PuK - Process Technology & Components 2024

Leading article
and frequently attacks sensors, expands
pipes or deforms surfaces.
This often leads to small relative
movements between components
that can degrade the structure. All
other pumps including rotary pumps
also cause pulsation, which can even
be severe under some conditions but
is usually tolerated. To improve the
durability of pumps, we need to optimise
the damping of oscillations and
surges, and of course prevent cavitation.
Since such dampers unfortunately
do not exist yet, this is an engineering
challenge.
leading engineering challenges for
compressors. Leakages may also occur
depending on the type of gas.
This is more of an issue the smaller
the gas molecules are. Hydrogen, for
example, is being discussed a great
deal today. Its lubrication properties
are extremely poor and it gets
warmer, not colder, when the pressure
is relieved at the start of the intake
stroke. A liquid seal is highly suitable
here but does of course require
gas purification on the pressure side.
In view of the energy loss through
heat and leakage close to a factor of
Fig. 2: Compression types of compressors: d1 isothermal; d2 polytropic, adiabatic with efficient
cooling; d3 adiabatic or isentropic; d4 polytropic with additional heat input, e. g. from
seal friction.
The efficiency factor of compressors
is highly dependent on cooling during
the entire compression process.
When seal friction occurs in addition,
this results in polytropic compression
in which the gas is additionally
heated beyond compression heating.
The hotter the gas, the more energy
is needed for conveying. Heating
of the gas during intake into the
working chamber with hot walls is
also problematic since it reduces the
intake volume. Compared to isothermal
compression in which the working
chamber is cooled, ideally using
a liquid, polytropic compression consumes
at least twice the energy. Selecting
appropriate cooling or perfectly
separating coolant droplets
of the internal coolant are thus the
two, gas purification on the pressure
side should be amortised quickly.
The energy demand for a target pressure
also increases the lower the actual
intake pressure is. Therefore,
the pressure loss on the intake side
should be minimised as far as possible.
The same applies for the seal
friction. The area below the curves
and lines in Figure 2 represents the
required compression energy.
The efficiency factor of compressors
depends on the following aspects:
1) A lack of effective cooling results in
adiabatic compression. When relatively
high seal friction on the piston
is added, we have polytropic
compression. The consumption of
energy is 150 % higher compared
to isothermal compression. The
hotter the gas at the end of conveying,
the more conveying energy.
2) Gas heating during intake is also
problematic; compression heating
results in hot working chamber
walls. The incoming gas is heated
and expands during intake. This
considerably reduces the intake
volume.
3) Every piston compressor in the
classic design has a dead space.
This is the remaining space at upper
dead centre, which first has
to be depressurised on the intake
stroke before the intake as such
can begin. When conveying hydrogen,
it also heats up when the
pressure is relieved.
4) The intake with pressure loss
means that the actual intake pressure
is lower than the static intake
pressure. While the difference is
small as a rule, it nevertheless has
a negative effect since compression
starting at a lower pressure
consumes the most energy per
compression stroke.
5) A pressure loss also occurs on the
pressure side. However, it occurs
in the upper pressure range and is
therefore the smallest loss in this
list.
6) Leakage on the piston: Depending
on the gas type and its lubricating
properties, the seal suffers
considerable wear and thus also
an appreciable leakage flow. Both
are particularly high for hydrogen
since it does not lubricate. A lot if
research is therefore being done in
this area as well.
7) With poorly lubricating gases
(such as hydrogen), sliding movements
of the compressor valves
can be expected to occur during
valve closing, from the initial contact
until the final limit of travel is
reached, which can cause wear.
The functionally best solution is a
liquid piston or a layer of liquid over
the metallic piston (piston works upward).
This can reduce the dead space
to zero. Leakage is also zero due to
the barrier effect of the liquid and the
piston seal is lubricated, which can
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PROCESS TECHNOLOGY & COMPONENTS 2024