Efficient Plant Jan./Feb. 2024 - 16

feature | compressed air
The heat that's generated
by your air-compression
system can be used in a
variety of ways.
Compress Air,
Recover Heat
GENERATING COMPRESSED AIR typically consumes as much
as 10% of a plant's total energy cost. Th is energy usage can be kept
in check by patching leaks, regular maintenance, or by using capacity-control
technology to help match air output with demand.
A signifi cant benefi cial component of compressing air is heat. As
much as 93% of the energy required to compress air is converted to
heat. Th is heat energy is typically dissipated into a compressor room
where it adds no real value. By designing in and using effi ciency- and
heat-recovery tools in a compressed-air system, you can re-purpose
extraneous heat to warm process water and provide facility heat, contributing
to energy savings and sustainability benefi ts. Th e return on
investment is typically as little as 3 to 5 yr.
BRINGING THE HEAT
Rotary-screw air compressors are especially well-suited to heat recovery
due to their operating principles and design. Both oil-fl ooded and
oil-free compressors can lend themselves to eff ective heat recovery.
In the case of fl ooded screw compressors, the heat of compression is
transferred to the fl uid injected into the compression chamber. Th is
heat is then recovered from the fl uid, using a number of techniques.
Additional heat can be recovered from the compressed air itself.
In an oil-free compressor, the heat of compression is contained
within the compressed air. Due to the higher operating temperatures of
an oil-free compressor, as well as piping considerations, heat recovery
within can be more challenging and require more investment.
16 | EFFICIENTPLANTMAG.COM
Jeremy Sickmiller
Hitachi Global Air Power
APPLICATIONS ABOUND
Applications for the rotary-screw compressor's heat byproduct are
many. Each off ers unique savings opportunities, as well as installation
considerations and investment. One use is for pre-heated make-up air.
Th is solution is applicable to an air-cooled machine and involves ducting
the compressor cooling air inlet to the outside of the building. Th e
outside air is then heated by pulling the heat of compression from the
compressor fl uid and compressed air. For every cubic foot of air pulled
in by this heat recovery system, a cubic foot of air that would have to be
heated from the outside is eliminated.
Because the plant's primary heating system does not have to heat the
outside air, the savings can stack up considerably. Th is is a fairly simple,
easy-to-install system with typically no need for extensive duct work or
booster fans, so the payoff is fast. One note of caution: In cold environments,
care must be taken to ensure the outside air temperature is not
so low that it will cause the air or moisture in the lines to freeze.
Some heat-recovery systems can automatically recirculate warm air
to maintain a constant temperature inside the compressor system. Th is
functionality can also provide a compressor with the ability to operate
in unheated spaces as well as maintain a more comfortable exhaust
temperature.
Supplementary heat is another use for compressor heat. Th is heat use
supports a plant's primary heating system. By considering compressor
placement and adding ductwork to a compressed-air system, the residual
hot air byproduct discharged from an air-cooled compressor can be
JAN/FEB 2024
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Efficient Plant Jan./Feb. 2024

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