D.C. 3 in 2075 29 1775 80.3 2.08 D.C. 4^ in. 6 in 1730 66 1780 125 1.89 A.C. 4Vi(> in. 1100 111 1680 201 1.82 A.C. 9 in 9 in. 1610 38 815 360 9.48 D.C. 9 in 9 in. 1550 66 1185 523 7.93 A.C. 12 in 12 in. 1140 58 818 643 11.08 D.C. 12 in 12 in. 1620 48 520 489 10.18 A.C. 12 in 12 in. 1400 67 1170 921 13.7 A.C. 16 in. 16 in. 1030 81.5 518 805 9.88 A.C. Condensing Pressure for Air Cooled Compressors. — Fig. 11 shows the condensing pressure in pounds per square inch gauge for air-cooled sulphur dioxide refrigerating machines, equipped with copper tube condensers. The curves show graphically how the condensing pressure increases with the increase of the room temperature. The space between curves A and B shows the result when the proper tube condensers are exposed to still air, while the space between curves B and C shows the results when forced air circulation over the con- denser is used. The curve D is the saturated vapor curve for sulphur dioxide and represents the corresponding condensing temperatures for the pressures shown on the left-hand side of the diagram. The relative distances between curve D and the curves A, B, and C show how nearly the condenser pres- sure approaches the theoretical possibilities. Flintlock Condensers.— Fig. 12 shows a new type condenser developed for air-cooled electric refrigerators by Flintlock Corporation of Detroit, Michigan. One lineal foot of this finned tubing has been found to have the equivalent cooling capacity of ten feet of copper tub- ing of equal size, when air is drawn through at an average velocity of 500 feet per minute. Tests have proven that draw fans are more efficient than blow fans. Only that amount of air which can be drawn through the free area of the condenser need be handled by the fan. <Callout type="tip" title="Efficient Air Circulation">Draw fans are more efficient for air-cooled refrigeration systems.</Callout> 146 HOUSEHOLD REFRIGERATION Fig. 13 sliows a cross section of tubes also the internal fins. The construction i> of ])rass tinned inside and out. The FIG. 12.— FT,l.ÉTI.orK .MR COOLED COXDEXSET?. tubes are an integral part of the fins. Heat transmission does not pass through a soldered joint. Ay Ik. A FIG. U.— CROSS SECTIOX OF TUBES— SHOWING IXTERAL FIXS. Fig. 14 is a t}pical installation of this type condenser on a compressor unit. Tubes and Spiral Fin Tubes. — The use of drawn seamless tubes or coils made into simple, or sometimes fairly compli- cated forms, is very extensive thrcnmit the refrigerating industry. Considering household machines, the conventional condenser and evaporator consists of many feet of seamless copper tubes, or steel tubes in case ammonia is used as the refrigerant. The copper tubes used ordinarily are 1/4 inch outside diameter, 5/16 inch up to 1/2 inch outside diameter with a wall thickness of about 0.015 to 0.032 inch. These tubes PIG. 14.— TYPICAL INSTALLATION OF FLINTLOCK CONDENSER. have ample bursting strength, are soft, easy to work with and when formed into coils present an attractive appearance. In some designs the tubing is flattened before or while it is being formed into a coil; the object in flatting the tubes i-. of course, for a given tube spacing to increase the area of the air passages between the tubes. For example, if a coil is formed with 3/8 inch tubes the center lines of which are 5/8 inch apart, the air passage between the tubes will be 2/8 or 148 HOUSEHOLD REFRIGERATION 1/4 inch. However, if the same tubes are flattened to a thick- ness of 3/16 inch the air passage will be increased from 1/4 inch to 7/16 inch. Further, if desired, the tubes can be placed closer together so that the air passage is still 1/4 inch as be- fore, but the overall dimensions of the coil, consisting of a mm 1 :"'• • • i''i««wf((((((((„, IIHb 1, ^^l [^hHH FIG. 13.— CROSS SECTION OF TUBES— SHOWING INTERNAL FINS. given number of feet of tubing, will obviously be reduced. In any case it is clear that there is a definite gain in the use of flat tubes and whether or not this gain is sufficient to war- rant the expense of flattening the tubes should be decided in each case. Instead of using plain tubing for condensers, evaporators, etc., it is possible and very advisable under certain conditions REFRIGERATING SYSTEMS 149 to use so-called spiral fin tubes. As the name indicates, a spiral fin, about 1/4 inch wide and 0.006 to 0.008 inches thick, is wound spirally around the tube and attached to it securely •,u4J»;,^' piatammmmmmimMiMm ^^tiH< FIG. 12.— SPIRAL FIN TUBE CONDENSER. by means of solder. The finished product is known as a spiral fin tube. Such a tube can be wound and formed into various shapes as shown in Figs. 15, 16 and 17 showing typical con- densers made by the MoCord Radiator C()m|)any of Detroit, Mich. FIG. 17.