Posted by Mike Hinz on February 10, 2000 at 19:22:01:
In Reply to: How can I measure the heating value of a boiler?(Boiler efficiency) posted by Ming-G on February 10, 2000 at 16:08:22:
Where we are (large government boiler plant)this is determined by dividing the total BTU output (measured in pounds of steam/hour) into the total BTU input (measured in cubic feet of gas/hour). Other plants may use meteric measurements, but the principle remains the same.
Steam out of the boiler is measured by means of a "steam flow meter." Any number of systems can be used to determine the steam flow, including orifice plate, target meter, flow nozzle, venturi tube, vortex and several others. Likewise, fuel flow can be found by means of various instruments, depending upon the type of fuel used and other factors. We use a vortex flowmeter for steam and a differential pressure system to measure the flow of gas past a certain point in the gas line. The greater the flow of gas, the greater the change in pressure across two orifices. The meter is calibrated in cubic feet/hour.
Since the heat content of steam varies at different pressures, it must be decided for whatever pressure you are dealing with. At our plant this is 1003 BTUs/pound at 160 pounds. The formula for determining this is enthalpy of steam minus the enthalpy of feedwater/1,000,000. (These enthalpy figures are available from steam tables found in engineering handbooks.)
We obtain our fuel BTUs daily from our local gas supplier. Usually, it averages 1020 BTUs/cubic foot.
EXAMPLE= On January 31st we used 2,606,800 cubic feet of gas(we usually note it simply as 2606.8 CF/gas) to produce 2,100,000 pounds of steam (2100.0 lbs/steam). 2606.8*1019 (the BTU content per cubic feet that day)= 2656.3 BTUs (times a million)input. 2100.0*1003=2106.3 BTUs (again, times a million) output. 2106.3/2656.3= 79.3%,which was our plant efficency for that day.
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