CHAPTER VI REFRIGERATING SYSTEMS. (Part 4)

customary to stop the mechanical unit for certain periods every few weeks to permit this frost to melt off the evaporating surface. It is an advantage to have an evaporator which will func- tion so that the surface will have a high enough temperature to defrost each inoperative period of the refrigerating cycle. Some of the most important advantages are : 1. Eliminates food odors from cabinet. 2. Cooling element operates more efficiently. 3. Cooling effect more uniform. The water vapor in the circulating air absorbs large quan- tities of gases and odors from the foods. Some of this water vapor is constantly being condensed on the surface of the cooling element. It is preferable to discharge this water to the drain as soon as possible. Freezing the water liberates a large per cent of the gases. Therefore the circulating air 158 HOUSEHOLD REFRIGERATION will be greatly benefited if the condensed water vapor is dis- charged to the drain each inoperative period. B.t.u. per pound water vapor 1. To cool water vapor (50° to 32°) 18 2. To condense water vapor 970 3. To freeze water vapor 144 4. To cool ice or frost (32° — 20°) 6 Total 1,138 It recjuires a relativel>" large qnantity of heat to condense, freeze, and cool the water vapor deposited on the evaporator surface, as shown in the table on the preceding page. The heat loss under Items 1 and 2 are necessary in order to have a dry food compartment with a relative humidity of approximately 60 to 80 per cent. The heat loss under Items 3 and 4 could be saved by operating the evaporator at a surface temperature so that it will automatically defrost during the inoperative part of the cycle. The efificiency of the evaporator surface for cooling the circulating air gradually decreases as the thickness of the layer of frost on it increases. The ice acts as a heat insulator. It is estimated that a layer of frost ]/> inch in thickness will decrease the effectiveness of the cooling surface about twenty per cent. Much difficulty has been experienced in returning lubri- cant from the evaporator to the compressor. In the usual household system there is a tendency for the lubricant to enter the evaporator, while if no special method is used for its return to the compressor it may collect in excessive quan- tities in the evaporator. An excessive amoimt of lubricant in the evaporator will reduce its heat-absorbing efficiency. Some household plants have a special oil return system, while others use oil traps to prevent this condition. It is an advantage to have the evaporator located above the compressor so that any oil in the suction line will drain to the compressor. The rate of heat transmission between the coil and the brine in a direct expansion type of brine tank is from ten to fifteen B.t.u. per square foot per degree F. per hour. In a REFRIGERATING SYSTEMS 159 flooded type tank the rate of heat transfer is about double this amount. When direct expansion coils are used to cool unagitated air the rate of heat transmission is I/2 to 2 B.t.u. per square foot per degree F. per hour. With brine pipes the rate is 2 to 21/ B.t.u. In designing an evaporator it is of importance to note the relative thermal conductivity of the following materials: Corkboard ^ ' ca Half inch air space = J- ^4 One inch air space = 1-5d Water = J^- Brine (calcium or sodium) — lo. Ice = ^'^• Iron = 1-100- Copper =8600. Brine Tank Data.— Table LVI gives the properties of solution of calcium chloride in water. The gravity expressed in degrees Beaume and in degrees salometer, per cent of cal- cium chloride, freezing point in degrees F., and the corre- sponding ammonia gauge pressure in pounds per square inch (corresponding to the freezing point) are given. Table LVII gives data on the properties of solutions of common salt (sodium chloride) in water. Table LVIII gives interesting brine tank data, relative to the heat-storing capacity and cost of various materials, which might be used to replace calcium chloride or sodium chloride brine. Specific gravity, specific heat, B.t.u. heat-stor- ing capacity per pound of material in cents, and B.t.u. stored for each cent cost of material are given