Dehumidification Capacity Calculator

How is dehumidification capacity calculated?

When air passes over a surface colder than its dew point, part of its water vapour condenses. Cooling coils in air handling units and portable dehumidifiers work this way. The moisture removed is the dry air mass flow times the difference between entering and leaving humidity ratios: ṁ_w = ṁ_a × (W_1 − W_2).

Dry air flow is the volumetric flow divided by the entering specific volume: ṁ_a = Q / v_1. Humidity ratios follow from temperature, relative humidity and air pressure through the psychrometric relations.

Cooling load

The total load the coil must remove is the enthalpy difference of the air less the energy carried away by the condensate: Q_t = ṁ_a × (h_1 − h_2) − ṁ_w × h_w. Part of it cools the air (sensible) and the rest condenses vapour (latent). The lower the sensible heat ratio, the larger the share of capacity spent on drying.

Example

Air at 18 000 cubic metres per hour enters a coil at 30 °C and 50 percent relative humidity and leaves saturated at 10 °C. The entering humidity ratio is 13.3 g/kg and the leaving one 7.66 g/kg. Dry air flow is 5.70 kg/s. About 116 kg of water condenses per hour and the cooling load is 197 kW, which is the result of the worked example in the ASHRAE Handbook of Fundamentals.

Things to watch

The tool warns when the leaving air is wetter than the entering air: air at 25 °C and 50 percent only gives up moisture once it is cooled below its dew point of 13.9 °C. The litres per day on an equipment label apply to one test condition, and capacity in your own room will differ.

Limits

The tool gives the load and the moisture removed for the leaving state you enter. Whether a particular coil reaches that state is decided by the manufacturer's selection software.