Automatic compressor performance test bench as a gas loop with low charge

The new F-Gas Regulation [Regulation (EU) No. 517/2014 on fluorinated greenhouse gases], which has been in effect since early 2015, imposes relatively strict regulations on the use of fluorinated refrigerants with high GWP values, resulting in the increased use of natural refrigerants and low-GWP refrigerants. Many of these substitutes are more or less highly flammable or, in some cases, toxic. Depending on these properties, the relevant safety standards (DIN EN 378, ISO 5149) specify maximum charge levels based on installation conditions, etc. For refrigeration test benches, there are various approaches to reducing the charge size through process design. This makes it possible to achieve relatively high capacities in test specimens—such as refrigerant compressors—with relatively small charge sizes. As a result, test benches can be installed in areas that were previously unsuitable for this purpose.

Project Objective

The use of a gas loop to determine compressor performance offers the following advantages:

• small refrigerant charge quantities
• minimal equipment requirements
• low energy consumption
• rapid attainment of steady-state conditions

On the other hand, there are difficulties in control and limited flexibility in adjusting the thermodynamic operating parameters due to the small refrigerant charge.

Approach and Development Steps

The challenges associated with operating gas loops with low fill levels required the development of new methods and equipment, with a focus on the following areas:

• Configuration of the replenishment system for fill level control
• Specific dosing method, including process automation
• Configuration and methods for adjusting the internal volume of the gas loop

Project Results

As part of the funded project, the new automatic compressor performance test bench was developed, built, commissioned, and tested as a gas loop. Its functionality was verified, and a great deal of experience and insight was gained regarding the new system configuration and, above all, the handling of flammable refrigerants in general. The refrigerant charge for the actual process is lower than in other system configurations; however, storing refrigerant in a reservoir to cover any potential replenishment needs results in an increase in the total refrigerant volume, so this advantage is smaller than expected. There is still room for optimization here, though. The basic suitability of the control system has been demonstrated, and it can also be applied to other test benches. One of the most important outcomes of the project is the body of knowledge gathered in connection with safety considerations regarding flammable refrigerants (A2, A2L, A3 safety classes according to DIN EN 378) for refrigeration test benches.