Sunday, February 14, 2016

Suggested Watt Densities for Electric Heating Elements

watt density in electric heaters
Always consider proper watt density
for your electric heater application.
Reprinted with permission from Hotwatt

The rates below are recommended watt densities for use with various materials. Safe values vary with operating temperature, flow velocity, and heat transfer rates. In general, the higher the material temperature, the lower the watt density should be, especially those materials which coke or carbonize, such as oils. Watt densities should be low if a material is being heated to a temperature near where the change of state to a vapor occurs (water to steam @ 212°F) since the vapor state has much poorer heat transfer capabilities.

Material being heated Maximum Operating Temp.°F Maximum Watts Per Sq. In.*
Acid Solutions:
   Acetic
   Chromic (5%)
   Citric
Ferric
   Chloride (40%)
Hydrochloric
Nitric (50%)
Sulphuric

212
Boiling
Boiling
Boiling

150
Boiling
Boiling

40
40
40
40

30
40
30
Alkali & selected oakite cleaning solution 212 40
Asphalt binder, tar, other viscous compounds 200
300
400
500
8
7
6
5
Caustic Soda 2%
                    10%
                    75%
210
210
180
45
25
25
Coffee (Direct Immersion) Boiling 90
Dowtherm A®
   flowing at
   1 ft/sec or more
   Non-flowing


750
750


22
10
Ethylene glycol 300 30
±Fuel Oils
   Grades 1 & 2 (Distillate)
   Grades 4 & 5 (Residual)
   Grade 6 & Bunker C
   (Residual)
200
200
160
22
13
8
Gasoline, kerosene 300 20
Glue (heating indirectly using water bath Lead-Stereotype pot) 600 35
on
casting
Liquid ammonia plating baths 50 25
** Lubrication Oils
   SAE 10, @ 130°F
   SAE 20, @ 130°F
   SAE 30, @ 130°F
   SAE 40, @ 210°F
   SAE 50, @ 210°F

250
250
250
250
250

22
22
22
13
13

* * Some oils contain additives that will boil or carbonize at low watt densities. Where oils of this type are encountered, a watt density test should be made to determine a satisfactory watt density.


Material being heated Maximum Operating Temp.°F Maximum Watts Per Sq. In.*
Metal melting pot 500 to 900 20-27
Mineral oil 200
400
20
16
Molasses 100 2-3
Molten salt bath 800-950 40
Molten tin 600 20
Oil draw bath 600
400
20
24
Paraffin or wax 150 16
Photographic solutions 150 70
Plating solutions:
   Cadmium plating
   Chrome plating
   Copper plating
   Nickel plating
   Tin plating
   Zinc plating

40
40
40
40
40
40
Salt Bath 900 30
Sea Water Boiling 90
Sodium cyanide 140 40
Steel tubing cast into aluminum 500 to 750 50
Steel tubing cast into iron 750 to 1000 55
Heat transfer oils
   flowing at 1 ft/sec or more
500
600
650
750
22
22
22
15
Trichloretylene 150 20
Vapor degreasing solutions 275 20
Vegetable oil (fry kettle) 400 30
Water (process) 212 60
Water (washroom) 140 80-90

* Maximum watt densities are based on heated length, and may vary depending upon concentration of some solutions. Watt density should be kept as low as possible in corrosive applications since higher watt densities accelerate corrosive attack on element sheaths. Consult BCE for limitations.

Important: The above values are estimates. It is strongly suggested that you discuss your requirement with an application expert before you apply any electric heating element in to a process where the proper watt density is unknown.