OBJECTIVES After studying Chapter 7, the reader should be able to: Understand the relationship between the A/C cycle and the components on the low side and high side of a system. Identify a thermal expansion valve and orifice tube system. Understand the functions for the low side and high side of a system. Explain the role of each A/C component.

INTRODUCTION The automotive A/C system uses the physical principles described in earlier chapters to move heat from the passenger compartment to the condenser and then to the ambient air moving through the condenser. The heating system uses some of the same principles to move heat from the engine’s cooling system to the passenger compartment.

INTRODUCTION

THERMAL EXPANSION VALVE (old school) TWO BASIC TYPES Uses a Receiver/Drier

ORIFICE TUBE AKA: Cycling Clutch Orifice Tube Uses an accumulator

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

In an operating system, you can identify the low and high sides by: 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

LOW-SIDE OPERATION Expansion Devices Evaporators Accumulator Thermal Expansion Valves Orifice Tubes Evaporators Accumulator Refrigerant Charge Level Evaporator Icing Controls Cycling Clutch Systems Evaporator Pressure Controls Variable Displacement Compressors

EXPANSION DEVICES 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.

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

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 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 Condensers Receiver–Drier 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

HIGH-SIDE OPERATION FIGURE 7-23 (CONTINUED) Piston compressors can drive the piston through a crankshaft (a), Scotch yoke (b), swash plate (c), or wobble plate (d). A rotary compressor can use vanes (e) or a pair of scrolls (f). (a and e are courtesy of Toyota Motor Sales USA, Inc.; c and d are courtesy of Zexel USA Corporation)

VANE

VANE MOVEMENT

SCROLL A 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

Like and Auger or drill bit SCROLL OPERATION Like and Auger or drill bit

CAN BE SAME TYPES AS BELT DRIVEN COMPRESSORS ELECTRIC DRIVEN CAN BE SAME TYPES AS BELT DRIVEN COMPRESSORS

CONDENSER A 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 or 8,10 & 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.

LINES AND HOSES FIGURE 7-42 A double-wall heat exchanger is a tube within a tube; the inner tube is convoluted so it won’t kink as the tubing is bent.

SUMMARY Mobile HVAC systems use either an OT or TXV system with a low (pressure and temperature) and high (pressure and temperature) side. The low side has an expansion device (either a TXV or OT and accumulator), evaporator with icing controls, suction line, and compressor. The high side has compressor, condenser, receiver–drier (if a TXV is used), discharge line, and liquid line. The accumulator and receiver–drier contain desiccant and provide volume for thermal expansion. Lines can be leakproof rubber hoses or metal tubing with a variety of connections.

REVIEW QUESTIONS Automotive A/C systems use either a(n) _____ _____ or a(n) _____ _____ _____ for the expansion device. The refrigerant _____ or _____ in the low side and _____ in the high side. When it is operating, the high side will have a high _____ and _____. An accumulator is used in an OT system because of the probability of _____ the compressor with _____ refrigerant. In a TXV system, the refrigerant will have a few degrees of _____ as it leaves the evaporator. The job of the _____ in the _____ or _____ _____ is to remove all traces of water from the refrigerant.

REVIEW QUESTIONS A(n) _____ is used with OT systems and a _____ _____ is used with a TXV to hold a small refrigerant reserve. A CC system _____ the compressor _____ to prevent _____ of the evaporator. The major styles of compressors use _____, _____, or a pair of _____ for pumping members. Reed valves control the refrigerant flow in and out of the _____ of a(n) _____ compressor. In a(n) _____ condenser, the refrigerant follows a single, winding path from the top to the bottom; a _____ _____ condenser has manifolds at the sides to control the cross-flow. The refrigerant line leaving a compressor is called the _____ _____; the line from the condenser to the evaporator is called the _____ _____; and the line from the evaporator back to the compressor is called the _____ _____.

