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Duct Heaters: Standard and Finned Tubular

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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. Forced 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.

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Typical Applications:

Air Drying/Curing Operations
Annealing
Autoclaves
Booster Air Heater

Breaking Resistor
Core Drying
Dehumidification
Forced Air Comfort Heating

Heat Treating
Make-Up Air Heating
Re-Heating
Resistor Load Banks

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 = CFX ×	P	×	530
	14.7		T + 460

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:

KW =	SCFM × Temperature Rise (°F)
	3000

KW =	SCMM × Temperature Rise (°C)
	47

For compressed air use equations:

KW =	CFM* × Density* (lbs/cu. ft.) × Temperature Rise (°F)
	228
*At heater inlet temperature and pressure

KW =	CMM* × Density* (kgs/cu. m) × Temperature Rise (°C)
	57.5
*At heater inlet temperature and pressure

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