1. Fixed Orifice Tube Cycling Clutch System (FOTCC)
Topics covered in this presentation:
Basic FOTCC System Components
Basic FOTCC System Operation
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2. Introduction
The “Fixed Orifice Tube Cycling Clutch System” (FOTCC) provides an economical way of producing a vehicle Air Conditioning System (A/C).
Because the Thermal Expansion Valve (TXV) is complex and expensive to produce, some valves costing as much as the whole of the rest of the system and compressors sapping power from the engine, A/C used to be found only on high powered luxury vehicles.
Today, however, A/C is found on all types of vehicles, this is partly because smaller engines are able to handle the extra load of the compressor, using their complex engine management systems and advances in A/C control.
The TXV system works by varying the amount of refrigerant sprayed into the evaporator to control the system.
The FOTCC system works by having a constant spray of refrigerant into the evaporator and turning the compressor on and off (cycling the clutch) to control the system.
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3. Basic FOTCC System Components
Component and layout used with a FOTCC system.
Condenser
Evaporator
Clutch cycling pressure switch
Compressor
Fixed orifice tube
Suction accumulator
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4. Basic FOTCC System Operation - 1
Compressor starts and creates high and low pressure sides in the FOTCC system.
High pressure
Air flow
High pressure vapour refrigerant flows from the pump to the condenser.
Vapour is cooled within the condenser, turning to a liquid. High pressure liquid leaves the condenser.
The fixed orifice tube restricts the flow of the high pressure liquid.
The high pressure hose has a service valve connection for a pressure gauge.
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5. Basic FOTCC System Operation - 2
High pressure liquid flows into the fixed orifice tube and enters the evaporator as a vapourised liquid.
Air flow
The vapourised liquid within the evaporator absorbs heat energy, and most of the refrigerant turns to low pressure vapour.
The vapour is then sucked into the accumulator/ drier by the compressor, where any moisture is removed.
The low pressure refrigerant vapour is then sucked into the compressor and pumped on to the condenser. The cycle repeats.
Low pressure
Accumulator
The low pressure hose has a service valve connection for a pressure gauge.
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6. Fixed Orifice Tube Construction
Evaporator core
Fixed orifice tube
High pressure
refrigerant liquid
Internal diameter of fixed orifice
Output filter
Input filter
Low pressure refrigerant spray
O rings
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7. Fixed Orifice Tube Operation
The fixed orifice tube provides a restriction against liquid refrigerant flow, builds pressure in the condenser and lowers pressure in the evaporator.
It also meters the flow of refrigerant spray into evaporator. If flow is not metered, the following may occur:
Starved Evaporator
If refrigerant flow is too low, all of it is vapourised before it reaches the evaporator outlet.
Cooling is insufficient, because not all surrounding heat is absorbed.
Flooded Evaporator
If refrigerant flow is too high, less vapourisation takes place because there is no room for complete vapourisation and expansion.
Cooling is insufficient, because of less vapourisation.
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8. Cycling Clutch Pressure Switch Construction
Suction accumulator/drier
Supply from function selector switch
Feed to compressor magnetic clutch field coil
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9. Cycling Clutch Pressure Switch Operation
Uncontrolled, the A/C system will run continuously, and temperature in the evaporator will drop below freezing. Ice forming on the fins will block airflow through the evaporator.
The pressure switch controls evaporator temperature at just above freezing, stopping ice forming.
The switch opens and closes in response to refrigerant pressure changes in the suction accumulator.
The switch opens at suction pressure below 1.5 to 1.7 BAR, depending on the system.
Switch remains open until pressure rises above 3 BAR.
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10. Suction Accumulator/Drier Construction
Pressure switch fitting
From evaporator
To compressor
Plastic cap
Vapour return tube
Desiccant bag
Refrigerant oil
Oil filter
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11. Suction Accumulator/Drier Operation
Refrigerant from evaporator is mostly vapour with small amounts of liquid.
Inlet from evaporator
Outlet to compressor
Plastic cap
Refrigerant enters from top, vapour refrigerant swirls around plastic cap, and exits to compressor via vapour return tube.
Vapour return tube
The heavier liquid refrigerant drops to bottom and then has a second chance to vapourise.
Desiccant bag
Oil mixes with vapour in the vapour return tube to lubricate system. The desiccant bag removes moisture from the system.
Refrigerant oil
Oil filter
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12. Compressor Clutch Components
Pulley
Circlip
Lock nut
Nut
Clutch field coil
Bearing
Shims
Hub
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13. Compressor Clutch Assembly
Pin
Pulley
Circlip
Coil supply
Nut
Shims
Lock nut
Hub
Coil
Compressor
Bearing
Clutch field coil locks onto compressor with circlip. It is prevented from turning by a pin in the end of the compressor. The coil has two supply wires.
Clutch pulley is fitted to a bearing on the compressor shaft, and rotates freely.
The clutch hub is keyed and locked onto the compressor shaft with a nut.
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14. Compressor Clutch Operation - 1
Final assembly is shown opposite. A drive belt is attached to the pulley.
Pulley
When the engine is started, the drive belt rotates the pulley.
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15. Compressor Clutch Operation - 2
When the coil is energized the hub is pulled toward the pulley by the resulting magnetic field.
The magnetic force locks the hub and pulley together, and the compressor shaft rotates.
When the coil is de-energized, the hub is pulled away from the pulley by springs, or a rubber bush.
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