## 13.20. NA7.5.16 Condenser Water Temperature Reset Controls Acceptance

At-A-Glance

NA7.5.16 Condenser Water Supply Temperature Reset Controls Acceptance
Use Document NRCA-MCH-17-A
Purpose of the Test
The intent of the test is to verify that the condenser water supply (entering condenser water) temperature is automatically reset as indicated in the control sequences; based upon building loads, outdoor air wet-bulb temperature, or another appropriate control variable. All cooling tower system components (e.g. fans, spray pumps) should operate per the control sequences to maintain the proper condenser water temperature and pressure set points. Chilled water plants serve many buildings, responding to the varying cooling loads throughout the year. As the loads vary, the chilled water supply temperatures adjust to satisfy the new operating conditions. Often, water-cooled chilled water plants can decrease the condenser water temperature in times of low cooling load. This occurrence can be demonstrated by running the cooling tower fans at a higher speed, staging on additional fans, or varying water distribution across the tower fill by closing and opening bypass valves. As a result, the cooling tower produces an energy penalty, however the chiller efficiency and the overall plant efficiency improves. The requirement for condenser water reset acceptance only applies to those chilled water systems with a cooling tower implementing some kind of condenser water temperature reset control. There is no code requirement that chilled water plants employ this type of control. However, if condenser water temperature reset is implemented, then it must be tested per the Energy Standards. The purpose of this test is not to evaluate whether a particular control sequence is the most appropriate for the facility, but whether the system follows the intended control sequence.
Instrumentation
Performance of this test will require measuring water temperatures, and possibly air temperature, relative humidity, system pressures, and system flow rates. The instrumentation needed to perform the task may include, but is not limited to: Hand-held temperature probe to calibrate or check existing sensors Humidity sensor or wet bulb temperature probe / psychrometer Installed sensors should be checked for accuracy, and may be used for testing where appropriate. Any instruments used for testing or checking other sensors must be calibrated within the past year, with date of calibration noted on the Acceptance Document.
Test Conditions
To perform the test, it may be necessary to use the building automation system (BAS) to manipulate system operation to achieve the desired control. BAS programming for the operation of the chillers, cooling towers, air handling units, and pumps must be complete, including but not limited to: Chilled water and condenser water temperature control Equipment start-stop control All installed and calibrated control sensors Tuned Control loops All systems must be installed and ready for system operation, including: Chillers, cooling towers, pumps, air handling units, valves, and piping. Control sensors (temperature, humidity, flow, pressure, valve position, etc.) Safeties, interlocks, and alarms (e.g. high/low water alarms, vibration, back-up system operation) Verify all piping is pressure tested, balanced, flushed, cleaned, and filled with water. Verify electrical power is supplied to all equipment. Confirm start-up procedures for all equipment must be complete, per manufacturer’s recommendations. At a minimum, all components and systems served by the chiller and cooling tower should have completed pre-functional checks and be capable of safe operation. Document the initial conditions before overrides or manipulation of the BAS. Return all systems to their initial condition after test.
Estimated Time to Complete
Construction inspection: 1 to 3 hours (depending on availability of construction documentation – i.e. plumbing drawings, material cut sheets, specifications, etc. – as well as sensor calibration records.) Functional testing: 2 to 5 hours (depending on familiarity with BAS, method employed to vary operating parameters, ambient conditions, building loads, and time interval between control command and system response)
Acceptance Criteria
Construction Inspection: All ambient temperature and relative humidity sensors used by the controller must be either calibrated (manufacturer calibrated with calibration certificates or field calibrated by TAB technician), or field checked against a calibrated sensor by the person performing the test. Functional Test: System must meet the following criteria during the test: Condenser water temperature controls modulate as intended. Actual condenser water supply temperature decreases to meet new set point within ± 2°F. Cooling tower fan(s) stage properly and/or adjust speed accordingly to meet lower set point. Chiller load amps decrease.
Potential Problems and Cautions
Condenser water temperature reset is most effective on a moderately warm day. When testing during cold weather conditions, make sure that freeze protection controls are installed and functional to prevent equipment damage. Also ensure the conditioned spaces do not fall below safe temperatures, as this may cause discomfort or unsafe working conditions. If conducting this test during hot weather conditions, make sure the chiller load amps don’t increase as the condenser water temperature decreases. If so, you will need to conduct this test on a cooler day. Likewise, stop the test if the chiller begins to surge. This test does not require operation of the plant equipment across all operating stages, so it is not necessary, nor desirable, that the system experience peak load conditions. However, the system cooling load must be sufficiently high to run the test. If necessary, artificially increase the load to perform the functional tests, or wait until a time of stable chiller operation. If necessary, reverse Steps 1 & 2 in the functional test based on atmospheric conditions and building loads. If the system is designed to employ variable flow simultaneously with temperature reset, allow the system to operate as programmed but take care that the water flow rate stays within the minimum and maximum flow rate limits for the chiller(s) and cooling tower(s). Minimum flow through a cooling tower is important to provide even water distribution and full wetting of the fill to prevent scaling. Exemption: There is an important exemption associated with this functional test to provide flexibility given the range of chilled water plant operations, as follows: If the control sequence differs significantly from that implied by the tests, and / or has already been tested during the building commissioning process, attach a description of the control sequence, a description of the tests that were done to verify the system operates according to the sequence, the test results, and a plot of any associated trend data.

