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The evolution of refrigeration has improved the economy of almost all areas because it’s a means of preserving products while they are being shipped to customers. Refrigeration has played a large role in the development of agricultural regions because of the greater demand for products. The dairy and livestock-producing areas have also enjoyed the growth brought about by the use of refrigeration.
We can define refrigeration as the process of removing heat from an enclosed space or material and maintaining that space or material at a temperature lower than its surroundings. As heat is removed, a space or material becomes colder. The more heat is removed, the colder the object becomes. Cold, therefore, is a relative term signifying a condition of lower temperature or less heat.
There are more than 100 basic elements that make up every thing around us. There are 92 natural elements; the rest are synthetic. Everything in nature on or around the earth, moon, sun, stars, and the human body is made up of these basic elements. In most cases two or more elements are combined to make a substance.
Each element in nature is made up of billions of tiny particles known as atoms. An atom is the smallest particle of which an element can be made up and still maintain the characteristics of that element. An atom is so small that it cannot even be seen with a very powerful microscope. An atom, for our purposes, is considered to be invisible an unchangeable. An atom cannot be divided (broken up) by ordinary means. Atoms of all elements are different. That is, iron is composed of iron atoms, and hydrogen is com posed of hydrogen atoms.
Scientists know many things about atoms. How they know these things is outside the scope of this text; we must accept these as true in order to understand the subject being studied.
The molecule is the next larger particle of a material. It consists of one or more kinds of atoms. When a molecule contains atoms of only one kind, it’s said to be a molecule of that element; two or more elements combined are molecules of a chemical Compound. Usually a molecule of an element contains only one atom. However, a molecule can contain several atoms of the same kind. For example, a molecule of iron contains only one iron atom, while a molecule of sulfur usually contains eight sulfur atoms.
A small piece of any element---Consists of billions of molecules. Each of these molecules consists of one or more atoms of that element.
Fgr. 1 Three states of a substance. (a) Solid; Force; (b) Liquid; (c) Gas
Chemical compounds---The molecule of a chemical compound consists of two or more atoms of different elements. For example, carbon monoxide (CO) is a simple compound with one atom of each element. This combining of different elements causes the material to become entirely different. The new material does not resemble either of the elements that make it up. For example, a molecule of water (H2O) contains two atoms of hydrogen and one atom of oxygen.
Many of the substances used in our daily living are chemical compounds. Some of these substances are table salt, baking soda, and calcium chloride. Likewise, the refrigerants used in air-conditioning and refrigeration units are chemical compounds.
Example 1 A molecule of Refrigerant-12, a colorless gas, consists of one carbon atom, two chlorine atoms, and two fluorine atoms.
Example 2 A molecule of Refrigerant-22 consists of one carbon atom, one hydrogen atom, one chlorine atom, and two fluorine atoms.
Scientists have found that all matter is made up of small particles called molecules. These molecules may exist in three states: solid, liquid, and gas. Molecules can be broken down into atoms. Atoms are discussed in more detail in Section 7.
However, we will study the theory of molecular movement and action because it’s involved in air-conditioning and refrigeration systems. A molecule is the smallest particle to which a compound can be reduced before breaking down into its original elements. For instance, water is made up of two elements, hydrogen and oxygen. The movement or vibration of these molecules determines the amount of heat present in a given body. This heat is caused by the friction of the molecules rubbing against each other. The attraction of these molecules to each other is reduced as the tempera ture increases. When a substance is cooled to absolute zero, all molecular motion stops. At this temperature the sub stance contains no heat.
Molecules vary in weight, shape, and size. They tend to cling together to form a substance. The substance will assume the character of the combining molecules. Because molecules are capable of moving around, the substance will be, to a degree, dependent on the space between them. The molecules in a solid have less space between them than the molecules in either a liquid or a gas. A liquid has more space between the molecules than a solid and less space than a gas. A gas has more space between the molecules than either a solid or a liquid. Many substances can be made to exist in any of these three forms depending on their temperature and pressure. Water is a very common example of this type of substance.
In solids, the vibrating rate of the molecules is very slow. Therefore, the attraction of the molecules to each other is very strong and a solid must have support or it will fall.
In liquids, the vibrating motion of the molecules is faster than in solids. Therefore, the attraction of the molecules to each other is less and a liquid must be kept in a container of some type. See Fgr. 6—lb. The higher the temperature of the molecules, the faster they vibrate. The warmer molecules will move upward in the container to ward the surface of the liquid because they are less attracted to each other and require more space. Therefore, they become lighter and rise upward.
