Refrigerant comparison to R404A/R507A − 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 refrigerants R404A and R507A are differing only very little. Placed slightly below are R448A and R449A. Significantly below are the refrigerants R513A, R1234yf, R134a and R450A. The named refrigerants can all be used without problems with the maximum allowable pressure of the system. The slightly lower pressure values of R448A and R449A indicate a slightly lower refrigerating capacity, at least when referring to the dew point. The lower pressure values of R513A, R1234yf, R134a and R450A make them feasible for applications from -20°C evaporation and up.
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.
The refrigerating capacity of R507A is approximately 2 to 3% above that of R404A, see bold dash dot line. With reference to the dew point for evaporating and condensing temperature, the refrigerating capacity of R448A and R449A is 2 to 4% below that of R404A, see bold dash line and bold dotted line. If the temperature glide can be utilized, the mid point temperature can be used as reference for evaporating and condensing temperature. In this case, the refrigerating capacity of R448A and R449A in such a system is 1 to 2% above that of R404A, see the lines with diamonds.
The comparisons of compressor performance data can differ slightly from these curves. Due to the lower suction pressure the capacity data for R448A and R449A can be 2 to 5% lower than theory for low temperature application, depending on the compressor design.
For systems with high useful superheat, e.g. 20°C suction gas temperature due to large internal heat exchangers, the differences increase a little. R404A and R507A gain more from useful superheat, than R448A and R449A do. In this case, the refrigerating capacity of R448A and R449A at low temperature application and reference to dew point can be up to 10% below R404A. Referring to mid point for evaporating and condensing temperature, the refrigerating capacity might be only approximately 5% below that of R404A.
For low temperature systems utilizing economizer operation for subcooling, the ratio of the refrigerating capacities is similar to the systems with internal heat exchanger. The refrigerating capacity with R448A and R449A would be approximately 10% below that of R404A, depending on the evaporating temperature. The refrigerating capacity of the refrigerants R134a, R513A and R1234yf for medium temperature application is approximately 60% of the capacity of R404A.
The capacity of R450A is slightly above 50% of that of R404A. These refrigerants might be feasible, if the refrigeration system delivers significantly more capacity than the current demand. As the flow speed in the liquid line and the vapour density in suction and discharge lines (influence on pressure loss) are lower and condenser and evaporator get "relatively bigger", an increase of refrigerating capacity could be considered, up to for example 70% and higher. This could be done by using a frequency inverter or by adding a compressor to a compound. A re-calculation of the suction and discharge lines regarding pressure drop will be necessary to apply this.
A comparison of calculations of the refrigerating capacity of the applied compressors with the data of R404A or R507A and comparing with R448A or R449A, with the BITZER SOFTWARE can give a more accurate information. For this also realistic values for superheat, subcooling aso. have to be used.
Dew point and mid point temperature
The refrigerants R448A and R449A 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 codensers, 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 valves 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. Due to the slightly lower suction pressure, the suction gas density is slightly lower. The impact on the oil transport should be small. The mass velocity of R448A and R449A is only 65% to 70%, compared to R404A at same displacement. As the liquid density is approximately 5% higher, the flow velocity in the liquid line drops to approximately 65%, which is maybe even advantageous here.
When using R450A the flow velocity in the liquid line will be approximately 35%, for R513A approximately 45% and for R1234yf approximately 50%.
Application limits and discharge gas temperatures
The application limits of R448A and R449A reach down to approximately -40°C only, due to the lower vapour pressure, while R404A and R507A reach down to -45°C. The upper limits are the same.
The discharge gas temperature of R448A and R449A is up to 20 K higher than that of R404A, but lower than e.g. that of R407A and R407F. Thus, the application of additional cooling might be necessary. A discharge gas temperature monitoring is recommended. Additional fans or refrigerant injection can be installed, if necessary.
Application of VARIPACK frequency inverter
In 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.
Application of R134a and its successing refrigerants
If the determination of the status (Necessary preparations) shows that the installed refrigerating capacity of a medium temperature refrigeration system is significantly too high, a retrofit with a different refrigerant can be reasonable. This might be caused by e.g. added glass doors on refrigerated shelves.
If the demand is approximately 60% of the installed capacity, the capacity can be delivered by using R134a or the successing refrigerants R450A or R513A. These refrigerants are not flammable. The two latter have GWPs of 605 and 631 and will thus stay available in the market in large amounts for longer. In systems where flammable refrigerants can be applied in a few years, a retrofit with R1234yf with GWP of 4 can be done later.