for some common sub- stances, such as calcium and salt brine, water, cast iron, lead, copper, aluminum, concrete, sandstone, paraffin, oil, and kero- sene. In reference to the heat stored per pound of material, it will be noted that water has the highest value. This is, of course, due to the high specific heat. Oil and kerosene are lowest, with approximately 0.4 B.t.u. per pound of material. In reference to the cost of material, it will be observed that the sandstone has the smallest cost, with sodium and calcium chloride brine next, and with aluminum as the highest cost. In reference to the B.t.u. stored for each cent cost of materials. 160 HOUSEHOLD REFRIGERATION it will be noted that lead has the lowest value, this being 0.006, and that sandstone is the highest, with the value of 4.4 B.t.u. TABLE LVI. — PROPERTIES OF SOLUTION OF CALCIUM CHLORIDE IN WATER ' Lbs. of Calcium Specific O-avitv I'er Cent Pure Freezing Temp. Chloride Crystals Weight, .Ijs. per at bO°F. Calcium Chloride Degree F. (73 to 75';,) in one Gal. of Brine Gal. at 60°F. 1 000 0 00 32.00 8 33 1 0!0 1 40 31 50 8 44 1 020 2 30 30 50 8 50 1.030 3 80 29 50 8.59 1.040 5 00 27 50 8.67 1.050 6 20 26 00 8.76 1.060 7,20 24 75 8.84 1.070 8.20 23 75 8.92 1.080 9 60 22 50 9.00 1.090 10.60 21.00 9.10 1.100 11.80 18,50 1 43 9 18 1.110 12 80 16 50 1 60 9.25 1.120 13 80 14 50 1,75 9.34 1.130 15 00 12 00 1 88 9.42 1.140 16 00 10 30 2 05 9.49 1.150 17.20 + 7,52 2.18 9,58 1.160 18 30 + 3 75 2.35 9.67 1.170 I'J 20 + 1 50 2.50 9.77 1.175 19 85 - 1 50 2.56 9.80 1.180 20 20 - 2 50 2.65 9.85 1.190 21.20 - 5 50 2.80 9 93 1.200 22 20 - 9 50 2.95 10 00 1.210 23.20 -14.00 3.10 10.09 1.220 24.20 -18.00 3.30 10.10 1.230 25.10 -23.50 3.45 10 22 1.240 26.00 -27.04 3 60 10.34 1.250 27.00 -32.62 3 76 10.42 1.260 27.85 -39.00 4 00 10.52 1.270 28.80 -44.50 4.10 10.60 1.280 29.70 -52.50 4.35 10.68 1.290 30 60 -54.40 4.50 10.76 1.300 31.60 -42.50 4.70 10.84 1.310 32 40 -32.50 4.90 10.92 1.320 33 40 -17.00 5 10 11.00 1.330 34.20 - 4.00 5.25 11 08 1.340 34.50 + 3.50 5 40 11.16 1.350 36.10 + 14.37 5 60 11.23 From "Practical Refrigerating Engineers' Pockethook." Xickerson & Collins Co. The freezing points of some brine tank solutions arc given by Fig. 8. Curves showing the freezing points as the percent- age by volume of solute are given for glycerin, denatured alcohol, calcium chloride, and one-half wood alcohol and one- half glycerin. Prime Mover. — Electric motors are used to drive practically all household refrigerating machines. Most of the machines REFRIGERATING SYSTEMS 161 TABLE LVII.— PROPERTIES OF SALT (SODIUM CHLORIDE) SOLUTIONS IN WATER Specific Gravity at 39°F. 1.010 1.020 1.030 1.040 1.050 1.060 1.070 1.080 1.090 1.100 1.110 1.120 1.130 1.140 1 . 150 1.160 1.170 1.180 1.190 1.191 1.200 1.204 Pb.- Cent of Sodium Chloride 15 2.6 4 0 5 2 6.5 7.8 9.1 10.4 11 8 13 0 14 1 15 5 16.8 18 0 19 2 20 5 21.8 23 0 24 . :i 24 .i 25 6 26 U Freezing Temp. Degree F. 30.25 28.40 26.60 J5.20 23.40 21.60 19.90 18.40 16 40 14.60 13.4 11.6 10.0 8.0 7.0 5.9 3.8 2.4 1.0 + 0.8 + 0.2 - 1.1 Weight, Lbs. Specific Heat per Gallon 8.44 0.986 8 50 0.979 8.59 0.968 8.67 0.958 8.70 0.945 8.84 0.938 8 . 92 0.922 9 OU 0.912 9 10 0.902 9.18 0.886 9.25 0 876 9.34 0.865 9.42 0.856 9.49 0.846 9.58 0.832 9.67 0.824 9.77 0.817 9.85 0.806 9.9:i 0.794 9.91 0.792 10 00 0.776 10.04 0.771 This table varies sHghtly from 4°F. to 20°F. from those usually pubhshod, which are considered more correct. The differences would affect, only calculations on congealing tanks, as it is customary in ice making to make the brme as strong as possible, or u^ar 25% or 26%. From "Practical Refrigerating Engineers' Pocketbook," Nickerson & Collins Co. TABLE LVIII. — BRINI : TANK DA TA Relative heat storing capacity- and cost of various materials which might replace calcium or salt brine. B.t.u. Heat B.t.u. Storing Cost per Stored Specific Specific Capacity Pound of for Each '^'•avity Heat per Material Cent Pound of Cents Cost of Material Materials Salt brine 1.2 0.78 0.93 0.5 1.9 Calcium Brine 1.2 0.70 0.84 0.5 1.68 Water 1.0 1.00 1.00 Cast Iron 7.1 0.13 0.92 5.0 0.18 Lead 11.4 0.03 0.34 6.0 0.006 Copper 8.9 0.093 0.83 20.0 0.041 Aluminum 2.6 0.22 0.57 30.0 0.019 