INTRODUCTION

TWO BASICTYPES

THERMALEXPANSION VALVE(old school)

Uses a Receiver/Drier

ORIFICE TUBEAKA:CyclingClutchOrifice Tube

Uses anaccumulator

The high and low pressure sides of an A/C system are divided by the compressor (where the pressure is increased) and either a TXV or an OT

(where the pressure drops).

Refrigerant changes state from a liquid to a vapor as it absorbs heat in the low side and into a liquid as it loses heat

in the high side.

HIGH PRESSURE

LOW PRESSURE

Refrigerant boils or evaporates (Gas) in the low side and it condenses (Liquid) in the high side.

In an operating system, you can identify the low and high sides by: Pressure (depending on type of system & ambient temp)

High (175 psi) Low (30 psi )

Location High (out) Low (in)

Temperature High (hot) Low (cold)

Tubing size High (small) Low (large)

LOW-SIDE OPERATION

When the A/C system is in full operation, the goal of most systems is to maintain an evaporator temperature just above the freezing point of water, 32°F (0°C).

This temperature produces the greatest heat exchange without ice formation on the evaporator fins (evaporator icing significantly reduces the heat transfer).

The cold temperature in the evaporator is produced by boiling the refrigerant (lowering pressure).

Remember that R-12 and R-134a have very low boiling points, well below 0°F, and that when a liquid boils, it absorbs a large amount of heat, the latent heat of vaporization.

To produce cooling, liquid refrigerant must enter and boil inside the evaporator.

The amount of heat an evaporator absorbs is directly related to the amount of liquid refrigerant that boils inside it

As liquid refrigerant enters the evaporator, the boiling point will try to drop as low as 32°F because of the drop in pressure. The cold temperature causes the refrigerant to absorb heat from the air circulated through the evaporator.

If the proper amount of refrigerant enters the evaporator, it has a slight superheat as it leaves.

A starved condition, in which not enough refrigerant enters the evaporator, does not produce as much cooling.

If too much refrigerant enters, the evaporator floods because the refrigerant will not all boil.

The low side begins at the TXV and includes the evaporator, receiver/drier and suction line to the compressor.

The OT system is the same but has an accumulator

A TXV is controlled by the pressure on the diaphragm from the heat-sensing tube, the pressure spring, and evaporator pressure through the equalizer pipe.

EXPANSION DEVICES

An H-type valve is essentially the same except evaporator pressure goes through an internal passage to the bottom of the diaphragm.

SUCTION THROTTLING VALVE ON TXV

Some systems use a suction throttling valve to keep evaporator pressure from dropping to the point at which icing can occur.

ORIFICE TUBE

An OT is a simple restriction that limits the flow of refrigerant into the evaporator. The locating dimple keeps the OT from moving

downstream.

Two views of a typical OT system; (a) is somewhat realistic and (b) is schematic. Both show the arrangement of the components and the refrigerant flow.

EVAPORATOR

Each type has a large contact area for heat to leave the air and enter the refrigerant.

TUBE & FIN PLATE

Accumulators are designed so that vapor from the top leaves to the compressor. They contain desiccant to absorb water from the refrigerant and many include a fitting for low-side pressure and the clutch cycling switch.

Water in an A/C system can combine with refrigerant to form acids. These acids can etch and dissolve components, cause rusting of metal parts, and cause ice blockage at the

expansion device.

An automotive A/C system has the potential to lose refrigerant through hoses, the compressor shaft seal, and line fittings

A system with the proper charge has the receiver–drier (a) or the accumulator (b) about half full of liquid

A properly charged system has the condenser filled with condensing vapor and some liquid, a liquid line filled with liquid, a receiver–drier

about half full of liquid, and an evaporator with vaporizing liquid

An overcharge with too much liquid causes liquid to partially fill the condenser

An undercharge has vapor in the liquid line and a starved evaporator

The compressor clutch allows us to cycle the compressor off and on to control evaporator temperature and to shut the

system off

Most TXV systems use a thermal switch to cycle the compressor out when the evaporator gets too cold. Most OT systems use a pressure switch to cycle the

compressor out when the low-side pressure drops too low.

