Limits to mechanical dehumidification efficiency

When designing drinking-water-from-air systems it is useful to know the limits to mechanical dehumidification efficiency. Mechanical dehumidifiers use chilled water coils, direct-expansion refrigerant coils or thermoelectric devices to provide a cooled surface over which flows the air to be dehumidified. Systems are designed usually with defrost controls to avoid frosting of the coil surface. Practically, the minimum temperature for coil operation is about 5 °C. The air leaving a wet coil is saturated so the state of leaving air may be, for example, dry bulb = 5 °C with 100% relative humidity. This combination of temperature and humidity is associated with air having a water vapour density (WVD) of 6.8 grams of water per cubic metre of moist air. The blue curve in the chart below shows how efficiency of water collection varies depending on the water vapour density of the ambient (entering) air.

Let's use an example to explain how the curve was constructed. Ambient air at standard testing conditions of 26.7 °C, 60% relative humidity enters the dehumidifier (atmospheric water generator). At standard atmospheric pressure of one atmosphere (1.013 bar), psychrometric calculations show the ambient air is capable of holding 15.3 grams of water vapour in a moist air volume of one cubic metre. As this unit volume of air flows across the 5 °C chilled surfaces of the coils the mass of condensate collected =  (15.3 g per cubic metre - 6.8 g per cubic metre) x 1 cubic metre = 8.5 g. The table below shows a series of similar calculations encompassing the natural range of water vapour densities in the atmosphere at the Earth's surface (about 4 to 22 g per cubic metre).

Ambient temperature together with the refrigeration capacity of the dehumidifier will determine whether or not a chilled surface temperature of 5 °C can be achieved. Therefore, the chart also has efficiency curves for leaving air at 10 °C and 15 °C. In Belize City air temperature was about 32 °C and the 40 Ton refrigerant capacity machine I was testing for my client had a leaving air temperature (similar to coil temperature) of 16 °C. Efficiency of water production was about 45%, near the limit of what could be expected given the weather conditions and equipment capacity. The atmospheric water generator (about the size of a 20-foot shipping container) was producing drinking water at the rate of about 2500 L/day—its designed capacity.

Note: You may click on the chart and table to enlarge them.