— SPIRAL FIN TUBE. A glance at Table LIV will .show that the total outside sur- face of the spiral fin tubes is nearly seven times as large as the surface of the plain tubes from which they are made. REFRIGERATING SYSTEMS 151 Since heat transfer from metal to a fluid such as air or brine depends upon the surface, it is clear that the spiral fin tube should have some advantage over the plain tube. 'I'his advan- tage is particularly large in such cases as that of condensing a refrigerant inside of a tube, over which a blast of air is di- rected by means of a fan or a blower. In a case of this kind the heat absorbed by the air per square foot of tube surface is very small compared to the heat transferred by the refrig- erant to the tube. For example, if the latter is 20 times as large as the former it is clear that the factor limiting the over- all heat transfer is the rate at which heat is absorbed by the air. However, suppose we increase the surface exposed to the air, while the surface in contact with the refrigerant is maintained the same ; then one square foot of the inner sur- face of the tube will furnish heat to seven square feet of the outer surface of the tube, instead of one square foot of the outer surface, and conditions will evidently be greatly im- proved. TABLE LIV STANDARD SIZES OF FLINTLOCK CONDENSERS Square Inch Size Width No. Tubes ID. Tubes No. Fins Radiating Surface 6"x 6" II4" IS -K 43 609 7" X 7" II4" 20 50 S14 8" X 8" II4" 24 "32" .37 1114 9"x 9" V4" 26 64 1364 10" X 10" iH" 30 1^32" 71 1682 10" X 12" IM" 36 "•sV' 71 2016 12" X 12" I'A" 36 ^Ih" S5 2415 14" X 14" IV2" 32 ''4^' 99 3778 16" X IS" 1 w 36 ' (%" 113 4937 The heat transfer from the condensing refrigerant to the tube can very aptly be compared to a boulevard 140 feet wide, terminating at a large square which would correspond to the tube which has a high conductivity ; if this square connects only with one pavement, say 20 or 30 feet wide, we shall have the case of the plain tube, but if we have seven such streets radiating from the square, we shall have the case of a spiral fin tube. 152 HOUSEHOLD REFRIGERATION If the temeperature of the fin surface were the same as the temperature of the tube surface then a square foot of the fin surface would be equivalent to a square foot of the tube surface. But this is not the case, and therefore, a spiral fin tube having one square foot of tube surface and six square feet of fin surface will have an effective heat transfer capacity of 1 + (0.60 X 6) = (1 + 3.6) 4.6 instead of a capacity of (1 } 6) == 7, assuming that the efficiency of the fin surface is 60 per cent of that of the tube, while the plain tube would have a heat transfer capacity of one. Another advantage of the spiral fin tube is the adaptability to compact designs. If 30 feet of spiral fin tubing replace 120 feet of plain tubing, as it has been done in practice, then it is clear that there will result compactness of design, and econ- omy of space. Further this compactness of design makes possible the improvement and control of the air flow through the coils. A very good example of this is Fig. 18 where the round con- denser can be made to cover the fan and thus use its air blast very efficiently. Calculation of the Surface of a Spiral Fin Tube. — Consider a 3/8 inch outside diameter tube wound spirally with a fin 1/4 inch wide and 1/6 inch pitch. The surface per foot length will be : 7r3/8 X 12 == 14.18 square inches per foot length of tube. Suppose that in winding the ribbon around the tube the out- side diameter is maintained at (1/4 -f- 3/8 -f 1/4) = 7/8 inch, and the excess material next to the tube is crimped. Then, the length of the ribbon, per turn will be practically, tt (7/8) and its area, facing upward, tt (7/8) (1/4). Thus the total fin or indirect surface, as it is sometimes called will be: TT 7/8 X 1/4 X 2 X 6 X 12 = 99 square inches per foot length of tube. Where the factor 2 is introduced because there are two surfaces, one facing upward and the other facing downward; the factor 6 is used because we have 6 turns per inch length of tube and 12 in order to get the surface per foot REFRIGERATING SYSTEMS 153 fength of tube. Adding the direct and indirect or fin surface we have 14.18 + 99=113.18 square inches per foot length. = 0.785 square feet per foot length. FIG. 18.— ROUND CONDENSER. DESIGNED TO COVER THE FAN. Next suppose that instead of crimping the fin on the inside, we draw it through a die, and force it to assume a flat ring- like shape around the tube. The surface of the ring will be ^{7/2>y (1/4) -.