DESCRIPTION

Technical Description

Refrigeration systems with two-stage compression are used for the generation of particularly low temperatures. At very low temperatures large pressure differences are required between the evaporator and condenser. In a compressor the volumetric efficiency drops significantly at high pressure ratios. Therefore, two compressors are connected in series, with each compressor only having a relatively low pressure ratio. This makes a more favourable dimensioning of the low pressure stage compressor possible. Due to the large specific volume it requires a larger capacity at lower drive power. 

In addition, intercooling between the low pressure compressor (LP) and the high pressure compressor (HP) reduces the outlet temperature of the HP compressor to harmless values and improves the efficiency of the compression. 

The trainer ET 430 uses injection intercooling. A small amount of liquid refrigerant is injected from the receiver into the outlet line of the LP compressor. The liquid refrigerant evaporates and thus cools the intake gas for the HP compressor. Via an add-on heat exchanger in the injection cooler the supercooling of the liquid refrigerant can be increased upstream of the expansion valve. This allows for an increase in the evaporator capacity.

Manometers and temperature measuring locations at all relevant circuit points enable the evaluation of the two-stage process. To determine the compressor capacity, both compressors are equipped with an electrical power measurement. Valves allow for the injection intercooling or the heat exchanger for refrigerant supercooling to be switched off. This can demonstrate their effect on the system. 

The online representation of the refrigeration process on the PC makes it easy for the student to understand the processes.

The well-structured instructional material sets out the fundamentals and provides a step-by-step guide through the experiments.

Learning Objectives / Experiments

- design and function of a refrigeration system with two-stage compression and injection intercooling 
- effect of the inlet temperature at the HP compressor on the efficiency of the compression
  * with intercooling
  * without intercooling
- effect of the additional refrigerant intercooling
- distribution of the compressor pressure ratios
- represent and understand the refrigeration cycle process in the log p-h diagram