HVAC Technology

Dx system and chiller
A chiller is a machine that removes heat from a liquid via a vapour compression or absorption refrigeration cycle. Dx or Direct expansion system works in such a way that the evaporator coil comes in direct contact with air to be supplied. When the refrigerant in the evaporator coil expands, it will cool the air by absorbing the heat from it. The expansion of the liquid refrigerant is handled by the flow control device. Whether Dx system with outdoor unit or a chiller, it consists of 4 major components:
1. Expansion device
2. Evaporator
3. Compressor
4. Condenser
The refrigerant high pressure is then reduced to the evaporator pressure as it passes through the expansion device. The pressure drop results in immediate lowering of temperature creating the liquid suitable for heat exchange.
Evaporator or heat exchanger transfers the heat of air to refrigerant and cools down the air, so to supply in rooms or areas. At this stage, the liquid is converted into vapour or semiliquid. For air conditioning purposes, DX system is chosen to be the best in heat transfer from liquid to air.
The compressor will extract the vapour from the evaporator into the suction inlet of the compressor. The vapour is compressed in the compressor causing it to rise in temperature and pressure.
The vapour then flows into the condenser where heat is transferred to the cool air that is drawn into the surface of the condenser coil by the condenser fan. When this happens, the temperature of the vapour drops to the new saturation temperature. Selection of condenser is done on the basis of degree of coolness to be provided or the temperature of refrigerant after passing through the compressor.
Types of condensers:
• Air cooled
• Water cooled
• Evaporative cooled
Advantages of DX system
· Low installation costs.
· Ease to test, adjust and balance the system.
· Minimum ceiling or wall space needed.
· Low energy consumption.
· Low maintenance costs.
· Individual sections can be operated without running the entire system in the building.
· Comfort under varying load conditions.
· Low noise level (NC 35).
· Good relative humidity control.