Book - School of Science and Technology

672 Running costsRunning hours for the auxiliaries will be greater than those of boiler output but even ifthe total were to be half as much again, the consumption noted above would representonly about 1% of the overall energy input. In the case of solid fuel firing, the same orderof power consumption for auxiliaries would apply but for gas firing, using a packagedburner with an associated pressure booster, the power consumed would be only about athird of that amount.Similarly, the current used for driving circulating pumps, etc., is of relatively smallmagnitude and indeed most of the energy paid for emerges as heat somewhere in thesystem and so is not altogether lost. A boiler plant of 1 MW capacity would probably beassociated with a heating system requiring pumping power of the order of 2 or 3 kW,running continuously throughout the season and thus perhaps equivalent to another 1%of the overall energy input.As a very broad approximation, pending a proper calculation from manufacturers'ratings of the various drives, etc., an allowance for a power consumption equivalent to5% of the overall energy input as fuel should cover all auxiliary equipment.Direct heating systemsFor direct systems of convective type, methods similar to those used for calculation ofenergy consumption in central plants may be used, using appropriate values from Table23.9, provided that equivalent arrangements for thermostatic and time switch control areprovided. If the system were to be of radiant type, a true comparison must be based uponequation of demand to maintenance of dry resultant temperature. An estimate may thenbe made of running cost taking the time when the heaters are likely to be in use andmultiplying this by their rated capacity. There is often no true temperature control, in theaccepted sense, provided for such systems, and data from comparable installations areprobably the best guide.Mechanically ventilated buildingsEssentially, the thermal energy requirements for mechanically ventilated buildings may betaken to be the same as for the equivalent heated and naturally ventilated spaces, withlimited correction and with some additions. It is assumed that no cooling or humidificationis to be provided.Heat requirementsThe following items should be considered in turn:. Reduction in air infiltration rate, possibly due to sealed windows.. Additional load due to ventilation air, i.e. the quantity of outside air introduced by theventilation system.. Reduction in load due to incidental gain to the air from the fan (fan gain).. Allowance for the benefit arising from the use of any heat recovery device.. Correction to the effective length of the heating season.The first two items above will be self-explanatory.