The slopes of constant-wet-bulb lines for different gas mixtures are shown in
the upper left area of Figures 6-21 and 6-22. The molar specific heats of
air, nitrogen, and oxygen are approximately equal at 6.95 Btu/pound-mole, as
represented by the highest line, and the molar specific heat of helium is
lower, at 4.96 Btu/pound-mole, represented by the lowest line. Lines for gas
mixtures containing different percentages of helium are plotted between these
limits. To determine a constant-wet-bulb-temperature line, draw a line
parallel to the appropriate line in the upper left area of the chart,
starting at the intersection of the wet-bulb temperature value and the proper
pressure curve. Extend the line to the right until it reaches the desired
value of dry-bulb temperature. The mole fraction of water vapor in the
mixture at the dry-bulb temperature is read from the right scale. The
relative humidity can then be calculated as the ratio of this value to the
value corresponding to the intersection of the dry-bulb temperature with the
proper pressure curve. If both the dry-bulb temperature and the relative
humidity are known, wet-bulb temperature can be found by the following
procedure: (1) read moisture mole fraction for saturated gas at dry-bulb
temperature and proper pressure curve; (2) multiply this value by relative
humidity, and plot the result on the dry-bulb temperature line; (3) draw a
constant-wet-bulb temperature line from this point to the left, to intersect
with the proper pressure curve. The wet-bulb temperature can be read below
this intersection on the temperature scale.
Both enthalpy and moisture concentration appear on the vertical scale of
Figures 6-21 and 6-22, with the temperature correction for the effect of
mixture shown in Figure 6-23. Most of the enthalpy of the water vapor is in
the latent heat of vaporization. Consequently, the water-vapor enthalpy is
almost directly proportional to the amount of water vapor present. Enthalpy
is expressed in Btu/mole of dry gas.
7.
GAS DISTRIBUTION. The following items are important considerations in
designing systems for distributing compressed gas to, from, and between the
gas system components.
a.
Flexibility. Flexibility is necessary in the routing of gasses to
provide back-up in the event of critical equipment malfunction. This
includes emergency by-pass valves around filters, regulators, or any other
equipment where by-pass may be necessary in event of malfunction, and
redundant gas supply lines so that loss of one line will not jeopardize the
safety of the chamber occupants.
b.
Line Size. Proper line sizes are necessary to supply maximum gas
flows required by the various system components.
c.
Valves. Proper valve sizes and opening rates are necessary to
allow system operators to be able to control flows and pressures required to
maintain chamber habitability.
d.
Dead Ends. Dead ends, bends, and low points should be kept to a
minimum in the piping and distribution system but should permit cleaning of
the system in place.
e.
Exhaust Lines. Exhaust lines should be provided from all gas
storage banks. Exhaust lines designed for oxygen service should discharge to
the atmosphere at a location which will minimize fire hazards and
contamination to the system.
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