Steam, Its Generation and Use Part 30

UTILIZATION OF WASTE HEAT

While it has been long recognized that the reclamation of heat from the waste gases of various industrial processes would lead to a great saving in fuel and labor, the problem has, until recently, never been given the attention that its importance merits. It is true that installations have been made for the utilization of such gases, but in general they have consisted simply in the placing of a given amount of boiler heating surface in the path of the gases and those making the installations have been satisfied with whatever power has been generated, no attention being given to the proportioning of either the heating surface or the gas pa.s.sages to meet the peculiar characteristics of the particular cla.s.s of waste gas available. The Babc.o.c.k & Wilc.o.x Co. has recently gone into the question of the utilization of what has been known as waste heat with great thoroughness, and the results secured by their installations with practically all operations yielding such gases are eminently successful.

TABLE 52

TEMPERATURE OF WASTE GASES FROM VARIOUS INDUSTRIAL PROCESSES

+-----------------------------------------------------+ |+-----------------------------------+---------------+| ||Waste Heat From |Temperature[50]|| || | Degrees || |+-----------------------------------+---------------+| ||Brick Kilns | 2000-2300 || ||Zinc Furnaces | 2000-2300 || ||Copper Matte Reverberatory Furnaces| 2000-2200 || ||Beehive c.o.ke Ovens | 1800-2000 || ||Cement Kilns | 1200-1600[51]|| ||Nickel Refining Furnaces | 1500-1750 || ||Open Hearth Steel Furnaces | 1100-1400 || |+-----------------------------------+---------------+| +-----------------------------------------------------+

The power that can be obtained from waste gases depends upon their temperature and weight, and both of these factors vary widely in different commercial operations. Table 52 gives a list of certain processes yielding waste gases the heat of which is available for the generation of steam and the approximate temperature of such gases. It should be understood that the temperatures in the table are the average of the range of a complete cycle of the operation and that the minimum and maximum temperatures may vary largely from the figures given.

The maximum available horse power that may be secured from such gases is represented by the formula:

W(T-t)s H. P. = ------- (23) 33,479

Where W = the weight of gases pa.s.sing per hour, T = temperature of gases entering heating surface, t = temperature leaving heating surface, s = specific heat of gases.

The initial temperature and the weight or volume of gas will depend, as stated, upon the process involved. The exit temperature will depend, to a certain extent, upon the temperature of the entering gases, but will be governed mainly by the efficiency of the heating surfaces installed for the absorption of the heat.

Where the temperature of the gas available is high, approaching that found in direct fired boiler practice, the problem is simple and the question of design of boiler becomes one of adapting the proper amount of heating surface to the volume of gas to be handled. With such temperatures, and a volume of gas available approximately in accordance with that found in direct fired boiler practice, a standard boiler or one but slightly modified from the standard will serve the purpose satisfactorily. As the temperatures become lower, however, the problem is more difficult and the departure from standard practice more radical.

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 pa.s.sage over the heating surfaces. In securing the velocity necessary to give a heat transfer rate with low temperature gases sufficient to make the installation of waste heat boilers show a reasonable return on the investment, the frictional resistance to the gases through the boiler becomes greatly in excess of what would be considered good practice in direct fired boilers. Practically all operations yielding waste gases require that nothing be done in the way of impairing the draft at the furnace outlet, as this might interfere with the operation of the primary furnace. The installation of a waste heat boiler, therefore, very frequently necessitates providing sufficient mechanical draft to overcome the frictional resistance of the gases through the heating surfaces and still leave ample draft available to meet the maximum requirements of the primary furnace.

Where the temperature and volume of the gases are in line with what are found in ordinary direct fired practice, the area of the gas pa.s.sages may be practically standard. With the volume of gas known, the draft loss through the heating surfaces may be obtained from experimental data and this additional draft requirement met by the installation of a stack sufficient to take care of this draft loss and still leave draft enough for operating the furnace at its maximum capacity.

