BASIC REFRIGERATION PRINCIPLES
Most users normally associate refrigeration with cold and cooling, yet the practice of refrigeration engineering deals almost entirely with the transfer of heat. This seeming paradox is one of the most fundamental concepts that must be grasped to understand the workings of a refrigeration system. Cold is really only the absence of heat, just as darkness is the absence of light, and dryness is the absence of moisture.
Thermo-dynamics is that branch of science dealing with the mechanical action of heat. There are certain fundamental principles of nature, often called laws of thermodynamics, which govern our existence here on Earth, several of which are basic in the study of refrigeration.
The first and most important of these laws is the fact that energy can neither be created or destroyed, but can be converted from one type to another. A study of thermodynamic theory is beyond the scope of this manual, but the examples that follow will illustrate the practical application of the energy law.
Heat is a form of energy, primarily created by the transformation of other types of energy into heat energy. For example, mechanical energy turning a wheel causes friction which creates heat.
Heat is often defined as energy in transfer, for it is never content to stand still, but is always moving from a warm body to a colder body. Much of the heat on the Earth is derived from radiation from the sun. A spoon in ice water loses its heat to the water and becomes cold; a spoon in hot coffee absorbs heat from the coffee and becomes warm. But the terms warmer and colder are only comparative. Heat exists at any temperature above absolute zero, even though it may be in extremely small quantities. Absolute zero is the term used by scientists to describe the lowest theoretical temperature possible, the temperature at which no heat exists, which is approximately 460 degrees below zero Fahrenheit. By comparison with this standard, the coldest weather we might ever experience on Earth is much warmer.
Temperature is the scale used to measure the intensity of heat, the indicator that determines which way the heat energy will move. In the United States, temperature is normally measured in degrees Fahrenheit, but the Centigrade scale (sometimes termed Celsius) is widely used in other parts of the world. Both scales have two basic points in common, the freezing point of water, and the boiling point of water at sea level. Water freezes at 32° F. and O° C., and water boils at sea level at 212° F. and 100° C. On the Fahrenheit scale, the temperature difference between these two points is divided into 180 equal increments or degrees F., while on the Centigrade scale the temperature difference is divided into 100 equal increments or degrees C, The relation between Fahrenheit and Centigrade scales can always be established by the following formulas:
Fahrenheit 9/5 (Centigrade + 32°)
Centigrade 5/9 (Fahrenheit - 32°)
The measurement of temperature has no relation to the quantity of heat. A match flame may have the same temperature as a bonfire, but obviously the quantity of heat given off is vastly different.
The basic unit of heat measurement used today in the United States is the British Thermal Unit, commonly expressed as a BTU. A BTU is defined as the amount of heat necessary to raise one pound of water one degree Fahrenheit. For example, to raise the temperature of one gallon of water (approximately 8.3 pounds) from 70° F. to 80° F. will require 83 Btu's.
8.3 x (80 70) ---- 83
The second important law of thermodynamics is that heat always travels from a warm object to a colder one. The rate of heat travel is in direct proportion to the temperature difference between the two bodies.
Assume that two steel halls are side by side in a perfectly insulated box. One ball weighs one pound and has a temperature of 400° F., while the second ball weighs 1,000 pounds and has a temperature of 390° F. The heat content of the larger ball is tremendously greater than the small one, but because of the temperature difference, heat will travel from the small ball to the large one until the temperatures equalize.
Heat can travel in any of three ways: radiation, conduction, or convection.
Radiation is the transfer of heat by waves similar to light waves or radio waves. For example, the sun's energy is transferred to the Earth by radiation. One need only step from the shade into direct sunlight to feel the impact of the heat waves, even though the temperature of the surrounding air is identical in both places. There is little radiation at low temperatures, and at small temperature differences, so radiation is of little importance in the actual refrigeration process. However, radiation to the refrigerated space or product from the outside environment, particularly the sun, may be a major factor in the refrigeration load.
Conduction is the flow of heat through a substance. Actual physical' contact is required for heat transfer to take place between two bodies by this means. Conduction is a highly efficient means of heat transfer as any service-man who has touched a piece of hot metal can testify.
Convection is the flow of heat by means of a fluid medium, either gas or liquid, normally air or water. Air may be heated by a furnace, and then discharged into a room to heat objects in the room by convection.
In a typical refrigeration application, heat normally will travel by a combination of processes, and the ability of a piece of equipment to transfer heat is referred to as the overall rate of heat transfer. While heat transfer cannot take place without a temperature difference, different materials vary in their ability to con-duct heat. Metal is a very good heat conductor, while asbestos has so much resistance to heat flow it can be used as insulation.
