Light, heat and power in buildings

Preface

In this volume the object is to present in compact form the main facts on which selection of the sources for light, heat and power in buildings should be based. The problem for which a solution is sought is to determine the kind of equipment that will yield the service required in any case at the least total cost Such a purpose leaves little room for discussions of theory relating to any particular class of apparatus, which has already been done in separate and larger volumes. It follows that the only novelty to be expected here is that of arrangement, by which the costs of service from widely different sources are set down side by side.

Should this arrangement prove convenient for those charged with the selection of apparatus for light, heac and power, the labor spent on the following pages will have accomplished its purpose.

ALTON D. ADAMS.

Contents

Chapter I. — Cost of heat, light and power from public gas and electrical supply and from coal. Cost of light from gas and electrical supply. Cost of heat from gas and electrical supply. Cost of power from steam plant and from gas. Cost of power from electrical supply. Efficiency, heating effect and required labor with motors and engines. Pages 9 — 13

Chapter II. — ^Gas, electricity, steam and hot water in the distribution of heat, light and power. Gas as a means of illumination. Electricity for illumination. Distribution of heat by gas. Distribution of heat by air, steam and hot water. Distribution of power by gas, electricity, belts and shafting and by steam. Conclusions 15 — 30

Chapter III. — Advantages of the combined production of light, heat and power from steam. Light, heat and power from a single plant. Fuel required with boilers for light, heat and power. Heat from exhaust steam. Power and heating with given amount of steam. Heating and illumination with given amount of steam. Times of demand for light and heat 31 — 39

Chapter I

COSTS OF HEAT, LIGHT AND POWER FROM PUBLIC GAS AND ELECTRICAL SUPPLY AND FROM COAL.

An open gas flame of sixteen candle power consumes five cubic feet of average gas per hour. At one dollar pe** i,ooo cubic feet, the cost of this gas flame is loo x .005 = 0.5 cent hourly. Ten cents per kilowatt-hour is a moderate rate for electrical energy. Fifty-six watts is a fair rate of energy consumption for an incandescent lamp of sixteen candle power. Such a lamp requires an hourly expense of 10 x .056 = 0.56 cent at the rate for energy just named. Simple, non-condensing engines, with good boilers, will readily yield each horse-power hour of work with a consumption of four pounds of fairly good coal. If this coal costs three dollars per ton of 2,000 pounds, the expense for fuel per horse-power hour amounts to 300 x 0.002 = 0.6 cent. This brake horse-power, when delivered on the shaft of a dynamo which has an efficiency of 90 per cent., produces an output of 746 x .90 = 671.4 watts. At 56 watts each, the number of sixteen candle power lamps that may be supplied from this output is 671.4 -f- 56 = 12. As the fuel cost of the horse-power hour is 0.6 cent, the charge against each sixteen candle power lamp is 0.6 --^ 12 = 0.05 cent hourly.

Gas from public supplies usually contains 20 to 40 per cent, of the heating power of coal, from which it is derived, according to its variety. It seems at once evident from this fact that gas is ill-suited for general wanning in buildings, and when the cost of heat derived from gas at current pubHc rates is considered, its common use to heat buildings is seen to be entirely impracticable. As an illustration, take average city gas, yielding 650 heat units per cubic foot and selling for one dollar per 1,000 cubic feet. This gas yields, therefore, 650 x 1,000 == 650,- 000 heat units for one dollar on perfect combustion. Good anthracite coal has a heating power of 13,000 units per pound, or 13,000 x 2,000 = 26,000,000 heat units per ton. The amount of gas to supply heat equivalent to that from one ton of coal is therefore 26,000,000 -~ 650 = 40,000 cubic feet, costing forty dollars at the rate named. Considered as a general heating agent, electric energy is in a much worse position as to the portion of the energy of coal that it can deliver, and as to its cost at usual rates, than is gas. Ten cents per kilowatt-hour is a low average rate for electric energy, and as the kilowatt-hour is the equivalent of 3,412 heat units, this rate gives 34,120 units of heat for one dollar. To equal in heating power one ton of coal, the kilowatt-hours necessary are 26,000,000 -f- 3,412 = 7,620, costing 7,620 X .10 == 760.00 dollars at the Tate named. In practice, the actual cost of electrical energy at ten cents per kilowatt-hour, when used for general warming, is less than the cost of coal at 760 dollars per ton, because nearly all of the electrical energy is available as heat in the apartments warmed, while more than 75 per cent, of the total energy of coal is seldom so available.

