Application Guide
Electric Heaters
•Recognize that more heatingcapacity exists than is being
utilized. (A short start-up time
requirement needs more wattage
than the process in wattage.)•Identify where most energy is go-ing and redesign or add insulationto reduce wattage requirements.
Power Evaluation
ContinuedHaving considered the entire system,a re-evaluation of start-up time,
production capacity, and insulating
methods should be made.
Review of Heater ApplicationFactors
Silicone Rubber Heater Example: 1000 watts are required for heating a 150°C (300°F) block.From the silicone rubber heater wattdensity chart in the flexible heatersection of the Watlow Heaters catalog,page 170.Maximum Watt Density =16 W/in Safe/Permissible Watt Densities A heater’s watt density rating gives us
an indication of how hot a heater willoperate. We use this information toestablish limits on the application of
heaters at various temperatures andunder a variety of operating
conditions.The maximum operating watt densityis based on applying a heater such
that heater life will exceed one year. In conjunction with desired life, wattdensity is used to calculate both therequired number of heaters and their size.
2 for wirewound on-off(2.5 W/in
2 ) or 38 W/in
2 (6 W/cm
2 ) foretched foilThis means 63 in
2 of wirewound (five 3 inch
• 5 inch heaters) or 27 in
2 of etched foil (two 3 inch
• 5 inchheaters) are required. Mechanical Considerations Full access must be provided (in thedesign process) for ease of heaterreplacement. This is usually done with
shrouds or guards over the heaters.
These guards also serve a secondarypurpose in that they may minimizeconvective heat losses from the back
of heaters and increase efficiency of the system.In all applications where the heatermust be attached to a surface, it isextremely important to maintain as
intimate a contact as possible to aidheat transfer. Heaters mounted on the
exterior of a part should have
clamping bands or bolts to facilitatethis contact. Heaters inserted in holes
should have hole fits as tight aspossible. Whenever possible, theholes should exit through the opposite
side of the material to facilitateremoval of the heater.choice of the external sheath ma-terial is very important to heaterlife. A corrosion guide is provided,
page144, and should be consultedin order to avoid using materials
that are not compatible with aparticular environment.•Explosive environments generallyrequire that the heater be com-pletely isolated from potentially
dangerous areas. This is accom-plished by inserting the heater in pro-
tective wells and routing the wiringthrough sealed passage-ways outof the hazardous area. Very closefusing is recommended in these
cases to minimize the magnitude ofthe failure, should it occur. Example: Heat is required to main-tain molten zinc in the passagewaysof a zinc
die casting machine. The
possible contaminants for this appli-cation are as follows:a.molten zinc metalb.zinc vaporc.hydraulic oils
d.high temperature anti-seizematerialse.moisture, if die cooling is aidedby water circulationAll of these factors indicate that ahighly sealed heater construction is required.•The corrosiveness of the materialsheated, or the materials that willcontact the heater must also be
taken into consideration. Even if a heater is completely sealed, the Operating Environment Factors •Contaminants are the primarycause of shortened heater life.
Decomposed oils and plastics
(hydrocarbons in general), con-
ductive pastes used as anti-seize
materials, and molten metals and
metal vapors can all create situa-
tions that affect heater life. Some
heater constructions are better
sealed against contaminants thanothers. In analyzing applications,all possible contaminants must be
listed in order to be able to fullyevaluate the proposed heater. 20