Condensation & Dehumidification Control

I. Controlling Condensation

As air temperature increases, it can hold more moisture in the form of invisible water vapour. Pool hall temperatures can hold more moisture than air at ordinary room temperatures, causing condensation to occur as the warmer, moist air moves over cooler surfaces.

Condensation is rarely an issue in most buildings and even then is predominantly a cosmetic problem which can be caught before it does too much damage.

Swimming pools however have additional complications. For instance, the condensate may contain aggressive chemicals from the water treatment process or there may be a suspended ceiling concealing the structure, so that damage may occur unnoticed.

There are two ways to avoid this happening:

• Insulate the cold surface on which condensation might occur so that the surface temperature is always above the dew point. The ‘dew point’ refers to the temperature at which the air is saturated with water vapour, corresponding to 100% relative humidity – as the air temperature falls towards dew point this is when condensation occurs. The insulation should preferably be located outside the structure so that the whole of it remains warm.
• Reduce the moisture in the air so that the relative humidity is less than 100% when it contacts the cold surface.

Condensation-related damage can occur out of sight and pool roofs have been known to collapse due to undetected corrosion. Invariably the cause is "Stress Corrosion Cracking" of stainless steel. It is important to carry out regular preventive checks and always consult a qualified specialist where necessary.

II. Controlling Humidity

The control of Humidity in any modern pool hall is carried out by an Air Handling Unit that also controls the Air and Pool Water temperatures and manages the Dehumidification process desribed below.

Regardless of whether air flow patterns have been arranged to reduce evaporation, it is important to control moisture condensing on cold surfaces by limiting humidity.

The alternative methods of dehumidification are:

• To dilute the air close to surfaces which are prone to condensation with dry air from outside the building
• To directly dehumidify the air

Dilution:

Outside air normally has much less moisture in it than pool hall air as it has a lower temperature. Mixing outside air with pool hall air can reduce overall humidity. The outside air has to be heated up to the pool hall temperature to make it comfortable for bathers, which uses energy. To make room for this incoming air, the air already in the pool hall must be extracted.

Pool hall air has a high temperature and also contains the latent heat of the moisture evaporated from the pool so it can be wasteful. However, some of this energy can be captured and returned to the pool hall, thus achieving savings. The volume of incoming air used to dilute the pool hall air can be reduced to the minimum consistent with avoiding condensation by using variable speed fans.

Direct Dehumidification:

This approach strips moisture out of the air within the pool hall system, rather than simply diluting it with outside air. In this way, the heat required to maintain conditions in the pool hall is reduced to what is needed to replace direct losses through the building structure and the heat needed to warm fresh air needed for respiration.

The two main ways to achieve this are through the use of a dehumidifying heat pump or a Cross Flow Heat Recuperator (the incoming air is fed past the outgoing air in separate but adjacent ducts so that heat transfer takes place).Modern Air Handling Units can incorporate either of these.

The cost effectiveness of both options needs to be evaluated carefully in life cycle terms. In either case they need to be able to cope with the corrosive air they have to handle, which can considerably reduce plant lifespan if not addressed. In addition, both options need to be controlled to achieve the correct function, and the control regime must be understood by plant operators as direct dehumidification is a radically different approach from that traditionally employed.

III. Dehumidifying Heat Pumps

A dehumidifying heat pump consists of an electrically driven heat pump where air from the pool hall passes over the evaporator or cold coil. Moisture condenses out of the air at the same time as it is cooled and the water is run to drains (or can be stored for cleaning or toilet flushing). The air is then reheated by passing it over the condenser or hot coil of the heat pump. Thus the air is dehumidified and returned to the pool hall at the right comfort temperature.

The considerable latent heat in the air being dehumidified produces a surplus that can be used to heat the pool. This can make up for the cooling effect caused by surface water evaporation from the pool in ordinary use. The recycling of otherwise wasted energy from the latent heat of the air used to heat the pool water makes this system highly energy efficient.