Mechanical power may be produced in buildings by means of either steam, gas or electrical energy. Power from steam implies both a boiler and engine. Gas develops power by means of an engine only, and electrical energy is transformed into mechanical work by a motor.

For apparatus of equal quality, to develop a given power, the steam boiler and engine will usually cost most, the gas engine a somewhat less amount, and the electric motor least. The cost of either of these equipments for power production is very small compared with the fuel or ■energy it consumes during its useful life, so that a moderate advantage in efficiency, or in the cost of power development, may more than offset a considerable excess in the first cost of the plant.

It has been previously shown that a good steam plant should deliver a horse-power hour at a fuel cost of 0.6 cent, when using fair coal at a price of three dollars per ton. Gas engines of small and moderate capacity, such as are commonly used in city buildings, may be fairly expected to consume twenty cubic feet of gas per delivered horse-power hour. The gas impHed in this rating is of the quality generally distributed for illuminating purposes in towns and cities, having a heating power of not less than 650 units per cubic foot, on perfect combustion. If a gas of lower heating power is used, as one of the socalled fuel or producer gases, which may develop as little as 150 heat units per cubic foot, the consumption per unit of work will increase in inverse ratio to the energy of combustion. The lowest rate common for illuminating gas in the United States is one dollar per 1,000 cubic feet, and, while this rate is in force for but few cities, the cost of power may be stated for it as a convenient basis from which to compute the cost at other rates. At one dollar per 1,000 cubic feet, twenty feet of gas, to develop one brake horse-power hour, cost 100 x 0.20 = 2 cents. As coal to produce this same unit of work was found to cost 0.6 cent, the fuel outlay for power from gas at one dollar per 1,000 cubic feet is more than three times as great as that for coal at three dollars per ton. Electrical energy for power production in motors can usually be had at materially lower rates than those charged when it is devoted to lighting purposes.

A frequent rate for energy suppUed to electric motors is 3.33 cents per electrical horse-power hour, corresponding to a charge of loo dollars for a horse-power year of 3,000 working hours. Electric motors do not, of course, deliver as mechanical work the equivalent of all of the electrical energy that they absorb, and the average efficiency, of small and medium sizes may be taken at 80 per cent., under the conditions of use. The exact figure for motor efficiencies increases slowly with the capacity of the motor. The average loss of 20 per cent, in motors raises the cost of their delivered work to 3.33 -^ .80 = 4.16 cents per horse-power hour. It thus appears that for the rates named the primary development of power in buildings with electric motors costs more than twice as much as that from gas and nearly seven times as much as that from coal, so far as the outlay for fuel and energy is concerned. In this comparison it should be noted that the steam is developed on the premises, while the gas and electrical energy are procured from the public supply. Fuel or energy is obviously only one of the items that go to make up the cost of power in buildings, and whether it is the most important should be decided on the circumstances of each case. Notwithstanding the low cost of fuel for the production of steam power, such power would be the most expensive possible where its total was small, no heating was required and it was necessary to employ an engineer for the care of the engine and boiler.

