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 TB 740-97-4
CHAPTER 6
DYNAMIC DEHUMIDIFICATION INSTRUCTIONS
137. General. a. Dynamic dehumidification involves the use of a machine dehumidifier. The machine removes moisture from air
circulated through it by means of built-in beds of desiccant. The machine periodically reactivates the desiccant by driving off
accumulated moisture by heating.
b. General instructions for the installation of dynamic dehumidification systems on vessels being preserved for storage are
provided in this chapter.
c. Detailed instructions for installation of a dynamic dehumidification system on each standard design vessel, as applicable,
are provided in appendices B through V.
d. Dynamic dehumidification (D/H) drawings illustrated in the appendices were developed for standard design vessels on a
type/design basis. D/H system designs depicted herein may be changed or modified by the local storing activity when such changes
are required due to vessel modifications.
e. Development of D/H system designs and installation is locally authorized for new design craft entering the inventory for
which D/H design drawings have not been prepared.
f. When the D/H design configuration is changed or modified from that shown in the drawings illustrated in the appendices,
such changes shall be penciled in red on a reproduced copy of the drawing. A copy of this marked-up drawing along with supporting
narrative instructions describing the changes will be maintained by the local storage facility storing the craft. A copy of the marked-
up drawing including changes in narrative instructions will be forwarded to the publication preparing activity DRSTS-SDP.
138. Installation of Dynamic Dehumidication. a. Dehumidification Machine. There are two sizes of machines norm ally used for
the dehumidification of floating equipment. The smaller machine, which has a nominal capacity of 10,000 cubic feet, will be used in
most cases. The larger machine, which has a nominal capacity of 150,000 cubic feet, will be used primarily on large vessels such as
floating repair shops and ocean going tugs Y-Tankers and liquid or dry vessels. When multiple 10,000 CFM dehumidification units
are used to service one zone they may be wired through a relay box as shown in figure 1.
NOTE
Multiple DH units wired through relay must be synchronized in order that no more than two (2) units go into the
reactivation cycle at the same time. Failure to synchronize the units will result in blown fuses. A suitable cover will
be fabricated as shown in figure 2 to provide protection for the relay box.
(1) Location of machines. The machines will be located outside the areas to be dehumidified, and will be situated to
require a minimum of duct work. The machines will be protected from the weather by suitable shelters. At storage sites insulated to
reflect solar heat. The desiccant beds cast off moisture within the machines when exposed to the excessively high temperature and
lose their capacity to absorb moisture during the drying cycle if overheated at that time. Machine covers will be designed to
encourage air circulation. Care will be taken not to obstruct the reactivation outlets of the machine. A short extension running
outward and downward from one of the reactivation outlets will aid in the exchange of air required to reactivate the desiccant.
(2) Ductwork and connections. Air will be circulated between the dehumidification machine and the area to be
dehumidified through galvanized sheet metal ducts. Four-inch diameter ducts will be used for the 10,000 cubic feet machine, and six
inch diameter ducts with the larger machines. The ducts will be attached to the machines by the use of slip joints or couplings which
permit easy removal of the machines for servicing or repair. Where flexibility is required, as in carrying air between wet stored
vessels, lengths of hose conforming to MIL-H-8796 will be used. Maximum use will be made of existing openings for connecting the
duct work to the area to be dehumidified. Suitable galvanized sheet metal or exterior grade plywood blanks will be used in
introducing the air lines through airports, accesses and other openings. Holes will be made in bulkheads and other components of
the vessels only when there is no other means of ingress. Normally, dry air will be introduced near the top of the area penetrated by
the ductwork, with moist air being withdrawn from a point near the bottom of the area to be dehumidified. The dry air and moist air
lines will be positioned to encourage circulation of air and to prevent short circuiting of the air between the two openings. In rare
cases, booster blowers may be needed to provide satisfactory dehumidification. Complete circulation of air is not necessary;
difference in vapor pressure tend to equalize humidity regardless of air flow.
b. Humidistats. The operation of each
Change 2
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