SUCTION THROTTLING VALVE

A suction throttling valve (STV) stops evaporator pressure from dropping below 30 psi, and this keeps ice from forming on the evaporator.

HOT GAS BY-PASS

A hot gas bypass system diverts high-side pressure into the evaporator to keep the pressure from dropping to the point at which

icing can occur.

HIGH-SIDE OPERATION

The high side of an A/C system takes the low-pressure vapor from the evaporator and returns high-pressure liquid to the expansion device.

To do this, the compressor must raise the pressure and concentrate the heat so that the vapor temperature is above ambient.

This causes heat to flow (exchange) from the refrigerant to the air passing through the condenser.

Removing the latent heat from the saturated vapor causes it to change state, to a liquid.

HIGH-SIDE OPERATION

Compressor Crank Piston Rotary Piston Vane Scroll Electric or Belt Driven

Condensers Receiver–Drier High-Pressure Control

CRANKSHAFT PISTON COMPRESSOR(OLD SCHOOL)

REED VALVE

IN OUT

ROTARY CRANK

WOBBLE PLATE PISTON

VARIABLE DISPLACEMENT

When the evaporator cools and low-side pressure drops, the piston stroke of a variable displacement compressor is reduced so that compressor output matches the cooling load.

PISTONLESS WOBBLE PLATE

VANE

VANE MOVEMENT

SCROLLA cutaway view of a scroll compressor.

Note that one scroll is secured to the housing and the other can be moved through its orbit by the drive shaft.

SCROLL OPERATION

Like and Auger or drill bit

ELECTRIC DRIVEN

CAN BE SAME TYPES AS BELT DRIVEN COMPRESSORS

CONDENSERA condenser is a heat exchanger that transfers heat from the refrigerant to the air flowing through it. Typically located in front of radiator

REFRIGERANT FLOW

Refrigerant follows a winding path through a serpentine condenser.

Refrigerant follows a back-and-forth path through a parallelflow condenser.

LIQUID EXPANSION

The volume of gas that enters a condenser is about 1,000 times the volume of liquid leaving it.

DUAL CONDENSER

refrigerant flows from the condenser portion through the modulator/receiver–drier portion and then through the subcooling portion.

RECEIVER DRIER

The outlet of a receiver–drier is close to the bottom so liquid flows on to the TXV. Many units include a sight glass so we can observe this flow.

HIGH PRESSURE RELIEF VALVE•A high-pressure relief valve contains a strong spring that keeps the valve closed unless high-side pressure forces it open & the valve closes when the pressure drops

• The fusible plug contains a meltable metal insert that will blow out if pressure gets too high. (no reset)

•Accumulator has no need for valve

LINES AND HOSES

The various system components must be interconnected so that refrigerant can circulate through the system. In modern vehicles, the majority of the lines are metal; hose is used only where flex is necessary.

Both flexible rubber and rigid metal hoses are used to link the components.

The connections to the compressor must be flexible to allow for engine and compressor movement.

A refrigerant hose contains one or two reinforcing braid layers around the rubber tube .

A barrier hose includes an impervious nylon layer to reduce leakage

THE THREE MAJOR HOSES/LINES ARE THE DISCHARGE, LIQUID, AND SUCTION LINES. MANY SYSTEMS HAVE TWO LIQUID LINES.

Typical hose sizes.

Most systems use three of these four(6,8 &10 or8,10 & 12)

Various types of fittings are used to seal the refrigerant line connections.

The service fitting is used for metal line repairs or to insert an inline filter.

Automotive Heating and Air Conditioning, Fifth EditionBy Tom Birch

© 2010 Pearson Higher Education, Inc.Pearson Prentice Hall - Upper Saddle River, NJ 07458