The force exerted by a liquid is toward the sides and to the bottom of the container. The force will be greater on the bottom than on the sides because of the weight of the liquid. As the surface of the liquid is approached, the force will decrease because of the reduced weight of the liquid.
In gases, the vibrating motion of the molecules is even faster than in liquids. Therefore, the attraction of the molecules to each other is very small and a gas must be kept in a closed container or it will escape to the atmosphere. A gas will take the shape of the container on all sides. See Fgr. 6—1c. The molecules of gas have little or no attraction for each other as well as molecules of other substances.
The force exerted by a gas is equal in all directions. Most gases are lighter than air. Therefore, they tend to float upward, causing a weightless condition.
With the proper regulation of temperature, any sub s can be made to remain in any of the three forms: solid, liquid, or gas. Also, any substance can be made to change from one form to another by the proper use of temperature and pressure. This change in form is known as the change of state.
CHANGE OF STATE
The addition of heat to a substance may cause, in addition to a rise in the temperature of that substance, a change of state of that substance. That is, an addition of heat may cause a substance to change from a solid to a liquid, or from liquid to a gas. There are three states of any substance. The states of water are: ice, water, and steam, that is, solid, liquid, and gas.
There are two terms that should not be confused. They are teat and temperature. Heat is considered as the measure of quantity; temperature is the measure of degree or intensity. For instance, if we have a 1-gallon container of water and a 2-gallon container of water and both are boiling, the 2-gal of water will contain twice as much heat as the 1 gallon, en though they are both 212°F (or the same temperature).
Temperature is measured in degrees with a thermometer. Heat is measured in Btu’s (British Thermal Units). A btu is defined as the amount of heat required to raise the temperature of 1 pound of pure water 1°F.
Methods of Heat Transfer:
It’s important to know that heat always flows from a warmer object to a cooler object. The rate of heat flow de ends on the temperature difference between the two objects. For example, consider two objects lying side by side r an insulated box. One of the objects weighs 1 pound and as a temperature of 400°F, while the second object weighs 1000 pounds and has a temperature of 390°F. The heat content of the larger object will be far greater than that of ne smaller object. However, because the temperatures are different, heat will flow from the smaller object to the larger object until their temperatures are the same.
The three ways that heat travels are: (1) conduction, convection, and (3) radiation.
Conduction---Conduction is the flow of heat through an object. For heat transfer to take place between two objects, the objects must touch. This is a very efficient method of heat transfer. If you have ever heated one end of a piece of metal and then touched the other end, you felt the heat that had been conducted from the heated end of the metal.
(a) Heat Flow; Single Object; Warmer Heat (b) Two Objects -- Flow; Colder
Fgr. 2 Heat transfer by conduction.
Convection---Convection is the flow of heat by the use of a fluid, either gas or liquid. The fluids most commonly used with this method are air and water. The heated fluids are less dense and rise, while the cooler fluids are more dense and fall, thus creating a continuous movement of the fluid. Another example of convection is the heating furnace. The air is heated in the furnace and blown into a room to heat the objects in the room by convection.
Radiation---Radiation is the transfer of heat by wave motion. These waves can be light waves or radio-frequency waves. The form of radiation that are most familiar with is the sun’s rays. When heat is transferred by radiation, the air between the objects is not heated, as can be noticed when a person steps from the shape into the direct sunlight. The air temperature is about the same in either place. However, you feel warmer in the sunlight. See Fgr. 6—3. This is because of the heat being conducted by the rays of the sun.
Fgr. 3 Heat transfer by radiation.
Table 1 Specific heat of foods.
There is little radiation at low temperatures and at small temperature differences. Therefore, heat transfer by radiation is of little importance in actual refrigeration applications. However, if the refrigerated space is located in the direct rays of the sun, the cabinet will absorb heat. This heat absorption from direct sunlight can be a major factor in the calculation of the heat load of a refrigerated space.
Heat will travel in a combination of these processes in a normal refrigeration application. The ability of a piece of refrigeration equipment to transfer heat is known as the overall rate of heat transfer. As was learned earlier, heat transfer cannot take place without a temperature difference. Different materials have different abilities to conduct heat. Metal is a good conductor of heat.
