cpp – Process technology for the chemical industry 01.2019

The compact, flow-optimised tube bundle
heat exchangers are located immediately
downstream of the assembly on the
discharge side
intake temperature of 20 °C, a pressure ratio
of three and an isentropic compressor efficiency
of 74 %, the air is heated to approximately
166 °C during compression. The
higher the temperature, the wider the range
of applications for waste heat recovery.
Thermodynamically, the quality of the heat
is described by the so-called Carnot factor,
which includes the ratio of the absolute
waste heat and heat sink temperatures, i.e.
the use of waste heat. The amount of heat
contained in the process air typically accounts
for about 85 % of the total that can
potentially be recovered in plants. The remaining
15 % are distributed more or less
equally between losses from the drive
motor, the mechanics and radiation from
the hot compressor stage underneath the
acoustic hood.
Waste heat for heating rooms
However these 15 % can also be used to heat
adjacent office or production areas in connection
with heat recovery measures. Aerzen
equips the exhaust air louvres of the Delta
Blower positive displacement blower, the
Delta Hybrid rotary lobe compressor and the
Delta Screw compressor with an exhaust air
duct. The heated air which previously cooled
the compressor stage, the silencer, and the
piping system underneath the acoustic hood
escapes from this duct. The return flow in
the central exhaust air duct, from which
branching ducts supply the rooms to be
heated, has a temperature of between 30
and 60 °C. Regulated flaps are used in this
system for individual temperature control.
Tube bundle heat exchangers
The first method of heat recovery is to exploit
the 85 % share of thermal energy, i.e.
the compressor air itself. Aerzen already successfully
uses flow-optimised tube bundle
heat exchangers for a large number of applications.
These compact units are located
immediately downstream of the assembly
on the discharge side and are simple to install.
Consequently, this technology is also
recommended for retrofitting, as a way to
noticeably improve the energy efficiency of
existing plants as well as the CO 2 balance.
The tube bundle heat exchangers used by
Aerzen are designed such that the perfor -
mance loss due to the increased duct pressure
(less than 2 to 3 %, depending on the
application and requirements) is negligible
– at least in terms of the savings as a result
of heat recovery.
While the thermal energy from the pure exhaust
air can be utilised most effectively for
direct room heating, tube bundle heat exchangers
represent an energy source with
significantly higher temperatures, opening
up a multitude of potential uses. Supporting
heating systems, preparing hot water for
showers and washrooms or heating service
water in industrial environments are typical
applications. In wastewater technology, the
heat is used to dry sewage sludge. When
planning these systems, they must always be
sized to match the base load. This can easily
be read off using long-term characteristic
curves.
The starting point for optimum waste heat
utilisation is to determine the usable heat.
This obviously depends on the usable temperature
difference, the prevailing volume
flow (i.e. the quantity available over a period
of time) and the degree of simultaneity
of supply and use. In view of these interdependencies,
Aerzen offers smart heat recovery
solutions for each application, so
that the available energy is effectively channelled
into production processes, hot water
preparation or heating systems.
Summary
Compressed air generation is automatically
linked to waste heat due to the law of
thermodynamics. Since this law cannot be
repealed, solutions are needed to integrate
previously unused heat into a plant’s own
energy demand. Plant operators often fail to
recognise the full extent of the potential
savings. In practice, however, this technology
pays for itself quickly and also reduces
CO 2 emissions.
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AUTHOR:
THORSTEN SIENK
Freelancer