Page 424 - Handbook of Energy Engineering Calculations

required for an electric heater used to melt a solid of any kind—lead, tin, type
metal, solder, etc. When the substance being heated is a liquid—water, dye,
paint, varnish, oil, etc.—use the relation H = (weight of liquid heated, lb)
[specific heat of liquid, Btu/(lb · °F)] (temperature rise desired, °F), when the
liquid is heated to approximately its boiling temperature, or a lower

temperature.
For space heating of commercial and residential buildings, two methods
3
used for computing the approximate wattage required are the W/ft and the
“35” method. These are summarized in Table 4. In many cases, the results
given by these methods agree closely with more involved calculations. When
the desired room temperature is different from 70°F (21.1°C), increase or
decrease the required kilowatt capacity proportionately, depending on

whether the desired temperature is higher than or lower than 70°F (21.1°C).

TABLE 4 Two Methods for Determining Wattage for Heating
Buildings Electrically *

For heating pipes with electric heaters, use a heater capacity of 0.8 W/ft 2
2
(8.6 W/m ) of uninsulated exterior pipe surface per °F temperature difference
between the pipe and the surrounding air. If the pipe is insulated with 1 in
2
(2.5 cm) of insulation, use 30 percent of this value, or 0.24 (W/(ft · °F) [4.7
2
W/(m · °C)].
The types of electric heaters used today include immersion (for water, oil,
plating, liquids, etc.), strip, cartridge, tubular, vane, fin, unit, and edgewound
resistor heaters. These heaters are used in a wide variety of applications
including liquid heating, gas and air heating, oven warming, deicing,