Fixed Orifice Tube and Thermal Expansion Valve Air Conditioning

CDX Diesel HVAC

Module 2: Fixed Orifice Tube and Thermal Expansion Valve Air Conditioning

ÂÂFixed Orifice Tube Air-Conditioning System ÂÂControl Devices ÂÂThermostatic Expansion Valve System ÂÂThermal Expansion Valves Fixed Orifice Tube Air-Conditioning System ready for review

Fixed orifice tube system components Boiling and evaporating • Condenser • A drop in pressure causes the low-pressure, low- • Evaporator temperature liquid to boil. • Compressor • Liquid mist evaporates rapidly in the evaporator • Orifice tube coils. • Accumulator • Absorbs heat from air passing across the external • Connecting pipes and hoses fins • Thermostat Cooled air to the vehicle interior • Blower fans • Cooled air is directed to the interior of the vehicle • Pressure switches by blower fans. Orifice tube • Refrigerant in the evaporator flows on to the • Located between the condenser and the evaporator accumulator. • Provides the restriction to flow necessary for Accumulator system operation • Acts as a storage volume for the refrigerant Switching on the air-conditioning system • Separates the liquid from the vapor • The electromagnetic clutch is energized on the Refrigerant vapor enters the accumulator. compressor drive plate. • Enters at the top of the accumulator • The compressor is driven by the engine crankshaft. • Any liquid refrigerant falls to the bottom. Flow of vaporized refrigerant Final evaporation • Drawn from the low-pressure side of the system by • The accumulator is made from aluminum and has a the compressor large surface area. From the compressor to the condenser • Assists the final evaporation of any liquid • Discharged from the compressor as high-pressure, refrigerant leaving the evaporator high-temperature vapor Drawn through the outlet tube • Flows to the condenser • Refrigerant vapor is drawn off from the top of • Cooled by air flowing across the condenser coils the accumulator through the outlet tube by the and fins compressor. Condensation Hole in the outlet tube • A reduction in temperature causes the refrigerant • Small hole at the bottom of the outlet tube vapor to condense. • For small amounts of lubricating oil that settle at • Turns into liquid, which flows to the filter and then the bottom of the accumulator to the orifice tube • Drawn to the compressor Loss of pressure Accumulator filters and desiccants • Liquid refrigerant loses pressure by flowing • The accumulator contains filters and desiccants. through the fine tube. • Removes impurities and moisture from refrigerant • Forced to expand suddenly by passing through the passing through nozzle • Enters a larger volume of the evaporator

Control Devices ready for review

Purpose of control devices Cycling • To ensure maximum efficiency • Cycling adjusts the system to the heat load. • To protect components from damage High-pressure switch system protection Thermostat • System protection is needed against high • Used to sense the temperature of evaporator fins pressures. • Helps ensure that the temperature does not drop • Provided by the high-pressure switch below 33.8°F (1°C) • Mounted in the high-pressure line between the • If it drops to 32°F (0°C) or less, moisture condenser and the orifice tube condensing out of the air will freeze on the unit. High temperatures and pressures • Will block the flow through the evaporator • May develop if the condenser is unable to transfer When the temperature of the fins drops its heat effectively to the outside air • The fins of the evaporator drop to 33.8°F (1°C). • May be due to blockage of the condenser fins or • The thermostat contacts open. an inoperative electric fan • Breaks the electrical circuit to the compressor High-pressure switch clutch • If this occurs, the high-pressure switch open- • Stops the compressor from circulating refrigerant circuits the compressor clutch. When the temperature rises • Prevents the compressor from circulating • The evaporator temperature rises to about 39.2°F refrigerant (4°C). If a leak develops • The thermostat contacts close. • A leak may develop and refrigerant is low in • The electrical circuit to the compressor clutch is charge. restored. • The system pressure falls below a set value. Heat load increases. • The low-pressure switch on the low-pressure side • The fixed orifice tube cannot alter the amount of of the system turns the compressor off. refrigerant entering the evaporator. Lubricating oil • Therefore, extra cooling is required and heat load • A lack of lubricating oil can damage the on the system increases. compressor. • The compressor cycles on for longer periods. • Carried around the system by refrigerant Heat load decreases. • Prevents damage to the compressor • As heat load decreases, the compressor cycles off for longer.

