The refrigerator was not manufactured until the 1920s. Before that time, ice was the primary source of refrigeration. A block of ice was kept in the icebox. The icebox was similar to the modern refrigerator in construction. It was well insulated and had shelves to store perishables. The main difference was the method of cooling.
The iceman came about once a week to put a new 50- or 100-lb block of ice in the icebox. How much cooling effect does a 50-lb block of ice produce? The latent heat of melting for 1 lb of ice is 144 Btu. The latent heat of melting for a 50-lb block is 50 ´ 144, or 7200 Btu. The latent heat of melting for the 100-lb block was 14,400 Btu. The refrigeration was accomplished by convection in the icebox.
One of the first refrigerators is shown in Fig. 2-9. The unit on the top identified it as a refrigerator instead of an icebox. Some of these units, made in the 1920s, are still operating today.
Fig. 2-9 Early modification of the icebox to make it a refrigerator unit.
Refrigeration from Vaporization (Open System)
The perspiration on your body evaporates and cools your body. Water kept in a porous container is cooled on a hot day. The water seeps from the inside. There is a small amount of water on the outside surface. The surface water is vaporized—it evaporates.
Much of the heat required for vaporization comes from the liquid in the container. When heat is removed this way the liquid is cooled. The heat is carried away with the vapor.
Basic Refrigeration Cycle
A substance changes state when the inherent amount of heat is varied. Ice is water in a solid state and steam is a vapor state of water. A solid is changed to liquid and a liquid to a vapor by applying heat. Heat must be added to vaporize or boil a substance. It must, be taken away
to liquefy or solidify a substance. The amount of heat necessary will depend on the substance and the pressure changes in the substance.
Consider, for example, an open pan of boiling water heated by a gas flame. The boiling temperature of water at sea level is 212ºF (100°C). Increase the temperature of the flame and the water will boil away more rapidly, although the temperature of the water will not change. To heat or boil a substance, heat must be removed from another substance. In this case, heat is removed from the gas flame. Increasing the temperature of the flame merely speeds the transfer of heat. It does not increase the temperature of the water.
A change in pressure will affect the boiling point of a substance. As the altitude increases above sea level, the atmospheric pressure and the boiling temperature drop. For example, water will boil at 193°F (89.4°C) at an altitude of 10,000 feet. At pressures below 100 psi, water has a boiling point of 338°F (170°C).
The relationship of pressure to refrigeration is shown in the following example. A tank contains a sub- stance that is vaporized at atmospheric pressure. How- ever, it condenses to a liquid when 100 lb of pressure are applied. The liquid is discharged from the tank through a hose and nozzle into a long coil of tubing to the atmosphere. See Fig. 2-10.
Fig. 2-10 Basic step of refrigeration. (Johnson)
As the liquid enters the nozzle, its pressure is reduced to that of the atmosphere. This lowers its vaporization or boiling point. Part of the liquid vaporizes or boils, using its own heat. The unevaporated liquid is immediately cooled as its heat is taken away. The remaining liquid takes heat from the metal coil or tank and vaporizes, cooling the coil. The coil takes heat from the space around it, cooling the space. This unit would continue to provide cooling or refrigeration for as long as the substance remained under pressure in the tank.
All of the other components of a refrigeration system are merely for reclaiming the refrigeration medium after it has done its job of cooling. The other
parts of a refrigeration system, in order of assembly, are tank, or liquid receiver, expansion valve, evaporator coil, compressor, and condenser.
Figure 2-11 illustrates a typical refrigeration system cycle. The refrigerant is in a tank or liquid receiver under high pressure and in a liquid state. When the refrigerant enters the expansion valve, the pressure is lowered, and the liquid begins to vaporize. Complete evaporation takes place when the refrigerant moves into the evaporator coil. With evaporation, heat must be added to the refrigerant. In this case, the heat comes from the evaporator coil. As heat is removed from the coil, the coil is cooled. The refrigerant is now a vapor under low pressure. The evaporator section of the system is often called the low pressure, back pressure, or suction side. The warmer the coil, the more rapidly evaporation takes place and the higher the suction pressure becomes.
