CHAPTER V HEAT TRANSFER (Part 1)

CHAPTER V HEAT TRANSFER Heat Transmission. — Heat is transmitted through a sub- stance when there is a temperature difference, and is caused by the natural tendency of heat toward a temperature equiHbrium. The heat flow is always from a region of higher temperature to a region of lower temperature, and may occur in three ways: Conduction, radiation and convection. The rate of heat transfer from one region to another, de- pends on the amount of surface, the difference in temperature and the material through which the flow^ occurs. The rate of transfer through various materials has been determined experimentally by many scientists, the most reliable of which are given in Table XLH as compiled by the Bureau of Stan- dards. From this table it will be noted that a coefhcient "C" is given, which is the overall transmission of heat based on a unit of time, surface, thickness and temperature difference or B.t.u. per hour per square foot per inch of thickness per degree F. As the heat transfer is practically proportional to the thickness, the fundamental law can be expressed in a very simple formula : "C" (1) Transmission in B.t.u./Hr. = X average sq. ft. X thickness temperature difference From the foregoing it will be noted, that if the temperature and area of a transmitting surface are known and held con- stant, the heat transfer depends upon conduction, radiation and convection. 101 102 HOUSEHOLD REFRIGERATION TABLE XEII. INTERNAL THERMAL CONDUCTIVITIES OF VARIOUS MATERIALS (c)* Material Description B.t.u. perj 24 hours , 4 2 Air Cell, 1 inch. . Aluminum Ammonia Vapor. Aqua Ammonia . Asbestos Mill Bd Asbestos Paper . . 110 12.0 Air Ideal air space Air Cell, Vi inch. . . Asbestos paper and air spaces . Asbestos paper and air spaces Cast 24.000 32°F 3.19 64° F 75.90 Pressed asbestos — not very flexible .•••.■■■ 20.00 Asbestos and organic bind- er 12. Asbestos Wood Asbestos and cement 65 . 0 Balsa Wood Very light and soft— across grain 8.4 EsS.^^^^^::::::::::::::::::;:::::::::::;|.ooo Brick Heavy 120 Brick Light, dry »4 Brine Salt ,• • ■ v ' L' ' " ^'^ Cabot's Quilt Eel grass enclosed in bur- _ lap ' ■ ' Calorax Fluffy finely divided min- eral matter 5.3 Celite Infusorial earth powder. . . 7.4 Cement Neat Portland, dry 150.0 Charcoal Powdered 10.0 Charcoal Flakes 14-6 Cinders Anthracite, dry 20. J Concrete 125.0 B.t.u. per hour Concrete Of fine gravel 109.0 Concrete Of slag 50.0 Concrete Of granulated cork ;„„„„ Copper 50.000 Cork '".'.'. Granulated K-3/ 16 inch.. 8.1 Cork Regranulate X/Xd-Ys inch. 8.0 CorklDoard No artificial binder — low density o- ' Corkboard No artificial binder— high density ' -^ Cotton Wool Loosely packed 7.0 Cypress Across grain lo.O Fibrofelt Felted vegetable fibers ... 7.9 Fire Felt Roll Asbestos sheet coated with cement 15.0 Fire Felt Sheet Soft, flexible asbestos sheet 14.0 FlaxHnum Felted vegetable fibers ... 7.9 Fullers Earth Argillaceous powder 17.0 Glass 24.0 Glass 178.0 Granite 600 Granulated Cork . . About 3/16 inch 7.5 Gravel Dry, coarse 62 .0 Gravel Dry. fine 39.0 Ground Cork ' • i Gvpsum Plaster ^f" Hair Felt "^ ■ ^ Hard Maple Across grain 27.0 Ice 408 Infusorial Earth. . . Natural blocks 14.0 Insulex Asbestos and plaster blocks— porous. ..... . 22.0 Insulite Pressed wool pulp— rigid. . 7.1 Iron Cast 7 740 Iron Wrought 11.600 Kapok Imp. vegetable fiber — loosely packed . 5.7 Keystone Hair Hair felt confined with building paper 6.5 Limestone Close grain 368 Limestone Hard 214. u Lb. per cu. ft. 0. 175 0 458 0 500 1000 000 0. 133 3.160 0.08 8.80 8.80 .62 0.21 56.50 0.830 61.00 0.500 31.0 3.700 123.0 0 350 12.700 625.000 5 000 3 . 500 1.130 0.321 0 221 0.308 6 250 0.417 0.613 0.845 5 200 4.540 2.080 1.790 2083 . 000 0.337 0.333 0 279 0.308 0.292 0.666 0.329 0.625 0.583 0.329 0.708 5.160 7.420 25.000 0.313 2.582 1.630 0.294 2.250 0 246 1.125 17.000 0.583 0.916 0.296 321.500 483.000 0.238 0.271 15.300 9.330 7.5 250' " 131. 115. 73.4 16.0 4 0 10.6 170. 