Central air conditioning plants are used for cooling whole buildings comprised of many floors, big hotels, large halls with a seating capacity of hundreds or even thousands of people, cinema halls, auditoriums, banks, offices, airports, large hospitals etc. Designing the central air conditioning plant for a particular application is a long and tedious process as the HVAC designers have to find out heat load in each and every room or space that is to be calculated. Such systems can be designed only by expert HVAC design companies who have extensive experience of installing these plants.
Central air conditioning plants have a plant room where
the huge compressor, depending on the capacity of the plant is installed. The shell and tube type of water cooled condenser is also housed in the plant room. The throttling devices are usually automatic electrically operated (solenoid valves) expansion valves. The evaporator is also shell and tube type.
There are two types of central air conditioning plants or systems:
1)
Direct expansion or DX central air conditioning plant: In this system the huge compressor, the condenser, the throttling valve and the evaporator or the cooling coil are all housed in the plant room. The cooling coil is fixed in the air handling unit, which also has large blower housed in it. The blower sucks the hot return air from the room via ducts and blows it over the cooling coil. The cooled air is then supplied through various ducts and into the spaces which are to be cooled. This type of system is useful for small buildings.
2)
Chilled water central air conditioning plant: This type of system is more useful for large buildings comprising of a number of floors. It has the plant room where all the important units like the compressor, condenser, throttling valve and the evaporator are housed. The evaporator is a shell and tube. On the tube side the Freon fluid passes at extremely low temperature, while on the shell side the brine solution is passed. After passing through the evaporator, the brine solution gets chilled and is pumped to the various air handling units installed at different floors of the building. The air handling units comprise the cooling coil through which the chilled brine flows, and the blower. The blower sucks hot return air from the room via ducts and blows it over the cooling coil. The cool air is then supplied to the space to be cooled through the ducts. The brine solution which has absorbed the room heat comes back to the evaporator, gets chilled and is again pumped back to the air handling unit.
To operate and maintain central air conditioning systems you need to have good operators, technicians and engineers. Proper preventative and breakdown maintenance of these plants is vital.