**1. Introduction**

In central air conditioning, air, water, or both are used as working fluids to produce the required heating and/or cooling, and therefore based on working fluids, central air conditioning systems can be classified into three groups [1–5], namely:


It is may be of interest to note that each type of the central air conditioning systems has several systems of sometimes different configurations, and the use of any system depends on its advantages and disadvantages [1–5]. The following sections will provide descriptions of all types of central air conditioning systems.

• **Filters**. Filters or strainers are placed at the early air path in AHU. Filter type

• **Mixing box**. It is place where fresh air is mixed with zone return air or with fresh conditioned air. Mixing processes are performed to obtain the desired air temperature and humidity or to maintain energy-efficient performance.

• **Dampers**. Dampers are used to control the amount and direction of air before

• **Heating coil**. Coil placed in AHU where hot liquid or vapor water from boilers

• **Preheating coil**. Preheating coil is placed at AHU entrance before cooling and heating coils. The task of preheating coil in hot days is to reduce the entering fresh air's relative humidity, thus preventing possibly the condensation of water vapor on cooling coil, hence preventing frost formation on the cold coil. In addition, preheater coil will prevent the freezing of water inside the coils in

• **Humidifier**. It is a system which is responsible for increasing the humidity in the conditioned zone. Humidifiers are usually used in cold days where

• **Centrifugal pumps**. Centrifugal pumps are used for cooling and heating air processes. They are used to maintain pumping cycle of hot water from boilers to the heating or preheating coils and back to boiler and/or pumping cycle of cold water from the chiller or cooling tower to cooling coil in AHU and back to

• **Control systems**. Control systems can vary from simple control system to advanced control system that use the latest technologies such programmable logic controllers. Usually controllers are used to control the temperatures and humidity of the supply air to the zones. It also controls the damper systems in the AHU. Moreover, advanced control systems can even control the fan rotation and thus the rate of air supplied to the zone based on the required temperature in the zone and zone exit damper. In such systems, pressure sensor connected to the control system will be placed in the air duct, and as attaining the required temperature in the zone, dampers will get closer increasing the duct-sensed pressure through which the fan connected to the control system will reduce its speed and thus maintain energy-efficient

• **Casing**. Casing is a kind of AHU cover that includes all the above AHU

will not reduce the conditioned hot air provided to the zones.

maintaining hot climate is desired; however this will accompany low-humidity levels, and thus the use of humidifier becomes essential to maintain the comfort edge. Humidifier can provide humidity either as hot vapor or water spray. The first one is more preferable particularly in healthcare applications as hot vapor will prevent the growth of biological organisms such as bacteria or algae besides hot vapor compared to the water spray. Besides that hot vapor is more preferable as provided humidity will have higher temperature and thus

used may depend on application type.

*Central Air Conditioning: Systems and Applications DOI: http://dx.doi.org/10.5772/intechopen.89455*

or after conditioning is performed.

is circulating.

cold days.

the chiller or cooling tower.

performance [7, 8].

components.

**3**
