1. Fixed Orifice Tube Cycling Clutch System (FOTCC)
Topics covered in this presentation:
Basic FOTCC System Components
Basic FOTCC System Operation

2. Introduction
Air conditioning systems (A/C) used to be only found on high powered luxury vehicles.
Today, they are common place, because of engine management systems and advances in A/C control.
Fixed orifice tube
Two main types:
Thermal Expansion Valve (TXV) System (complex, expensive).
Fixed Orifice Tube Cycling Clutch (FOTCC) System (economical).
The TXV system monitors temperature to control the amount of refrigerant sprayed into the evaporator.
The FOTCC system monitors pressure to control the amount of refrigerant sprayed into the evaporator. This is performed by switching the compressor on and off (cycling the clutch).
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3. Question 1
The TXV control of an air conditioning system is not as popular now as the FOTCC method. Why is this?
A) TXV is more expensive to produce
B) TXV does not offer as good control
C) TXV has to be switched on and off frequently
Correct Answer = A
D) TXV only works on high powered luxury vehicles

4. Basic FOTCC System Components
Component and layout used with a FOTCC system:
Condenser
Evaporator
Clutch cycling pressure switch
Compressor
Suction accumulator
Fixed orifice tube
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5. Question 2
Where is the fixed orifice tube fitted in an air conditioning system?
A) Between the clutch pressure switch and the evaporator
B) Between the compressor and the condenser
C) Between the condenser and the evaporator
Correct Answer = C
D) Between the clutch pressure switch and the compressor

6. Basic FOTCC System Operation
Compressor pumps vapour, creating high and low pressure sides.
Air flow through condenser changes high pressure vapour to liquid.
Air flow through heat exchanger (condenser)
High pressure
side
Compressor
Air flow through heat exchanger (evaporator)
FOTCC valve restricts liquid flow into evaporator.
The low pressure evaporator liquid absorbs heat from vehicle interior and changes back to vapour.
Low pressure side
FOTCC valve
Vapour is sucked into accumulator where moisture is removed. Cycle repeats.
Accumulator
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7. Question 3
Which of the following sections of an a/c system will be at low pressure?
A) Between the compressor and the condenser
B) Between the condenser and the fixed orifice tube
C) Between the compressor and the fixed orifice tube
Correct Answer = D
D) Between the evaporator and the accumulator

8. Fixed Orifice Tube Construction
Evaporator core
Fixed orifice tube
High pressure
refrigerant liquid
Internal diameter of fixed orifice
Input filter
Output filter
O rings
Low pressure refrigerant spray
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9. Fixed Orifice Tube Operation
The fixed orifice tube restricts the refrigerant flow. This builds pressure in the condenser and lowers pressure in the evaporator.
The fixed orifice tube meters refrigerant flow into evaporator.
Fixed Orifice Tube - Incorrect Metering Problems
Starved Evaporator
Refrigerant flow is too low. It has vapourised before it reaches the evaporator outlet.
Cooling is insufficient.
Flooded Evaporator
Refrigerant flow is too high. There is less vapourisation.
Cooling is insufficient.
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10. Question 4
Which of the following is NOT a function of the fixed orifice tube?
A) To restrict the refrigerant flow
B) To produce insufficient cooling
C) To raise the pressure in the condenser
Correct Answer = B
D) To lower the pressure in the evaporator

11. Cycling Clutch Pressure Switch Construction
Pressure switch controls the system by switching the compressor on and off.
Suction accumulator/drier
Cycling clutch pressure switch
Feed to compressor magnetic clutch field coil
Supply from function selector switch
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12. Question 5
Which property causes the cycling clutch pressure switch to turn on and off?
A) Temperature
B) Humidity
C) Pressure
Correct Answer = C
D) Flow rate

13. Cycling Clutch Pressure Switch Operation
If the A/C system runs continuously, evaporator temperature will drop below freezing and ice will form on the fins, blocking airflow.
The switch opens and closes (switches the compressor on and off) in response to refrigerant pressure changes.
Typically, it opens at suction pressures below 1.5 to 1.7 bar.
Typically, it closes at pressures above 3 bar.
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14. Question 6
Above what pressure, in bar, will a typical cycling clutch pressure switch close?
45 Bar
Correct Answer = 45

15. Suction Accumulator/Drier Construction
Pressure switch fitting
Cross section view of a typical accumulator/drier.
From evaporator
To compressor
Plastic cap
Vapour return tube
Desiccant bag
Refrigerant oil
Oil filter
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16. Question 7
What is normally found in the very bottom of a suction accumulator/drier?
A) Desiccant
B) Vapour
C) Water
Correct Answer = D
D) Refrigerant oil

17. Suction Accumulator/Drier Operation
Refrigerant from evaporator is mostly vapour, it enters the drier from the top.
Inlet from evaporator
Outlet to compressor
Plastic cap
Vapour swirls round the plastic cap and exits via the vapour return tube.
Vapour return tube
Liquid refrigerant drops to the bottom.
Desiccant bag
Oil mixes with vapour to lubricate system.
Refrigerant oil
Oil filter
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18. Question 8
Which is the longer tube inside a suction accumulator drier?
A) Inlet from evaporator
B) Vacuum return tube
Correct Answer = B

19. Compressor Clutch Components
Pulley
Circlip
Lock nut
Clutch field coil
Nut
Bearing
Shims
Exploded view of the compressor clutch components.
Hub
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20. Compressor Clutch Assembly & Operation
Coil
Circlip
Pin
Coil supply
Bearing
Pulley
Lock nut
Hub
Nut
Shims
The clutch field coil locks onto the compressor with a circlip.
The clutch pulley is fitted to a bearing on the compressor shaft.
Hub is locked onto the compressor shaft with a nut.
When engine is started, the pulley rotates.
When the field coil is energized, the hub is pulled toward the pulley.
They then lock together and the compressor rotates.
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21. Question 9
What causes the compressor to engage with the hub, when the engine is rotating?
A) Energizing the field coil
B) Rotating the pulley
C) Fitting the clutch pulley
Correct Answer = A
D) Locking the hub onto the compressor shaft

22. Summary You should now be aware of: Basic FOTCC System Components
Basic FOTCC System Operation
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