CHAPTER I. Introduction (Part 10)

air was admitted sufficient to raise the temperature of the air to 60\ while the walls, eta, were 45*, we should experience the same sensation of cold; for our bodies would immediately radiate their heat to the colder walls, etc., through the warm air, and would continue to do so until by some means the walls, etc., became warmed. Our bodies would also lose heat by convection to the air passing over us in draughts or currents caused by unequally heated columns of air cooled by the cold walls. If we suppose such a room thoroughly warmed before our entrance, so that walls, floor&, and furniture were at 70' temperature, and that suddenly all the warm air was with- drawn and cold air admitted at a temperature of 45*, we should be conscious only of a delightful freshness of the air ; our bodies would not lose any heat by radiation, but we should keenly observe cold draughts, the air next the walls becoming warmed by convection, the cold air in the centre of the room falling and causing draughts or currents, and removing heat by convection from any parts of our body exposed to the passing colder air. 90 DOMESTIC SANITARY DRAINAGE AND PLUMBING. The amount of radiant heat emitted and received varies greatly with the nature of the surface afffected ; the power of radiating and the power of absorbing heat are equal in the same surface. The power of giving and receiving radiant heat to and from surfaces at ordinary temperatures is simply propor- tional to the difference of temperature between the giving and receiving surface at high temperatures, with which we have not much concern. Dulong has shown great variation from this rule. For low temperatures, P&let's experiments give the following results, showing units of heat given and received per square foot per hour for each d^ree Fahrenheit of difference of temperature: — Copper, silver - plated an<] I Chalk •678 polished . -0266 Wood sawdust, fine •721 Copper •0827 Stone, plaster, brick •785 Tiu . . . '0439 Fine sand •74 Zinc and brass, polished •049 Calico •746 Tinned iron '0858 Woollens, any colour •762 Sheet iron, polished . •092 Silks . •758 Sheet lead •1328 Oil paint •758 Sheet iron -566 Paper, any colour •77 Cast iron •648 Lampblack •82 Rusty iron •687 Water r085 Glass •596 on . . . ViS We thus find that sheet iron, and particularly rusty sheet iron, will absorb and give out more heat than sheet lead, in a proportion which we can calculate from P^clet's table of results. Metals possess the least radiant power. The effect of colour is slight; white lead radiates as well as lampblack. Polish influences and lessens radiation, roughness of surface increases radiating power. We have pointed out that radiant heat is transmitted in straight lines and at right angles from the heating surface, ELEMENTARY SCIENCE FOR PLUMBERS. 91 through the air or in vacuo, to any distance, however remote, until intercepted and absorbed by some intervening matter. A heated sphere of iron, suspended in the centre of a room, so long as its temperature is higher than the walls of the room, will transmit rays of radiant heat in straight lines and in all directions through the air, which they do not afiTect until they strike and are absorbed or reflected by the walls. If the temperatures are reversed, the iron sphere having a temperature lower than that of the walls of the room, then the walls will transmit effective rays of radiant heat in straight lines and in all directions at right angles with the surfaces until the temperature is equalised. The effect in each case will be very different, all the rays pro- ceeding from the iron sphere taking effect on the cooler walls, while a very small proportion of the rays from the walls reach the sphere directly, the remainder practically negativing each other, neither losing nor gaining, and not directly affecting the sphere; indeed, where temperatures are like, radiant heat action is considered to cease. It will be seen that if the sphere was placed inside a spherical chamber the converging rays should meet in the sphere, and the effect on its temperature be at its maximum 80 long as the sphere remained cooler. The same laws appear to govern both radiant heat and light. The lens of a telescope may be used to collect heat rays as well as light rays, and to concentrate them on one point, as in a burning-glass. All bodies, whatever be their temperature, emit or radiate heat. Opposite surfaces of bodies radiate towards each other; the heat which each receives is partly reflected or diffused, partly absorbed by conduction from particle to particle, raising the temperature of the body, and partly trans- mitted through the substance of the body to other bodies beyond. 92 DOMESTIC SANITARY DRAINAGE AND PLUMBING. Bodies of unequal temperature in a confined space interchange their heat till equilibrium of temperature is established. The heat radiating and absorbing powers of bodies are equal, and the heat-reflecting power of a body not trans- parent to radiant heat is the complement of the radiant and absorbing power. If the intensity of the ray of heat be represented by 100 we are given the following numbers : — Polished silver Reflecting Radiating power. power. . 97 ... 8 Steel . ReflecUng Radiating power. power. . 88 ... 17 Red copper Polished brass Platinum . 98 ... 7 . 93 ... 7 . 88 ... 17 Glass. LampbUck . . 10 ... 90 0 ... 