TM
5-858-7
4-4. Humidity.
by volume, and acts on the human nervous system
to maintain involuntary respiration. At levels in
a. The humidity gains in the closed-ventilation
excess of 1 percent it begins to cause hyperventila-
system of well-constructed facilities will originate
tion, increased oxygen consumption, and increased
largely from the respiration and perspiration of
respiratory carbon dioxide production; concentra-
personnel, and are treated as a waste-heat load.
tions higher than about 4 percent are toxic. The
Where there are no other overriding considera-
natural air content of 0.03 percent carbon dioxide
tions, 50 percent relative humidity should be main-
should not be reduced by the carbon dioxide-
tained for human comfort and control of air-borne
control system, but this is not likely to be a prob-
bacteria.
lem. The maximum carbon dioxide conent of the
b. Chemical dehumidification will usually be un-
room exhaust air should not exceed 1 percent and
desirable because nearly all commercial absorbent
the corresponding concentration in return air
and adsorbent systems must be thermally regener-
should be less than 0.08 percent.
ated, which adds additional heat load to the waste-
b. Carbon dioxide can be removed readily from
heat rejection system. Where there is already a
air by causing the gas to react with strong bases
substantial load of sensible heat to be removed,
(caustics) to produce carbonates. The reactions are
consider dehumidifying by direct extraction of la-
exothermic and will add significantly to any
tent heat via condensation.
personnel-dependent heat sources. From several
c. Although the use of water spray towers may
feasible combinations of reactant and process, the
be the most efficient means of dehumidification at
optimum method will be chosen by considering
low water temperatures, a possible rise in heat-
cost, convenience, and minimum release of waste
sink water temperature during the facility mission
heat, in approximately that order. In a gas absorp-
period would require the use of mechanical refrig-
tion system widely used by industry, a sodium hy-
eration for dehumidification. Dehumidification sys-
droxide solution is recirculated counter-current to
tem design should be based on the maximum heat-
the carbon dioxide-contaminated air stream,
sink temperature allowable at the end of the
through a spray or packed scrubber tower. This
mission. At cooling coil temperatures approaching
system is recommended for large-capacity carbon
the freezing point, provide the automatic de-
dioxide removal systems for hardened facilities on
frosting of dehumidifier coils.
the basis of reactant, and low heat of reaction
d. To minimize space requirements, specify inte-
(about 1020 Btu per pound of carbon dioxide
grated cooling and dehumidification units.
absorbed).
c. If the scrubber solution temperature exceeds
4-5. Oxygen. If the principal oxygen demand is
the desired dew point temperature of the air, an
by personnel, us the values given in table 41 to
additional load will be placed on the humidity con-
estimate the rate of consumption. In view of the
trol system. In addition to the heat of carbonate
probable low cost of oxygen for this purpose, an ar-
formation, latent heat and sensible heat trans-
bitrary allowance of 3 lb per man-day is reason-
ferred to or from the air stream must also be con-
able. The most practical storage will usually be in
sidered in the heat-load calculations for the scrub-
3
standard, 250-ft , high-pressure cylinders up to a
ber. In any case, the scrubber design should be
total of about 12,000 ft3 . For larger storage re-
coordinated with the design of the air temperature
quirements, specify larger capacity cylinders to im-
and humidity control systems.
prove the packing factor and to simplify ducting.
d. The wet scrubber system is highly amenable
Use standard two-stage pressure regulators to set
to continuous process application with automatic
the flow rate. Since variation of oxygen content be-
control. In such a system, carbon dioxide removal
tween the normal 21 percent and about 17 percent
efficiency and outlet concentration are controllable
is acceptable, highly accurate or automatic control
and adjustable while the system is in operation by
of oxygen flow is not essential. Periodic checking of
varying solution concentration, temperature, flow
oxygen concentration and manual adjustment of
r a t e s , a n d air-stream velocity through the
flow rate is adequate.
scrubber.
4-6. Carbon dioxide.
e. Dry process absorption in attractive for rela-
tively small-capacity carbon dioxide rem-oval re-
a. Carbon dioxide buildup in hardened-facility
quirements (less than about 5 lb per hour). These
ventilation air will originate largely, if not exclu-
systems consist of trays or canisters of dry granu-
sively, from personnel. Formation rates can be es-
lar absorbents through which air is circulated at
timated from the data in table 41. Carbon dioxide
low velocity. Since absorbers must be replaced pe-
is present in atmospheric air at about 0.03 percent
4-2
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