THE COMPRESSOR
By: Richard M. Dumais
Ammonia Fact or Myth!!
The compressor is a key component of most refrigeration systems and A/C units.
Other means of moving refrigerant gas/vapor are employed but for this
explanation we will stick
to a mechanical compressors.
What does a compressor do in a refrigeration system?
A compressor is used as a means to move Vapor/gas within a refrigeration
system. There are many types of compressors with as many designs. But the
basics here are the same. We will not try to cover all the types, sizes,
designs etc. but attempt to explain how it is used in an operating
refrigeration system and it's relationship to head pressure or system pressure.
Compressors move Gas/Vapor within a refrigeration system using a compression
mechanical action created by pistons, rotors etc. They have a suction side
or intake side and a discharge side or high side "Discharge Pressure side".
They all have ways to cool and lubricate the compressor motors and moving
parts. The specifics are far beyond this document and I will not even go there.
There are fixed volume compressors and variable volume compressors. Their
basic functions are the same and produce the same results. The reason I
have written this document is that I have found by speaking with many
Technicians and system Operators that there seems to be a misunderstanding
about how they work in relationship to head pressure or system pressure.
Here is a wakeup call for some of you! The compressor does not create the
head pressure as seen on a PSIG gauge, that's right it does not. Most
compressors have the ability to create a specific amount of discharge
pressure. In other words if the discharge side of the compressor is shut
and the compressor is run it will produce a specific amount of pressure.
It can be as high as just a few pounds to 400 of 500 pounds or higher.
The total amount of pressure it can produce is not the question here but
the use of this pressure. The pressure the compressor can produce is
controlled by the manufacturer and the specific use of the compressor.
Once the compressor reaches it's pressure maximum usually it will either
by-pass internally or unload or shut-down, depending on what the manufacturer
designs into the unit. To make the explanation easier we will use water as
a refrigerant and allow the compressor to use liquid water instead of vapor
or gas. Just for the sake of this explanation suppose that you shut the
"line out" from the condenser and start the compressor which is using liquid
water as a refrigerant. The compressor will build up pressure because the
line out of the condenser is closed and not allowing any liquid to leave the
system. The head pressure as read at the proper gauge would read the highest
pressure the compressor could create. The compressor would in a short period
of time shut down on high discharge pressure. Let's say that the pressure it
shut-down at 220lbs. Now open the line out at the condenser and run the
compressor again, the results will quite different. The compressor will be
able to push water thru the condenser and out into the system. The pressure
required to do this let's say would be 150lbs. and as long as nothing changed
the compressor would continue to do this work forever. Now that 150lbs. of
pressure would be considered the head pressure or system pressure. The problem
is that in a real system you are not using water but rather a refrigerant that
is affected by temperature and pressures. Water is also affected by the same
temperatures and pressures but in no way as sensitive as refrigerants. The other main difference also is the fact that we do not run the system with the line
out shut on the condenser.
Ok think about this next statement, if you are running a system and the head
pressure is 200lbs. You suddenly turn off the compressor, does the head
pressure drop off right away or does it maintain for quite awhile? Here
is the reason it does not drop off. Yes it does bleed off slowly because
of restriction etc. but the real reason is because the refrigerant sent up
to the condenser is a super heated vapor and vapor takes up lot more
space than liquid does. Also hot vapor or gas expands more & more as it is
heated. Expansion means pressure and pressure is measured in PSIG.
So what does all this mean? Well the pressure created in the condenser is
actually produced by the expansion and contraction of the super heated gas/
vapor. In an adjustable volume compressor the compressor will adjust to
the settings required by the operator. So if 150 pounds of head pressure
is where you want to be then the compressor unit will load and unload to
maintain that pressure. It does this by reducing the amount of Super heated
gas sent to the condensers thus maintaining a specific system pressure.
Think about this, why is the head pressure higher on a real hot day and lower
on a cool day, the compressor has no way of knowing the outside temperature
it just keep right on pumping away. Another way of looking at it is what can
cause high head pressure. Non-condensible gases such as air, why is that?
Because
the non-condensibles take up space and the condenser is designed for a specific
expected ambient temperature "It's coil size" and the non-condensibles steal
space as if you shortened the tubing, less surface lower condensation ratio.
The longer it takes to condense vapor to liquid the higher the pressures will
get. So on a hot day the system includes fans and water cooling to assist in
the overall heat reduction thus improving the condensation rate and maintaining
an acceptable head pressure. It's easy when you really think about it.
The final lay description is to use water and fire to explain what really is
happening in a condenser. Take a metal tank without any outlets except for
a cork. remove the cork and pour water into it filling it about halfway.
Now insert the cork into the opening and
build a fire under it. As the heat boils the water the resulting steam expands
and starts creating pressure in the container. Within a few minutes the cork
will be blown
out of the hole and the steam will escape to the atmosphere. If you had a
PSIG gauge attached you could have seen a constant pressure rise until the
cork went into space. The fire can be compared to the compressor, the means to
which the vapor is moved from the bottom of the container to the top. Next do
the same thing but this time put a different cork into the container one with
a small tube in the center of it to allow pressure or steam to escape. As you
build pressure in the container take a water hose and cool off the container.
The results will be less pressure and a controlled venting of steam. Increase
the flame or heat simply increase the amount of cooling water, reduce the heat
remove the cooling water. Yes the fire creates the initial action that forms
the steam but the actual pressure is created by the amount of expanding steam
within the confines of the container.
COMMENT:
I have written this article in the most raw and basic lay terms possible.
The reason for this type of lay approach is not because I really think that
our Web Site users require a simple explanation, but rather that the lay
approach offers the best way to produce the results I am attempting to get
across. Please do not be offended by my approach to this problem.
Thanks for taking the time to review this Article..
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