PuK - Process Technology & Components 2024

Compressors and Systems
Heat recovery
Heat recovery
Save money and benefit the environment
Dipl. Betriebswirtin Daniela Koehler, Dipl.-Ing (FH) Gerhart Hobusch
Compressors and the compressed
air they generate are used in a multitude
of industrial applica-tions.
However, the fact that compressor
exhaust heat can be harnessed often
remains forgot-ten. The magic
words here are “heat recovery”: up
to 96 % of the drive energy supplied
to a compressor is available for reuse.
This not only saves energy and
costs, but also reduces the operator’s
CO 2
footprint.
Fully 100 % of the drive energy supplied
to a compressor is converted
into heat. Both air- and fluid-cooled
rotary screw compressors are exceptionally
well-suited to comprehensive
recovery and reuse of this energy;
around 76 % of their energy input remains
as heat in the cooling fluid and
is removed in the fluid cooler. A further
15 % can be recovered as heat
via the compressed air aftercooler.
Up to 5 % of the heat produced is
emitted by the electric motor – with
targeted cooling, fully enclosed rotary
screw compressors can even recover
this energy as well. Only 2 % of the total
energy input is lost as heat radiation,
whilst a further 2 % remains as
heat in the compressed air.
Of course, this heat could simply
be conveyed away. However, there
are plenty of ways to make use of this
readily available energy source that
occurs as a by-product of the compression
process. The simplest and
most efficient method is to use the
compressor exhaust heat directly,
e. g. for heating adjoining rooms or
spaces. Here, instead of discharging
hot air from the compressed air station
outside, an air ducting system directs
it to neighbouring warehouses
or work-shops. When no hot air is
required, the heated exhaust air is
simply conveyed outdoors by means
of a flap or louvre. A thermostatically
controlled louvre enables hot air to
be provided as and when required
in order to maintain a constant temperature.
In addition to providing full or supplementary
heating for operating spaces,
hot compressor exhaust air can
be used to support applications such
as drying processes, generating hot air
curtains or preheating burner air for
heating systems. The corresponding
investment costs can often be amortised
within a period of one year.
Compressor exhaust heat can also
be used to supply existing hot water
heating and service water systems; depending
on the available storage capacity,
water temperatures of 70 ºC
and even higher can be gener ated.
There are several ways to achieve this.
The most cost-effective method is to
use a plate-type heat exchanger integrated
into the compressor, which is
connected to the compressor cooling
fluid circuit and transfers energy from
the heated cooling fluid to the water
that requires heating. Depending on
whether the hot water is required for
particularly sensitive production or
cleaning processes, for showering and
washing, or for general heating systems,
special safety heat exchangers
or conventional plate-type heat exchangers
may be used. These enable
70 – 80 % of the installed compressor
output to be used for heating purposes
without the need for any additional
expenditure on energy.
Fig. 1: Virtually the full amount of energy supplied for compressed air generation can be
used for heat recovery.
Fig. 2: Heated compressor cooling air can be used for simple and effective heating of neighbouring
spaces via air ducting.
90 PROCESS TECHNOLOGY & COMPONENTS 2024