FREQUENTLY ASKED QUESTIONS
An update on refrigerant blends and how to use them

Can I add an HFC or HCFC refrigerant blend on top of the CFC charge in my system?
Do I need to change the oil in my system when I retrofit to a blend?
Why are there so many refrigerant blends? Why don't the chemical producers get together to supply one or two refrigerant blend options?
Will a perfect drop-in be developed? A refrigerant blend that requires no oil change, no glide, or fractionation?Something that has better capacity and efficiency, and that won't require adjustments to the system?
What is the proper charging method for refrigerant blends? If I charge by liquid, won't I slug my compressor?
If I am putting in the whole refrigerant bottle, can I feed vapor then?
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?
Why do bubbles appear in the sight glass when I use a blend? Does this mean I don't have enough refrigerant?
What do "bubble point" and "dewpoint" mean?
Why are there two columns on a PT chart, and how are they used?

Can I add an HFC or HCFC refrigerant blend on top of the CFC charge in my system?
There is no chemical or legal reason why you can't add an HFC or HCFC refrigerant blend on top of an existing CFC charge, but you'll be left with a mixture of refrigerants with no Pressure Temperature (PT) chart or table of properties to tell you how it should behave. You may also have problems with lubrication or safety if the resulting mixture has a higher pressure than the original refrigerant. Finally, you'll lose the value of the pure CFC refrigerant, and instead create a recycle/reclaim liability in the form of mixed refrigerants.
Back to top
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 the mineral oil flushed out and replaced with polyolester (POE). Most manufacturers recommend less than 5% residual mineral oil, and in some cases down to 1%. With the retrofit blends (401A, 401B, 402A, 402B, 408A, 409A, and similar blends) the answer isn't as clear. With one exception, compressor manufacturers recommend that some of the mineral oil be changed to either alkylbenzene (AB) or POE (usually 50% or more). On the other hand, some refrigerant manufacturers have made conflicting claims that "no oil change is needed." These are based on oil-refrigerant miscibility tests or measuring oil return in actual systems.

The main concern about lubricant choice is protecting the compressor, which means providing for oil return from the system. The two factors that will affect oil return are chemical mixing of refrigerant and oil and physical system design which promotes "mechanical" oil return. Smaller, warmer-evaporator systems will generally show better oil return than larger, colder-evaporator systems.

Because we generally can't change the system design on a retrofit, the best way to improve oil return is to change the less miscible mineral oil to the more miscible AB or POE. This is true even with R-502, which often has oil return problems. Generally, the more mineral oil changed, the better the oil return.

Given the properties of the refrigerant/oil-type mixture, and after reviewing the system design, the contractor or technician should be able to decide how much, if any, of the oil should be changed.
Back to top

Why are there so many refrigerant blends? Why don't the chemical producers get together to supply one or two refrigerant blend options?
Each refrigerant manufacturer has tried to differentiate itself by blending a technically better alternative refrigerant. Actually, most blends fall into a few categories and types. There are CFC-12 retrofit blends, and R-502 retrofit blends. The CFC-12 type blends either match CFC-12 in automotive A/C (hot) conditions, or refrigeration conditions. A few "low-temp" CFC-12 blends are available as well. Then there are the longer-term HFC blends for CFC-12, R-502, and HCFC-22 applications. The blends in any given category/ temperature range are very similar in properties, behavior, and they all present the same challenges such as fractionation (composition change).
Back to top
Will a perfect drop-in be developed? A refrigerant blend that requires no oil change, no glide, or fractionation? Something that has better capacity and efficiency, and that won't require adjustments to the system?
No. We've mixed it all and haven't found a perfect blend. Each blend has advantages and disadvantages which must be balanced to pick the best overall choice for your specific application. Although certain blends can be used in some applications with little or no changes, you should at least check the glide, oil miscibility, and performance properties for problems.
Back to top
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.
Back to top
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.
Back to top
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 probably better 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.
Back to top

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.
Back to top

What do "bubble point" and "dewpoint" 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 dewpoint 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." Dewpoint 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."
Back to top

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 dewpoints equal to each other, and we simply call this the boiling point. When there is only one column on the PT chart, lowglide 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.
Back to top