8/12/2019 Compressor Failures
1/19
Compressor Failures
The failure of an a/c compressor can be the result of several factors. When a
compressor needs to be replaced, care should be given to diagnose the
reason for the failure. The replacement compressor might also fail rapidly if
the problem, which caused the replacement, has not been diagnosed and
corrected. One of the more common failures is caused by the compressor
functioning without the proper amount of lubricant. The technician installing
a compressor needs to comply with the manufactures recommendation and
insure that the proper amount of lubricant has been placed in the a/c system.
When the compressor is operating normally, the lubricant does not stay in
the compressor. The lubricant continually flows through the a/c system. The
oil flowing through the system in necessary to keep the compressor properly
lubricated. If a problem develops that impedes the proper flow of the
lubricant, a rapid premature compressor failure will occur. There are many
reasons why the lubricant's ability to flow might be changed.
The refrigerant takes small portions of the lubricant with it as it moves
through the system. This flowing of oil keeps the a/c system working
properly. As the compressor moves the refrigerant gas from the low side to
the high side, it also carries the oil. If a failure occurs which lets the oil
escape from the refrigerant's grip, or if some problem impedes the flow of
oil, the a/c system is headed for failure.
Most compressors are being shipped without the proper amount of oil
recommended by the manufacturer. It is up to the installer to determine the
proper amount to be added to the compressor. If the installer does not follow
the manufactures' recommendation, the compressor could be damaged due to
8/12/2019 Compressor Failures
2/19
lack of lubricant. Install of the recommended oil amount in the
compressor and the other in the accumulator.
The A/C system with an improper amount of refrigerant can affect the flow
of lubricant. If there is not enough refrigerant in the system the movement of
lubricant will be greatly affected. There will not be enough lubricant carried
with the smaller amount of refrigerant. If the system is over charged, the
flow of lubricant can be adversely affected by the higher head pressure, and
the possibility of pooling in the condenser and or drier.
If a leak develops anywhere in the pressurized system, the oil will also leakout. A considerable amount of oil can leak out in a very short period of time.
In many systems, a compressor failure can occur after a very small amount
of oil has leaked out. When installing a replacement compressor, remember
that improper compressor mounting torque can contribute to leaks and/or
noise.
A problem can arise when the condenser has been impacted with a heavy
load of contaminates. The installer flushes the condenser and assumes that
since the flush came out clean the condenser was clean. Most modern
condensers are "dual pass". This means that the high-pressure line from the
compressor comes into the condenser at the top and splits into at least two
parallel passages. If one of these passages happens to be clean, and the other
is totally clogged, the flush will follow the path of least resistance and flowthrough the open side. This leaves a tremendous amount of contaminates in
the system unnoticed by the installer. If a significant amount of these
contaminates leaves the condenser, it will flow to other components and will
8/12/2019 Compressor Failures
3/19
cause the slowing or stoppage of the lubricant flow. The proper installation
of an inline filter can help to avoid this problem.
Design of Vapor-Compression
Refrigeration Cycles
Goal
We want to design a vapor-compression refrigeration cycle to absorb heat
from a cool environment and reject it to a warm environment. The design isto be based upon the ideal vapor-compression refrigeration cycle, with four
components: a cooler (where we reject the heat), a throttle, a heater (where
we absorb the heat), and a compressor.
Basics of Vapor-Compression Refrigeration Cycles
The general idea
The challenge in refrigeration (and air conditioning, etc.) is to remove heat
from a low temperature source and dump it at a higher temperature sink.
Compression refrigeration cycles in general take advantage of the idea that
highly compressed fluids at one temperature will tend to get colder when
8/12/2019 Compressor Failures
4/19
they are allowed to expand. If the pressure change is high enough, then the
compressed gas will be hotter than our source of cooling (outside air, for
instance) and the expanded gas will be cooler than our desired cold
temperature. In this case, we can use it to cool at a low temperature and
reject the heat to a high temperature.
