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AUT 236 – Automotive HVAC

AUT 236 – Automotive HVAC. Cpt 4 – Heat Movement Theory. OBJECTIVES. After studying Chapter 4, the reader should be able to: Discuss how heat can be transferred from one location to another. Describe the three states of matter and the effect that heat has on them.

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AUT 236 – Automotive HVAC

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  1. AUT 236 – Automotive HVAC Cpt 4 – Heat Movement Theory

  2. OBJECTIVES After studying Chapter 4, the reader should be able to: • Discuss how heat can be transferred from one location to another. • Describe the three states of matter and the effect that heat has on them. • Discuss what latent heat is and why it is important to A/C. • Discuss how pressure is measured. • Discuss the effect that pressure has on boiling points.

  3. INTRODUCTION • In practical terms, an HVAC system moves heat. • The A/C system simply transfers the heat out of a place where it is not wanted, and the heating system moves the heat into a place where it is wanted. • Heat always moves from hot to cold. • Remember that heat is energy, and cold is lack of energy. • The rate or speed that heat moves, then, is simply a factor of the difference in the temperature between the hot and cold areas.

  4. FIGURE 4-1 Heat always moves from hot to cold. In this example, it will travel from the fire to the cooler water and from the hand to the cooler ice. INTRODUCTION

  5. FIGURE 4-2 Heat can enter a vehicle’s passenger compartment from several sources. The A/C system allows us to move excess heat out of the vehicle. (Courtesy of Toyota Motor Sales USA, Inc.) INTRODUCTION

  6. HEAT MOVEMENT • Heat can travel on one or more of three paths as it moves from hot to cold: conduction, convection, or radiation. • Heat travel can be beneficial and intentional, or it can be detrimental. • Conduction • Convection • Radiation

  7. HEAT MOVEMENT • The transfer of heat directly through a material is called conduction.

  8. HEAT MOVEMENT • We move heat from the heater to the passengers or from the passengers to the A/C system by convection. Heat movement from the engine to the radiator using circulating coolant is also an example of convection.

  9. HEAT MOVEMENT • Heated air next to the stove will rise and cooler air will move in to replace it. This creates a convection current to move the air and heat.

  10. HEAT MOVEMENT • Heat is transferred from the sun to things on Earth through radiation.

  11. HEAT MOVEMENT • Radiant heat entering a vehicle through the windows can add a lot of heat to a car’s interior

  12. STATES OF MATTER • The air-conditioning process works through a fluid, called a refrigerant, that continuously changes state from liquid to gas and back to liquid. • These changes of state are where the movement of heat needed for cooling occurs. • All basic materials exist in one of the states of matter—solid, liquid, or gas—and most of them can be changed from one state to another by adding or removing heat. • Solid • Liquid • Gas

  13. STATES OF MATTER • We can cause matter to change state by adding or removing heat.

  14. STATES OF MATTER • A water molecule contains two oxygen atoms and one hydrogen atom; • R-12 is a combination of one carbon, two chlorine, and two fluorine atoms; • R-134a is a combination of two carbon, four fluorine, and two hydrogen atoms; and • R-152a has two more hydrogen and two less fluorine atoms than R-134a.

  15. STATES OF MATTER • Ice is a solid form of water with a low temperature and slow molecular action.

  16. STATES OF MATTER • Water is warmer than ice and has a much freer molecular action. • Liquid: H2O at temperatures between 32deg and 212deg F (0 and 100 dec C ) • Liquid/Fluid takes the shape of the container • Exerts pressure downward and to the sides

  17. STATES OF MATTER • Adding heat to water produces steam, the gas state, with a much freer molecular action. • GAS • H20 at temperatures above 212 deg F, 100 deg C • Gas has no shape • Can exert pressure in all directions.

  18. LATENT AND SENSIBLE HEAT • Sensible heat makes sense; it can be felt and measured on a thermometer. • If we have 1 lb of water at 40°F and add 1 Btu of heat to it, the temperature will increase to 41°F; adding another Btu of heat will increase the temperature to 42°F; and adding another 170 Btu (212 – 42) will increase the temperature to 212°F, the boiling point.

