Central Air Conditioning Plants
In our
department, these plants (Air‐cooled or water‐cooled) are commonly available
above 10 TR up to 100 TR. These types of plants are more suitable for large
installations such as AIR Radio Studio / TV Studio Buildings and High Power
Transmitter Buildings. In water‐cooled plants, external cooling towers / water
spray ponds with water softening plants are the common features. These are
invariably provided with AHU (Air Handling Unit) and supply & return ducts
for carrying air.
The central
air conditioning plants or the systems are used when large buildings, hotels,
theaters, airports, shopping malls etc are to be air conditioned completely.
The window and split air conditioners are used for single rooms or small office
spaces. If the whole building is to be cooled it is not economically viable to
put window or split air conditioner in each and every room. Further, these
small units cannot satisfactorily cool the large halls, auditoriums, receptions
areas etc.
In the
central air conditioning systems there is a plant room where large compressor,
condenser, thermostatic expansion valve and the evaporator are
kept in the large plant room. They perform all the functions as usual
similar to a typical refrigeration system. However, all these parts are larger
in size and have higher capacities. The compressor is of open reciprocating
type with multiple cylinders and is cooled by the water just like the automobile engine.
The compressor and the condenser are of shell and tube type. While in the small
air conditioning system capillary is used as the expansion valve, in the
central air conditioning systems thermostatic expansion valve is used.
The chilled
is passed via the ducts to all the rooms, halls and other spaces that are to be
air conditioned. Thus in all the rooms there is only the duct passing the
chilled air and there are no individual cooling coils, and other parts of the
refrigeration system in the rooms. What is we get in each room is the
completely silent and highly effective air conditions system in the room.
Further, the amount of chilled air that is needed in the room can be controlled
by the openings depending on the total heat load inside the room.
The central
air conditioning systems are highly sophisticated applications of the air
conditioning systems and many a times they tend to be complicated. It is due to
this reason that there are very few companies in the world that specialize in
these systems. In the modern era of computerization a number of additional
electronic utilities have been added to the central conditioning systems.
There are two types of central air conditioning
plants or systems:
In this
system the huge compressor, and the condenser are housed in the plant room,
while the expansion valve and the evaporator or the cooling coil and the air
handling unit are housed in separate room. The cooling coil is fixed in the air
handling unit, which also has large blower housed in it. The blower sucks the
hot return air from the room via ducts and blows it over the cooling coil. The
cooled air is then supplied through various ducts and into the spaces which are
to be cooled. This type of system is useful for small buildings.
This type
of system is more useful for large buildings comprising of a number of floors.
It has the plant room where all the important units like the compressor,
condenser, throttling valve and the evaporator are housed. The evaporator is a
shell and tube. On the tube side the Freon fluid passes at extremely low
temperature, while on the shell side the brine solution is passed. After
passing through the evaporator, the brine solution gets chilled and is pumped
to the various air handling units installed at different floors of the
building. The air handling units comprise the cooling coil through which the
chilled brine flows, and the blower.
The blower
sucks hot return air from the room via ducts and blows it over the cooling
coil. The cool air is then supplied to the space to be cooled through the
ducts. The brine solution which has absorbed the room heat comes back to the
evaporator, gets chilled and is again pumped back to the air handling unit. To
operate and maintain central air conditioning
systems we need to have good operators, technicians and engineers.
Proper preventative and breakdown maintenance of these plants is vital.
Air
cycle
Indoor air
may be too cold, too hot, too dry, too wet, too drafty or too still. These
conditions are changed by rotating the air and these treatments are provided in
the air‐conditioning air cycle.
Air distribution system directs the
treated air from the air conditioning equipment to the space to be conditioned
and then back to the equipment. The main components in the air cycle are
(i) Fan
(iii) Supply Outlets
(v) Return duct
(ii) Supply duct
(iv) Return outlets
(vi) Filter
The total
resistance of these components to the flow of the air plus the friction
resistance caused by the air passing through the duct run are major factors in
determining the size of the fan and fan motor and the amount of air pressure
that is required.