Where the temperatures are low, the added frictional resistance will ordinarily be too great to allow the draft required to be secured by additional stack height and the installation of a fan is necessary. Such a fan should be capable of handling the maximum volume of gas that the furnace may produce, and of maintaining a suction equivalent to the maximum frictional resistance of such volume through the boiler plus the maximum draft requirement at the furnace outlet. Stacks and fans for this cla.s.s of work should be figured on the safe side. Where a fan installation is necessary, the loss of draft in the fan connections should be considered, and in figuring conservatively it should be remembered that a fan of ample size may be run as economically as a smaller fan, whereas the smaller fan, if overloaded, is operated with a large loss in efficiency. In practically any installation where low temperature gas requires a fan to give the proper heat transfer from the gases, the cost of the fan and of the energy to drive it will be more than offset by the added power from the boiler secured by its use.

Furthermore, the installation of such a fan will frequently increase the capacity of the industrial furnace, in connection with which the waste heat boilers are installed.

In proportioning heating surfaces and gas pa.s.sages for waste heat work there are so many factors bearing directly on what const.i.tutes 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 pa.s.sages for the specific set of conditions.

[Graph: Per Cent of Water Heating Surface pa.s.sed over by Gases/Per Cent of the Total Amount of Steam Generated in the Boiler against Temperature in Degrees Fahrenheit of Hot Gases Sweeping Heating Surface

Fig. 31. Curve Showing Relation Between Gas Temperature, Heating Surface pa.s.sed over, and Amount of Steam Generated. Ten Square Feet of Heating Surface are a.s.sumed as Equivalent to One Boiler Horse Power]

Fig. 31 shows the relation of gas temperatures, heating surface pa.s.sed over and work done by such surface for use in cases where the temperatures approach those found in direct fired practice and where the volume of gas available is approximately that with which one horse power may be developed on 10 square feet of heating surface. The curve a.s.sumes what may be considered standard gas pa.s.sage areas, and further, that there is no heat absorbed by direct radiation from the fire.

Experiments have shown that this curve is very nearly correct for the conditions a.s.sumed. Such being the case, its application in waste heat work is clear. Decreasing or increasing the velocity of the gases over the heating surfaces from what might be considered normal direct fired practice, that is, decreasing or increasing the frictional loss through the boiler will increase or decrease the amount of heating surface necessary to develop one boiler horse power. The application of Fig. 31 to such use may best be seen by an example:

a.s.sume the entering gas temperatures to be 1470 degrees and that the gases are cooled to 570 degrees. From the curve, under what are a.s.sumed to be standard conditions, the gases have pa.s.sed over 19 per cent of the heating surface by the time they have been cooled 1470 degrees.

When cooled to 570 degrees, 78 per cent of the heating surface has been pa.s.sed over. The work done in relation to the standard of the curve is represented by (1470 - 570) (2500 - 500) = 45 per cent. (These figures may also be read from the curve in terms of the per cent of the work done by different parts of the heating surfaces.) That is, 78 per cent - 19 per cent = 59 per cent of the standard heating surface has done 45 per cent of the standard amount of work. 59 45 = 1.31, which is the ratio of surface of the a.s.sumed case to the standard case of the curve. Expressed differently, there will be required 13.1 square feet of heating surface in the a.s.sumed case to develop a horse power as against 10 square feet in the standard case.

The gases available for this cla.s.s of work are almost invariably very dirty. It is essential for the successful operation of waste-heat boilers that ample provision be made for cleaning by the installation of access doors through which all parts of the setting may be reached. In many instances, such as waste-heat boilers set in connection with cement kilns, settling chambers are provided for the dust before the gases reach the boiler.

By-pa.s.ses for the gases should in all cases be provided to enable the boiler to be shut down for cleaning and repairs without interfering with the operation of the primary furnace. All connections from furnace to boilers should be kept tight to prevent the infiltration of air, with the consequent lowering of gas temperatures.

Auxiliary gas or coal fired grates must be installed to insure continuity in the operation of the boiler where the operation of the furnace is intermittent or where it may be desired to run the boiler with the primary furnace not in operation. Such grates are sometimes used continuously where the gases available are not sufficient to develop the required horse power from a given amount of heating surface.

