Refrigerant comparison to R22 − operating conditions and system design
When evaluating and selecting a refrigerant for retrofit, operating conditions and system design have to be considered. The results of the comparison can differ, depending on whether a medium or low temperature application is considered, and if the system is using an internal heat exchanger, economizer or non of these options.
The comparison of performance data with the BITZER SOFTWARE shall be made according to the real system configuration and operating conditions.
Refrigerant circuit in a p,h diagram
The above figure shows the simple circuit with low superheat and subcooling.
Like shown above the efficiency improves significantly in systems with screw compressors and economiser or two stage compressors with liquid subcooler.
Using an internal heat exchanger, liquid subcooling is reached by heating up the suction gas, see figure above. Most refrigerants improve in efficiency with internal heat exchange, especially R134a, R404A and R507A.
However, in systems with Economiser or with 2-stage compressors with refrigerant subcooler this only applies to short circuits, provided that the liquid side of the heat exchanger is installed between condenser and subcooler. In case of longer pipe layouts and usual placement of the heat exchanger directly at the evaporator, the efficiency is significantly reduced due to the strongly subcooled refrigerant. This is especially true for low temperature systems in which the condenser is designed for a low temperature difference.
Vapour pressure
An important point when retrofitting is comparing pressures in the system. The figure below shows the vapour pressure curves of several refrigerants according to their dew point.
The lower pressure values of R513A, R1234yf, R134a and R450A make them feasible for applications from -20°C evaporation and up.
When choosing the refrigerant, it should be considered that in most systems the maximum allowable pressure must not exceed 28 bar!
Refrigerating capacity
When comparing refrigerating capacity values for the same displacement, the consideration of the system design is important. The figure below shows a comparison for a simple circuit based on refrigerant properties only. Conditions chosen are 40°C condensing temperature, 10 K superheat, no sub-cooling, variable evaporating temperature.
A comparison of calculations of the refrigerating capacity of the applied compressors can give a more accurate information, when calculating using the data of R22, with the BITZER SOFTWARE. For this also realistic values for superheat, subcooling aso. have to be used.
Dew point and mid point temperature
The refrigerants R448A,R449A, R407A, R407C, R407F, R417A, R422A, R422D, R427A, R438A change temperature during the evaporating or condensing progress at constant pressure. They have a so called temperature glide. During evaporation this will be approximately 3 to 4 K. When using a large internal heat exchanger or economizer operation, it will be up to 5 K. At condensation the temperature glide is approximately 5 to 6 K.
In systems with generously sized evaporators and condensers, the temperature difference between refrigerant and heat transfer medium or coolant is not large. Thus, the temperature glide can lead to deviations from the expected refrigerating capacity or efficiency. For dry expansion evaporators, also the necessary temperature difference for the superheat should be considered.
For an air cooler evaporator with only 5 to 6 K cooling of the air and a small temperature difference to the air, the temperature glide leads to worse utilization of the evaporator and probably to increase in frost formation at the injection end. In this case, the refrigerant comparison referring to dew point on the suction side is feasible.
When evaporating with slightly higher temperature difference and pure counter flow between refrigerant and heat transfer medium, reference to the mid point evaporating temperature might be feasible.
For systems with a large refrigerating capacity control range, like a parallel compound, the temperature glide in air cooled condensers will be disadvantageous in the lower capacity range, as the temperature difference to the air and the heating of the air are small, the refrigerant however, condenses fully only at the end of the temperature glide. This is especially true for low temperature systems where condensers are generally designed for a low temperature difference.
When evaluating the system operation, special caution is to be put on always to determine the superheat with reference to dew point and the subcooling with reference to bubble point.
Material compatibility
The refrigerants R448A and R449A contain amounts of the refrigerants R1234yf, R448A and R1234ze(E). These refrigerants have different compatibility behaviour in relation to plastic gasket materials than the components of the up to now common refrigerant blends, like R404A or R407F. Thus it is necessary to gather compatibility and applicability statements from the manufacturers of the system components. In case of compatibility problems, components might become leaky, to the outside or in case of solenoid vales also internally. Swelling gaskets might block control valves. Softening of valve seats can lead to wear at the seals and to malfunction after some time.
For many components a problem free operation with the new refrigerants is possible (Compatibility of BITZER products).
Flow velocity
When retrofitting an existing system with an other refrigerant, the displacement stays almost the same. Thus also the flow velocities inside the suction lines stay almost the same. The impact on the oil transport should be small. The flow velocity of R407C, R407A, R407F, R417A, R427A, R438A, R448A, R449A in the liquid line will stay approximately the same. For R404A, R507,R422A, R422D, it can be expected that flow velocities will be 20-60% higher.
When using R134a, the flow velocity in the liquid line will be approximately 60%, for R513A approximately 70%.
Application limits and discharge gas temperatures
- The refrigerantsR134a, R513A and R450A are suitable for applications with evaporation temperatures as low as -25°C.
- For application down to -40°C, the remaining alternatives are suitable (substitutes).
- R404A, R507A and R422A can be used down to -45°C.
- For many low temperature applications, an additional fan is necessary.
to | Evaporating temperature [°C] |
toh | Suction gas temperature [°C] |
Δtoh | Suction gas superheat [K] |
tc | Condensing temperature [°C] |
| Additional cooling or max. 0°C suction gas temperature |
| Additional cooling & limited suction gas temperature |
Application of VARIPACK frequency inverter
ln case the refrigerating capacity reached after the retrofit shows to be slightly too low, a frequency inverter of the VARIPACK range can be used to increase the compressor speed and gain some additional refrigeration capacity. For compressors of the ECOLINE range, performance data with variable speed control with VARIPACK are included in the BITZER SOFTWARE and the matching model is easily selectable.