Improve Efficiencies With Careful Compressor Selection
By AUTUMN NICHOLSON AND MIKE SAUNDERSHVAC Systems Refrigeration
How to match a commercial refrigeration system with design conditions.
• Trans-critical booster systems are among the most complex systems in use today and use low temperature (LT) compressors to raise (or boost) the vapour pressure to the level of the medium temperature (MT) evaporators. CO2 refrigerant is used for both LT and MT parts of the system.
Another important variable in the compressor selection equation is the increasing prevalence of zeotropic refrigerants. Zeotropic refrigerants (commonly referred to as glide refrigerants) are comprised of a blend of refrigerants, each with a unique boiling point that creates temperature fluctuations as the refrigerant enters and exits the evaporator. The difference between these temperatures is often referred to as temperature glide. This makes the awareness of mid-point, saturated suction temperature (SST) and saturated condensing temperature (SCT) very important to compressor selection.
Refrigeration system designers have traditionally selected compressors to match the capacity anticipated on the hottest days of the year, accounting for a design Btu load and high ambient temperatures. While this approach ensures required performance in extreme conditions, it does not always equate to achieving system efficiencies. In light of the changing refrigeration landscape, designers should evaluate compressor selection on three important considerations.
1 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. In refrigerants with no glide (such as R22), assuming negligible pressure drop, gas enters and exits the evaporator at the same temperature (i.e., inlet and outlet temperatures are the same). But with a glide refrigerant (such as 407A), there is a temperature differential between the inlet and outlet of the evaporator, necessitating the measurement of mid-point. Mid-point temperature in the evaporator, then, is the average of refrigerant inlet and outlet temperatures, and is the best way to approximate the middle state of evaporation (see Figure 1).
A compressor, conversely, actually runs at dew point in the vapour state, so 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 and aid in the appropriate compressor selection.
Refrigerants with glide (zeotropic) have differing temperatures at evaporator entrance and exit points. Mid-point is an average of the two.
2 Evaluate evaporator and compressor capacities and superheats. Understanding the evaporator and compressor loads you are trying to match is critical to the selection process. Even though the same mass flow is going through the compressor, system designers must consider the capacities of the evaporator and compressor.
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). Return gas (RG) refers to the temperature of the gas as it enters the compressor and is the sum of the SST and superheat.
Similarly, there are two components of superheat. Evaporator superheat refers to the points between when 100 per cent of the liquid has become saturated vapour and the evaporator outlet. In order to guarantee that liquid does not flood back to the compressor, systems are always designed with a positive superheat coming off the evaporator. 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 quite a bit of additional compressor superheat.
It is important for retailers to realize that there is an inverse relationship between compressor superheat and the capacities of the evaporator and compressor. Increased compressor superheat results in increased compressor capacity and decreased evaporator capacity. This is due to changing density of the gas going into the compressor as temperature increases, which effectively decreases evaporator capacity. Conversely, decreased compressor superheat results in decreased compressor capacity and increased evaporator capacity.
System designers should be aware of compressor superheat in the design condition so that they can make compressor selections to match that condition. But, it is equally important to consider the system’s evaporator capacity and compare that against the design load requirements.
3 Increase capacity through mechanical sub-cooling or vapour injection. Sub-cooling refers to the process of cooling the liquid refrigerant prior to evaporation. This is achieved in one of two ways: vapour injected sub-cooling via the compressor (e.g., a vapour-injected compressor); or mechanical sub-cooling via a separate cooling cycle.
Regardless of the method, the net effect of sub-cooling is additional refrigeration capacity. Because the liquid is sub-cooled before entering the evaporator, there is more potential to warm it up as it goe
s through the evaporator. A mechanical or electronic expansion device is used to lower the refrigerant pressure, and at that point it goes into the evaporator and starts to boil. Sub-cooling means there is more liquid to boil (versus vapour), or put another way, more potential for latent heat to be transferred (versus sensible heat).
Essentially, sub-cooling increases compressor capacity without changing its displacement. 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 maximizes the efficiency of the refrigeration system.
CONSIDER AEER VERSUS EER
When sizing compressors, refrigeration system designers tend to focus primarily on capacity. While capacity is important, efficiency should be factored in as well – and not just the energy efficiency ratio (EER) but the annual energy efficient ratio (AEER).
AEER takes into account ambient conditions throughout the year based on the installation’s geographic location. When designers consider efficiency from an annual perspective, they can select a refrigeration system that has the potential to run more efficiently in winter months (and much of the calendar year) at much lower condensing pressures.
Many systems are not designed to exploit this potential, and compressors are selected to perform best in high condensing conditions (see Figure 3). This represents a condition that only occurs on the hottest days of the year and in actuality may be a poor match for most common annual operating conditions. While compressors and evaporators must be sized to match the load at the hottest days of the year, the system should also be configured for low condensing conditions to maximize energy efficiencies.
ADVANTAGES OF PROPER COMPRESSOR SELECTION
While regulatory changes and the energy efficiency initiatives may be forcing retailers to adopt new refrigeration system architectures, careful compressor selection has become more important than ever. There are many advantages to carefully sizing compressors to match the load. If smaller compressors match design conditions, retailers can save on equipment and installation costs.
Appropriately matched compressors will result in fewer compression cycles, longer compressor life spans, and an increased potential for energy savings. Finally, retailers should check with their equipment manufacturers to utilize software tools that can accurately match compressor selection to design conditions. <>
Compressor Selection — A Real-life Scenario
Traditionally, refrigeration system designers base compressor selections on dew point conditions, Btus required at max load condition and a design safety factor. But often, the compressor selected is not the best match for the operating conditions. Figure 2 is an example that demonstrates the selection options available in the design process. This scenario uses R-407A refrigerant and represents traditional design conditions found in supermarkets with large centralized racks:
• Required load: 40 000 Btu/hr
• Design conditions: +20F evaporating temperature/105F condensing temperature/65F RG (evaporator temperature plus 45F superheat)/0F sub-cooling/10F evaporator superheat
You can see in Figure 2 that the compressor selected has a capacity of 47 800 Btu/hr. Based on dew point, the compressor selected is 19 per cent oversized and results in seven per cent excess capacity for the evaporator. At the mid-point selection, it is even more oversized: 26 per cent for the compressor and 13 per cent for the evaporator. The third row (45F RG/10F eSH) shows a newer smaller system with lines more closely coupled to the evaporator loads; it won’t pick up as much compressor superheat. You can see that compressor capacity decreases and evaporator capacity goes up, but the result is still close to 20 per cent excess capacity versus the load.
The final row shows the best match for the design conditions and is accomplished with the selection of a smaller compressor. The more appropriately sized compressor allows the refrigeration system to achieve improved efficiencies.