Question

Explain the central air conditioning system.

Hint:

Name the chiller, cooling tower, air handling unit and ducts. Then explain how the chilled water cools the air that is sent to the rooms.

Answer 1

Step 1: Understanding the Concept:
A central air conditioning system has one large plant at a central place. It cools or heats, cleans and humidifies or dehumidifies the air. Then ducts carry the conditioned air to many rooms or to the whole building.

Step 2: Main components.
1. Refrigeration plant (chiller) with compressor, condenser and evaporator.
2. Cooling tower and pump, for a water-cooled condenser.
3. Air handling unit (AHU) with filter, cooling coil, heating coil, humidifier and blower.
4. Supply duct, return duct and fresh air duct with dampers.
5. Supply diffusers and return grills in the rooms.
6. Controls such as thermostat and humidistat.

Step 3: Working.
The chiller cools water to about 7 degrees C. A pump sends this chilled water to the cooling coil in the AHU.
The blower draws a mix of return air and fresh air through the filter and across the coil. The air gets cooler and drier.
The blower sends this air through the supply duct to the rooms.
Room air returns through the return duct to the AHU. Some of it is thrown out, and some is mixed with fresh air again.
The warm chilled water returns to the chiller. The condenser heat goes out through the cooling tower.

Step 4: Merits and demerits.
Merits: it controls temperature and humidity in all the rooms, saves floor space in the rooms, and is quiet in the rooms.
Demerits: it has a high first cost, needs long ducts and skilled operators, and one fault can affect the whole building.

Final Answer:
The central air conditioning system uses one central plant and a duct network to supply conditioned air to the whole building. It is used in malls, hospitals, hotels and offices.