Watlow FIREROD Application Guide

Electric Heaters

7

W A T L O W

Application Guide

Electric Heaters

Product Overview

This section of the Application Guide
is devoted to electric heaters; their
different types, methods of use and
general calculations for determining
specifications. If you’re unable to find
or determine which type of Watlow
heater will best suit your needs, call
your nearest Watlow sales represent­ative. Sales offices are listed on the
back cover of this Application Guide.

Heaters

Band and Nozzle Heaters

Led by the high performance MI Band heater, the patented, flexible
THINBAND®heater and the standard mica band heater for specialized
constructions, Watlow’s band and nozzle heaters are ideal for every type of
plastic processing equipment.

Sheath materials available include stainless steel with mica insulation, stainless
steel with mineral insulation and aluminized or zinc steel with mica insulation.

Performance Capabilities

• Maximum operating temperatures
to 760°C (1400°F)

• Typical maximum watt densities from

8.5 W/cm2(55 W/in2) to

35.7 W/cm2(230 W/in2)

Applications

• Extruders

• Blown film dies

• Injection molding machines

• Other cylinder heating applications

Cable Heaters

The versatile Watlow cable heater can be formed to a variety of shapes as
dictated by its many applications. These small diameter, high performance units
are fully annealed and readily bent to your desired configuration.

Sheath materials available include Inconel®and stainless steel.

Performance Capabilities

• Typical maximum watt densities to

4.6 W/cm2(30 W/in2)

• Maximum operating temperatures
to 650°C (1200°F)

Applications

• Plastic injection molding nozzles

• Semiconductor manufacturing and
wafer processing

• Hot metal forming dies and punches

• Sealing and cutting bars

• Restaurant and food processing
equipment

• Cast-in heaters

• Laminating and printing presses

• Air heating

• Heating in a vacuum environment

• Textile manufacturing

Inconel®is a registered trademark of the
Special Metals Corporation.

8

Application Guide

Electric Heaters

Product Overview

Continued

Cartridge Heaters

The Watlow FIREROD

®

heater enters its 50th year of industry leading expertise
as the premier choice in swaged cartridge heating. With premium materials
and tight manufacturing controls, the FIREROD heater continues to provide
superior heat transfer, uniform temperatures and resistance to oxidation and
corrosion in demanding applications and high temperatures.

Sheath materials available are Incoloy®and stainless steel.

Performance Capabilities

• Typical maximum watt densities up
to 62 W/cm2(400 W/in2)

• Maximum operating temperatures
to 760°C (1400°F)

Applications

• Molds • Life sciences

• Dies • Aerospace

• Platens • Semiconductor

• Hot plates • Foodservice

• Sealings

• Fluid heating

Cast-in Heaters

When Watlow creates a custom-engineered cast-in product, the result is more
than just a heater. It’s a “heated part” that becomes a functional component of
your equipment, designed in the exact shape and size you need. The IFC
heated part consists of a Watlow heater element built into custom metal shapes
designed specifically for your application.

Sheath materials available are 319 and 356 aluminum, pure aluminum and
IFC (stainless, nickel, Inconel®, aluminum, copper and bronze).

Performance Capabilities

• Typical maximum watt densities to

15.5 W/cm2(100 W/in2)

• Maximum operating temperatures
to 400°C (752°F) to 760°C (1400°F)
depending on material

Applications

• Semiconductor manufacturing

• Foodservice equipment

• Plastics processing

• Medical equipment

• Hot glue melt

• Circulation heating

Incoloy®and Inconel®are registered
trademarks of the Special Metals Corporation.

equipment

Electric Heaters

9

W A T L O W

Application Guide

Electric Heaters

Product Overview

Continued

Circulation and Process Heaters

Watlow’s circulation heaters are compact heating solutions for fluids such as
purified and inert gases, supercritical fluids and liquids like de-ionized water
for use in semiconductor and electronics industries as well as for general liquid
and gas heating applications. Watlow’s industrial process heater lines of
immersion, circulation and duct heaters are used to heat a myriad of high and
low viscosity fluids ranging from de-ionized and process water, oils, solvents,
rinse agents, caustic solutions, etc. to process gases like air, nitrogen, purified
and inert gases as well.

