Heat exchanger

“ All that is necessary is a current of air over your salt water to remove the water vapor collected there, and then the carrying of this confined current into a cool reservoir where the water may condense.
The thermos-bottle idea applied to a water tank As little or no artificial heating is required, a great saving can be effected in the matter of fuel. It is extraordinary how wasteful we are in our means of producing heat and in retaining it after it has been produced. It is safe to say that a great deal more heat goes up the chimney than we utilize from a fire. ”

“ Heating Unit. The heating unit of a hot-water system is like any stove consisting of a fire pot and grate. Some are adjustable so that different kinds of fuel may be [66] used. A gas burner is sometimes placed in the fire pot and used for heating a furnace, but this is one of the most wasteful ways of using gas. A real gas furnace is much more economical. The fire and heat from the fire circulate around the coils containing the water. If the coils are not constantly kept full of water, they will be injured by the heat. ”

“ The water formed by the condensation of steam in the heating system is carried back through the main return pipe to some kind of receiver, and is pumped into the boilers. It is generally arranged to pass through some kind of an exhaust-steam feed-water heater on its way to the boiler. The pump is also usually operated automatically, as will be discussed later.
The feed-water heater is an essential in all steam plants, and its purpose is to utilize as much as possible of the heat in the exhaust steam in heating the water fed to the boiler. ”

“ Similarly, fuels in hot coals and ashes can be moved, or the hot material can be scraped away to deprive them of heat.
Water is highly efficient as a cooling agent because of its great heat-absorbing capacity, in both liquid and vapor form. ”

“ If the proportion of free oxygen (i.e. excess of air) is large, more air is passed through the furnace than is required for complete combustion, and the heating of this excess is clearly a waste of heat. The position of the econometer in testing should be as near the furnace as possible, as there may be considerable air leakage through the brickwork of the flues.
The air supply to modern furnaces is usually delivered hot, the inlet air being first passed through an air-heater the temperature of which is maintained by the waste gases in the main flue. ”

“ It does not necessarily require all the exhaust steam—or the whole volume of waste steam passing from the engine to bring the feed water up to the temperature desired, and the larger the heating appliance the smaller proportion is needed—hence heaters are best made with two exits nicely proportioned to avoid back pressure and at the same time utilize enough of the exhaust to heat the feed water. ”

“ As the production of water gas implies the absorption of a large quantity of sensible heat, it is accompanied with a rapid fall of temperature in the chambers, B', and eventually also in the generator, A, while at the same time the chambers, B, are but moderately heated by the sensible heat of the current of gas produced. ”

“ On these grounds it is not so economical, so far as the consumption of fuel is concerned, to use steam instead of water, either water heated to a high temperature under pressure, or to heat air for warming purposes, because the gases from the fire employed to produce the higher degree of heat will pass off at a high temperature, and the heat they contain be wasted. On the other hand, the capital outlay required, where highly-heated pipes are used, is smaller than with hot-water pipes, because a smaller heating surface, and therefore smaller pipes, will suffice when the temperature is high ”

“ These consist of a motor and a machine for compressing gas, a chamber which is to be cooled, and sometimes coils of pipe containing brine.
When the gas—for example, ammonia or carbon dioxide—is compressed, it heats the pump which compresses it. That is, when a liquid or gas is being compressed, it gives up heat. When a liquid or gas expands, it takes heat from somewhere. In refrigerating plants, the expanding gas is made to take the heat either directly from the refrigerator or storeroom, or from brine which is then used for cooling the refrigerator or room. ”

“ We are easily convinced, à posteriori, that this re-establishment of equilibrium causes a loss of motive power if we reflect that it would have been possible to previously heat the feed-water by using it as condensing-water in a small accessory engine, when the steam drawn from the large boiler might have been used, and where the condensation might be produced at a temperature intermediate between that of the boiler and that of the principal condenser. ”

“ In the preceding Section, we learned that many houses heated by hot water are supplied with fresh-air pipes which admit fresh air into separate rooms or into suites of rooms. In some cases the amount which enters is so great that the air in a room is changed three or four times an hour. The constant inflow of cold air and exit of warm air necessitates larger radiators and more hot water and hence more coal to heat the larger quantity of water, but the additional expense is more than compensated by the gain in health. ”

“ The principal advantage of gas-heating is that stoking is dispensed with, and, if the pressure in the pipes is pretty constant, the apparatus may be left for many hours without attention, as the heat given off will be constant, and to a great extent determinable beforehand. ”

“ Some firms have now made percolators and water heaters with fuse-nut or safety fuse devices which melt if overloaded or allowed to heat up without any liquid in them to be heated. You must not let a percolator “perc” without any water in it. People complain more about good percolators because their heating element [11] burns out, either because they do this or because they have it connected up with too many other devices. Even if you do the right thing in these respects, don’t forget to disconnect the electricity by pulling out the plug. ”

“ As long as the temperature at the heated side of the furnace wall is kept constant, the temperature gradient and the rate of heat conduction also remain constant. Increasing the thickness of the wall lengthens the path through which the heat must travel, decreasing the temperature gradient and the rate at which heat appears at the outside surface. Raising the temperature in the furnace increases the temperature gradient and the rate of heat conduction, much as raising the inlet pressure increased the rate of flow of water in the pipe. ”

