(1) Sizing. The total weight of the absorbent can be selected on
the basis of the total weight of carbon dioxide to be absorbed. The volume
of a tightly-packed absorbent bed will then depend upon the absorbent
density, Figure 6-18, and the residence time will be the same for any
configuration of this volume. (Residence time is equal to absorbent bed
volume divided by gas volume flow.)
If the bed volume is selected on the basis of absorbent weight, then the
residence time of gas in the bed will be proportional to the operating period
for which the scrubber is designed, and the rate of ventilation through the
absorber. As a general rule, the volume flow rate required for CO2 removal
should be the same at all depths, matching respiratory volume
characteristics.
(2) Pressure Drop. The pressure drop through an absorbent bed will
depend upon the relation of flow cross section and bed depth for a fixed bed
volume. A large cross section and small depth will result in low pressure
drop. However, a uniform flow distribution over the cross section depends
upon uniformity of pressure drop. This may be difficult to control if the bed
is too thin or if the absorbent material is not packed properly in the
canister. These difficulties can be minimized by using a perforated plate at
the inlet of the bed to provide controlled pressure drop and flow
distribution, and by establishing proper packing techniques. Figure 6-18
shows the relation of pressure drop to superficial velocity, pressure, and
bed depth for air passing through a bed of 4 to 8 mesh Sodasorb or Baralyme
pellets. The superficial velocity is defined as the calculated velocity for
flow through the empty bed space. The data of Figure 6-18 are experimental
data for air only and are taken in a flow regime of variable friction and
momentum losses such that the data cannot be readily interpreted for
helium-oxygen mixtures.
(3) Canister Design. Figure 6-19 shows two typical canister
designs. Annular canisters are used with radial flow in either the inward or
the outward direction. Cylindrical canisters usually contain annular baffles
as shown to minimize by-passing of flow at the canister walls. The canister
requires frequent servicing due to limited absorbent life. The cover should
be of the quick-opening type, such as shown in Figure 3-23, and to facilitate
removal and replacement of the absorbent, the designer should consider the
use of cartridge-type replacement units. Servicing of the canister will be
necessary also due to the corrosive effects of the absorbents. Careful
selection of materials will reduce this tendency. However, the canister
should be accessible for cleaning.
6.
TEMPERATURE-HUMIDITY CONTROL. A typical temperature-humidity control as
shown in Figure 6-20 includes the following elements:
a.
Cooling coil.
b.
Cooling coil control valve.
c.
Cooling coil by-pass.
d.
Water trap.
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