Always Learning

CHAPTER 4 Heat Movement Theory

Discuss how heat can be transferred from one location to another. OBJECTIVES After studying Chapter 4, the reader will 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.

KEY TERMS Bar Boiling point Chlorofluorocarbon (CFC) Conduction Convection Critical pressure Critical temperature Dew point Fractionization Gas Gauge pressures Glide Heat exchanger Hydrochlorofluorocarbon (HCFC) Hydrofluorocarbon (HFC) Hydrofluoro-olefin (HFO) Insulator Kilopascal (kPa) Latent heat Latent heat of condensation Latent heat of evaporation Latent heat of fusion Liquid Pounds per square inch absolute (psia) Pounds per square inch gauge (psig) Pressure Pressure–temperature (PT) Radiant heat Refrigerant Retrofit Saturated liquid Saturated vapor Sensible heat Solid Subcool Superheat Vacuum Zeotrope

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. Several physical principles are involved. If we understand these basic principles, the operation of the A/C system becomes easily understandable. This understanding, in turn, makes diagnosing and servicing an A/C system relatively easy and quite interesting.

HEAT MOVEMENT CONDUCTION CONVECTION RADIATION the transfer of heat between two parts of a stationary system, caused by a temperature difference between the parts. CONVECTION the transfer of heat by the circulation or movement of the heated parts of a liquid or gas. RADIATION the complete process in which energy is emitted by one body, transmitted through an intervening medium or space, and absorbed by another body.

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. SOLID LIQUID GAS

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.

LATENT AND SENSIBLE HEAT Sensible heat is fairly easy to understand, but if we add more heat, an odd thing occurs. SEE FIGURE 4–12 . If we add another Btu of heat to water at 212°F, some of the water will boil, but the temperature of both the water and the steam produced will remain at 212°F. The added heat has caused some of the water to change state, but it has not changed temperature. This is an example of latent, or hidden, heat.

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 3°F for each pound per square inch (psi) of pressure, or about 1°C for each 5 kPa. CRITICAL TEMPERATURE

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.

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).

The working fluid of an A/C system is refrigerant. REFRIGERANTS The working fluid of an A/C system is refrigerant. Refrigerants were first developed by the DuPont Corporation using the name Freon. This term is used improperly by many people to mean refrigerant. A new term, SUVA, refers to DuPont’s newer refrigerants: SUVA MP52 (a blend) and SUVA Trans A/C (134a). There are many refrigerants, but the three main ones used in automotive and other mobile systems are R-12, R-134a, and R-1234yf.

REFRIGERANTS R-12 R-22 R-134A HFO-1234YF R-152A R-290, PROPANE, & R-600 BUTANE R-744, CO2 BLENDS REFRIGERANT OILS

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.

SUMMARY The boiling points of liquids can be increased or decreased by changing the 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.

REVIEW QUESTIONS 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 ______________ .

REVIEW QUESTIONS 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.

1. Which of the following is true about heat? CHAPTER QUIZ 1. 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.

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

CHAPTER QUIZ 3. The process of transferring heat by circulating the heated media is called ______________ . radiation convection Conduction None of these

4. The movement of heat through heat rays is called ______________ . CHAPTER QUIZ 4. The movement of heat through heat rays is called ______________ . Radiation convection Conduction None of these

5. Molecules are composed of ______________ . CHAPTER QUIZ 5. Molecules are composed of ______________ . Atoms protons Electrons compounds

6. Which of the following is a form of matter? CHAPTER QUIZ 6. Which of the following is a form of matter? Solid Liquid Gas All of these

7. Which of the following describes sensible heat? CHAPTER QUIZ 7. Which of the following describes sensible heat? It causes a change of state. It causes a temperature change. It causes a liquid to boil. All of these

8. ______________ causes a change of state in matter. Sensible heat CHAPTER QUIZ 8. ______________ causes a change of state in matter. Sensible heat Latent heat Which is correct? A only B only Both A and B Neither A nor B

CHAPTER QUIZ 9. It takes ______________ Btu of heat to cause a change of state from 1 lb of water to 1 lb of steam. 100 212 970 32

CHAPTER QUIZ 10. The boiling point of a liquid can be increased by raising the ______________ . Temperature latent heat Pressure All of these

CHAPTER QUIZ 11. Superheat refers to temperature increases in a vapor after all of the liquid has boiled. True False

12. Zero psig is ______________ . CHAPTER QUIZ 12. Zero psig is ______________ . the zero point on all gauges the lowest point on a vacuum gauge equal to about 15 psi on the absolute pressure scale All of these

13. Which of the following is not a CFC or HCFC? CHAPTER QUIZ 13. Which of the following is not a CFC or HCFC? R-12 R-22 R-134a None of these

CHAPTER QUIZ 14.Two students are discussing heat. Student A says that heat always travels from hot to cold. Student B says that heat is a form of energy. Who is correct? Student A Student B Both A and B Neither A nor B

15. R-12 is normally used with ______________ oil. CHAPTER QUIZ 15. R-12 is normally used with ______________ oil. Mineral PAG POE Any of these