Air conditioning systems can be categorized according to the means by which the controllable cooling is accomplished in the conditioned space. They are further segregated to accomplish specific purposes by special equipment arrangement. In selecting a suitable air conditioning system for a particular application, consideration should also vent to the following like system constraints such as cooling load, zoning requirements, heating and ventilation. Architectural Constraints depended on size and appearance of terminal devices, acceptable noise level, Space available to house equipment and its location relative to the conditioned space, and acceptability of components obtruding into the conditioned space. Financial constraints including capital cost, operating cost, and maintenance cost. There are four basic system categories of air conditioning namely central chilled water air conditioning system. An all-air system provides complete sensible and latent cooling capacity in the cold air supplied by the system. Heating can be accomplished by the same air stream, either in the central system or at a particular zone. All-air systems can be classified into 2 categories namely single duct systems and dual duct systems. The single duct system as shown in figure 1 is used for a large room such as an atrium, a banking hall, a swimming pool, or a lecture, entertainment or operating theatre. It can be applied to groups of rooms with a similar demand for air conditioning, such as offices facing the same side of the building. A terminal heater coil under the control of temperature sensor within the room can be employed to provide individual room conditions. A variable value (AV) system has either an air volume control damper or a centrifugal fan in the terminal unit to control the quantity of following into the room in response to signals from a room air temperature sensor. Air is sent to the terminal units at a constant temperature by the single duct central plant, according to external weather conditions. A reducing demand for heating or cooling detected by the room sensor causes the damper to throttle the air supply or the fan to reduce speed until either the room temperature stabilizes or the minimum air flow setting is reached. Air flow from diffuser is often blown across the ceiling to avoid directing jets at the occupant. As a result of the Coanda effect the air stream forms a boundary layer along the ceiling and entrains room air to produce thorough mixing and temperature stabilization before it reaches the occupied part of the room. When the VAV unit reduces air flow, there may be insufficient velocity to maintain the boundary layer, and in summer cool air can dump or drop from the ceiling onto to the occupants, resulting in complaints of cool draughts. Figure 1 : Single- duct all-air installation in a false ceiling. The dual duct systems as shown in figure 2 used, in order to provide for wide-ranging demand for heating and cooling in multi-room buildings. Air flow in the two supply ducts may, of necessity, be at high velocity (10-20 m/s) to fit into services ducts of limited size. Air turbulence and fan noise are prevented from entering the conditioned room by an acoustic silencer. In summer, the hot duct will be for mixed fresh and recirculated air, while the cold duct is for cooled and dehumidified air. The two streams are mixed in variable proportions by dampers controlled from a room air temperature detector. During winter, the cold duct will contain the untreated mixed air in the hot duct will contain the untreated mixed air and the air in the hot duct will be raised in temperature in the plant room. The system is used for comfort air conditioning as it does not provide close humidity control. It reacts quickly to changes in demand for heating or cooling when, for example, there is a large influx of people or rapid increase in solar gain. Figure 2 Dual-duct installationnin a false ceiling and detail of the mixing box. The advantages of the system is, the central plant is located in unoccupied areas, hence facilitating operating and maintenance, noise control and choice of suitable equipment. No piping, electrical wiring and filters are located inside the conditioned space. Allows the use of the greatest numbers of potential cooling season house with outside air in place of mechanical refrigeration. Seasonal changeover is simple and readily adaptable to climatic control. Gives a wide choice of zonability, flexibility, and humidity control under all operating conditions. Heat recovery system may be readily incorporated. Allows good design flexibility for optimum air distribution, draft control, and local requirements. Well suited to applications requiring unusual exhaust makeup. Infringes least on perimeter floor space. Adapts to winter humidification.
However, the system still has disadvantages, between is the system it requires additional duct clearance which can reduce the usable floor space, air-balancing is difficult and requires great care, and accessibility to terminals demands close cooperation between architectural, mechanical and structural engineers.
A second category is central chilled water air conditioning systems that use air and water systems. The water side consists of a pump and piping to convey water to heat transfer surfaces within each conditioned space. The water is commonly cooled by the introduction of chilled water from the primary cooling system and is referred to as the secondary water loop. Individual room temperature control is by regulation of either the water flow through it or the air flow over it.
System contained in these categories like induction system. The inducting system is designed for use in perimeter rooms of multi-story, multi-room building that may have reversing sensible heat characteristics. It is especially adapted to handle the loads of skyscrapers with minimum space requirements for mechanical equipment. In the induction system, ducted primary air is fed into a small plenum chamber where its pressure is reduced by means of a suitable damper to the level required at the nozzles.
The plenum is acoustically treated to attenuate part of the noise generated in the duct system and in the unit. The primary air is then delivered through nozzles as high velocity jets which induce secondary air from the room and over the secondary coil. Induction units are usually installed at a perimeter wall under a window. Some hotel rooms are providing with induction coils. The induction system as shown in figure 3.

