What is the relationship between temperature and pressure as applied to refrigerants at saturation?

One of the most important aspects of designing a refrigeration system is the selection of an optimum evaporating temperature for the application at hand. This criterion has a domino effect on other parameters such as suction & discharge pressures, current consumption, refrigeration capacity & mass flow rate. Although each parameter is crucial to the system in its own way, this article delves into how the capacity of the compressor is affected by variations in the evaporating & suction temperatures, which in turn is governed by the refrigerant's pressure at saturation.

A source of secrets

Datasheets from compressor manufacturers reveal an extensive amount of hidden information if looked upon with a curious eye. One such example is of a German compressor manufacturer, SECOP, whose specification sheets personifies this point. An excerpt from one such sheet for an R290 reciprocating compressor [table below] highlights the capacity variation at two separate condensing temperatures. Table A represents the capacities at a condensing temperature of 45°C & table B represents those at 55°C. The vapour specific volume column was not part of the specification sheet and was added to better explain the concepts below.

Off the bat, the most striking variation that one would notice is the evaporating temperature as a function of compressor capacity. Graphing this relationship separately for both specified condensing temperatures helps ease our analyses.

Reiterating the findings from the graph, the compressor’s capacity increases with an increase in evaporating temperature. We will now try to uncover the reason for this trend.

Simplified representations for the win

A higher evaporating temperature would mean that the system is functioning at a higher evaporating pressure. Since the entire evaporator & suction line is exposed to this same pressure (assuming no drop), its effect has a profound impact on the molecules of gas in the suction line. The diagram below is a simplified representation of reciprocating compressor arrangement executing a suction stroke at an evaporating temperature of -35°C at its respective saturation pressure [for simplicity, let us assume that only saturated vapour is entering the compressor with no superheat].

Let us look at the same arrangement when the system is function at a higher evaporating temperature (-5°C), hence higher evaporation pressure (Saturation pressure of R290 at -5°C).

The space occupied by the R290 gas molecules at the -35°C configuration is greater than that occupied by the molecules at -5°C [at their corresponding saturation pressures]. The higher overall pressure in the evaporator & suction line causes the molecules to arrange themselves in a closely packed condition with each other due to the compressible nature of gas molecules in general. Let us isolate the compressor representations of the 2 configurations for comparison.

Graphing things make them simpler to understand

A look at the saturation properties of the refrigerant under consideration (R290) paints a clearer picture of this scenario.

At saturation, the specific vapour volume of gaseous R290 drops from 0.3121 m3/kg to 0.1122 m3/kg when its saturation temperature is raised from -35°C to -5°C corresponding to their respective saturation pressures.

The lower specific volume of the -5°C condition causes a very dense refrigerant vapour to enter the compressor in its suction stroke. Hence more mass of refrigerant flows into the compressor during its suction stroke than during the -35°C case. This is a direct implication that the mass flow rate handled by the compressor at -5°C is much greater than that at -35°C at the same condensing pressure/temperature. This is easy to visualize with the help of a graph [evident from the spec sheet].

Observe that the mass flow handled by the compressor is higher for the 45°C condensing temperature configuration. This is because the compressor needs to compress the refrigerant to a lower pressure compared to the 55°C configuration before returning for the suction stroke. Thereby executing the suction & discharge strokes faster and hence handling more refrigerant in the process.

The greater mass of molecules handled by the refrigerant in the suction stroke [for the -5°C configuration] signifies that as the compression stroke begins, the compressor will need to do more work compared to the -35°C configuration. This is characterized by the increase in input power of the compressor & the amp draw of the compressor with an increase in evaporating temperature.

Refrigerant gas behavior at super heated conditions

Beyond saturation i.e. in the superheated state, a rise in suction temperature is associated with an increase in specific volume at a constant pressure. This is because the gas expands as it absorbs heat while the pressure in the suction line is maintained constant. This means that the capacity of the compressor decreases with higher superheat, but its current consumption lowers due to the lower mass of vapor entering the suction stroke. Although superheat is an absolute necessity to ensure only vapour enters the compressor, it has negative implications on refrigeration capacity as a side effect. This variation in the specific volume of the refrigerant at constant pressure, as a comparison to that at varying pressures, needs to be understood to get a thorough grasp of what is happening inside the system. This trend is clear upon close analysis of the super-heated properties of the refrigerant.

A balancing act

It is clear, that working at elevated evaporating temperatures & pressures could improve compressor capacity. The evaporating temperature is usually selected by maintaining a dT of about 10°C to 12°C from the target temperature that we need to maintain in the refrigerated space. However, raising the evaporating temperature significantly, in the name of increasing its compressor capacity does not always correlate to improved performance in practice.

If the refrigerant evaporates at a significantly higher temperature than the designed evaporating temperature, the cooling needs of the room may not be met. This is due to the lower delta T between the room & the refrigerant. So, there is a fine balancing act that needs to be made to ensure the best results.

In the case of expansion valves sensing superheat values from downstream of the evaporator, the suction temperature is usually maintained constant. But in certain cases where the load rises greater than designed limits, there will be a rise in suction temperature that correlates to higher discharge temperatures when functioning at elevated evaporating temperatures.

For capillary systems, the effect is more critical. As the higher evaporating temperature proportionally raises the suction temperature due to a lack of regulating valves in between like the TXV or EEV.

In the case of capillary systems, raising the evaporating temperature & pressure, raises the suction pressures & a proportional rise in discharge pressures are noticed. This may knock the pressures outside the envelope of the compressor.

An interesting trend that I have noticed in my experience with capillary systems is that if the suction pressure is raised slightly from its designed point (by reducing the capillary length), it is possible to achieve a so-called "sweet spot" that helps achieve the initial pull down time faster than the usual configuration. But any further increase may put the system into imbalance. It is a fine range of operation that needs to be discovered.

The relation between temperatures and boiling pressuress for 

• R-717 : Ammonia
• R-134a : Tetrafluoroethane
• R-22 : Hydrochlorofluorocarbon
• R-507 :
• R-290 : Propane
• R-744 : CO2

are indicated in the diagrams below:

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This page can be cited as

• Engineering ToolBox, (2007). Refrigerants - Saturation Pressures vs. Temperatures. [online] Available at: https://www.engineeringtoolbox.com/constant-boiling-refrigerants-d_1184.html [Accessed Day Mo. Year].

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