Why does cooling capacity change with temperature?
The cooling capacity of a recirculating chiller is not a fixed value under every operating condition.
A chiller may be described by a nominal cooling capacity in kW, but the actual cooling capacity available depends on operating conditions, particularly the required process fluid temperature and the ambient temperature around the chiller.
This is why chiller selection should be based on the cooling capacity available at your required operating conditions, rather than simply selecting a unit from its nominal kW rating.
What does cooling capacity mean?
Cooling capacity describes the amount of heat that a chiller can remove from a process, usually expressed in kilowatts (kW).
For example, if an application continuously generates a particular amount of heat, the chiller needs sufficient cooling capacity to remove that heat while maintaining the required process fluid temperature.
However, a chiller’s refrigeration system operates differently as temperatures and conditions change. The amount of heat it can remove therefore varies across its operating range.
Why does fluid temperature affect cooling capacity?
The required process fluid temperature affects the conditions under while the refrigeration system has to operate.
As a general principle, producing colder process fluid places greater demands on the refrigeration system. The cooling capacity available at a low fluid temperature can therefore be different from the capacity available at a warmer fluid temperature.
This means that a chiller described by a particular nominal kW rating should not automatically be assumed to provide that same capacity at every setpoint.
For applications requiring low fluid temperatures, it is particularly important to check the model-specific cooling performance at the required operating point.
Why does ambient temperature affect cooling capacity?
For an air-cooled chiller, the refrigeration system ultimately rejects process heat into the surrounding air.
As ambient temperature increases, rejecting this heat becomes more challenging. The cooling capacity available from the chiller can therefore change as the temperature of the surrounding air changes.
The relevant ambient temperature is the temperature of the air entering the chiller, which may not always be the same as the general room temperature.
Poor ventilation, nearby heat-generating equipment or recirculation of hot discharge air can all increase the local temperature experienced by the chiller.
What does nominal cooling capacity mean?
A nominal cooling capacity is a useful way of describing and comparing chiller models, but it relates to defined operating conditions.
It should not be treated as a guarantee that the same cooling capacity will be available at every combination of process temperature and ambient temperature.
When comparing chillers, check the conditions associated with the stated capacity and use the model-specific performance information to determine whether sufficient cooling is available for your application.
How do I select the correct chiller capacity?
Start by determining the heat load of the application: the amount of heat that needs to be removed during operation. 7
ATC then needs to understand the conditions under which that heat must be removed.
When requesting a quotation, please provide:
- Required cooling capacity or process heat load in kW
- Required process fluid temperature
- Coolant or process fluid
- Required flow rate
- Known system pressure requirement or pressure drop
- Expected ambient temperature
- Available electrical supply
- Details of the application being cooled
These factors allow the required cooling duty to be considered against the performance of an appropriate chiller.
Should I simply choose the next larger chiller?
Selecting a chiller should be based on the actual application requirements and operating conditions, rather than simply choosing a model with a larger nominal kW figure.
Correct specification also involves coolant flow, pump selection, temperature range, ambient conditions, electrical requirements, and how the chiller will integrate with the customer’s process.
Providing ATC with accurate information allows these factors to be considered together.
What if my operating conditions are unusual?
If your application needs to operate at particularly high or low fluid temperatures, in challenging ambient conditions, or outside the standard capabilities of the current product range, tell ATC when requesting a quotation.
Compatible models may have predefined configurations or Option Packs for particular operating conditions, including High Ambient, Water-Cooled, Low Temperature, and High Temperature solutions.
Where the requirement falls outside ATC’s standard product capabilities, it can be reviewed by Sales and Engineering as a non-standard requirement.
How can ATC help me to select the right cooling capacity?
You do not need to select a chiller from the nominal kW rating alone.
Tell us what you are cooling, the heat load, required fluid temperature, coolant flow, process fluid, and expected ambient conditions, and our team can use the relevant product performance information to help to identify a suitable chiller.
Contact ATC to discuss the cooling requirements and operating conditions for your application.
