Book - School of Science and Technology

348 Ventilationsimilar plant. This will have been designed to balance the heat loss through the buildingfabric and to maintain an internal temperature of, say, 20 C during winter weather. Inthese circumstances, the ventilation air supply should be delivered to the room at about17 C, or 3 K lower than the desired temperature, so that it has the potential to reach 20 Cas it absorbs unwanted heat prior to leaving the room.Under summer conditions, without provision for mechanical cooling, no control can beexercised over the temperature of the air delivered to the room other than by the smallreduction made possible by a process of evaporation, referred to later.HumidityVentilation air admitted from outside will have the same moisture content as thatprevailing at the source. In cold weather, as was mentioned in an earlier chapter, thismoisture content may be lowand although the quantity will not alter in the absolute sensewhen the air is heated to say 17 C, the relative humidity will have fallen. This situationmay be corrected, where mechanical inlet plant is used, by incorporating humidificationequipment in the form of water sprays or some other device for producing water vapour.By such means, the amount of water added may be controlled to give a desirable relativehumidity of 40±60%, at room temperature.In the normal mid-season mild weather of spring and autumn in the British Isles, thehumidity prevailing outside will generally be at a level which will permit some temperatureadjustment without any necessity to take corrective action. The human body, as wasexplained earlier, is not too critical of humidity variations at the temperature levels thenprevailing. There is, in addition, a reservoir effect in most buildings as a result of thehygroscopic retention of fabrics, timber, paper, etc., which serves to steady changes inhumidity taken over a long period.In summer, when the humidity outside is high, there can be no complete control ofinternal humidity without full air-conditioning which, by definition, includes means fordehumidification. Humidifiers, of whatever type, can only add moisture but, in doing so,some may cool the air by evaporation to a temperature approaching that of the wet bulb.Thus, with outside air at, say, 24 C dry bulb and 18 C wet bulb, (relative humidity ˆ52%), a humidifier may be able to reduce the temperature to 19 C, but at the same timeincreasing the relative humidity to 90%. This second condition may well be moreoppressive than the first and it is often found that humidifiers, where provided, are shutoff in hot weather for that reason. Reference to Figure 1.2 (p. 13) will show that the firstcondition noted above is just inside the summer comfort zone, but that the secondcondition is well outside.It is necessary to add a note of caution here since, as a result of the investigations intothe incidence of Legionnaires' disease which are discussed more fully in both Chapter 14and Chapter 20, the use of water spray types of humidifier in air streams is now lookedupon with suspicion. For the use here mentioned, it is probable that the critical temperatureband 25±45 C will be avoided but the margin is too narrow to be ignored.Air purityMost buildings requiring some form of ventilating system are in the centre of towns orcities where atmospheric pollution is at the highest level normally encountered. As a resultof the action which followed the implementation of the 1968 Clean Air Act, the soot, tar,ash and sulphur dioxide which had previously been a major problem as a result of