Applications

• Oil and gas field equipment

• Refineries & petrochemical plants

• Chemical and industrial gas plants

• HVAC duct heating

• Open tanks and heat treat baths

• Textile drying

• Heat transfer and lube oil systems

• Semiconductor processing equipment

• Precision cleaning equipment

• Power generation systems

• Emissions control systems

• Supercritical fluid heating

• In-line water boilers

Ceramic Fiber Heaters

Ceramic fiber heaters integrate a high temperature iron-chrome-aluminum
(ICA) heating element wire with ceramic fiber insulation. Numerous stock,
standard and/or custom shapes can be provided, achieving the “heated
insulation” concept for your high temperature, non-contact applications. The
ceramic fiber insulation isolates the high temperatures inside the heated
chamber from the outside. The heaters are low mass, fast heating, with high
insulation values and the self-supported heating elements that offer some of
the highest temperature heating capabilities within the Watlow family of heater
designs.

The sheath material available is molded ceramic fiber.

Performance Capabilities

• Typical maximum watt densities to

1.8 W/cm

2

(11.5 W/in2)

• Maximum operating temperatures
to 1205°C (2200°F)

Applications

• High temperature furnaces

• Metal melting, holding and transfer

• Semiconductor processing

• Glass, ceramic and wire processing

• Analytical instrumentation

• Fluidized beds

• Laboratory and R&D

• Other high temperature process
applications

10

Multicell Heaters

The multicell heater from Watlow offers independent zone control for precise
temperature uniformity, loose fit design for easy insertion in and removal from
the equipment and extreme process temperature capability. The heaters are
available with up to eight independently controllable zones and one to three
internal thermowells for removable sensors. Custom assemblies are available.

Incoloy®sheath material is available.

Performance Capabilities

• Typical maximum watt densities to

6.2 W/cm2(40 W/in2)

• Maximum operating temperatures
to 1230°C (2250°F)

Applications

• Super plastic forming and diffusion
bonding

• Hot forging dies

• Heated platens

• Furnace applications

• Superheating of air and other gases

• Fluidized beds for heat treating

• Glass forming, bending and tempering

• Long heater needs (1219 cm (40 foot))

• Soil remediation

• Aluminum processing

Application Guide

Electric Heaters

Product Overview

Continued

Flexible Heaters

Flexible heaters from Watlow are just what the name implies: thin, bendable and
shaped to fit your equipment. You can use your imagination to apply heat to the
most complex shapes and geometries conceivable without sacrificing efficiency
or dependability.

Sheath materials available include silicone rubber, Kapton®, HT foil and neoprene.

Performance Capabilities

• Typical maximum watt densities from

1.7 W/cm2(11 W/in2) to 17.0 W/cm

2

(110 W/in2)

• Maximum operating temperatures
to 595°C (1100°F)

Applications

• Medical equipment such as blood

analyzers, respiratory therapy units and
hydrotherapy baths

• Freeze protection for military hardware,

aircraft instrumentation and hydraulic
equipment

• Battery heating

• Foodservice equipment

• Factory bonding / subassemblies

• Any application requiring a flexible shape

or design

Kapton®is a registered trademark of
E.I. du Pont de Nemours & Company.

Electric Heaters

11

W A T L O W

Application Guide

Electric Heaters

Product Overview

Continued

Radiant Heaters

With Watlow’s diverse RAYMAX

®

heater line, we have a solution for almost any
application requiring radiant heat. Our capabilities cover a wide range of
needs, from contamination-resistant panel heaters to fast-responding quartz
tubes to rugged tubular elements and high temperature ceramic panels.

Incoloy®tubular, molded ceramic fiber, quartz tube and stainless steel emitter
strip sheath materials are available.

Performance Capabilities

• Typical maximum watt densities from

4.6 W/cm2(30 W/in2) to 7.0 W/cm

2

(45 W/in2)

• Maximum operating temperatures to
1095°C (2000°F)

Applications

• Thermoforming

• Food warming

• Paint and epoxy curing

• Heat treating

• High temperature furnaces

• Tempering and annealing processes

Polymer Heaters

For the latest in heating technology from Watlow, specify a heated plastic part
in your next product. Watlow’s heated plastic parts combine resistive heating
elements with a wide range of thermoplastic compounds to yield a part that is
both heater and structure. Watlow utilizes typical injection molding techniques
and patented resistive element construction methods to produce heated plastic
parts that are durable, safe and cost-effective.

