Types Of Cooling Systems

The most common types of cooling system can been classified as centralized or partially centralized air/water systems, or local systems.

Centralized plant

A centralized cooling system has all the plant located in single area, for example in a basement or roof-top plant room. One or more air-handling units (AHUs) condition the air which is then supplied by ductwork to the floors/spaces within the building.

The air-handling units typically contain heating and cooling coils, a humidifier, filters, and fans to move the air.

One or more chillers will typically be located nearby to provide chilled water for the cooling coil(s). Hot water for the heating coil(s) is provided by a heat-raising system (such as gas boilers or heat pumps).

Centralized air systems

Centralized air systems can be categorized as:

Constant volume (CV)
Variable air volume (VAV)

A centralized air system showing cold and hot water supply from chiller and boiler and distribution of air through VAV units.

Partially centralized air/water systems

In a partially centralized air/water system the bulk of the cooling/heating is carried out within the occupied space by individual room units such as fan coils. These are supplied with hot/chilled water from a central plant area via a pipework system.

Partially centralized air/water systems consist of:

Fan coils
Chilled beams
Chilled ceilings
Room-based heat pumps

Central plant also supplies outside air throughout the building by means of ductwork for ventilation dilution of odors and prevent build up of CO2 . The size of the ductwork installation and associated air-handling plant is smaller than that required by the centralized air system. Unlike a centralized air system, air is only required for ventilation. Consequently, the high volume of air necessary to carry the building’s heating/cooling requirements is avoided.

A partially centralized air/water system (the example is a fan coil system).

Local systems

Local systems are not linked to any centralized plant and only provide cooling in the immediate space where they are located. They may or may not provide ventilation depending on their level of complexity. The common type of local systems are local systems

Split units
Variable refrigerant flow (VRF).


What are the advantages of centralized cooling systems?
Centralized cooling systems offer several advantages, including easier maintenance, improved air quality, and better humidity control. With all the plant located in a single area, maintenance personnel can access and service equipment more efficiently. Additionally, centralized systems can provide better air quality by allowing for more effective filtration and humidification. Furthermore, centralized systems can maintain a consistent humidity level throughout the building, which is particularly important in applications such as data centers or laboratories where precise environmental control is critical.
How do variable air volume (VAV) systems differ from constant volume (CV) systems?

Variable air volume (VAV) systems and constant volume (CV) systems are two types of centralized air systems. In a CV system, the airflow rate remains constant, while the temperature is controlled by adjusting the cooling or heating coil output. In contrast, VAV systems vary the airflow rate to match the changing cooling or heating demands of the building. VAV systems are more energy-efficient and can provide better zone control, but they require more complex controls and may be more expensive to install. CV systems, on the other hand, are simpler and less expensive, but may waste energy by constantly circulating air at a fixed rate.

What are the typical components of an air handling unit (AHU) in a centralized cooling system?

An air handling unit (AHU) in a centralized cooling system typically consists of several components, including heating and cooling coils, a humidifier, filters, and fans. The heating coil is usually connected to a hot water supply, while the cooling coil is connected to a chilled water supply. The humidifier adds moisture to the air to maintain a desired humidity level, while the filters remove airborne contaminants. The fans, which can be axial or centrifugal, move the conditioned air through the ductwork to the building spaces.

How do local cooling systems differ from centralized cooling systems?

Local cooling systems differ from centralized cooling systems in that they are decentralized and typically serve a single room or zone. Local systems can include window units, split systems, or packaged terminal air conditioners (PTACs). They are often less expensive to install and can provide more precise temperature control, but may be less efficient and more difficult to maintain than centralized systems. Local systems are often used in applications where a centralized system is not feasible or cost-effective, such as in small buildings or in areas with limited mechanical space.

What are the benefits of using chillers in centralized cooling systems?

Chillers are a crucial component of centralized cooling systems, providing chilled water for the cooling coils. The benefits of using chillers include high cooling capacities, precise temperature control, and energy efficiency. Chillers can also be designed to operate at part-load conditions, making them suitable for buildings with varying cooling demands. Additionally, chillers can be configured in multiple arrangements, such as in series or parallel, to provide redundancy and improve system reliability.

How do heat raising systems, such as gas boilers or heat pumps, support centralized cooling systems?

Heat raising systems, such as gas boilers or heat pumps, support centralized cooling systems by providing hot water for the heating coils in the air handling units. During the heating season, the hot water is circulated through the coils to warm the air, which is then distributed to the building spaces. The heat raising system can also provide hot water for other building services, such as domestic hot water or space heating. In some cases, the heat raising system can be designed to recover heat from the chiller condenser, improving overall system efficiency.