MIL-HDBK-1133
time in the fall shortly after winter startup. Plant operating
records that indicate steam or hot water supply temperatures and
flows during these times can be used to assess losses. During
no-use periods, the amount of fluid cycled and associated supply
and return temperatures should be collected. Generally, the more
periods of data collected and used, the more accurate the
estimate. The difference in temperatures (assuming constant
pressure) can be used with any standard steam table to determine
an estimated heat loss per quantity of fluid. This value
multiplied by the fluid cycled will provide an estimate of heat
losses per hour. This method must assume the accuracy of the
flow and temperature measurements. Any condensate return in
steam systems must also be accounted for. Equation 9 can be used
to calculate estimated losses from steam and hot water systems:
[ E supp * Flowsupp ] - [ E ret * Flowret ]
Loss - rate=
1000
(9)
where
Loss-rate = estimated line loss rate in kBtu/h
Esupp
= enthalpy of steam OR hot water flow at
exit temperature in Btu/lb
Flowsupp = flow rate of supply fluid in lb/h
Eret
= enthalpy of condensate OR hot water flow
at return temperature in Btu/lb
Flowret
= flow rate of return fluid in lb/h
The loss rate value can then be multiplied by the plant operating
hours for the time period being analyzed, to derive an estimated
loss value.
2.8.4
Central Plant Efficiency. Central plant efficiency is
another energy use that is not accounted for in building
estimates. As with line losses, if this energy is to be
allocated to buildings, it need not be calculated. However, if
this energy use is to be accounted for separately, it may be
estimated. Plant efficiency is a combination of other component
efficiencies such as combustion, transfer losses, and auxiliary
load uses. Assessments of large central plant operations and
efficiencies may have been done recently at the installation by
installation or contractor staff. This information provides the
best values and is the preferred method. If these assessments
have not been done recently, the operations staff may roughly
estimate efficiency by dividing the thermal content of fuel used
by the thermal content delivered.
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