The Economics of Preheated Air for Stokers

Abstract

Abstract The development of the regenerative cycle, the operation of boilers at higher and higher outputs, and the adoption of higher pressures and the single-pass boiler, have reduced the heat-reclaiming possibilities of the economizer and have resulted in the installation of air heaters for most high-efficiency steam-generating units. Highly preheated air has increased the maintenance of underfeed stokers beyond reasonable figures. The purpose of this paper is to determine to just what extent stoker maintenance is a function of air temperature. Maintenance data have been supplied by 33 operating companies operating 122 underfeed stokers supplied with hot air. The data cover stokers operating with average air temperatures varying from 120 to 460 F and maximum air temperature as high as 600 F and representing 475 stoker-years of operation. Maintenance has been plotted against average air temperature and a fairly satisfactory relation found. In order to remove the influence of other variables affecting maintenance, stoker maintenance has been plotted against other operating and design factors, and each corrected for temperature. From this data it was found that the variables, other than average air temperature, which most seriously affect maintenance are: (1) Area of stoker (2) Average coal burned in lb per sq ft per hr (3) Percentage of ash in the coal burned. The data were corrected for these three variables, and a new curve of maintenance vs. average air temperature was obtained with some of the influence of these three variables removed. From this corrected curve the following conclusion can be drawn: Up to 300 F, the preheating of the air supplied to underfeed stokers does not seem to increase maintenance above reasonable figures, but above 300 or 350 F, there is a sharp increase in maintenance with increase in air temperature. Information is also given in the paper on the changes made and operating methods used by various companies to reduce stoker maintenance. Data were also obtained on the maintenance costs of 77 chain-grate stokers supplied with hot air, representing 440 stoker-years of operation. When plotted, the data did not show any consistent relation between maintenance and average air temperature. However, a study of the data seems to indicate that chain-grate stokers, supplied with hot air and burning the types of fuel for which they are suited, can match the underfeed stoker in coal-burning capacity in lb per sq ft per hr, and apparently operate with lower maintenance costs.