For a
Broadcast Studio set up this resistance is of the order of 25 mm to 50 mm of
water gauge. Centrifugal fan is most commonly used in commercial and
residential installations. It consists of a scroll, a shaft and a wheel. The
scroll is actually a housing for the shaft and wheel and the shaft serves as an
axle for the wheel. The wheel is cylindrical in shape and has many blades.
Centrifugal fans are available with forward or backward curved blades. A
forward curved fan can deliver a required quantity of air at low fan speed. The
air velocity and speed of the fan wheel (tip speed) not only play a large part
in determining the efficiency of the fan but also affect the level of noise
generated by the fan. High tip speed and high velocity usually result in more
noise.
Remote
location of the fan reduces the noise but the system become more expensive.
Ducts may be circular, rectangular or square in shape. From the appearance and
practical point of view, rectangular ducts are generally adopted. Ducts are
fabricated from a wide variety of materials. Ducts made of sheet metal are very
common. The ducts are lined with glass wool or mineral wool slabs of 25 mm
thickness wrapped in copper naphthanate treated cloth.
Outlets are
another major part of the air distribution system. They are important from the
point of view of appearance, functions and performance. The primary function of
the outlets is to provide properly controlled distribution of air to the room
and removing the air from the room.
Ceiling diffusers, grilles and registers are used as
supply outlet and grilles are used as return outlets.
Operation
Before
starting the plant, ensure that proper functioning of safety controls including
interlock circuit have been checked and correctly set, and that all motors are
megger‐tested, direction of rotation verified, all bearings lubricated and
refrigeration system fully charged. The crank case heater must be energised
well in advance.
Proceed step by step for operating the system as
follows:
• Start the air handling unit,
ensuring that dampers in the supply duct are fully open.
• Open all water valves and
start the water pump. Observe pressures at condenser inlet and outlet.
•
Open hot‐gas valve on the condenser and the discharge
service valve on the compressor. Open discharge gauge valve to read the
pressure.
• Follow the same procedure
and read the suction pressure.
• Open liquid line valve.
Observe standing pressure on the gauges. This should be approximately 7.03
2 2
kg/cm (100
psi) for R‐12 and 10.5 kg/cm (150 psi) for R‐22 to indicate that the system is
tight with no leakage.
•
Open suction service valve and start the compressor.
Observe the refrigerant and oil pressures. Check the current drawn by the
compressor motor, observe the oil level in the compressor sight glass. Oil
should be clear without foam after operation has stabilised.
It is
essential to collect the refrigerant in the condenser with isolation to prevent
its loss before opening the compressor or any other part of the system. This is
called pump‐down and the operation involves the following procedure:
•
Short the low pressure switch with a temporary jumper
wire so that the compressor does not stop before the refrigerant from it is
emptied.
• Slowly close the suction
valve with the compressor running.
2
• When the
suction pressure drops to about 0.15 kg/cm (2 psi), stop the compressor.
2
•
Never pump the compressor below 0.15 kg/cm to prevent
infiltration of moisture and dirt into the crank case.
•
After a few minutes, the dissolved refrigerant will
leave the crank case raising the suction pressure. This additional refrigerant
can be pumped to the condenser by operating the compressor again for a short
while.
2
•
Repeat the above procedure till the suction pressure
does not rise above 0.15 kg/cm after closing the service valves.
Removing
Refrigerant from the System
It may be necessary to remove the
refrigerant from the system into a cylinder if there is an excess charge or
there is a leak in the condenser. Take the following steps for this operation:
(a)
Connect a suitable line between the angle valve
provided for charging and an empty refrigerant cylinder.
(b) Purge the air from the
connection line.
(c)
Keep the cylinder cold by immersing it in ice cold
water to ensure a faster refrigerant flow from the system.
