Issue:
What can be done to help with short cycling of compressors?
Resolution:
CGAM Chillers:
Short cycling of compressors is typically attributed to system-related issues. CGAM chillers require a minimum loop duration of 2 minutes. In simpler terms, this means that the actual gallons per minute (GPM) through the evaporator should be doubled to meet the minimum requirement of water/glycol in the loop. The presence of closed isolation/bypass valves in the chilled water loop may also contribute to "short loop" problems.
The chiller regulates the runtime of the compressors based on a "Wear Number". Each circuit and compressor has its own wear number.
Compressor Wear Number = starts + hours
Circuit Wear Number = (compressor A Wear + Compressor B Wear)/2 (Note: for a 3-compressor circuit, it would be (A Wear + B Wear + C Wear)/3)
To enable Balanced Starts and Hours:
- Access Kestrelview
- Go to Unit View
- Select the Capacity Control Tab
- Choose the Circuit Staging Option for Balanced Starts-Hours and the Compressor Staging Option for Balanced Starts-Hours.
Setting the design delta T of the chiller is crucial for compressor staging. Compressors stage based on the Design Delta T divided by the number of compressors. Incorrectly setting the delta T can result in compressor short cycling. To determine the appropriate design Delta T, the GPM through the evaporator must be measured. Measure the pressure drop across the evaporator and use the pressure drop charts in the IOM to obtain the GPM. Then, refer to the nominal tonnage from the unit nameplate and calculate the GPM/ton.
For example, if the chiller is a CGAM100 with a GPM of 190, the GPM/ton would be 190/100 = 1.9 GPM/ton. Referring to the table below, the chiller should be programmed with a Design Delta T of 13.3°F.
After calculating the GPM/ton, consult the table below to determine the appropriate Delta T setting that can help address short cycling issues.
The utilization of Minimum and Maximum Differential to Start/Stop setpoints can be beneficial in mitigating the occurrence of short cycling.
It is of utmost importance to exercise caution when increasing the differential to Stop to avoid potential issues such as low evaporator refrigerant temperature or low pressure diagnostics caused by excessively cooling the water (below 42F Leaving water temperature). The differential to Start should be determined based on the specific application. This setting should be configured to allow for the warmest water temperature that the application can tolerate before the compressors are required to cool the water.
If, even after configuring the design delta T, ensuring an adequate volume of water in the loop, and setting the differential to Start/Stop setpoints, any issues or inquiries persist, please do not hesitate to contact Pueblo Technical Service.
TechTV Video:
Other Question/Problem/Cause Statements:
- what can be done to help with short cycling of compressors?
- Chiller his high number of starts and low hours?
- Compressor starts and hours?
- Starts/Hours?
- Compressors are short cycling, why?
- Compressors are staging on and off?
- How does balanced starts and hours work?
- How does the chiller balance compressor run time?
- How does balance starts/hours work?
- Short cycle of compressors?
- Short Cycling
Facts:
- Air-Cooled Chiller (Pueblo) (CGAM 20-130 Ton)
Document Identifier: DOC-107808
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.