Page 262 - Facility Piping Systems Handbook for Industrial, Commercial, and Healthcare Facilities

HEAT TRANSFER, INSULATION, AND FREEZE PROTECTION

5.46 CHAPTER FIVE

9. Area where the cable is to be installed
10. Type of temperature control, if any
11. Whether a dedicated or emergency power supply will be needed
These factors are discussed in detail in the following paragraphs:

1. The minimum temperature to which water should be allowed to fall is 40°F. This is a
safe temperature, which provides a safety factor to allow for variables over which there
is no control, such as new record low temperatures, higher than expected wind velocities,
uneven heat distribution of the cable selected, and the types of control devices used.
2. The ambient temperature in the area where the heating cable is to be in stalled can be
obtained from Fig. 5.3. This is the mean low temperature, not the absolute low.
3. A wind velocity of 20 mph (9 mi) (12 km/h) has been selected as an average value.
For indoor installations, the requirements for heating can be reduced by 10 percent.
Conversely, if a velocity of 40 mph is used, the value should be increased by 10 percent.
4. Pipe or vessel size speaks for itself. The material of the container affects the transmis-
sion of heat from the cable to the liquid. The charts provided here are based on metal.
When plastics are used, the heat produced must be increased by 30 percent. This value
is acceptable for pipes up to 6 in and most plastic tanks. Be careful to check the surface
temperature of the cable and make sure the plastic material is capable of withstanding
that temperature without harm.
5. Tolerance of various control elements, such as sensing devices (used to detect the
temperature of liquids) and control thermostats, is important if the liquid temperature
must be closely maintained.
6. The type and thickness of the insulation is an important factor in the heat loss from the
pipe and cable. The charts provided are based on fiberglass. If another type of insula-
tion is used, refer to Table 5.6 for the insulation factor.
7. Special monitoring of the operation of the heating cable system will affect the selec-
tion of the heating cable type. If close control is required, the use of an ammeter and
an adjustable current sensing monitor (set to between 3 and 20 percent of base current)
will be required. If a self-regulating system is used, there will be no base current since
the system current varies. Therefore, the only kind of monitoring that could be used
would be the continuity type, which detects a break in the conductor.
8. If an alarm is desired, the type (for example, a bell, a light, or both) and the location
must be chosen.
9. The area in which the cable will be installed will affect the choice of cable. The condi-
tions of the area, such as whether the cable will be immersed or merely become wet,
whether there is very high humidity or not, whether the location is hazardous or subject
to explosions are very important when specifying the system.
10. The type of temperature control desired will affect the specific device used to turn the
system on. There are two possible modes of sensing: the air temperature or the surface
temperature of the pipe. Although there may be some special case in which the air tem-
perature is most important, sensing the actual temperature of the liquid in the pipe is
usually much more significant. There are two types of sensors, capillary-bulb and elec-
tronic. Both of these types can be used for either application, but the electronic device
is selected mostly for surface measurement. The bulb has a tolerance of 2 percent full
scale. The electronic type is very accurate, on the order of hundredths of a degree.
Sensing bulbs generally have a tolerance of about 4°F, and a repeatability error of
about 1°F. The longer the distance from the bulb to the actual controller, the greater
the possibility of error. Therefore, it is not good practice to use these types of sensors
if the distance is longer than 10 ft.

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