CHANGE OF STATE
Most common substances can exist as a solid, a liquid, or a vapor, depending on their temperature and the pressure to which they are exposed. Heat can change their temperature, and also can change their state. Heat is absorbed even though no temperature change takes place when a solid changes to a liquid, or when a liquid changes to a vapor. The same amount of heat is given off when the vapor changes back to a liquid, and when the liquid is changed to a solid.
The most common example of this process is water, which exists as a liquid, can exist in solid form as ice, and exists as a gas when it becomes steam. As ice it is a usable form of refrigeration, absorbing heat as it melts at a constant temperature of 32° F. If placed on a hot stove in an open pan, its temperature will! rise to the boiling point (212° F. at sea level). Regardless of the amount of heat applied, the temperature cannot be raised above 212° F. because the water will completely vaporize into steam. If this steam could be enclosed in a container and more heat applied, then the temperature could again be raised. Obviously the boiling or evaporating process was absorbing heat.
When steam condenses back into water it gives off exactly the same amount of heat that it absorbed evaporating. (The steam radiator is a common usage of this source of heat.) If the water is to be frozen into ice, the same amount of heat that is absorbed in melting must be extracted by some refrigeration process to cause the freezing action.
The question arises: Just where did those heat units go? Scientists have found that all matter is made up of molecules, infinitesimally small building blocks which are arranged in certain patterns to form different substances. In a solid or liquid, the molecules are very close together. In a vapor the molecules are much farther apart and move about much more freely. The heat energy that was absorbed by the water became molecular energy, and as a result the molecules rearranged themselves, changing the ice into water, and the water into steam. When the steam condenses back into water, that same molecular energy is again converted into heat energy.
Sensible heat is defined as the heat involved in a change of temperature of a substance. When the temperature of water is raised from 32° F. to 212° F., an increase in sensible heat content is taking place. The Btu's required to raise the temperature of one pound of a substance 1° F. is termed its specific heat. By definition the specific heat of water is 1.0, but the amount of heat required to raise the temperature of different substances through a given temperature range will vary. It requires only .64 BTU to raise the temperature of one pound of butter 1 degree F., and only .22 BTU is required to raise the temperature of one pound of aluminum 1 degree F. Therefore the specific heats of these two substances are .64 and .22 respectively.
LATENT HEAT OF FUSION
A change of substance from a solid to a liquid, or from a liquid to a solid involves the latent heat of fusion. It might also be termed the latent heat of melting, or the latent heat of freezing.
When one pound of ice melts, it absorbs 144 Btu's at a constant temperature of 32° F., and if one pound of water is to be frozen into ice, 144 Btu's must be removed from the water at a constant temperature of 32° F. In the freezing of food products, it is only the water content for which the latent heat of freezing must be taken into account, and normally this is calculated by determining the percentage of water content in a given product.
LATENT HEAT OF EVAPORATION
A change of a substance from a liquid to a vapor, or from a vapor back to a liquid involves the latent heat of evaporation. Since boiling is only a rapid evaporating process, it might also be called the latent heat of boiling, the latent heat of vaporization, or for the reverse process, the latent heat of condensation.
When one pound of water boils or evaporates, it absorbs 970 Btu's at a constant temperature of 212° F. (at sea level) and to condense one pound of steam to water 970 Btu's must be extracted from it.
Because of the large amount of latent heat involved in evaporation and condensation, heat transfer can be very efficient during the process. The same changes of state affecting water apply to any liquid, although at different temperatures and pressures.
The absorption of heat by changing a liquid to a vapor, and the discharge of that heat by condensing the vapor is the keystone to the whole mechanical refrigeration process, and the movement of the latent heat involved is the basic means of refrigeration.
LATENT HEAT OF SUBLIMATION
A change in state directly from a solid to a vapor without going through the liquid phase can occur in some substances. The most common example is the use of "dry ice" or solid carbon dioxide for cooling. The same process can occur with ice below the freezing point, and is also utilized in some freeze-drying processes at extremely low temperatures and high vacuums. The latent heat of sublimation is equal to the sum of the latent heat of fusion and the latent heat of evaporation.
The condition of temperature and pressure at which both liquid and vapor can exist simultaneously is termed saturation. A saturated liquid or vapor is one at its boiling point, and for water at sea level, the saturation temperature is 212° F. At higher pressures, the saturation temperature increases, and with a decrease in pressure, the saturation temperature decreases.
After a liquid has changed to a vapor, any further heat added to the vapor raises its temperature so long as the pressure to which it is exposed remains constant. Since a temperature rise results, this is sensible heat. The term superheated vapor is used to describe a gas whose temperature is above its boiling or saturation point. The air around us is composed of superheated vapor.