Performance Capabilities

• Typical maximum open watt densities
from 0.08 W/cm2(0.5 W/in2) to

0.59 W/cm2(3.8 W/in2)

• Typical maximum immersion watt
densities from 0.62 W/cm2(4.0 W/in2)
to 9.30 W/cm2(60 W/in2)

• Maximum operating temperatures to
220°C (428°F)

Applications

• Medical • Battery heating

• Analytical • Foodservice

• Aerospace • Transportation

• Freeze protection • Semiconductor

• Any heated part application requiring a

flexible shape

Alcryn®is a registered trademark of
Ferro Corporation.

Santoprene®is a registered trademark of
Advanced Elastomer Systems.

12

Strip Heaters

Watlow’s mica and 375 strip heaters are the versatile solution for a number of
applications. They can be bolted or clamped to a solid surface for freeze and
moisture protection, food warming and other applications or utilized as a non­contact radiant heater. The 375 finned strip heaters are commonly used for air
heating, drying ovens and space heaters.

Performance Capabilities

• Typical maximum watt densities from

7.8 W/cm2(50 W/in2) to 15.5 W/cm

2

(100 W/in2)

• Maximum operating temperatures to
760°C (1400°F)

Applications

• Dies and molds

• Tank and platen heating

• Thermoforming

• Packaging and sealing equipment

• Ovens

• Food warming equipment

• Vulcanizing presses

• Duct, space and air heaters

• Incubators

• Autoclaves

• Freeze and moisture protection

Application Guide

Electric Heaters

Product Overview

Continued

Thick Film Heaters

Watlow layers thick film resistor and dielectric materials on quartz, stainless steel
and ceramic substrates to produce high performance industrial heaters. The
thick film heaters provide very fast temperature response and uniformity on a
low-profile heater. Thick film heaters are ideal for applications where space is
limited, where conventional heaters can’t be used, when heat output needs vary
across the surface, or in ultra-clean or aggressive chemical applications.

430 stainless steel (open air), 430 stainless
steel (immersion), aluminum nitride, quartz
(open air) and quartz (clamp-on) sheath
materials are available.

Performance Capabilities

• Typical maximum watt densities from
3 W/cm2(20 W/in2) to 27 W/cm

2

(175 W/in2)

• Maximum operating temperatures to
550°C (1022°F)

Applications

• Ultra pure aggressive chemicals

• Large panel processing

• Analytical equipment

• Foodservice equipment

• Packaging sealing equipment

• Life sciences sterilizers and GC/mass

spectroscopy

• Semiconductor wafer process equipment

• Plastics hot runners nozzles and

manifolds

Electric Heaters

13

W A T L O W

Tubular and Process Assemblies

Watlow’s WATROD tubular heater elements and flat FIREBAR elements are
designed primarily for direct immersion in liquids such as water, oils, solvents
and process solutions, molten materials as well as air and gases. By generating
all the heat within the liquid or process, these heaters are virtually 100 percent
energy efficient. These versatile heaters can also be formed and shaped into
various geometries for radiant heating and contact surface heating applications.
UL®and CSA component recognized elements available.

Applications

• Furnaces and ovens

• Molten salt baths

• Foodservice equipment

• Semiconductor equipment

• Die casting equipment

• Metal melt and holding

• Fluidized beds

• Boilers

• Radiant heating

• Process air heating

• Drying and warming

Application Guide

Electric Heaters

Product Overview

Continued

14

Application Guide

Electric Heaters

Most electrical heating problems
can be readily solved by determin­ing the heat required to do the job.
To do this, the heat requirement must
be converted to electrical power and
the most practical heater can then
be selected for the job. Whether the
problem is heating solids, liquids or
gases, the method, or approach, to
determining the power requirement
is the same. All heating problems
involve the following steps to their
solution:

Define the Heating Problem
Calculate Power Requirements

System Start-up and Operating Power
Requirement

System Maintenance Power
Requirements

Operating Heat Losses
Power Evaluation

Review System Application Factors

Safe/Permissible Watt Densities
Mechanical Considerations
Operating Environment Factors
Safety Factor
Heater Life Requirements
Electrical Lead Considerations

Defining the Problem

Your heating problem must be clear­ly stated, paying careful attention to
defining operating parameters. Gather
this application information:

• Minimum start and finish
temperatures expected

• Maximum flow rate of material(s)
being heated

• Required time for start-up heating
and process cycle times

• Weights and dimensions of both

heated material(s) and containing
vessel(s)

• Effects of insulation and its thermal
properties

• Electrical requirements—voltage

• Temperature sensing methods and
location(s)

• Temperature controller type

• Power controller type

• Electrical limitations

And since the thermal system you’re
designing may not take into account
all the possible or unforeseen heating
requirements, don’t forget a safety
factor. A safety factor increases
heater capacity beyond calculated
requirements. For details on safety
factor, please see “Safety Factor
Calculation” under the portion of this
section dealing with “Review of Heater
Application Factors,” on page 20.

