Watlow WATROD Duct Data sheet

W A T L O W

Tubular and
Process Assemblies

Duct Heaters

Constructed of sturdy 0.430 inch
(11 mm) diameter WATROD heating
elements mounted to a 1⁄4 inch
(6 mm) thick steel flange, duct
heaters are easily adapted to many
non-pressurized, air-heating
systems.

They are easily installed in
applications requiring a wide range
of temperature vs. air flow
combinations.

Watlow duct heaters offer
advantages over gas or oil fired and
open coil electric units with:

• Installation flexibility—
no flues or fuel lines.

• 100 percent energy efficient—
no energy loss up the flue.

• Universal availability of electricity.

• Resistance coil in Incoloy
sheath is protected from corro­sive environments.

Performance Capabilities

• Watt densities to 40 W/in
(6.2 W/cm2)

• Recommended process
temperatures from -20 to 1200°F
(-7 to 650°C)

• Wattages to 2.2 megawatts

• Voltages to 600VÅ(ac)

Features and Benefits

• Long life Incoloy®sheath

resists corrosion/oxidation while
protecting resistance coils
against contamination.

• MgO insulation filled elements,

compacted to rock hard density
maximize dielectric strength, heat
transfer and life.

• Field replaceable heating

elements permit easy service
and reduce downtime. Element
change-out is made simple by a
single screw clamp.

Incoloy®is a registered trademark of
Special Metals Corporation.

®

2

1

⁄2 inches (90 mm) thick

• 3
mineral insulation keeps wiring

cooler and reduces heat loss.

• Vented general purpose

(NEMA 1) terminal enclosure

ensures cooler terminations.

• A 1⁄4

inch (6 mm) inside

diameter thermowell accepts

an optional Type J or K
thermocouple for accurate
sheath temperature sensing.

• Rigid stainless steel suppor ts
prevent element sagging or
deformation in various mounting
positions.

1

• A

⁄4 inch (6 mm) thick steel

flange, with 3⁄8 inch (10 mm)

diameter mounting holes, easily
bolts to the duct wall.

®

is a registered trademark of

UL
Underwriter's Laboratories, Inc.

On stock chart units:

• Three to five working days on most heaters

• 10 working days on special voltages and/or
wattages

• 15 working days on special element lengths

Bussing

Single
Screw
Clamp

Threaded
Hole

Mineral

Insulation

• WATROD hairpins are
repressed (recompacted) after

bending to assure MgO density
that eliminates hot spots and
electrical insulation voids.

• Stock heaters feature 6, 12, 18,

24, 30, 36, 42, 48, 54, and 60
elements to meet a wide variety

of kW demands.

• One or three phase voltages to
meet local power supplies.

• Maximum 48 amps per circuit
complies with National Electrical
Code (NEC).

• Duct heaters with general

purpose enclosures meet UL
and CSA component recogni­tion to 480 and 600VÅ(ac)

maximum respectively—UL
and CSA file numbers are
E52951 and 31388.

Duct Heaters

®

®

411

Tubular and
Process Assemblies

Duct Heaters

Applications

• Dr ying ovens

• Autoclaves

• Furnaces

• Load banks

• Heat treating

• Reheating

•HVAC

• Paint dr ying

Choosing a Duct Heater

The following English and metric
graphs, shown on pages 413 to

414, will help you to select the

correct duct heater. These graphs
include: Watt Density vs. Air

Temperature/Velocity, Watt Density
vs. Sheath Temperature and
Pressure Drop vs. Air Velocity.

These graphs, with the quick
formulas on this page, along with
information specific to your
application, will determine the
correct duct heater specifications.
However, if engineering assistance
is needed, contact your Watlow
representative.

Required Application Information

• Desired outlet air temperature

• Inlet air temperature

• Delta T—the temperature
difference between inlet and
desired outlet temperature

• Air volume (CFM/CMM)
measured at both inlet
temperature and pressure

• Air velocity in feet per minute
(FPM); meters per minute (MPM)
which equals:

• Minimum duct heater wattage
(kW). This can be determined by:

English

CFM measured at standard conditions

FPM =

Duct cross section area at heater in ft

Metric

MPM =

kW =

kW =

CMM measured at normal conditions

Duct cross section area at heater in m

English

CFM x Delta T (°F) x 1.1(safety factor)

3000

Metric

CMM x Delta T (°C) x 1.1(safety factor)

48

2

2

412

Note: The duct heater, or
combination of duct heaters, used
for the process should be equal to
or exceed the minimum wattage
calculation.

W A T L O W

WATROD Duct Heater

Watt Density vs. Air Temperature/Velocity

Process Temperature—°F

Air Velocity—FPM

100

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

1400

1500

1600

200 300 400 500 600 700 800 900 1000 1100 1200

40 W/in230 W/in

2

23 W/in

2

20

W/in

2

16 W/in

2

8 W

/in

2

Catalog Duct Heaters

Catalog Duct Heaters

Estimated Sheath Temperature = 1400°F

Process Temperature—°C

Air Velocity—MPM

100

500

Estimated Sheath Temperature = 760°C

WATROD Duct Heater

Watt Density vs. Air Temperature/Velocity

400

300

200

100

200 300 400 500 600 700

4.65 W

/cm

2

3.5 W/cm

2

3.1 W/cm

2

2.5 W/cm

2

1.25 W

/cm

2

Catalog Duct Heaters

6.2 W

/cm

2

Catalog Duct Heaters

Sheath Temperature—°F

Watt Density—W/in

2

100

4

WATROD Duct Heater

Watt Density vs. Sheath Temperature

8

12

16

20

24

28

32

36

40

44

48

52

56

60

0

200 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

Inlet Temperature at
75 Degrees Fahrenheit

1200 FPM

760 FPM

680 FPM

300 FPM

60 FPM or Natural Convection

1550 FPM

Sheath Temperature—°C

100

WATROD Duct Heater

Watt Density vs. Sheath Temperature

Watt Density—W/cm

2

5

4

3

2

1

6

7

8

9

10

200 300 400 500 600 700 800 900 1000

365 MPM

235 MPM

180 MPM

90 MPM

18 MPM or Natural Convection

475 MPM

Inlet Temperature at
25 Degrees Celsius

Tubular and
Process Assemblies

Duct Heaters

Watt Density vs. Air
Temperature/Velocity

To decide watt density
requirements, first determine the
desired outlet air temperature and
velocity in feet per minute. Then

follow the lines on the graph for
velocity and process temperature to
the watt density curve’s intersecting
point. This shows the recommended
watt density based on a maximum

sheath temperature of 1400°F
(760°C). For longer heater life,

lower watt densities should be
chosen.

Watt Density vs. Sheath
Temperature

The Watt Density vs. Sheath
Temperature graph shows the air

velocity (FPM or MPM) required to
operate a WATROD duct heater at

specific watt densities or sheath
temperatures. Also depicted is the
appropriate watt density vs. sheath
temperature at a specified air flow.

Duct Heaters

413