The compressor then takes the low-pressure vapor and builds up the pressure sufficiently to condense the refrigerant. This starts the high side of the system. To return the refrigerant to a liquid state (to condense it), heat picked up in the evaporator coil and the compressor must be removed. This is the function of the con- denser used with an air- or water-cooled coil. Being cooler than the refrigerant, the air or water absorbs its heat. As it cools, the refrigerant condenses into a liquid and flows into the liquid receiver or tank. Since the pressure of the refrigerant has been increased, it will condense at a lower temperature. In some systems, the liquid receiver may be part of another unit such as the evaporator or condenser.
Capacity
Refrigeration machines are rated in tons of refrigeration. This rating indicates the size and ability to pro- duce cooling energy in a given period. One ton of refrigeration has cooling energy equal to that produced by one ton of ice melting in 24 hours. Since it takes 288,000 Btu of heat to melt 1 ton of ice, a 1-ton machine will absorb 288,000 Btu in a 24 hour period.
Refrigerants
Theoretically, any gas that can be alternately liquefied and vaporized within mechanical equipment can serve as a refrigerant. Thus, carbon dioxide serves as a refrigerant on many ships. However, the piping and machinery handling it must be very heavy-duty.
Practical considerations have led to the use of several refrigerants that can be safely handled at moderate pressures by equipment having reasonable mechanical strength and with lines of normal size and wall thick- ness. While no substance possesses all the properties of an ideal refrigerant, the hydrocarbon (Freon) refrigerants come quite close
Fig. 2-11 High and low sides of a refrigeration system..
Refrigerant 12 is made of carbon (C), chlorine (Cl), and fluorine (F). Its formula is CC12F2. It is made of a combination of elements. Refrigerant 22 is made of carbon (C), hydrogen (H), chlorine (Cl), and fluorine (F). Its formula CHClF2 is slightly different from that of R-12.
Each of these manufactured refrigerants has its own characteristics, such as odor and boiling pressure. Refrigerants are the vital working fluids in refrig. Today it is no longer used. It will explode when allowed to combine with air. It is nontoxic.
Refrigerant Replacements and the Atmosphere
Refrigerants such as ammonia are used for low- temperature systems. These include food and process cooling, ice rinks, and so forth. Propane has been used for some special applications. Now that chlorinated hydrocarbons have been determined to be harmful to the earth’s ozone layer, R-11 (CCl3F), R-12 (CCl2F2), and other similar compounds that in were common use refrigeration systems. They transfer heat from one place to along with the less harmful refrigerant R-22 (CHClF )
Many substances can be used as refrigerants, including water under certain conditions. The following are some common refrigerants:
- Ammonia. The oldest commonly used refrigerant, still used in some It is very toxic.
- Sulphur dioxide. First to replace ammonia and to be used in small domestic It is very toxic.
- Refrigerant 12 . The first synthetic refrigerant to be
tional protocols (standards) have set schedules for the elimination of damaging refrigerants from commercial use. Replacements have been, and are being, developed. Part of the challenge is technical and part is economic. First, to find a fluid that has optimal characteristics and is safe is a challenge. Second, to encourage manufacture in sufficient quantities to produce and distribute the fluid at an affordable price is another. R-123 (CHCl2CF3) has been developed as a near-equivalent replacement for R-11, with R-134a (CH FCF )
- Refrigerant Used in many of the same applications as R-12. Its lower boiling point and higher la- tent heat permit the use of smaller compressors and refrigerant lines. It is nontoxic.
- Refrigerant Methylchloride is used in the commercial refrigeration field, particularly in small
some chlorine in it. R-134a can be bought at auto sup- plies stores for automobile air conditioners. Most new cars are required to have R-134a in their A/C systems.
R-22 is used widely in residential and commercial air- conditioning scroll compressor systems. It too will be phased out someday (probably during the period 2020–2030). However, finding a suitable, widely accepted replacement has not come as quickly as first thought.
