Thermodynamics

1. CHAPTER Thermodynamics 3 Instructor Name:(Your Name)

2. Learning Objectives • Describe how heat is transferred. • Explain thermal equilibrium. • Define a British thermal unit (BTU). • Explain the difference between sensible and latent heat. • Describe the three changes of state in which a substance may be formed.

3. Learning Objectives(continued) • Describe the differences between latent heat of vaporization and latent heat of fusion. • Explain the term superheating. • Explain the term subcooling. • Describe the effects of pressure on boiling point. • Define atmospheric pressure.

4. Learning Objectives(continued) • Describe what a vacuum is and how it is measured. • Define the term humidity and explain its effects upon an air-conditioning/refrigeration system.

5. Introduction • The terms air conditioning and refrigeration may be used to describe virtually the same process • These processes use the same components to perform the task of maintaining the temperature of a controlled space. • The difference between the two systems is that air conditioners are not used to maintain low temperatures.

6. Introduction(continued) • An air-conditioning system is designed to maintain temperatures that are suited to the average person’s comfort zone (70F to 80F/21C to 27C). • When most people think of air conditioning, they think of a fan that blows cold air out of the vents on the dash of their vehicle or out of the vents in their home.

7. Introduction(continued) • This is what air conditioning feels like to the human body. • Very few people understand how air conditioning really functions. • After examining the thermodynamics of air conditioning, however, you will discover that there is actually something quite different going on within the system.

8. System Overview • Before technicians can perform any service work or begin to diagnose problems in air-conditioning or refrigeration systems, they must have a basic understanding of thermodynamics. • When we talk about refrigerants boiling, we are talking about a substance that can boil at –20F (–28.8C).

9. System Overview(continued) • In the field of air-conditioning service and repair, learning the basics includes understanding the principles behind how an air-conditioning system works. • The principles that will be discussed in this chapter are the basis for any air-conditioning or refrigeration system.

10. System Overview(continued) • Understanding these principles will help a technician improve their diagnostic skills. • The more you know about the air-conditioning system, the easier it will be for you to identify problems within the system.

11. Heat • The word cold is often used to describe the temperature of a substance or of the ambient air. • Theoretically, the lowest temperature obtainable is 459F ( 273C). • This is known in scientific terms as absolute zero (no one has yet reached this temperature). • To date, the lowest temperature achieved is 457F ( 272C).

12. Heat(continued) • Cold is a term used to indicate an absence of heat. • Molecular action slows down as a substance becomes cooler (Figure 3-1). • When all molecular motion is stopped, it is considered to be at absolute zero. • Anything warmer than 459F (273C) contains heat.

13. Heat(continued) Figure 3-1.Molecular motion increases with heat intensity.

14. Heat(continued) • Cold, as the term is used, does not really exist because everything contains some heat (except at absolute 0). • When heat is removed from a substance, that substance becomes cold as a result. • Heat is often defined as molecules in motion.

15. Heat(continued) • If the temperature of a substance increases, so too does the molecular motion within the substance. • Likewise, as the temperature decreases, molecular motion is decreased.

16. Heat Transfer • The basis of all air-conditioning or refrigeration equipment is that heat flows from a warmer object to a colder object. • When an object is being cooled, it is because the heat of the object is being transferred to another object. • Just as water always runs downhill, heat always flows from a warm object to a colder object (Figure 3-2).

17. Heat Transfer(continued) Figure 3-2. The direction in which heat flows is always from warm to cold.

18. Heat Transfer(continued) • Heat transferhappens in one of three ways: • Conductionis heat flow through a solid object. Conduction transfers heat from molecule to molecule through a substance by chain collision. • Convection is the transfer of heat by a flowing substance. Because solids don’t flow, convection heat transfer occurs only in liquids and gases.

19. Heat Transfer(continued) • Radiation is the transfer of heat from a source to an absorbent surface by passing through a medium (air) that is not heated. The efficiency with which an object radiates or absorbs radiant heat depends on the color of the surface. • Lighter colors reflect more heat. • Darker colors absorb more heat and, when warm, they are excellent radiators of energy.

20. Thermal Equilibrium • If two objects are placed together, and neither one of them undergoes any temperature change, then they are said to be at the same temperature. • If the objects are not at the same temperature, the hot one gets cooler and the cold one gets warmer until both of them stop changing temperature. • When the heat intensity is equal and no further change takes place, the objects are said to be in a state of thermal equilibrium (heat balance).

21. EXAMPLE • If we place a cup of hot coffee in a room, the heat from the coffee will transfer to the cooler ambient air of the room. • In time, the temperature of the coffee reaches the temperature of the room and no further change in temperature takes place. • The coffee and the air in the room are now in a state of thermal equilibrium.

22. EXAMPLE (continued) • The same thing happens to a cold can of soda if it is forgotten on the kitchen table. • The heat energy of the warmer air in the room transfers to the cooler can and its contents until both are the same temperature. • When there is no more heat transfer between the room temperature and the can of soda, they too are said to be in a state of equilibrium.

23. Rate of Heat Transfer • The speed of heat transfer from one object to another is affected by the temperature difference between the two objects. • Heat energy travels fast when there is a great temperature difference between two objects.

24. Temperature • Temperatureindicates the average velocity (movement) of the molecules of a substance. • When the heat energy in a substance increases, its molecules vibrate more intensely. • A thermometer measures the intensity of this vibration. • Temperature is not a measurement of heat energy.

25. Temperature (continued) • Temperature is a measure of the intensity of heat energy in a substance.

26. Temperature Scales • The Fahrenheit scale, in the customary system, divides the difference from the freezing point of water to the boiling point of water into 180 equal divisions, each division being one degree. • The Fahrenheit scale sets the freezing point of water at 32F and the boiling point of water at 212F.