A. Test Application

Newly Constructed and Additions/Alterations: All new condenser water temperature reset controls installed on new or existing systems must be tested.

Some control variables used to reset supply water temperature include, but are not limited to:

• Outdoor air wet-bulb temperature. A common control strategy is to reset supply water temperature based on outdoor wet bulb temperature. For example, the entering condenser water set point may be reset at a fixed amount (e.g. 7°F) above the outdoor air wet bulb temperature, with limits to meet the chiller and cooling tower operation. The cooling tower may then meet the set point by increasing or decreasing the amount of water circulating through the tower, staging on or off cooling tower fans, or adjusting tower fan motor speed for VFD-equipped fan motors. The designer determines nominal supply water and outdoor air temperatures, which are available in the design narrative, specifications or control drawings.

• Condenser water and chilled water temperatures. A cooling tower may operate to maintain a certain temperature difference between the condenser water supply and chilled water return. This process maintains chiller lift or pressure across the compressor. For example, the control may cycle tower fans on and off, or modulate fan speed, to maintain a 14°F difference between condenser water supply of 70°F – 78°F and chilled water return of 56°F – 62°F.

• Load signal from chiller. The condenser water temperature may follow a load signal from the chiller. For example, condenser water temperature may follow a “horseshoe” shape, increasing in times of highest and lowest load, and decreasing during low and moderate chiller loading. This strategy enables the chiller to maintain capacity at high load, benefiting from increased efficiency during times of moderate load, and maintaining adequate lift during times of lowest load.

B. Construction Inspection

Prior to functional testing, verify and document the following:

• Check if the condenser water supply system and control system are installed per the system design, as documented on the building plans or as-builts.

• Check if condenser water supply temperature control sequence, including condenser water supply high and low limits, are available and documented in the building documents.

• Check if all cooling tower fan motors are operational, cooling tower fan speed controls are installed, operational, and connected to cooling tower fan motors per OEM start-up manuals and sequence of operation.

• Check if cooling tower fan control sequence, including tower design wetbulb temperature and approach, are available and documented in the building documents.

• Check if the following temperature sensors are installed per plans: outdoor air drybulb and wetbulb, entering condenser water, and leaving chilled water. Note any discrepancies on the Acceptance Document.

All ambient dry bulb temperature, and relative humidity/wet bulb sensors used by controller must be factory calibrated (with certificate), field calibrated by TAB technician or other technician (with calibration results), or field checked against a calibrated reference standard by test technician (with results). Attach supporting documentation to the Acceptance Document.

When field calibrating temperature sensors, it is recommended that you perform a “through system” calibration that compares the reference reading to the reading at the EMCS front end or inside the controller (e.g. it includes any signal degradation due to wiring and transducer error).

Document the following from the control system or using test sensors:

• Current outdoor air dry bulb and wet bulb temperatures

• Current entering condenser water supply temperature

• Current leaving chilled water temperature

C. Functional Testing

If the control sequence differs significantly from that implied by the tests, and/or has already been tested during the building commissioning process, attach a description of the control sequence, a description of the tests that were done to verify the system operates according to the sequence, the test results, and a plot of any associated trend data.

Document reset control parameter (e.g. outside air wet-bulb temperature) on the Acceptance Document.

Step 1: Adjust the reset control parameter to decrease the condenser water temperature (toward the lower supply temperature limit).

Within the programmed reset strategy, change the reset control variable to its minimum value to decrease condenser water supply temperature downward towards the lower limit. For example, if the control strategy calls for the condenser water supply to reset downwards from 85°F to 70°F with a difference of 10°F above current ambient wet bulb temperature of 75°F, override the sensor reading to read a wet bulb temperature below 70°F.

If the reset control variable input cannot be modified, then change the limit of the variable around the currently occurring value. For example, in the example above, adjust the sequence to a difference of 6°F between the condenser water supply temperature and ambient wet bulb temperature.

Take care not to allow condenser water temperature to drop below the chiller low temperature limit. Allow time for the system to stabilize.

Verify and Document

• Condenser water temperature controls modulate as intended.

• Actual condenser water supply temperature decreases to meet new set point within ± 2°F.

• Cooling tower fan(s) stage properly and/or adjust speed accordingly to meet lower set point.

• Chiller load amps decrease.

Step 2: Adjust the reset control parameter to increase the condenser water temperature (toward the upper supply temperature limit).

Using the desired reset strategy, override reset control variable towards its maximum value to increase the condenser water supply temperature upward to its high limit. If the reset control variable input cannot be modified, then change the limit of the variable around the currently occurring value. Allow time for the system to stabilize.

Verify and Document

• Condenser water temperature controls modulate as intended.

• Actual condenser water supply temperature increases to meet new set point within ± 2°F.

• Cooling tower fan(s) stage properly and/or adjust speed accordingly to meet upper set point.

• Chiller load amps increase.

Step 3: Restore reset control parameter and system to automatic control.

Restore all controls and equipment to original settings, and/or restore the high and low limits of the reset control variable. Remove all system overrides initiated during test.

Verify and Document

• Condenser water temperature controls modulate as intended.

• Actual condenser water supply temperature changes to meet new set point within ± 2°F.

• Cooling tower fan(s) stage properly and/or adjust speed accordingly to meet set point.

• All equipment returns to normal operation.