CHAPTER XI COLD STORAGE DUTY (Part 2)

case, air cooling involves cooling not only the mechanical mixture of oxygen and nitrogen, but a large quantity of water vapor as well. If the air contains just sufficient moisture so that the cooling brings it to the point of saturation, the heat that must be abstracted from the water vapor will be only that rep- resented by the specific heat of the vapor and the number of de- grees cooled through. If it is cooled below the point of saturation, as is usually the case in cold storage practice, not only the specific heat of the vapor but the latent heat of that part of the vapor precipitated as well must be removed. Generally the process is carried still farther and the precipitated moisture is chilled to the freezing point and finally frozen, when not only the specific heat of the liquid but the latent heat of fusion is involved. In case the ice is cooled to a lower temperature the specific heat of the ice is also involved. Example. — It is required to cool 2,000 cu. ft. of air per minute from 80"* Fahr. to 36'' Fahr. In the following calculations it is assumed that the amoimt of air to be cooled is 2,000 cu. ft. before it is cooled, and not, as it might be construed to mean, 2,000 cu. ft. of cooled air. For the sake of simplicity the air is assumed to be dry. Dry air at SO** F. weighs .0731 pounds per cu. ft. 2,000 cu. ft. would weigh 146.2 lbs. The specific heat of air is 0.2377 B. t. u. required to cool 2,000 cu. ft. 1° F 34.75 B. t. u. reqxured to cool 2,000 cu. ft. 44** F 1529. One ton of refrigeration is sufficient to dispose of heat at the rate of 288,000 B. t. u. per twenty-four hours, or (dividing this number by 1440, the number of minutes in 24 hours) gives the equivalent rate per minute or 200 B. t. u. per minute. On this basis, the cooling of 2,000 cu. ft. of air per minute from SO"" Fahr. to 36*" Fahr. would require the expenditure of 1529 -^ 200 = 7.64 tons of refrigeration. Digitized by V^OOQIC COLD STORAGE DUTY 157 Had the requirements been for 2,000 cu. ft. of cooled air, the amount of refrigeration needed would have been 8.36 tons, the difference being accounted for by the diflEerence of weight per cu. ft. of air at 80® Fahr. and 36® Fahr., respectively. Cold Losses Through Cold Storage Doors There is no known means of accurately determining loss of refrigeration through the opening of cold storage doors. It is possible that it might be roughly approximated from formulae giving the flow of gases under sUght diflEerences in pressures, in which case some delicate form of draft gauge might be employed to show the excess pressure of the cold air on the inside of the cold storage compartment over that of the outside air. The area through which the outward flow due to the observed diflEerence of pressure would take place would be probably about one-half of that of the opening oflEered by the door, because in a single opening the upper part would be given up to the inward current of warm air. While it would be difficult to estimate the velocity at which cold air rushes out of a cold storage compartment, it is apparent that it will increase as the diflEerence between the inside and out- side temperatures, and with the increase in height of the cold air colmnn, both of these factors acting to afiEect an unbalancing of the atmospheric pressures and consequently tending to produce a flow. In this connection it may be remarked that the circulation of air in cold storage compartments, as well as currents of air entering and leaving the compartment, can be conveniently studied by using smoke as an indicator. It might be possible by means of a puflE of smoke and a stop watch, in the absence of a delicate an- emometer, to roughly determine the velocity of the air currents. The inward current would have a maximum velocity at the top of the opening and the outward current at the bottom, while somewhere near midway would be found a place with no per- ceptible current. From this it follows that the volume of air lost through the opening might be determined by multiplying one-half the area of the opening by one-half the maximum velocity. This product of the average velocity