(d)
Start the compressor and open the liquid line charging
valve, allowing the liquid into the empty cylinder. If excess refrigerant is to
be removed, hold the charging valve open only until the discharge pressure
reaches the normal reading. After this operation, remove the charging line and
close the charging valve.
Do not overcharge the cylinder as excessive pressure
is dangerous.
Purging
Non Condensible Gases
Presence of
non‐condensibles gases such as air causes high discharge pressure, resulting in
reduction of capacity and high power consumption. In case such symptoms are
present, the following check should be done:
• Shut down the system
overnight, long enough for the temperature of all components to level off.
• Read the standing pressure
and compare it with the refrigerant saturation pressure corresponding to
the
temperature of the system. If the standing pressure exceeds the saturation
pressure by 0.75
2
kg/cm (10
psi) or more, the non condensibles are excessive and must be removed.
For example, if R‐22 is used and the
system temperature is 85 F (29.4 C) and standing pressure is 12.8
2
kg/cm (175 psi), then there is
excess of non condensibles. Saturation pressure for R‐22 corresponding to a
|
o
|
2
|
2
|
2
|
temperature 85 F is 11 kg/cm . The
difference is 1.05 kg/cm more than 0.75 kg/cm , indicating corrective purging.
For purging, take the following steps:
• Pump down the system as
described earlier.
• Immediately after stopping
the compressor, close the compressor discharge valve.
• Run the water through the
condensor for condensation of refrigerant vapour.
• Crack open the purge valve
on the top of the condensor for an instant, shut it again.
•
Allow the system to stabilize for a few minutes before
reopening and closing the purge valve. Repeated purging and closing operation
should clear the system of non condensible.
•
Restore normal system operation, check the improvement
in discharge pressure. Check refrigerant charge and compressor oil pressure.
Refrigerant
Charging
A correct
operating charge of refrigerant in the system is essential. Loss due to leakage
in the system has to be made up. It may be necessary to replace the entire
charge. An over charge results in unduly high temperatures, pressures and
operating costs and may damage the system components. An undercharged system
leads to insufficient cooling, high operating cost, and, in hermetic system,
the compressor motor may fail.
Refrigerant
may be added to the system either as a vapour or liquid depending upon the
location of charging point and quantity required. Generally, for adding make‐up
refrigerant, vapour charging method is more convenient. For total system
charge, liquid charging at the high side followed by vapour charging at
compressor low side will be quicker.
Under no
circumstances should liquid refrigerant be allowed to enter the compressor to
avoid damage to the compressor. The procedure for vapour charge method is
described below:
•
Open the suction and discharge shut‐off valves of the
compressor. Install a gauge in the discharge gauge port and open the gauge line
if a gauge port has not been provided.
•
Connect a refrigerant cylinder and the connection with
a compound gauge, to the charging valve provided on compressor suction line.
Purge the air from the lines and tighten the connections.
•
Admit the refrigerant by slowly opening the
refrigerant cylinder. The cylinder should be kept in upright position to
prevent the refrigerant from entering the compressor in liquid state.
• Start the compressor.
•
As the cylinder gets emptied, its pressure will drop
to the same level as the suction pressure. The remaining refrigerant can be
drawn from the cylinder by closing the suction shut off valve and pulling a vacuum
on the cylinder with the compressor running.
•
Check the quantity of refrigerant charge by noting the
difference in the weight of the cylinder and observing the pressure.
Algae/slime
scale and corrosion on the water side of the heat transfer equipment retards
heat transfer causing general loss of efficiency and breakdowns. Oxygen, Carbon
Dioxide, Sulphur Dioxide absorbed from the air and dissolved in water cause
corrosion, reducing the capacity of lines, increasing frictional losses and
pumping cost. Hard water causes scaling problem. When heated, the minerals are
left behind, which form a deposit on the heat exchanger surface. The heat
transfer rating of the scale is very much lower than metal. Retarded heat
transfer results in increased discharge pressure causing loss in capacity and
increased power consumption.