Sensible heat---Heat added to or removed from a sub stance, resulting in a change in temperature but not change of state, is called sensible heat. The word sensible as applied to heat refers to that which can be sensed with a thermometer. An example of sensible heat is when the temperature of water is raised from 42 to 212°F. There was a change in temperature of 170°F. This change is sensible heat. It can be measured with a thermometer.
Latent heat---Heat added to or removed from a sub stance, resulting in a change of state but no change in temperature, is called latent heat. The types of latent heat are:
(1) latent heat of fusion, (2) latent heat of condensation, (3) latent heat of vaporization, and (4) latent heat of sublimation.
Latent heat of fusion---Latent heat of fusion is the amount of heat that must be added to a solid to change it to a liquid at a constant temperature. Latent heat of fusion is also equal to the heat that must be removed from a liquid to change this liquid to a solid at a constant temperature.
Latent heat of condensation Latent heat of condensation is the amount of heat that must be removed from a vapor to condense it to a liquid at a constant temperature.
Latent heat of vaporization Latent heat of vaporization is the amount of heat that must be added to a liquid to change it to a vapor at a constant temperature.
Latent heat of sublimation Latent heat of sublimation is the amount of heat that must be added to a substance to change it from a solid to a vapor, with no evidence of going through the liquid state. This process is not possible in all substances. The most common example of this is dry ice. The latent heat of sublimation is equal to the sum of the latent heat of fusion and the latent heat of vaporization.
Specific heat---The amount of heat required to raise the temperature of 1 pound of any substance 1°F is called specific heat. Specific heat is also the ratio between the quantity of heat required to change the temperature of a substance 1°F and the amount of heat required to change an equal amount of water 1°F.
Specific Heat Food (Unfrozen) (Frozen)
Btu: Veal Beef Pork Fish Poultry Eggs Butter Cheese Whole Milk
From the definition of a Btu given earlier, it can be seen that the specific heat of water must be 1 Btu per pound. The specific heat values of some of the more popular foods are given. If you will note, after the foods are frozen their specific heat values drop considerably. It may be assumed that the specific heat is a little more than one- half of what it was before the foods were frozen.
Superheat Heat added to a vapor after the vapor is no longer in contact with its liquid is called superheat. For example, when enough heat is added to a liquid to cause all the liquid to vaporize, any additional heat added to the vapor is termed superheat.
As stated earlier, temperature is a measure of the degree or intensity of heat. The device used to measure temperature is a thermometer. There are two types of thermometers:
Fahrenheit and Centigrade ---- In the United States the Fahrenheit thermometer is the most often used, while in Europe the Centigrade thermometer is more common.
Because refrigeration systems depend basically on pressure differences inside the system, a basic understanding of pres sure and the laws that govern it’s very important to the designer and the technician.
Pressure is defined as the weight of force per unit area and is generally expressed in pounds per square inch (psi) or pounds per square foot. The normal atmospheric pressure at sea level is 14.7 psi.
All substances exert pressure on the materials that support them. A guide exerts pressure on the table. A liquid exerts pressure on the bottom and the sides of its container, and a gas exerts pressure on all the surfaces of its container, such as a balloon.
Vapor at 222°F (105.6°C)
Fgr. 4 Explanation of superheat.
If we had a cube of 1 inch in all dimensions that weighed 1 pound, it would exert a pressure of 1 psi on a the top when it’s placed on it.
The liquid in a container maintains a greater pressure on the bottom and sides of its container as the liquid levels raised. However, gases don’t always exert a constant pressure on the container because the amount of pressures determined by the temperature and the quantity of gas inside the container.
The air around us exerts pressure, which is called atmospheric pressure. All liquids have a definite boiling temperature at atmospheric pressure. If the pressure over a liquid is increased, the boiling temperature will also be increased. If the pressure over a liquid is lowered, the boiling temperature will also be lowered. All liquids have a definite boiling temperature for each pressure. This is one of the basic principles used in refrigeration work.
The pressure exerted on the earth by the atmosphere above us is called atmospheric pressure. At any given point, this atmospheric pressure is relatively constant, except for changes caused by the weather. As a basic reference for comparison, the atmospheric pressure at sea level has been universally accepted as being 14.7 psi. This pressure is equal to that exerted by a column of mercury 29.92 inches high.
The depth of the atmosphere is less at altitudes above sea level; therefore, the atmospheric pressure is less on a mountain. For example, at 5000 feet elevation the atmospheric pressure is only 12.2 psi.
