The beginning of 2020 marked an important milestone on our F-gas calendars: the ban on the use of virgin refrigerants with a GWP higher than 2500 for refrigerant charges greater than or equal to 40 tonnes of CO₂ equivalents. Considering the large number of supermarkets still using high GWP refrigerants such as R-404A and R-507A, the impact will certainly not be minor.

 
Retailers who decide to continue with R-404A (or other refrigerants with a GWP higher than 2500) by using regenerated or recycled refrigerant for maintenance of their systems, as is allowed for another 10 years, will have to pay attention to the highly-likely increase in prices of regenerated refrigerants. The price of regenerated R-404A at the end of 2019 was already almost twice as high as the new refrigerant! 

If the chosen option is to replace the entire system, other restrictions in force from 1 January need to be taken into account: only refrigerants with a GWP lower than 2500 are allowed in stationary systems. The use of R-744 (CO₂), with a GWP of just one, can be considered a long-term option.

However, if wanting to choose an intermediate alternative, retrofits can be done using refrigerants with a GWP lower than 2500. R-448A, R-449A, R-407F, R-407A or R-452A are possible candidates for retrofitting R-404A or R-507A systems. These refrigerants have a GWP between 1200 and 2200 and are mixtures with glide. This means that the start and end evaporation and condensing temperatures are different, which needs to be taken into account when programming the controllers. Let’s see why.

What is “glide” when talking about refrigerants?

When using pure refrigerants or azeotropic mixtures, evaporation and condensing temperatures are practically the same, while with zeotropic mixtures there is a difference due to the different evaporation and condensing temperatures of the components in the mixture. In essence, the difference between the saturated vapour temperature, dew point, and the saturated liquid temperature, bubble point, gives the glide value. For instance, R-448A and R-449A refrigerants have a glide of around 4-6 °C, depending on the working pressure, whereas R-404A has a negligible glide value. 

In general, when retrofitting a system with a different refrigerant, it is important to revise the parameters set on the controller to ensure that there is not a decline in the performance of the unit. This becomes especially relevant when retrofitting from a refrigerant without glide (or with negligible glide, such as R-404A) to a refrigerant with glide such as R-448A, or vice versa. 

What needs to be taken into account for compressor control?

The objective of compressor control is to control the average evaporation temperature. The related phenomena are the P-T refrigerant curve, which is different for each refrigerant, and the glide value. Pressure drop must be also considered. 

Compressor control is normally performed using a probe located on the suction side, with the dew point temperature used as the reference. For refrigerants without glide, this temperature corresponds approximately to the evaporation temperature. However, when using refrigerants with glide, these temperatures are not the same, because the evaporation temperature is the average between the dew and bubble point temperatures. 

 
This can be seen more clearly with two examples, one with pressure control and the other one with temperature control. Both cases look at retrofitting from R-404A to R-448A and R-449A.  

• Pressure control: if keeping the same suction pressure as with R-404A, for example 0.7 bars (and considering a pressure drop of 0.8 bars), the average evaporation temperature drops to -27 °C when using R-448A or R-449A, as shown in the following table. It is thus advisable to lower the suction pressure set point, approximately to 0.5 bars, in order to keep the same evaporation temperature.

It should be noted that with pressure control, the difference comes from the fact that the refrigerants are different, but glide has no notable influence.

• Temperature control: if considering an evaporation temperature set point of -32 °C for the three refrigerants, the average evaporation temperature rises by about two degrees when using R-448A or R-449A, as shown in the following table. To avoid this, the evaporation temperature set point must be raised to -30 °C for R-448A or R-449A.

Superheat and subcooling measurements: is there any change?

The superheat measurement is calculated as the difference between the temperature of the refrigerant entering the compressor and that of the saturated vapour, where the saturated vapour temperature (dew point) is used as the reference. This means that the set point remains the same when retrofitting from a refrigerant without glide to one with glide. 
As regards the subcooling measurement, which is calculated as the difference between the temperature of the saturated liquid and that of the refrigerant leaving the compressor, the saturated liquid temperature (bubble point) is used as the reference. Once again, this means that the set point remains the same.

Optimisation of condenser fan control

Regardless of the refrigerant, it is advisable to activate a function available on many controllers such as CAREL’s, called “floating condensing set point”. This allows the condensing temperature to be decreased, at a given outside temperature, and thus reducing compressor energy consumption.
The floating condensing set point control is defined as the sum of the outside temperature and a temperature differential called the offset. This value is usually specified by the condenser designer.

For pure refrigerants, condensing temperature is approximated to the saturated liquid temperature, bubble point. However, when using refrigerants with glide, condensing temperature is the average between bubble and dew point temperatures and thus the condenser works at a higher average temperature than the bubble point temperature. Consequently, a lower floating condensing offset must be indicated. For example, if the offset with R-404A is 10°C, the offset to set for R-448A or R-449A is around 7.5°C.

Caution! 

Refrigerants usually used as retrofits for R-404A have a GWP between 1200 and 2200, as the ones used as examples in this post. This means that increasing prices and decreasing availability over the next few years due to F-gas quota can be expected. The market, regulations and the environment require sustainable solutions. In this regard, the conversion of some stores from R-404A to R-744 (CO₂) have successfully been completed. As an example, the conversion of a store in Sofia (Bulgaria) from R-404A to an ejector-based CO₂ transcritical refrigeration system has been reported.
On the road to HFC phase-out… this year is the key, this decade will be decisive!

The content of this post was the subject of a webinar (in Italian) that can be seen at this link.
 

Related Posts
 

How will commercial refrigeration develop in the future?