Page 243 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION 5.27

The use of electric immersion heaters is practical only when the cost of the electricity
is low. An advantage is the lowest initial cost of the heating methods discussed when used
for potable water.

Calculating Heat Loss from Elevated Tanks
A method to determine the amount of heat lost from water tanks developed by the National
Fire Protection Association (NFPA) has become the standard for design, and is very con-
servative in its approach. The basic consideration is not to allow the temperature of the
water to fall below 42°F. This is a safe temperature above freezing that makes allowance
for tolerances in various equipment and for variations in weather design data.
The procedures required to calculate the amount of heat necessary to replace the heat
that was lost to the atmosphere are as follows:
1. Determine the lowest 1-day mean temperature at the proposed site. Refer to Fig. 5.3 for
this information.
2. Determine the number of gallons the tank will hold. Use the nominal, not actual,
capacity.
3. Using the lowest temperature and the capacity from steps 1 and 2, refer to either Table 5.9
(for elevated wood or steel tanks or wood tanks on grade) or Table 5.10 (for steel tanks
on grade) for the heat loss in thousands of Btu/h.
4. Refer to Table 5.11 to find the heat loss in Btu/h from the riser for each foot of length
from grade to the bottom of the tank.
5. Add the two figures obtained in Steps 3 and 4 together to calculate the total heat loss for
the installation. This figure is the capacity in thousands of Btu/h that the heating system
must be capable of providing.

SEWER AND WATER SUPPLY PIPING DESIGN

Designing a complex piping system to prevent freezing is a very involved procedure that
requires a thermal analysis of the piping network. The types of systems discussed in this
book are not considered complex.
In current practice, precise analysis is not necessary and therefore is not made. The
empirical methods discussed have been verified under actual field conditions and are con-
sidered sufficiently accurate for engineering design calculations. Although the following
discussions are for a flowing sewage system, the same calculations can be made for flowing
water piping.
The following assumptions have been made to simplify the solution of the heat transfer
process. These are:
1. The thermal characteristics of sewage are essentially the same as that of water.
2. The sewer is assumed to be flowing full and at a minimum velocity of 2 ft/s.
3. There are critical flow periods that will last about 6 to 8 h, during which the flow will
be approximately 25 percent of the 24-h average flow, and a 1- to 2-h period when the
flow is about 5 percent of the 24-h flow rate.
4. The losses through manholes, catch basins, cleanouts, and the like are the same as those
through the pipe run itself.
5. Heat is transferred mainly through conduction.

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