DIRECT EXPANSION COIL ( HVAC )

 

DIRECT EXPANSION

REFRIGERANT - INSIDE TUBES

AIR - OVER TUBES

 

ACCOMPLISHES MOST OF ITS HEAT ABSORPTION   BY  BOILING   WITHIN   TUBES   -  PHASE CHANGE.

Ø SINCE LOW TEMP. / PRESSURE LIQUID REFRIGERANT EVAPORATES INTO GAS, IT IS A LATENT HEAT TRANSFER.

Ø ANY LATENT HEAT TRANSFER DOES NOT ACCOMPANY ANY CHANGE IN TEMPERATURE.

 

o  THE TEMPERATURE AT WHICH REFRIGERANT BOILS IS PRESSURE DEPENDENT. SINCE THERE IS A PRESSURE DROP, AS REFRIGERANT FLOWS THROUGH A TUBE, BOILING TEMPERATURE DROPS.

o  AFTER LIQUID REFRIGERANT HAS COMPELETY EVAPORATED, THERE IS SOME SENSIBLE HEAT ADDITION CALLED SUPER HEAT.

A direct expansion (DX) coil is another type of coil used in HVAC systems for cooling. Unlike a chilled water coil that uses water as a medium for heat exchange, a DX coil uses a refrigerant to directly cool the air. It is also known as a "evaporator coil" or "cooling coil."

Here's how a DX coil works in an HVAC system:

1. Refrigerant flow: The DX coil is connected to the outdoor condensing unit, which contains a compressor and a condenser coil. The compressor pressurizes the refrigerant and sends it to the DX coil as a high-pressure, high-temperature gas.

2. Expansion valve: Before entering the DX coil, the high-pressure refrigerant passes through an expansion valve. The expansion valve reduces the pressure of the refrigerant and controls its flow rate into the DX coil.

3. Heat exchange: The low-pressure refrigerant enters the DX coil, which is typically made of copper or aluminum fins and tubes. As the air from the space moves across the coil, heat is transferred from the warm air to the cold refrigerant. This causes the refrigerant to evaporate and absorb the heat from the air.

4. Cooling effect: The process of heat exchange between the air and the refrigerant causes the air temperature to decrease. The cooled air is then distributed back into the space through ductwork or vents, providing a comfortable environment.

5. Return refrigerant: After absorbing heat from the air, the now-gaseous refrigerant exits the DX coil and returns to the outdoor condensing unit. There, it undergoes a compression process to become a high-pressure, high-temperature gas again, and the heat is released into the outdoor environment through the condenser coil.

DX coils are commonly used in smaller HVAC systems, such as residential air conditioners and some commercial systems. They offer a more compact and efficient cooling solution compared to chilled water coils because they eliminate the need for a separate chiller unit and water distribution system. However, DX systems may require additional components like a fan or blower to ensure proper airflow across the coil.