Air Cooling. Washing, Humidifying, Evaporative Cooling

Used principally for industry when constant temperature and humidity must be maintained over the whole year, also for commercial buildings and office blocks, theatres and cinemas in summer.

(a) Cooling of the air with mains water or spring water. At a temperature of 13°C, spring water should be allowed to drain back again as much as possible on account of the ground water table level. In most towns, the use of mains water for cooling is not permitted and is uneconomical anyway, due to the high price of water. Spring water systems require the approval of the water authorities.

(b) Compression cooling systems for room air conditioning must accord with strict regulations and must use nonpoisonous refrigerants such as Freon 12 or Freon 22 (F12, F22), etc. If the cooling plant is in the direct vicinity of the central air conditioning area, direct evaporation of the refrigerant should take place in the cooling radiators of the air conditioning plant. Since 1995, substances containing CFCs are prohibited.

(c) In large installations, cooling of the water takes place within a closed circuit, with distribution by pumps. Advantages: the central cooling plant can be in an area where noise and vibration are not troublesome; very safe in operation. Today, compact cold water systems and prefabricated air conditioning/cooling units are available.

For large cooling installations (d) Compression of the refrigerant in a sealed unit turbo compressor (complete machine installation with compressor, water-cooler and condenser), low vibration and very low noise levels.
(e) Absorption cooling facility with lithium bromide and water. Due to the vaporisation of the water, heat is extracted from the water to be cooled; water vapour is absorbed by the lithium bromide and continuously evaporated in the cyclic process, then condensed again and passed to the first vaporisation process. Very low noise levels; vibration-free system requiring little space.

(f) Steam jet cooling: A high velocity steam jet induces a negative pressure in a vessel. Circulating cooling water becomes atomised and vaporised, with simultaneous cooling. The cold water is transferred to the air coolers of the air conditioning plant. This method of cooling is employed in industrial applications.

The condenser heat must be disposed of in all mechanical cooling systems. Various means are employed for this purpose, e.g. water cooled condensers, which are cooled by spring water or circulating water, and air cooled condensers. On water- cooled condensers, the spring water installation requires approval by the local water authorities. Also, careful checks should be made as to whether the spring water contains any aggressive substances which would damage the condensers in the cooling installation. If appropriate, sea water resistant condensers must be used (cost factors).

A return cooling system is necessary on circulating water installations (cooling tower). In the cooling tower, circulating water is sprayed by jets. The water then flows over layers of granular material and is blown through with air (evaporative cooling). The cooling towers should be sited away from buildings or, better still, be sited on the roofs of buildings, due to the level of noise generated. The same applies to air cooled condensers.

Washing, humidifying, evaporative cooling. Air washers provide humidification for dry air (when correctly set) and, to a certain degree, they can also provide air cleaning. By means of saturation, i.e. increasing the absolute water content of the air in the washer, 'evaporative cooling' can take place at the same time; this provides the possibility of cheap cooling for industrial air conditioning facilities in areas where the outside air is of low humidity. The water is very finely atomised in the air washer, through the use of pumps and jet sprays. The sprays are housed in galvanised steel sheeting or watertight masonry or concrete. An air rectifier or water-control sheeting prevents the escape of water into the conditioning chamber.

Other humidifying devices
(a) Evaporation vessels on heating elements or atomisers.
(b) Centralised device with steam or electrically heated evaporation vessels (disadvantage is scaling).
(c) Rotating atomisers (aerosol apparatus) - only usable where low volumes of air are involved

The efficiency of a good ventilation design can be 80-90%, depending on the application. Both radial and axial fans produce the same noise levels up to a total delivery pressure of approx. 40mm head of water. Above this level, axial fans are louder and they are used particularly in industrial construction. Special foundations are provided with damping elements to isolate vibration levels.