For the case just named, where the power is quite small, and especially if it is fluctuating in amount and intermittent in point of time, electrical energy from the public supply would usually be the most economical source of power. Where heat is necessary in season to an amount nearly equal to or exceeding the capacity of the exhaust steam that would result from the desired power production, the steam plant is usually the most economical primary source of power. In some other cases, where the necessary amount of heat is less than that which would be available from the exhaust of a steam engine of sufficient power capacity, a gas engine may be the means best suited for cheap primary power. This last is quite apt to be true where the use of power, though at a considerable rate, takes place during only a very few hours each day, or at times of day that are far apart. The gas engine is especially suited to the case just cited, because its waste heat is available for warming; it can be started at once without previous preparation, such as is necessary to get up steam in a boiler, and because the labor involved in its operation is less in amount and less exacting as to skill than that necessary on a steam plant. As an offset to the cost of energy for its operation, when employed as a primary source of mechanical power, the electric motor requires only a trifling amount of attention, involving but a moderate degree of skill, and is ever ready for immediate use without previous preparation. The efficiency of the electric motor is six to ten times as great as that of steam or gas engines, and its wasted heat, that might be devoted to warming, is a very trifling matter. The primary source of mechanical power in city building is usually located in the basement, in a space especially devoted to it, and certain features, such as heat, noise and vibration, that may belong to the apparatus are of minor importance compared with what they would have if it were located at various points through the building. For the great majority of large city buildings that require mechanical power for a variety of purposes, such as electric lightings elevators and ventilation, as well as a large amount of heat for general warming, the steam plant is the most economical source of power.

CHAPTER 11.

GAS, ELECTRICITY, STEAM AND HOT WATER IN THE DIS- TRIBUTION OF HEAT, LIGHT AND POWER.

Gas and electrical energy are usually available for distribution as lighting agents in modern buildings. The electrical energy is capable of ready production in buildings and is also generally available from the pubHc supply. As illuminating gas cannot be readily produced in most buildings, it is open to the disadvantages of high cost. Three main disadvantages, aside from its cost, attach to the use of gas for illuminating purposes. These are its effect to heat and vitiate the air, the secondary quality of gas illumination, and the limits imposed on lamp arrangements, due to the presence of an open flame. If air is to be maintained at a fair standard of purity in a building, the use of gas for extensive illumination largely increases the volume of air to be handled per hour, because of the products of combustion given ofif. As is well known, the quality of ordinary gas light is decidedly inferior to artificial illumination produced by some other means, both as to the sensation produced on the eye and the permanent effects on it of extensive use. In some cases of illumination, particularly where an artistic effect is desired, it is difficult or impossible to attain the result with gas, because of the flame and combustion. Electrical energy is almost an ideal source of illumination. Electric light is more pleasing to the human eye and less harmful on continued use than any other artificial illumination. As incandescent electric lamps involve no combustion whatever, and arc lamps only a trifling amount, their use adds nothing to the volume of air necessary to maintain any required standard of purity.

Incandescent lamps, having their hot parts entirely enclosed in an air-tight globe, may be readily placed in any position or among any surroundings desired for artistic effects. Where it is necessary to place lamps so that they are difficult of access, the incandescent electric variety is particularly suitable, because the Hght can be instantly produced or extinguished without any electro-magnetic mechanism at each lamp, as is necessary for gas burners under such circumstances. The wires required for the distribution of electric energy throughout a building occupy little more space than is required for gas pipes, and should be the subjects of scarcely more attention after they are properly installed. Heat is distributed in buildings for two distinct classes of service, general warming and industrial purposes, such as cooking and the chemical and mechanical arts. For general warming the demand for heat extending over a long period daily is fairly constant during such periods and its total amount is large, though very high temperatures at any particular point are not necessary. In the chemical and mechanical arts, and also frequently for cooking, the demand for heat is variable and intermittent, and its total amount is comparatively small. Industrial operations are very exacting, however, as to the degree of heat required at certain points, and temperatures must often be much above those necessary for general heating. A serious objection to the use of gas for general warming, in addition to its high cost, without the employment of some agent, as air, water or steam, to distribute the heat, is that a fire must be maintained in every room.