Electric Heaters

15

W A T L O W

Short Method

Start-up watts = A + C + 2⁄3

L + Safety Factor

Operating watts = B + D + L + Safety Factor

Safety Factor is normally 10 percent to 35 percent based on application.
A = Watts required to raise the temperature of material and equipment

to the operating point, within the time desired

B = Watts required to raise temperature of the material during the

working cycle

Equation for A and B (Absorbed watts-raising temperature)

Specific heat

Weight of material (lbs)

•

of material • temperature rise (°F)

(Btu/lb • °F)

Start-up or cycle time (hrs) • 3.412

C = Watts required to melt or vaporize material during start-up period
D = Watts required to melt or vaporize material during working cycle

Equation for C and D (Absorbed watts-melting or vaporizing)

Weight of material (lbs) • heat of fusion or vaporization (Btu/lb)

Start-up or cycle time (hrs) • 3.412

L = Watts lost from surfaces by:

• Conduction-use equation below

• Radiation-use heat loss curves

• Convection-use heat loss curves

Equation for L (Lost conducted watts)

Thermal conductivity Temp. differential

of material or insulation Surface area to ambient

(Btu

•

in./ft

2

•

°F • hr) (ft2)(°F)

Thickness of material or insulation (in.) • 3.412

Application Guide

Electric Heaters

Power Calculations

Calculations for Required
Heat Energy

When performing your own calcu­lations, refer to the Reference Data
section (begins on page 127) for
values of materials covered by
these equations.

The total heat energy (kWH or Btu)
required to satisfy the system needs
will be either of the two values shown
below depending on which calculated
result is larger.

A. Heat Required for Start-Up
B. Heat Required to Maintain the

Desired Temperature

The power required (kW) will be the
heat energy value (kWH) divided by
the required start-up or working cycle
time. The kW rating of the heater will
be the greater of these values plus a
safety factor.

The calculation of start-up and oper­ating requirements consist of several
distinct parts that are best handled
separately. However, a short method
can also be used for a quick estimate
of heat energy required. Both methods
are defined and then evaluated using
the following formulas and methods:

16

Equation 1

QA or QB=w •C

p

•

∆T

3.412

QA= Heat Required to Raise Temperature of Materials During

Heat-Up (Wh)

QB= Heat Required to Raise Temperature of Materials Processed

in Working Cycle (Wh)
w = Weight of Material (lb)
Cp= Specific Heat of Material (Btu/Ib •°F)
∆T = Temperature Rise of Material (T

Final

- T

Initial

)(°F)

This equation should be applied to all materials absorbing heat in the
application. Heated media, work being processed, vessels, racks, belts,
and ventilation air should be included.

Example: How much heat energy is needed to change the temperature
of 50 lbs of copper from 10°F to 70°F?

Q= w •C

p

•

∆T

= (50 lbs) •(0.10 Btu/Ib •°F) •(60°F) = 88 (Wh)

3.412

Application Guide

Electric Heaters

Power Calculations—
Conduction and Convection
Heating

Equation 1—Absorbed Energy,
Heat Required to Raise the
Temperature of a Material

Because substances all heat differ­ently, different amounts of heat are
required in making a temperature
change. The specific heat capacity
of a substance is the quantity of heat
needed to raise the temperature of a
unit quantity of the substance by
one degree. Calling the amount of
heat added Q, which will cause a
change in temperature ∆T to a weight
of substance W, at a specific heat of
material Cp, then Q =w •C

p

•

∆T.

Since all calculations are in
watts, an additional conversion
of 3.412 Btu = 1 Wh is introduced
yielding:

Equation 2—Heat Required to
Melt or Vaporize a Material

In considering adding heat to a
substance, it is also necessary to
anticipate changes in state that might
occur during this heating such as
melting and vaporizing. The heat
needed to melt a material is known
as the latent heat of fusion and
represented by Hf. Another state
change is involved in vaporization
and condensation. The latent heat of
vaporization Hvof the substance is
the energy required to change a sub­stance from a liquid to a vapor. This
same amount of energy is released as
the vapor condenses back to a liquid.

