Vapour pressure of air / Vapour pressure of saturated (air at the same temperature) x 100.

A more convenient definition, although technically incorrect is : -

Moisture content of air / moisture content of saturated air (at the same temperature) x 100

This definition although more convenient is actually the percentage saturation. For the majority of conditions the relative humidity and the percentage saturation are virtually identical and rarely differ by more than 1%

Saturated air is air which currently holds the maximum quantity of water possible (for a given temperature and pressure). To attempt to evaporate more water into saturated air simply means that some water will "drop out" .

To demonstrate the above, and hopefully clarify the definitions, the following figures should be considered.

Air at 30 degrees C (a common pool room temperature) when fully saturated, will hold a maximum of 27.309 grammes of water in each kg of dry air. (at these conditions 1kg of air occupies 0.896 cubic metres)

Air at 30 degrees C and 50% saturation holds 13.655 grammes of water in each kg of dry air. ie exactly half as much .

(As a point of interest, the relative humidity at the above condition is 51%, and this minor deviation would probably be considered small enough to consider the definitions of relative humidity and percentage saturation as "near enough" for most calculations)

The study of the properties of moist air is known as Psychrometry. The Psychrometric chart is an important tool for the air conditioning engineer and the dehumidification system designer. Using this chart the engineer can calculate any property of moist air and can plot on this chart any process such as heating, cooling, addition of moisture (humidification) and removal of moisture (dehumidification). Most importantly plotting these processes in this way will allow the engineer to calculate the amount of power required and the air flow and equipment size required to perform the job!

The Psychrometric Chart

The most important elements of the above for the pool engineer are the following.

Right hand axis : Moisture content in kg / kg dry air varying from 0.00kg/kg to 0.030 kg/kg ( 30 grammes / kg)

Bottom axis : Dry bulb temperature in degrees C (this is the conventional temperature as measured by a dry mercury in glass thermometer)

The 10 bold curved lines sweeping up from left to right are lines of constant percentage saturation. The last one, which bounds the graph to the left is the 100% relative humidity line, also known as the saturation line. The other 9, predicatbly enough, are the 10%, 20% etc etc.

Dehumidification systems

There are a number of popular methods to control the humidity in an indoor pool.

Full fresh air systems

The traditional means has been to take external (called ambient) air and drive this into the pool room. For this to be worthwhile the ambient air needs to be drier than the pool room air. With pool room conditions of 30 degrees C and 60% relative humidity the room air will contain around 16 grammes for each kg of dry air. Ambient air rarely contains more than 11 grammes per kg so it can be seen that each kg of air introduced into the pool room 5 grammes of moisture can be removed.

For a pool evaporating 5 litres per hour, equal to 5000 grammes per hour, then 1000 kg of fresh air would be required every hour. At typical conditions this would equate to around 1100 - 1200 cubic metres per hour.

The disadvantage of such a simple scheme, and the reason that it is no longer popular, is that the incoming fresh air, whilst usefully dry, is also generally cold. To bring this air up to a usable temperature large quantities of heat have to be added. As energy costs rise this method has fallen from favour for alternative less energy wasteful systems.

Recuperator based systems

The recuperator based system is a fresh air scheme, with a difference. The recuperator includes a heat exchanger that is used to reduce the amount of heat required to heat up the fresh air. The heat exchanger uses the warm pool room air to heat up the incoming fresh air. The heat exchanger would be located on the exhaust outlet and also the fresh air intake and under preferential conditions heat transfer efficiencies as high as 65% can be achieved.

Refrigeration based "heat pump" systems

Refrigeration based systems are the only schemes that do not require any form of fresh air input. Refrigeration systems work by drawing the humid pool air over a refrigerated coil. This chills the air and in so doing the air drops below its "dew point". When this occurs water condenses on the coil and hence the moisture content of the air is reduced. The chilled air, with a reduced moisture content, is then heated and returned to the pool room. Where these units are able to offer great thermal efficiency is that the energy used to heat the air, is the same energy that was used to chill the air in the first stage. Thus with this type of system the room humidity can be reduced by simply condensing water vapour from the room and using the extracted heat from the water vapour (known as latent energy) to provide heating for the room air or the pool water

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