Chiller Terms

Air cooled

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“Air-cooled” refers to a chiller system that leverages the surrounding ambient air to condense the refrigerant back into its liquid state, effectively cooling the system.

Water Cooled

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“Water-cooled” refers to a type of chiller system that absorbs heat from process water and transfers it to a separate external water source like a cooling tower, river, or pond. This type of chiller is often employed for large-capacity applications, especially where the heat produced by an air-cooled chiller would cause issues. Water-cooled chillers are the preferred choice when a cooling tower is already present or when the goal is to optimize energy consumption efficiency. However, they require regular condenser water treatment to prevent mineral buildup, which can impede heat transfer and reduce the system’s overall efficiency.


“Capacity” in the context of a chiller refers to the maximum cooling output that the chiller is designed to provide under peak load conditions. The cooling output or capacity can usually be regulated in most chillers to match the real-time cooling demand closely. This capacity is often expressed in units of kilowatts (kW) or tons of refrigeration (TR), signifying the cooling power of the chiller system.



The “Evaporator” is a crucial component in a chiller system where the unwanted heat from the environment (such as a building) is absorbed before being conveyed to the condenser. As this excess heat enters the evaporator, it prompts the refrigerant within to boil and evaporate, effectively capturing and carrying away the heat towards the condenser. In this process, the refrigerant enters the evaporator as a low-pressure liquid and exits as a low-pressure vapor, effectively absorbing and transporting the heat away from the source.

Cooling Tower

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A “Cooling Tower” functions as a substantial heat exchanger within the chiller system. It facilitates the cooling of water, which, in turn, helps extract heat from the coolant in the chiller. As this cooling water interacts with air within the tower, a portion of it evaporates, consequently reducing its overall temperature – a process often referred to as “evaporative cooling”. This cooled water is then recycled back into the system, effectively managing heat levels within the chiller.



“Refrigerant” is a term given to any substance used in a chiller to cool water. This process occurs within a heat exchanger or evaporator. Characterized by a low boiling point, these substances, which include the likes of Freon and ammonia, facilitate the heat transfer process, efficiently reducing the temperature of the water in the chiller system.


screw compressor

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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

“Chilled Water” is the water generated by the chiller, circulating in a closed-loop system between the chiller’s evaporator and the cooling coils within the structure. This circulation is facilitated by a pump, which drives the chilled water around the building, towards the coils in the Air Handling Units (AHUs) and Fan Coil Units (FCUs). Here, unwanted heat from the air transfers into the water, thereby cooling the air and warming the chilled water.

This warmed chilled water then returns to the chiller evaporator, where it dispels the unwanted heat. This heat dispersion causes the refrigerant to boil, carrying the heat away and subsequently cooling the water once again. The water then continues its cycle, collecting more heat.

The typical temperature of the chilled water varies; however, the average flow and return temperatures are approximately 6°C (42.8°F) and 12°C (53.6°F), respectively. These figures can vary depending on the specific circumstances and setup.

Condenser(Cooling) water

“Condenser Water” refers to the water flowing between the cooling tower and the condenser in a water-cooled chiller system. It collects the unwanted heat from the condenser, transferred from the refrigerant, and in some designs, also absorbs heat from the compressor. The condenser water then travels to the cooling tower, where the captured heat is expelled into the atmosphere. After releasing the heat, the water returns to the condenser to continue the heat collection process.

Typically, the condenser water flow temperature is around 32°C (89.6°F) and the return temperature is around 27°C (80.6°F). However, these temperatures can fluctuate based on specific system configurations and operational conditions.


“COP,” or Coefficient of Performance, is a measure of chiller efficiency. It’s a ratio representing the amount of cooling you obtain per unit of electricity input. The formula to calculate COP is:

COP = kW of Refrigeration / kW of Electricity

For instance, if a chiller provides 2500 kW of cooling and uses 460 kW of electricity, the COP would be 5.4. This means that for every 1 kW of electricity consumed by the chiller, it generates 5.4 kW of cooling.

The COP is not constant; it fluctuates based on the cooling load on the chiller. Thus, it’s most helpful for gauging efficiency at a specific moment or under particular conditions.


“Load” refers to the cooling demand placed on a chiller.

When a chiller is at “full load,” it operates at its maximum cooling capacity. However, it’s worth noting that chillers typically run at full load for only about 1-2% of the year.

On the other hand, “part load” refers to a chiller operating at less than its maximum cooling capacity. This is the most common operating condition for chillers throughout the year.

A chiller at “low load” operates at a very low capacity. In these conditions, chillers often run inefficiently and are more prone to faults. If a chiller operates at low load for extended periods, it indicates that the chiller is oversized for its application. Exploring alternative options may be beneficial for energy savings and reduced operating costs.

The cooling load is typically measured in units such as BTUs per hour, refrigeration tons, or kilowatts.

Setpoint, active chilled water setpoint

A “setpoint” in a chiller context refers to the target temperature or pressure – most commonly, this is concerned with the chilled water supply temperature. This target value is set within the chiller’s controls, and the chiller aims to achieve this temperature.

An integral component of this system is a temperature sensor located near or at the chilled water supply outlet of the evaporator. This sensor measures the actual temperature, and the chiller’s controls use this information to make necessary adjustments to meet the setpoint. In essence, the goal is to have the actual temperature align as closely as possible with the active chilled water setpoint.

Chilled water pump and condenser water pump

Chilled and condenser water pumps play a crucial role in circulating water throughout the building. They facilitate the movement of water between the chiller, cooling coils, and cooling tower. These pumps can operate under constant or variable flow, depending on the system’s design.

Variable flow systems are becoming increasingly popular, particularly in secondary side systems. The main advantage of variable flow systems is their potential for substantial energy savings and decreased operating costs. They adjust the flow rate based on the cooling demand, thus improving efficiency and reducing energy consumption.


“Lift” in the context of chillers refers to the pressure difference between the refrigerant in the condenser and the refrigerant in the evaporator. A larger pressure difference signifies that the compressor has to work harder. Lift is determined by the chilled and condenser water temperatures, as well as approach temperatures.

By lowering the condenser water setpoint and raising the chilled water setpoint, you can reduce the lift, and consequently decrease the energy consumption of the compressor. This concept of optimizing setpoints can improve the overall energy efficiency of the chiller system.

Approach(Evaporating) temperature

The “Evaporating Approach Temperature” in chillers refers to the temperature differential between the chilled water supply as it leaves the chiller and the temperature of the refrigerant within the evaporator. For instance, if the chilled water supply temperature is 7°C (44.6°F) and the refrigerant’s temperature is 3°C (37.4°F), the approach temperature is 4°C or 7.2°F. A typical evaporating approach temperature lies within the range of 3-5°C or 5-9°F. This metric is crucial for assessing the chiller’s performance and operational efficiency.

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).