THE VALUE OF MECHANICAL DRAFT.

Progress, Volume II, Issue 2, 1 December 1906, Page 55

THE VALUE OF MECHANICAL DRAFT.

Low grade fuels can be burned only by steady and intense draft. 'Ihus, it is difficult with a chimney to obtain sufficient blast to burn the smallest size of anthracite coal, which require a strong and concentrated draft. Ihe lower efficiency of poorer grades of fuel may readily be offset by the decrease in their cost, provided the fuel is burned under proper conditions; and these conditions can scarcely fail to be supplied by mechanical draft. It has been stated that a simple change in grate bars is all that is required to adapt a boiler to burn practically clear yard screenings by means of forced or induced draft. In general, better results may be expected with automatic stokers when they are used with mechanical draft, on account of the positive and, perchance, automatically controlled air supply. With the chimney damper wide open, an increase in draft and resulting additional output of the plant can be secured only by adding to the chimney's height. The admission of a little more steam to the cylinder of the fan engines solves the problem with mechanical draft. A further advantage lies in the fan's independence of outside weather and temperature conditions. Additional economy m fan-engine operation may be secured by utilising the exhaust steam for heating purposes. Mechanical draft finds a special field of usefulness in connection with power plants which are operated wholly by water-falls during part of the

yeai, and which are remiorced by steam engines in the dry seasons. In most cases it is much cheaper to install a fan system for the allowable purpose of forcing the boiler output for a comparatively short time than it is to invest in additional boilers. As a substitute for the chimney in case of accident, artificial draft may be quickly and easily applied. In solving the draft problem it is wise to provide every possible precaution to obtain continuous service This is far more important than an increase in operating efficiency. A flexible arrangement is a combination of chimney and mechanical draft, each of which will serve as a supplement and relay in case of trouble or even in regular operation. Piobably the greatest simplicity is secured by driving fans for mechanical draft by steam engines. It would seem that the possible greater economy of an electric motor drive would be somewhat offset by the increased complication of the regulating and controlling mechanism. Published information in regard to motor-drive draft fans, their economy, and cost of operation in comparison with the singlefan engines largely in use to-day would be welcome to designers. Enough has been said to indicate the importance of thoroughly going over the draft question before deciding off-hand to use either the chimney or its rival If more space has been given in these comments to the advantages of mechanical draft over chimneys, it is only because the good points of the artificial method are as yet unrecognised in main places Experience is not wanting with either method, and there would seem to be no excuse for not analysing both sides of the draft problem m deciding which combination to use.