Page 241 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION 5.25

point, an inversion takes place. The coldest water, at a lesser density, now rises to the surface.
Continued cooling of the surface water further reduces its density, and the temperature
rapidly falls to the point of freezing.
Ice generally appears first at the sides of the tank, then quickly forms a continuous layer
over the surface. This same progression is true for similar installations such as pools and
lagoons. A formula for the determination of ice formation on the surface of an open vessel
has been developed. It considers the insulating effect that the forming layer of ice will have
on the transfer of heat from the surface of the water to the air. One assumption is that the
time is constant. The following formula calculates the thickness of ice formation in any
period of time. The formula is:

X = 25 l (5.8)
32 − T
where X = time, h
I = ice thickness, in (A generally accepted figure of 1/16 in is required to stop
operation of float valves, etc.)
T = design air temperature, °F
A design air temperature value based on the mean of the high and low reading is con-
sidered adequate for design of tanks storing potable water. For tanks used to store fire
protection water only, it would be advisable to use the lowest 1-day mean temperature. The
lowest one-day mean temperature can be found in Fig. 5.3.

Selecting the Heating System Type
Selecting the most economical type of heating system depends on the tank height, amount
of heat required, and the availability and cost of any particular fuel or heating medium. The
basic methods or devices are:

1. Direct discharge of steam into the water
2. Steam coils inside the tank
3. Hot water coils inside the tank
4. Electric immersion heating elements inside the tank
The direct discharge of steam into a tank is the method used most often for nonpotable
water where steam is available. A steam supply line of adequate size is piped directly into
the tank. This line ends in a tee placed about one-third of the height of the tank from the
bottom. A condensate return line may be required, but it does not penetrate the tank.
When there is a potable water supply in combination with fire reserve water in a tank,
the possibility of cross-contamination from an outside source must be minimized to the
greatest extent possible. As a result, the following three methods should be considered.
The use of steam in coils at the bottom of a tank is mostly limited to tanks that have a flat
bottom, are not elevated to a great height, and do not need too much heat transferred. Based
on past experience, this method is not considered very reliable because a large number of
problems have been reported.
Gravity circulation of hot water requires a heat exchanger or a hot water generator to be
placed in close proximity to the tank, usually under the tank in the valve pit, if provided, or
in a separate building adjacent to the tank. Cold water is taken from the discharge pipe and
run through the heater. Since the now heated water is lighter than the colder supply, a natural
circulation is obtained. Long runs of heater piping are not practical due to the heat loss from
the piping and the expense of insulation needed to keep the heat from being lost. In addition,
the hot water generator takes space in what might be a small valve pit under the tank.

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