27. Temperature Scales (continued) • The Celsius or centigrade scale in the metric system divides the difference from the freezing point of water to the boiling point of water into 100 equal divisions. • The Celsius scale sets the freezing point of water at 0C and the boiling point of water at 100C.

28. British Thermal Unit • The quantity of heat transfer cannot be measured with a thermometer. • The measurement for the quantity of heat in a substance is the British thermal unit, or BTU for short. • The higher the BTU rating, the better a product will perform.

29. British Thermal Unit (continued) • A BTU is the quantity of heat required to raise the temperature of 1 pound of water by 1F. • The metric equivalent of the BTU is the kilocalorie (kcal). • One kcal is the amount of energy required to raise the temperature of 1 kilogram of water by 1C.

30. EXAMPLE 1

31. EXAMPLE 1 (continued) Figure 3-3.It requires 180 BTUs of heat energy to change the temperature of 1 pound of water at 32F to 212F.

32. EXAMPLE 2 • You have 15 pounds of water at 70F and you want to cool the water to 35F. How many BTUs of energy would be required? • Answer: It would take 525 BTUs of energy to cool 15 pounds of water from 70F to 35F.

33. British Thermal Unit (continued) These formulas show you how to convert between English and metric units of heat: ___ Watts BTUs per hour ___ Bkilocalories (kcal) ___ Bwatts

34. British Thermal Unit (continued) • In reference to an air-conditioning system capacity, smaller systems are rated in BTUs and larger systems are rated in tons of refrigeration capacity. • This refers to the amount of cooling required to change 1 ton (2,000 pounds) of water to 1 ton of ice in 24 hours.

35. British Thermal Unit (continued) • This is calculated by using the following method: • This example would be based on a 24-hour period. Normally, we speak of a 1-hour duration, and this then would be 12,000 BTUs.

36. British Thermal Unit (continued) • Most truck air-conditioning systems are in the range of one and three-quarters to three and a quarter tons of capacity.

37. Types of Heat Sensible Heat • This type of heat causes a change of temperature in a substance. • This is a kind of heat that can be felt, such as the temperature of the ambient air. • Sensible heat can be measured with a thermometer.

38. Types of Heat (continued) Latent Heat • This is the heat required to change the state of matter from either a liquid to a vapor or a liquid to a solid. • Latent heat is hidden heat. • It cannot be felt nor be measured with a thermometer.

39. EXAMPLE • A way of demonstrating latent heat simply would be to put a thermometer into a pot of water. • As heat is applied to a pot, the water gets warmer and the change in temperature can be seen on the thermometer. • When the water reaches its boiling point of 212F (100C), the water will start to change states. • The whole time the water is boiling (changing states), the temperature of the thermometer will not rise above 212F (100C).

40. Types of Heat (continued) • This is latent heat, the change of state of a substance without a change in temperature. • Latent and sensible heats are the two most important types of heat with respect to refrigeration.

41. Superheat • Whenever the steam in a closed vessel is raised above the temperature at which it changes states, it is considered to be superheated. • As stated earlier, the water must be under pressure. • If the water in a radiator is under pressure and the temperature of the water is 214F (101C), for example, then the water is considered to be 2F (-17C) above its boiling point. • This water would be considered superheated.

42. Subcooling • Subcoolingis a term used to describe the temperature of a liquid. • A liquid that is at a temperature below its boiling point is considered to be subcooled. • The graph in Figure 3-4 illustrates how heat energy is used to convert 1 pound of ice at 0F (–17.7C) to steam at 212F (100C). • The graph starts with a 1-pound (0.45 kilograms) piece of ice at 0F (17.7C).

43. Subcooling (continued) Figure 3-4. This graph shows how heat energy is used to convert 1 pound of ice at 0F (-17.8C) to steam at 212F (100C).

44. Subcooling (continued) • Sixteen BTUs of sensible heat are added to the ice, which causes the ice to change temperature. • Ice has a specific heat of ‘‘0.5’’; therefore, it will take 16 BTUs of energy to raise the temperature from 0F to 32F (–17.7C to 0C) .

45. Subcooling (continued) • Once the ice reaches 32F (0C), it begins to melt. • It requires the addition of 144 BTUs of heat to convert all the ice into water. • Note that the entire time the ice is melting, the temperature of the ice and water mixture does not change from 32F (0C). • The 144 BTUs of added heat could not be measured with the thermometer, and therefore the mixture is absorbing latent heat.

46. Subcooling (continued) • If heat is added after all of the ice has changed into water, it will change the temperature of the water. • To increase the temperature of the water to 212F (100C), 180 BTUs of heat energy must be absorbed. • Water has a specific heat of 1.0 and,in this case, the change in temperature is 180F (212F – 32F).

47. Subcooling (continued) • During the time that the water absorbs the 180 BTUs, the temperature of the thermometer increases. Thus the water is absorbing sensible heat. • When the water temperature reaches 212F (100C), it will begin to boil. • The water will absorb an additional 970 BTUs of energy to convert all the water into steam.

48. Subcooling (continued) • The entire time the water is turning to steam, the temperature of the thermometer will not rise above 212F (100C). • The fact that the thermometer does not change means that the water absorbs latent heat energy while it is changing states. • If heat is still added to the steam, it will become superheated.

49. Subcooling (continued) • Once the temperature of the steam rises above 212F (100C), it is considered to be superheated (Figure 3-4).

50. Change of State • Matter can commonly be found in three different states: solid, liquid, and gas. • The greatest amount of heat movement (heat transfer) occurs during a change of state. • Three processes describe a change of state: • Evaporation: The change in state from a liquid to a gas (vapor).