Book - School of Science and Technology

166 Indirect heating systemsTable 6.9 Approximate properties of dry air (at atmospheric pressure)Re massRe volumeTemperature( C)Specificmass(kg/m 3 )Specificheat capacity(kJ/kg K)Specificvolume(m 3 /kg)Specificheat capacity(kJ/m 3 K)10 1.247 1.011 0.802 1.25720 1.205 1.012 0.830 1.21930 1.165 1.013 0.858 1.18040 1.128 1.013 0.887 1.14350 1.093 1.014 0.915 1.10860 1.060 1.015 0.943 1.07670 1.029 1.015 0.972 1.04580 1.0 1.016 1.0 1.01690 0.973 1.016 1.028 0.988100 0.946 1.017 1.057 0.963110 0.922 1.018 1.085 0.938120 0.898 1.018 1.113 0.915130 0.876 1.019 1.142 0.893140 0.855 1.020 1.170 0.838150 0.835 1.020 1.198 0.851either at or just below that required in the space:heating is provided by some quiteseparate system.The plenum system, conversely, introduces a supply of air at a temperature well inexcess of that required in the space:ventilation, in effect, is a by product. In cooling to thespace temperature, the air provided surrenders the heat it carried to windows, walls, floorand roof, etc., in just the same way that heated air convected from a hot water `radiator'cools in circulation within a room. A further difference is that in the case of the plenumsystem, either all or a considerable proportion of the air provided is taken from outsidewith intent to produce a positive pressure differential, inside to outside, and thus reduceinward leakage through cracks around windows, doors etc.The traditional plenum systemThis is best discussed using, as a simple illustration, the building shown in Figure 2.3 (p.28). As may be seen from Table 2.13, the component of the calculated heat loss associatedwith the building fabric is all that is required, in the case of a fully convective system, todetermine the value of the two necessary temperature ratios. Thus:From earlier calculations (pp. 40 and 56):(AU/A) ˆ 240/419 ˆ 0:57 W/m 2 Kand by reference to Table 2.13:thusF 1 ˆ 0:97 and F 2 ˆ 1:10Q u ˆ 0:97 240 22 ˆ 5122 WQ v ˆ 1:10 2091 0:33 22 ˆ 16 699 W