The steam engine and other heat-engines by Ewing James Alfred Sir 1855-1935

Author:Ewing, James Alfred, Sir, 1855-1935
Language: eng
Format: epub, pdf
Tags: Steam-engines, Heat-engines
Publisher: Cambridge [Eng.] University Press
Published: 1910-03-25T05:00:00+00:00

Fig. 96.

' Throttling Calorimeter" (Peabody).

An appropriate value of k should be taken from the table in § 65.

The throttling should be regulated so that the temperature t' is not greater than t 2 by more than a

trifling amount—just enough to ensure that the steam in the chamber is perfectly dry. It is only when the steam is nearly dry to begin with that it can be superheated or even completely dried by throttling. Even when the steam is no more than dried by throttling the limit of wetness beyond which the apparatus cannot be used is not high. With steam at 100 lbs. pressure, for instance, only 4 per cent, of moisture can be removed by throttling if the pressure in the chamber is as low as that of the atmosphere: but if a condenser is available the pressure in the chamber may be reduced far enough to deal with about 6 per cent.

In a modified form of the apparatus by Prof. Barrus 1 the

1 American Soc. Meeh. Eng. 1890.

vessel B becomes a mere tube, separated from the steam-pipe by a diaphragm with a small aperture through which the steam is wire-drawn. A separator is added, between the throttling apparatus and the steam-pipe, to allow as much as possible of the original moisture to be deposited before throttling takes place. With this addition it becomes possible to apply the method to steam that is originally very wet, for the separator leaves so little moisture still in the steam as to make throttling suffice to dry it completely. The water collected by the separator is to be added in reckoning the original wetness.

A porous plug forms a better means of throttling than the stop-valves and pin-hole orifices which have been used in instruments of this kind. The thermometer by which the temperature is taken after throttling should be placed as close as possible to the plug, for the steam quickly loses its superheat by conduction to the outside, and this cannot so well be done when a pin-hole orifice is used instead of a plug, since the kinetic energy of the stream through the orifice must be destroyed before it is allowed to come into contact with the thermometer. When all precautions are taken to secure that there shall be no losses of heat between the point of throttling and the thermometer, the method only serves, at the best, to show what was the wetness of the steam when it was on the point of entering the throttling plug. Whether its state then is the same as the average state of steam in the steam-pipe is another question. Unless special precautions be taken in connecting the apparatus to the steam-pipe, the sample of steam taken off for examination is liable to suffer some condensation before it reaches the plug, and therefore to give an exaggerated impression of the wetness of the supply. On the other hand the steam supplied by a steam-pipe to an engine is liable to carry along with

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