Unit 11
Direct Expansion (DX) systems
Chilled water (indirect) systems. 1. Direct Expansion System
It is the system where the refrigerant is utilised to cool the air directly. The common examples of this system are :
Room air conditioner.
Packaged units.
Central direct expansion plants.
The main components of the direct expansion system are :
(i) Compressor (hermetic or open type).
(ii) Condenser (air cooled or water cooled).
(iii) Evaporater or cooling coil with fan.
(iv) Cooling tower or spray panel (in case of water-cooled condensor)
(v) Condenser water pump set.
(vi) Air distribution ducting and grills.
Room air conditionrs
There are self-contained air conditioning units, comprising of a compressor, evaporator fan and air-cooled condenser. This type of plant is used for single rooms having limited occupancy. These units are ideally suited for bed rooms and rooms of similar application, where very close control of temperatures and relative humidity is not required. Room air conditioner is mounted at the window sill on an external wall where hot air from the condenser can be discharged without causing a nuisance.
Packaged air Conditioners
Packaged air conditioner comprises of a compressor, water cooled condenser, evaporator and fan, all mounted in a sheet metal cabinet. They are ideally suited for residences, shops, banks, offices and some industrial applications. Window units are available upto a limited capacity. For higher capacity it is economical to operate the air-cooled window system and the equipment also becomes bulky, which does not make if possible to install it at the window. Floor mounted self-contained packaged units are made to meet the requirements for large capacities. The packaged units can be mounted within the air-conditioned space or remote in a separate enclosure. The installation becomes economical if the unit is mounted within the space. They are normally mounted on a resilient pad which prevents vibration of the compressor from being transmitted to the building.
Central direct expansion plants
In this system, all the equipment pertaining to air conditioning is installed in a central plant room, and the conditioned air is distributed to the rooms/halls/enclosures by ducts. The system is more useful for factories, hotels, assembly halls, cinema halls and big residential buildings where it is uneconomical to install separate units for each room.
2. Chilled Water System
In this system, secondary medium, such as water is used to cool the air. The refrigerant first cools the water and then the water cools the air. The main components of this system are the same as direct expansion system but a chiller is the additional item. Following is the list of the components of the system :
(i) Compressor (reciprocating or centrifugal),
(ii) Condenser (mostly water-cooled).
(iii) Chiller (direct expansion or flooded type).
(iv) Air handling unit with chilled water coil and/or fan coil unit or chilled water air washers.
(v) Cooling tower.
(vi) Chilled water and condenser water pump sets.
(vii) Air distribution ducting and grills.
Chilled water is produced in the refrigeration plant housed in the main plant room. Chilled water is then carried through insulated chilled water piping to air-handling unit or fan coil unit, where the return-air and the fresh-air mixture is filtered, cooled and dehumidified and' then distributed to the conditioned space through galvanized iron and aluminium ducting and grills/diffusers. The return air is brought back to the air-handling unit through the annular space formed around the supply air duct and the false ceiling enclosure. Here it is mixed with the fresh air or ventilation air and then passed through the filters and the cooling coil.
Selection of air-conditioning system
The selection of correct air-conditioning system for a particular building depends upon the following factors :
(i) Capital cost.
(ii) Running and operating cost.
(iii) Space for the location of the equipment.
(iv) Type of application : Whether comfort application or
industrial application.
(v) Type of controls required : Whether rigid control on
the inside of the conditioned space is required as in
the case of some industrial applications.
(vi) Acoustic considerations, such as in auditoriums, radio
and TV studios, conference rooms, etc.
(vii) Type of filtration, whether sterile room or clean room
application.
Table 2.2 gives general guidelines for the selection of proper air conditioning system for various applications. In the table, v7 indicates the recommonded type of air conditioning system.
TABLE 2.2. SELECTION OF AIR CONDITIONING SYSTEM