SUB COOLED LIQUID
Any liquid which has a temperature lower than the saturation temperature corresponding to its pressure is said to be sub cooled. Water at any temperature less than its boiling temperature (212 degrees F. at sea level) is sub cooled.
The atmosphere surrounding the Earth is composed of gases, primarily oxygen and nitrogen, extending many miles above the surface of the Earth. The weight of that atmosphere pressing down on the Earth creates the atmospheric pressure in which we live. At a given point, the atmospheric pressure is relatively constant except for minor changes due to changing weather conditions. For purposes of standardization and as a basic reference for comparison, the atmospheric pressure at sea level has been universally accepted, and this has been established at 14.7 pounds per square inch, which is equivalent to the pressure exerted by a column of mercury 29.92 inches high.
At altitudes above sea level, the depth of the atmospheric blanket surrounding the Earth is less, therefore the atmospheric pressure is less. At 5,000 feet elevation, the atmospheric pressure is only 12.2 pounds per square inch.
Absolute pressure, normally expressed in terms of pounds per square inch absolute (psia) is defined as the pressure existing above a perfect vacuum. Therefore in the air around us, absolute pressure and atmospheric pressure are the same.
A pressure gauge is calibrated to read 0 pounds per square inch when not connected to a pressure producing source. Therefore the absolute pressure of a closed system will always be gauge pressure plus atmospheric pressure. Pressures below 0 psig are actually negative readings on the gauge, and are referred to as inches of vacuum. A refrigeration compound gauge is calibrated in the equivalent of inches of mercury for negative readings. Since 14.7 psi is equivalent to 29.92 inches of mercury, 1 psi is approximately equal to 2 inches of mercury on the gauge dial.
It is important to remember that gauge pressures are only relative to absolute pressure. Table 1 shows relationships existing at various elevations assuming that standard atmospheric conditions prevail.
PRESSURE RELATIONSHIPS AT VARYING ALTITUDES
The absolute pressure in inches of mercury indicates the inches of mercury vacuum that a perfect vacuum pump would be able to reach. Therefore, at 5,000 feet, elevation under standard atmospheric conditions, a perfect vacuum would be 24.89 inches of mercury, as compared to 29.92 inches of mercury at sea level.
At very low pressures, it is necessary to use a smaller unit of measurement since even inches of mercury are too large for accurate reading. The micron, a metric unit of length, is used for this purpose, and when we speak of microns in evacuation, we are referring to absolute pressure in units of microns of mercury.
A micron is equal to 1/1000 of a millimeter and there are 25.4 millimeters per inch. One micron, therefore, equals 1/25,400 inch. Evacuation to 500 microns would be evacuating to an absolute pressure of approximately .02 inch of mercury, or of standard conditions, the equivalent of a vacuum reading of 29.90 inches mercury.
PRESSURE-TEMPERATURE RELATIONSHIPS, LIQUIDS
The temperature at which a liquid boils is dependent on the pressure being exerted on it. The vapor pressure of the liquid, which is the pressure being exerted by the tiny molecules seeking to escape the liquid and becomes vapor, increases with an increase in temperature until at the point where the vapor pressure equals the external pressure, boiling occurs.
Water at sea level boils at 212° F.,. but at 5,000 feet elevation it boils at 203° F.' due to the decreased atmospheric pressure. If some means, a compressor for example, were used to vary the pressure on the surface of the water in a closed container, the boiling point could be changed at will. At 100 psig, the boiling point is 327.8' degrees F., and at 1 psig, the boiling point is 102° F.
Since ail liquids react in the same fashion, although at different temperatures and pressure, pressure provides a means of regulating a refrigerating temperature. If a cooling coil is part of a closed system isolated from the atmosphere and a pressure can be maintained in the coil equivalent to the saturation temperature (boiling point) of the liquid at the cooling temperature desired, then the liquid will boil at that temperature as long as it is absorbing heat -- and refrigeration has been accomplished.
PRESSURE-TEMPERATURE RELATIONSHIPS, GASES
One of the basic fundamentals of thermodynamics is called the "perfect gas law." This describes the relationship of the three basic factors controlling the behavior of a gas--pressure, volume, and temperature. For ail practical purposes, air and highly superheated refrigerant gases may be considered perfect gases, and their behavior follows the following relation:
Pressure x Volume/Temperature = Pressure x Volume/Temperature
Although the "perfect gas" relationship is not exact, it provides a basis for approximating the effect on a gas of a change in one of the three factors. In this relation, both pressure and temperature must be expressed in absolute values, pressure in psia, and temperature in degrees Rankine or degrees Fahrenheit above absolute zero. (degrees F. plus 460 degrees). Although not used in practical refrigeration work, the perfect gas relation is valuable for scientific calculations and is helpful in understanding the performance of a gas.
