The electric refrigerator operates on a rather simple principle. A volatile fluid, the refrigerant, flows under high pressure from a flask through a nozzle into a coiled pipe, the freezing coil. The liquid absorbs heat from the coil and evaporates. The vapor is continuously carried away by a pump. Evaporation chills the coil, much as evaporating water cools the human body. The action continues as long as liquid refrigerant flows from the flask, which is known as the receiver.

Keeping the receiver filled with liquid is the job of the rest of the machine. Vapor from the freezing coil is compressed by the pump, thereby raising the vapor's temperature. The hot vapor flows through a second pipe, usually bent in the form of a grid, called the condenser. The condenser is cooled by the ambient air. Heat is transferred by the compressed vapor to the relatively cold condenser. The cooled vapor condenses into liquid on the wall of the tubing just as moisture condenses on cold objects during hot, humid weather. The condensed refrigerant trickles into the receiver for another trip through the machine.

The actions are nicely balanced. The rate at which the liquid refrigerant flows into the receiver matches the rate at which it drains from the receiver into the freezing coil, and the rate at which heat is absorbed by the freezing coil matches the rate at which the atmosphere absorbs heat from the condenser. In effect the machine pumps heat out of the freezing coil and into the air.

Freezing coils can be made in any reasonable size and shape from copper tubing, and they can be connected to the machine with flexible hoses. The freezing coils can be interchanged by fitting their ends with screw connections. The complete assembly need weigh no more than 60 pounds; when it is mounted on casters, it is easily portable.

You need not buy the mechanism. Cast-off refrigerators in good working condition are available in every city dump and often from one's neighbors. The essential mechanisms can be salvaged with ordinary hand tools. Usually the compressor and the condenser are assembled as a unit in the bottom of the cabinet. A pair of tubes connect this unit with the freezing compartment in the top of the refrigerator.

The smaller tube of the pair resembles a thick wire, but it is actually a capillary. It carries warm liquid refrigerant at high pressure from the receiver to the freezing coil. The capillary resists the flow of refrigerant: its length and bore determine the rate at which refrigerant flows through the machine. The larger tube carries cold vapor at low pressure from the freezer to the inlet of the compressor. The tubes are usually soldered together through most of their length, so that they function as a heat exchanger. Cold vapor in the low-pressure tube chills the warm liquid refrigerant in the high-pressure capillary and prevents this heat from entering the freezer-a clever trick for improving the efficiency of the machine.

To convert the machine into a portable cooler, cut the low-pressure tube and the capillary at the point where they enter the freezing coil and provide the cut ends with screw fittings for connection to freezing coils of your own design. Copper tubing in a range of diameters is available from hardware stores. Mount the compressor and condenser assembly on casters.

The job involves a few tricks, but it is not difficult. For example, the compressor and its motor are housed in a sealed tank partly filled with special refrigerator oil and with refrigerant at a pressure that varies with the temperature of the tank. If the refrigerant is Freon 12, as is usually the case, the vapor pressure will amount to about one pound per square inch per degree Fahrenheit, or roughly 75 pounds per square inch at a temperature of 75 degrees F. Freon dissolves in oil much as carbon dioxide dissolves in water. If the pressure is lowered abruptly, the oil foams for the same reason that carbonated water bubbles when a bottle is opened. If the tubes connected to the freezer are cut before the compressed vapor is re]eased, foaming oil will squirt out and empty the compressor.

The loss of oil can be prevented by lowering the pressure slowly. This procedure requires the use of a tool known as a piercing valve, which is available from dealers in refrigeration supplies. Measure the diameter of the inlet and outlet tubes of the compressor and obtain a piercing valve that fits each pipe. Installation instructions come with the valves.

Equip the outlet of the piercing valve on the high-pressure side with a short hose and insert the open end of the hose into a quart bottle that contains an inch or so of water. Open the valve until the gas gently bubbles through the water. Pour the water out but leave the hose in the bottle to catch any oil that escapes. Gas may leak from the machine for a day or more. Do not hurry the job. When the machine is empty, measure the amount of oil that escaped. You will replace it later.

Cut the high-pressure and low-pressure tubes at the point where they enter the freezing coil. Do not use a hacksaw, because particles of copper may enter the tubes, find their way into the compressor and jam it. Cut the low-pressure tube with a pipe cutter of the wheel type. Cut the capillary by filing a deep nick and then bending the tube back and forth at the nick until it breaks. Promptly close the cut ends with adhesive tape.

Remove the compressor and condenser assembly from the cabinet and mount them on an improvised base fitted with casters. Obtain a 15-foot length of copper tubing with an outside diameter of 1/4 inch, cut a 10-foot length from it and wind the cut portion into a helical coil of convenient diameter, say six inches. Put a screw connector on one end. A short length of capillary tubing must be soldered in the other end to serve as a nozzle. I did it with a nail of slightly larger diameter than the capillary tube. I put the nail in the open end of the 1/4 inch tubing and, with a pair of pliers, squeezed the side of the tubing until the copper pinched tight around the nail. When the nail was withdrawn, the opening made a snug fit with the capillary.