Boiling Temperature of Water
Freezing Temperature of Water
Fgr. 5 Comparison of the Fahrenheit and Centigrade thermometer scales.
The pressure measured from a perfect vacuum is called absolute pressure. Atmospheric pressure is 14.7 psi. Absolute pressure is normally expressed in terms of pounds per square inch absolute (psia). Absolute pressure is equal to gauge pressure plus atmospheric pressure. To find absolute pressure, add 14.7 to the gauge pressure reading.
Constant atmospheric pressure
Fgr. 6 Application of Charles’ law.
Table 2 Comparison of Atmospheric and Absolute Pressure at Varying Attributes.
Gauge pressure is zero pounds per square inch gauge (psig) when the gauge is not connected to a source of pres sure. Pressures below 0 psig are negative gauge readings and are commonly referred to as inches of vacuum. Refrigeration compound gauges are calibrated in inches of mercury (inch Hg) for readings below atmospheric pressure. Since 14.7 psi is equal to 29.92 inches Hg, 1 psi is equal to 2 inches Hg: 29.92/14.7 = 2.03 inches Hg
It should be remembered that gauge pressures are only relative to absolute pressures.
The measurement of low pressures requires a unit of measurement smaller than the pound or the inch of mercury. The micron is commonly used for measuring low pressures. A micron is a metric measurement of length and is used in measuring the vacuum in a refrigeration system. It’s considered as being absolute pressure.
One micron is equal to 1/1000 of a millimeter. There are 25.4 millimeters in 1 inch. Therefore, one micron is equal to 1/25,400 of an inch. A system that has been evacuated to 500 microns would have an absolute pressure of 0.02 inch Hg. At standard conditions this would be equal to a vacuum reading of 29.90 inches Hg, which is impossible to read on a refrigeration compound gauge.
Effects of temperature---The effects of temperature on the pressure of gases is of great importance in the refrigeration industry. It must be thoroughly understood before a good knowledge of the refrigeration cycle can be obtained. There are several scientific laws that govern the effects of temperature on the pressure of a vapor within a confined space. These laws are as follows:
Charles’ law Charles’ law of gases states that “With the pressure constant the volume of a vapor is directly proportional to its absolute temperature.” In mathematical form this is stated as V T where V is the old volume, V is the new volume, T is the old temperature, and T is the new temperature.
In practical applications this can be proven by use of a properly fitted piston within a cylinder. In this example the cylinder is fitted with a sliding piston.
(a) Constant atmospheric pressure; moving piston
Refrigerant Boiling Points (°F); Altitude; Boiling Point of Water
Fgr. 7 Application of Boyle’s law.
The cylinder is full of vapor at atmospheric pressure. Heat is applied to the cylinder, causing the temperature to rise. Because the piston is easily moved, the volume of vapor increases but the pressure remains constant at atmospheric pressure. On the other hand, if the gas is cooled, the volume of vapor will become smaller. If it were possible to cool the vapor to absolute zero temperature, -460°F, the volume would be zero because there would be no molecular movement at this temperature.
• The mechanical equivalent of heat is the heat produced by the expenditure of a given amount of mechanical energy.
• The six components of a basic refrigeration system are: compressor, condenser, flow control device, evaporator, connecting tubing, and refrigerant.
Boyle’s law Boyle’s law of partial gases states that “With the temperature constant, the volume of a gas is inversely proportional to its absolute pressure.” In mathematical form this is stated as P = P where P is the old pressure, P is the new pressure, V is the old volume, and V is the new volume.
As before, this can be proven by the use of a cylinder with a properly fitted piston. See Fgr. 6—7a. By taking simultaneous temperature and pressure readings as the vapor is slowly compressed within the cylinder so that no temperature increase will be experienced, each side of the equation will always be equal. This is possible because a decrease in volume will always be accompanied by an in crease in pressure.
Dalton’s law of partial pressures Dalton’s law of partial pressures states that “Gases occupying a common volume each fill that volume and behave as though the other gases were not present.” This law, along with the combination of Charles’ law and Boyle’s law, forms the basis for deriving the psychrometric properties of air.
A practical application of Dalton’s law is: The total pressure inside a cylinder of compressed air, which is a mixture of oxygen, water vapor, and carbon dioxide, is found by adding together the pressures exerted by each of the individual vapors.