Concrete 2.2 0.25 0.55 0.14 3.9 Sandstone 2.2 0.20 0.44 0.1 4.4 Paraffin 0.9 0.69 0.62 10.0 0.062 Oil 0.9 0.4 0.36 6.0 0.06 Kerosene 0.8 0.5 0.40 2.0 0.20 162 HOUSEHOLD REFRIGERATION on the market today use Y^ horse power motors. This size motor with a reasonably efficient refrigerating system should be capable of refrigerating properl} fifty cubic feet of food storage space. Refrigerating systems of this capacity in use todav recjuire from three to six times the amount of current necessary to ])erform this duty on a large commercial plant. More efficient machines should be developed; however, it is not necessary to ver}- closely approach the efficiency of the large plant. Some machines have been placed on the market using 1/6 horse power motors. This size has now proven successful for the smaller units up to twenty cubic feet of food storage space. It is assumed that the food storage spaces are properly insulated. For fotul compartment temperatures of 4O°-50° F., the insulation should be at least o inches thickness of cork- board or its equivalent. The starting torque and the overload capacit} are impor- tant features in the choice or design of the motor. The over- load may l^e double the normal operating load and it may be necessary to operate at this overload for several hours. This condition usuall}- occurs when the machine is i)laced in operation in a warm environment temperature. The starting torque is high when the unit is first placed in operation on account of the high pressure on the evaporating side of the system. In normal operation the starting torque may be greatl}- increased if either the expansion valve or the com- pressor discharge valve leaks. Air-cooled machines have a more severe starting condition than water-cooled machines especially Avhcn a dead air condenser is used. It is customary to use repulsion-induction t>pe of a.c. motors for driving household refrigerating machines because of their rclati"cly high starting torque. Split-phase motors have been used to a 'ery limited extent on some of the smaller machines. Some machines have been made with the entire motor housed inside a gas tight metal casing, thus eliminating the packing of a drive shaft. Considerable difficultv has been experienced, however, in operating a motor enclosed with the REFRIGERATING SYSTEMS 163 refrigerant gas. A later design has the stator outside a thin metal casing, the rotor being inside, thus eliminating pack- ing a drive shaft. Lubrication of the motor is an important feature as it usu- ally operates from six to twelve hours a day. With this service condition, the motor should be oiled at least once a month. Some motors are oiled automatically through copper tube lines from a gear case pump ; the oil is forced or splashed into the tube by the rotating gear. This method is only ap- plicable on a direct-connected motor compressor unit. The efficiencies of fractional horsepower alternating' cur- rent motors of the repulsion-indution type at full rated load are usually ^^•ithin the following limits: Horsepower Efficiency per cent Yf, 50-60 ^ 60-75 y. 65-80 Direct current motors should have efficiencies considerably higher than given in this table. It is customary to limit the normal operating load to 300 watts on the )4 hp. and to 200 watts on the 1/6 hp. size. These motors will usually stand 100 i)er cent overload for short periods of operation. Table LIX gives the amjK-re ratings of alternating current motors of capacities ranging from ^ to 5 h]). on both single and three-phase current, at 110 and 120 volts. TABLE LIX — AMPERE RATING OF ALTERNATING CURRENT MOTORS SINGLE PHASE THKEE pha.se Horsepower no Volts 220 Volts 110 Volts 220 Volts "34 4 2 ^ 7.5 3.75 4.4 2.2 H 10 5 1 12.5 6.25 8 4 IV2 18 9 10.3 5.1 2 - 24 12 12.5 6.25 3 34 17 18 9 4 43 22 24 12 0 55 28 30 15 164 HOUSEHOLD REFRIGERATION The Drive. — Some of the more important types of drives in use are : belt, gear, and direct. The belt drive has several important advantages. It gives an easier starting torque than a direct-connected or gear drive. Some motors operate at a rather small load and therefore at a low efficiency, simply because they must be large enough to insure