Figure 3 Induction unit installation in a multi-storey building and section of perimeter induction unit installation.
The induction system employs air ducts to convey treated air with higher pressure levels and of the right adjustable quantities to various cooling/heating coil units. These coil units are built in with induction nozzles such that when high pressure air goes through them, air room the room is inducted across the fin surface of the water-circulated coils. This inducted air stream is either cooled or heated after passing through the coil, and then mixed with the air coming out of the nozzle. The right quantity of high pressure air is adjusted automatically in response to a thermostat located in the conditioned space. The system is well suited to provide temperature control for individual spaces or zones.
The benefit of this system is individual room temperature control. It also separate sources of heating and cooling for each space available as needed to satisfy a wide range of load variations. Low distribution system space required as a result of reducing the air supply by use of secondary water for cooling and high velocity air design. Reduced size of central air handling equipment. Dehumidification and filtration performed in a central plant room remote from conditioned space. Outdoor air supply is positive. Minimal maintenance required for individual induction units which have no moving parts, for example, no fans. Air duct dimensions are smaller than VAV systems or CAV systems. Zoning of central equipment is not required. No fan comes together with the coil, making the conditioned space quiet.
The system also has a weakness that cannot be overcome, such as, limited to perimeter space. The primary air supply is usually constant with no provision for shutoff. Not applicable to spaces with high exhaust requirement. Higher energy consumption due to increased power required by the primary pressure drop in the terminal unit. Controls tend to be more complex than for all-air systems. A low chilled water temperature is needed to control space humidity adequately. Seasonal changeover is necessary and initial cost is usually higher than fan coil systems.
The third system in these categories is central chilled water air conditioning systems are used all water systems, including cooling towers which can also be applied to systems one and two above. Fan coil units including in this system. Fan coil units is use when heating and cooling loads that proves to be too great for induction units can be dealt with by separate fan and coil units fitted into the false ceiling of each room or buildings module. Better air filtration can be achieved than with the induction unit. A removable access hatch below the unit is required to facilitate motor and filter maintenance.
Care is taken to match the fan-generated noise to the required acoustic environment. As with the other systems, the extracted air can be taken through ventilated luminaires to remove the lighting heat output at source and avoid overheating the room. The supply and extract ducts only carry the fresh air. All recirculation is confined to the room. A typical layout is shown in figure 4.

Figure 4 Fan coil unit installation in false ceiling and vertical unit for fan coil unit system
A fourth category is direct expansion systems for split type and package air conditioning systems. A package air conditioning systems are used in factory assembled (floor mounting) package, placed indoor, containing direct expansion coil, controls, fan and compressor, with the condenser remotely placed outdoor. A package air unit is a self-contained air-conditioning unit comprising a hermetically sealed refrigeration compressor, a refrigerant evaporator coil to cool room air, a hot-water or electric resistance heater battery, a filter, a water or air cooled refrigerant condenser and automatic controls. Packaged units can either be completely self-contained, needing only a supply of electricity, or piped to central heating and condenser cooling-water plant. Small units are fitted into an external wall and have a change-over valve to reverse the refrigerant flow direction. This enables the unit to cool the internal air in summer and the external air in winter.
Heat rejected from the condenser is used to heat the internal environment in winter. In this mode of operation it is called a heat pump. A separate ventilation system may be needed. Compressor and fan noise levels are compared with the acceptable background acoustic environment. Maintenance requirement are filter cleaning, bearing lubrication and replacement of the compressor when it becomes too noisy or breaks down.
Split system unit have a separate condenser install outside the building. Two refrigerant pipes of small diameter connect the internal and external equipment boxes. This allows greater flexibility in siting noise-producing compressor. Ducted models provide conditioning and ventilation and are often sited on flat roofs. Figure 5 shows a typical through-the-wall installation. Split system used at same place with package air system.

Figure 5 Packaged air-conditioning unit.