Fgr. 8 Application of Pascal’s law.
Pascal’s law Pascal’s law states that “The pressure applied upon a confined fluid is transmitted equally in all directions.” A practical application of Pascal’s law is shown with a cylinder of liquid and a properly fitted piston. See Fgr. 6—8. The piston has a cross-sectional area of 1 square inch. With 100 psig pressure applied to the piston, the pres sure gauges show that the pressure exerted in all directions is equal.
This is the basic principle used in most hydraulic and pneumatic systems.
The general gas law---The general gas law is made by combining Boyle’s law and Charles’ law of gases. The general gas law is sometimes expressed as where P is the absolute pressure of the vapor, in pounds per square feet, V is the volume of the given quantity of vapor, in cubic feet, M is the weight of the given amount of vapor, in pounds, R is the universal gas constant of 1545.3 divided by the molecular weight of the vapor, and T is the absolute temperature of the vapor.
P = P
A more useful form of the equation is:
PV = MRT
The general gas law may be used to study changes in the conditions of a vapor as long as absolute temperature and absolute pressure are used. Gauge pressure cannot be used. This law is used in calculating the psychrometric properties of air in air-conditioning systems.
Pressure—temperature relationships Of vital importance in refrigeration equipment design and servicing is the relationship between temperature and pressure. The temperature at which a liquid will boil is dependent on the pressure applied to it, and the pressure at which it will boil is dependent on its temperature. From this it can be seen that for each pressure exerted on a liquid there is also a definite temperature at which it will boil, providing an uncontaminated liquid is being measured.
Old pressure and volume; New pressure and volume
100 Pound Force
In practice, because all liquids react in the same manner, pressure provides us with a convenient means of regulating the temperature inside a refrigerated space. When an evaporator is part of a refrigeration system that is isolated from the atmosphere, pressure can be applied to the inside of the evaporator which is equal to the boiling pressure of the liquid at the desired cooling temperature. The liquid will boil at that temperature and, as long as heat is absorbed by the liquid, refrigeration is being accomplished.
This process is also reversible. When the pressure over a vapor is increased enough to cause the temperature of the vapor to be higher than the surrounding medium, heat will be given up and condensation of the gas will occur. This is the principle used in the condenser of a refrigeration system.
As stated above, cooling is merely the removal of heat from a substance. This cooling can be accomplished in several ways. However, we will discuss only the evaporation and expansion methods at this time.
Evaporation---The process that causes the water left in an open container to disappear is called evaporation. Evaporation of water depends on two things: temperature and moisture in the air. When water is left in an open container in the summertime, it dries rapidly because of the high temperature. If the temperature drops, the rate of evaporation will decrease. If the temperature goes up, the rate of evaporation will increase. Even at temperatures of —40°F evaporation takes place.
If the air is nearly saturated with moisture, it will absorb additional moisture very slowly. If the air is dry, it will absorb moisture very rapidly. During a hot spell when the air is muggy or humid, evaporation of the perspiration from the human body is slow because the saturated air absorbs moisture very slowly.
When water evaporates, a sufficient amount of heat will be absorbed in order to supply the latent heat of vaporization. This heat is absorbed from the water itself or from any object (or air) in contact with the water.
The cold, clammy feeling experienced by a person wearing a wet bathing suit is caused by evaporation of the water and absorption of heat from the material of the suit and from the skin of the person.
Expansion---The process that causes a cooling effect by compressing a vapor, then rapidly reducing the pressure on the vapor is called expansion. When a vapor is com pressed, heat is generated in an amount equivalent to the amount of work done in compressing the vapor. This may be demonstrated by the bicycle pump that gets hot due to the heat developed by compressing air into the tire. The greater the compression, the higher the temperature of the compressed air.
The cooling action that takes place when compressed air is allowed to expand is just the reverse of the compression effect. After a long ride over hot roads, the air in an automobile tire becomes very hot and the pressure within the tire is increased. If the valve stem were opened and the hot air allowed to escape, the air would become cool as it was released due to the expansion and resulting reduction in pressure of the air.
Some of the early refrigeration systems used this principle. The air was compressed and then cooled by passing it through a water-cooled coil. The water was used to absorb and carry away the heat from the compressed air. When the air was allowed to expand, the temperature dropped in relation to the amount of heat that was removed by the water while it was in the compressed state.
