TM-5-8554
4-5. Refrigeration equipment.
a. The refrigeration cycle can be used for both cooling and dehumidification by absorbing heat at the
evaporator and for heating by rejecting heat at the condenser. A chilled-water system is preferable for
underground installation because breaks or leaks in the distribution lines will not create a critical
condition within the structure, either from the standpoint of the replacement of the coolant or the
generation of potentially dangerous gases.
b. Provision will be made for at least one or more water chiller units complete with motor,
compressor, condenser, and evaporator for each structural unit to be conditioned. Refrigerant
compressors of the reciprocating type will have three stages of capacity reduction. Centrifugal type units
will have a capacity control system providing for continuously variable capacities of from 10 to 100
percent. Absorption type units will have automatic steam generation and control Water chiller units of
the same type will be interconnected to ensure maximum utilization of capacity control.
c. If the supply of condenser water is drawn from underground wells or reservoirs, the chemical
content of this water must be analyzed to determine the fouling factor which must be considered in the
condenser design and in determining the requirement for water treatment equipment in the system.
4-6. Fans.
a. Underground facility design requires a powered air moving device such as a fan for ventilation
and exhaust. Standard fan designs are available for most fan requirements. Heavy-duty ventilating
-
- fans will be used for more severe conditions.
(1) Fan type selection depends on application. When space is not a factor centrifugal fans will
backwardly curved and air foil shaped blades will be used for maximum efficiency. Vaneaxial fans are
used when space is at a premium and nonturbulent inlet conditions can be obtained. The use of inlet
vane straightness for this purpose is recommended. Propeller fans are generally limited to applications
requiring 2 in. wg will be mounted on inertia blocks.
(2) In selecting the proper fan, consideration will be given to airflow, head pressure, noise, and
available space. In the majority of protective structures, space will govern fan selection. However, space
must be balanced against noise and efficiency of operation.
b. In selecting the proper fan, consideration will be given to airflow, head pressure, noise, and
available space. In the majority of protective structures, space will govern fan selection. However, space
must be balanced against noise and efficiency of operation.
(1) Head pressure requirements will be determined carefully, particularly with regard to
seasonal variations of the air temperature, CB filters and such other items as blast-closure valves,
prefilters, and structure pressurization. Because of the high efficiency of the CB filter, it is recommended
that the higher resistance of 6 in. wg be used in determining fan capacities:
(2) Noise is of major importance in a closely occupied structure. Therefore, duct work and fan
mounts will be carefully designed. Ducts will be connected to fan and filter inlets and outlets by means
of butyl rubber or butyl coated nylon cloth material. Main supply fans will be remote from occupied areas
and provided with resilient sound-absorbing bases. Noises due to high-velocity ducts, abrupt turns, and
rigid connections to fans will all be considered. Where high-velocity minimum-size equipment must be
used, an adequate acoustical and vibration treatment will be employed.
(3) ASHRAE Handbook, Equipment, will be consulted for fan sizing, selection, application, and
control. Parallel start and operation of fans will be done in accordance with fan manufacturer's
recommendation in particular when using pitch controlled vaneaxial fans.
4-7. Duct systems.
a. Ducts which may carry contaminated air or run through areas which may become contaminated
will be gastight. Duct systems will be designed within prescribed limits of available space, friction loss,
noise level, heat loss or gain, and pressure containment. Both high velocity and low velocity duct
systems will be designed in accordance with ASHRAE Handbook Fundamentals. The aspect ratio R of
rectangular ducts defined as the ratio of the longer to the shorter side of the cross-section is an important
parameter in the optimization of duct systems.
b. The economic factors of first cost and operating cost will be evaluated in conjunction with
available space to determine the best duct system. Each application is different and will be analyzed
separately. Normally square or round ducts will be preferred to rectangular ducts of larger aspect ratio
because they are less subject to heat pick up and more economical as shown below.
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