The unit can be operated in different modes. 11 manually operated valves allow corresponding changes to be made to the refrigeration circuit.  The contains a hermetic compressor, 4 heat exchangers, 3 expansion valves and 4 different interchangeable capillaries. The relevant measured values are displayed directly on indicator instruments. This heat exchanger is used to heat or cool a glycol/water mixture circuit. The relevant measured values are displayed directly on indicator instruments. Therefore, sequences and consequences of switching operations within a cooling unit can be directly investigated. A coaxial tube bundle heat exchanger can be operated as a counter flow condenser or as a parallel flow evaporator. Sight glasses in the refrigerant circuit allow direct observation of the refrigerant status.