Book - School of Science and Technology

94 The building in summer(a)(b)Figure 3.16 Dry bulb (a) and wet bulb (b) isothermsIsotherms (contours of equal temperature) showing those dry bulb and wet bulbtemperatures which are exceeded for only 1% of the summer months in the British Islesare shown in Figure 3.16. These isotherms relate to sea level and the temperatures shownshould be reduced by 0.6 C for each 100 m of height above that base. Other similarfigures have been produced for temperatures which are exceeded for greater proportionsof those months and, for instance, if a 2 1 2% level were to be chosen, then temperatureswould be about 1 K lower in each case.For a more accurate assessment of the most appropriate summer design conditionsGuide Section A6 provides data to enable the designer to plot the percentage frequenciesof combinations of hourly dry bulb and wet bulb temperatures on a psychrometric chart.This enables the frequency with which the specific enthalpy exceeds given values to bedetermined, from which summer design conditions may be established.Intermittent operationIn Chapter 2, when considering intermittent heating, the matter of excess plant andsystem capacity over and above that calculated for steady state heat losses in order toprovide for morning preheating, was discussed in some detail. In the case of plantprovided for cooling a building, the situation is not the same since the peak load (forthe whole building) will normally occur somewhat later in the day, part way through theoccupancy period after a steady rise in heat gain and requirements for cooling. Inconsequence, it is unlikely in most instances that any useful purpose would be servedwere the capacity of the plant to be increased above that required to meet the selecteddesign condition.It is nevertheless possible to go some way towards overcoming a deficit in plant size andto reduce running cost during warm daytime periods, by setting out deliberately to cool abuilding overnight when unoccupied. This may be achieved to some extent by circulating