Differences between Dry Coils And Wet Coils

Differences between Dry Coils And Wet Coils

Dry coils are devices used to provide cooling or heating to indoor return air. To achieve the “dry” function, the chilled water temperature used in dry coils is higher than that in standard coils, generally preventing condensation—hence the name “dry coil.”

The inlet water temperature of a dry coil is generally about 2°C higher than the indoor air dew point (e.g., inlet 13°C, outlet 18°C). An air velocity of 2 m/s through the dry coil is optimal for heat exchange efficiency. A drain pan is recommended to prevent condensation buildup under high humidity conditions or when pipe insulation is inadequate.

Primarily used in constant temperature and humidity air conditioning systems for cleanrooms, this system generally consists of three components: MAU, FFU, and DC:

MAU (Make-up Air Unit) maintains positive pressure and fresh air volume in the cleanroom while achieving indoor humidity control requirements through dew point control;

FFU (Fan Filter Unit) ensures the cleanliness grade of the indoor environment. Achieving the required cleanliness grade necessitates the combined processing of primary, intermediate, and high-efficiency filters in the MAU; relying solely on the FFU is insufficient;

DC (Dry Coil), primarily eliminates sensible heat (heat generated by people, lighting, machinery, etc.) to control indoor temperature.

Wet coils, categorized as cooling coils or heating coils, are devices that cool (or heat) and dehumidify fresh air or return air while regulating supply air temperature and humidity. Condensate water typically accumulates during operation.

Within cleanroom HVAC systems, dry coils handle sensible heat loads within the clean area, while the wet coils in the MAU manage fresh air loads.

Coil materials typically consist of copper tubes with aluminum fins. Five coil types exist: 2-row copper tubes (2U), 4-row copper tubes (4U), 6-row copper tubes (6U), 8-row copper tubes (8U), and 10-row copper tubes (10U).

Consider the following points when selecting coils:

- Determine the number of copper tube rows based on enthalpy difference;

- Determine the required airflow volume to select the appropriate coil surface area;

- Coil face velocity should generally not exceed 2.5 m/s;

- Coil fins are aluminum fins, requiring blue wave treatment for corrosion resistance.