There are three steps involved in the expansion refrigeration cycle:
1. Air or gas is compressed to a high pressure.
2. The heat produced by compression is removed.
3. The air or vapor is expanded, causing a reduction in temperature through the absorption of heat by the air.
Sub-cooling---Sub-cooling occurs when a liquid is at a temperature lower than the Saturation temperature corresponding to its pressure. Water at any temperature below its boiling temperature (212°F) at sea level is said to be sub-cooled.
In refrigeration systems this sub-cooling may occur while the liquid refrigerant is temporarily stored in the con denser or the receiver. Some of the heat may also be dissipated to the ambient temperature while passing through the liquid line on route to the flow control device. The use of a sub-cooler will pay for itself through the increased capacity and efficiency of the refrigeration system.
There has been a great amount of money spent on re searching the effects of heat and non-condensables in a refrigeration system. Even now, many of the effects are still a mystery. We do know that their presence can result in many forms of damage in a refrigeration system such as sludging, corrosion, oil breakdown, carbon formations, and copper plating. These contaminants are usually the cause of compressor failure. Dehydration, also known as evacuation, is the removal of air, moisture, and non-condensables from a refrigeration system.
Purposes of dehydration---Dehydration protects the refrigeration system as much as possible from contaminants, and causes it to operate as efficiently as possible with a minimum amount of equipment failure.
The methods of dehydration are many and varied. As stated above, two methods used to cause a liquid to boil are lowering the pressure exerted on it and applying heat to the liquid. Some of the ways to eliminate moisture from a refrigeration system by the boiling process are:
Flow Control Device:
1. Move the system to a higher elevation where the ambient temperature is high enough to boil water at the existing pressure.
2. Apply heat to the system, causing the moisture inside it to boil.
3. Use a vacuum pump to lower the pressure inside & refrigeration system so that the ambient temperature will boil the moisture.
In practice, the first two choices are impractical. Therefore, the vacuum pump method is the most desirable means that removing moisture and non-condensables from a system. To accomplish effective dehydration, the refrigeration system must be evacuated to at least 500 microns.
Before attempting to charge a system with refrigerant, it must be evacuated with some type of vacuum pump. This s especially true since manufacturers of refrigeration and air-conditioning units have changed most of their equipment to air-cooled condensers. This change has resulted n higher operating head pressures and temperatures and higher condensing pressures, especially when hermetic compressors are used. Also, the greater use of low-temperature refrigeration equipment results in higher compression ratios. When single-stage compressors are used, still higher discharge temperatures are present.
These higher head temperatures make it even more important to remove all the air, as well as the moisture, from ne system to a point below the critical point. The process of removing the air from the system can be referred to as removing the non-condensables, or “degassing” the unit. The worst contaminant is the oxygen in the remaining air. Oxygen is one of nature’s most chemically active elements, and its rate of reaction with the refrigeration oil in the system increases very rapidly with any increase in temperature above 200°F.
Because of this, the most important factor is removing i of the oxygen from the system, or at least to a very minimum. In the process of removing all of the oxygen, the system will also become adequately dehydrated. filter—driers are installed in the refrigerant lines to pick up any contaminants, such as soldering flux, any moisture that nay exist in the oil and refrigerant charge, and materials of :
BASIC REFRIGERATION SYSTEM
The basic components of a refrigeration system are the compressor, condenser, flow control device, evaporator, connecting tubing, and the refrigerant. The compressor is known as the heart of the system. It causes the refrigerant to circulate in the system. The refrigerant is pushed by the compressor to the condenser where both sensible and latent heat are removed. The liquefied refrigerant then goes to the flow control device where the pressure as reduced, allowing the refrigerant to expand and absorb heat from within the refrigerated cabinet. This low-pressure, heat-laden refrigerant vapor is then drawn to the compressor where the cycle is repeated.
Fgr. 9 Basic refrigeration system.
The vapors or liquids used in refrigeration systems are known as refrigerants. The evaporation of the refrigerant within the evaporator extracts heat from the surrounding objects. The various parts of the refrigeration unit compress and condense the refrigerant so that it can be used over and over again. Even though there are many different types of refrigerants, only the more common fluorocarbons will be considered here.
For practical purposes, a refrigerant is a fluid that absorbs heat by evaporating at a low temperature and pressure and gives up that heat by condensing at a higher temperature and pressure.