Scaling of
the condensor tubes in a re‐circulated water system is unavoidable. De‐scaling
has to be carried out as a preventive maintenance once every 12 months or
earlier depending on the hardness of the water. De‐scaling can be carried out
quite conveniently by circulating mild inhibited acid solution with the help of
a small pump connected across the condensor inlet and the water valves are
closed to confine the circulation to the condenser only.
Chemical
compounds are available which suspend minerals of dissolved scale. Algae attach
themselves to the surfaces, and since they are living plants, they grow until
they clog the passages of the system. Bacteria forms slime and close the system
in much the same way as algae. Algae/Slime is controlled by use of toxic. A
specialist should be consulted to determine the algae/slime.
The trouble
should be diagnosed as accurately as possible before any repair is attempted.
Definite symptoms will accompany a faulty operation in the system. The
following trouble shooting chart will help in fault location and prompt
correction:
********
TONNAGE MEASUREMENT OF AC PLANTS
(I) By air-flow method
Tonnage
of refrigeration (TR) = A x V x (H1 - H2) {FPS units}
S 200
A = Cross sectional area of duct through which
air is passing in sq. ft.
V = Air velocity per minute, in Ft. per minute,
measured by anemometer in ft./min
S = Specific volume of (return) air
H1 = Enthalpy for return air,
in Btu/lb
H2 = Enthalpy for supply air,
in Btu/lb
Note:
Both H1 and H2 are determined from the
psychometric chart with help of Dry bulb temperature (Tdb in deg F.) and Wet bulb temperature (Twb in deg F.) Similarly Specific volume (S) is
determined from the psychometric chart
Example
1:
Calculate Tonnage of AC Plant having the
following measurement figures:
A = 30.25 sq. ft.
V = 293 Ft. per minute
S = Specific volume of return air = 13.7 cubic
Ft./ Lb.
For Return duct,
Tdb = 73 0F and Twb = 670 F. ------------- (X)
For Supply duct,
Tdb = 53 0F and Twb = 490 F. ------------- (Y)
Calculations:
H1 = Enthalpy for return air,
in Btu/ Lb, determined from psychometric chart in r/o (X)
= 31.8 Btu/Lb.
H2 = Enthalpy for supply air,
in Btu/ Lb, determined from psychometric chart in r/o
(Y)
= 19.8 Btu/Lb.
S = Specific volume of return air = 13.7 cubic
Ft./ Lb.
Therefore
Tonnage = Tonnage of refrigeration = A x V x (H1 -H2)TR
S 200
=
Tonnage of refrigeration = 30.25x293 x (31.8 – 19.8) TR
13.7 200
= 38.8 TR (Answer)
(II)
By Water-flow method
|
Points to be remembered:
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||||
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•
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1Watt
|
=
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0.86 k Cal / Hr** (unit of power i.e. rate of
energy)
|
|||
|
•
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1
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Watt
|
=
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3.412
|
Btu / Hr*
|
or [1 Btu = 1÷ 3.412 Watts]
|
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•
|
1 k Watt
|
=
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3412
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Btu / Hr
|
or [1 Btu = 1÷ 3412 k Watts] #
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•
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1
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Btu
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=
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0.252 k Cal
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•
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1
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Ton
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=
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12000 Btu / Hr
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=
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200 Btu / Min
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=
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50 k Cal / Min
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[200 Btu x 0.252 k Cal]
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=
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3024 k Cal / Hr
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[200 Btu x 0.252 k Cal x 60
Min]
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=
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3.561 kW
|
# [12000 ÷ 3412 = 3.561]
|
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Heat
gained by water = { Q x Sp. Heat x (Th – Tc) x 60} Btu/Hr ---- (A) = heat rejected by the
refrigerant in the condenser
Heat developed due to work done by compressor = {√ 3 V x I x Cos Φ} Watts
= {√ 3 V x I x Cos Φ x
3.412}*Btu/Hr
----- (B)
Or = {√ 3 V x I x Cos Φ x 0.86} ** k Cal /Hr
Refrigeration capacity in TR =
[Heat
gained by water in Btu/Hr] – [Heat developed due to work done by compressor in
Btu/Hr] 12000
= (A) - (B) 12000
= { Q x Sp. Heat x (Th – Tc) x 60} – { √ 3 V x I x
Cos Φ x 3.412}
12000
Where Q = Quantity of water flowing through the water
cooled condenser in Ltr/ Min
Th = Temperature after
condenser in0 F Tc = Temperature before
condenser in 0 F
Sample measurements
Q = Quantity of
water flowing through the water cooled condenser = 620 Ltr/ Min Th =
Temperature at condenser outlet = 990 F
V
=
compressor Voltage = 390 Volts
I = compressor Current = 60
Amp Cos Φ = Power Factor = 0.85
Calculations:
(A) = Heat rejected by the refrigerant in the
condenser
= Q x Sp. Heat x (Th – Tc) x 60 Btu/Hr
= 620 x 2.204 x (99 – 92) x 60 = 57,3922 Btu/Hr
(B) = Heat developed due to work done by compressor
= {√ 3 V x I x Cos Φ x 3.412} Btu / Hr
= {√ 3 x 390 x 60 x .85 x 3.412} Btu / Hr = 11,
7476 Btu / Hr
|
Refrigeration capacity in
TR = (A) - (B)
|
= (57,3922) -
(11,7476) = 38 TR, ANSWER
|
|||
|
|
12000
|
|
12000
|
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***********