in feet per minute and the area of the current will be the number of cubic feet per minute lost through the opening, and since 4,000 cubic feet per minute cooled one degree requires refrigeration at the rate of one ton per Digitized by V^OOQIC 158 ELEMENTARY MECHANICAL REFRIGERATION 24 hours, it follows that outside air at a temperature of 80® Fahr., rushing into the cold storage compartments to take the place of cold air escaping at a temperature of 40° Fahr., requires an addi- tional ton of refrigeration for every 100 cubic feet of flow. To reduce this excessive loss to a minimum, vestibules sufficiently large to permit one door to be closed before the other is opened are often provided for doors communicating directly with the outside. Where products alone are to be passed, rotary doors, or in the case of ice storage rooms automatically closing swing doors, may be employed to advantage. Insulation Losses Good heat insulators are simply poor heat conductors and poor heat insulators, good heat conductors— this property of each being numerically the reciprocal of that of the other. Since all heat insulators are to some extent heat conductors, the flow of heat through insulated walls cannot be prevented but only re- duced in proportion to the thickness and efficiency of the insulation employed. The amount of heat that will pass through a square foot of cold storage insulation per 24 hours, like that through other more or less imperfect conductors, is practically proportional to the difference in temperature on the two sides of the insulation and to the efficiency of the material not as a heat insulator but as a heat conductor. Products cooled to the temperature of the cold storage com- partment, where uniform temperatures are maintained, require the expenditure of no further refrigeration.* The necessity for operating the refrigerating plant for the preservation of such products is therefore due largely to the en- trance of heat through the insulated walls of the cold storage compartments and the insulation should be made as efficient as economy will permit. True economy is found at the point where the cost of the refrigeration that would otherwise be lost is bal- anced up against the cost of the insulation effecting the saving. Obviously, the more it costs to produce a ton of refrigeration the more it is economy to spend for insulation to conserve the refrigera- tion produced. * Exceptions to this general rule are products which are fermented while in storage, the process of fermentation giving rise to the evolving of a con- siderable quantity of heat. Digitized by V^OOQIC COLD STORAGE DUTY 169 Thermal conductivity varies widely among various so called insulating materials and even with the same material when vary- ing amounts of air and moisture are present. Table XXIII shows the rate of transmission expressed in B. t. u. per square foot per 24 hours per degree difference in temperature between the two sides of the insulation. These values represent efficiencies under best conditions. In making computations for determining the capacity of refrigerating machines it is customary among some builders to increase these values by frpm 25 to 50 per cent., accord- ing to the physical condition of the insulation. TABLE XXIII.— HEAT CONDUCTIVITIES, C, OF COLD STORAGE INSULATION Transmission in B.t.u. per sq. ft. per degree difference in temperature inside and out per 24 hours. Compiled largely from Information published by the Armstrong Cork Co. Instdating Slabs. B.t.u. 1" "Pure Cork Sheets" (Granulated Cork united by heat and pressure) 6.5 1" "Rock Wool Composition Boards" (Waterproofed); 7.4 1" Impregnated Cork Board (Granulated Cork and Asphaltic binder) 8.9 1" Indurated Wood Pulp Board 10.0 Built-^p Insulation (Wood and Air space) 1" American Spruce 16 . 80 (%" Dressed and Matched Spruce) (% Sp.) (paper, % Sp.) (% Sp. paper. % Sp.) . . 4.75 (% Sp. paper, % Sp.) (1" air space) {% Sp., paper,% Sp.) 4.25 6 thicknesses % Sp., 3 papers, 2 air spaces arranged as above 3 . 