Equation 2

QCor QD=w •H

f

OR

w

•

H

v

3.412 3.412

QC= Heat Required to Melt/Vaporize Materials During Heat-Up (Wh)
QD= Heat Required to Melt/Vaporize Materials Processed in

Working Cycle (Wh)
w = Weight of Material (lb)
Hf= Latent Heat of Fusion (Btu/Ib)
Hv= Latent Heat of Vaporization (Btu/lb)

Example: How much energy is required to melt 50 lbs of lead?
Q = w •H

f

Q = (50 lbs) •(9.8 Btu/Ib) = 144 (Wh)

3.412 Btu/(Wh)

Changing state (melting and vaporizing) is a constant temperature process.
The Cpvalue (from Equation 1) of a material also changes with a change in
state. Separate calculations are thus required using Equation 1 for the material
below and above the phase change temperature.

Electric Heaters

17

W A T L O W

Equation 3A—Heat Required to Replace Conduction Losses

Q

L1

=k •A • ∆T • t

e

3.412 • L

QL1= Conduction Heat Losses (Wh)
k = Thermal Conductivity

(Btu • in./ft

2

•

°F • hour)
A = Heat Transfer Surface Area (ft2)
L = Thickness of Material (in.)
∆T = Temperature Difference Across Material

(T2-T1) °F

te = Exposure Time (hr)

This expression can be used to calculate losses through insulated walls of
containers or other plane surfaces where the temperature of both surfaces
can be determined or estimated. Tabulated values of thermal conductivity are
included in the Reference Data section (begins on page 134).

Convection Heat Losses

Convection is a special case of
conduction. Convection is defined
as the transfer of heat from a high
temperature region in a gas or liquid
as a result of movement of the masses
of the fluid. The Reference Data
section (page 127) includes graphs
and charts showing natural and
forced convection losses under
various conditions.

Equation 3B—Convection Losses

Q

L2

= A • F

SL

•

C

F

QL2= Convection Heat Losses (Wh)
A = Surface Area (in2)
F

SL

= Vertical Surface Convection Loss Factor

(W/in2) Evaluated at Surface
Temperature (See Ref. 9, page 26)

CF = Surface Orientation Factor

Heated surface faces up horizontally = 1.29
Vertical = 1.00
Heated surface faces down horizontally = 0.63

Application Guide

Electric Heaters

Power Calculations

Continued

Conduction Heat Losses

Heat transfer by conduction is
the contact exchange of heat from
one body at a higher temperature to
another body at a lower temperature,
or between portions of the same body
at different temperatures.

Radiation Heat Losses

For the purposes of this section,
graphs are used to estimate radiation
losses. Charts in the Reference Data
section (page 127) give emissivity
values for various materials.
Radiation losses are not dependent
on orientation of the surface.
Emissivity is used to adjust for a
material’s ability to radiate heat
energy.

Equation 3C—Radiation Losses

Q

L3

=A • F

SL

•

e

QL3= Radiation Heat Losses (Wh)
A = Surface Area (in2)
FSL= Blackbody Radiation Loss Factor at Surface Temperature (W/in2)
e = Emissivity Correction Factor of Material Surface

Example:

Using Reference 139, page 155, we find that a blackbody radiator (perfect
radiator) at 500°F, has heat losses of 2.5 W/in2.
Polished aluminum, in contrast, (e = 0.09) only has heat losses of 0.22 W/in

2

at the same temperature (2.5 W/in

2

•

0.09 = 0.22 W/in2).

18

Application Guide

Electric Heaters

Power Calculations

Continued

Combined Convection
and Radiation Heat Losses

Some curves in Reference 139
(page 155) combine both radiation
and convection losses. This saves
you from having to use both Equations
3B and 3C. If only the convection
component is required, then the
radiation component must be
determined separately and subtracted
from the combined curve.

Total Heat Losses

The total conduction, convection and
radiation heat losses are summed
together to allow for all losses in the
power equations. Depending on the
application, heat losses may make up
only a small fraction of total power
required... or it may be the largest
portion of the total. Therefore, do not
ignore heat losses unless previous
experience tells you it’s alright to do.

Equation 3E—Total Losses

Q

L

= QL1+ QL2+ Q

L3

If convection and radiation losses are calculated
separately. (Surfaces are not uniformly insulated

and losses must be calculated separately.)
OR
QL= QL1+ Q

L4

If combined radiation and convection curves are used.

(Pipes, ducts, uniformly insulated bodies.)