(3/8)^' (1/4) or Approximately tt (5/8) (1/4) 154 HOUSEHOLD REFRIGERATION Where 5/8 is the average diameter of the ring and 1/4 its width. Thus the total indirect surface will be TT 5/8 X 1/4 X 2 X 6 X 12 = 70.7 square inches per foot length. The total surface of the spiral fin tube will be 70.7 + 14.18 = 84.88 square inches per foot length. Thus the total surface TABLE LV — DATA ON COMMERCIAL FIN TUBES Tube Sizes •>i'6 ^8 'ie Vi ^ Outside Diameter of Tube.s. inches 0.312 0.375 0.437 0 . .'0 ) 0.625 Outside surface oi tubes, square inches per foot length. 11 . 78 14.18 16.49 18.85 23.56 Fins per inch length of tube 6 6 6 6 6 Width of fins, inches 0.1S7 0.2.50 0.250 0.250 0.250 Outside surface of fins when crimped, square inches per foot length 58.31 99.0 106.0 113.1 127.2 Total outside surface (crimped fins), square inches per foot Iciigth. . . . Square feet per foot length. . 70.09 4.85 113.18 7.87 122.49 8.50 131.95 9.15 150.76 10.46 Outside surface of fins when not crimped, square inche.- per foot length 42.3 70.7 77 . 7 S5 •>9 Total outside surface (fins not crimped) , square inches per foot length 54.08 3.76 84.98 5.9 94.19 6.54 103.85 721 122.56 Square feet per foot length.. , 8.72 of the crimped spiral fin tube is 113.2 square inches i)er foot length of tube, while that of the uncrimped spiral fin tube is 84.9 square inches or 75 per cent of the former. Table LV gives in detail data on commercial spiral fin tubes, which were calculated as those outlined above. The Evaporator. — There are two types of evaporator or cooling elements in general use. The type operating with an expansion valve is sometimes called the "dry" system. The REFRIGERATING SYSTEMS 155 other type, in which a relatively larger amount of liquid re- frigerant is retained in the evaporator, is the "flooded" system. The "flooded" system has several important advantages. H'eat transfer is more rapid through surfaces contacting with liquid than through surfaces contacting with a gas or a mix- ture of a gas and a liquid. The additional liquid refrigerant in the evaporator has a certain heat storage capacity which may prove advantageous. A direct expansion system for a household machine usually requires a much smaller quantity of refrigerant. This is an advantage, if any danger is involved should the gas escape in the home. The direct expansion system has an advantage in giving an easier starting load when the machine is first placed in operation. This condition is very important when an air-cooled condenser is used. This system usually oper- ates with a more uniform suction pressure, thus automatically regulating the refrigerating load more closely than with the flooded system. It is customary to control the supply of liquid refrigerant to the flooded system by a float valve. A float on the liquid refrigerant surface drops when the liquid refrigerant is vapor- ized and removed by the compressor. This opens a valve, allowing sufficient liquid to enter the evaporator to maintain the liquid level required by the float to close the valve. This valve may be placed in a reservoir forming part of the flooded evaporator, or in the liquid sump or reservoir below the condenser. When the valve is placed outside of the refrigerator, it is necessary to insulate the liquid line to the evaporator. In order to avoid this insulated line, most designs show this valve located in a header forming part of the cooling unit. An evaporator in common use consists of pipes or tubes immersed in a solution of calciurii or salt brine contained in a sheet-metal tank. This tank is placed in the ice compart- ment of a refrigerator and usually functions at a surface tem- perature colder than ice. The average brine temperature found to be suitable for household refrigerators is about 20° F. The temperature may vary as much as 10° above or below this amount during the 156 HOUSEHOLD REFRIGERATION operating period without any objectionable results in oper- ation. It has been found that with a 20° F. average brine tem- perature, ice and desserts can be frozen in quantities sufficient for household use within the shortest time intervals between meals, that is, five or six hours. Experience has indicated that the food compartment of the average ice refrigerator will accommodate a large enough brine tank for the cooling with a 20° brine tank surface. There are three principal factors involved in determining the amount of cooling performed by the evaporator : 1. Amount of effective evaporator surface. 2. Temperature of evaporator surface. 