If gas is employed to heat air, water or steam at some suitable point, it ceases to be an agent of heat distribution in a building and can be considered only as a fuel. Air, water and steam are readily heated in suitable apparatus with any desired fuel, and the ease with which either may be distributed through a building renders each of them a good agent for general warming. Where hot water or steam are used for warming, their heat is derived directly from the gases of combustion and the radiation from the incandescent fuel. If air is to be the agent of heat distribution, it is usually desirable and necessary, especially in large plants, to prevent contact of the air with the heated surfaces of boilers and furnaces, because their temperatures are so much greater than that desired for heated air that the quaHty of the air is injuriously affected. In most large plants, therefore, where air is the agent of heat distribution, it is heated by contact with coils that contain either hot water or steam. This water or steam is supplied from a boiler, and the loss incident to the second transfer of heat, or that from water or steam to air, is quite small in good apparatus. In the matters of first cost and required space, the distribution of heat by hot air is at a disadvantage compared with that by some other means, especially where it is used exclusively. This disadvantage is due to the large volume of air necessary to supply the required heat and to the consequent sizes and arrangements of conduits for it.

Where due provision must be made for ventilation, the primary disadvantages as to the size and cost of air conduits are largely offset by theif double service in the distribution of both heat and pure air. Probably the most economical arrangement, as to both first cost and that of subsequent operation, where both heating and ventilation are desired, includes conduits of sufficient size to transmit the heated air necessary for ventilation and the necessary steam or hot water radiators to supply the remainder of the required heat. Steam possesses many advantages for the distribution of heat in buildings, and is probably more extensively used for that purpose in large plants than any other agent. The cost of pipes for steam heating in buildings and the room they require are moderate, but such a system is by no means ideal. One difficulty with steam distribution for general warming is the lack of heat regulation in the coils and radiators. Steam does all of its work as a heating agent at nearly constant temperature. Thus, steam at five pounds gauge pressure, or one that is seldom necessary to exceed in the heating system of a building, has a temperature of 22.J degrees Fahr., while, after it has condensed to water at atmospheric pressure, thus yielding 970 heat units per pound, the temperature is still 212 degrees. An inevitable result of the nearly constant temperature of steam heated surfaces is the waste of heat in mild weather, and more or less discomfort in over-heated apartments.

Among the small disadvantages connected with a steam heating system are noises due to the collection of water in pipes and radiators, and an unpleasant odor of the steam that at times escapes through air valves on radiators. The system of heat distribution, by means of hot water circulating through the pipes and radiators, compares very faborably with steam heating in several particulars. The pipes for hot water heating, where the circulation of the water is maintained by the difference in temperature between the water in the outflow or supply pipe and that in the return, are usually somewhat larger than steam pipes would be for an equal supply of heat. This excess in the size of hot water pipes over those for steam and equivalent service, adds only a moderate per cent, to their cost and still less to their required space Two of the most distinct advantages of the hot water heating system are its capacity for temperature regulation and to maintain a gradually diminishing supply of heat during some hours after the fire in the boiler furnace has been banked or gone out. While the temperature of steam is nearly constant at all of the pressures commonly employed in warming buildings, the temperature of water in the low pressure system, or that open to the air, may be at any desired point below 212 degrees. In mild weather, or when less than the maximum rate of heat delivery is desired for general warming, a hot water system may be readily operated at any temperature that gives the heat supply desired.

