testing for Condensers and Cooling Towers

Key Commissioning Test Requirements

General

Regardless of whether the system is an open-circuit or closed-circuit cooling tower, heat rejection is based on both convective and evaporative heat transfer principles. Condenser water temperature is controlled primarily by the modulation of air flow through the tower. The purpose of the tests is to ensure that individual components are installed and integrated to operate on a system level per the design intent and sequence of operations.

Attention to the following items during the commissioning process can result in significant improvements in system operation and energy efficiency:

Safeties, Interlocks, and Alarms

Verify that all safeties, interlocks, and alarms are programmed (or hard-wired, if applicable) and function correctly.

Sensors

Verify that sensor installation and calibration is sufficient to achieve the design control strategies. The DDC control system relies on input from various sensors (including temperature, pressure, and flow) in order to achieve the desired system operation. However, if sensors are not located correctly, or the measured value from any sensor to the control algorithm is incorrect, the system will not respond as intended.

Unit Capacity

Verify that the unit meets the manufacturer's stated part load performance under the actual test conditions. In some instances, verifying cooling tower efficiency or capacity may be required. Tests targeted at verification of as-tested conditions (such as part load performance) will allow the components to perform as intended and may prove more cost-effective than tests targeted to document absolute capacity under design conditions.

Actuation and Sequencing

1 Verify proper stroke for control valves to ensure that they open and close completely (for example, isolation valves and coil valves). Control valve leakage testing should reveal no detectable leakage when valve is commanded closed under normal operating conditions.

2 Verify proper cooling tower staging, water control, and fan control (including water distribution across the fill and fan modulation) to maintain design condenser water temperature setpoint per the specific sequence of operations. When an individual cooling tower is not operating, the isolation valve should be closed to prevent condenser water from circulating through the unit. Depending on the control sequence, this configuration should reduce pumping energy and prevent control problems.

3 Verify proper control of both the spray pump and tower fan for evaporative and “fluid” coolers to maintain fluid temperature setpoint per the specific sequence of operations. Typically, the first stage of heat rejection in a closed-circuit system is to spray water over the coils and then modulate the tower fan to achieve condenser water temperature setpoint. This control strategy may be altered during winter months to prevent operational problems or tower freezing: Some portion of the condenser water may flow through a bypass valve, rather than over the tower fill. Testing will ensure proper system performance throughout all operating conditions. Refer to Test Conditions, Considerations and Cautions, below, for a detailed discussion.

4 A control strategy with potential to reduce cooling tower fan energy is to distribute condenser water across the entire fill (in multiple cooling tower applications) before modulating the fans to maintain water temperature. Care must be exercised to ensure that the expected performance is achieved.

5 Verify proper cooling tower fan control and staging, especially if multiple units are installed. Regardless of the type of tower, optimizing the control strategy used to maintain the condenser water at setpoint is important. For example, every tower system will vary the amount of air flowing across the heat transfer elements as part of the control strategy. Commissioning the system and making sure the particular control strategy employed (fan cycling, 2-speed motor, or VFD) is optimized will reduce system energy usage.

6 Verify proper condenser water pump staging and VFD control (if applicable). Many condenser systems are designed to provide a constant flow of water through the chiller condenser bundle and the condenser water pumps should stage ON and OFF per the design sequence. However, variable condenser water flow is becoming increasingly accepted as a way to further reduce energy consumption and improve chiller performance. For example, the flow rate may be modulated based on maintaining a constant temperature differential across the chiller condenser. Another example of variable flow would be a heat pump loop: As individual heat pumps cycle ON and OFF, 2-way valves open and close to vary the flow of condenser water through the entire loop. Testing will ensure that the control strategy operates per design.

Setpoints and Reset Controls

Verify optimum condenser water temperature setpoint. The condenser water temperature setpoint is sometimes held constant based on the performance characteristics of the chiller being served. Some chillers use less energy with lower condenser water temperature, but the cooling tower will expend additional energy to achieve the lower temperature setpoint. Testing will help determine the optimum setpoint to minimize overall system energy usage.