Cut two inches of capillary from the machine, insert an inch of it in the pinched end of the coil and solder the two together. In the same manner solder the other end of the capillary into a three-inch length of 1/4-inch tubing and equip the other end of the 1/4-inch tubing with a screw fitting. Finally, put similar fittings on the cut ends of the low pressure and high-pressure tubes of the machine.

The receiver of some machines is quite small and the required amount of refrigerant is correspondingly critical. I equip such machines with a second receiver. It is a six-inch length of 3/4-inch pipe closed with pipe caps. I drill the caps and thread the holes to accept tubing connectors. The modified machine must be re charged with oil and refrigerant. Mea sure the amount of oil that escaped and put the same amount of fresh refrigeration oil in a jar with a wide mouth. Refrigeration oil is specially treated to remove waxes and minimize the tendency to foam when the oil is charged with refrigerant. Put a small, clean hose on the inlet tube of the compressor, which you have supplied with a screw fitting, insert the open end of the hose in the fresh oil and apply power to the motor momentarily. The oil will be sucked into the compressor.

Install the new freezing coil. Be sure to connect the nozzle end to the high pressure tube (the one that includes the capillary). The nameplate of the ma chine specifies the kind and amount of refrigerant required, such as "Freon 12 #1" or "R-12 16 oz." Both indicate that the machine requires one pound of Freon 12. Certain old machines may specify sulfur dioxide or methyl chloride, and some special machines may require R-22, R-500 or R-502.

Freon 12 comes in one-pound and two-pound cans and in 12-pound flasks. It costs about $1.50 per pound. Access to canned refrigerant is had by means of a special piercing valve that costs about $4. Flasks are fitted with a Schroder valve, which is similar to the one in an automobile tire. Vapor is transferred from the containers through a high-pressure hose provided with screw fittings, one of which is designed to depress the stem of the Schroder valve.

Close the piercing valve that was installed on the high-pressure side of the compressor. Attach the high-pressure hose to the piercing valve that you installed on the low-pressure side of the compressor. Connect the other end of the hose to the refrigerant dispenser. Place the can or flask, with the hose attached, on a spring balance and make a note of the gross weight. Start the compressor. Open the piercing valve on the low-pressure side, admitting refrigerant to the system. When the spring balance indicates a loss in weight of two ounces, shut off the supply of refrigerant and stop the compressor. Open the piercing valve on the high-pressure side and let the vapor escape slowly. This flushes most of the trapped air and moisture from the system.

When vapor stops leaking from the machine, close the high-pressure valve, start the machine and admit refrigerant as before. Continue until the machine has been charged with the specified amount. If all has gone well, the freezing coil will have accumulated a coating of frost. If you admit too much gas, frost will form on the low-pressure line, perhaps as far as the point at which it enters the compressor. No harm is done. Let vapor escape through the high pressure valve until the frost line moves to within a foot or two of the freezing coil.

To prevent the wasteful absorption of heat from the air, it is advisable to insulate the freezing coils. For example, a convenient apparatus for chilling glass beakers can be made by winding the freezing coil on a tin can of appropriate size. Solder the coil to the can and place it in a wooden box along with a generous packing of rock wool. If you want to, you can fit the top of the box with a hinged lid. You can insert high-pressure hoses between the machine and the freezing coil. The hoses enable you to manipulate the freezers as you would a hand torch. If the hoses are more than three feet long, you may have to compensate for their volume by adding refrigerant.

The machine can be modified in various ways to make it more versatile, convenient or efficient. For instance, you can substitute 1/4-inch copper tubing for the capillary by installing a needle valve at the freezer end of the tube. The valve enables you to regulate the pressure in the freezing coil. Temperature varies with pressure. In the case of Freon 12 you can adjust the valve for any temperature down to -30 degrees F.

The same pressure-temperature relation exists on the high-pressure side of the machine, where the temperature is determined in part by the effectiveness of the condenser. The condensers of domestic refrigerators have cooling fins and rely on air convection to remove heat. Normally they operate at roughly 90 degrees F., but the temperature of the vapor inside the pipe averages 20 to 30 degrees higher, and the corresponding pressure ranges from 135 to 155 pounds per square inch. By forcing air through the fins of the condenser with an electric fan the temperature of the vapor can be lowered 20 to 30 degrees. The pressure goes down correspondingly, thereby substantially reducing the load on the compressor. Much the same result can be achieved by connecting two condensers in series. Better yet, you can improvise and install a water-cooled condenser. The capacity of the machine can be increased by connecting two compressors in parallel. Don't overlook the fun of experimenting with refrigerants other than Freon 12.

