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 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.
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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:
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| Where | W | = | the weight of gases passing 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 [Pg 233] 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 passage 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 passages 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 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 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.