Book - School of Science and Technology

Heat meters 225H.P. isolating valveStrainerL.P. isolating valveReducing valveRelaypipeRelief valveBy passFigure 8.24 Steam pressure reducing `set'. a fixed charge plus a unit rate based upon metering. a unit rate based wholly upon metering.The first two of the alternatives offer the advantage that they may be assessed beforethe event and thus included in any periodic exercise of forward planning. In an era ofrising fuel and labour costs, however, they must, in equity, be subjected to a periodicreview and may be thought not to reflect any individual efforts made towards conservationof energy. In consequence, the situation overall must be considered in relation to thepracticability of using some form of meter to determine heat consumption.Metering of steam systemsIn a context which is wholly industrial and where steam is the medium for heat distribution,the boiler plant will most probably be equipped with an accurate inferential typesteam flow meter. Although it would be possible to use similar equipment at the remotepoints of use, the application of a mechanical meter of some sort to condensate flowwould be a more economic solution. Rotary piston, rotary vane and rotary helix patternsare available and have an accuracy of about plus or minus 2%. The rotary vane type hasgenerous clearances and is thus particularly suitable for use with condensate.Heat meters for hot water systemsComplex hot water meters have been produced but few have performed up to expectationsfor any length of time. An acceptable level of accuracy in the integration of the twovariables, flow quantity and temperature difference, has been the problem requiring aneconomic solution. Three quite different methods have been adopted by various manufacturersas follows:Mechanical. Where the drive between components of a conventional water meter isadjusted mechanically by temperature sensitive elements.Electrical. Where both flow rate and temperature difference are measured and integratedby electronics.Inferential. Where a measured but very small quantity of return water is heated electricallyback to the supply temperature, as sensed by thermostat. The energy then taken ismeasured by a simple kWh meter.