Refrigeration, Cooling or Desiccant?

Desiccant Dehumidifier

A desiccant dehumidifier operates on a totally different principle to a refrigerant type. The main benefit of a desiccant dehumidifier is that it performs exceptionally well when used in cooler climates, or when a low dewpoint is required.

As there is no actual water produced during the process, these units can work effectively at sub-zero temperatures. The rotor (the heart of the dehumidifier), is manufactured from alternate layers of flat and corrugated sheets, impregnated with an active component (desiccant). It is made to form a vast number of axial air channels running parallel through the structure.

As air passes through these channels, moisture is transferred between the air and the desiccant in its vapour form. The rotor is generally divided into two air zones which are separated by seals, the process sector (typically 75%), and the reactivation sector (typically 25%). The rotor is rotated slowly (approximately 8 to 12 rph) using a small geared drive motor.

System air is pulled through the larger process sector where moisture is adsorbed from the air by the rotor material. The air leaves the dehumidifier warm and dry. Most of the heat gain is due to energy exchange during adsorption.

A smaller heated air stream (usually fresh air) is pulled through the reactivation sector. This hot air is used to drive out moisture adsorbed into the rotor from the process air. The reactivation air leaves the rotor warm and wet. This air is normally exhausted to outside.

As these two opposing airflows pass through the rotor simultaneously, a continuous and automatic dehumidification process is achieved. Drying capacity is normally controlled by regulation of the reactivation heater power.

System efficiency can be further improved by inclusion of a heat recovery or purge sector. This third sector preheats the incoming reactivation air by recovering heat from the rotor material prior to passing across the reactivation heater. Less energy is required for heating the reactivation air and the dry air produced is both drier and cooler than that achieved in traditional desiccant systems (see Recusorb).

Refrigerant Dehumidifier

The most common dehumidifier used is the refrigerant type. It is cheap to produce, easy to operate and effective in most domestic and commercial applications.

The primary benefit of a refrigerant dehumidifier is that it performs exceptionally well when used in warm, humid conditions.

One of the most important things to understand about a refrigerant dehumidifier is that the performance is directly related to the difference between the air dry bulb temperature and the air dewpoint temperature.

A refrigerant dehumidifier is essentially a re-arranged portable air conditioning unit, where the air is first passed across the evaporator coil to cool it, then across the condenser coil to heat it. The air becomes both warmer and drier.

The cold evaporator coil reduces the air temperature to a point where condensation occurs. Condensation formed on the cold coil then drips into a collection tray. Water is either drained away or collected in a container in the base of the unit. 

As the surrounding air becomes drier the dewpoint is lowered, so the temperature necessary to create condensation on the cold evaporator coil, also becomes lower. Even with an evaporator coil temperature of 0°C, it is unlikely that the air will be reduced much below 10°C dewpoint (it is worth noting that air at 20°C, 50% RH already has a dewpoint below 10°C).

If the dewpoint of the air is already low, the coil temperature necessary to create condensation can easily be sub-zero. At this point, the operating efficiency of a refrigerant dehumidifier is greatly reduced. The evaporator begins to freeze as airborne water vapour makes contact with the cold tube surface. Defrosting of the coil is then necessary. Defrosting is normally achieved by allowing hot refrigerant gas from the compressor directly into the evaporator coil. During the defrost cycle, the dehumidifier process ceases to remove water from the air.

Desiccant Drying with Pre-Cooling

As with refrigerant cooling, chilled water cooling coils can also be used to remove moisture efficiently down to 10°Cdp. Whenever the inlet condition permits, it makes sense to use cooling coils in addition to the desiccant unit.

It is generally accepted that cooling methods can be used safely to create a dew point down to 10°C. With high quality latent coils, slightly lower dew points could be achieved. This is often unnecessary and risks condensate freezing on the coil surface. Ice creates a thermal barrier that reduces heat transfer efficiency. Eventually the coil will freeze up and impede or totally choke the airflow.

On fresh air systems that require drying below 10°Cdp, it is now standard practice to use chilled water (cw) coils to extract moisture only to that level. Cool, saturated air from the cooling coil can be dried further by passing it directly into a desiccant dehumidifier.

By utilising both cooling and desiccant technologies, very dry air can be produced (as low as minus 60°Cdp).

This system works well in summer when the incoming air is warm and humid.

During the cooler winter months the pre-cooling system is disabled and dehumidification is achieved using the desiccant dehumidifier only.

As moisture is removed, energy is released in the form of heat into the air. The warm dry air can be cooled by installation of another cooling coil, which serves to control temperature only (no latent duty).