Chromalox Technical Documents
Technical
Technical Information
Watt Density & Heater Selection - Guidelines
Understanding Watt Density Maximum Sheath Temperature ers or with tubular or strip heaters clamped
Sheath Material to tank walls. The final choice of heater type
Watt density (W/in2) is the heat flux emanating Recommended Maximum Watt Density will involve process considerations, appear-
from each square inch of the effective heating ance, available space both inside and outside,
area (heated surface) of the element. Maximum Sheath Temperature The sheath
temperature of an electric element should be economy, maintenance, etc. The following
W/in2 = Rated Watts Effective heating area limited to prevent damage to the heater and pages cover the procedures for selecting heat-
provide reasonable life. To a large extent, the ers for clamp-on applications, liquid immer-
The effective heating area is the surface area maximum sheath temperature of the heating sion heating, oil immersion heating, air or gas
per linear inch of the heater multiplied by the element is determined by the final operating heating and cartridge or platen heating.
heated length. For strip heaters which are temperature of the process. In direct immer-
rectangular in shape, the surface area per sion applications, the sheath temperature will Clamp-On Heater Applications
linear inch is: approximate the temperature of the heated
media. In clamp-on, air and gas heating ap- The limiting factor in most clamp-on heater
1-1/2" wide = 3.45 in2 per linear inch applications is the operating temperature of
plications, the operating sheath temperature
1" wide = 2.31 in2 per inch. the heater sheath. Selecting heaters for clamp
can be estimated using factors derived from
empirical charts and graphs. on applications requires an analysis of the
The heated length (HL) of strip heaters is
maximum expected sheath temperature based
calculated as follows:
Sheath Material Element sheath material on the estimated ambient temperature and
< 30-1/2" long HL = Overall Length less 4" is selected based on the maximum allowable the temperature of the material being heated.
30-1/2" long HL = Overall Length less 5" sheath temperature, the material being heated Graph G-175S provides a method of estimat-
and corrosion resistance required. Depend- ing the sheath temperature and allowable watt
For tubular elements, watt density is deter- ing on the sheath material and construction, densities for tubular heaters for various ambi-
mined by the following formulas. metal sheathed electric resistance elements ent temperatures and wattage ratings.
will operate satisfactorily at temperatures from
Effective heating area = x Dia. x Heated Length
less than -300F (cryogenic) to approximately Graph G-175S Clamp-On
The surface area per linear inch of standard 1500F. Copper sheath elements are com- Tubular Heaters
diameter tubular elements is shown below: monly used for low temperature and direct
immersion water heating. Steel is used for 40
Size (Dia.) In2/in. oil immersion and strip heater applications. Sheath Te
mp. 1
Maximum Sheath (W/In2)
30 400
0.246 inch (1/4) 0.77 Stainless steel and INCOLOY are used for F
Sheath Te
0.315 inch (5/16) 0.99 corrosive solutions, high-temperature gas or mp. 1
200
20 F
0.375 inch (3/8) 1.18 air heating and cartridge heaters. The table
Sheath Te
0.430 inch (7/16) 1.35 below lists the maximum recommended mp. 1
10 Shea 000
th Temp F
0.475 inch 1.49 operating temperatures for common sheath . 80 0
F
0.500 inch (1/2) 1.57 materials (UL 1030): 0
The following example illustrates the proce- Copper 350F Chrome Steel 1200F 0 200 400 600 800 1000 1200 1400
dure for determining the watt density of a Iron 750F Stainless 300 1200F
Estimated Ambient Temp. (F)
typical tubular heater. Steel 750F INCOLOY 1600F1 (Sheath Temp. + Material Temp.)
MONEL 900F INCONEL 1700F1 2
Example A 12 kW screw plug heater has The example on the following page illustrates
three 0.475" diameter elements with a B Maximum Recommended Watt Density
the procedure. 12 kW is required to heat mate-
dimension of 32 inches and a 2 inch cold end. Some materials such as water, vegetable
rial in a steel tank from 70F to 800F. Heat
The watt density is: oils and salt baths can tolerate relatively high
is to be supplied by tubular electric elements
sheath watt densities. Other materials such as
clamped to the side of the tank. Since the
0.475 x x (32 in. - 2 in.) x 3 x 2 (Hairpin) = petroleum oils or sugar syrups require lower
watt densities. These solutions have high vis- material is heated to 800F, INCOLOY sheath
268 in2
cosity and poor thermal conductivity. If the watt elements must be used.
12,000 Watts 268 in2 - 45 W/in2 density is too high, the material will carbonize Note 1 For sheath temperatures above
For convenience in selecting equipment, all or overheat, resulting in damage to the heating 1500F, contact your Local Chromalox Sales
equipment or material being heated. Other office for application assistance.
heaters in this catalog have the watt density
sections of this catalog provide guidelines and
specified for standard ratings.
suggestions for sheath materials and recom-
Heater Selection Guidelines mended watt densities for many common
INFORMATION
heating problems.
