Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 410

compressor and the capacity of the heat pump respectively. Possible designs are
thermostatic or electronic expansion valves or capillary tubes.
− Thermostatic expansion valves are used for dry evaporation. They are con-
trolled by the evaporation pressure and the superheating of the refrigerant dis-
charged from the evaporator. As superheating takes place in the evaporator and
requires a corresponding heat transfer surface, superheating has to be kept as
low as possible with regard to high evaporator efficiency.
− Electronic expansion valves are regulated by using mathematical algorithms
containing the thermophysical properties of the refrigerant and the relevant pa-
rameters of the heat pump. These are the transit time of the refrigerant through
the evaporator, control characteristics, compressor data, etc. They enable a con-
trollable evaporation temperature and superheating.
− Capillary tubes are used in combination with fully hermetic compressors. They
are very thin (mostly between 1 and 2 mm internal diameter). They are up to 1
or 2 m long in order to guarantee the required throttling effect. However, capil-
lary tubes can only guarantee supercooling in the condenser, but not superheat-
ing in the evaporator. Therefore they need a low-pressure accumulator to en-
sure that the compressor does not take any liquid refrigerant in. Capillaries are
also used for coarse regulation in combination with thermostatic or electronic
expansion valves. Refrigerators are, for example, almost exclusively built with
capillaries.
− Newer developments, where a small turbine is installed instead of the valve and
thus the compression energy is regained in parts ("Expander"), have only been
known from larger heat pumps that are hardly ever used for shallow geother-
mal energy utilisation.
Lubricants. The use of the lubricant should minimise the wear and tear of the
compressor. Depending on the compressor design, lubricants (oil) and refrigerants
have more or less contact.
− In turbo compressors the separation of refrigerant and oil is easily achieved.
Oils that do not blend with the refrigerant can be used.
− Screw-type compressors require large amounts of oil for sealing off as they are
directly in contact with the refrigerant. Oil separators are used here to avoid oil
losses.
− Piston compressor surfaces are continuously moistened with oil by the piston
movement. Lubricant and refrigerant are in contact here, too.
− Scroll compressors are also lubricated with oil. Lubricant and refrigerant are in
contact.
With the exception of turbo compressors, an optimum adjustment of the lubricant
characteristics to those of the refrigerant is of particular importance. As a small
amount of oil is always discharged into the refrigerant circuit in the case of oil-
lubricated condensers, it has to be ensured that it is transported through the entire
refrigerating circuit.