The capacity of water to store heat and then to give it ofif gradually renders it unnecessary to keep boiler fires In active operation during the entire period of each day that heat Is necessary. Owing to the same property of water, the temperature of buildings heated with it does not fall as low during the daily period when fires are not In action, as is the case where steam is the heating agent. One result of the use of water instead of steam for heat distribution Is that the daily hours of labor In the boiler room may be materially reduced. Reduction of the heat given off by a hot water system in mild weather, to the amount actually necessary to maintain the required temperature in a building, by lowering the temperature of the water and radiating surfaces, obviously tends to economy in fuel. The less the temperature of the heating surfaces of a boiler the larger is the amount of heat that is extracted from the gases of combustion. In steam heating above atmospheric pressure the temperature of at least a part of the boiler must be higher than 212 deg^rees at all times when any heat whatever is wanted. In hot water heating the temperature of the circulating water is varied to meet the demands for heat, being only 212 degrees at its highest point. It follows that the average temperature of boiler surfaces are materially lower in a hot water than in a steam heating system, and the gases of combustion may therefore be delivered to the flue with less of the fuel energy in them.

This tends to high efficiency. Where a steam plant is used for the double purpose of heat and power, and the exhaust steam from engines is to be employed in the heating system, the system of distribution by hot water presents some especial advantages. To circulate the exhaust steam through the pipes and radiating surfaces of a building, it is most common to operate the engines on two to five pounds back pressure. This back pressure reduces both the capacity and efificiency of engines, especially when they are of the compound type. There are certain special equipments on the market for use in connection with steam heating systems that make practicable the circulation of the steam at or below atmospheric pressure, but unless these equipments are used the back pressure in the case of engines has to be reckoned with. If, instead of circulating the steam from engines through the entire heating system of a building, it simply goes to a bank of coils for heating hot water, the back pressure on engines may be so reduced, without any special equipment, as to become a very small factor. Hot water may thus be raised to a temperature of 212 degrees, or that of a regular low-pressure hot water system. Circulation from these hot water coils through the heating system of a building takes place just as though the water was heated directly by the fire under a boiler. If it seems desirable in any case to reduce the sizes of pipes necessary for the hot water, where the circulation depends entirely on the difference in temperature in the outflow and return pipes, the hot water may pass through a pump that maintains any desired rate of circulation, through a heating system of any dimensions, and pipes of any size.

If the average amount of exhaust steam available is less than that required by the heating system, the small addition to the exhaust, resulting from the use of a pump, will usually be condensed in the heating coils, so that the cost of pumping the hot water will be very slight. In this way the cost of pipes for a large system of hot water heating may be materially reduced below that of steam pipes for equivalent service. Another decided advantage of the system of hot water heating, in connection with a steam power plant, is the ability, by means of a hot water tank, to store the heat of exhaust steam, produced during one period of the day, for use at another. Some of the demands for power in large buildings are far from evenly distributed throughout the usual working hours of the day, and this is particularly true of the electric lighting load, which is mostly concentrated in three or four hours. Where exhaust steam is distributed through the heating system oi a building, any excess above the amount that the system will condense during any particular period must be devoted to some other purpose or wasted. With a system of hot water distribution, if the total heat of the daily product of exhaust steam does not exceed the total daily requirement for heat in the building for general warming, the surplus heat of the exhaust during any period of the day may be stored and subsequently utilized. Probably the best system for both heating and ventilation in a large building, where both first cost and the expense of operation is considered, includes hot water and hot air, each circulated by mechanical means, under average circumstances. In the distribution of heat for mechanical and other industrial purposes, considerations as to the supply of heat at just the right time and to the required degree are paramount, and the sum total of heat consumed is usually a secondary matter. The decided advantages of electrical energy and gas for heat production in many industrial operations, owing to the effective control made possible wath them, of the time and degree of heat supply, often more than offset their high cost per unit of heat energy actually furnished. For some industrial purposes, where a nearly uniform and moderate degree of heat is required in comparatively large amounts through considerable periods, as in cooking and drying operations, hot water and steam are more available than gas and electrical energy, because of the low cost per unit of the heat they contain. The extended discussion of industrial applications of heat is beyond the limits of the present work.