Thermostatic Expansion Valve System ready for review

Components of the system • Also provides for the complete vaporization of all • Condenser liquid refrigerant entering the evaporator • Evaporator Refrigerant and lubricating oil • Compressor • The system is charged with refrigerant. • Receiver drier • Also charged with lubricating oil • Thermostatic expansion valve • Circulates with refrigerant at all times • Connecting pipes and hoses The air-conditioning system is switched on. • Thermostat • The electromagnetic clutch is energized on the • Blower fans compressor drive plate. • Pressure switches • The compressor is driven by the engine crankshaft. Thermostatic expansion valve Vaporized refrigerant is drawn in. • Also called a TX valve • Drawn in from the low-pressure side of the system • Located at the entry to the evaporator by the intake side of the compressor • Provides a throttling or restricting function to Discharged from the compressor control the quantity of refrigerant entering the • Discharged from the compressor as high-pressure, evaporator high-temperature vapor

• Flows to the condenser Receiver drier • Cooled by air flowing across the condenser coils • Acts as a storage volume for liquid refrigerant and fins • Contains a filter and desiccant (drying material) Condensation and flow to the receiver drier Flow to reach the pick-up tube and TX valve • A reduction in temperature causes refrigerant • Liquid refrigerant passes through the filter and vapor to condense. desiccant. • Turns into liquid, which flows to the receiver drier • Reaches the pick-up tube • Flows on to the TX valve

Thermal Expansion Valves ready for review

TX valve Changes in pressure • Determines the amount of liquid refrigerant flow • Reduces pressure in chamber “B” • A spring-loaded valve is controlled by different • Allows pressure in chamber “A” to move the valve pressures on each side of the connecting away from its seat by action of the diaphragm diaphragm. • High-pressure liquid flows through the orifice. Pressure in the chamber “A” side of the valve • Enters the evaporator as low-pressure liquid • Determined by the temperature sensing bulb Liquid expansion and then vaporization • The bulb is taped to the evaporator outlet. • Liquid expands into the larger volume of the • Insulated from air temperatures evaporator. Sensing bulb • Causes a reduction in its pressure • Filled with refrigerant • Heat from the air passes over the evaporator fins. • Connected to chamber “A” by the capillary tube • Both actions cause liquid to vaporize. • The temperature at the evaporator outlet Low temperature of refrigerant leaving the determines the pressure in the bulb. evaporator Higher temperatures • Interpreted by a sensing bulb attached to the • Will give higher pressures outlet • Will tend to open the TX valve wider • If the temperature is low, refrigerant in the sensing • Allow more refrigerant to flow through bulb will contract. Lower temperatures • Pressure in chamber “A” of the TX valve will • Will give lower pressures reduce. • Will tend to close the TX valve Diaphragm movement • Reduce the flow of refrigerant • The diaphragm moves and the valve moves Pressure in the chamber “B” side of the valve towards the valve seat. • Determined by the refrigerant pressure in the • Reduces the quantity of refrigerant entering the evaporator evaporator • Also determined by the force supplied from the High temperature of refrigerant leaving the valve spring evaporator • The spring is often called the superheat spring. • Refrigerant in the sensing bulb expands. Superheat spring • Exerts more pressure in chamber “A” • Designed to ensure that the temperature of • Causes the valve to move away from its seat refrigerant leaving the evaporator is between • Allows more refrigerant to enter the evaporator 35.6°F to 48.2°F (2°C to 9°C) higher than Amount of refrigerant flow the boiling point of refrigerant at the current • Depends on the quantity of heat to be removed operating pressure from air passing over the evaporator fins Superheat • More heat means more refrigerant is required to • The difference in temperature is called remove it. “superheat.” • Less heat means less refrigerant is required. • Ensures that all liquid refrigerant entering the Control devices evaporator is vaporized before leaving • Fitted into the system When the compressor switches on • Ensure maximum efficiency • Suction from the compressor removes refrigerant • Operate as described in the section on the fixed vapor from the evaporator. orifice system • Lowers its pressure