Watlow Screw Plug Data sheet

W A T L O W

Tubular and
Process Assemblies

Screw Plug
Immersion Heaters

Screw plug immersion heaters are
ideal for direct immersion heating of
liquids, including all types of oils and
heat transfer solutions.

Available in a variety of stock and
made-to-order sizes, Watlow screw
plug immersion heaters feature both
WATROD round and FIREBAR®flat
tubular elements.

Heating elements are hairpin bent
and either welded or brazed into the
screw plug—depending on element
sheath and plug material
compatibility.

General purpose (NEMA 1) terminal
enclosures are standard; with
optional moisture resistant
(NEMA 4), explosion resistant
(NEMA 7) and explosion/moisture
resistant (NEMA 7/4) enclosures
available to meet specific
application needs.

Optional thermostats provide
convenient process temperature
regulation.

Performance Capabilities

• Watt densities to 120 W/in
(18.6 W/cm2)

• Wattages to 38kW

• UL®and CSA component
recognition to 480VÅ(ac) and
600VÅ(ac) respectively

• Incoloy®sheath temperatures to
1600°F (870°C)

• Passivated 316 stainless steel
sheath temperatures to 1200°F
(650°C)

• 304 stainless steel sheath
temperatures to 1200°F (650°C)

• Steel sheath temperatures to
750°F (400°C)

• Copper sheath temperatures to
350°F (175°C)

Incoloy®is a registered trademark of
Special Metals Corporation.

®

is a registered trademark of

UL
Underwriter's Laboratories, Inc.

2

Features and Benefits

• Screw plug and element sizes:

1" NPT 0.315" WATROD

1

1

⁄4" NPT 0.315" WATROD

1" FIREBAR

2" NPT 0.475" WATROD
21⁄2" NPT 0.475" WATROD

1" FIREBAR

• A variety of element sheath and
screw plug materials to meet
application needs.

• Integral thermowells provide
convenient temperature sensor
insertion and replacement without
draining the fluid being heated.

• Terminal enclosures can be
rotated to simplify connection with
existing conduits.

On stock chart units:

• Same day on most heaters

• 10 working days on special voltages
and/or wattages

• 15 working days on special element lengths

NPT
Pipe
Thread

Half
Coupling

Weld

WATROD Heating Element

FIREBAR
Heating
Element

• Welding or brazing WATROD
and FIREBAR elements to the
screw plug provides a pressure
tight seal.

• WATROD hairpins are
repressed (recompacted) to
maintain MgO density, dielectric
strength, heat transfer and life.

• 21⁄2" NPT screw plug assemblies
feature element support(s) to
help ensure proper spacing for
maximizing heater performance
and life.

• Phase capability:

1" NPT 1-Phase
11⁄4", 2", 21⁄2" NPT 1- or 3-Phase

• UL®and CSA component
recognition under file numbers

E52951 and 31388 respectively.
See pages 268-271 for details.

321

Screw Plug

Tubular and

View A-A

Thermowell

Element
Bundle

A

A

View A-A

Thermowell

Element
Bundle

A

A

View A-A

Thermowell

Element
Bundle

A

A

View A-A

Thermowell

Element
Bundle

A

A

View A-A

Thermowell

Element
Bundle

A

A

Process Assemblies

Screw Plug
Immersion Heaters

Screw Plug Orientation

Correct element/thermowell
orientation assures proper process
temperature sensing.

Correct horizontal mounting of
WATROD and FIREBAR screw plugs
is shown to the right. Correct
orientation assures optimum
performance and maximum heater
life. Additional mounting information
is provided in the Installation and
Maintenance Instructions.

