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

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). Next >

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

Basic FOTCC System Components Component and layout used with a FOTCC system: Condenser Evaporator Clutch cycling pressure switch Compressor Suction accumulator Fixed orifice tube Next >

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

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 Next >

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

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 Next >

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. Next >

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

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 Next >

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

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. Next >

Question 6 Above what pressure, in bar, will a typical cycling clutch pressure switch close? 45 Bar Correct Answer = 45

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 Next >

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

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 Next >

Question 8 Which is the longer tube inside a suction accumulator drier? A) Inlet from evaporator B) Vacuum return tube Correct Answer = B

Compressor Clutch Components Pulley Circlip Lock nut Clutch field coil Nut Bearing Shims Exploded view of the compressor clutch components. Hub Next >

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. Next >

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

Summary You should now be aware of: Basic FOTCC System Components Basic FOTCC System Operation End >