Book - School of Science and Technology

228 Pumps and other auxiliary equipmentreading taken, and factors representing the heat capacity of the individual units powered,are totalled as for other apportioning meters and used to determine the overall runningcost. Allowance has to be made of course for any useful heat output from the meteredcomponents, by radiation or natural convection, which may take place when the fan orpump is not running.Air venting, etc.Mention has been made in Chapter 6 (pp. 161±2) of the adverse effect of a stagnant airfilm upon steam-side heat transfer. As far as water systems are concerned, there will be aninitial presence of both dissolved and free air and, since the solubility of air in water fallswith increasing temperature, the free component will increase rather than decrease as thesystem is brought into service. The water content of an open system, or of one pressurisedby equipment incorporating an open spill cistern, will be subjected to a process ofcontinual re-aeration as a result of expansion and contraction. Furthermore, smallleakages at pump and valve glands etc. and evaporation from almost any type of cisternwill require that raw water be admitted regularly to make up the deficit.If the presence of air is not to impede pipe circulation; reduce heat output fromemitters; lead to cavitation difficulties in low pressure areas such as pump suction connectionsand, primarily in domestic systems, produce `kettling' noises in boilers; thenactive measures must be taken towards its elimination. The matter of corrosion arisingfrom air in the water content will be considered separately.Air ventingThe traditional approach to the disposal of air from low temperature hot water systemswas to route a full-bore flow pipe immediately from the boiler plant to the highest levelin the building and, at that point, to provide an open air vent to above the feed andexpansion cistern. This approach, although a counsel of perfection in some respects, israrely practicable in modern buildings and in any event would be a considerable impedimentto design development.For low temperature hot water systems, it is current practice to take advantage of anysensible opportunity to dispose of air without using components which require maintenance.A simple open vent taken straight from the boiler plant, as shown in Figure 10.14(p. 277), although provided for a different purpose, is favourably placed to carry away airAir outletat topAir outletat sideWater connectionWater connectionFigure 8.27 Simple automatic air valves (air eliminators)