Fear has at times been expressed that certain waste gases, such as those containing sulphur fumes, will have a deleterious action on the heating surface of the boiler. This feature has been carefully watched, however, and from plants in operation it would appear that in the absence of water or steam leaks within the setting, there is no such harmful action.

[Ill.u.s.tration: Fig. 32. Babc.o.c.k & Wilc.o.x Boiler Arranged for Utilizing Waste Heat from Open Hearth Furnace. This Setting may be Modified to Take Care of Practically any Kind of Waste Gas]

CHIMNEYS AND DRAFT

The height and diameter of a properly designed chimney depend upon the amount of fuel to be burned, its nature, the design of the flue, with its arrangement relative to the boiler or boilers, and the alt.i.tude of the plant above sea level. There are so many factors involved that as yet there has been produced no formula which is satisfactory in taking them all into consideration, and the methods used for determining stack sizes are largely empirical. In this chapter a method sufficiently comprehensive and accurate to cover all practical cases will be developed and ill.u.s.trated.

Draft is the difference in pressure available for producing a flow of the gases. If the gases within a stack be heated, each cubic foot will expand, and the weight of the expanded gas per cubic foot will be less than that of a cubic foot of the cold air outside the chimney.

Therefore, the unit pressure at the stack base due to the weight of the column of heated gas will be less than that due to a column of cold air.

This difference in pressure, like the difference in head of water, will cause a flow of the gases into the base of the stack. In its pa.s.sage to the stack the cold air must pa.s.s through the furnace or furnaces of the boilers connected to it, and it in turn becomes heated. This newly heated gas will also rise in the stack and the action will be continuous.

The intensity of the draft, or difference in pressure, is usually measured in inches of water. a.s.suming an atmospheric temperature of 62 degrees Fahrenheit and the temperature of the gases in the chimney as 500 degrees Fahrenheit, and, neglecting for the moment the difference in density between the chimney gases and the air, the difference between the weights of the external air and the internal flue gases per cubic foot is .0347 pound, obtained as follows:

Weight of a cubic foot of air at 62 degrees Fahrenheit = .0761 pound Weight of a cubic foot of air at 500 degrees Fahrenheit = .0414 pound ------------------------ Difference = .0347 pound

Therefore, a chimney 100 feet high, a.s.sumed for the purpose of ill.u.s.tration to be suspended in the air, would have a pressure exerted on each square foot of its cross sectional area at its base of .0347 100 = 3.47 pounds. As a cubic foot of water at 62 degrees Fahrenheit weighs 62.32 pounds, an inch of water would exert a pressure of 62.32 12 = 5.193 pounds per square foot. The 100-foot stack would, therefore, under the above temperature conditions, show a draft of 3.47 5.193 or approximately 0.67 inches of water.

The method best suited for determining the proper proportion of stacks and flues is dependent upon the principle that if the cross sectional area of the stack is sufficiently large for the volume of gases to be handled, the intensity of the draft will depend directly upon the height; therefore, the method of procedure is as follows:

1st. Select a stack of such height as will produce the draft required by the particular character of the fuel and the amount to be burned per square foot of grate surface.

2nd. Determine the cross sectional area necessary to handle the gases without undue frictional losses.

The application of these rules follows:

Draft Formula--The force or intensity of the draft, not allowing for the difference in the density of the air and of the flue gases, is given by the formula:

/ 1 1 D = 0.52 H P |--- - -----| (24) T T_{1}/

in which

D = draft produced, measured in inches of water, H = height of top of stack above grate bars in feet, P = atmospheric pressure in pounds per square inch, T = absolute atmospheric temperature, T_{1} = absolute temperature of stack gases.

In this formula no account is taken of the density of the flue gases, it being a.s.sumed that it is the same as that of air. Any error arising from this a.s.sumption is negligible in practice as a factor of correction is applied in using the formula to cover the difference between the theoretical figures and those corresponding to actual operating conditions.

The force of draft at sea level (which corresponds to an atmospheric pressure of 14.7 pounds per square inch) produced by a chimney 100 feet high with the temperature of the air at 60 degrees Fahrenheit and that of the flue gases at 500 degrees Fahrenheit is,

/ 1 1 D = 0.52 100 14.7 | --- - --- | = 0.67 521 961 /