One of the problems of refrigeration is disposing of the heat which has been absorbed during the cooling process, and a practical solution is achieved by raising the pressure of the gas so that the saturation or condensing temperature will be sufficiently above the temperature of the available cooling medium (air or water) to insure efficient heat transfer. When the low pressure gas with its low saturation temperature is drawn into the cylinder of a compressor, the volume of the gas is reduced by the stroke of the compressor piston, and the vapor is discharged as a high pressure gas, readily condensed because of its high saturation temperature.
Specific volume of a substance is defined as the number of cubic feet occupied by one pound, and in the case of liquids and gases, it varies with the temperature and the pressure to which the fluid is subjected. Following the perfect gas law, the volume of a gas varies with both temperature and pressure. The volume of a liquid varies with temperature, but within the limits of practical refrigeration practice, it may be regarded as incompressible.
The density of a substance is defined as weight per unit volume, and in the United States is normally expressed in pounds per cubic foot. Since by definition density is directly related to specific volume, the density of a gas may vary greatly with changes in pressure and temperature, although it still remains a gas, invisible to the naked eye. Water vapor or steam at 50 psia pressure and 281 ° F. temperature is over 3 times as heavy as steam at 14.7 psia pressure and 212° F.
PRESSURE AND FLUID HEAD
It is frequently necessary to know the pressure created by a column of liquid, or possibly the pressure required to force a column of refrigerant to flow a given vertical distance upwards.
Densities are usually available in terms of pounds per cubic foot, and it is convenient to visualize pressure in terms of a cube of liquid one foot high, one foot wide, and one foot deep. Since the base of this cube is 144 square inches, the average pressure in pounds per square inch is the weight of the liquid per cubic foot divided by 144. For example, since water weighs approximately 62.4 pounds per cubic foot, the pressure exerted by 1 foot of water is 62.4 / 144 =.433 pounds per square inch. Ten feet of water would exert a pressure of 10 X .433 4.33 pounds per square inch. The same relation of height to pressure holds true, no matter what the area of vertical liquid column. The pressure exerted by other liquids can be calculated in exactly the same manner if the density is known.
Fluid head is a general term used to designate any kind of pressure exerted by a fluid which can be expressed in terms of the height of a column of the given fluid. Hence a pressure of 1 psi may be expressed as being equivalent to a head of 2.31 feet of water. (1 psi / 433 psi/ft. of water). In air flow through ducts, very small pressures are encountered, and these are commonly expressed in inches of water. 1 inch of water = .433 / 12 = .036 psi.
PRESSURE EQUIVALENTS IN FLUID HEAD
In order for a fluid to flow from one point to another, there must be a difference in pressure between the two points to cause the flow. With no pressure difference, no flow will occur. Fluids may be either liquids or gases, and the flow of each is important in refrigeration.
Fluid flow through pipes or tubing is governed by the pressure exerted on the fluid, the effect of gravity due to the vertical rise or fall of the pipe, restrictions in the pipe resisting flow, and the resistance of the fluid itself to flow.
For example, as a faucet is opened, the flow increases, even though the pressure in the water main is constant and the outlet of the faucet has no restriction. Obviously the restriction of the valve is affecting the rate of flow. Water flows more freely than molasses, due to a property of fluids called viscosity, which describes the fluid's resistance to flow. In oils, the viscosity can be affected by temperature, and as the temperature decreases the viscosity increases.
As fluid flows through tubing, the contact of the fluid and the walls of the tube create friction, and therefore resistance to flow. Sharp bends in the tubing, valves and fittings, and other obstructions also create resistance to flow, so the basic design of the piping system will determine the pressure required to obtain a given flow rate.
In a closed system containing tubing through which a fluid is flowing, the pressure difference between two given points will be determined by the velocity, viscosity, and the density of fluid flowing. If the flow is increased, the pressure difference will increase since more friction will be created by the increased velocity of the fluid. This pressure difference is termed pressure loss or pressure drop.
Since control of evaporating and condensing pressures is critical in mechanical refrigeration work, pressure drop through connecting lines can greatly affect the performance of the system, and large pressure drops must be avoided.
EFFECT OF FLUID FLOW ON HEAT TRANSFER
Heat transfer from a fluid through a tube wall or through metal fins is greatly affected by the action of the fluid in contact with the metal surface. As a general rule, the greater the velocity of flow and the more turbulent the flow, the greater will be the rate of heat transfer. Rapid boiling of an evaporating liquid will also increase the rate of heat transfer. Quiet liquid flow on the other hand, tends to allow an insulating film to form on the metal surface which resists heat flow, and reduces the rate of heat transfer.