System pressure and temperature controls
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System Pressure and Temperature controls. Evaporator freezing. Evaporator temperature cannot be allowed to drop below 32 deg. F. There is always condensation on the surfaces of objects that are cooler than the dew point of the air surrounding them.

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System Pressure and Temperature controls

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System pressure and temperature controls

System Pressure and Temperature controls


Evaporator freezing

Evaporator freezing

  • Evaporator temperature cannot be allowed to drop below 32 deg. F.

  • There is always condensation on the surfaces of objects that are cooler than the dew point of the air surrounding them.

  • In normal operation the evaporator surface is covered with liquid water.


Evaporator freezing1

Evaporator freezing

  • If the temperature of the evaporator drops below 32 deg. F this water will turn to ice.

  • Ice on the surface of the evaporator will block to flow of air through the evaporator.

  • Even though the evaporator is now a solid block of ice the temperature of the air inside the car will be uncomfortably warm.


Preventing evaporator icing

Preventing evaporator icing

  • There are two ways to prevent the evaporator from icing.

    • Cycling the compressor on and off to control evaporator temperature

    • Regulating the evaporator pressure

      • Evaporator pressure is controlled by:

        • A pressure regulating valve

        • A variable displacement compressor


What s wrong with cycling the compressor

What’s wrong with cycling the compressor ?

  • Each time the compressor turns on there is an audible click that can be heard in the cabin.

  • If the engine is idling the rpm will drop momentarily and the engine may appear to run rough for a second or two.

  • Manufactures of luxury cars prefer A/C designs that run the compressor constantly whenever the A/C system is turned on due to the perception of smooth and quiet operation.


Preventing liquid refrigerant entering the compressor

Preventing liquid refrigerant entering the compressor

  • The compressor will be destroyed if liquid refrigerant is allowed to enter it.

  • There are two methods of preventing liquid entering the compressor.

    • Using an expansion valve to throttle the amount of liquid entering the evaporator

    • Using an accumulator as a liquid trap between the evaporator and compressor.


Expansion valve

Expansion valve

  • The bulb, capillary tube and diaphragm chamber are filled with a refrigerant [usually R-22] that expands as it is heated.

  • The pressure of this fluid pushing on the diaphragm lifts the spring loaded ball off of its seat opening the orifice.

  • The thermal bulb is clipped onto the evaporator outlet and wrapped with butyl tape to insulate it from the surrounding air.

Capillary tube

Thermal Bulb

Diaphragm

Pin

Ball

Spring


Expansion valve1

Expansion valve

Bulb

Vapor – to compressor

  • The expansion valve opens the orifice when the temperature of the evaporator outlet is a few degrees above the design temperature limit.

  • When the temperature of the outlet falls below the design limit the expansion valve closes.

Capillary tube

Expansion Valve

Liquid – from condenser


Expansion valve2

Expansion valve

  • When the temperature at the outlet is colder than the design limit the fluid in the bulb contracts, reducing the pressure on the diaphragm.

  • The spring pushes the ball into it’s seat – closing the valve.

  • With the flow of refrigerant through the evaporator stopped any liquid in the evaporator will remain there.

Capillary tube

Bulb

Diaphragm

Pin

Ball

Spring


Expansion valve with equalizer line

Expansion valve with equalizer line

  • Some expansion valves have an equalizer line that connects the underside of the diaphragm to the outlet of the evaporator.

  • This compensates for difference in pressure between the evaporator inlet and outlet when there is a lot of liquid refrigerant and oil flowing through the evaporator.

Vapor – to compressor

Equalizer line

Liquid – from condenser


Expansion valve with equalizer line1

Expansion valve with equalizer line

Equalizer line

Evaporator outlet

Note: Butyl tape normally wrapped around thermal bulb has been removed for photography

Expansion valve


Thermal cycling switch

Thermal cycling switch

To A/C switch

Thermal cycling switch

  • The thermal cycling switch prevents evaporator freeze-up by turning off the compressor when the evaporator core temperature approaches 32 deg. F.

Bellows

Capillary tube for thermal cycling switch is clipped onto the evaporator core

Sight glass

Constant displacement type compressor

Receiver/drier


Accumulator

Accumulator

Splash shield

  • The accumulator is a liquid trap that is installed between the evaporator and compressor.

  • Accumulators are used on systems that have an orifice tube instead of an expansion valve.

Vapor and liquid from evaporator

Service fitting

Desiccant bag

Vapor – to compressor


Ccot system pressure controlled

CCOT System – pressure controlled

To A/C switch

  • The compressor cycling switch on the accumulator turns the compressor off when pressure falls below 27 psi.

To radiator fan

Clutch cycling switch

Constant displacement type compressor


Pressure actuated cycling switch

Pressure actuated cycling switch

  • Compressor cycling switches are normally found on older A/C systems that are not control by a computer.

  • The advantage of a pressure actuated cycling switch is that if the systems leaks and all the refrigerant is lost the compressor will be automatically shut down.