3. Rate of air circulation in the cabinet. A brine tank will usually maintain a food compartment temperature under 50° F. under usual service conditions. If the brine tank has a surface equivalent to the average ice sur- face, it should, of course, produce lower food compartment temperatures, as the 20° F. brine tank surface is 12° colder than ice. Some manufacturers use an evaporator made of pipes or tubing directly exposed to the air. This system eliminates the brine. Much difficulty has been experienced in making tanks to hold the brine solution, as there is a chemical and electro- lytic action which frequently causes tanks to leak. This effect is especially bad with copper and solder exposed to the action of calcium chloride brine. The brineless evaporator usually has a smaller heat stor- age capacity. However, with an automatic machine, this is not considered so important, as frequent operation is not ob- jectionable. Sometimes this heat storage condition is im- proved by the addition of a heavy cast-iron sleeve to contain the ice trays and to also serve as a heat storage element. A large amount of refrigerant is stored in the evaporator by some manufacturers to function as a heat storage capacity. When the heat storage capacity of the evaporator is rela- tively low, the cycles of operation are usually lengthened by increasing the temperature differential of the evaporating unit. A brine system might operate with a brine differential temperature of 4° (22° — 18°). Nearly the same results would REFRIGERATING SYSTEMS 157 be obtained on a brineless evaporator, say of half the heat storage capacity, but with a temperature diflferential of 8° (24° — 16°). There would be some loss in efftciency in the lat- ter case, as the compressor operates at lower efficiency at the lower suction pressure required to cool to 16° F. rather than 18° F. It is very important to properly place the evaporator in the ice compartment. It should not project above or block the warm air flues. The warm air entering these flues should pass over the top of the evaporator with little or no restric- tion, so that it can drop along the four sides of the brine tank to replace the cold air passing out of the compartment. The sides of the evaporator should clear all side walls by at least two and, preferably, three inches. The clearance at the bot- tom should be at least three inches and preferably more. The frost collecting on the evaporator sometimes inter- feres with the normal operating of the refrigerating system. As the evaporating surface is usually below 32°, moisture from the circulating air is deposited and freezes .to the cold surfaces of the evaporator. This frost will gradually build up unless the evaporating surface temperature reaches 32° F. during the inoperative period of the cycle. This layer of frost acts as a heat insulator and increases the temperature in the food compartments. It <Callout type="warning" title="Frost Build-Up">Frost on the evaporator can increase temperatures in the food compartment.</Callout> is
Key Takeaways
- Air-cooled refrigeration systems use condensers and evaporators to regulate temperature.
- Spiral fin tubes offer increased surface area for better heat transfer.
- The flooded system in evaporators provides more rapid heat transfer through liquid surfaces.
Practical Tips
- Use draw fans instead of blow fans for air-cooled refrigeration systems as they are more efficient.
- Properly place the evaporator to ensure optimal airflow and prevent frost build-up.
- Consider using spiral fin tubes in your condensers for improved heat transfer efficiency.
Warnings & Risks
- Be aware that copper and solder can leak when exposed to calcium chloride brine, leading to system failures.
- Frost on the evaporator can increase temperatures in food compartments, affecting cooling performance.
- Ensure proper insulation of liquid lines if using a float valve outside the refrigerator.
Modern Application
While the specific refrigerants and equipment mentioned are outdated, the principles of air-cooled systems, condensers, and evaporators remain relevant. Modern refrigeration techniques have improved in efficiency and safety but still rely on these fundamental concepts. Understanding these basics can help in troubleshooting and maintaining modern cooling systems during emergencies or when conventional power is unavailable.
Frequently Asked Questions
Q: What are the advantages of using spiral fin tubes in air-cooled refrigeration systems?
Spiral fin tubes offer a significant increase in surface area, which enhances heat transfer efficiency. This can lead to more compact designs and improved overall performance.
Q: How does the flooded system differ from the dry system in evaporators?
The flooded system retains more liquid refrigerant in the evaporator, allowing for faster heat transfer through liquid surfaces rather than gas. It also provides better temperature regulation due to a larger heat storage capacity.
Q: What is the importance of proper placement of the evaporator in an ice compartment?
Proper placement ensures that warm air can circulate freely over the top of the evaporator, allowing it to cool more effectively. This prevents frost build-up and maintains optimal cooling performance.