Characteristics of Refrigerants:
Most of the commonly used refrigerants exist in a vaporous state under ordinary atmospheric pressures and temperatures. To change these vapors to liquid form, it’s necessary to compress and cool them as is done by the condensing unit on a refrigeration system. A fluid is a liquid, gas, or vapor. The words gas and vapor are ordinarily used interchangeably, although to be perfectly technical, perhaps we should explain that a gas near its condensation point is called a vapor. All fluids have both a liquid and a gaseous state. Some fluids have high boiling points, which means that they exist as a gas only when heated to a high tempera ture or when placed under a vacuum. Fluids that have low boiling points are in the form of vapor at ordinary room temperatures and pressures. Many of the more common refrigerants such as those in the freon group are in this category. If these vapors are to be liquefied, they must be compressed and cooled, or condensed.
Water is a fluid that exists as a liquid at atmospheric pressure and temperature. The boiling point of water under atmospheric pressure at sea level is 212°F. If the water is left in an open basin, it will evaporate very slowly. If heat is applied to the water and its temperature raised to its boiling point, it will then evaporate, or boil, very rapidly. The water will change to the gaseous form of water known as steam or water vapor. If the water is boiled in an open container, its temperature won’t rise above 212°F. All the heat sup plied by the flame is used to boil off or vaporize the water.
If a liquid refrigerant is similarly placed in an open container, it will immediately begin to boil vigorously and vaporize, but at a very low temperature. Liquid Refrigerant- 12, under atmospheric pressure, will boil at —21.6°F. It will absorb sufficient heat from the container and the surrounding air to enable it to boil. It would not be necessary to heat it with a flame as is done with the water.
A vaporizing refrigerant will absorb heat equal to the amount of energy necessary to change the physical form of the refrigerant from the liquid state to the gaseous state. Each refrigerant will absorb an amount of heat per pound of refrigerant equal to its latent heat of vaporization.
Effects of Pressure on Boiling Point
The boiling point of any liquid may be raised or lowered in accordance with the amount of pressure applied to the liquid. The greater the pressure, the higher is the boiling point; the lower the pressure, the lower is the boiling point. Thus, a liquid can be caused to boil at a low temperature by placing it in a partial vacuum.
The capacity of any refrigeration unit will vary as the refrigerant temperatures vary on the high- and low-pressure sides of the system. The latent heat of the refrigerant, its condensing pressure, and its vaporizing pressure will also vary as the temperature of the refrigerant varies. For purposes of comparing different refrigerants and refrigeration units, certain standard conditions have been developed. The refrigeration industry has set forth conditions known as standard conditions. These conditions are established with the following temperatures at various locations in the refrigeration cycle: a temperature in the evaporator of 5°F; a temperature in the saturated portion of the condenser of 86°F; a liquid temperature at the flow control device of 77°F; and a suction gas temperature of 14°F.
Now if we use the following factors in comparing any two refrigerants, with these temperatures as a basis, we have a true comparison and can arrive at correct conclusions.
Condensing pressure---The condensing pressure will depend on the temperature at which the vapor will liquefy. In refrigeration work, it’s desirable to avoid high condensing pressures if at all possible. Therefore, the condensing medium (air or water) must be as cool as possible. Ordinarily, a water-cooled condenser will operate at a lower condensing temperature and pressure than an air-cooled condenser. Because of this, there is some difference in the operating pressures for these two types of condensers.
In general, it may be assumed that the condensing temperature and pressure of an air-cooled unit will be approximately 25 to 35°F higher than ambient temperatures.
Table 3 Vaporizing pressures of refrigerants at 5°F.
Pressure at —15°C
The actual temperature and pressure will, however, depend on the efficiency of the condenser itself, the location of the condenser, whether or not sufficient air circulation is obtained, and the cleanliness of the condenser surface.
Vaporizing pressure The vaporizing pressure of a refrigerant is important because the refrigerant must evaporate without requiring too low a suction pressure. A temperature of 5°F is considered the temperature of most domestic refrigerator evaporators. This is the same temperature as that set for the standard conditions used for comparison of different refrigerants and refrigeration units. In general, a refrigerant is desired that has an evaporating pressure at or near atmospheric pressure. See Table 6—3. A refrigerant that requires a vacuum to produce evaporation is not practical under ordinary conditions because of the tendency for air to leak into the system. The air won’t condense and will cause the condensing pressure to become very high. This high pressure will reduce the efficiency of the refrigeration unit. Refrigerants that have a vaporizing pressure above atmospheric pressure don’t allow air to be drawn into the system through a leak.
