We know that the air conditioning is the preparation and distribution of air as per the requirements of the human beings — comfort air conditioning — and different industrial processes and preservation of certain raw materials — Industrial air conditioning. The requirements of both the applications are different.
There are various systems that are used for the preparation of air as required.
These systems are mainly classified as to equipment arrangement:
1. Unitary System, Which Includes:
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(a) Room air conditioner
(b) Unitary packaged air and water cooled systems
(c) Unitary system with combination of heating and cooling
(d) Packaged heat-pump unit
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(e) Unitary split system for cooling.
2. Central Air Conditioning System, which Includes:
(a) Chilled water system
(b) Chilled water air washers
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(c) Centrifugal chillers system
(d) DX system.
Unitary Air Conditioning Systems:
If we make a survey of the market of air conditioning equipment or systems being sold, we will see that there is a continually increasing use of unitary systems.
These systems make use of air conditioners which are factory assembled or packaged. Many of the current unitary or packaged units are designed to be located outside the rooms or space they serve.
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(a) Room Air Conditioner (Window Air Conditioner):
A room air conditioner is an encased assembly designed as a unit primarily for mounting in a window or through a wall. These units are made to deliver cool or warm conditioned air to the room, generally without ducts. This unit includes the main source of refrigeration, dehumidification and means of circulating and cleaning air and also may include means of heating.
The basic function of any air conditioning plant is to provide comfort by cooling or heating, dehumidifying or humidifying, filtering or cleaning and recirculating the space air. All these functions are present in a room air conditioner or window air conditioner.
It may provide ventilation by introducing outside air into the room and/or exhausting the room air to the outside. Room temperature can be controlled by providing a thermostatic setting in the window air conditioner. The conditioner may provide heating by heat pump operation, electric resistance elements or by a combination of both.
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Figure 37.13 shows the schematic diagram of a typical room air conditioner or window air conditioner. Working or operation of this air conditioner is illustrated in Fig. 37.13 (b).
Warm room air (for recirculation) passes over the cooling or evaporator coil and in the process, gives up its sensible and latent heat also (in case of dehumidification is required). This conditioned air along with the fresh air (ventilation air) is then re-circulated in the room by a fan or blower.
The heat from the warm air-vaporises the cold liquid refrigerant flowing through the evaporator. The vapour then carries heat to the compressor, which compresses the vapour and increase its temperature to a value higher than temperature of the outdoor air. In the condenser, the hot refrigerant vapour liquefies and gives up the heat from the room air to the outdoor air.
The high pressure liquid refrigerant then passes through a restrictor—thermostatic expansion valve or capillary tube—which reduces its pressure and temperature. The cold and low pressure liquid refrigerant then re-enters the evaporator to repeat this refrigeration cycle.
The cooling capacities or commercially available window conditioners range from 0.75 to 10 kW.
Most room air conditioners are designed for bringing in outside air, exhausting room air or both. Controls usually permit these features to function independently.
Shortly, we say that:
(a) Temperature is controlled by an adjustable built-in thermostat.
(b) One control operates the unit electrically.
(c) Additional knobs or levers operate louvers, deflectors, ventilation system, etc.
(d) Some air conditioners are of slide-out chassis design or integral chassis design.
(e) Filter replacement is the essential service requirement.
(f) Sound level, particularly in bed-room, is to be minimised.
Lastly, the room unit serves a definite need; spot cooling at a minimum installed cost and mobility of location, both of which preclude the consideration of central system cooling units.
(b) Unitary Split-System for Cooling:
The split system, as the name implies, consists of two parts—an indoor cooling component and an outdoor condensing section. The two are connected by liquid and suction refrigerant lines. The indoor unit can be varied widely, first as an evaporator coil with upflow, downflow or a horizontal air flow arrangement depending on the heating system, or it can be equipped with a blower to provide its own air handling capability. The adaptability of these systems made them an immediate success and there is fast growth in their use.
During the same period of development of the package and split-system cooling equipment, the industry also converted many of these cooling systems to reverse cycle operation and called them heat pumps.
In many apartments, it is very difficult to find outdoor space to put packaged units except on the roof: running ducts down several stories is not a practical and economical consideration when compared to running refrigerant lines. The same situation is true for many multi-floor commercial buildings. So, to meet many variations, the industry developed the split system, which consists of an indoor cooling section, outdoor condensing unit, and interconnecting refrigeration piping.
Figure 37.14 shows the schematic diagram of a unitary split-system for cooling and is self-explanatory. The same unit can be used as a heat pump by providing a reversing valve.