100 Good reflectors of heat are therefore bad radiators. The theory of radiant heat has been stated thus: If an enclosure be kept at a uniform temperature, any substance within it will attain that temperature. All bodies are constantly giving out radiant heat, independently of the temperature of the bodies which surround them. Therefore, when a body is kept at a uniform temperature, it receives back as much heat as it gives out. Bodies when cold receive the same rays which they give out when hot. The intensity of radiant heat varies inversely as the square of the distance, as with radiant light, and also as with the attraction of gravitation. When bodies are heated the first radiant heat is given oflf in obscure or dark rays, conveying the sensation of heat, only without light; as the temperature rises the rays begin to affect the eye in increasing numbers, making the body assume in turn a red heat, yellow heat, and white heat. ELEMENTARY SCIENCE FOR PLUMBERS. 93 The Transmission of Hbat by Convection. The conductivity of liquids and gases for heat is very slight — in the case of gases it has not been fully proved to exist — nevertheless heat is rapidly transfused throughout their volume, owing to their qualities of expansion and mobility, by direct transport of the heated particles. These particles, expanding and becoming lighter or less dense by contact with heat, are forcibly displaced by the greater weight of any colder particles above them, and are driven upwards, carrying heat with them, and transmitting heat by convection. Heat applied at the lowest point of any vessel or apparatus has, therefore, a better effect than when applied at the sides or top, for as the colder particles fall and dis- place the heated ones, they in turn receive the heat by the closest and most direct contact at the bottom, become lighter, and are displaced in rotation by other colder and denser particles setting up a circulating current. Heat applied at the side of a vessel, as in the side flues of boilers, causes the currents of side-heated particles to start only from that point of the sides where the heat is applied, consequently the lower down the side flues are formed the greater will be the heating effect. Heat applied at the top of a boiler is almost useless, except in checking upward outward radiation of heat from the boiler. Heat applied at the bottom is, therefore, the most rapidly diffused, and is the really effective heat in any boiler. The particles of heated water, being driven away from contact with the heated bottom surface, rise through the longest section of colder water in the boiler, transfer- ring heat by convection as they pass among the descending particles. Similar convected or transported heat action takes place in air with much greater rapidity, and is the principal 94 DOMESTIC SANITARY DRAINAGE AND PLUMBING. effective cause of the warming of the air, whether it be the outside open air, or confined in buildings. The Transmission of Heat by Conduction can be observed easily by placing one end of a bar of metal in a fire, and holding the other end in the hand. The heat will pass from the fire and be transmitted or conducted from particle to particle of iron, until in time the hand must be removed, unless the length of the bar be such as to exceed the conducting power, which varies in different metals, and in some is soon overcome by the radiating power emitting the heat as rapidly from the surface of the metal as it is transmitted to it by conduction. The following list gives the degree of conductivity of various substances: — Silver 1000 Steel . . 116 Copper 776 Lead . . 85 Gold . 532 Platinum 84 Brass . 236 Palladium 68 Zinc . » 190 German silver 60 Tin . 145 Bismuth . 18 Iron . 119 Of solid bodies, metals — excepting bismuth — are the best conductors of heat. As an easy experiment, place a silver spoon and a German silver spoon in one vessel of hot water, each having a grain of phosphorus on the end. That on the silver will quickly ignite, while that on the German alloy will remain unaffected, showing the difference clearly between the conducting powers of these two metals. This knowledge is useful to plumljers, enabling them to select suitable metals to effect in the best way certain definite purposes in heating apparatus, and to use suitable materials to prevent waste and diffusion of heat, which they may have contracted to convey and deliver at distances from the sources of heat at their command. elementary science for plumbers. 95 Latent Heat. The processes of melting solids and vapourising liquids take place at certain defined temperatures, which are the same for the inverse processes of solidification and lique- faction. Thus, ice melts at 32' to water, and water vapour- ises at 212'' to steam ; and by inverse process steam liquefies at 212" to water, and water solidifies at 32° to ice — the critical point of the change of state being close to the given temperatures. When heat is continuously applied to ice the ice melts, and continues melting into water at 32° ; but the heat does not raise the apparent temperature, nor affect the thermo- meter, which remains at 32° constant. The heat is absorbed by the ice and water and becomes latent or concealed, and is termed the latent heat of liquefaction. This latent heat is disengaged when the water is changed back to ice. One pound of water at 212° in an open vessel requires 966 units of heat to convert it into steam at 212°, and this is termed the latent heat of vapourisation. If the steam be condensed again to water the same quantity of heat will be disengaged. This fact is taken advantage of in warming buildings, and in heating large quantities of water in tanks by steam. Ice requires latent heat to convert it into water; water requires latent heat to convert it into steam. One pound of ice at 32° takes as much heat to change it into water at the same temperature as would raise