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Charging Refrigeration Systems With Blended Refrigerants
Today’s techs need to understand how to work with newer blends
In some respects, servicing today’s refrigeration systems is no more complicated than it ever has been. What has changed, however, is the need to understand the differences in the newer refrigerant blends. Many of the commonly used refrigerants were single component, including R-12, R-22, and R-134a — whether a vapor or liquid, the chemical is still the same.
All these refrigerants have been or will be phased out. While there are still units using some of these refrigerants, it is simply a matter of time before they will need to be repaired or replaced. That is why service technicians need to understand how to work with the alternative blends in order to retrofit existing equipment, or replace equipment that has failed.
Differences
The need for new refrigerants to replace the single component refrigerants, while mimicking their performance and pressure-temperature relationships, has led to the development of blended refrigerants. While these blends may seem like they work the same, there are differences that require alternative methods of charging compared to a single-component refrigerant.
There are two types of blended refrigerants: azeotropic and zeotropic blends. Azeotropic blends are made of two components and are given a number in the 500 Series of refrigerants. These can usually be treated the same as a single-component refrigerant. Zeotropic blends are comprised of multiple components and do not have common boiling points for the various components. Understanding these characteristics can save service technicians a lot of trouble when dealing with blended refrigerants.
Zeotropic refrigerants are subject to temperature glide across the various boiling points of the different chemical components. The pressure-temperature chart may have a bubble point and a dew point, and the temperature glide is the difference between the two. Bubble point is when the first bubble of vapor forms and the refrigerant becomes saturated liquid. Dew point is when the first drop of liquid condenses from vapor and the refrigerant becomes saturated vapor. If the bubble point (saturated liquid) is used instead of the dew point (saturated vapor) when calculating superheat, it will result in an incorrect value.
Recharging a system after a refrigerant leak can often lead to the need to not just top off a system, but to completely replace the refrigerant charge. Since the various components of the refrigerant have different boiling points, a leak in a system can result in losing more of one of these components than the others, throwing the blend out of the designed ratios.
A good practice to follow is if more than half of the charge is lost, then recover it and replace it with all-new refrigerant.
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Recovering and Charging
When recovering refrigerants, first determine if the refrigerant will be reused; for example, if a repair is performed and no charge was lost. As always, use a clean and evacuated recovery tank, and do not mix the refrigerant to be reused with any other refrigerant — even of the same type.
After the repair, replace the liquid line drier and vacuum the system, then return the refrigerant to the system. If the charge is found to be low, it may be advisable to use new refrigerant. Again, never mix distinct types of refrigerants in a recovery tank.
When charging after a repair or component replacement, the system should be properly leak-tested and vacuumed. In a water-cooled system, make sure water is flowing through the heat exchanger. The refrigerant drum should have arrows indicating upside down for liquid and upright for vapor. Introduce enough vapor to bring the pressure above the freezing point. If it is an air-cooled system, liquid may be introduced. When charging in liquid, introduce the liquid on the high side, preferably at the receiver service valve before the system is started.
If the amount of charge is known, it should be weighed in using an accurate scale. If this is a conversion to a blend from a single-component refrigerant, remember that the volume of refrigerant may need to be adjusted. Start by using about 80% of the volume of the refrigerant that was replaced. Once there is sufficient refrigerant in the system, the compressor can be started. Remember to allow the suction pressure to come up into positive pressure before starting the system. Once the system is running, if additional charge is required, it must be charged on the suction side.
The danger of introducing liquid on the suction side is the risk of liquid slugging of the compressor. The use of a flash charger or similar device to evaporate the liquid while charging may be desirable. Technicians may also be able to use the manifold to cause the liquid to evaporate by just cracking the manifold valve slightly and adding refrigerant in short intervals.
The goal is to keep a solid column of liquid in the liquid line to the TXV or metering device. Give the system time to stabilize and the box temperature to begin to come down. If the unit has a sight glass and receiver, add just enough refrigerant for the sight glass to just clear. Once the box has come down to the desired temperature, verify the operating pressures are within normal range for the ambient temperature.
Maintain Ratios
The key to using any blended refrigerant is to maintain the proper ratio of the different components of the refrigerant and understand how charging in liquid is vital to maintaining this balance, as well as protecting the compressor from damage.
Understanding how leaks in various parts of the system can affect these ratios and knowing what temperature glide is and how it affects superheat measurement will make servicing and using blended refrigerants much easier.
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