Refrigeration and Air Conditioning Technology PDF

Section 1: Theory of Heat Unit 1: Heat, Temperature, and Pressure Objectives (1) After studying this unit, the student should be able to: – Define temperature – Make conversions between Fahrenheit and Celsius scales – Describe molecular motion at absolute zero – Define the Briti...

Section 1: Theory of Heat Unit 1: Heat, Temperature, and Pressure Objectives (1) After studying this unit, the student should be able to: – Define temperature – Make conversions between Fahrenheit and Celsius scales – Describe molecular motion at absolute zero – Define the British thermal unit Objectives (2) – Describe heat flow between substances of different temperatures – Explain the transfer of heat by conduction, convection, and radiation – Discuss sensible heat, latent heat, and specific heat – State atmospheric pressure at sea level and explain why it varies at different elevations Objectives (3) – Describe two types of barometers – Explain psig and psia as they apply to pressure measurements Heat, Temperature, and Pressure Heat energy moves from one substance to another as well as between the molecules of a single substance When we need to be more specific than “hot” or “cold,” we refer to temperature “Heat” and “temperature” are not the same System pressures are obtained by using a refrigeration gauge manifold Temperature The level of heat or heat intensity Measured with thermometers – English system: Fahrenheit (F) – Metric system: Celsius (C) – Fahrenheit absolute scale: Rankine (R) – Celsius absolute scale: Kelvin (K) – Absolute zero: Temperature at which all molecular movement stops (-460°F) Thermometers Figure 1–5 (A) A Fahrenheit and Rankine thermometer. (B) A Celsius and Kelvin thermometer. Converting Celsius to Fahrenheit To convert a Celsius temperature to Fahrenheit – ºF = (1.8 x ºC) + 32º Example: Convert Celsius temperature of 20ºC – ºF = (1.8 x ºC) + 32º – ºF = (1.8 x 20ºC) + 32º – ºF = 36º + 32º – ºF = 68º – So, 20ºC = 68ºF Converting Fahrenheit to Celsius To convert a Fahrenheit temperature to a Celsius temperature – ºC = (ºF - 32º) ÷ 1.8 Example, Convert Fahrenheit temperature of 50ºF – ºC = (ºF - 32º) ÷ 1.8 – ºC = (50ºF - 32º) ÷ 1.8 – ºC = 18º ÷ 1.8 – ºC = 10º – So, 50ºF = 10ºC Introduction to Heat Heat is the motion of molecules Heat cannot be created or destroyed Heat can be measured and accounted for Heat can be transferred from one substance to another Heat travels from a warmer substance to a cooler substance British Thermal Units Figure 1–6 One British thermal unit (Btu) of heat energy is required to raise the temperature of 1 lb. (pound) of water from 68°F to 69°F. Conduction Heat energy travels from one molecule to molecule within a substance Heat energy travels from one substance to another Heat does not conduct at the same rate in all materials Example of Conduction Example of conduction: heat will travel through a copper rod when placed near fire Figure 1–8 The copper rod is held in the flame only for a short time before heat is felt at the far end. Convection Heat transfers through a fluid from one substance to another Natural convection utilizes natural fluid flow, such as the rising of warm air and the falling of cooler air Forced convection uses fans or pumps to move fluids from one point to another Example of Convection Example of convection: baseboard heating Figure 1–11 Natural convection occurs when heated air rises and cool air takes its place. Radiation Radiant heat passes through air, heating the first solid object with which the heat comes in contact These heated objects, in turn, heat the surrounding area Radiant heat can travel through a vacuum Example of Radiation Radiant heat can travel through space without heating it Example of radiation: – An electric heater that glows red Sensible Heat Heat transfer that results in a change in temperature of a substance Sensible heat transfers can be measured with a thermometer Example of a sensible heat transfer: – Changing the temperature of a sample of water from 68°F to 69°F Latent Heat Latent heat transfers result in a change of state of a substance with no change in temperature – Also referred to as hidden heat – Latent heat transfers cannot be measured with a thermometer – Example of a latent heat transfer: Changing 1 pound of ice at 32°F to 1 pound of water at 32°F Three Terms for Latent Heat Three terms that are important to understand when referring to latent heat transfers: – Latent heat of vaporization – Latent heat of condensation – Latent heat of fusion How Water Responds to Heat Figure 1–14 The heat/temperature graph for 1 lb. of water at atmospheric pressure explains how water responds to heat. An increase in sensible heat causes a rise in temperature. An increase in latent heat causes a change of state, for example, from solid ice to liquid water. Specific Heat Defined as the number of Btus required to raise the temperature of 1 pound of a substance 1 degree Fahrenheit – Specific heat of water is 1.00 – Specific heat of ice is approximately 0.50 – Specific heat of steam is approximately 0.50 – Specific heat of air is approximately 0.24 Sizing Heating Equipment: Formula Q = Weight x Specific Heat x Temperature Difference – Where Q = Quantity of heat needed for the temperature change Sizing Heating Equipment: Example Example: 1000 pounds of steel must be heated from 0°F to 70°F – How much heat is required to accomplish this? The specific heat of steel is 0.116 Btu/lb Substituting in the above formula gives us Q = 1000 pounds x 0.116 Btu/lb x (70°F - 0°F) Q = 1,000 x 0.116 x 70 = 8,120 Btu Pressure Defined as the force per unit area Often expressed in pounds per square inch – Example: If a 100-pound weight rests on a surface of 1 square inch, the pressure is 100 psi – Example: If a 100-pound weight rests on a surface of 100 square inches, the pressure is only 1 psi Illustration of Pressure Figure 1–16 Both weights are resting on a 1-square-inch (1-in2) surface. One weight exerts a pressure of 1 psi, the other a pressure of 100 psi. Atmospheric Pressure The atmosphere we live in has weight The atmosphere exerts a pressure of 14.696 psi at sea level (often rounded off to 15 psi) 14.696 psi at sea level is known as the standard condition Measured with a barometer A Mercury Barometer Figure 1–20 Mercury (Hg) barometer. Pressure Gauges Bourden tube: measures pressure in a closed system – Used to measure the pressures in an air conditioning or refrigeration system Gauges read 0 psi when opened to the atmosphere Gauge pressures are measured in pounds per square inch gauge, psig How a Bourdon Tube Functions Figure 1–23(A) The Bourdon tube is made of a thin substance such as brass. It is closed on one end, and the other end is fastened to the pressure being checked. When pressure increases, the tube tends to straighten out. When attached to a needle linkage, pressure changes are indicated. A Picture of a Bourdon Tube Figure 1–23(B) An actual Bourdon tube. Photo by Eugene Silberstein Summary (1) Thermometers measure temperature The higher the temperature, the faster the molecular movement One BTU raises the temperature of one pound of water one degree Fahrenheit Heat can be transferred by conduction, convection or radiation Summary (2) Sensible heat transfers change the temperature of a substance Latent heat transfers result in a change of state with no change in temperature Pressure is the force per unit area Barometers measure atmospheric pressure in inches of mercury Gauges measure pressures in enclosed systems