Psychrometric process – overcooling and reheating

In some situation, using just one equipment is not satisfied the supply air condition (required temperature and humidity level), the combination of more than one HVAC equipment is required. This note illustrate the overcooling and reheating process, it is the method used to precisely control indoor tempeture and humidity at the same time. It is use where humidity control is critical, such as hospitals, laboratores, cleanroms, and some commercial buildings.

Overcooling by cooling coil: Air is cooled below the required supply temperature to remove the moisture at the desired level.

Reheating by heating coil or heater: The air is then heat back to the desired supply temperature, so the space does not become too cold.

Disadvantage: Because the system cools the air too much and then heats it again, so it is the inefficient energy. Therfore, using heat recovery to reheat are the alternatives to be considered.

Reheat methods: Reheating can be done using different heat sources:

1. Hot water coil
2. Steam coil
3. Electric heater

Alternative methods: These system improve humidity control with lower energy consumption:

1. Dedicated outdoor air system (DOAS)
2. Variable air volume system (VAV)
3. Heat recovery system
4. Heat pipe
5. Run around coil
6. Hot gas bypass of DX system
7. Heat pump

Related notes:

Psychrometric process – cooling and dehumidification, mechanical based