HVAC‐ Air Conditioning Troubleshooting and Repair
The
following is an general A/C system troubleshooting guide. Realize that it is
generic and many of the things listed here may not apply to the 944.
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Symptom / Possible Cause
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Solutions
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Low Compressor Discharge Pressure
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Repair
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1.
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Leak in system
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1.
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Repair leak in system
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2.
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Defective expansion valve
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2.
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Replace valve
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||||
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3.
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Suction valve closed
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3.
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Open valve
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4. Freon shortage
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4. Add freon
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5.
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Plugged receiver drier
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5.
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Replace drier
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Compressor suction valve
leaking
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6.
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Replace valve
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7. Bad reed valves in
compressor
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7. Replace reed valves
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High Compressor Discharge Pressure
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Repair
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1.
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Air in system
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1. Recharge system
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2. Clogged condenser
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|||||||||
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2. Clean condenser
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3.
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Discharge valve closed
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3. Open valve
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4. Overcharged system
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|||||||||
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4. Remove some refrigerant
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5.
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Insufficient condenser air
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|||||||||
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5.
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Install large fan
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|||||||||||
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6.
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Loose fan belt
|
|||||||||||
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6.
|
Tighten fan belt
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|||||||||||
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7. Condenser not centered
on fan or too far from
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|||||||||||
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7. Center and check
distance
|
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radiator
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|||||
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Low Suction Pressure
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Repair
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1.
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Refrigerant shortage
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1.
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Add refrigerant
|
||||||||
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2. Worn compressor piston
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2. Replace compressor
|
||||||||
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3. Compressor head gasket
leaking
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3. Replace head gasket
|
||||||||
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4.
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Kinked or flattened hose
|
4.
|
Replace hose
|
|||||||||
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5.
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Compressor suction valve
leaking
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5.
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Change valve plate
|
|||||||||
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6.
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Moisture in system
|
6.
|
Replace drier
|
|||||||||
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7. Trash in expansion
valve or screen
|
7.
|
Replace drier
|
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||||||
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High Suction Pressure
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Repair
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1.
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Loose expansion valve
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1.