Verify condenser water temperature setpoint reset strategy per the sequence of operations. The sequence may be revised to optimize system operation relative to atmospheric conditions, chiller energy, and tower fan (energy). For example, if the current atmospheric conditions will not allow the condenser system to achieve setpoint, then the setpoint may be raised to reduce condenser energy in exchange for a slight increase in chiller energy usage. A condenser water reset control strategy can be complicated and must be tested in order to achieve design intent and proper system operation.

Control Accuracy and Stability

Verify proper control sequence and integration of all components (such as setpoints and reset strategies, start-up / shut down procedures, and time delays).

All cooling tower and evaporative fluid cooler system components (control valves, fans, spray pumps) operate per sequences to maintain condenser water temperature setpoints under varying atmospheric and load conditions.

All PID control loops achieve stability (i.e., no hunting) within a reasonable amount of time after a significant load change, such as start-up and automatic or manual recovery from shut down.

Verify cooling tower make-up water control functions correctly.

Water treatment/chemical treatment system functions correctly.

Key Preparations and Cautions

Prefunctional Checklists and Start-up

Prefunctional checklists should be completed throughout construction during normal commissioning site visits as installation of the various components and systems are completed. Sensor and actuator calibration is typically considered to be part of the prefunctional checklist.

In addition to the prefunctional checklists, all component start-up procedures must be complete in order to conduct functional test procedures. Both the air-side and water-side TAB must also be complete prior to functional testing.

Valve leakage tests and tests that are targeted at verifying valve stroke, spring range, and sequencing should be conducted with the pumping system operating at its peak differential pressure. The differential pressure across the valve plug can have a significant impact on the close-off rating and shift the operating spring range of the valve. These tests should be performed prior to temporary system operation to ensure that equipment will not be damaged during functional testing.

Test Conditions, Considerations and Cautions

Functional performance testing for a cooling tower condenser system can occur during virtually any atmospheric conditions except, perhaps, during extreme cold (unless the system is intended to operate even when outside air temperature is low, such as an HVAC system serving a 24/7 computer room load). However, getting a chiller to operate when conditions typically don’t warrant system operation (for example, if the chiller is not intended to run when outdoors air temperature is less than 50°F ) may be more difficult. Complete performance verification may require a phased-testing approach to check system operation under various ambient conditions. Care should be taken to ensure that testing conditions will not have an adverse impact on the equipment (for example, low condenser water temperature entering the chiller).

The following points should be noted to avoid testing complications:

1 If testing the cooling tower condenser system during cold weather conditions, be sure that freeze protection controls are functional to prevent adverse impact on equipment.

2 If the condenser water system employs a variable flow strategy or a control strategy in which water flows across all cooling towers before the fans are enabled,, ensure that the control sequence does not allow the water flow rate to be lower than the minimum flow specified by the manufacturer for each piece of equipment served (both chiller and cooling tower). Minimum flow through a cooling tower is important to provide even water distribution and full wetting of the fill to prevent scaling. Further, pump speed should not drop below the motor manufacturer’s minimum speed requirement.

3 Safety and interlock tests, as well as some test procedures and loop-tuning efforts (for example, high or low refrigerant cut-out setpoints, emergency shut-down procedures, and failure/back-up system operation) could place the system at risk if the sequences do not function as intended. Appropriate precautions and procedures should be in place to protect personnel and machinery, including plans for quickly aborting the test if necessary.

Instrumentation Required

Instrumentation requirements will vary from test to test and will typically include, but are not limited to, the following:

Temperature measurement devices

Differential pressure measurement devices

Amperage and voltage measurement devices

Tachometer

Flow measurement devices

Data loggers

Time Required to Test

Overview

The time necessary to execute functional tests on an entire cooling tower condenser system depends greatly on the size and complexity of the installation and specified control sequences. For example, the number of system components (such as cooling towers, condenser fans, condenser water pumps and spray pumps), as well as complexity of the sequence of operation (including reset strategies, VFD fan and/or pump controls, staging parameters, and safeties/alarms) will significantly impact the time associated with testing the entire system. Therefore, time estimates have been separated out by component on a per unit basis as well as on an overall system level. Component-level tests typically refer to discrete functions of each piece of equipment (such as start/stop procedures, safeties, operational and failure interlocks, and alarms), whereas system-level tests focus on evaluating proper integration of each component to satisfy the desired control strategy (including staging, setpoints, and reset strategies).