the temperature of 142 pounds of water one degree ; and as one pound of water at 212° takes as much heat to change it into steam at the same temperature as would raise 966 pounds of water one degree, the latent heat of water is said to be 142, and of steam 966. The latent heat of vapourisation is measured from the 96 DOMESTIC SANITARY DRAINAGE AND PLUMBING. number of units of heat required to change liquids from their boiling point into vapour under an atmospheric pressure of thirty inches of mercury. The latent heat of water = 966 ; alcohol, 457 ; ether, 313 ; naphtha, 184 Heat is disengaged when substances enter into chemical combination, as instanced by carbon and oxygen in combus- tion, water and quicklime, water and sulphuric acid. Heat is also disengaged by mechanical action — friction, percussion, and compression. Measurement of Heat. Heat cannot be measured by our sensations. It does not affect our bodies equally ; it produces sensations differ- ing not only according to the degrees of the heat itself, but also to the varying condition of our bodies and of our surroimdings. If we enter a room at 60° directly from frosty air outside at 30* we feel the inner air too' warm ; yet the very same air will feel cold if we enter it directly from a Turkish bath heated to 150\ If we plunge one hand in water at 120* and the other in water at 40*, and, suddenly withdrawing them, plunge both together into water at 80*, we shall experience the opposite sensations of heat and cold from the same water at the same moment. Heat, therefore, cannot be measured by our sensations; hence the value and necessity of thermometers, or heat- measurers. Heat causes bodies affected by it to expand, or to become lighter bulk for bulk, as they increase in tem- perature. There is one exception to this rule. Heat applied to water at 32* F. causes the water to contract until it reaches 39° F., at which temperature it commences to ex- ELEMENTARY SCIENCE FOR PLUMBERS. 97 pand until it reaches the boiling point, 212^ after which, at ordinary atmospheric pressure, it expands into steam. The temperature of water and air is measured in various ways. The mercury thermometer is the instrument usually employed. In France, and generally for scientific research, the thermometer adopted is marked on the scale called centigrade, in which freezing point is zero, or 0\ and boiling point 100°, with 100 equal degrees marked between. In Germany the thermometer is marked on the scale called after Eeaumur, in which freezing point is zero, or 0', and boiling point 80°, with 80 equal degrees marked between. In England the thermometer is marked on the scale called after Fahrenheit, in which freezing point is 32° and boiling point 212°, with 180 equal degrees marked between, the zero, or 0° point, being therefore 32° below freezing point, and chosen by Fahrenheit as zero because it was at that time supposed to be the point of greatest cold. Plumbers often require to convert centigrade into the Fah- renheit scale, and vice versd. This is easily done. Multiply the centigrade degrees by 9, and divide the product by 5, and add 32, the result being the corresponding degrees on Fahrenheit scale— 9 C.° ooo x^o — - — + oZ = r . 5 Thus, to convert the boiling point of 100° centigrade to Fahrenheit — 100° X 9 = 900 - 5 = 180° + 32° = 212° F., boiling point ; which, being translated, is — multiply 100 by 9, equals 900 ; divide this by 5, equals 180 ; add to this 32, equals 212. 100° centigrade boiling point. 9 5)^00 180 32 212" Fahrenheit boiling point. H 98 DOMESTIC SANITARY DRAINAGE AND PLUMBING. The same courae is adopted to convert any other given temperature. To reverse the proceeding, and convert Fahrenheit scale into centigrade, subtract 32* from the given temperature, multiply the remainder by 5, and divide the product by 9. Thus, to convert 104* Fahrenheit into centigrade — 104* - 32* = 72* X 5 = 360'* -j- 9 = 40^ C. Subtract 32 from 104, equals 72; multiply by 5, equals 360; divide by 9, equals 40. 104* Fahrenheit. 32 72 5 9)360 40* centigrade. 4 C* To convert centigrade to Reaumur: - - - =R.* To convert Eeaumur to centigrade : 5R.* = C.' .0.4 To convert Fahrenheit to Reaumur : (F.* - 32r = R."" 9 To convert Reaumur to Fahrenheit: -A +32 = F.* 4 The boiling point of liquids at atmospheric pressure is given by various authorities: — Degrees. Degrees. Water . . 212 Nitric acid (Dalton) . . 220 Mercury (Regnault) . 662 Muriatic acid (Dalton) . . 222 Linseed oil (Ure) . 600 Alcohol (Ure) . . . 173 Sulphur (Ure) . . 670 Ether, sulphuric (G. Lussac) . 100 Naphtha (Ure) . . 306 Water saturated with chloride Oil turpentine (Ure) . 316 of calcium . . . 355 Sulphuric acid (Dal ton) 240-620 The boiling point of water at diflferent pressures varies as Regnault has proved, in the following proportions : — ELEMENTARY SCIENCE FOR PLUMBERS. 99 At half atmospheric pressure, or with mercury at 15 inches, water boils at 180" F. At atmospheric pressure, or with mercury at 30 inches, water boils at 212'. At a pressure of 2 atmospheres, or 60 inches of mercury, or 14*7 lbs. per square inch, water boils at 249'. At a pressure of 3 atmospheres, or 90 inches of mercury, or 29 lbs. per square inch, water boils at 273°. At the following pressures water boils: — 44 lbs., 291°; 59 lbs., 306^ 73 lbs., 319'; 88 lbs., 330'; 103 lbs., 339'; 117 lbs., 348'; 132 lbs., 357'. In high-pressure small-bore heating apparatus this pro- portion may be observed. The melting points of solids given by M. Pouillet seem to be the most reliable :— Wrought i Steel ron DegTe«*8. 2910-2730 2550-2370 Bismuth Tin Degrees. 518 455 Cast iron 2190-1920 Sulphur 239 Gold 2280-2156 W^ax