Vapor-compression refrigeration cycles specifically have two additional
advantages. First, they exploit the large thermal energy required to change a
liquid to a vapor so we can remove lots of heat out of our air-conditioned
space. Second, the isothermal nature of the vaporization allows extraction of
heat without raising the temperature of the working fluid to the temperature
of whatever is being cooled. This is a benefit because the closer the working
fluid temperature approaches that of the surroundings, the lower the rate of
heat transfer. The isothermal process allows the fastest rate of heat transfer.
More details
An ideal refrigeration cycle looks much like a reversed Carnot heat engine
or a reversed Rankine cycle heat engine. The primary distinction being that
refrigeration cycles lack a turbine, using a throttle instead to expand the
working fluid. (Of course, a turbine couldbe incorporated into a
refrigeration cycle if one could be designed to deal with liquids, but the
useful work output is usually too small to justify the cost of the device.)
The cycle operates at two pressures, Phighand Plow, and the statepoints are
determined by the cooling requirements and the properties of the working
fluid. Most coolants are designed so that they have relatively high vapor
8/12/2019 Compressor Failures
5/19
pressures at typical application temperatures to avoid the need to maintain a
significant vacuum in the refrigeration cycle.
The T-s diagram for a vapor-compression refrigeration cycle is shown
below.
Figure 1: Vapor-Compression Refrigeration Cycle
T-s diagram
Below is a possible CyclePaddesign of a refrigeration cycle. The layout
shown below is a clickable image. To jump to the part of this page that
details the assumptions of a particular device or statepoint, just click on it.
8/12/2019 Compressor Failures
6/19
Figure 2: Basic refrigeration
cycle layout
Jump To:
Cooler Inlet
Cooler (Condenser)
Cooler Outlet
Throttle
Heater (Evaporator)
Compressor Inlet
Compressor
Example Design Constraints
Cooling requirements
For purposes of illustration, we will assume that a refrigeration system used
to cool air for an office environment. It must be able cool the air to 15.5C
(about 60F) and reject heat to outside air at 32C (90F).
the working fluid
We have several working fluids available for use in refrigeration cycles.
Four of the most common working fluids are available in CyclePad: R-12,
R-22, R-134, and ammonia. (Nitrogen is also available for very low
temperature refrigeration cycles.) We will choose R-22 for this example.
Description of Cycle Stages
We will examine each statepoint and component in the refrigeration cycle
where design assumptions must be made, detailing each assumption. As we
can see from the example design constraints, very few numbers need be
http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#coolerhttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#coolerhttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-outlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-outlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#throttlehttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#throttlehttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#heaterhttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#heaterhttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comp-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comp-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comphttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comphttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comphttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comp-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#heaterhttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#throttlehttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-outlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#coolerhttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-inlet8/12/2019 Compressor Failures
7/19
specified to describe a vapor-compression refrigeration cycle. The rest of the
assumptions are determined by applying reasoning andbackground
knowledge about the cycle. The two principle numerical design decisions are
determiningPhighand Tlow, at thecooler outletand thecompressor inlet.
Cooler (Condenser) inlet (S1
This state need not involve any design decisions, but it may be important to
come back here after the cycle has been solved and check that T2, which is
the high temperature of the cycle, does not violate any design or safety
constraints. In addition, this is as good a place as any to specify the working
fluid.
Cooler (Condenser): Heat Rejection (CLR1)
The cooler (also known as the condenser) rejects heat to the surroundings.
Initially, the compressed gas (at S1) enters the condenser where it loses heat
to the surroundings. During this constant-pressure process, the coolant goes
from a gas to a saturated liquid-vapor mix, then continues condensing until it
is a saturated liquid at state 2. Potentially, we could cool it even further as a
subcooled liquid, but there is little gain in doing so because we have already
removed so much energy during the phase transition from vapor to liquid.
Cooler (Condenser) outlet (S2)
We cool the working fluid until it is a saturated liquid, for reasons statedabove. An important design question arises at this state: how high should the
high pressure of the cycle be?
http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-outlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-outlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-outlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comp-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comp-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comp-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#comp-inlethttp://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#cooler-outlet8/12/2019 Compressor Failures
8/19
We choose Phighso that we can reject heat to the environment.Phighis the
same as P2, and P2determines the temperature at state S2, T2. (T2is just the
saturation temperature at Phigh). This temperature must at least be higher than
that of the cooling source, otherwise no cooling can occur.