Equation 3D—Combined Convection and Radiation Heat Losses

QL4= A •F

SL

QL4= Surface Heat Losses Combined Convection and Radiation (Wh)
A = Surface Area (in2)
FSL= Combined Surface Loss Factor at Surface Temperature (W/in2)

This equation assumes a constant surface temperature.

Equation 5—Operating Power (Watts)

Po=

QB+ Q

D

+(QL)•(1 + S.F.)

t

c

QB= Heat Absorbed by Processed Materials in Working Cycle (Wh)
QD= Latent Heat Absorbed by Materials Heated in Working Cycle (Wh)
QL= Conduction, Convection, Radiation Losses (Wh)
S.F. = Safety Factor
tc= Cycle Time Required (hr)

Equation 4—Start-Up Power (Watts)

P

s

=

QA+ Q

C

+

2

(QL)

•

(1 + S.F.)

t

s

3
QA= Heat Absorbed by Materials During Heat-Up (Wh)
QC= Latent Heat Absorbed During Heat-Up (Wh)
QL= Conduction, Convection, Radiation Losses (Wh)
S.F. = Safety Factor
ts= Start-Up (Heat-Up) Time Required (hr)
During start-up of a system the losses are zero, and rise to 100 percent at

process temperature. A good approximation of actual losses is obtained when
heat losses (QL) are multiplied by 2⁄3 .

Equations 4 and 5 Start-Up
and Operating Power Required

Both of these equations estimate
required energy and convert it to
power. Since power (watts) specifies
an energy rate, we can use power to
select electric heater requirements.
Both the start-up power and the
operating power must be analyzed
before heater selection can take
place.

Electric Heaters

19

W A T L O W

Application Guide

Electric Heaters

Power Calculations—
Radiant Heating

When the primary mode of heat trans­fer is radiation, we add a step after
Equation 5.

Equation 6 is used to calculate the net
radiant heat transfer between two
bodies. We use this to calculate either
the radiant heater temperature
required or (if we know the heater
temperature, but not the power
required) the maximum power which
can be transfered to the load.

Equation 6—Radiation Heat Transfer
Between Infinite Size Parallel Surfaces

P

R

=

S (T

4

- T4) F

A (144 in2/ft2) (3.412 Btu/Wh)

P

R

= Power Absorbed by the Load (watts) - from Equation 4 or 5
A = Area of Heater (in2) - known or assumed
S = Stephan Boltzman Constant

= 0.1714 • 10

-8

(Btu/Hr. Sq. Ft. °R4)
T1(°R) = Emitter Temperature (°F + 460)
T2(°R) = Load Temperature (°F + 460)
e

f

= Emissivity Correction Factor - see below

F = Shape Factor (0 to 1.0) - from Reference 139, page 155

(1)

e

f

1

2

Emissivity Correction Factor (ef)

Plane Surfaces

Concentric Cylinders
Inner Radiating Outward

Concentric Cylinders
Outer Radiating Inward

1 1

eSe

L

ef = - 1+

1

e

S

ef =

+

e

L

- 1

1

()

D

S

D

L

1

e

S

ef =

+

- 1

()

e

L

1

D

S

D

L

•

eS= Heater Emissivity (from Material

Emissivity Tables)

eL= Load Emissivity (from Material

Emissivity Tables)
DS= Heater Diameter
DL= Load Diameter

Power Evaluation

After calculating the start-up and
operating power requirements, a
comparison must be made and
various options evaluated.

Shown in Reference 1 are the start-up
and operating watts displayed in a
graphic format to help you see how
power requirements add up.

With this graphic aid in mind, the
following evaluations are possible:

• Compare start-up watts to

operating watts.

• Evaluate effects of lengthening

start-up time such that start-up
watts equals operating watts (use
timer to start system before shift).

Power (Watts)

Time (Hours)

Start-Up

7

Process

10% Safety Factor

Losses

2

/3 of

Conduction
Convection
Radiation

{

Initial Heat to
Melt or Vaporize

(Q

L)

(QC)

Initial Heat to
Raise Equipment
and Materials

(Q

A)

Operating Process Heat to
Raise Product from T

1

to T

2

Process Heat—Melting or Vaporizing

Operating Losses

Conduction
Convection

Radiation

10% Safety Factor

(Q

D)

(QB)

S.F.

(Q

L)

8

9

10

6

5

4

3

2

1

Comparison of Start-Up and Operating Power Requirements

Ref. 1

20

• Recognize that more heating
capacity exists than is being
utilized. (A short start-up time
requirement needs more wattage
than the process in wattage.)

