The presence of moisture in steam causes a loss, not only in the
practical waste of the heat utilized to raise this moisture from the
temperature of the feed water to the temperature of the steam, but also
through the increased initial condensation in an engine cylinder and
through friction and other actions in a steam turbine. The presence of
such moisture also interferes with proper cylinder lubrication, causes a
knocking in the engine and a water hammer in the steam pipes. In steam
turbines it will cause erosion of the blades.
The percentage by weight of steam in a mixture of steam and water is
called the _quality of the steam_.
The apparatus used to determine the moisture content of steam is called
a calorimeter though since it may not measure the heat in the steam, the
name is not descriptive of the function of the apparatus. The first form
used was the "barrel calorimeter", but the liability of error was so
great that its use was abandoned. Modern calorimeters are in general of
either the throttling or separator type.
Throttling Calorimeter--Fig. 14 shows a typical form of throttling
calorimeter. Steam is drawn from a vertical main through the sampling
nipple, passes around the first thermometer cup, then through a
one-eighth inch orifice in a disk between two flanges, and lastly around
the second thermometer cup and to the atmosphere. Thermometers are
inserted in the wells, which should be filled with mercury or heavy
cylinder oil.
[Illustration: Fig. 14. Throttling Calorimeter and Sampling Nozzle]
The instrument and all pipes and fittings leading to it should be
thoroughly insulated to diminish radiation losses. Care must be taken to
prevent the orifice from becoming choked with dirt and to see that no
leaks occur. The exhaust pipe should be short to prevent back pressure
below the disk.
When steam passes through an orifice from a higher to a lower pressure,
as is the case with the throttling calorimeter, no external work has to
be done in overcoming a resistance. Hence, if there is no loss from
radiation, the quantity of heat in the steam will be exactly the same
after passing the orifice as before passing. If the higher steam
pressure is 160 pounds gauge and the lower pressure that of the
atmosphere, the total heat in a pound of dry steam at the former
pressure is 1195.9 B. t. u. and at the latter pressure 1150.4 B. t. u.,
a difference of 45.4 B. t. u. As this heat will still exist in the steam
at the lower pressure, since there is no external work done, its effect
must be to superheat the steam. Assuming the specific heat of
superheated steam to be 0.47, each pound passing through will be
superheated 45.4/0.47 = 96.6 degrees. If, however, the steam had
contained one per cent of moisture, it would have contained less heat
units per pound than if it were dry. Since the latent heat of steam at
160 pounds gauge pressure is 852.8 B. t. u., it follows that the one per
cent of moisture would have required 8.5 B. t. u. to evaporate it,
leaving only 45.4 - 8.5 = 36.9 B. t. u. available for superheating;
hence, the superheat would be 36.9/0.47 = 78.5 degrees, as against 96.6
degrees for dry steam. In a similar manner, the degree of superheat for
other percentages of moisture may be determined. The action of the
throttling calorimeter is based upon the foregoing facts, as shown
below.
Let H = total heat of one pound of steam at boiler pressure,
L = latent heat of steam at boiler pressure,
h = total heat of steam at reduced pressure after passing
orifice,
t_{1} = temperature of saturated steam at the reduced pressure,
t_{2} = temperature of steam after expanding through the orifice
in the disc,
0.47 = the specific heat of saturated steam at atmospheric pressure,
x = proportion by weight of moisture in steam.
The difference in B. t. u. in a pound of steam at the boiler pressure
and after passing the orifice is the heat available for evaporating the
moisture content and superheating the steam. Therefore,