  • The switch is often mounted to a Schrader valve fitting on the accumulator. The Schrader valve allows the switch to be replaced without having to evacuate and recharge the system.


Temperature actuated cycling switch

Temperature actuated cycling switch

  • Older orifice tube A/C systems sometimes used an electromechanical temperature actuated switch to cycle the compressor on and off.

  • Electromechanical switches use a small bellows connected to a capillary tube filled with R-22.

  • The capillary tube is clipped to the evaporator core surface.

  • When evaporator temperature rises the R-22 pressure builds causing the bellows to expand.

  • The expansion of the bellows closes an electrical contact the triggers the compressor relay.


Thermal cycling switch1

Thermal cycling switch

To A/C switch

To radiator fan

Thermal cycling switch

  • This type of system is controlled by the evaporator temperature via an electro-mechanical switch.

Bellows

Capillary tube

Constant displacement type compressor


Vdot variable displacement orifice tube

VDOT – Variable displacement orifice tube

To A/C switch

  • In the VDOT system the A/C compressor runs constantly whenever the A/C switch is turned on.

  • When evaporator pressure drops below 27 psi the compressor shaft still turns but the pistons do not move in their bores.

To radiator fan

Variable displacement type compressor


Evaporator pressure control

Evaporator pressure control

  • To keep the A/C system quiet manufactures prefer that the A/C compressor run continuously and not cycle on and off.

  • Prior to the introduction of variable displacement compressors an evaporator pressure control valve allowed the compressor to run continuously while the pressure in the evaporator never dropped below 27 psi.


Evaporator pressure control1

Evaporator pressure control

  • A valve was placed between the evaporator outlet and the compressor inlet that closed when the pressure in the evaporator core dropped below 37 psi.

  • Without the suction from the compressor evaporator pressure would build up – opening the valve when the pressure exceeded 27 psi.

  • An aneroid inside the valve controlled the opening and closing of the valve.


Evaporator pressure control2

Evaporator pressure control

Aneroid

  • The aneroid is a sealed brass bellows that contains partial vacuum.

  • When the surrounding pressure is high it contracts.

  • When the surrounding pressure is low it expands.

From evaporator

To compressor

Valve


Expansion valve w evaporator pressure control

Expansion valve w/ evaporator pressure control

To A/C switch

Evaporator pressure control valve – prevents evaporator freeze-up

Sight glass

Constant displacement type compressor

Expansion valve – prevents liquid refrigerant from leaving the evaporator

Receiver/drier – holds a reserve of liquid refrigerant and absorbs moisture


Terminology

Terminology

  • The names and design details of evaporator pressure control system varies but they all do basically the same thing.

  • They prevent evaporator freeze-up yet allow the compressor to run constantly.

  • They can be called:

    • EPR [Evaporator Pressure Regulator]

    • POA [Pilot Operated Absolute]

    • VIR [Valves in receiver]

    • STV [Suction Throttling Valve]

    • H-valve

      • This valve gets its name from the layout of the valve. It looks like the letter ‘H’.


H valve

H-valve

Evaporator pressure control valve

  • The H-Valve is a combination of an expansion valve and an evaporator pressure control valve.

Expansion valve

From receiver/drier

  • The valve is normally located on the firewall where the lines for the evaporator pass through the firewall.


H valve1

H-valve

  • The H-valve is a convenient place to mount pressure switches and service fittings.

  • H-valves that do not contain an evaporator pressure control valve are often used on cycling clutch systems.


H valve2

H-valve

  • The H-valve is connected the evaporator core on the engine compartment side of the fire wall.

  • This greatly simplifies service and replacement of the A/C hoses and expansion valve.


Electronic compressor control

Electronic compressor control

  • The A/C compressor in most modern cars is controlled by the BCM [Body Control Module].

  • Manufactures may use other names for the module but the BCM controls most of the system in the vehicle that are not related to the drive train.

  • The BCM normally controls:

    Lights

    Horn and wipers

    Door locks and windows

Heating and air conditioning

Power seats

Power mirrors etc.


Bcm controlled compressor

BCM controlled compressor

PCM

BCM

To A/C switch

MAP

TPS

CTS

High side pressure sensor

Low side pressure sensor


Electronic compressor control1

Electronic compressor control

  • Electronic control systems use pressure sensors instead of switches.

  • Pressure sensors use a piezoresistive semiconductor element that changes electrical resistance as pressure on the element increases and decreases.

  • The BCM sends a 5 volt reference signal to the sensor and monitors the drop in voltage as the electric current passes through the sensor.

  • Most electronic systems monitor the pressure on both the high side and the low side.


Pcm data

PCM data

  • The BCM is networked to the PCM [Power train Control Module] so that data can be exchanged.

  • If the PCM indicates that the engine is overheating the BCM will shut down the compressor to reduce the heat load at the condenser.

  • If the PCM sees data from the MAP and TPS sensor that indicates the engine is running at full power the BCM will shut down the compressor so that maximum power is available at the wheels.

  • The BCM tells the PCM the compressor is about to be turned on so that the PCM can open the throttle slightly in anticipation of the additional load.


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