• Identify where most energy is go­ing and redesign or add insulation
to reduce wattage requirements.

Application Guide

Electric Heaters

Power Evaluation

Continued

Having considered the entire system,
a re-evaluation of start-up time,
production capacity, and insulating
methods should be made.

Review of Heater
Application Factors

Safe/Permissible Watt Densities

A heater’s watt density rating gives us
an indication of how hot a heater will
operate. We use this information to
establish limits on the application of
heaters at various temperatures and
under a variety of operating
conditions.

The maximum operating watt density
is based on applying a heater such
that heater life will exceed one year.

In conjunction with desired life, watt
density is used to calculate both the
required number of heaters and
their size.

Silicone Rubber Heater Example:

1000 watts are required for heating
a 150°C (300°F) block.

From the silicone rubber heater watt
density chart in the flexible heater
section of the Watlow Heaters catalog,
page 170.

Maximum Watt Density =
16 W/in2 for wirewound on-off

(2.5 W/in2) or 38 W/in2 (6 W/cm2) for
etched foil

This means 63 in2 of wirewound
(five 3 inch • 5 inch heaters) or 27 in

2

of etched foil (two 3 inch • 5 inch
heaters) are required.

Mechanical Considerations

Full access must be provided (in the
design process) for ease of heater
replacement. This is usually done with
shrouds or guards over the heaters.

These guards also serve a secondary
purpose in that they may minimize
convective heat losses from the back
of heaters and increase efficiency of
the system.

In all applications where the heater
must be attached to a surface, it is
extremely important to maintain as
intimate a contact as possible to aid
heat transfer. Heaters mounted on the
exterior of a part should have
clamping bands or bolts to facilitate
this contact. Heaters inserted in holes
should have hole fits as tight as
possible. Whenever possible, the
holes should exit through the opposite
side of the material to facilitate
removal of the heater.

choice of the external sheath ma­terial is very important to heater
life. A corrosion guide is provided,

page 144, and should be consulted

in order to avoid using materials
that are not compatible with a
particular environment.

• Explosive environments generally
require that the heater be com­pletely isolated from potentially
dangerous areas. This is accom­plished by inserting the heater in pro­tective wells and routing the wiring
through sealed passage-ways out
of the hazardous area. Very close
fusing is recommended in these
cases to minimize the magnitude of
the failure, should it occur.

Operating Environment Factors

• Contaminants are the primary
cause of shortened heater life.
Decomposed oils and plastics
(hydrocarbons in general), con­ductive pastes used as anti-seize
materials, and molten metals and
metal vapors can all create situa­tions that affect heater life. Some
heater constructions are better
sealed against contaminants than
others. In analyzing applications,
all possible contaminants must be
listed in order to be able to fully
evaluate the proposed heater.

Example: Heat is required to main­tain molten zinc in the passageways
of a zinc die casting machine. The

possible contaminants for this appli­cation are as follows:

a. molten zinc metal
b. zinc vapor
c. hydraulic oils
d. high temperature anti-seize

materials

e. moisture, if die cooling is aided

by water circulation

All of these factors indicate that a
highly sealed heater construction
is required.

• The corrosiveness of the materials
heated, or the materials that will
contact the heater must also be
taken into consideration. Even if
a heater is completely sealed, the

Electric Heaters

21

W A T L O W

Application Guide

Electric Heaters

Review of Heater
Application Factors

Continued

Below are the estimated life expect­ancies for mineral insulated heater
types: FIREROD

®

, FIREBAR®,

Tubular, MI Cable, MI Strip, MI Band.

815 (1500) 3

1

⁄2 yrs.

870* (1600)* 1 yr. (2000 hrs.)

925 (1700) 4 mos.
980 (1800) 1 1⁄2 mos.

1040 (1900) 2 wks.
1095 (2000) 1 wk.
1150 (2100) 2 days

* Application charts and operating recommen-

dations use maximum 870°C (1600°F) internal
temperature to insure expected heater life
greater than one year.

Internal Element

Temperature Approximate

°C (°F) Life

Heater Life Requirements

Temperature

The higher the temperature, the
shorter a heater’s service life. Mineral
insulated heaters using traditional
alloys for resistance elements are
subject to the life limiting factor of
wire oxidation. The winding wire
oxidizes at a rate proportional to the
element temperature. If the element
temperature is known it is possible to
project a heater life as shown on the
table in Reference 2.

