Book - School of Science and Technology

Capacity of central stores 127In conclusion, it is worth re-emphasis that since the operational characteristics of floorwarming are relatively critical, it is necessary to pursue the full routine of design calculationsalthough these are somewhat tedious, particularly for multi-storey buildings. Thenotes here, therefore, are no more than a summary of the necessary steps.Capacity of central storesThe principal differences between the capacity required of a room store and that of acentral store are that it may be necessary in the latter case to take account of distributionlosses: this penalty may be offset in some instances, however, by allowing for any diversityin the primary demand for output, room by room. A further penalty, of more significancein most cases, is that static losses by both night and day may be truly lost if the store issited outside the spaces to be heated. It is of course not possible to generalise as to thesenegative or positive adjustments: they are functions of the application rather than thestore.Warm air unitsThe required capacity for such units is determined in the same way as that for storage fanheaters except in circumstances where a ventilation requirement exists and is met byadapting a unit to introduce an outside air supply. In those circumstances, the designheat requirement would be increased during the hours of occupation and the unit outputrating would have to be adjusted to suit. Some units have elements rated in excess of theoutput required to charge the store: the additional capacity may then be used to providepreheat to the spaces served, still using the low-cost supply.Dry core boilersThe particular characteristics of stores of this type, in conjunction with those of theconnected indirect system, lead to a departure from the practice previously noted in thatthe capacity of the store is normally related to a charging period of about 5 1 ¤ 2 hours. Thisperiod is determined not by the time when a reduced-price supply is available, which ispresumed to remain at 7 hours, but in consequence of the capability of the unit to producea direct output to the connected system. This output is used during the last 1 1 ¤ 2 hours ofthe low-cost supply period to provide preheating to the space served without drawingupon the capacity of the store.Since the 24 hour output of the connected indirect system is dissociated from thethermal cycle of the store, it may be assessed quite independently as a function of timesof building use, etc. Thus, the required rating (R s ) is determined by:R s ˆ (YHQ ‡ L)/nwhereH ˆ period of use or occupancy (hours)Y ˆ factor from Table 5:3 taken as 0:75 for this applicationL ˆ 24 hour static heat loss from store when not useful emission (kWh)