  19. LATENT AND SENSIBLE HEAT • Heat that causes a temperature increase is called sensible heat.

  20. LATENT AND SENSIBLE HEAT • If we add 970 Btu of heat to 1 lb of water at 212°F, we will have 1 lb of steam at the same temperature.

  21. LATENT AND SENSIBLE HEAT • To change 1 lb of water at 212°F into 1 lb of steam at 212°F, we must add 970 Btu’s of heat. Using metric terms, to change 1 g of water at 100°C into 1 g of steam at 100°C, we must add 540 calories of heat. This is called the latent heat of evaporation. • To reverse this and change 1 lb of steam back into 1 lb of water, we have to remove this same 970 Btu of heat; this is called the latent heat of condensation.

  22. LATENT AND SENSIBLE HEAT • If we start with 1 lb of water at 32°F, adding 180 Btu will increase the temperature to 212°F. It will take another 970 Btu (the latent heat of evaporation) to boil that pound of water.

  23. LATENT AND SENSIBLE HEAT • In an A/C system, the refrigerant changes state and absorbs heat in the evaporator and releases heat as it changes state again in the condenser.

  24. LATENT AND SENSIBLE HEAT • If we start with a 1-lb block of ice at 32°F, it will take 144 Btu (the latent heat of fusion) to melt all of the ice.

  25. LATENT AND SENSIBLE HEAT • The amount of heat movement required to change 0°F ice to steam, or vice versa. • Adding 16 BTU will warm ice from 0 deg F to 32 deg F. • Adding 144 BTU more will change the ice to water with no temp change • Adding 180 BTU will raise the water from 32 deg F to 212 deg F. • Adding 970 BTU will change the water to steam with no temperature change.

  26. LATENT AND SENSIBLE HEAT • A refrigeration cycle absorbs heat as the refrigerant boils in the evaporator and removes heat as it changes state back to a liquid in the condenser.

  27. BOILING POINTS • Boiling points can be increased or decreased by raising or lowering the pressure on the liquid. • With water, the boiling point will rise about 2 1/2°F for each pound per square inch (psi) of pressure, or about 1°C for each 5 kPa. • Critical Temperature • The critical temperature is the maximum point at which a gas can be liquefied or condensed by raising the pressure. • The critical pressure is the pressure that is necessary to liquefy a gas at that temperature.

  28. BOILING POINTS • The boiling point of a liquid changes as the pressure changes. • Water will boil at 183°F with a pressure of 13 psi on a mountain. • It will boil at 212°F at an atmospheric pressure of 14.7 psi. • In a radiator at a pressure of 30 psi (absolute), it will boil at 250°F.

  29. SATURATED VAPORS AND THE PRESSURE–TEMPERATURE RELATIONSHIP • Saturated vapor, also called a saturated liquid, is the term used to describe a liquid and gas inside a closed chamber, which is the condition in an A/C system. • When discussing saturated vapors, we need to learn two additional terms: subcool and superheat. • Subcool refers to a liquid whose temperature is well below its boiling point. • Superheat refers to the temperature increases of a vapor above its boiling point.

  30. SATURATED VAPORS AND THE PRESSURE–TEMPERATURE RELATIONSHIP • R-12 in a container is a saturated vapor with gas in contact with a liquid. The pressure in the container is in direct relation to the temperature (a); • a chart can be used to determine the temperature if we know the pressure, or vice versa (b).

  31. PRESSURE: GAUGE AND ABSOLUTE • Pressure is defined as a certain amount of force exerted on a unit area. • Traditionally in the United States, pressure is given in pounds and the unit area in square inches, so pressure has been given in pounds per square inch (psi). • Most pressure gauges disregard atmospheric pressure and are calibrated to read 0 at their starting point, which is the normal atmospheric pressure surrounding us. • The pressure created by the weight of the air in our atmosphere generates a pressure of 14.7 psi at sea level, which is often rounded off to 15 psi

  32. PRESSURE: GAUGE AND ABSOLUTE • The weight of the air in our atmosphere generates a pressure of about 15 psi at sea level. • Atmospheric pressure and the boiling point of water are lower at higher altitudes .