45 8 " " 4 " 3 " " " " 2.70 10 " " 5 " 4 " •• •• " 2.70 (8 thicknesses being }i and 2 thicknesses being %" thick) Built-up Insulation Wood, Paper and Fill . paper, Dry ) ( " '^ " ) 1.35 (8" Granulated Cork) " " " ) 1.90 (1" Pure Sheet Cork) ' ) 3.10 (% Sp. paper) (1" Pure Sheet Cork) (paper, fg Sp.) 3.26 ' " •• > (2" " " )( ^' " ) 2.60 (3" " ")( " " ) 2.25 1.20 (% Sp. paper, Ji Sp.) (?^ Sp. paper,Jg Sp.) 4.75 ( " ^' " ) (4" Mineral Wool) (% Sp. paper, % Sp..) 2.20 { " ;; •• ) (8" Mill Shaying8,_Damp) (^ Sp. paper, K Sp.) 2 . 10 ^ ) d" ? " " ) (3" ( " " ) (4" (H Sp.) d" Pitch) (% Sp.) 4.90 I " )(2" ")( " ) 4.25 Built-up Insulation (Wood, Paper, Air Space and FiU) i% Sp. paper, % Sp.) (1" Air Space) i% Sp.) (6" Min. Wool) (% Sp.. paper, % Sp.) . 1 . 49 ( " '^ " )( " M " ) (6" Gran. Cork) (% Sp., paper, Ji Sp. ) 1.46 ^ '^ " " )( " )(2"PureSh. " )( " " " ) 1.60 " ) (2" Pure Sheet Cork) (paper, % Sp.) 2 . 10 " )(3" " " )( " ) 1.70 " )(4" " " )( " " ) 1.20 " )(5" " " )( " " ) 90 Brick Wall and Sheet Cork (13" Brick Wall) (2" Pure Sheet Cork) 2.75 ( )(4" •• •• ) 1.47 Assuming, for example, a cold storage box 10' X 10' X 10', the superficial surface exposed is 600 square feet. The insulation may consist of two courses of |-" dressed and matched spruce with a course of paper between, a 1" air space and two more courses of spruce with paper between. The con- Digitized by V^OOQIC 160 ELEMENTARY MECHANICAL REFRIGERATION ductivity of insulation of this construction is given in the table as 4.25 B. t. u. If the insulation is found to be moist, about 20% may be added to the above value bringing the heat trans- mission up to about 5 B. t. u.* The 24-hour duty is now found by multiplying 600, the number of square feet surface, by 5, the heat transmission per square foot, giving 3,000 the niunber of B. t. u. per 24 hours per degree difference in temperature. This multiplied by the difference between the outside and inside tem- peratures (say 90''-36° or 54*") gives 162,000 B. t. u. bs the total heat absorbed. This divided by 144 B. t. u., the amount of heat required to melt a pound of ice, gives 1,125 pounds or 0.5625 tons as the amount of refrigeration required per 24 hours to make up for insulation losses. A simple expression for pounds of refrigeration K per 24 hours, per square foot of insulation having a B. t. u. conductivity C per 24 hours per degree difference in temperature (^— ^i), as given in Table XXIV, is (t-ti) [17] K = C 144 Substituting in this expression the values given in the above example gives which result multiplied by the total square feet of surface, 600, gives 1,125 pounds as before. Table XXIV gives similar values of K for different insulation conductivities, ranging from 1 to 10 B. t. u. per square foot and for differences in temperature ranging from 40° to 100°. *As a matter of fact, five B. t. u. per square foot per degree difference in temperature is often employed where the exact value of the insulation cannot be determined, as an approximate factor for estimating the total cold storage duty required for small and medium sized boxes with insulation of the average inferior quality usually employed in market and hotel refrigerators. The amount of refrigerating duty estimated on this basis should be ample to pro- vide not only for the insulation losses but for the cooling of the average small amount of product and the neutralizing of the amount of heat generated by lights, workmen and entering through the opening of doors. Digitized by V^OOQIC COLD STORAGE DUTY 161 To employ this table in the above example, find constant iC= 1.875 in the horizontal line opposite {t- <i) = 54® and in the vertical column mider C = 5. This factor multiplied by the surface, 600, gives, as before, 1,125 pounds, which divided by 2,000 gives 0.5625 tons of refrigeration as the required capacity to make up for insulation losses. TABLE XXrV. — Values op Constant, Ky Pounds Refrigerating Duty per Square Foot Wall Surface per 24 Hours for Different Insulation Con- ductivities and Differences in Temperature {t — ii), Inside and Out. 90° Fahr. (0, assumed outside temperature, minus inside temp. (<i), = (t — ti)f column 2. 