TM-5-855-4
3-6. Thermal properties of materials and structures.
Fundamentals are applicable in connection with hardened structures. For estimating, the outside surface
coefficients of exposed ceilings, walls, and floors of internal structures should have the same values as
those ordinarily used for inside surface coefficients.
b. For natural convection in an empty underground chamber the air- to-rock heat transfer coefficient
20
U, ranged from 1.4 to 1.0 Btuh/fi F.
Fundamentals. For forced convection on rough surfaces and air velocity v' in fph, a good approximation
20
in Btuh/ft F is
3-7. Conversion time duration.
a. The length of time required to bring the walls to the desired temperature depends on the thermal
inertia of the space. Aboveground the question is seldom significant because thermostatted conditions can
normally be established in less than 100 hours. For underground structures, this time is much longer and
depends on the sensible and latent heat loads during conversion and during the thermostatted period, as
well as the equipment available to carry these loads. Selection of the conversion duration will depend on
correlating load, cost, and time factors.
b. Within practical limits, conversion time can be reduced by increasing equipment capacities.
Available data indicates that both the removal of moisture from the surrounding rock mass and the
stabilization of temperature are relatively slow processes and that a point of marginal return is reached
beyond which the cost of acceleration, in terms of equipment capacities and power requirements, will be
excessive. A point often overlooked is the latent load due to moisture transfer at the walls of the
underground space, which will normally require some form of dehumidification to maintain acceptable
conditions in the space much like in many residential basements.
c. In an actual case involving the conversion of an existing underground refrigerated space, closed
cycle dehumidifiers were installed when it appeared that the space humidity would not come down to
reasonable levels, even after rock temperatures had returned to their original levels. In the ensuing
period of 630 days, 3,600,000 pounds of water were recondensed and 3,780 million (MBtu) returned to the
space before a steady state was reached of 40 percent RH and the moisture emission gradually reduced to
less than 3,000 gallons per week from an initial 16,000 gallons per week. This example reveals that a
considerable amount of time may be required to prepare a given space for its intended function, a factor
which precludes the utilization of many existing underground structures for immediate shelter.
3-8. Trend Analysis.
a. General. In order to show the influence of the different parameters such as space size and thermal
properties on the heat transfer to the rock, cylindrical models were fitted to three spaces in various
combinations of the parameters given in table 3-2 and the results shown graphically in figures 3-16 to 3-19.
(1) The cylindrical configuration is selected because most underground spaces are elongated and
therefore best modelled by the cylinders. It could be shown that the spherical model would exhibit similar
properties with respect to the parameters.
(2) Figures 3-16 and 3-17 show solutions for the constant flux during the warm-up period and are
The last factor or denominator in the right side of this equation consists of two terms proportional to
thermal' resistance or insulation. The first represents the insulation proper to the rock and the second that
due to the surface air film and any other additional wall insulation features included.
b. Temperature differential (T3 -T1). The figures are based on a differential of 25 0 F. The warm-
up flux is directly proportional to this differential. The correction factor for the other differential is
simply the ratio of the new differential to the 25 0 F base reference.
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