However, if T2is too high (that is, higher than the critical temperature TCfor
the working fluid), then we will be beyond the top of the saturation dome
and we will loose the benefits of the large energy the fluid can reject while it
is being cooled. Furthermore, it is often impractical and unsafe to have very
high pressure fluids in our system and the higher P2we choose, the higher T1
must be, leading to additional safety concerns. To find an applicable
pressure, use the saturation tables to find a pressure which is somewhere
between the saturation pressure of the warm air yet still in the saturation
region.
For reference, TCfor our four working fluids are given below.
Critical Temperatures
of some refrigerants
substance TC(C)
R-12 (CCL2F2) 111.85
R-22 (CHCLF2) 96.15
R-134a (CF3CH2F) 101.05
ammonia (NH3) 132.35
8/12/2019 Compressor Failures
9/19
For our example using R-22, we must be able to reject heat to air that is
32C. We can choose if T2to be anywhere between that number and the
96C TC. We'll choose it to be 40C for now.
Figure 3: Vapor-Compression Refrigeration Cycle
COP versus Thighin the cooler
The figure above gives a general idea of the improvements we can expect
with lower temperatures in the cooler. Keep in mind that the practical
limitation here is heat transfer to the surrounding air. While lower
temperatures will make the cycle more efficient theoretically, setting Thigh
too low means the working fluid won't surrender any heat to the
environment and won't be able to do its job.
Throttling (THR1)
The high-pressure, saturated liquid is throttled down to a lower pressure
from state S2to state S3. This process is irreversible and there is some
inefficiency in the cycle due to this process, which is why we note an
increase in entropy from state S2to S3, even though there is no heat
transfer in the throttling process. In theory, we can use a turbine to lower the
pressure of the working fluid and thereby extract any potential work from
the high pressure fluid (and use it to offset the work needed to drive the
compressor). This is the model for the Carnot refrigeration cycle. In practice,
turbines cannot deal with the mostly liquid fluids at the cooler outlet and,
even if they could, the added efficiency of extracting this work seldom
justifies the cost of the turbine.
8/12/2019 Compressor Failures
10/19
Heater (Evaporator): Heat Absorption (HTR1)
The working fluid absorbs heat from the surroundings which we intend to
cool. Since this process involves a change of phase from liquid to vapor, this
device is often called the evaporator. This is where the useful "function" of
the refrigeration cycle takes place, because it is during this part of the cycle
that we absorb heat from the area we are trying to cool. For an efficient air
conditioner, we want this quantity to be large compared to the power needed
to run the cycle.
The usual design assumption for an ideal heater in a refrigeration cycle is
that it is isobaric (no pressure loss is incurred from forcing the coolant
through the coils where heat transfer takes place). Since the heating process
typically takes place entirely within the saturation region, the isobaric
assumption also ensures that the process is isothermal.
Compressor Inlet (S4)
Where do we want S4?
Typically, we want state S4to be right at the saturated vapor side of the
saturation dome. This allows us to absorb as much energy from the
surroundings as possible before leaving the saturation dome, where the
temperature of the working fluid starts to rise and the (now non-isothermal)
heat transfer becomes less efficient.
Of course, we would get the same isothermal behavior if we were to start the
compression before the fluid was completely saturated. Further, there would
seem to be a benefit in that statepoint S1(seeFigure 1) would be closer to
the saturation dome on the Phighisobar, allowing the heat rejection to be
http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#fig-1http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#fig-1http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#fig-18/12/2019 Compressor Failures
11/19
closer to isothermal and, therefor, more like the Carnot cycle.
It turns out that, for increased efficiency, we can choose S4such that S1is
on the saturation dome, instead of outside of it in the superheat region.
Figure 4 shows the T-s diagrams for two refrigeration cycles, one where S4
is a saturated vapor and the other (in light green) where S4has been moved
further into the saturation dome to allow S1to be a saturated vapor.
