Part 30 (1/2)
UTILIZATION OF WASTE HEAT
While it has been long recognized that the reclaases of various industrial processes would lead to a great saving in fuel and labor, the probleiven the attention that its importance merits It is true that installations have been eneral they have consisted si surface in the path of the gases and thosethe installations have been satisfied hatever power has been generated, no attention being given to the proportioning of either the heating surface or the gas passages to meet the peculiar characteristics of the particular class of waste gas available The Babcock & Wilcox 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 Frorees || |+-----------------------------------+---------------+| ||Brick Kilns | 2000-2300 || ||Zinc Furnaces | 2000-2300 || ||Copper Matte Reverberatory Furnaces| 2000-2200 || ||Beehive coke Ovens | 1800-2000 || ||Ce Furnaces | 1500-1750 || ||Open Hearth Steel Furnaces | 1100-1400 || |+-----------------------------------+---------------+| +-----------------------------------------------------+
The power that can be obtained froht, and both of these factors vary widely in different coives a list of certain processes yielding waste gases the heat of which is available for the generation of steaases It should be understood that the tee of a complete cycle of the operation and that the ures given
The ases is represented by the formula:
W(T-t)s H P = ------- (23) 33,479
Where W = the weight of gases passing per hour, T = te surface, t = teases
The initial teas will depend, as stated, upon the process involved The exit temperature will depend, to a certain extent, upon the teovernedsurfaces installed for the absorption of the heat
Where the te that found in direct fired boiler practice, the problen of boiler beco surface to the voluas to be handled With such teas 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 froases, to obtain a heat transfer rate at all comparable with that found in ordinary boiler practice, the lack of teases in their passage over the heating surfaces In securing the velocity necessary to give a heat transfer rate with low teases sufficient to make the installation of waste heat boilers show a reasonable return on the investh the boiler becoood practice in direct fired boilers Practically all operations yielding waste gases require that nothing be done in the way of iht interfere with the operation of the primary furnace The installation of a waste heat boiler, therefore, very frequently necessitates providing sufficient mechanical draft to overco surfaces and still leave ample draft available to meet the maximum requirements of the priases are in line hat are found in ordinary direct fired practice, the area of the gas passages as 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 as that the furnacea suction equivalent to the h the boiler plus the maximum draft requirement at the furnace outlet Stacks and fans for this class 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 se loss in efficiency In practically any installation where low teive the proper heat transfer froy 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 hich the waste heat boilers are installed
In proportioning heating surfaces and gas passages for waste heat work there are sodirectly 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 ht in the deteras passages for the specific set of conditions
[Graph: Per Cent of Water Heating Surface passed over by Gases/Per Cent of the Total Aainst Te Surface
Fig 31 Curve Showing Relation Between Gas Te Surface passed over, and A Surface are assu 31 shows the relation of gas te surface passed over and work done by such surface for use in cases where the temperatures approach those found in direct fired practice and where the voluas available is approximately that hich one horse powersurface The curve assue 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 assu the case, its application in waste heat work is clear Decreasing or increasing the velocity of the gases over the heating surfaces froht be considered nor the frictional loss through the boiler will increase or decrease the a surface necessary to develop one boiler horse power The application of Fig 31 to such use as teases are cooled to 570 degrees From the curve, under what are assuases have passed over 19 per cent of the heating surface by the tirees
When cooled to 570 degrees, 78 per cent of the heating surface has been passed 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 = 131, which is the ratio of surface of the assumed case to the standard case of the curve Expressed differently, there will be required 131 square feet of heating surface in the assuainst 10 square feet in the standard case
The gases available for this class of work are almost invariably very dirty It is essential for the successful operation of waste-heat boilers that a 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 ce chaases reach the boiler
By-passes 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 froht to prevent the infiltration of air, with the consequent lowering of gas terates 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 prirates are soases available are not sufficient to develop the required horse power fro surface