An update on refrigerant blends and how to use them

Do I need to change the oil in my system when I retrofit to a blend?
To begin, HFCs (134a and 404a / 507) MUST have some of the mineral oil replaced with polyolester (POE). Most manufacturers recommend less than 5% residual mineral oil, although that is an arbitrary number and recent studies have show larger amounts of residual mineral oil will still work fine. Some blends contain hydrocarbons, which help with mineral oil circulation even though the HFC blend will not mix with the MO. Larger, more complicated systems will still require the addition of POE to help mix the oil with liquid refrigerant, for example in a receiver.

With the older R-12 and R-502 retrofit blends (401A, 401B, 402A, 402B, 408A, 409A, and similar blends), replacement of some of the mineral oil with alkylbenzene (AB) or POE is recommended at lower temperatures. Evaporators running above 0ºF will generally be able to return mineral oil with these blends.

What is the proper charging method for refrigerant blends? If I charge by liquid, won't I slug my compressor?
In a cylinder, a zeotropic blend will have a different vapor composition sitting above the bulk of the liquid. If you remove this vapor, you will: 1) take the wrong composition refrigerant out of the cylinder, and 2) leave behind the wrong composition refrigerant for future use. Liquid must be removed from the cylinder in order to avoid this fractionation effect. Somewhere between the cylinder and the compressor the liquid refrigerant should be flashed to vapor to avoid slugging. This can be done, for example, by just cracking open the valve on the gauge set while charging.

If I am putting in the whole refrigerant bottle, can I feed vapor then?
You can feed vapor, however, at any point in time the compressor will be seeing the wrong composition gas. At first the vapor will be rich in the higher pressure, higher capacity component. This will cause high discharge pressure and temperature, high motor amps, etc. As the cylinder empties, the compressor will see the lower capacity gas which is left behind, changing the operating conditions the other way.

It will take some time for the "locally fractionated" gas to get mixed back into the original composition. Besides, if you need to charge the whole bottle, it's faster to put it in as a liquid.

If a blend leaks out of the system, will I need to pull the remaining charge and recharge, or can I top-off the existing charge after repairs?
It depends. Studies were done a few years ago to show how higher glide blends behave during leakage and they showed significant fractionation, which affected the properties of the blend. When the system was topped off, the properties came back close to original. The cycle was repeated to see how many times the system could leak before topping off became a problem (the recommendation was not more than five). These studies were done on containers at rest, which promotes the worst case of fractionation.

Another study was performed recently on a system running full time, then cycling normally (2/3 on, 1/3 off), which found that the blend did not fractionate when the refrigerant is moving around inside, and not much fractionation occurred when cycling. Low-glide blends didn't show much fractionation in any case.

What this means is that running systems found to be low on charge have probably not fractionated the blend much, and can be repaired and recharged directly. If the system has been off for a long period (more than a day) and found to have leaked (worst case is about half the charge), it's best to pull what's left and charge with fresh, unless very little is gone, or very little is left. Low-glide blends won't cause any fractionation-related problems.

Why do bubbles appear in the sight glass when I use a blend?
Does this mean I don't have enough refrigerant?

There are several reasons for bubbles in the sight glass. If one of the traditional refrigerants showed vapor in the sight glass it often meant there wasn't enough liquid refrigerant being fed to the valve, and more refrigerant was added to the system.

Blends could show flashing for the same reason, however, they can also flash when there is plenty of liquid in the receiver. Ironically, this liquid in the receiver could be causing the problem, particularly when the equipment is in a hot environment. Blends will come out of the condenser slightly subcooled — at a temperature below the saturated temperature of the blend at the existing high side pressure.

Yet when the blend sits in the receiver, it can "locally fractionate," or change composition slightly by shifting one of the components into the vapor space of the receiver. This will effectively produce a saturated liquid in the receiver, at the same pressure you had before, which flashes when it hits the expanded volume of the sight glass. In most cases these bubbles will collapse when the blend gets back into the tubing which feeds the valve, and the system will operate just fine.

Check other system parameters such as pressures, superheat and amperage to confirm whether you have the right charge. Don't rely solely on the sight glass.

What do "bubble point" and "dew point" mean?
A single component refrigerant always had a "boiling point." Zeotropic blends change composition when they boil or condense, and therefore have a continuously changing boiling point. The most useful temperatures to know are where the boiling starts and ends. Bubble point and dew point are terms used in the chemical industry to define these two temperatures.

Bubble point is the temperature where the saturated liquid starts to boil off its first "bubble" of vapor. (Picture a pot of liquid with the first bubbles starting to appear.) It is also called the "liquid side temperature/ pressure relationship." Dew point is the temperature where saturated vapor first starts to condense, or the last drop of liquid evaporates. (Picture a room full of vapor with a few drops forming on the ceiling.) This is also called the "vapor side temperature/ pressure relationship."

Why are there two columns on a PT chart, and how are they used?
The two columns on the PT chart give the liquid and vapor pressures at the listed temperatures. Single component refrigerants and azeotropic blends have bubble points and dew points equal to each other, and we simply call this the boiling point. When there is only one column on the PT chart, low glide blends would have very similar numbers in the two columns, and often the PT chart will only have one column as well for them.

How a two-column PT chart is used is straightforward. Most times you're interested in knowing the saturated temperature of the refrigerant at the system pressure, so you can compare it to a measurement you're making on the system (for example, to check a superheat or subcool setting). Simply keep track of the condition of the refrigerant where you're measuring, and cross-reference the same side of the PT chart.

Superheat measurements check the line temperature of superheated refrigerant vapor coming out of the evaporator versus the saturated vapor temperature, so you would use the vapor side of the PT chart.

Subcool measurements check the temperature of subcooled liquid refrigerant coming out of the condenser versus the saturated liquid temperature, so you would use the liquid side of the PT chart.