Chiller Terms

Air cooled

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Refers to a chiller system that uses the ambient air surrounding the condensing unit to cool and condense the refrigerant back into a liquid.

Water Cooled

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Water cooled chillers absorb heat from process water and transfer it to a separate water source such as a cooling tower, river, pond, etc. They are generally used for large capacity applications, where the heat generated by an air cooled water chiller creates a problem. They are also considered when a cooling tower is already in place, or where the customer requires optimum efficiency of power consumption. Water cooled chillers require condenser water treatment to eliminate mineral buildup. Mineral deposits create poor heat transfer situations that reduce the efficiency of the unit.


This refers to how much cooling the chiller is designed to provide at maximum load. The capacity can be controlled in most chillers so that it closely matches the actual cooling demand at the current time. The capacity is usually stated in units of kW or TR tons of refrigeration.



The evaporator is where the unwanted heat of the building is collected before being transferred over to the condenser. As the unwanted heat enters the evaporator it will cause the refrigerant to boil and evaporate, as it evaporates it will carry the heat away to the condenser. The refrigerant enters the evaporator as a low-pressure liquid and as it evaporates it leaves as a low pressure vapour.

Cooling Tower

cooling tower

A cooling tower is a large heat exchanger unit that provides cooling water to remove heat from a coolant in your chiller. When the cooling water meets the air, a small portion evaporates, lowering its temperature. This is known as “evaporative cooling”.



A refrigerant is any substance used to cool the water in a chiller through a heat exchanger or evaporator. The substance typically has a low boiling temperature and includes Freon and ammonia.


screw compressor

scroll compressor

A compressor in a refrigeration circuit compresses cool low pressure refrigerant gas to hot high pressure refrigerant gas that is then condensed back into a liquid to be used again.

Chilled water

The chiller generates chilled water, this water flows in a closed circuit between the chillers evaporator and the cooling coils within the building. A pump forces the chilled water around the building to the coils within the AHU’s and FCU’s where the unwanted heat in the air will transfer into the water, this cools the air down and warms the “chilled water” up, this warm chilled water then returns to the chiller evaporator to dump this unwanted heat. As the heat is dumped it causes the refrigerant boil and carry’s this heat away which causes the water to cool down again. It then repeats the cycle and collects more heat. Typical temperatures of the chilled water are flow: 6°C (42.8°F) Return: 12°C (53.6°F) These numbers can and will vary from this.

Condenser(Cooling) water

Condenser water is the water that flows between the cooling tower and the condenser of the water-cooled chiller. This picks up all the unwanted heat in the condenser which was transferred over via the refrigerant. It also collects the heat from the compressor in certain designs. The condenser water is sent to the cooling tower where the heat is removed and rejected to atmosphere and it then returns back to the condenser to pick up more heat. A typical temperature would be: Flow: 32°C (89.6°F) Return 27°C (80.6°F) these numbers are typical, they can and vary from this.


COP stands for Coefficient of performance. It’s simply the ratio of how much cooling you get per unit of electricity you put in which is a way of measuring the chillers efficiency.

COP = kW of Refrigeration / kW of electricity
2500kW of cooling / 460kW of electricity = COP:5.4, so for every 1kW of electricity you put into the chiller, it will generate 5.4kW of cooling.

The COP varies with the cooling load on the chiller. It is useful for measuring the efficiency at a specific point in time or under specific conditions.


Load refers to the cooling demand on the chiller.

Full load means the chiller is operating at its maximum cooling capability, this is only around 1-2% of the year typically.

Part load means the chiller is operating at less than its maximum capability, this is normal for most of the year in typical applications.

Low load means the chiller is operating at very low capacity, faults can often occur at these conditions and chillers typically do not operate efficiently at these conditions. If a chiller is operating at low load for long periods throughout the year then it is oversized and alternative options should be investigated to replace it which will save energy and operating costs.

Cooling load is typically measured in BTU/s refrigeration Tons or kW.

Setpoint, active chilled water setpoint

Setpoints within chillers refer to a desired temperature or pressure, usually this has a focus on the chilled water supply temperature. The desired temperature is defined within the controls and the chiller tries to meet this temperature. A temperature sensors at or near to the chilled water supply outlet of the evaporator will measure the actual temperature and the chillers controls will make adjustments to meet this or operate as close as possible to it.

Chilled water pump and condenser water pump

These are the pumps which distribute the chilled and condenser water around the building between the chiller, cooling coils and cooling tower. They can be either constant or variable flow depending on the design of the system. Variable flow is increasingly popular in secondary side systems as it can provide significant reduction in energy and operating costs.


Lift refers to the difference in pressure between the refrigerant in the condenser and the refrigerant in the evaporator. The higher the difference the more work the compressor is having to do in order to achieve this. The chilled and condenser water temperatures and the approach temperatures set the required lift. Reducing the condenser water setpoint and increasing the chilled water setpoint will reduce the energy consumption of the compressor.

Approach(Evaporating) temperature

This refers to the temperature difference between the chilled water supply temperature as it exits the chiller compared to the temperature of the refrigerant within the evaporator.

For example the chilled water supply temperature might be 7°c (44.6°F) and the refrigerant might be at 3°C (37.4°F) Approach therefore equals 4°C or 7.2°F. A difference 3-5°C or 5-8°F is typical.

Flow rate

This is referring to the quantity of water passing through the chiller or a specific part of the distribution pipework. It’s a measurement of volume per unit of time. Example a gallon per minute (gpm) a litre per second (l/s) or cubic meter per second (m3/s).