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.
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| 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
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.
are there so many refrigerant blends? Why don't the chemical
producers get together to supply one or two
refrigerant blend options?
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).
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?
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.
is the proper charging method for refrigerant blends?
If I charge by liquid, won't I slug my compressor?
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
I am putting in the whole refrigerant bottle, can I feed
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
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.
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.
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
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.
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/
are there two columns on a PT chart, and how are they
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.
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.