Tempco Custom Heater Manufacturing

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Duct Heaters: Standard & Finned Tubular

Heavy wall Incoloy® tubular heating elements (field replaceable) provide protection against corrosive air environments and resistance to vibration when compared to open coil elements. Air duct heaters can be designed specifically for high pressure and/or hazardous locations. Turnkey systems including the duct heater, power and temperature control panel, and the temperature and over-temperature sensors can also be provided.

Design Features: Standard Tubular Duct Heaters

  • NEMA 1 General Purpose Ventilated Enclosure
  • Painted Steel Mounting Flange
  • Single- and Three-Phase Wiring
  • 3-1/2" (89 mm) Insulation
  • Field Replaceable Incoloy® 840 Elements
  • Element Bends Re-pressed
  • 1/4" (6 mm) Inside Diameter Thermowell
  • Stainless Steel Support Plate and Corner Posts

Design Features: Finned Tubular Duct Heaters

  • NEMA 1 General Purpose Ventilated Enclosure
  • Stainless Steel Mounting Flange and Terminal Box
  • Single- and Three-Phase Wiring
  • 1" (25 mm) Insulation
  • Field Replaceable .430 Diameter Stainless Steel Elements
  • 9/32" (7 mm) ID Sensor Thermowell
  • Stainless Steel Support Plate and Corner Posts
  • Stainless Steel Insulation Housing

 

Duct Heater Cut out

Compression Fittings

Bulkhead Fittings

A

NEMA 1 terminal box enclosure with vented cover to help keep wiring cooler. Optional enclosures: NEMA 4 (moisture resistant), NEMA 7 (explosion resistant)  and NEMA 12 (dust resistant).

B

3-1/2 inches (89 mm) of mineral insulation in a stainless steel enclosure below the mounting flange, minimizes heat losses while keeping the electical wiring cooler.

C

The heavy duty frame is composed of a 1/4 inch (6 mm) thick steel mounting flange, stainless steel support plate and corner posts to securely hold the heating elements rigid in any mounting position.

D

Standard field replaceable elements are held in place with single-screw quick-release “V” clamps. Pressure resistant designs utilizing welded elements,  bulkhead fittings, or compression fittings to attach elements to the flange are available to limit leakage of ducted air or gases into the terminal enclosure. Welded elements are used for gas tight applications.

E

A 9/32" (7 mm) inside diameter thermowell accessed through a 1/8" NPT tapped hole in the flange allows installation of an optional Type J or K thermocouple  for sensing temperature within the element bundle. It can be clamped directly to an element for use as a high limit providing a faster response. An excellent safeguard for your system.

F

The .430" (11 mm) diameter elements are silicone resin sealed. High temperature tubular duct heaters utilize Incoloy®  sheath material for excellent high temperature scaling and corrosion resistance. The medium temperature finned duct heaters have stainless steel fins on a corrosion resistant stainless steel sheath. High temperature Incoloy®  elements have all bends repressed in special dies to recompact the MgO refractory to eliminate any electrical insulation voids and hot spots.

Sizing a Duct Heater

To properly match a duct heater to an application, the wattage, air velocity and element watt density must be determined.

Formulas and graphs that will aid in your design include:

  • Wattage calculation formulas and table
  • Element Watt Density vs. Sheath Temperature and Air Velocity Graph
  • Pressure Drop vs. Air Velocity Graph

In most applications the following design limitations should be adhered to:

  • Maximum watt density of 40 watts/in2  (6.2 watts/cm2 )
  • Maximum element sheath temperature of 1400°F (760°C)
  • Minimum air velocity of 200 feet per minute (61 meters per minute)
  • Maximum voltage for UL certified heaters is 480V.
  • Maximum voltage for CSA certified heaters is 600V.

Calculating Minimum Wattage Requirement

 This table is for quick-estimation purposes and is based on air under standard conditions (70°F inlet air temperature at 14.7 PSIA). 

If flow is given in CFM at operating temperature and pressure it can be converted to SCFM (Standard Cubic Feet per Minute) with the following formula (use the equations to the right for compressed air):

SCFM equation
P = operating pressure (gauge pressure + 14.7)
T = operating temperature

Note: Remember when calculating wattage to use the maximum anticipated air flow and to compensate for any heat losses.

For free air use equations:

Free Air KW equation (English)

Free Air KW equation (Metric)

 

For compressed air use equations:

Compressed Air KW equation (English)

Compressed Air KW equation (English)

 

KWH to Heat Air at Selected Flow Rates


Amt. of Air
CFM
Temperature Rise (°F)
50 100 150 200 250 300 350 400 450 500 600
Kilowatt Hours to Heat Air
100 1.7 3.3 5 6.7 8.3 10 11.7 13.3 15 16.7 20
200 3.3 6.7 10 13.3 16.7 20 23.3 26.7 30 33.3 40
300 5.0 10 15 20 25.0 30 35.0 40.0 45 50.0 60
400 6.7 13.3 20 26.7 33.3 40 46.7 53.3 60 66.7 80
500 8.3 16.7 25 33.3 41.7 50 58.3 66.7 75 83.3 100
600 10 20 30 40 50.0 60 70.0 80.0 90 100 120
700 11.7 23.3 35 46.7 58.3 70 81.7 93.3 105 116.7 140
800 13.3 26.7 40 53.3 66.7 80 93.3 106.7 120 133.3 160
900 15 30 45 60 75.0 90 105 120 135 150 180
1000 16.7 33.3 50 66.7 83.3 100 116.7 133.3 150 166.7 200
1100 18.3 36.7 55 73.3 91.7 110 128.3 146.7 165 183.3 220
1200 20 40 60 80 100 120 140 160 180 200 240

 

Note: For additional information or help with your application please contact TEMPCO.

Element Watt Density vs. Air Temperature and Air Velocity

Use graph (English or Metric) to plot Outlet Air Temperature vs. Outlet Air Velocity to determine Element Watt Density.

The recommended watt density is based on a maximum element sheath temperature of 1400°F (760°C). Air and other gases that are poor conductors of heat require watt densities matched to the velocity of the gas flow to prevent element overheating. Selecting a lower watt density for the heating elements will extend heater life expectancy.

Element Watt Density is the wattage dissipated per square inch of the element sheath surface and is calculated with the following formula.

Watt Density equation

Air Velocity (feet per minute)

English

Air Velocity Graph (English)
Process Temperature °F – Approximate Sheath Temperature 1400°F

Air Velocity (meters per minute)

Metric

Air Velocity Graph (Metric

Process Temperature °C – Approximate Sheath Temperature 760°C

Element Watt Density vs. Sheath Temperature and Air Velocity

Use graph (English or Metric) to plot Watt Density vs. Air Velocity to determine Sheath Temperature.

Use graph (English or Metric) to plot Watt Density vs. Sheath Temperature to determine the required Air Velocity.
 

Watt Density (W/in2)

English

 

Watt Density equation (Metric)
Sheath Temperature (°F)

Watt Density (W/cm2)

Metric

 

Watt Density equation (Metric)
Sheath Temperature (°C)

Pressure Drop vs. Air Velocity

Use graph (English or Metric) to plot Pressure Drop vs. Air Velocity for standard duct heater sizes used to properly Size Blowers.
 

Air Velocity equation

Air Velocity (feet per minute)

English

 

Pressure Drop vs. Air Velocity Graph (English)
Approximate Pressure Drop (inches of water)

Air Velocity (meters per minute)

Metric

 

 

Pressure Drop vs. Air Velocity Graph (Metric)

Approximate Pressure Drop (Kilopascals)

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