Steam Engines

by Anonymous

CHAPTER IV

STEAM ENGINE ECONOMY

Under the general heading of steam engine economy, such items as cylinder condensation, steam consumption, efficiency, ratio of expansion, under- and over-loading, condensing, etc., are treated.

The principal waste of steam in the operation of an engine is due to condensation during the first part of the stroke. This condensation is due to the fact that during expansion and exhaust the cylinder walls and head and the piston are in contact with comparatively cool steam, and, therefore, give up a considerable amount of heat. When fresh steam is admitted at a high temperature, it immediately gives up sufficient heat to raise the cylinder walls to a temperature approximating that of the entering steam. This results in the condensation of a certain amount of steam, the quantity depending upon the time allowed for the transfer of heat, the area of exposed surface, and the temperature of the cylinder walls. During the period of expansion the temperature falls rapidly, and the steam being wet, absorbs a large amount of heat. After the exhaust valve opens, the drop in pressure allows the moisture that has collected on the cylinder walls to evaporate into steam, so that during the exhaust period but little heat is transferred. With the admission of fresh steam at boiler pressure, a mist is condensed on the cylinder walls, which greatly increases the rapidity with which heat is absorbed.

The amount of heat lost through cylinder condensation is best shown by a practical illustration. One horsepower is equal to 33,000 foot-pounds of work per minute, or 33,000 × 60 = 1,980,000 foot-pounds per hour. This is equivalent to 1,980,000 ÷ 778 = 2,550 heat units. The latent heat of steam at 90 pounds gage pressure is 881 heat units. Hence, 2,250 ÷ 881 = 2.9 pounds of steam at 90 pounds pressure is required per horsepower, provided there is no loss of steam, and all of the contained heat is changed into useful work. As a matter of fact, from 30 to 35 pounds of steam are required in the average simple non-condensing high-speed engine.

There are three remedies which are used to reduce the amount of cylinder condensation. The first to be used was called steam jacketing, and consisted in surrounding the cylinder with a layer of high-pressure steam, the idea being to keep the inner walls up to a temperature nearly equal to that of the incoming steam. This arrangement is but little used at the present time, owing both to the expense of operation and to its ineffectiveness as compared with other methods.

The second remedy is the use of superheated steam. It has been stated that the transfer of heat takes place much more rapidly when the interior surfaces are covered with a coating of moisture or mist. Superheated steam has a temperature considerably above the point of saturation at the given pressure; hence, it is possible to cool it a certain amount before condensation begins. This has the effect of reducing the transfer of heat for a short period following admission, and this is the time that condensation takes place most rapidly under ordinary conditions with saturated steam. This, in fact, is the principal advantage derived from the use of superheated steam, although it is also lighter for a given volume, and therefore, a less weight of steam is required, to fill the cylinder up to the point of cut-off. The economical degree of superheating is considered to be that which will prevent the condensation of any steam on the walls of the cylinder up to the point of cut-off, thus keeping them at all times free from moisture. The objections to superheated steam are its cutting effect in the passages through which it flows, and the difficulty experienced in lubricating the valves and cylinder at such a high temperature. The third and most effective remedy for condensation losses is that known as compounding, which will be treated under a separate heading in the following.

Multiple Expansion Engines

It has been explained that cylinder condensation is due principally to the change in temperature of the interior surfaces of the cylinder, caused by the variation in temperature of the steam at initial and exhaust pressures. Therefore, if the temperature range be divided between two cylinders which are operated in series, the steam condensed in the first or high pressure cylinder will be re-evaporated and passed into the low-pressure cylinder as steam, where it will again be condensed and re-evaporated as it passes into the exhaust pipe. Theoretically, this should reduce the condensation loss by one-half, and if three cylinders are used, the loss should be only one-third of that in a simple engine. In actual practice the saving is not as great as this, but with the proper relation between the cylinders, these results are approximated.

Engines in which expansion takes place in two stages are called compound engines. When three stages are employed, they are called triple expansion engines. Compounding adds to the first cost of an engine, and also to the friction, so that in determining the most economical number of cylinders to employ, the actual relation between the condensation loss and the increased cost of the engine and the friction loss, must be considered. In the case of power plant work, it is now the practice to use compound engines for the large sizes, while triple expansion engines are more commonly employed in pumping stations. Many designs of multiple expansion engines are provided with chambers between the cylinders, called receivers. In engines of this type the exhaust is frequently reheated in the receivers by means of brass coils containing live steam. In the case of a cross-compound engine, a receiver is always used. In the tandem design it is often omitted, the piping between the two cylinders being made to answer the purpose.