Operating conditions

To be able to properly compare refrigerants via their thermodynamic properties, it is important to fix the conditions, where the refrigerant is situated. This would be e.g. the evaporating temperature, the suction superheat and other refrigerant state data. Pressure drops and parasitic heat flows into the system are not covered by the calculation. Trying to e.g. fix a more realistic cold room temperature would lead to the necessity to make a whole series of assumptions on refrigerant dependent evaporator and system design, which influence the comparison significantly. Parameters like these can be estimated well, based on the results of the pure refrigerant comparison if the conditions have been sufficiently varied. The variation is possible with a spread sheet software.

Here, different operating conditions for medium temperature refrigeration (MT) and low temperature refrigeration (LT) from standards for compressors and condensing units were used and additionally some of the values were varied. As the standard reference conditions for carbon dioxide (R744) were partly not defined in a comparable way, it will not be evaluated in the following.

Example operating conditions for the comparison of refrigerants. Values t_o evaporating temperature, t_c condensing temperature.
Point	t_o in °C	t_c in °C	Return gas temperature in °C / superheat in K	liquid t / subcool.	compression	Standard
MT 10 K	-10	45	0	0 K		EN12900
MT 20°C	-10	45	20	0 K		EN12900
LT 10 K	-35	40	-25	0 K		EN12900
LT 20°C	-35	40	20	0 K		EN12900
LT 2st 10 K	-35	40	-25	0 K	2-stage eco	EN12900
LT 2st 20°C	-35	40	20	0 K	2-stage eco	EN12900
evaporation	-40 .. +10	40	10 K	0 K
condensing MT	-10	20 .. 60	10 K	0 K
condensing LT	-35	20 .. 60	10 K	0 K
SEPR CU LT	-35	t_a+15 K	20°C	0 K		EN13215
SEPR CU LT 2st	-35	t_a+15 K	10 K	0 K	2-stage eco	EN13215
SEPR ambient temperatures		32, 25, 15, 5°C

These operating conditions are mostly values from standards established and proven for comparing compressors and condensing units of different manufacturers, with same refrigerants at same refrigeration applications. However, the standards were not developed to compare refrigerants at a single operating condition point. A comparison of refrigerants that makes sense is only possible when taking several different operating conditions and maybe further variations into a common view.

Comparing refrigerants at only one operating condition in only one system layout carries the risk that the conditions are considerably more favourable for one refrigerant than for another one. This way, the comparison would not be neutral and not lead to the best possible choice.

Refrigerants with temperature glide

Zeotropic refrigerant blends change their temperature when evaporating or condensing at constant pressure. In the refrigerant tables, there is the value for the change of temperature, meaning the increase in temperature, from boiling point to dew point at atmospheric pressure, listed as temperature glide. In real evaporators the temperature glide is smaller, as already a certain vapour content appears at the evaporator inlet.

The impact of the temperature glide on the energy efficiency is strongly depending on the design of the system. Running air coolers in cold rooms with high air humidity is often done with very low temperature difference between refrigerants and air and with low degree of cooling of the air flow. In such case, the dew point would be the relevant value for the evaporating temperature. When cooling air or liquid flows with larger cooling temperature difference, the impact is different. With large average temperature difference in heat exchangers, which usually is not beneficial efficiencywise, the impact of the temperature glide is small. In such cases, the medium evaporating temperature would be the relevant value.

In the shown diagrams, the dew point was used as reference. With the selected refrigerant blends, the efficiency using medium evaporating temperature would be ca. 3 .. 6% higher.