Chiller System Basics

Chillers in a district cooling at University of Rochester in Rochester, New York.
Chillers in a district cooling at University of Rochester in Rochester, New York.

Small air conditioning units are often suitable for general residential and light industrial applications, but once a building reaches a certain size, it becomes more efficient to use a centralized chilled water system.

Chillers use a refrigerant gas to move the unwanted heat between the evaporator and the condenser. The chilled water is generated in evaporator and this is sent around the building by a pump to collect the unwanted heat and bring it back to the evaporator to be cooled down.

The refrigerant collects this heat and moves it to the condenser. The condenser puts this unwanted heat into another loop which is sent by a pump to the cooling towers to send it into atmosphere or it will send it to a cooling coil where a fan blows the heat away, much like you blow onto a hot spoon of soup to cool it down.

Chillers are typically located in the basement or on the roof. Rooftop chillers tend to be air cooled where as basement chillers tend to be water cooled. Water cooled chillers will use cooling towers where as air cooled chillers will blow air across their condenser, much like an A/C unit, to disperse the unwanted heat into the atmosphere.

Water Cooled Chiller Key Concepts

  • Water cooled chillers generate cold water for AC in buildings and contain 3 Main Loops: Refrigeration Loop (Compressor), Chilled Water Loop (Building AHU), Condenser Circuit (Cooling Tower).
  • Refrigeration Loop: Compressor → Condenser → Expansion Valve → Evaporator (Insulated)
  • Chilled Water Loop (Leaves chiller around 43F, hits AHUs, returns to chiller around 54F)
  • Condenser Water loop (Leaves chiller around 90F, hits Cooling Tower, returns to chiller around 81F).
  • Chilled water and condenser systems are isolated from each other, heat is transferred by refrigerant.
  • Most buildings typically have N + 1 chillers, where N = number of chillers required to handle worst case conditions. This allows for a backup chiller in the event of failure or the ability to cycle duty for maintenance. Critical environments may have N + 2 or N + 3.
  • The oil system in a chiller moves oil around the system to cool and lubricate high friction areas in mechanical components (compressor bearings). Connected to chiller cooling to regulate oil temp.


Compressors are the driving force behind a chiller and is responsible for pushing refrigerant through the system to keep producing chilled water. There are several different types of compressor:

Centrifugal Compressors

  • Mainly found on water cooled chillers with medium to large loads.
  • Use one or two rotating impellers to compress the refrigerant and force it around the chiller.
  • Sucks in refrigerant, angular velocity causes centrifugal force (dispersed at all angles) forcing the particles into a diffuser.
  • Speed and diameter are important characteristics of the compressor impeller for tangent velocity.
  • Amount of work is determined by the difference between suction and discharge.
  • Capacity control is achieved through speed control and vane guides.

Turbocor Compressor

  • Found on air or water-cooled chillers, in all cooling loads from large to small buildings.
  • Uses two rotating impellers to compress the refrigerant.
  • Capacity control is achieved through speed control and vane guides.

Reciprocating Compressor

  • Found in air or water cooled chillers – old technology, less common now.
  • Used in small to medium cooling loads – common in simple low-cost refrigerators.
  • Utilizes a piston and chamber to compress refrigerant.
  • Capacity control is achieved through compressor staging or cylinder unloading and speed control.

Scroll Compressor

  • Found in air or water-cooled chillers, with small to medium cooling loads.
  • Contains one or more compressors, fixed or variable speed, staged or speed controlled.
  • Use two spiral plates to compress the refrigerant, one fixed in place, one rotates.
  • Capacity controlled via momentarily separating scrolls with solenoid valve and electronic modulation.

Screw Compressor

  • Found in Air or water-cooled chillers, with small to medium cooling loads.
  • Typically, 1 compressor on water cooled, 1 or 2 compressors on air cooled.
  • Uses two interlocking rotating helical rotors to compress the refrigerant.
  • Capacity is controlled via speed control or slider.


The evaporator is the component that actually produces the chilled water that is sent throughout a building. To maximize efficiency, and not pick up stray heat, the chilled water lines will be completely insulated.

Inside the chiller shell, the evaporator is filled with tubes containing the chilled water, refrigerant surrounds the tubes and both are isolated from each other. Heat is transferred from chilled water loop to liquid refrigerant, which easily boils and evaporates into a gas.

Evaporators are classified by the number of passes required for the refrigerant to move all the way across the evaporator (1 pass, 2 pass, etc.). A sight glass allows visual inspection of refrigerant and tubes. Relief values are fitted to the evaporator to allow the release of excess pressure.


The chiller condenser collects the unwanted heat from building via the compressed refrigerant to condensed water loop inside the chiller shell. The condensed water loop collects the heat and travels to cooling tower.

Refrigerant must be at a higher temperature than the condensed water return in order to effectively transfer heat. Because condensed water is exposed to the outside environment, sediments can build up in the condenser, which requires regular to keep clean and efficient.

Expansion Valve

The expansion valve on a chiller controls the refrigerant flow, responds to suction line superheat, and adjusts to cooling load.

  • Pilot expansion valves are found on larger chillers and offer better control. They are connected from condenser to main valve, and may also contains a thermal bulb connected at suction line. The pilot valve works to control a larger main expansion valve.
  • Thermal expansion valves are more common on smaller chillers and feature a similar design to the pilot valve, except there is no main valve. Everything is contained in single unit.
  • Electronic expansion valves offer the best performance by utilizing a step motor to precisely change the expansion valve. An electronic controller monitors temperature and pressure from evaporator to calculate superheat and will adjust accordingly.

Air Cooled Chillers

Air cooled chillers are very common, especially in small to medium size commercial and office type buildings. They are usually located externally, either up on the roof or at ground level.

Air Cooled Chillers do not use cooling towers, instead they dump their heat into the ambient air and therefore need access to a lot of fresh air, in order to reject the unwanted heat from the building.

The chillers will produce “chilled water” which is pumped out around the building to the Air Handling Units (AHU’s) and Fan Coil Units (FCU’s) Which remove the unwanted heat from the building and transfer it into the chilled water loop before repeating the cycle.

Air Cooled Chiller Main Components

  1. Compressor – Pushes a refrigerant round the inside of the chiller.
  2. Condenser – Coils which contain the hot refrigerant, heat is distributed away from the tubes and into the air which is blown across the condenser.
  3. Condenser fans – Sucks outside air across the condenser coils, entering from the sides, and exists the top of the unit upwards into the ambient atmosphere.
  4. Expansion Valve – Responsible for expanding the refrigerant, changing its pressure before it enters the evaporator.
  5. Evaporator – Produces chilled water and extracts heat from a conditioned space, sending it to the condenser.

Vapor Absorption Chillers

Vapor absorption chillers will use a heat source to move the refrigerant around the system rather than using a mechanical compressor. The refrigerant in these chillers move around between areas of different temperature and pressure. Absorption chillers should only be used where there is an abundance of high quality waste heat or cheap heat.