Component Level Testing

One to two hours per cooling tower

One to two hours per condenser water pump

Thirty minutes to one hour per spray pump (closed-circuit units only)

One hour or less per isolation, control, and/or by-pass valve, including individual stroke from full open to full close

System Level Testing

Three to four hours are needed to verify proper condenser water control strategies (including water temperature setpoint, fan modulation, by-pass valve control, and spray pump operation). Typically, testing entails verifying that the PID loop maintains setpoint without hunting, as well as optimizing system operation, if applicable.

One to three hours are needed to verify proper condenser water pump staging and control. Constant volume systems shouldn’t take more than an hour, but variable volume systems could take upwards of three hours to verify all aspects of the control strategy.

Capacity testing of individual components like chillers or cooling coils can require many hours and several team members to set up and monitor all of the necessary operating points.

Discussion

The time necessary to develop a specific functional test, or to adapt a generic test procedure to meet the specific needs of the current project, have not been included in the estimates above. A rough estimate is two to four hours for each component type.

The time associated with completing prefunctional checklists has not been included in the estimates above. These checks should be made throughout construction during normal commissioning site visits as installation of the various components and systems are completed. Sensor and actuator calibration, coil/piping flushing and valve stroke tests are typically considered to be part of completing the prefunctional checklists.

Key Commissioning Test Requirements

General

The purpose of the tests is to ensure that the individual components are installed and integrated to operate on a system level per the design intent and sequence of operations. Commissioning an air-cooled or evaporative-cooled condenser system can result in significant improvements in system operation and energy efficiency. Some universal and technology-specific issues and projected benefits are included in the list below:

Attention to the following steps during the commissioning process can result in significant improvements in system operation and energy efficiency:

Safeties, Interlocks, and Alarms

Verify that all safeties, interlocks, and alarms are programmed (or hard-wired, if applicable) and function correctly.

Low-ambient controls for condenser fans operate correctly to protect refrigeration compressor or chiller from experiencing operational problems.

Sensors

Verify proper sensor installation and calibration. The DDC control system relies on input from various sensors (including temperature, pressure, and flow) in order to achieve the desired system operation. However, if sensors are not located correctly or the measured value from any sensor to the control algorithm is incorrect, the system will not respond as intended. Commissioning will ensure all sensors are located, installed, and calibrated correctly, so that the DDC system will have accurate data from which to execute each control sequence.

Unit Capacity

In some instances, verifying condenser capacity and/or efficiency at peak load may be required. Verifying part load performance can be an easier and more cost-effective solution than attempting to test peak load performance since most systems operate at part load a majority of the time.

Actuation and Sequencing

Verify proper condenser control and staging, especially if multiple units are installed. Regardless of the type of condenser installed, optimizing the control strategy used to maintain the condenser at setpoint is important. For example, every condenser system will vary the amount of air flowing across the heat transfer elements as part of the control strategy. Testing the system and making sure the particular control strategy employed (fan cycling, 2-speed motor, or VFD) is optimized will reduce system energy usage.

Setpoints and Reset Controls

Verify head pressure setpoint and optimize if necessary. Typically this is only applicable to air-cooled condensers serving large chiller and refrigeration systems (not package HVAC systems with air-cooled condensers). The condenser in a DX application is typically controlled to hold a minimum head pressure setpoint. Minimum head pressure setpoint is typically determined by the pressure at which liquid refrigerant is guaranteed to be delivered to the metering device (such as thermal expansion valve and electronic expansion valve) and to ensure oil is returned back to the compressor. As the condensing temperature rises due to elevated atmospheric temperature, the head pressure will float above the minimum setpoint as necessary to deliver liquid refrigerant to the expansion device. As the condensing temperature lowers due to colder atmospheric temperature, the condenser fans will modulate as necessary to maintain minimum head pressure setpoint. Head pressure control strategies can be complicated and must be tested in order to achieve design intent and proper system operation under all atmospheric conditions.