11.8 15.0 40.0 136.0 124.0 94.5 7.5 556.0 5.3 10 0 6.9 29.0 11.3 43.0 26.0 11.3 33.0 150.0 185.0 166.0 8.1 115.0 91.25 9.4 44.0 57.4 43.0 29.0 11 .9 450.0 485.0 0.88 19.0 185.0 159.0 'From "Principles of Refrigeration," Nickerson & Collins Co., Chicago. HEAT TRANSFER 103 TABLE XLII. — INTERNAL THERMAL CONDUCTIVITIES OF VARIOUS MATERIALS (c) — (CONTINUED)* Material Description B.t.u. per 24 hours B.t.u. per hour Lb. per cu. ft. .Soft 100.0 4.167 113 0 Linofelt . Vegetable fiber confined with paper 7.2 0.300 11.3 Lithboard . Mineral wool and vegeta- 9.1 22.0 0.379 0.916 n 5 Mahogany . Across grain 34.0 Marble .Hard 445 18.530 175.0 Marble .Soft 104 4.330 156.0 Mineral Wool. . . . . Medium Packed 6.6 0.275 12.5 Mineral Wool. . . . . Felted in blocks 6.9 0.288 18.0 Oak . Across grain 24.0 1.000 38.0 Paraffin ."Parowax," melting point 52° C 38.0 1.582 56.0 Petroleum .55°F 24.7 1.030 50.0 Plaster 132.0 5.500 105.0 Plaster . Ordinary mixed 90 3.750 83.5 Plaster .Board 73 3.040 75.0 Planer Shavings. . .Various 10.0 0.417 8.8 . Stiff pasteboard 11 0 0 458 Pumice .Powdered 11.6 0.483 20.0 Pure Wool 5.9 5.9 0.246 0.246 6 9 Pure Wool 6.3 Pure Wool 6.3 7.0 16.0 0.263 0.292 0.667 5 0 Pure Wool 2 5 Rice Chaff 10 0 Rock Cork . Mineral wool and binder — rigid 8.3 0.346 21.0 Rubber .Soft 45 7.875 94.0 ' Rubber .Hard, vulc 16.0 0.667 59.0 Sand . River, fine, normal 188.0 7.830 102.0 Sand . Dried by heating 54.0 2.250 95.0 Sandstone 265 11.100 138.0 Sawdust .Dry 12.0 0.500 13.4 Sawdust .Ordinary 25.0 1.040 16 0 Shavings . Ordinary 17.0 0.707 8.0 Silicate Cotton. . . 14.0 18.0 0.583 0.750 8 55 Slag Wool 15.0 Snow on Ref . Coils 75 17 0 3.130 0 707 Tar Roofing 55 0 Vacuum . Silvered vacuum jacket. . . 0.1 0.004 Virginia Pine . Across grain 23.0 0.958 34.0 Water .Still, 32° F 100 4.166 62.4 White Pine . Across grain 19.0 0.791 32.0 Wool Felt . Flexible paper stock 8.7 0.363 21.0 Conduction. — Heat transfer by conduction occurs by means of molecular transmission due to the different intensities of ir- regular vibration of the molecules, causing the higher tempera- ture or more rapid moving molecules to strike the lower tem- perature or slower moving molecules and cause them to move at the same rate. Due to friction, adhesion, etc., the intensity decreases as it passes from the faster to the slower molecules. The interchange of heat in this way may occur between differ- ent parts of the same body or between two separate bodies in actual contact. When one end of a bar of iron is held in a fire the other end willsoon become too hot to hold in the hand. The heat 104 HOUSEHOLD REFRIGERATION has been tiausierred by conduction. One end of a wooden stick can be held in the fire without the other end becoming warm. In general, metals are good conductors, while lighter weight materials are poor conductors, so that comparative transmission can be made from their densities. A recent the- ory for the better insulating properties of substances contain- ing air cells, is that there is a very intense atomic resistance at the junction of a solid and gas, thus oftering greater retarda- tion to the molecular activity transfer. Radiation. — Radiation is the transfer of heat b} means of continuous and irregular ether vibrations and the transforma- tion, in whole or in part, of the energy of light into heat energy by imjjact upon tlie surface of a substance. It is an electro- magnetic phenomenon, in which the longest heat waves are about 0.042 centimeters while the shortest solar waves that can pass through the atmosphere are 0.00003 cm. The range of the radio waves is about 3 meters to 20,000 meters. When heat or solar radiation strikes a bod_' it is in general partly reflected, partly absorbed and partly transmitted. The part which is transmitted is nil in case of metals, unless they are made into exceedingly thin almost transparent foils, it is very small in case of water and ice and large in case of quartz, rock salt, etc. Thus in most practical cases part of the radiation is absorbed and part of it reflected, the amount of which is smaller the more dull and black the surface is. In the ideal limiting case which is closely approached b- lampblack, the entire amount of radiation is absorbed. The amount of heat transferred by radiation depends upon the character of the radiating surfaces; whether hot or cold, dark or light, temperature difiference, absorbing properties, etc. The blacker an object the more heat it will lose by radia- tion. Stoves and radiators intended to give out heat