It should be noted that the pressure in the evaporator and the low side of the system will be the same. Also, the temperature of the evaporating refrigerant will correspond to the temperature shown on the pressure—temperature chart on the gauges, the pressure in the evaporator, or in the low side of the system.
Latent heat of vaporization The amount of heat (in Btu) required to change a liquid to a vapor, the change taking place at a constant temperature, is known as the latent heat of vaporization. This definition can now be developed so as to apply to 1 pound of refrigerant, the vaporization taking place at atmospheric pressure and the liquid to be at a temperature equal to its boiling point when the operation begins. Thus, the latent heat of vaporization of a liquid is the amount of heat (in Btu) required to vaporize 1 pound of the liquid at atmospheric pressure, the liquid to be at its boiling point when the operation begins. To convert 1 pound of water at 212°F into steam at the same temperature and pressure, the water must absorb 970 Btu per pound This quantity of heat is equal to the total latent heat of 1 pound of water at atmospheric pressure.
Table 4 Latent heat of vaporization.
R-502 at 40°F
Any refrigerant, when evaporating in the evaporator, must absorb heat from within the cooled space exactly equal r its latent heat of vaporization. When a refrigerant has a high latent heat, it will absorb more heat per pound of liquid and a refrigerant with a lower latent heat of vaporization. Thus, if a refrigerant with a high latent heat of vaporization s used, a smaller compressor, condenser, and evaporator an be used.
The latent heat of vaporization of a liquid will vary with the pressure and the corresponding temperature at which the vaporization occurs. When lower temperatures and pressures are encountered, the latent heat of vaporization increases.
Types of Refrigerants:
There are many types of commercially available ref rig rants. The types that are commonly used in domestic refrigerators and freezers are nontoxic and are not dangerous, rr the most part. For normal domestic refrigerators and freezers Refrigerant-12 and Refrigerant-22 are the most pop . Both of these refrigerants are nontoxic and are safe to in food-handling applications. They operate at relatively .cw pressures with evaporating pressures above atmospheric pressure.
Handling of Refrigerant Cylinders:
The pressure created by a liquid refrigerant in a sealed container is equal to its saturation pressure at that liquid temperature as long as there is space above the liquid for rte vapor. However, if the refrigerant cylinder is overfilled,
1 the cylinder is gradually and uniformly overheated, the liquid refrigerant will expand until the cylinder becomes full ± liquid. When this occurs, hydrostatic pressure builds up rapidly, producing pressures well above saturation pres s. After the cylinder becomes full of the expanded liquid under gradual and uniform overheating, the true pres sure—temperature relationship no longer exists.
The extremely dangerous pressures that can result under such circumstances can cause the rupture of the refrigerant cylinder. Under uniform heating conditions the cylinder can rupture at approximately 1300 psi. If, however, heat is applied with a welding torch, the area of the cylinder wall where the heat is applied may be weakened and the danger of rupture increased.
1. In what three states may a molecule exist?
2. What determines the amount of heat present in a given body?
3. At what temperature does all molecular motion stop?
4. When heat is added to a substance, what other than a rise in temperature happens to that substance?
5. By the regulation of heat, can any substance be caused to remain in any of its three states?
6. To what does the degree or intensity of heat refer?
7. With what is temperature measured?
8. Name the three ways that heat travels.
9. What is the heat added to or removed from a substance resulting in a change in temperature but no change of state known as?
10. What is the amount of heat that must be added to a solid to change it to a liquid called?
11. What is the amount of heat that must be removed from a vapor to condense it to a liquid at a constant tempera ture known as?
12. What is the amount of heat required to raise the temperature of 1 lb of any substance one degree known as?
13. What is the specific heat of water?
14. What is the heat added to a vapor after the vapor is no longer in contact with its liquid known as?
15. Define pressure.
16. Define atmospheric pressure.
17. What is absolute pressure?
18. What pressure is read when a gauge is not connected to a source or pressure?
19. Name the six components of a refrigeration system.
20. How will an increase in pressure on a liquid affect its boiling point?
21. What is the removal of heat from a substance known as?
22. Define subcooling.
23. What must be done to a refrigeration system be fore charging it with refrigerant?
24. What is considered to be the evaporating tempera ture of most domestic refrigerators?
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