Читать книгу Henley's Formulas, Recipes and Processes (Applied Chemistry) - Various - Страница 551

It hardly pays to reduce pressure on boilers, except in very extreme cases, but if it can be done by throttling before the steam reaches the cylinder of the engine it would be an advantage, because this retains the heat units due to the higher pressure in the steam, and the throttling has a slight superheating effect. As a matter of fact, tests go to show that for light loads and high pressure a throttling engine may do better than an automatic cut-off. The ideal arrangement is to throttle the steam for light loads; for heavier loads, allow the variable cut-off to come into play. This practice has been carried into effect by the design of Mr. E. J. Armstrong, in which he arranges the shaft governor so that there is negative lead up to nearly one-quarter cut-off, after which the lead becomes positive, and this has the effect of throttling the steam for the earlier loads and undoubtedly gives better economy, in addition to making the engine run more quietly.