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Tighten valve
|
||||||||
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2. Overcharged system
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2. Remove some refrigerant
|
||||||||
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3. Expansion valve stuck
open
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3. Replace expansion valve
|
||||||||
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4. Compressor reed valves
|
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4. Replace reed valves
|
||||||||
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5. Leaking head gasket on
compressor
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5. Replace head gasket
|
||||||||
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|
|||||
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|
Compressor Not Working
|
|
Repair
|
|
||||||||
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1.
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Broken belt
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|
||||||||||
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1.
|
Replace belt
|
|||||||||
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2. Broken clutch wire or
no 12v power
|
|||||||||||
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2. Repair wire or check
for power
|
|||||||||
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3. Broken compressor
piston
|
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|||||||||
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3. Replace compressor
|
|||||||||
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4. Bad thermostat
|
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|||||||||
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4. Replace thermostat
|
|||||||||
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5.
|
Bad clutch coil
|
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|
|||||||||
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5.
|
Replace clutch coil
|
|||||||||||
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6.
|
Low Refrigerant ‐ low
pressure switch has cut
|
|||||||||||
|
6.
|
Add refrigerant
|
|||||||||||
|
|
off clutch power
|
|||||||||||
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|
||||||||
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|
||||||||
|
|
Evaporator
Not Cooling
|
Repair
|
|
|||||||||
|
|
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|
|
|
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|
|||||
|
|
1.
|
Frozen coil, switch set
too high
|
1.
|
Turn thermostat switch
back
|
||||||||
|
2.
|
Drive belt slipping
|
2.
|
Tighten belt
|
|||||||||
|
3.
|
Hot air leaks into car
|
3.
|
Check for holes or open
vents
|
|||||||||
|
4.
|
Plugged receiver drier
|
4.
|
Replace drier
|
|||||||||
|
5.
|
Capillary tube broken
|
5.
|
Replace expansion valve
|
|||||||||
|
6.
|
Shortage of refrigerant
|
6.
|
Add refrigerant
|
|||||||||
|
|
7. High head pressure
|
|
|
7. See problem #2
|
||||||||
|
|
8. Low suction pressure
|
|
|
8. See problem #3
|
||||||||
|
|
9. High suction pressure
|
|
|
9. See problem #4
|
||||||||
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|
Defective expansion valve
|
10.
|
Replace expansion valve
|
|
|
11.
|
Frozen expansion valve
|
11.
|
Evacuate and replace drier
|
|
|
|
|
|
Repair
Frozen Evaporator Coil
1. Replace
thermostat
1. Faulty
thermostat
2. Set to
driving condition
2. Thermostat
not set properly
3. Check for
excessive duct hose length,
3.
Insufficient evaporator air
kink or
bend.
AC System Gauge Readings
The
following table is a general guideline for A/C system pressures and
temperatures based on ambient outside temperature. Remember that these are a
guideline and your actual temperatures and pressures will vary depending on
humidity in the air and the condition of your system.
A/C System Pressure Readings
|
Ambient Temperature
|
Low Side Pressure
|
High Side Pressure
|
Center Vent Temperature
|
|
|
|
|
|
|
60 °F
|
28‐38 psi
|
130‐190 psi
|
44‐46 °F
|
|
|
|
|
|
|
70 °F
|
30‐40 psi
|
190‐220 psi
|
44‐48 °F
|
|
|
|
|
|
|
80 °F
|
30‐40 psi
|
190‐220 psi
|
43‐48 °F
|
|
|
|
|
|
|
90 °F
|
35‐40 psi
|
190‐225 psi
|
44‐50 °F
|
|
|
|
|
|
|
100 °F
|
40‐50 psi
|
200‐250 psi
|
52‐60 °F
|
|
|
|
|
|
|
110 °F
|
50‐60 psi
|
250‐300 psi
|
68‐74 °F
|
|
|
|
|
|
|
120 °F
|
55‐65 psi
|
320‐350 psi
|
70‐75 °F
|
|
|
|
|
|
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