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
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.
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-
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 0.5 rpm) 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).
Desiccant Drying with Pre-Cooling
As refrigerant and chilled water cooling coils work well down to 10°Cdp, it makes
sense to use one of these methods whenever conditions allow.
It is generally accepted that these methods are not suitable to produce conditions
below 8°Cdp, as there is high risk of the coils freezing.
On fresh air systems that require drying below 8 to 10°Cdp, it is now standard
practice to use either refrigerant (dx) or chilled water (cw) coils to extract moisture
only to that level (8 -10°C).
Cool saturated air leaving the cooling coil is then dried further by passing it through
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
Unwanted heat gains on the warm dry air supply can be removed by
installation of another cooling coil, which serves to control
temperature only (no latent duty).