  33. PRESSURE: GAUGE AND ABSOLUTE • As shown on this compound gauge, pressures below atmospheric are commonly called a vacuum (a). • A perfect vacuum is 29.92 inches of mercury (b)

  34. PRESSURE: GAUGE AND ABSOLUTE • The boiling point of water drops as pressure is reduced. At a near-perfect vacuum of 29.9199 δHg or 2.54 microns, the boiling point is -90°F

  35. PRESSURE: GAUGE AND ABSOLUTE • The relative pressures between seven different measuring systems.

  36. REFRIGERANTS • The working fluid of an A/C system is refrigerant. Refrigerants were first developed by the Du Pont Corporation using the name Freon. • Depending on the combination of carbon, chlorine, fluorine, or hydrogen, a refrigerant is classed as an HFC, CFC, or HCFC

  37. REFRIGERANTS • Comparison of the physical characteristics of R-12 and R-134a.

  38. REFRIGERANTS • R-12 • R-22 • R-134a • R-152a • HFO-1234yf • R-290,Propane, and R-600 Butane • R-744,CO2 • Blends • Refrigerant Oils

  39. REFRIGERANTS • Refrigerant is commonly available in small or large (30- or 50-lb) containers. A 30-pound, disposable container of R-12 and small, 12-ounce can of R-134a is shown.

  40. REFRIGERANTS • A refrigerant and its oil must be completely compatible with all of the materials and chemicals in the system.

  41. REFRIGERANTS • The boiling points for R-12, R-22, and R-134a vary depending on the pressure.

  42. SUMMARY • Heat travels by either conduction, convection, or radiation. • Many compounds can be found as a solid, liquid, or vapor. • Adding heat will cause water to change from a solid to a liquid or from a liquid to a vapor. • Removing heat will cause water to change from a liquid to a solid or from a vapor to a liquid. • Sensible heat changes the temperature, and latent heat causes a change of state.

  43. SUMMARY • The boiling points of liquids can be changed by changing pressure. • Changing the temperature of a saturated vapor will change the pressure. • Pressure is measured in psi, bar, or kPa. • Refrigerants have very low boiling points and are either a CFC, HFC, HCFC, or HFO.

  44. REVIEW QUESTIONS Heat • We must move _________ to the outside of the vehicle in order to cool it. • Heat transfer from one molecule to the one next to it is called _________, and moving heat by circulating hot air or water is called _________. • The three states of matter are _________, _________, and _________. • If we add enough heat to solid matter, it will change to a(n) _________. • Heat that is added to change water to gas is called _________, _________ of _________. Conduction Convection Liquid Solid Gas Liquid Latent Heat Evaporation

  45. REVIEW QUESTIONS Boiling • Adding pressure to a hot liquid will increase the _________ _________. • A liquid gas mixture in a closed container is called a(n) _________ _________, and heating this container will _________ the internal pressure. • Ten psi is equal to _________ kPa. • A pressure lower than atmospheric is called a(n) _________, and this pressure is measured in _________ or _________. • Most vehicle and compressor manufacturers recommend using _________ oil in R-134a systems. Point Saturated Vapor Raise 69.85 Vacuum HG, microns Mm, Hg PAG

  46. CHAPTER QUIZ • Which of the following is true about heat? • Heat always travels from something warm to something cold. • Heat is a form of energy. • Cold is the lack of heat. • All of these are true.

  47. CHAPTER QUIZ 2. _________ occurs when heat travels through a material, from one molecule to the one next to it. • Radiation • Convection • Conduction • None of these

  48. CHAPTER QUIZ 3. The process of transferring heat by circulating the heated media is called _________. • Radiation • Convection • Conduction • None of these

  49. CHAPTER QUIZ 4. The movement of heat through heat rays is called _________. • Radiation • Convection • Conduction • None of these

  50. CHAPTER QUIZ 5. Molecules are composed of_________. • Atoms • Protons • Electrons • compounds

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