should be black. Cooking utensils, coffee urns, etc., should be bright, (tinned or nickeled) in order to lose as little heat as possible. A stove nickel plated all over will give out only about half as much heat as the same stove at the same temperature if black. A brightly tinned hot air furnace pipe may lose less heat than when covered with one or two layers of asbestos paper, as the surface of the asbestos paper radiates heat much HEAT TRANSFER 105 more rapidly than the bright tin. The pipe should be black to prevent radiation to the inside surface of the asbestos paper, then the asbestos would be more effective. If asbestos paper of sufficient thickness is used, it will save heat, even on bright tin pipes. Further it has been found experimentally that a body as it is heated radiates heat waves the amount of which is equal to the amount absorbed. The table below gives the radiating and absorbing power, and the reflecting power of a few com- mon substances. It may be noted here that the radiating power is also called the emissivity, TABLE XLIII. — HEAT ABSORBING. RADIATING. AND REFLECTING POWER OF SLTBSTANCES ' Absorbing & Reflecting Substances Radiating Power Power gl-^'^'x'^ :•:::::::;;:::: Ifo S;?S i;le ;;;;;;;::;::::::::::::::.::: ss .is Polished Cast Iron 25 -75 Polished Wrought Iron f^ ■'' Polished Brass 07 .V^ Copper Hammered -^ -^^ Silver Polished Qj ZL According to Prevost's theory of heat exchanges a warm body radiates more heat to the surrounding cold bodies than it receives from them and thus its temperature drops, while a cold body also radiates heat but it radiates less than it re- ceives, and, therefore, its temperature rises. According to this theory, a body in a refrigerator placed near the ice radiates heat no faster than it would to a warmer body, but it receives less from the ice in return and, therefore, becomes colder. It is well established that the heat exchange by radiation between two bodies is given by: H = E (T2* — TiO X 16 X 10" Where H = B.t.u. per sq. ft. per hour. Ti & Tj = Absolute Temperatures of the two bodies in degrees F. E = An empirical constant called the emissivity of the surface considered: E = 1 for a black body. Radiation Between the Sun and the Earth. — The heat and light from the sun come to us through space in a form of wave motion called radiation. The atmosphere offers considerable 106 HOUSEHOLD REFRIGERATION obstruction to the passage of these waves. Even when the sky is very clear, rarely more than 65 per cent of the radiation penetrates to the surface of the earth, the part absorbed being expended in raising the temperature. The region near the upper limits of the atmosphere is one of intense cold. As the sun, having a much higher temperature than earth, radiates heat to the earth, so from the surface of the earth, heat is radiated to the much colder upper limits of the atmosphere. The radiation of heat from the earth is continuous both day and night when there are no clouds or other obstruction be- tween the earth and the upper atmosphere. During the day the amount of heat received from the sun is so much greater than the amount lost by radiation from the earth, that the tem- perature rises. After the sun sets, however, no heat is re- ceived to counterbalance the loss by outgoing radiation and the temperature falls. — (U. S. Department of Agriculture, Farmers' Bulletin, No. 1096). Convection. — Convection is the transfer of heat by displace- ment of movable media ; the heated medium moves and car- ries the heat energy with it. In other words heat is carried from one place or object to another by means of some outside agent, such as air or water or any moving gas or fluid. The hot air and hot water heating systems work on this prin- ciple. For example, in case of an ordinary household radia- tor, steam or hot water heats the radiator and it establishes a temperature differential between the metal and the adja- cent layer of air, the layer of air is heated and consequently its density is reduced as compared to the cooler layers of air. Thus the denser and cooler particles of air begin to descend while the warmer and less dense particles begin to rise and a natural upward movement of heated particles sets in. If