Applications

• Water:
Deionized
Demineralized
Clean
Potable
Process

• Industrial water rinse tanks

• Vapor degreasers

• Hydraulic oil, crude, asphalt

• Lubricating oils at API specified
watt densities

• Air and gas flow

• Caustic solutions

• Chemical baths

• Anti-freeze (glycol) solutions

• Paraffin

FIREBAR Heating Element

11⁄4" NPT–One Element 21⁄2" NPT–Three Elements

WATROD Heating Element

1" NPT–One Element

Options

Terminal Enclosures

General purpose (NEMA 1) terminal
enclosures, without thermostats, are
standard on all screw plug
immersion heaters. To meet specific
application requirements, Watlow
offers the following optional terminal
enclosures:

• General purpose (NEMA 1) with
single or double pole thermostat

11⁄4" & 2" NPT–Two Elements 2" & 21⁄2" NPT–Three Elements

• Moisture resistant (NEMA 4) or
corrosion resistant (NEMA 4X) —
available with optional single or
double pole thermostat

• Explosion resistant (NEMA 7)
class 1, groups C and D
explosion resistant—available
with optional single or double pole
thermostat. For class 1, group B
enclosures, consult your Watlow
representative or refer to CSA
specifications on page 271.

• Explosion/moisture resistant

available with optional single or
double pole thermostat
Note: Unless otherwise stated on
the accompanying illustrations,
both WATROD and FIREBAR
screw plugs are centered on the
terminal enclosure. To order, add
the suffix letter(s) to the screw
plug heater’s base code number.
This is depicted on the Stock and
Options ordering example on

page 336. Also, specify class and

group, if applicable.

(NEMA 7/4) combination—

322

W A T L O W

3

13

/

16

"

(97 mm)

3

/

4

"

Conduit Opening

4

5

/

8

"

(117 mm

)

4

"

(100 mm)

3

/

4

"

Conduit Opening

5

1

/

2

"

(140 mm)

4

1

/2

"

(114 mm)

2

5

/8

"

(67 mm)

1

5

/16

"

(33 mm)

1

7

/16

"

(37 mm)

2

7

/8

"

(73 mm) Ref.

(1) 1.093

"

(28 mm)

Dia. Hole

2

"

(51 mm)

2

"

(50 mm)

3

/4

"
Conduit
Opening

4

"

(100 mm)

13

/16

"

(21 mm)

6

9

/

16

"

6

9

/

16

"

(167 mm)

4

1

/

4

"

(108 mm)

6

"

(152 mm)

3

13

/16

"

(97 mm)

4

3

/8

"

(111 mm)

1

3

/8

"

(35 mm)

2

1

/2

"
(64 mm)
Dia.

6

1

/4

"

(159 mm)

5

3

/4

"

(146 mm)

1

1

/2

"

Conduit Opening

2

9

/16

"

(65 mm)

21

/32

"

(17 mm)

3

1

/16

"

(78 mm)

3

3

/4

"

(95 mm)

3

/4

"

Conduit Opening

Tubular and
Process Assemblies

Screw Plug
Immersion Heaters

Options

Continued

General Pupose (NEMA 1)

Single Pole Thermostat

All screw plug sizes

Double Pole Thermostat

All screw plug sizes

Caution:

Explosion-resistant terminal
enclosures are intended to provide
explosion containment in
the electrical termination/wiring
enclosure only. No portion of the
assembly outside of this enclosure
is covered under this NEMA rating.
NEMA rating effectiveness may
be compromised by abuse or
misapplication.

Moisture Resistant NEMA 4

Without Thermostat

All screw plug sizes

Single Pole Thermostat

1" & 11⁄4 " NPT–1 WATROD Element

Single or Double Pole Thermostat

11⁄4 " NPT–2 WATROD Elements
11⁄4 " NPT–1 FIREBAR Element
All 2" & 2 1⁄2 " NPT screw plugs

Note: The thermostat is not centered on the
WATROD screw plug immersion heater.

Note: The thermostat is not centered on the
screw plug immersion heater.

Explosion/Moisture Resistant (NEMA 7 or 7/4)

Without Thermostat

All WATROD screw plugs

Single or Double Pole Thermostat

11⁄4 " NPT–1 FIREBAR Element
All WATROD screw plugs

Screw Plug

➀

➀ All NEMA 7/4 rated terminal enclosures supplied with a gasket for the cover.

323

Tubular and

3

1

/8

"

(79 mm)

3

5

/16

"

(84 mm)

3

1

/2

"
(89 mm)
O.D.

5

"

(127 mm)

4

1

/2

"

(114 mm)

1

5

/32

"

(29 mm)

1

1

/4

"

Conduit Opening

2

9

/16

"

(65 mm)

3

1

/16

"

(77 mm)

21

/32

"

(17 mm)

2

1

/2

"
(64 mm)
O.D.