“ Desirable qualities are that the arrangements of the furnaces should be such that a reasonably perfect combustion of the fuel should be possible, and that the heating surfaces should be capable of transmitting a large proportion of the heat produced to the water so as to obtain a high evaporative efficiency. Further, the design generally should be compact, not too heavy or costly, and such that the cleaning necessary to maintain the evaporative efficiency can be easily effected. ”

“ For warming buildings, the furnace would become an electric generator, from which wires, instead of pipes for steam or hot air, would lead to the rooms to be heated, when, by interposing a suitable resistance, the energy of the current would be converted into heat. The probability of being able to convert the energy due to combustion of fuel into electric instead of heat energy may be very small ”

“ As the heat transfer does not, therefore, increase proportionately with the volume of gases, the exit temperature for a given furnace temperature will be increased as the volume of gases increases. As this is the measure of the efficiency of the heating surface, the boiler efficiency will, therefore, decrease as the volume of gases increases or the capacity at which the boiler is operated increases. ”

“ Cold air enters at the bottom of the frame, becomes warm by passing over the heating elements, and rises out at top and into the room. More cold air enters the frame and is heated in the same way, and in a very short time the whole of the air of the room becomes warmed. The full power of the heater is used in the preliminary warming of the room, but afterwards the temperature may be kept up with a much smaller consumption of current, and special regulating switches are provided to give different degrees of heat. ”

“ The metal pipes which convey the heated air from the furnace to the ducts are sometimes covered with felt, asbestos, or other non-conducting material in order that heat may not be lost during transmission. The ducts which receive the heated air from the pipes are built in the non-conducting walls of the house, and hence lose practically no heat. The air which reaches halls and rooms is therefore warm, in spite of its long journey from the cellar.
Not only houses are warmed by a central heating stove, but whole communities sometimes depend upon a central heating plant. ”

“ Hence, as coal-gas is assumed for the present purpose to possess invariably the same heating power, it appears that the same quantity of heat is always needed to convert a given amount of cold water at a certain temperature into steam ”

“ The plan of carrying the heat from the fire to the air to be warmed by means of hot-water pipes affords also a very economical method of warming air, because the best-constructed hot-water apparatus will enable the full heating value to be got out of the fuel. Fuel may be consumed to far greater advantage in a close furnace than in any open grate, because the admission of air for the combustion of the fuel can be regulated to any required extent. ”

“ The stream of air is usually accomplished by the use of ordinary chimney draft assisted by the warm products of heating from the heater. The hot air products of combustion pass through a tapered nozzle into the moist air exhaust pipe, and by the speed a suction is created which helps to pull the moist air out of the cabinet and up the chimney.
When you buy a dryer see to it that the exhaust pipe is large so that you will have wind and heat instead of just heat. Air circulation is what you are really buying. ”

“ With low temperature gases, to obtain a heat transfer rate at all comparable with that found in ordinary boiler practice, the lack of temperature must be offset by an added velocity of the gases in their passage over the heating surfaces. ”

“ All liquids do not evaporate at the same temperature as water. Some require a great deal more than 212 degrees; others, like, for instance, dioxide-of-carbon, will evaporate at 110 degrees, or about one half the heat necessary to turn water into steam.
On the other hand, all gases act alike so far as their heat absorption is concerned, so that by using a material with a low evaporative unit, less [195] fuel will be required to get the same expansion, which means the same power. ”

“ Owing to the fact that the heating takes place by means of coils, the steam passed through may be condensed, and thus each pound can be made to give up some 950 units of heat. With the absorption process worked by an efficient boiler, it may be taken that 200,000 thermal units per hour may be eliminated by the consumption of about 100 lb. of coal per hour, with a brine temperature in the refrigerator of about 20° Fahr. ”

“ Vapors of water can be formed only through the intervention of a boiler, while atmospheric air could be heated directly by combustion carried on within its own mass. Considerable loss could thus be prevented, not only in the quantity of heat, but also in its temperature. This advantage belongs exclusively to atmospheric air. Other gases do not possess it. They would be even more difficult to heat than vapor of water.
(3) In order to give to air great increase of volume, and by that expansion to produce a great change of temperature, it must first be taken under a sufficiently high pressure ”

“ One estimable feature in this system of gas-producing is that the production can be arrested for half a day without deranging the producer, the fuel and brickwork being sufficiently bulky to maintain a dull red heat for that period. Air cannot enter the grate unless the gases given off in the producer are withdrawn to the furnace; and when the gas valve of the furnace is reopened, the production of the gases is once more continued. ”

“ Keep the burner clean of dust and soot. Be sure that the valve is entirely closed when the gas is turned off, and that the pipes fit tight at all connections so that gas cannot leak into the room.
Fig. 40. Reflector gas heater.
81. Illuminating Flame and Bright Metal Reflector Heaters. These heaters are used with manufactured gas. They burn with an illuminating flame since there is no device for mixing air with the gas as it enters the stove. The bright metal reflector not only makes an attractive stove, but reflects the heat out into the room. ”

“ In proportioning heating surfaces and gas passages for waste heat work there are so many factors bearing directly on what constitutes the proper installation that it is impossible to set any fixed rules. Each individual installation must be considered by itself as well as the particular characteristics of the gases available, such as their temperature and volume, and the presence of dust or tar-like substances, and all must be given the proper weight in the determination of the design of the heating surfaces and gas passages for the specific set of conditions. ”