TECHNICAL
Once the total heat energy requirements have Using the values determined in the selection
been determined, the selection of the type of criteria, choose the type of heater best suited
electric heater is based on three criteria. to the application. For instance, water can be
heated by direct immersion, circulation heat-
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Technical
Technical Information
Allowable Watt Density & Heater Selection - Guidelines
Selecting Clamp-On Tubular Using the previous 12 kW example, determine General Recommendations for
Heaters (contd.) the number of strip heaters required. An 800F Liquid Heating Applications
material temperature requires chrome steel
From the chart, a maximum sheath tempera- strip heaters. From Graph G-130S, a maxi- Chromalox standard immersion heater ratings
ture of 1200F results in an average ambient mum sheath temperature of 1200F results in match the suggested watt densities for general
temperature of (800F + 1200F) 2 = 1000F. an ambient temperature of 1000F inside the
purpose immersion heating. Extended heater
From the curves, the allowable watt density is space between the thermal insulation and the
vessel, (800F + 1200F) 2 = 1000F. life will be obtained by using the lowest watt
9.5 W/in2. Based on size of container, 0.475 density practical for any given application.
inch diameter TRI elements 28 in. long are From the curve, the allowable watt density is
selected. 8 W/in2. Based on the tank size, chrome steel Standard Ratings
The 0.475 TRI element has 1.49 in per linear 2 sheathed strip heaters 24 inches long without
Water Heaters 45 - 75 W/in2
inch of sheath. The heated length is the overall mounting tabs were selected. To determine the
number and wattage of strip heaters needed, Corrosive Solution Heaters 20 - 23 W/in2
sheath length less 6.5 inches. The allowable Oil Heaters (Light Wt.) 20 - 23 W/in2
wattage rating on the element is (28 - 6.5) x use the formula: allowable watts per strip =
(overall length minus 4" cold section) x 3.45 Oil Heaters (Medium Wt.) 15 W/in2
1.49 x 9.5 = 305 watts. The total number of
in2 per lineal inch of sheath x 8 watts/in2. Thus Oil Heaters (Heavy Wt.) 6 - 10 W/in2
elements required is 12,000W 305W = 39
elements. Order 39 elements similar to TRI- (25-1/2" - 4") x 3.45 x 8 = 593 (600) watts. The
2845 except rated 305 watts. If the application total number of strips required is 12,000W Suggested Allowable Watt Densities
requires the use of tubular elements whose 600W = 20 strips. Order strips similar to for Liquids
overall length is not standard, each element OT-2507 in size but rated 600 watts. To avoid
rating would be determined as follows: a special order, consider using 24 standard Max. Max.
OT-2405, 500 watt strips. These heaters would Material Temp (F) W/In2
Heater Watts = (A - 2CE) (Area x 9.5W) have a watt density of: Acid solutions 180 40
Alkaline solutions (Oakite) 212 40
Where: 500W ([23-3/4 - 4] x 3.45) = 7.35 W/in2 Asphalt, tar, and other 200 10
A = Sheath length, overall heavy or highly viscous 300 8
CE = Cold pin length If the application uses 3 phase power, the total compounds 400 7
Area = Effective heated area (in2/in.) element count should be a multiple of 3 to 500 6
9.5 = recommended W/in2 from G-175S permit a balanced electrical load. Bunker C fuel oil 160 10
Caustic soda 2% 210 45
The nomograph below may also be used for 10% 210 25
Selecting Clamp-On Strips Heaters heater selection in clamp-on strip heating 75% 180 15
applications. Dowtherm A 750 23
Graph G-130S provides a method of estimat- Dowtherm A vaporizing 750 10
ing the maximum allowable watt density for
strip heaters for clamp on applications based
Strip Heater Nomograph Dowtherm J liquid 575 23
Electroplating tanks 180 40
on sheath operating temperature and various Strip Heater Selection Ethylene glycol 300 30
ambients. Clamp-on Applications
1000 20 Freon 300 3
900 18 Fuel oil pre-heating 180 9
Graph G-130S Clamp-On Strip 800 16 Gasoline, kerosene 300 20-23
Max. Work Temp. (F)
Heaters 700 14 Machine oil, SAE 30 250 18-20
22 600 12 Metal melting pot 500-900 20-27
Max. W/In2
Chrome Steel Mineral oil 200 20-23
20 500 Sheath 10
1200F 400 16
Sh
18 400 8 Molasses 100 4-5
ea
th
1100F 300 6 Molten salt bath 800-950 25-30
16 Rust-Resistant
Te
200
mp
Iron Sheath 4 Molten tin 600 20-23
14
Allowable (W/In2)
era
1000F 100 2 Oil draw bath 400 20-23
tur
12 0 0 600 16
e
900F
10 A B C Steel cast into aluminum 500-750 50
800F Steel cast into iron 750-1000 55
8
700F Heat transfer oils (Therminol, 500-650 23
6 To Use the Graph Mobiltherm, etc.)
600F
4 Vapor degreasing solutions 275 20-23
500F 1. Select the maximum desired work Vegetable oil (fry kettle) 400 20-30
2 temperature on A. Water (process) 212 40-75
0 2. Choose either chrome steel or rust- Water (washroom) 140 75-100
200 400 600 800 1000 1200 resistant iron sheath (points B) on the
Note The above watt densities are
Estimated Ambient Temp. (F) basis of operating temperatures.
based on non-circulating liquids. The
(Sheath Temp. + Material Temp.) 3. Draw a straight line through points A and
B to C. C gives the maximum allowable allowable watt density may be adjusted
2
watts per square inch. when heat transfer or flow rates are
4. Select desired length heater with equiva- increased.
lent or less watt density.
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