Evaporative cooling

Evaporative cooling exploits the principle that molecules of a substance in a vapour state contain much more energy than the same molecules in a liquid state. The amount of heat needed to change a substance like water into a vapour is its latent heat of evaporation. The heat is removed from the liquid and transferred to the vapour, causing cooling of surfaces in the process. This is the cooling system employed by nature in humans to lower the body temperature through perspiration.1

Direct evaporative cooling is achieved when incoming air is blown directly across a wetted medium or through a water spray. In this instance evaporative cooling provides 'sensible' cooling, i.e. experienced by the senses, whilst increasing the latent heat content of the air. The process is called 'adiabatic cooling' where the sensible heat removed from the air equals the latent heat absorbed by the water evaporated as the heat of vaporization.

This method of cooling can be created by intelligent landscaping, as when incoming air first passes over the surface of an external mass of water. An example where this technique is to be seen is at the University of Nottingham. In its Jubilee Campus the prevailing winds cross an artificial lake before being directed into the building by an inclined plane of glazing.2

Indirect evaporative cooling occurs when exhaust air is cooled using evaporative techniques and then used to cool the incoming air by being passed through a heat exchanger.

The Malta Stock Exchange offers an interesting variation on the theme of evaporative cooling. The interest lies in the fact that it is an existing chapel converted to this new function. Being a historic building, its walls have high thermal mass. The architect chosen was Brian Ford of WSP Environmental and formerly of Short Ford and Partners, who designed the celebrated Queen's Engineering Building at Leicester de Montfort University. He opted for a passive downdraught evaporative cooling (PDEC) solution. The major advantage of PDEC is that it is driven by buoyancy dynamics alone, removing the need for mechanical assistance.

A raised roof ridge accommodates the misting jets and cooling pipes. Fresh air is drawn through centre pivot vents in the side of the ridge which is then cooled by evaporation. This sets up a downdraught, cooling the atrium below. In summer, automatic high-level vents allow the night-time air entering at low level to pre-cool the building.

In the event that internal relative humidity level is above 65% the evaporation nozzles are automatically switched off and chilled water is directed to the cooling pipes. This causes cool air to descend into the atrium whilst the warm air rises up the sides of the atrium to be cooled and then descend. Energy has to be used to chill the water but there are no fans which typically use 30% of the energy of an air conditioning system. There is also no ductwork or air handling units.

Thermal modelling predictions indicated that electricity savings of 50% would be realized against a conventional air conditioning system (see Fig. 3.3).

Air handling systems which involve the warming of water carry a theoretical risk of legionella. Evaporative cooling can create pre-cooling of ambient air for dry air coolers. This lowers the temperature of the water in the system which reduces the risk of this disease compared to a wet cooling tower.

Advantages of evaporative cooling include:

• the fact that it can be combined with conventional systems

• the heat exchanger in indirect systems can be used for heat recovery in winter

• if the exchanger is located within the exhaust air pathway.

Figure 3.3 Sections and ventilation paths, Malta Stock Exchange. (a) summer cooling to the atrium is provided by a combination of passive evaporation, cooling coils and night-time cooling. When outside temperature is high and relative humidity below 60%, misting nozzles operate and cooling air is drawn through side vents in the ridge. (b) above relative humidity of 65%, nozzles switch off and chilled water in the cooling pipes takes over the cooling function. (c) at night, ridge vents opened and cool air is drawn from low level, rising by stack effect to exit at high level, pre-cooling the structure in the process (courtesy of WSP Environmental)

Figure 3.3 Sections and ventilation paths, Malta Stock Exchange. (a) summer cooling to the atrium is provided by a combination of passive evaporation, cooling coils and night-time cooling. When outside temperature is high and relative humidity below 60%, misting nozzles operate and cooling air is drawn through side vents in the ridge. (b) above relative humidity of 65%, nozzles switch off and chilled water in the cooling pipes takes over the cooling function. (c) at night, ridge vents opened and cool air is drawn from low level, rising by stack effect to exit at high level, pre-cooling the structure in the process (courtesy of WSP Environmental)

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Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

How would you like to save a ton of money and increase the value of your home by as much as thirty percent! If your homes landscape is designed properly it will be a source of enjoyment for your entire family, it will enhance your community and add to the resale value of your property. Landscape design involves much more than placing trees, shrubs and other plants on the property. It is an art which deals with conscious arrangement or organization of outdoor space for human satisfaction and enjoyment.

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