Quite distinct from the question as to the most economical source of the primary mechanical power in a build-, ing is that of the distribution of power to its several parts. While power may be developed with a steam plant in the basement of a building at a much less cost than that for which gas or electrical energy can be bought for equal results, it frequently happens that the power is required in a number of small units throughout the building and very little of it can be applied to its ultimate use near the steam engine. A complete solution of the question of power production in modern buildngs must therefore include the means for its distribution to the points where it is wanted.

There are four methods, either of which may be applied to power distribution in buildings, namely, mechanical equipment, such as belts and shafting, steam pipes to the several points where power is wanted, connected to engines there located, gas pipes running to suitably located gas engines, and electric circuits and motors. Distribution by mechanical appliances or by steam pipes and engines is only possible, for most instances, when the primary power plant is located in the building. Electrically distributed power may be had from either a local or a public plant. Gas pipes and engines must usually derive their supply from the public service mains.

Power distribution by mechanical means involves the cost of belts, shafting and pulleys. For steam power distribution, pipes and small engines must be provided. In like manner distribution by gas implies the necessary pipes and gas engines. If the energy is to be distributed electrically, wiring circuits and motors are necessary where the supply is taken from an outside source, and to these must be added an item for dynamos, when the primary source of the power is in the building. Where the power of a local steam plant is to be distributed the first cost of engine, shafting and belting, of steam pipes and a number of small engines, and of engine, dynamo, wiring and motors for the same delivery of power at the points where it is desired, should be compared. The cost of steam pipes and small engines connected at points where the power is wanted will not differ greatly in many cases from the cost of one or more larger engines of equal ultimate capacity, and the belts and shafting necessary to distribute the power. A large engine, dynamo, wiring and electric motors are quite sure, in most cases, to cost more than the same engine with the shafting and belting necessaxy for distribution. To fairly decide between these three methods of distribution, the efficiency, convenience and the objectionable features of each should be considered, as one or more of these items may much more than ofifset that of first cost. Where gas or electric energy from an outside source is to be distributed in a building for power purposes, gas pipes and engines must be paid for in one case and electric wiring and motors in the other.

As a rule the gas pipes and engines will cost more than the electric circuits and motors. The costs of operation for the gas and electric equipments when supplied from the public mains has already been shown, and they include the questions of efficiency. The main permanent objection to gas pipes for engines, over electric circuits for motors of equal capacity, is the, greater amount of room necessary to the former. Objections to gas engines are about the same as those to steam engines, and will be considered when the latter are taken up. When the distribution of power in a building from a local plant is considered the relative importance of the efficiency, convenience and objectionable features varies with the character of the building where the distribution takes place. In a factory building efficiency and convenience of operation, as they affect the cost of production, are of prime importance. For an office building, the absence of any very objectionable features is the most necessary requisite. Between these extreme classes of buildings there are a large number where convenience of operation and objectionable features are of variable importance, but efficiency as to the consumption of fuel is highly desirable in almost every case. Where steam is distributed to the various parts of a building through long pipes for a number of small engines, two serious sources of loss are encountered. The long pipes are certain to condense considerable steam, and if they are not well insulated for heat the loss by condensation rises to a large amount. Much more serious than the loss of heat from pipes with good insulation is that in the small engines themselves. While a large, simple engine should deliver a brake horse-power hour on not more than thirty pounds of dry steam, small engines of three to ten horse-power generally have a consumption of two to four times this rate. Including the losses from condensation in long pipes, it is probably safe to say that a considerable number of small engines, scattered over a large building and supplied from the main boilers, consume fully three times as much steam per unit of work as would one or more large, simple engines for the same aggregate power, when located near the boiler plant. If the conditions of service are such as to warrant the use of compound engines for large units, the equipment of numerous small engines would be at a still greater comparative disadvantage. So low is the efficiency of numerous small engines at considerable distances from a boiler plant that the use of such equipments is mostly confined to a few cases where special conditions seem to warrant it. The power necessary to keep belts and shafting in motion varies, among other factors, with their extent, and is nearly constant whatever the power transmitted at any particular time.