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MEEF |
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 In most CHP
applications, the exhaust gas from the electric generation equipment
is ducted to a heat exchanger to recover the thermal energy in the
gas. Generally, these heat exchangers are air-to-water heat
exchangers, where the exhaust gas flows over some form of tube and
fin heat exchange surface and the heat from the exhaust gas is
transferred to make hot water or steam. The hot water or steam is
then used to provide hot water or steam heating and/or to operate
thermally activated equipment, such as an
absorption chiller
for cooling or a
desiccant dehumidifier
for dehumidification.
Many of the thermal recovery technologies used in building CHP systems require hot water, some at moderate pressures of 15 to 150 psig. In the cases where additional steam or pressurized hot water is needed, it may be necessary to provide supplemental heat to the exhaust gas with a duct burner. In some applications air-to-air heat exchangers can be used. In other instances, if the emissions from the generation equipment are low enough, such as is with many of the microturbine technologies, the hot exhaust gases can be mixed with make-up air and vented directly into the heating system for building heating. In the majority of installations, a flapper damper or "diverter" is employed to vary flow across the heat transfer surfaces of the heat exchanger to maintain a specific design temperature of the hot water or steam generation rate.
Typical Heat Recovery
Installation
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In some CHP
designs, the exhaust gases can be used to activate a thermal wheel
or a desiccant dehumidifier.
Thermal wheels use the exhaust gas to heat a wheel with a medium
that absorbs the heat and then transfers the heat when the wheel is
rotated into the incoming airflow.
A professional engineer should be involved in designing and sizing of the waste heat recovery section. For a proper and economical operation, the design of the heat recovery section involves consideration of many related factors, such as the thermal capacity of the exhaust gases, the exhaust flow rate, the sizing and type of heat exchanger, and the desired parameters over a various range of operating conditions of the CHP system — all of which need to be considered for proper and economical operation.