Equipment for drying homes, offices and buildings

There are several different methods of drying buildings and it follows that there are different types of equipment that can be used.

This section is not intended to provide a comprehensive guide to all available types of equipment but to highlight two methods - dehumidification and convection drying. Other methods include heat transfer, heat exchangers, vacuum drying and 'open and closed' systems.

Types of dehumidifier

There are essentially two ways of accomplishing dehumidification of atmospheric air:

• By chilling air to below its dewpoint causing moisture to condense on cool surfaces
• By passing air over substances that have an affinity for moisture. These substances are called desiccants and are capable of extracting moisture directly from the atmosphere.

Refrigerant dehumidifiers

Using refrigeration to remove moisture from the atmosphere is a common method for dehumidifying air. Air, at its initial temperature and moisture content, is chilled by refrigeration when it has been cooled sufficiently to bring its condition to saturation. Further cooling causes moisture to condense. In a typical refrigeration unit this condensation occurs directly onto a finned cooling tube, and is collected and drained away.

In its final dehumidified condition the air is considerably cooler and contains less moisture but it's still nearly saturated with a relative humidity of close to 100%. This air is then reheated as it passes through the dehumidifier which, in turn, reduces the relative humidity.

Drying by using mechanical refrigeration can be quite efficient depending on the prevailing temperature and relative humidity (RH). The actual cooling effect or energy extracted can be several times greater than the energy input required to operate the system; this ratio is called coefficient of performance.

The best operating range for a refrigeration dehumidifier is 15 to 28 °C and 60 to 98% RH.

Desiccant dehumidifiers

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A desiccant is a substance characteristically having a high affinity for absorbing water without changing its chemical composition. So high, in fact, that it can draw moisture from the surrounding air.

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The desiccant dehumidifier uses a drying wheel that's impregnated with an adsorbent substance such as silica gel. The wheel, which has a honeycomb structure, is sectioned off into two zones.

• A working zone in which the air that requires drying is drawn through the honey comb structure and adsorbs the moisture within the air stream
• A second zone, which is usually called the reactivation zone, has preheated air drawn through it in the opposite direction. The moisture held by the desiccant is then absorbed by the air and driven off as a warm wet vapor.

The wheel rotates within the unit at 8 to 10 revolutions per hour, so as the wheel moves from the reactivation zone it's and warm and dry and ready to accept more moisture.

The optimum operating range of the desiccant dehumidifier is 0 to 25 °C and 40 to 90% RH.

Convection drying

As an alternative to dehumidifiers, convection drying uses three factors of drying - air movement, raised air temperature and very low moisture content, all combined with continual air changes.

More technically, convection drying is where the wet (high specific humidity) air within the flood affected area is replaced with drier air (lower specific humidity). This can be accomplished simply by heating incoming air; this reduces its relative humidity and results in a more hygroscopic (or drier) air which is able then to carry away the moisture generated from evaporation. The key to successful convection drying is to match the incoming dry air water capacity with the evaporated moisture caused by heating, and to remove it quickly before it condenses on surfaces or is adsorbed by hygroscopic materials.

Manufacturers and users of this method claim some key benefits over dehumidifiers, but it's beyond the scope of this document to comment on these, which will in any event be understood by a technically competent person.