desired, however, the movement of these heated particles can be ac- celerated and the heat transfer greatly increased by means of a fan or blower. In the first instance we have natural convec- tion and in the second forced convection. It is also clear that the increased heat transfer secured in the latter case is pro- duced by the external mechanical work supplied and here as in all other engineering work we pay for what we receive. HEAT TRANSFER 107 The food and containers in a refrigerator are cooled mostly by convection. The circulating air is the medium used to trans- fer the heat from the food and walls of the food compart- ment to the ice. This process of heat transfer is continuous. The air in passing through the food compartment absorbs suf- ficient heat to increase its temperature about 10° F. It is well known in a general qualitative way that heat flow by forced convection between a metal surface and a fluid depends upon the following: l_The velocity of the fluid; the higher the velocity the higher the heat flow. 2 — The temperature difference between the metal and the fluid; the higher this difference the higher is the heat flow. 3 — The thermal conductivity of the fluid. 4 Diameter of the tubes around or in which the fluid is assumed to flow. 5— The density of the fluid. 6 — The viscosity of the fluid. 7 — The depth or length of the device measured along the path in which the fluid flows. 8 — The temperature difference between fluid and body. 9 — The character of the surface. The values given in Table XLIV are for inside surface only, and are represented by the symbol K. Due to the more exposed outside surface and rapid movement of air, the coeffi- cient for this is much larger, generally 23^2 to 3 times, so that K2 can be used as 3 times K. TABLE XLIV. COEFFICIENTS OF RADIATION AND CONVECTION IN B.T.U. PER HR. PER » F. SQ. FT. University of Illinois Engineering Experimental Station. Brick Wall 1.40 Glass 2.00 Concrete 1.30 Tile plastered on both sides. . 1.10 Wood 1.40 Asbestos board 1.60 Corkboard 1.25 Sheet asbestos 1.40 Magnesia board 1.45 Roofing 1-25 Comparison of Heat Insulators. — Table XLV gives a com- parative idea of the thermal conductivity of insulators used in household cabinets. Air which cannot circulate and carry heat in that way (by convection) is one of the best heat insulators 108 HOUSEHOLD REFRIGERATION to be found. Cotton, wool, feathers, cork, etc., are good in- sulators because they contain a large amount of air in the cells or in the spaces between the fibers. Clothing keeps in the heat of the body chiefly because it contains air between the lay- TABLE XLV. COMPARISON OF THERMAL CONDUCTIVITY OF HEAT INSULATING MATERIALS USED FOR INSULATING HOUSE- HOLD REFRIGERATORS Relative Thermal Material Conductivity Vacuum jacket silvered 1 Mineral wool (medium packed) 66 Corkboard (low density) 67 Ground cork (ordinary) 71 Vegetable fibre (Linofelt) 12 Granulated cork (about 3/16 inch) 75 Eel grass (enclosed in burlap) 11 Balsa wood (medium weight) 92 Planer shavings ~ 100 White pine (across grain) 190 Oak (across grain) 240 ers and in the meshes of the cloth. Wlien the enclosed warm air is displaced and is replaced by colder air, as is the case in windy weather, the clothing no longer keeps one so warm. FIG. 4.— SHOWING RADIATION AND CONVECTION LOSSES. If the clothing is close-fitting, there is less room for an air layer between the layers of the clothes and therefore, it is less warm. To keep warm in cold, windy weather, the clothing should consists of loosely fitting garments, preferably of wool, with HEAT TRANSFER 109 some outside wrap which is nearly windproof, such as a very- close woven cloth, or even leather or rubber. A fur coat is very much warmer if the fur is on the inside, where the wind cannot disturb the air which is held among the hairs. Determination of Heat Loss Through a Wall or Refrig- erator.— As shown in the previous paragraphs, heat is trans- mitted through a substance from higher regions to a lower re- gion by means of conduction, radiation, and convection. Re- ferring to Fig. 4 it can readily be seen that the drop from T^ to T, is radiation and convection losses from the outer surface, 13 "Brick W^Li- 2 Cem emt- 2 "Cork Bo>\rd 2" — 2