Upto 7.5 tonnes, there is no distinct rule to classify a piece of equipment as commercial or residential, for there is a wide degree of application in both markets, using the same products. However, above 7.5 tonnes, the application becomes distinctly commercial, and the product designs are different in component standards. Commercial split- systems range from 7.5 tonnes to 100 tonnes and above.
Central Air Conditioning System:
The name central station system or equipment is also commonly called as applied system or applied machinery. Central station equipment is associated with installations where the cooling plant is located in the basement or in a penthouse on the roof of multistorey buildings.
It serves air handling equipment and air distribution systems throughout the building. Although size is not necessarily the crossover point between unitary and central station, it is usually acknowledged that central station equipment starts at 25 to 50 tonnes and extends upward to the multi-thousand- tonne systems.
Another distinguishing difference is that central station systems use the medium of liquid—mostly water— to transfer heating and cooling to a space air terminal, while unitary systems are based on distributing conditioned air directly to the conditioned space. Unitary equipment makes use of factory packaged, balanced and tested equipment which requires a minimum of onsite labour and. material to be operational.
Central station systems are made up of separate components such as chillers, air-handlers, water towers, controls, etc. which can become quite complex in terms of on-site installations and labour and related trades and crafts. Central station equipment is closely associated with the plan put on by engineering firms. Equipment is then selected and/or built in order to compliance with these specifications.
Conventional Central Station System:
Conventional Central Station System is shown in Fig 37.15.
The major components are:
1. Water chiller
2. Boiler
3. Air handling unit
4. Water cooling tower
5. Control system
The water chiller will produce 4°C – 7°C cold water, and by means of a pump, circulate it to the cold water coil in the air handler. Water off the coil will generally return at a 6°C rise.
Similarly, in winter, the boiler will produce hot water at 80°C to 90°C and pump it to the hot water coil. Note that it is possible to have the boiler and chiller operating at the same time, because in large buildings, there may be need for cooling and heating in different zones.
Condenser water off the chiller (36 – 37°C) is pumped to cooling tower spray nozzles where it is cooled within the tower to around 30°C and then is returned to the condenser.
The air handling unit or units, depending on the number of floors or zones, generally contain:
(a) Chilled water coils
(b) Main hot water coils (can be steam)
(c) Humidifier
(d) Filters
(e) Dampers for mixing return air and outside air
(f) Blower and motor.
A pre-heat coil is frequently required where large amounts of outside air at or below 0°C are needed.
The face and by-pass dampers either permit all the air to go through the humidifiers and the heating and cooling coils or allow some of it to be by-passed, depending on the particular situation. All air to be conditioned is always filtered and cleaned.
Water Chilling Equipment may be:
1. Packaged chillers – Lower capacity.
2. Centrifugal chillers (Hermetic) – Very large capacities upto 1300 tonnes.
3. Screw compressor chillers.
4. Absorption chillers.
Cooling coils in central air conditioning station may use either chilled water circulating through the coils or there may be chilled water evaporators (chilled water sprays) or air-washers. In this case, chilled water works as a secondary refrigerant.
DX System (Direct Expansion) for Central Air Conditioning Plant:
The process of heat removal from the substance to be cooled or refrigerated is done in the evaporator. The liquid refrigerant is vapourised inside the evaporator (coil and shell) in order to remove heat from a fluid such as air, water or brine. The fluid to be cooled can be made to pass over the evaporator surface inside which the refrigerant is boiling, such a system is called the Direct Expansion System (DX System).
In certain cases, such as in large air conditioning systems or in industrial processing, water or brine is chilled in the evaporator and the chilled fluid is circulated through copper or steel coils over which the air is passed. Such a system is called the indirect system. The coil, generally called cooling coil, acts as heat exchangers.
In cases, where condensing unit and evaporator are comparatively near to each other. Direct expansion (DX) system can be used.
Other Air Conditioning Systems:
Air Conditioning Systems in addition to the equipment arrangement—are classified according to the seasons in which the systems are used.
They are discussed as follows:
(a) Summer Air Conditioning System for Hot and Dry Weather:
In this plant, the process carried out will be cooling with humidification by using adiabatic humidifier. Places like Nagpur and North India use this type of the system.
(b) Summer Air-Conditioning System for Hot and Wet Weather:
In this plant, the process carried out will be cooling with dehumidification. Places like Mumbai, Chennai use this type of system because along with the high temperature, humidity is also more and hence ‘cooling and dehumidification’ process is carried out.
(c) Winter Air Conditioning System:
Winter air conditioning is altogether different than the summer air conditioning. For comfort, air has to be heated. By heating air, relative humidity decreases. To increase relative humidity, we have to humidify the air.
(d) Winter Air Conditioning System:
Here the heating process may be carried out in more than one stage.