The ideal working fluid or often called refrigerant would have favorable thermodynamic properties, be noncorrosive to mechanical components, and be safe, including freedom from toxicity and flammability. It would not cause ozone depletion or climate change.

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. 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. This is also called the “vapor side temperature/ pressure relationship.”

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.

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.

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.

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.