4

3

/4

"

(121 mm)

3

3

/4

"

(95 mm)

3

/4

" Conduit

Opening

3

13

/16

"

(97 mm)

4

3

/8

"

(111 mm)

3

1

/2

"
(89 mm)
O.D.

6

3

/8

"

(162 mm)

5

3

/4

"

(146 mm)

1

3

/8

"

(35 mm)

1

1

/2

"

Conduit Opening

Process Assemblies

Screw Plug
Immersion Heaters

Options

Continued

Explosion/Moisture Resistant (NEMA 7 or 7/4)

Without Thermostat

11⁄4 " NPT––1 FIREBAR element

➀

All NEMA 7/4 rated terminal enclosures supplied with a gasket for the cover.

Without Thermostat

21⁄2 " NPT–3 FIREBAR elements

➀

CSA Certified Enclosures

CSA certified moisture and/or
explosion resistant terminal
enclosures protect wiring in
hazardous gas environments. These
terminal enclosures, covered under

CSA file number 61707, are
available on all WATROD and
FIREBAR screw plug immersion
heaters. For additional information,
consult your Watlow representative.

Single or Double Pole Thermostat

21⁄2 " NPT–3 FIREBAR elements

To order, specify CSA certified
enclosure, process temperature

(°F), maximum working pressure of
application (psig), media being
heated and heater mounting
orientation (horizontal or vertical)
and screw plug size.

Pilot Light

The optional pilot light gives the
operator visual indication of heater
on or off power status.

The PL10 pilot light is configured to a
maximum 250VÅ(ac), and supplied
with six inch (150 mm) leads.

Thermostats

To provide process temperature
control, Watlow offers optional single
pole, single throw (SPST) and double
pole, single throw (DPST)
thermostats.

324

The PL11 pilot light is rated for
480VÅ(ac), and supplied with four
inch (100 mm) leads.

Pilot lights may be attached to either
single or double pole thermostats
with general purpose (NEMA 1)
enclosure only. For moisture or

Unless otherwise specified,
thermostats are mounted inside the
terminal enclosure. For details and
ordering information, refer to
Thermostats on pages 423 to 425.

explosion resistant terminal
enclosures (NEMA 4 or NEMA 7),
consult factory.

To order, refer to the Build-a-Code
chart on page 336.

Please verify that the thermostat’s
sensing bulb O.D. is compatible with
the screw plug’s thermowell I.D.

W A T L O W

Tubular and
Process Assemblies

Screw Plug
Immersion Heaters

Options

Continued

Thermocouples

Type J or K thermocouples offer
extremely accurate sensing of
process and/or sheath temperatures.
A thermocouple may be inserted into
the thermowell or attached to the
heater’s sheath.

Thermocouples are supplied with
120 inch (305 mm) leads (longer
lead lengths available). Unless
otherwise specified, thermocouples
are supplied with temperature
ranges detailed on the
Thermocouple Types chart.

Using a thermocouple requires an
appropriate temperature and power
control. These must be purchased

separately. Watlow offers a wide
variety of temperature and power
controls to meet virtually all
applications. Temperature controls
can be configured to accept
process variable inputs, too. Consult
your Watlow representative for
details.

To order, specify Type J or K
thermocouple and lead length.

Indicate if the thermocouple is for
process temperature sensing or
heater sheath high-limit protection.
Please specify if the screw plug will
be mounted vertical or horizontal in
the tank. If vertical, indicate if the
housing is on top or bottom.

If the screw plug heater is mounted
in an in-line circulation heating
application, indicate flow direction
relative to the heater’s enclosure.

Thermocouple Types

Recommended

ASTM Conductor Characteristics Temperature Range

Type Positive Negative °F (°C)

J Iron Constantan 0 to 1000 (-20 to 540)

(Magnetic) (Non-Magnetic)

K Chromel

(non-magnetic) (Magnetic)

➀

Type J and Type K thermocouples are rated 32 to 1382°F and 32 to 2282°F (0-750°C and
0-1250°C), respectively. Watlow does not recommend exceeding temperature ranges shown
on this chart for the tubular product line.

