Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 408

there into a so-called "pump container". Only there steam and fluid are fi-
nally separated. Thus even large heat transfer surfaces can be operated at
constant load.
− The condenser is the interface between the heat pump and the heat sink. It dis-
charges utilisable heat to the liquid or gaseous operating medium. Like the
evaporator it is designed as a heat exchanger. Its mean temperature gradient re-
flects the temperature difference between condensation of the refrigerant and of
the heat consumer (heat sink). Depending on the design, heaters of fluid (shell
and tube, coaxial or plate heat exchanger) and air heaters (mainly finned design
for air heaters) can be differentiated.
Compressors. Within the heat pump compressor the gaseous refrigerant moving
in a closed circuit between evaporator and condenser is compressed. Fully her-
metic, partly hermetic and open compressors can be differentiated here.
− For fully hermetic compressors the compressor and the drive motor are in-
stalled together in a gas tight, soldered/welded capsule (i.e. encapsulated hous-
ing). The drive capacity can be up to several kW.
− For semi-hermetic compressors the motor is flanged to the condenser. Like the
fully hermetic compressors, they have a common shaft. The drive capacities are
between 4 to 150 kW in this case.
− In open compressors the drive motor is outside of the actual condenser. Motor
and compressor are linked by a shaft and a coupling. Open compressors are
mainly used in larger systems. The drive can be driven electrically or through a
combustion engine.
Important compressor designs are piston, scroll, screw-type, and turbo compres-
sor.
− In piston compressors the pressure increase is achieved by reducing the size of
sealed off compressor spaces (i.e. by using displacement machines). They are
built as fully hermetic compressors with drive capacities of up to around
25 kW, as partly hermetic up to around 90 kW and as open machines for even
higher capacity levels. The suction volume flows can be up to 1,600 m 3 /h. The
machines are built for smaller and also larger capacities with 1 to 16 cylinders.
− In scroll compressors a disc with spiral fin moves eccentrically above a fixed
disc with the corresponding counter-fin. The spaces separated by the fins are
getting smaller in size during the operation of the compressor. Thus the encir-
cled gas is compressed and discharged again through openings before the gap
is enlarged again (i.e. the displacement machine). The production of the fin re-
quires high precision in order to maintain the compression spaces as gas-tight
as possible. The advantages of this design are the circular movements and the
few moving components and the good performance at partial load.
− Screw-type compressors can be differentiated as oil-free systems on the one
hand and systems with oil-injected cooling. Oil works as a cooling agent and a
lubricant and is supposed to seal off the gap between the rotor blades them-
selves and towards the frame. The part load efficiency of such compressors is