144 Inside (t—h) B.t.u. per Sq. Ft. per Degree Difference in Temperature per 24 Hours Temp. 1 2 3 4 5 6 7 8 9 10 50 48 40 42 .2772 .2916 .5564 .5832 .8346 .8749 1.111 1.166 1.391 1.458 1.669 1.75 1.926 2.041 2.222 2.333 2.5 2.635 2.777 2.916 46 44 .3055 .6110 .9165 1.222 1.527 1.833 2.139 2.444 2.749 3.055 44 42 40 38 46 48 50 52 .3194 .3333 .3492 .3611 .6388 .6667 .6944 .7222 .9582 .9999 1.042 1.083 1.278 1.333 1.389 1.444 1.597 1.667 1.736 1.805 1.916 2. 2.083 2.167 2.236 2.333 2.432 2.528 2.555 2.666 2.777 2.889 2.875 3.000 3.125 3.249 3.194 3.333 3.471 3.610 36 54 .375 .750 1.125 1.5 1.875 2.25 2.625 3.00 3.375 3.750 34 32 30 56 58 60 .3899 .4028 .4166 .7778 .8056 .8332 1.167 1.208 1.25 1.556 1.611 1.666 1.945 2.014 2.083 2.332 2.417 2.6 2.729 2.82 2.916 3.119 3.222 3.333 3.501 3.625 3.749 3.883 4.028 4.166 28 62 .4306 .8612 1.292 1.722 2.153 2.583 3.014 3.485 3.875 4.306 26 24 22 20 64 66 68 70 .4444 .4583 .4722 .4861 .8888 .9166 .9444 .9722 1.333 1.375 1.417 1.458 1.778 1.833 1.889 1.944 2.222 2.292 2.361 2.431 2.666 2.75 2.833 2.917 3.001 3.208 3.305 3.403 3.555 3.666 3.778 3.889 4.000 4.125 4.250 4.375 4.444 4.582 4.722 4.861 18 72 5. 1. 1.5 2. 2.5 3. 3.5 4. 4.5 5. 16 14 12 10 8 6 4 2 74 76 78 80 82 84 86 88 .5139 .5278 .5417 .5556 .5694 .5833 .5972 .6111 1.028 1.056 1.083 1.111 1.139 1.167 1.194 1.222 1.542 1.583 1.625 1.667 1.708 1.75 1.792 1.833 2.056 2.111 2.167 2.222 2.278 2.333 2.389 2.444 2.569 2.639 2.708 2.778 2.847 2.917 2.986 3.056 3.083 3.167 3.25 3.333 3.416 3.5 3.583 3.667 3.597 3.695 3.792 3.889 3.986 4.083 4.180 4.278 4.111 4.222 4.334 4.445 4.555 4.666 4.778 4.889 4.625 4.750 4.875 5.000 5.125 5.250 5.375 5.500 5.139 5.278 5.417 5.555 5.694 5.833 5.972 6.111 90 .625 1.25 1.875 2.5 3.125 3.75 4.375 5. 5.625 6.250 —2 —4 —6 —8 —10 92 94 96 98 100 .6388 .6526 .6666 .6805 .6942 1.277 1.301 1.333 1.361 1.388 1.916 1.958 2.000 2.165 2.083 2.553 2.610 2.666 2.72 2.778 3.193 3.262 3.333 3.470 3.192 3.835 3.913 4.000 4.82 4.167 4.468 4.565 4.666 4.762 4.861 5.110 5.22 5.333 5.442 5.555 5.750 5.875 6.000 6.125 6.250 6.388 6.526 6.666 6.805 6.942 Digitized by V^OOQIC Digitized by V^OOQIC INDEX Absolute zero, 8. Absorber, 56, 59. Absorbing mediums, heat and water, 64. Absorption machines, commercial, 54. counter current effect in, 60. elementary, 68. invention of, 14. members of, 56. Morris, 40. water, 40. and compression machines, origin of, 14. and compression machines, com- pared, 53, 62. and compression, use of NaCl brine in, 40. of heat in cold storage, 73. systems, 27, 52, 53, 97. Actual capacity of compressors, 139. displacement of compressors, 138. Adiabatic compression, 64. Agitation in plate ice plants, 93. Air, change in volume and weight in cooling, 19. circulation in refrigerator, 18. circulation due to difference in head, 19. circulation natural and forced, 19, 20. circulation in ice bunker system, 18. composition of, 154. cooling, 22, 154, 156. dry, in bunker systems, 20. expansion of, under low pressure, 103. moisture in, 156. pressure, testing with, 100. saturated, 154. Alcohol, boiling point of, 6, 78. freezing point of, 6. Ammonia absorption machine, in- vention of, 14. absorption machine, commercial, 54,56. absorption machine, elementary, 68. absorption, of, in silver chloride, 52. amount required per pound of ice, 25. amount required per 100 ft. ex- pansion pipe, 108. and sponges, absorbing mediums 64. and steam condensers 74. properties of, 69. boihng points of, 6, 76. charge for ice plants, 109. compression of, adiabatic, 64. compressors, capacity of, 139. compressors, displacement of, 130. compressors, efl&ciency of, 86. compressors, types of, 48. condition of, 116. condenser and steam radiator com- pared, 74. cooling capacity of liquid, 25. cooling of, 131. cubic feet per unit of refrigeration, 129. disassociation of, 3. explosions of, 105. freezing point of, 6. latent heat of vaporization of, 6. for water cooling, 25. liquid in compressor, 6, 11. path of, in absorption madhine, 59. pounds per minute per ton, 135. pounds per unit of refrigeration, 128. shipping drums, 104. 163 Digitized by V^OOQIC 164 INDEX Ammonia, specific heat of, 131. stiU, 59. temperatures and pressures of, 105. use of to expel air, 103. weight of, per cubic foot, 107. Amount of chaige of ammonia, 105, 107. Analyzer, 57, 59. Anhydrous ammonia.