Refrigeration Theory

1. Refrigeration Theory History and Development

2. What is Refrigeration? • Refrigeration is defined as the process of removing heat from an undesirable place and transferring it to a place where it is unobjectionable. • The process works somewhat like a sponge soaking up water. The water is removed from one place and kept in a less objectionable place (the sponge).

3. Application of Refrigeration • The most important application for refrigeration is the preservation of food. • It was discovered that food lasts longer when it is kept cold. • Food spoilage slows down as molecular motion slows down.

4. History of Refrigeration • Early refrigeration was obtained by the use of ice. It was cut from lakes and ponds in the winter and stored in insulated rooms for summer use. • Around 1834, Jacob Perkins patented the first refrigerated machine in America: a compression system. • 1890 - Shortage of ice during a warm winter emphasized the need for a mechanical ice making process.

5. History of Refrigeration • 1918 - Kelvinator sold the first domestic refrigerator to U.S. markets. That first year, there were 67 sold. Now, millions are sold each year in the U.S. alone. • 1928 - G.E. introduced the first hermetic or sealed refrigeration unit, called the monitor top. • People now take the refrigeration process for granted.

6. Temperature • Defined as the level of heat intensity measured in degrees. • “Absolute zero” is -460°F (-273°c) and is the state when all molecular motion stops, in theory. This has never been accomplished. • Cold low-intensity level of heat (absence of heat). • Everything has some level of heat (except absolute zero).

7. Fundamentals of Refrigeration • All matter has weight and mass. • There are three states of matter: • Solid • Liquid • Vapor (gas)

8. Solid Matter • Solid Matter: • Molecular movement is slow. • Molecules are closely packed together. • Temperature easily effected by small amounts of heat.

9. Liquid Matter • Liquid Matter: • Molecular movement is faster. • The molecules are further apart. • More heat is required to affect temperature.

10. Vapor Matter • Vapor (Gas) Matter: • Molecules are moving fastest. • Large amounts of heat required to effect temperature.

11. Thermodynamics Defined • Thermodynamics is the study of heat and its properties. • Heat is a form of energy • Energy can not be created or destroyed • Heat is a measurement of the speed of the molecules which make up all matter • Heat is measured in British Thermal Units (Btu).

12. Laws of Thermodynamics • There are three Laws of Thermodynamics: • First Law: Heat and mechanical energy are mutually convertible. • Second Law: Heat moves from a warmer substance to a cooler substance. • Third Law: All substances have heat energy as long as they are above absolute zero.

13. Types of Heat • There are three types of heat: • Sensible heat • Specific heat • Latent heat

14. Sensible Heat • Sensible heat is measurable with a thermometer. • Adding or removing sensible heat will raise or lower the temperature of a substance.

15. Specific Heat • Specific heat is the amount of heat required to raise or lower the temperature of 1 pound of a substance by 1 degree Fahrenheit. • It is expressed in Btu (British thermal units). • The specific heat of water in the liquid form is 1.

16. Latent Heat • Latent heat is heat that is not easily measured (also known as hidden heat). • It is the amount of heat required to change the state of a substance with out changing the temperature of a substance. • Types of latent heat include evaporation, condensation, fusion, melting, sublimation.

17. Evaporation, Condensation, Fusion • Evaporation covers the change in state from liquid to vapor. This change is known as vaporization. • Condensation covers the change in state from vapor to liquid. • Fusion covers the change in state from a liquid to a solid. This change is known as freezing.

18. Melting, Sublimation • Melting covers the change in state from a solid to a liquid. • Sublimation is the change in state from a solid to a vapor without passing through the liquid state.

19. Saturation • Saturation is a condition existing when a substance contains all of another substance it can hold at a given temperature and pressure. • Saturation also can be related to the state of a given substance when it is confined to a container in more than one state. For example, refrigerants in a tank or a system.

20. Heat Transfer Methods • The three principle heat transfer methods are Radiation, Conduction, and Convection. • Radiation is the transfer of heat with rays (as in a quartz heater). • Conduction is the transfer of heat through parts or fluids that are in contact with each other; also called radiant heat. • Convection is the movement of heat with fluids or air (as in a baseboard heating system)

21. Basics of Heat Measurement • The British Thermal Unit (Btu) is the recognized unit for heat measurement. • It is defined as the amount of heat required to raise one pound of water one degree Fahrenheit. • Temperature is the measurement of the speed of molecules times the number of molecules. • Ambient Temperature is the temperature of the air surrounding an object.

22. Temperature Scales • There are four commonly recognized temperature scales; they are Fahrenheit, Celcius, Rankine, and Kelvin. • Note that different scales set absolute zero at different number designations (see the following slide for more information).

23. Temperature Scales, Absolute Zero • Fahrenheit has absolute zero at -460°F • Rankine uses the same graduations as Fahrenheit, but sets absolute zero at 0°R. Therefore, freezing occurs at 32° on the Fahrenheit scale, and 492° on the Rankine scale. • Celsius sets absolute zero at -273°C. • Kelvin uses the same graduation as Celsius, but sets absolute zero at 0°.

24. Types of Energy • The types of energy are mechanical, electrical, thermal (heat), kinetic, potential, and enthalpy. • Kinetic energy is energy in motion. • Potential energy is energy at rest. • Enthalpy is the total of kinetic and potential energy. • The enthalpy of a substance is the amount of heat present, including sensible and latent heats.

25. Types of Energy, Equivalents • 1 Btu = 778 Foot LBS • 1 hp/hr = 2546 Btu • 1 hp = 746 watts • 1 watt = 3.414 Btu • 1 kg calorie = 29,685 Btu

26. Laws of Refrigeration • There are several laws used in understanding the principles of refrigeration: Charles’ Law, Boyle’s Law, Pascal’s Law, and Dalton’s Law.

27. Charles’ Law • Charles Law refers to the relationship of pressure, volume, and temperature. • Pressure is the force applied to a substance. • Volume is the space a body occupies. Volume of a given mass of gas at constant pressure varies according to temperature. • Changing the temperature of a substance will change its pressure when confined to a given volume. Temperature and pressure rise and fall together. • Density is the amount of a substance within a given volume.

28. Boyle’s Law • Boyle’s Law refers to the relationship of pressure to volume. • As pressure increases the volume decreases and vise versa, so long as temperature remains the same. • Reducing the pressure on a cylinder increases the amount a cylinder can hold at a given temperature.

29. Pascal’s Law • Pascal’s Law refers to the relationship of pressure within a given area. • Pressure imposed upon a fluid is transmitted equally in all directions (balloon principles behind hydraulics).

30. Dalton’s Law • Dalton’s Law refers to the mixing of vapor pressures for two different substances. • Vapor pressure created in a container by a mixture of gases is equal to the sum of the individual vapor pressures.