With the saving in fuel by the reduction of heat consumption of an engine indicated, it remains to be shown the effect of the use of superheated steam on such heat consumption. As already explained, the use of superheated steam reduces condensation not only in the mains but especially in the steam cylinder, leaving a greater quantity of steam available to do the work. Furthermore, a portion of the saturated steam introduced into a cylinder will condense during adiabatic expansion, this condensation increasing as expansion progresses. Since superheated steam cannot condense until it becomes saturated, not only is initial condensation prevented by its use but also such condensation as would occur during expansion. When superheated sufficiently, steam delivered by the exhaust will still be dry. In the avoidance of such condensation, there is a direct saving in the heat consumption of an engine, the heat given up being utilized in the developing of power and not in changing the condition of the working fluid. That is, while the number of heat units lost in overcoming condensation effects would be the same in either case, when saturated steam is condensed the water of condensation has no power to do work while the superheated steam, even after it has lost a like number of heat units, still has the power of expansion. The saving through the use of superheated steam in the heat consumption of an engine decreases demands on the boiler and hence the fuel consumption per unit of power.
Superheated Steam for Steam Turbines—Experience in using superheated steam in connection with steam turbines has shown that it leads to economy and that it undoubtedly pays to use superheated steam in place of saturated steam. This is so well established that it is standard practice to use superheated steam in connection [Pg 142] with steam turbines. Aside from the economy secured through using superheated steam, there is an important advantage arising through the fact that it materially reduces the erosion of the turbine blades by the action of water that would be carried by saturated steam. In using saturated steam in a steam turbine or piston engine, the work done on expanding the steam causes condensation of a portion of the steam, so that even were the steam dry on entering the turbine, it would contain water on leaving the turbine. By superheating the steam the water that exists in the low pressure stages of the turbine may be reduced to an amount that will not cause trouble.
Again, if saturated steam contains moisture, the effect of this moisture on the economy of a steam turbine is to reduce the economy to a greater extent than the proportion by weight of water, one per cent of water causing approximately a falling off of 2 per cent in the economy.
The water rate of a large economical steam turbine with superheated steam is reduced about one per cent, for every 12 degrees of superheat up to 200 degrees Fahrenheit of superheat. To superheat one pound of steam 12 degrees requires about 7 B. t. u. and if 1050 B. t. u. are required at the boiler to evaporate one pound of the saturated steam from the temperature of the feed water, the heat required for the superheated steam would be 1057 degrees. One per cent of saving, therefore, in the water consumption would correspond to a net saving of about one-third of one per cent in the coal consumption. On this basis 100 degrees of superheat with an economical steam turbine would result in somewhat over 3 per cent of saving in the coal for equal boiler efficiencies. As a boiler with a properly designed superheater placed within the setting is more economical for a given capacity than a boiler without a superheater, the minimum gain in the coal consumption would be, say, 4 or 5 per cent as compared to a plant with the same boilers without superheaters.
The above estimates are on the basis of a thoroughly dry saturated steam or steam just at the point of being superheated or containing a few degrees of superheat. If the saturated steam is moist, the saving due to superheat is more and ordinarily the gain in economy due to superheated steam, for equal boiler efficiencies, as compared with commercially dry steam is, say, 5 per cent for each 100 degrees of superheat. Aside from this gain, as already stated, superheated steam prevents erosion of the turbine buckets that would be caused by water in the steam, and for the reasons enumerated it is standard practice to use superheated steam for turbine work. The less economical the steam motor, the more the gain due to superheated steam, and where there are a number of auxiliaries that are run with superheated steam, the percentage of gain will be greater than the figures given above, which are the minimum and are for the most economical type of large steam turbines.
An example from actual practice will perhaps best illustrate and emphasize the foregoing facts. In October 1909, a series of comparable tests were conducted by The Babcock & Wilcox Co. on the steam yacht “Idalia” to determine the steam consumption both with saturated and superheated steam of the main engine on that yacht, including as well the feed pump, circulating pump and air pump. These tests are more representative than are most tests of like character in that the saving in the steam consumption of the auxiliaries, which were much more wasteful than the main engine, formed an important factor. A résumé of these tests was published in the Journal of the Society of Naval Engineers, November 1909.
The main engines of the “Idalia” are four cylinder, triple expansion, 111⁄2 × 19 inches by 2211⁄16 × 18 inches stroke. Steam is supplied by a Babcock & Wilcox marine boiler having 2500 square feet of boiler heating surface, 340 square feet of superheating surface and 65 square feet of grate surface.
The auxiliaries consist of a feed pump 6 × 4 × 6 inches, an independent air pump 6 × 12 × 8 inches, and a centrifugal pump driven by a reciprocating engine 57⁄16 × 5 inches. Under ordinary operating conditions the superheat existing is about 100 degrees Fahrenheit.
Tests were made with various degrees of superheat, the amount being varied by by-passing the gases and in the tests with the lower amounts of superheat by passing a portion of the steam from the boiler to the steam main without passing it through the superheater. Steam temperature readings were taken at the engine throttle. In the tests with saturated steam, the superheater was completely cut out of the system. Careful calorimeter measurements were taken, showing that the saturated steam delivered to the superheater was dry.