Compressor Failures

8/12/2019 Compressor Failures

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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


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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


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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


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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


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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.


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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
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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?
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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


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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.


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.


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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
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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.


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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?


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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
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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.


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.


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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.


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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.)


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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
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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.


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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.

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