Figure 4: T-s diagram for different compressor conditions
The advantage in the second case is that we have reduced the compressor
work. We have also reduced the heat transfer somewhat, but the reduced
compressor work has a greater effect on the cycle's coefficient of
performance. Figure 6 shows the cycle's COP versus the quality of S4. We
note that the change in COP is noticable, but not terribly impressive.
8/12/2019 Compressor Failures
12/19
Figure 5: COP versus compressor inlet quality
However, in setting S4below the saturated vapor line, we assume our
compressor can work with fluid that is substantially liquid at statepoint S4.
Since the liquid part of the fluid is incompressible, this is likely to damage
the compressor. It is for this reason that we choose the inlet to the
compressor to be completely saturated vapor, ensuring that the compressor
can do its work entirely in the superheat region. When we are told we have
compressors capable of dealing with fluids whose quality is slightly less than
100% (these are sometimes available), we can adjust the position of S4to
improve cycle efficiency.
How to choose Tlow
This brings us to another design issue: Now that we know that S4is on the
saturated vapor line, where on the line is it? In other words, how low can
Tlowgo?
8/12/2019 Compressor Failures
13/19
Tlowoccurs within the saturation dome, so it determinesPlowas well. We
know that Tlowmust at least be cooler than the desired temperature of the
stuff we wish to cool, otherwise no cooling will occur. An examination of
the saturation tables for ourrefrigerants shows that setting Tlowat, for
instance 15 C, still allows for fairly high pressures (4 to 7 atmospheres,
typically). So, while this tells us how low Plowmust be, it does not tell us
how low it can be.
There are several major practical considerations limiting Plow.
Fundamentally, we must concern ourselves with the properties of our
working fluids. Examination of the saturation table for R-22 shows that at
atmospheric pressure, the saturation temperature is already very cold (about
-40C). For small-scale air-conditioning applications, we have no desire to
create a stream of extremely cold air, both due to safety concerns and
because cold air holds very little moisture and can be uncomfortably dry. For
larger-scale applications, this is less of a concern because we can always mix
the cold, dry air with warmer, wetter air to make it comfortable.
Another hardware consideration is that it is fairly difficult to maintain a very
low-pressure vacuum using the same compressor that will achieve high
pressure at its outlet. Choosing a Tlowthat results in a Plowof 0.1 atmospheres
is probably not practical if we intend to have Phighup near 10 atmospheres.
This brings us to the other reason we cannot make Tlowtoo small. ExaminingFigure 1again, we see that the lower Plowis, the further out to the right
(higher entropy) the saturated vapor will be at statepoint S4. Statepoint S4
has the same entropy as S1, and the further to the right S1is along the Phigh
http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#fig-1http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#fig-1http://www.qrg.northwestern.edu/thermo/design-library/refrig/refrig.html#fig-18/12/2019 Compressor Failures
14/19
pressure isobar, the hotter S1must be. This high temperature is undesirable
from both efficiency and safety standpoints.
The figure below shows the relationship between Tlowand the cycle's
coefficient of performance (COP). We note that the higher Tlow, the better
the COP. The practical limit on Tlowis heat transfer rate in the evaporator;
having Tlowtoo close to the temperature of the stuff we wish to cool results
in low heat transfer rates.
Figure 6: Vapor-Compression Refrigeration Cycle
COP versus Tlow
So, ultimately, we want a low pressure such that its saturation temperature is
below the desired cool air temperature but high enough that the temperature
at state one is not too hot. For our example, where we need to cool air down
to 15.5C, we will choose Tlowto be 10C.
8/12/2019 Compressor Failures
15/19
Compressor (COMP1)
Ideal compressors are like ideal pumps, adiabatic and isentropic. We also
note that the compressor is the only device in the system that does work to
the fluid. For an efficient air conditioner, we want this quantity to be small.
Refrigeration
Now, we shall use our understanding of heat engines and phase
transitions to explain how refrigerators work. The enthalpy changes
associated with phase transitions may be used by a heat engine
(Figure 1) to do work and to transfer heat between (1) the substance
undergoing a phase transition and (2) its surrounding environment. In
a heat engine, a "working substance" absorbs heat at a high
temperature and converts part of this heat to work. In a secondary
process, the rest of the heat is released to the surroundings at a
lower temperature, because the heat engine is not 100% efficient.