When one makes modifications in the design of the machine, it is interesting to observe what happens inside the system as indicated by pressure measurements. Two systems of pressure measurement, known as gauge pressure and absolute pressure, are in use. For calibrating gauges zero pressure corresponds to atmospheric pressure, whereas in the absolute system zero indicates a perfect vacuum. Atmospheric pressure at sea level is assumed to be 14.7 pounds per square inch absolute and is equal to the weight of a column of mercury about 30 inches high. When air is fully exhausted from a tube that stands in a pool of mercury, the metal will actually rise to a height of 29.81 inches; in the approximate parlance of technicians a "30-inch vacuum" has been created. Subtract 14.7 from gauge pressure in pounds per square inch (abbreviated psig) to get absolute pressure (psia). The pressure-temperature relations of refrigerants are listed by reference texts in one system or the other but usually not in both.

An essential tool of the refrigeration technician is a pair of gauges that are calibrated in terms of gauge pressure and in the corresponding Fahrenheit temperature of Freon 12 and Freon 22. One gauge of the pair, used for measuring high pressures, is calibrated from 0 to 500 psig. The other, known as a compound gauge for measuring low pressures, is calibrated from 30 inches of vacuum through 0 to 80 psig. The instruments are mounted on a manifold that has a pair of two-way valves and three screw connections for three hoses. Two of the hoses provide continuous access to the gauges. By manipulating the valves either gauge or both can be connected simultaneously to the center hose. The hoses can be connected to the high-pressure and low-pressure sides of a sealed refrigerator by means of piercing valves, so that pressures can be observed while refrigerant is admitted to the machine through the third hose. Typically the reading on the high-pressure side is about 110 psig and on the low-pressure side about zero.

Valves can be installed at other points for isolating and interchanging various parts with minimum loss of refrigerant. For example, one may wish to use a needle valve to adjust the flow of liquid refrigerant to produce a certain temperature in the freezing coil and then, without substantial loss of refrigerant, substitute for the valve a capillary for automatically maintaining that temperature. I do this by first ascertaining the correct valve setting by experiment. The valve is removed without disturbing its setting and is connected to a water supply under low pressure. I count the number of drops that flow through the valve during a timed interval and then adjust the length of a capillary tube, by splicing or cutting, so that water at the same pressure flows through the capillary at the same rate.

The adjusted capillary is thoroughly dried and flushed with refrigerant before it is connected to the system. Dry Freon is relatively inert, but with water it becomes mildly corrosive. Moreover, moisture that is trapped in the system may freeze and block the flow of refrigerant.

Douglas Miller of Steubenville, Ohio, has made a device from similar parts for liquefying various gases. The basic principles of the machine are similar to those of a refrigerator, but the mechanism differs in that the gas is cooled by alternate compression and expansion until it condenses at atmospheric pressure and collects as liquid in an insulated container. Miller writes:

"My apparatus consists of a reservoir of gas to be liquefied, a deflated rubber balloon from which gas is admitted to the compressor at atmospheric pressure, a compressor capable of pumping about 900 cubic centimeters per second, a condenser cooled with a mixture of ice and salt, a heat exchanger known as the regenerator and a Dewar flask for collecting liquefied gas [see Figure 4].

"The capacity of compressors from old refrigerators can be measured with sufficient accuracy by observing the rate at which air from the machine displaces water. I fill a gallon jug with water and invert it in a bucket containing about six inches of water. I connect a hose to the outlet of the compressor, insert the other end in the bottle and time the displacement interval with a stopwatch. A volume of one gallon is equal to 3,785 cubic centimeters.

"To make the condenser I wound a 10-foot length of 1/4-inch copper tubing into a helix that would fit inside a one-gallon paint bucket. The ends of the helix were slipped through holes near the top and bottom of the bucket and soldered.

"The regenerator consists of a pair of concentric copper tubes five feet long. Compressed gas that has been cooled by the condenser enters the center tube, which is 1/4 inch in diameter, and expands to atmospheric pressure through a nozzle at the other end. The nozzle is made of capillary tubing. The expanded gas absorbs heat from the closed Dewar flask and lowers the temperature of the flask. The cold, expanded gas returns to the compressor via the 1/2-inch outer tube of the concentric pair, where it absorbs additional heat from compressed gas then flowing to the nozzle. Thus the temperature of the compressed gas is lowered both by the condenser and by the regenerator.

"As the operation continues the Dewar flask ultimately reaches the low temperature at which the gas can exist in the liquid phase at atmospheric pressure. The regenerator will function as a straight concentric tube, but I made it in the form of a helix to conserve space. The condenser, regenerator and Dewar flask were assembled as a unit on a simple wood frame [see Figure 6].

