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this is regained by the use of an open heater for the feedwater, into which the exhaust steam from the auxiliaries may pass, thus heating the feed-water and returning a part of the heat to the boilers.
Ques. 771.-What precautions must be observed in the operation of a condensing outfit in order to obtain the highest efficiency?
Ans.-A prime requisite to the maintenance of high vacuum, with the resultant economy in the operation of the condensing apparatus, is that all entrained air must be excluded from the condenser. There are various ways in which it is possible for air to find its way into the condensing system. For instance, there may be an improperly packed gland, or there may be slight leaks in the piping, or the air may be introduced with the condensing water. This air should be removed before it reaches the condenser, and it may be accomplished by means of the “dry" air-pump.
Ques. 772.-Describe some of the leading characteristics of the dry air-pump.
Ans.-This dry air-pump is different from the ordinary air-pump that is used in connection with most condensing systems. The dry air-pump handles no water, the cylinder being lubricated with oil in the same manner as the steam-cylinder. The clearances also are made as small as possible. These pumps are built either in one or two stages.
Ques. 773.-What particular features would be required in the design of a compound or stage-expansion reciprocating engine, in order to develop a high vacuum, for instance as high as 28.5 inches?
Ans.-In comparing the efficiency of the reciprocating engine and the steam-turbine it is not to be inferred that reciprocating engines would not give better results at high vacuum than they do at the usual rate of 25 to 26 inches, but to reach and maintain the higher vacuum of 28 to 28.5 inches with the reciprocating engine would necessitate much larger sizes of the low-pressure cylinder, as also the valves and exhaust pipes, in order to handle the greatly increased volume of steam at the low-pressure demanded by high vacuum.
Ques. 774.-What advantage has the turbine over the reciprocating engine, in the disposal of its exhaust steam?
Ans.—The steam-turbine expands its working steam to within 1 inch of the vacuum existing in the condenser, that is, if there is a vacuum of 28 inches in the condenser there will be 27 inches of vacuum in the exhaust end of the turbine cylinder. On the other hand, there is usually a difference of 4 or 5 inches (2 to 2.5 pounds) between the mean back pressure in the cylinder of a reciprocating condensing engine and the absolute back pressure in the condenser.
Ques. 775.-Mention the two principal sources of economy that the steam-turbine possesses in a high degree.
Ans.-Two of the main sources of economy that the steam-turbine possesses in a much higher degree than does the reciprocating engine are: First, its adaptability for using superheated steam, and second, the possibility of maintaining a higher degree of vacuum.
Ques. 776.-What can be said of the steam turbine, regarding friction of rubbing parts, such as reciprocating pistons, cross-heads, etc?
Ans.—There are no rubbing surfaces in the turbine except the bearings of the rotor.
Ques. 777.-Of what type is the Allis Chalmers steamturbine?
Ans.—It is of the reaction, or Parsons type, with a number of modifications in details of construction.
Ques. 778.-Give an elementary description of the "Parsons' steam-turbine.
Ans.-It consists essentially of a fixed casing, or cylinder, usually arranged in three stages of different diameters, that of the smallest diameter being at the highpressure, or admission end, and that of the largest diameter at the low-pressure or exhaust end of the casing.
Inside of this casing is a revolving drum, or rotor, the ends of which are extended in the form of a shaft, and carried in two bearings, just outside each end of the cylinder.
Ques. 779.- What causes the drum to revolve within the cylinder?
Ans.—The drum is fitted with a large number of small curved blades, or paddles arranged in straight rows around its circumference. The blades in each stage, or step, are also arranged in groups of increasing length, those at the beginning of each larger stage heing shorter than those at the end of the preceding stage, the change being made in such a manner that the correct relation of blade length to drum diameter is secured. These rows of
revolving blades fit in and run between corresponding rows of stationary blades that project from the walls of the cylinder. These stationary blades have the same curvature as the revolving blades, but are set so that the curves incline in the opposite direction to those of the revolving blades. The steam entering the cylinder at the smallest or first stage, is deflected in its course by the first row of stationary blades, and immediately impinges with a pressure but slightly reduced from boiler pressure, against the first row of revolving blades. It then passes
Main bearings, A and B. Thrust bearing, R. Steam pipe, C. Main throttle valve, D, which is balanced, and operated by the governor. Steam enters the cylinder through passage E, passes to the left through the alternate rows of stationary and revolving blades, leaving the cylinder at F and passes into the condenser, or atmosphere through passage G. 11, J and K are the three steps or stages of the machine. L M and N are the three balance pistons. 0, Pand Q are the equalizing passages, connecting the balance pistons with the corres. ponding stages.
to the next row of stationary blades, which again deflect its course so as to cause it to strike the next row of moving blades at the proper angle. Thus the continual pressure and reaction of the steam against the curved surfaces of the moving blades causes the drum, or rotor to revolve.