The changing refrigeration landscape has made compressor selection a critical process for designers. Mike Saunders and Autumn Nicholson of Emerson Climate Technologies provide tips on evaluating compressor performance.
Commercial retailers know that compressor selection is a vital part of achieving refrigeration system performance and efficiency objectives. Today’s changing refrigeration landscape is forcing retailers to reconsider traditional approaches.
Global regulations are phasing down the use of hydroflourocarbon (HFC)-based refrigerants and giving rise to many blended (zeotropic), natural and synthetic refrigerant alternatives.
An increasing focus on energy efficiency and sustainability is driving the adoption of more complex, unconventional refrigeration system architectures.
These new refrigeration alternatives introduce a variety of design conditions, including secondary, cascading and transcritical booster systems. As a result, compressor selection is more important than ever and refrigeration system designers must carefully select compressors to meet their unique system requirements.
First, identify the difference between mid-point and dew point temperature. The dew point refers to the moment at which the last drop of liquid evaporates upon exiting the evaporator. Zeotropic (commonly referred to as glide) refrigerants create temperature fluctuations between evaporator entrance and exit points.
With a glide refrigerant (e.g., 407A), there is a temperature differential between the inlet and outlet of the evaporator. Mid-point is the average of these temperatures.
Since the compressor runs at dew point in the vapor state, selecting at mid-point alone would incorrectly match the compressor to the required load.
System designers should perform a conversion between evaporator mid-point and compressor dew point conditions to account for the temperature differential. Compressor manufacturers often provide software tools to make this conversion.
Secondly, evaluate evaporator and compressor capacities and superheats. Evaporator capacity is often referred to as Net Refrigeration Effect (NRE) and is the available cooling generated from the refrigeration system.
Compressor capacity is the cooling capacity generated from the evaporator plus the heat gained between the exit of the evaporator and suction into the compressor (or superheat).
There are two components of superheat. Evaporator superheat refers to the points between when 100 per cent of the liquid has become saturated vapor and the evaporator outlet.
Compressor superheat refers to the additional heating that takes place to the gas after it leaves the evaporator and before it reaches the compressor. Systems with long suction line runs have the potential to pick up significant compressor superheat.
Designers should be aware of compressor superheat in the design condition and select the compressors to match it. But they should also consider the system’s evaporator capacity and compare that against load requirements.
Finally, increase capacity through mechanical sub-cooling or vapor injection. Sub-cooling refers to the process of cooling the liquid refrigerant prior to evaporation and is achieved in one of two ways: vapor injected sub-cooling via the compressor or mechanical sub-cooling via a separate cooling cycle.
The net effect of sub-cooling is additional refrigeration capacity. Since the mass flow going through the compressor is not changing, yet there is more potential for liquid evaporation, not only does sub-cooling increase compressor capacity, it also maximises the efficiency of the refrigeration system.
About the author (s)
Autumn Nicholson is an account manager for retail solutions while Mike Saunders is the director of end user technical sales and support.