starting under all conditions of service. The belt also gives a certain protection to the motor, as it will sometimes slip or come of? the pulleys with an excessive overload on the motor. Another important advantage of a belt drive is that it can be easily repaired or replaced without the services of an expert mechanic. A belt drive generally costs less than a gear drive. The belt drive is easier to manufacture and assemble as it does not require such close limits on lining up the motor. Some machines use a series of from two to five small belts. If one breaks it does not greatly ailect operation. This multiple belt system has not proven very satisfactory in actual use, probably because one of the belts is usually driving more than its share of the load. A belt drive is ordinarily from 95 to 98 per cent efficient. This is a much higher efficiency than is usually obtained with a gear drive. An exposed belt drive is dangerous on a machine which starts automatically, and every precaution should be taken to safeguard it. One method of obtaining this result is to make the condenser coil of tubing and arranging it so as to form a guard around the belt and its pulleys. Flat belts have been used on a large number of successful machines. They are generally made of either leather, canvas, or fabric. An idler is generally used with a flat belt drive. It is necessary in order to increase the angle of contact on the motor pulley. The idler is usually operated by a spring or a weight. It also serves another purpose in automatically keep- ing the belt tight by compensating for any stretching of the belt in service. One cannot depend upon attention being given to a belt by the user, especially in the way of making adjust- ments. One of the difficult features on a flat belt drive is to insure necessary lubrication of the idler pulley. REFRIGERATING SYSTEMS 165 The V-type rubber or fabric belt as developed for use in driving" the radiator fan on automobiles is being used with success on household plants. It has most of the features of a flat belt with the added advantage of not requiring an idler pulley. A belt of this type drives by means of friction on the side of the V-shaped groove. The inside face of the belt should not touch the pulley. These belts are generally of the endless type, they run quite loose and do not stretch enough in service to require any adjustment of pulley centers. Spiral gear drives arc used on compressors both with par- allel and right angle shafts. Spiral gears have an advantage over worm gears in that they do not require as close limits on shaft centers and can be made without a hob. Gear drives produce end thrust on the shafts which is usually carried on a thrust or ball bearing. It is difficult to keep the end clearance on shafts, subjected to a thrust load, to a small enough limit so that the noise from end play on the shafts will not be objectionable. The thrust bearings should be well lubricated. The start- ing torque ma}' sometimes be greatly increased when the thrust bearings have not received instant lubrication on start- ing. This may occur when the thrust bearings are lubricated by a splash system which does not function until the machine has started to operate. ' It has been difficult to build gear drives for the recipro- cating type compressors so that excessive noise would not result on account of backlash caused by the necessary clear- ance between the teeth. Gear drives usually operate at an efficiency of 70 to 90 per cent. The direct-connected drive is in common use on machines having a gear or rotary pump and on machines with the mov- ing parts enclosed in the refrigerant gas space. Most of the designers have placed the packing gland on the relatively slow-speed compressor shaft, as it is more difficult to pack the motor shaft which rotates at a much higher speed. When the motor or the moving part of the motor is enclosed in the gas space, this packing gland trouble is eliminated. Diffi- culties have been experienced in starting machines which have 166 HOUSEHOLD REFRIGERATION a thin metal shell between the rotor and field of the motor, especially on three-phase, alternating-current motors. The direct-connected