Utilisation of Ambient Air and Shallow Geothermal Energy - Renewable Energy: Technology, Economics and Environment - page 407

carrier on the condenser side (e.g. heating water) and the heat carrier of the evapo-
rator (e.g. brine) is not increased unnecessarily by temperature gradients that are
too big. Values of around 5 K have been accepted as a good compromise.
Heat exchangers can be differentiated according to the flow direction of the
substances involved: there are parallel, cross-current, or countercurrent flow heat
exchangers. Mixed types exist. If the heat carriers are brine or water, shell and
tube, plate or coaxial heat exchanger can be used.
− Shell and tube heat exchangers consist of a bundle of tubes usually connected
to plenums (sometimes called water boxes) through holes in tubesheets. They
are fitted into a jacketed tube (shell). The two media that are involved in the
process flow in the tubes, and around the tubes within the shell.
− Plate heat exchangers consist of plates that are welded, soldered or screwed to-
gether. The two media alternate in their flow between the plates. In comparison
with bundled shell and tube heat exchanger of the same capacity they require
less space.
− Coaxial heat exchangers consist of one internal tube and an external tube fitted
around it. One of the two media is flowing through the internal tube, whereas
the other - mainly in countercurrent flow - flows in the space between the in-
ternal and the external tube.
− Finned tube heat exchangers are air-to-liquid heat exchanger and consist of
several parallel tubes that are redirected a number of times in order to lengthen
the path. The entire stack of tubes is joined together by fins. The air mainly
flows in cross or countercurrent between the fins around the tubes, the fluid
flows within the tubes.
In the case of heat pumps, heat exchangers of these types are mainly used to trans-
fer heat between the heat source and the heat pump (i.e. the evaporator) or be-
tween the heat pump and the heat sink (i.e. the condenser). The respective charac-
teristic features are discussed in the following.
− The evaporator is the connecting element between the heat source and the heat
pump. The temperature difference between the heat source and the evaporation
temperature of the refrigerant determines its size. Dry evaporation, flooded
evaporation and evaporation during pumping can be differentiated.
• During dry evaporation as much refrigerant is injected by the expansion
valve into the evaporation tubes as can still be evaporated entirely. It is si-
multaneously overheated slightly (superheating means heating the working
medium up to a temperature above the evaporation temperature here). Super-
heating is a measurement for the injection of the refrigerant in this case.
• Within the flooded evaporator one part of the evaporator is flooded with a
liquid refrigerant. Evaporation takes place around the tubes. The saturated
steam is released from the heat exchanger, superheating is therefore not pos-
sible. In a separator connected at the end, drops of fluid that have been car-
ried along during the evaporation process have to be separated.
• For evaporation during pumping the refrigerant is evaporated in the tube. A
significant excess of fluid is pumped off into a secondary circuit and from