Control Accuracy and Stability

Verify proper head pressure control and integration over all components (including setpoints and reset strategies, fan staging, water spray control, and/or compressor splitting, start-up / shut down procedures, and time delays). Typically a chiller is operated under various conditions and loads to ensure head pressure control is stable.

Verify that all control loops achieve stability within a reasonable amount of time after a significant load change (such as start-up, automatic or manual recovery from shut down).

Key Preparations and Cautions

Prefunctional Checklists and Start-up

Prefunctional checklists should be completed throughout construction during normal commissioning site visits as installation of the various components and systems are completed. Sensor and actuator calibration is typically considered to be part of the prefunctional checklist. Verification that refrigerant piping is installed, pressure tested, and evacuated per design and manufacturer's recommendations also occurs as a part of the prefunctional checklist.

In addition to the prefunctional checklists, all component start-up procedures must be complete in order to conduct functional test procedures. Both the air-side and water-side TAB must also be complete prior to functional testing.

Test Conditions, Considerations and Cautions

The following points should be noted to avoid testing complications:

1 Functional performance testing for an air-cooled condenser system can occur during virtually any atmospheric conditions except, perhaps, during extreme cold (unless the system is intended to operate even when outside air temperature is low, such as an HVAC system serving a 24/7 computer room load or a large refrigeration system). Refer to Functional Testing Field Tips Chiller module for a discussion of this issue. Testing some control sequences, like low-ambient head pressure controls, can only be tested accurately when proper atmospheric conditions exist. Therefore, complete performance verification may require a phased-testing approach to check system operation under various ambient conditions. Care should be taken to ensure that testing conditions will not have an adverse impact on the equipment.

2 If testing an evaporative-cooled condenser system during cold weather conditions, ensure all freeze protection controls are functional to prevent damage to the equipment.

3 The minimum required system head pressure is based on the minimum pressure that will guarantee liquid refrigerant is available at an individual thermal expansion device. Use caution if optimizing head pressure controls during commissioning.

4 Safety and interlock tests, as well as some test procedures and loop tuning efforts (for example, high or low refrigerant cut-out setpoints, emergency shut-down procedures, and failure/back-up system operation) could place the system at risk if the sequences do not function as intended. Appropriate precautions and procedures should be in place to protect personnel and machinery, including plans for quickly aborting the test if necessary.

Instrumentation Required

Instrumentation requirements will vary from test to test and typically will include, but are not limited to, the following:
Temperature measurement devices

Refrigeration pressure measurement devices

Amperage and voltage measurement devices

Tachometer

Flow measurement devices (if needed for system capacity verification)

Data loggers

Time Required to Test

Overview

The time necessary to execute functional tests on an entire condenser system depends predominately on the size and complexity of the installation and specified control sequences. The number of condenser fans, coils, spray pumps, split coil circuits, as well as the complexity of the head pressure control sequence (including floating pressure, low-ambient controls, possibly VFD fan controls, and safeties/alarms) will significantly impact the time associated with testing the entire system.

For this reason, time estimates have been separated out by component on a per unit basis as well as on an overall system level. Component-level tests typically refer to discrete functions of each piece of equipment (such as start/stop procedures, safeties, operational and failure interlocks, and alarms), where system-level tests focus on evaluating proper integration of each component to satisfy the desired control strategy (for example, staging, set points and reset strategies).

The time necessary to develop a specific functional test, or to adapt a generic test procedure to meet the specific needs of the current project, have not been included in the estimates above. A rough estimate is two to four hours for each component type.

The time associated with completing prefunctional checklists has not been included in the estimates above. These checks should be made throughout construction during normal commissioning site visits as installation of the various components and systems are completed. Sensor calibration and chiller/piping flushing is typically considered to be part of completing the prefunctional checklist.

Component Level Testing

One to two hours is needed per condenser.

Capacity testing of individual condensers can require many hours and several team members to set up and monitor all of the necessary operating points.

System Level Testing

Two to three hours is needed to verify proper head pressure control strategies.