Environmental Engineering Reference
In-Depth Information
generally slightly lower than that of piston compressors. Heat pumps with a
screw-type compressor additionally require an oil separator on the pressure
side connected to the end of the compressor. After separation from the working
medium and the cooling in the oil cooler at the end, the oil is again available
for a renewed injection into the screw-type compressor. Screw-type compres-
sors have a comparatively long operational life as they have only a few mov-
able parts (i.e. no working valves).
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Turbo compressors are dynamical type compressors structured with one or sev-
eral stages of compression. One compression stage consists of a running wheel
with a fixed mounting of blades and guide blades to convert kinetic into poten-
tial energy. Within one turbo compressor up to 8 of these bladed wheels can be
installed. Thus a pressure between 8 and 11 bar can be achieved. Radial and
axial machines are used. However, mainly radial turbo compressors are utilised
as they achieve a higher pressure ratio per compression stage and the specific
production costs are lower than for axial machines. The power adjustment is
achieved through the size of the housing, the amount and the width of the
bladed wheels. For a steady adjustment of power, the rotational speed can be
changed and/or the guide blades in the air intake can be adjusted. As the supply
with lubricating oil is totally separated from the working medium in the turbo
compressor (lubricating oil free compression of the working medium), the sol-
vent capacity of the working media is not relevant for the lubricating oil. Ad-
vantages of the turbo compressors are: a low level of wear and tear due to their
simple construction, the steady power regulation from between 10 to 100 %.
They also require comparatively little space - even for higher capacities. Such
turbo compressors are only offered for large capacities.
If required, compressors can be coupled in two different ways. For the compres-
sion in several stages several compressors are connected in series if the pressure
difference between the evaporation and the condensation pressure cannot be man-
aged any longer by a single compressor. However, in the case of the heat pump
cascade, each compressor has its own condenser and evaporator. The ideal refrig-
erant can thus be used at the respective temperature. However, this type of con-
nection causes higher system costs and heat losses (temperature gradient) due to
the higher number of required heat exchangers.
For lower capacities that dominate ambient air and shallow geothermal energy
utilisation and near-surface ground energy, piston and scroll compressors are
normally used. They are generally built into a hermetically sealed off capsule to-
gether with an electric motor (i.e. fully hermetic compressors). In contrast, screw-
type, turbo and similar compressors with semi-hermetic or open design are gener-
ally reserved for larger capacities.
Expansion valves.
In the throttle or expansion valve the pressure of the liquid re-
frigerant is released from condenser pressure to evaporator pressure. Additionally,
the mass flow of the working medium circulating in the heat pump circuit is con-
trolled. Selecting the expansion valve depends on the refrigerant, the size of the
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