CHAPTER QUIZ Which of the following best describes the low side of a system? High pressure and high temperature High pressure and low temperature Low pressure and high temperature Low pressure and low temperature

CHAPTER QUIZ Which of the following best describes the low side of a system? High pressure and high temperature High pressure and low temperature Low pressure and high temperature Low pressure and low temperature

CHAPTER QUIZ 2. Which of the following best describes the high side of a system? High pressure and high temperature High pressure and low temperature Low pressure and high temperature Low pressure and low temperature

CHAPTER QUIZ 2. Which of the following best describes the high side of a system? High pressure and high temperature High pressure and low temperature Low pressure and high temperature Low pressure and low temperature

CHAPTER QUIZ 3. In a properly operating system, the evaporator temperature is about _____. 25°F 30°F 32°F 35°F

CHAPTER QUIZ 3. In a properly operating system, the evaporator temperature is about _____. 25°F 30°F 32°F 35°F

CHAPTER QUIZ 4. The TXV is located at the _____. evaporator outlet evaporator inlet condenser outlet condenser inlet

CHAPTER QUIZ 4. The TXV is located at the _____. evaporator outlet evaporator inlet condenser outlet condenser inlet

CHAPTER QUIZ 5. The refrigerant inside an evaporator is a _____. Gas Liquid liquid changing to a gas gas changing to a liquid

CHAPTER QUIZ 5. The refrigerant inside an evaporator is a _____. Gas Liquid liquid changing to a gas gas changing to a liquid

CHAPTER QUIZ 6. An accumulator is very similar to a(n) _____. receiver–drier Evaporator Condenser orifice tube

CHAPTER QUIZ 6. An accumulator is very similar to a(n) _____. receiver–drier Evaporator Condenser orifice tube

CHAPTER QUIZ 7. Two students are discussing orifice tubes. Student A says the OT is always placed close to the evaporator inlet. Student B says the OT is placed close to the condenser in some vehicles. Who is correct? Student A only Student B only Both A and B Neither A nor B

CHAPTER QUIZ 7. Two students are discussing orifice tubes. Student A says the OT is always placed close to the evaporator inlet. Student B says the OT is placed close to the condenser in some vehicles. Who is correct? Student A only Student B only Both A and B Neither A nor B

CHAPTER QUIZ 8. Two students are discussing an A/C problem. Student A says that an overcharge of refrigerant will cause a higher-than-normal high-side pressure. Student B says that a refrigerant overcharge will cause poor cooling. Who is correct? Student A only Student B only Both A and B Neither A nor B

CHAPTER QUIZ 8. Two students are discussing an A/C problem. Student A says that an overcharge of refrigerant will cause a higher-than-normal high-side pressure. Student B says that a refrigerant overcharge will cause poor cooling. Who is correct? Student A only Student B only Both A and B Neither A nor B

CHAPTER QUIZ 9. Most A/C systems cycle the compressor clutch in regular intervals to control _____. condenser temperature compressor pressure evaporator temperature None of these

CHAPTER QUIZ 9. Most A/C systems cycle the compressor clutch in regular intervals to control _____. condenser temperature compressor pressure evaporator temperature None of these

CHAPTER QUIZ 10. In a noncycling clutch system, evaporator freeze-up is prevented by controlling _____. TXV pressure evaporator pressure condenser pressure condenser temperature

CHAPTER QUIZ 10. In a noncycling clutch system, evaporator freeze-up is prevented by controlling _____. TXV pressure evaporator pressure condenser pressure condenser temperature

CHAPTER QUIZ 11. Two students are discussing variable displacement compressors. Student A says they are designed to control evaporator temperature. Student B says they are designed to control evaporator pressure. Who is correct? Student A only Student B only Both A and B Neither A nor B

CHAPTER QUIZ 11. Two students are discussing variable displacement compressors. Student A says they are designed to control evaporator temperature. Student B says they are designed to control evaporator pressure. Who is correct? Student A only Student B only Both A and B Neither A nor B

CHAPTER QUIZ 12. An automotive A/C compressor is of the _____ type. reciprocating piston Scroll rotary vane Any of these

CHAPTER QUIZ 12. An automotive A/C compressor is of the _____ type. reciprocating piston Scroll rotary vane Any of these

CHAPTER QUIZ 13. Two students are discussing the lines used to connect the A/C components. Student A says they are usually rubber hoses with metal ends. Student B says that metal tubing is often used. Who is correct? Student A only Student B only Both A and B Neither A nor B

CHAPTER QUIZ 13. Two students are discussing the lines used to connect the A/C components. Student A says they are usually rubber hoses with metal ends. Student B says that metal tubing is often used. Who is correct? Student A only Student B only Both A and B Neither A nor B