As shown in Figure 2, a refrigerator can be thought of as a heat
engine in reverse. The cooling effect in a refrigerator is achieved
by a cycle of condensation and vaporization of the nontoxic
compound CCl2F2(Freon-12).As shown in Figure 5, the refrigerator
contains (1) an electrically-powered compressor that does work on
Freon gas, and (2) a series of coils that allow heat to be released
outside (on the back of) the refrigerator or absorbed from inside the
refrigerator as Freon passes through these coils.
8/12/2019 Compressor Failures
16/19
Figure 5
This is a schematic diagram of the major functional components
of a refrigerator. The major features include a compressor
containing Freon (CCl2F2) gas, an external heat-exchange coil
(on the outside back of the refrigerator) in which the Freonpasses and condenses, an expansion valve, and a heat-exchange
coil inside the insulated compartment of the refrigerator (blue)in which the Freon is vaporized, absorbing heat from inside the
refrigerator (and thus lowering its temperature).
Figure 6 (below) traces the phase transitions of Freon and their
associated heat-exchange events that occur during the refrigeration
cycle. The steps of the refrigeration cycle are described below the
figure. (The numbers in the figure correspond to the numbered stepsbelow.)
8/12/2019 Compressor Failures
17/19
Figure 6
This diagram shows the major steps in the refrigeration cycle.
For a description of each step (indicated by the green numbers),
see the numbered steps below. In this figure, blue dots represent
Freon gas, and solid blue areas represent liquid Freon. Smallarrows indicate the direction of heat flow into or out of therefrigerator coils.
Please click on the pink button below to view a QuickTimemovie showing an animation of the refrigeration cycle shown in
the figure above and described below. Click the blue buttonbelow to downloadQuickTime 6.5 to view the movie.
1. Freon creasing the pressure of the gas. As the pressure of the
gas increases, so does its temperature (as predicted by the
ideal-gas law). Outside of the refrigerator, the electrically-run
compressor does work on the Freon gas,in
http://www.apple.com/quicktime/http://www.apple.com/quicktime/http://www.apple.com/quicktime/http://www.apple.com/quicktime/8/12/2019 Compressor Failures
18/19
2. Next, this high-pressure, high-temperature gas enters the coil
on the outside of the refrigerator.
3. Heat (q) flows from the high-temperature gas to the lower-
temperature air of the room surrounding the coil.This heat
loss causes the high-pressure gas to condense to liquid,as
motion of the molecules decreases and intermolecular
attractions are formed. Hence, the work done on the gas by
the compressor (causing an exothermic phase transition in
the gas) is converted to heat given off in the air in the room
behind the refrigerator. If you have ever felt the coils on the
back of the refrigerator, you have experienced the heat given
off during the condensation of Freon.
4. Next, the liquid Freon in the external coil passes through an
expansion valve into a coil inside the insulated compartment of
the refrigerator.Now, the liquid is at a low pressure (as a
result of the expansion) and is lower in temperature
(cooler) than the surrounding air (i.e., the air inside the
refrigerator).
5. Since heat is transferred from areas of greater temperature
to areas of lower temperature, heat is absorbed (from
inside the refrigerator) by the liquid Freon, causing the
temperature inside the refrigerator to be reduced.The
absorbed heat begins to break the intermolecular attractions of
the liquid Freon, allowing the endothermic vaporization process
to occur.
6. When all of the Freon changes to gas, the cycle can start over.
8/12/2019 Compressor Failures
19/19
The cycle described above does not run continuously, but rather is
controlled by a thermostat. When the temperature inside the
refrigerator rises above the set temperature, the thermostat starts the
compressor. Once the refrigerator has been cooled below the set
temperature, the compressor is turned off. This control mechanism
allows the refrigerator to conserve electricity by only running as much
as is necessary to keep the refrigerator at the desired temperature.
Top Related