Basic rules of refrigeration efficiency

Basic Thermodynamic Rules

There are some fundamental thermodynamic rules which apply to refrigeration, which determine both the optimal design and efficiency of all refrigeration systems. These rules are used to base the design of all efficient refrigeration systems. Some of the key rules are as follows:

Heat travels in one direction: Hot to cold

This is how we achieve cooling; the evaporating temperature of the refrigerant must always be lower than the medium required cooling. This also determines that the condensing temperature must be higher than the surrounding air/medium to achieve cooling/condensing effect.

Raising evaporating temperature:

Raising the evaporating temperature of a system will increase the compressor performance by approximately 4% per degree k. By increasing the compressor cooling capacity with minimal effect on its power consumption and therefore reducing the capital cost related to the compressor.

How – Increase cooling surfaces, surface efficiency, control or raise air/fluid/product temperatures if possible

Lowering condensing temperature:

Lowering the condensing temperature of a system will increase the compressor efficiency by approximately 3% per degree k, by lowering the pressure to which the refrigerant must be compressed. Therefore decreasing motor sizes and absorbed power on the compressor.

How – increase condensing surfaces, condenser efficiency and control

Choosing the right refrigerant:

All refrigerants have different thermal properties and vary in efficiency greatly. The refrigerant of choice however is quite an emotive subject and typically is chosen for emotional reasons and not for technically the correct one. Choices are made due to personal experience, legislation or quite often rumour!

Large is Good (well in many cases):

With mechanical items such as compressors, motors etc the larger the piece of equipment, the more inherently efficient it becomes (at the same conditions). Therefore larger systems are typically more efficient than smaller ones even working in the same conditions, and providing an appropriate level of utilisation.

Common systems are more efficient:

Common systems with common heat exchangers are more efficient than multiple, individual systems when designed and controlled properly. This is because the key pieces of capital plant (condensers, evaporators) can all be simultaneously utilised, improving efficiency under part load conditions.

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