What is the function of the heat exchanger in a fan coil unit

A heat exchanger is a device that enables heat transfer between two or more fluids (liquids or gases) without mixing with each other. It is an essential core component in refrigeration, air conditioning, heating, chemical, energy and other systems.

The main function of a heat exchanger is to handle the exchange of heat and cold, and it is the "heat exchange heart" of a fan coil unit. Simply put, cold/hot water flows through the pipeline, and air blows over the surface of the heat exchanger to complete heat transfer.

In short, a heat exchanger is like a 'heat mediator'. It neither generates nor eliminates heat, but is only responsible for efficiently and controllably "transporting" heat from the high temperature side to the low temperature side.

During cooling mode

The central air conditioning/heat pump sends cold water at around 7 ℃ flowing inside the copper pipe; The fan drives indoor hot air to pass through the finned heat exchanger, transferring heat from the air to the cold water, cooling the air, and releasing condensed water (dehumidification). The cold air is then sent to the room.

During heating mode

The heat pump sends 45~50 ℃ hot water flowing inside the copper tube; The indoor cold air passes through the heat exchanger, and the heat of the hot water is transferred to the air. The heated hot air is then sent into the room for heating. 

Nowadays, most fan coil units mainly use finned-tube heat exchangers. The composition structure of this heat exchanger mainly consists of adding thin metal fins outside the metal tubes to expand the heat exchange area. Due to the poor thermal conductivity of air, relying solely on copper tubes for heat transfer has low efficiency; Adding fins increases the surface area in contact with air, allowing for rapid heat transfer. 

Hot or cold water flows through the copper tubes of the heat exchanger, carrying heat or cold. The heat is conducted to the external aluminum fins, and convection is generated by the blowing of the fan. The air laterally flushes the surface of the fins, completing heat exchange between the air and the fins.

The characteristics of a fan coil heat exchanger are:

Advantages: Compact structure, large heat exchange area, moderate cost, suitable for air-water heat exchange (one side air, one side water), widely used in fan coil units, air conditioning outdoor units, and air source heat pumps.

Disadvantages: The gap between the fins is very small, which makes it easy for dust and oil stains to accumulate (especially in kitchen environments), and the heat transfer effect drops sharply when covered with dust; The fins will also oxidize in humid environments.

Heat exchangers also have some additional functions for fan coil units:

1. Dehumidification (during cooling): When the surface temperature of the heat exchanger is lower than the dew point of the air, water vapor condenses in the air, reducing indoor humidity. This is also the principle of dehumidification at the end of the air conditioner.

2. Thermal medium: Convert the water energy in the water pipe circuit into the cold and hot energy of the air, without direct contact between the water side and the air side.