"Gas is admitted to the machine through the deflated rubber balloon, which contains a few ounces of calcium chloride to absorb water vapor. Unless the gas is dry, water vapor may freeze and clog the nozzle. The drying procedure is particularly essential in the case of gases prepared at home by the reaction of substances in solution. The balloon is closed and suspended by a rubber stopper that contains two holes. Gas enters the balloon through a plastic tube that extends to the bottom of the balloon, where it is surrounded by calcium chloride. Gas filters through the chemical and enters the system through a short tube in the second hole of the stopper. Thus the balloon performs the dual function of drying the gas and maintaining it at atmospheric pressure.

"The Dewar flask is closed by a snugly fitting stopper of plastic that I molded around the nozzle end of the regenerator. The plastic comes as a putty-like epoxy resin called 'Black Magic' that hardens in a few hours. It is available from shops that specialize in the repair of automobile bodies. I qleaned the tubing so that the material would adhere to the metal and greased the rim of the Dewar flask to release the plug after it hardened. The plug need not make an airtight seal with the flask, because the compressed gas expands to atmospheric pressure.

"The regenerator and condenser must be well insulated to prevent the absorption of heat from the atmosphere. I use a commercial insulation of fiber glass in strip form; it is manufactured for wrapping hot-water pipes. Suppliers of refrigeration materials stock similar insulation in the form of tape that is coated on one side with a pressure-sensitive adhesive. All tube joints should be checked for leaks before the machine is placed in operation.

"Although the system will liquefy a number of common gases, I suggest that the novice begin with one of the Freons, because these materials are odorless, nonflammable and nontoxic. Freon 12 is used in domestic refrigerators and hence is available in most communities. To operate the machine, fill the condenser with a mixture of rock salt and ice in the proportions used for freezing ice cream at home. Start the compressor and admit gas until the balloon puffs outward, but not enough to stretch the rubber. Then relax. The machine must pump heat from its various parts before it can chill the gas to the temperature at which it liquefies at atmospheric pressure. Liquid should begin to collect in the expansion flask within two hours. The balloon will begin to collapse as the gas liquefies. Admit more gas and continue. Stop the machine after three hours, remove the flask and inspect the contents.

"A variety of Freons are available for experimentation. The starting material from which they are made is carbon tetrachloride, consisting of an atom of carbon bound to four atoms of chlorine. The structure of this molecule resembles that of methane, in which four atoms of hydrogen are bound to the atom of carbon. Carbon tetrachloride is made into Freon 12 by replacing two of the chlorine atoms with two atoms of fluorine. The chemical name of the resulting compound is dichlorodifluoromethane. Freon 11 is made by replacing only one of the chlorine atoms with fluorine. Similarly, three or four chlorine atoms can be replaced by fluorine atoms, and the resulting compounds can be mixed with still other preparations to produce vapors that liquefy at desired temperatures and absorb characteristic amounts of heat when they evaporate.

"In addition to the Freons I have liquefied propane, butane, sulfur dioxide, methyl chloride, ethyl chloride and ammonia. Propane and butane are extremely hazardous substances because they can combine with air to form a highly explosive mixture. Sulfur dioxide is a nonflammable and nonexplosive compound that, at atmospheric pressure and all temperatures above 14 degrees F., is a colorless gas with a strong, irritating odor. It corrodes copper but was a popular refrigerant in domestic refrigerators 25 years ago. If you intend to experiment with corrosive gases, substitute stainless steel for all copper parts. Methyl chloride, also formerly used in domestic refrigerators, is noncorrosive and nontoxic, although it can interfere with respiration if it is inhaled in large amounts. At atmospheric pressure it boils at -11.36 degrees F., and it can be chilled by expansion to form 'ice' at-132.7 degrees. Ethyl chloride is chemically similar but at atmospheric pressure it boils at 55.6 degrees F. Ammonia is somewhat flammable, and when it is combined with air in the correct proportion, it is explosive. It is toxic, attacks copper and is extremely soluble in water. On the other hand, ammonia absorbs a large amount of heat when it evaporates. For example, when a pound of ammonia vaporizes at five degrees F., it absorbs 565 B.T.U.'s of heat. (One B.T.U. of heat raises one pound of water one degree.) In contrast, one pound of Freon 12 that evaporates at five degrees F. absorbs only 69 B.T.U.'s. For this reason refrigeration machines that use ammonia as the refrigerant can be made substantially smaller than those that use Freon 12. Some characteristics of the gases with which I have experimented are plotted in the accompanying graph [Figure 7].

Bibliography

REFRIGERATION ENGINEERING. H. J. Macintire and F. W. Hutchinson. John Wiley & Sons, Inc., 1950.

PRINCIPLES OF REFRIGERATION. ROY J. Dossat. John Wiley & Sons, Inc., 1961.

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