®

Alumel

®

0 to 2000 (-20 to 1100)

➀

Wattages and Voltages

Watlow routinely supplies screw plug
immersion heaters with 120 to
480VÅ(ac) as well as wattages from

Sheath Materials

The following sheath materials are
available on WATROD and FIREBAR
heating elements:

External Finishing

Passivation

During the manufacturing process,
particles of iron or tool steel may
become embedded in the stainless
steel or alloy sheath. If not removed,
these particles may corrode,

250 watts to 38kW. If required,
Watlow will configure heaters with
voltages and wattages outside these

special voltage and wattage
configurations, consult your Watlow
representative.

parameters. For more information on

Standard Sheath Materials Made-to-Order Sheath Materials

WATROD

FIREBAR

Incoloy
316 stainless steel
Steel
Copper

®

Incoloy

WATROD

FIREBAR

Exotic Sheath Materials

304 stainless steel

®

Monel
304 stainless steel

®

Consult your Watlow representative
for details and availability.

Other Finishes

produce rust spots and/or
contaminate the process. For critical
applications, passivation will remove
free iron from the sheath. To order,

Simple belt polishing and glass
beading are available to meet
cosmetic demands. Consult factory
for details.

specify passivation.

Screw Plug

Alumel®and Chromel®are registered trade­marks of the Hoskins Manufacturing Company.

Monel®is a registered trademark of
Special Metals Corporation.

325

Tubular and

1" (25 mm)
Minimum No-Heat Section

Process Assemblies

Screw Plug
Immersion Heaters

Options

Continued

Screw Plug Materials

The following screw plug materials
are available:

To order, specify screw plug size
and material.

Screw Plug Sizes

Including European

1

• NPT–1, 1

• Gas–G11⁄4, G11⁄2, G 2 inch
(brass only)

⁄4, 2, 21⁄2 inch

Standard Screw Plug Materials Made-to-Order Plug Materials

WATROD

FIREBAR

• BSP–1

1

⁄2

inch

304 stainless steel
316 stainless steel
Steel
Brass
304 stainless steel

(stainless steel only)

Consult factory for sizes and
materials not listed.

For both WATROD and FIREBAR,
consult factory about details and
availability.

To order, specify size, style (NPT,
Gas or BSP) and material.

BSP = British Standard Pipe
Gas = Gas pipe standard

Screw Plug to Flange Adaptors

Screw plug to flange adaptors permit
replacing flange heaters with screw
plug heaters. To order, specify the
appropriate code number.

Application Hints

• Select the recommended sheath
material and watt density for the
substance being heated. Use the

Supplemental Applications
Chart on pages 263 to 266. If

unable to determine the correct
heater material and type, consult
your Watlow representative.

• Extend the element’s no-heat
section completely into the fluid
being heated to help prevent
premature heater failure. See
accompanying illustration for
proper no-heat section placement.

• Locate screw plug heater low in
the tank, but above the sludge
level.

Screw Plug to Flange Adaptors

Screw Plug Estimated

to Flange Shipping Wt. Code

Adaptor Sizes Material lbs (kg) Availability Number

1

⁄4 to 3”-150# Steel 13 (5.9) Stock 125X3SA

1

1

2

⁄2 to 3”-150# Steel 11 (5.0) Stock 250X3SA

1

⁄2 to 4”-150# Steel 16 (7.3) Stock 250X4SA

2
2 1⁄2 to 5”-150# Steel 25 (11.3) Stock 250X5SA
2 1⁄2 to 6”-150# Steel 33 (15.0) Stock 250X6SA

• Minimize problems associated
with low liquid level conditions by
using a low liquid level sensor or
sheath temperature high-limit
control.

• Periodically remove the screw
plug assembly for inspection and
clean the heating element(s). This
preventive maintenance will

• Choose a FIREBAR element when
your application requires a
smaller system package or lower
watt density.

• Ensure wiring integrity by making
sure terminal enclosure
temperature does not exceed
400°F (205°C).

• Keep electrical connections
clean, dry and tight.

reduce premature failure and
optimize heater performance.

• Refer to the Installation and
Maintenance Instructions for
correct orientation of FIREBAR
elements. Correct element
orientation to flow minimizes
pressure drop, increases
buoyancy force and heater
performance.

326