Heaters utilizing lower temperature
insulating materials (silicone rubber
and mica) have life limiting factors
associated with exceeding the
temperature limits of the insulation
and with thermal cycling. Flexible
heaters and mica strip and band
heaters must be properly sized and
controlled to minimize the temperature
swings during thermal cycling of the
elements.

Thermal Cycling

Excessive thermal cycling will
accelerate heater failure. The worst
cycle rate is one which allows full
expansion and full contraction of the
heater at a high frequency
(approximately 30 to 60 seconds on
and off).

Prevent excessive cycling by using
solid state relays (SSRs) or SCR
power controllers. If using SSRs, set
the temperature controller’s cycle time
to one second. If using SCR power
controllers (like Watlow’s DIN-A­MITE®), be sure to use the variable
time base, burst-firing version.

For Immersion Heaters

Use the Corrosion Guide, page 144,
and the Selection Guides in the
Tubular Elements and Assemblies

Ref. 2

Safety Factor Calculation

Heaters should always be sized for a
higher value than the calculated
figure, often referred to as adding in a
safety factor.

Generally speaking, the fewer
variables and outside influences—the
smaller the safety factor.

Here are some general guidelines:

• 10 percent safety factor for large
heating systems or when there are
very few unknown variables.

• 20 percent safety factor for small to
medium heating systems where
you are not 100 percent sure you
have accurate information.

• 20 to 35 percent for heating
systems where you are making
many assumptions.

section of the Watlow Heaters catalog,
page 262, to ensure that the sheath
material and watt density ratings are
compatible with the liquid being
heated.

Immersion heaters used in tanks
should be mounted horizontally near
the tank bottom to maximize convec­tive circulation. However, locate the
heater high enough to be above any
sludge build-up in the bottom of the
tank. Vertical mounting is not
recommended.

The entire heated length of the
heater should be immersed at all
times. Do not locate the heater in a
restricted space where free boiling or
a steam trap could occur.

Scale build-up on the sheath and
sludge on the bottom of the tank must
be minimized. If not controlled they
will inhibit heat transfer to the liquid
and possibly cause overheating and
failure.

Extreme caution should be taken not
to get silicone lubricant on the heated
section of the heater. The silicone will
prevent the “wetting” of the sheath by
the liquid, act as an insulator, and
possibly cause the heater to fail.

22

Application Guide

Electric Heaters

Review of Heater
Application Factors

Continued

Electrical Lead Considerations

General considerations in selecting
various lead types are:

• Temperature of lead area

• Contaminants in the lead

area

• Flexibility required

• Abrasion resistance

required

• Relative cost
Temperatures listed indicate actual

physical operating limits of various
wire types. Wires are sometimes rated
by CSA, UL®and other agencies for
operating at much lower tempera­tures. In this case, the rating agency
temperature limit is the maximum level
at which this wire has been tested. If
agency approvals are required, don’t
exceed their temperature limits.

Lead Protection

Metal Overbraid Average Good Excellent Moderate
Flexible Conduit Good Average Excellent Moderate

Lead Insulation

Ceramic Beads 650 (1200) Poor Poor Average High
Mica-Glass Braid

(Silicone or Teflon

®

Impregnated) 540 (1000) Poor Good Average High
Glass Braid

(Silicone
Impregnated) 400 (750) Poor Good Average Low

Silicone Rubber 260 (500) Good Good Poor Low
Teflon

®

260 (500) Excellent Good Good Low

PVC 65 (150) Good Good Poor Low

Maximum

Lead Area Contamination Abrasion Relative

Lead Types Temperature Resistance Flexibility Resistance Cost

°C (°F)

Lead Characteristics—Ref. 3

Teflon®is a registered trademark of
E.I. du Pont de Nemours & Company.

UL®is a registered trademark of the
Underwriter’s Laboratories Inc.

——
——

Select Heater

Heater Costs

After calculating wattage required and
considering various heater attributes,
the scope of possible heater types
should be narrowed considerably.
Now, several factors not previously
examined must be considered before
final heater type selection: installation,
operation and replacement costs.

Initial Installation Cost

Each heater type has specific
installation costs to be considered.

• Machining required—
mill, drill, ream

• Materials required—
heater, brackets, wiring

• Labor to mount and wire
heating elements

Operating Cost

The total system operating cost is a
composite of two factors. It is usually
best to examine cost on an annual
basis:

• Total cost of energy
(kW Hours) ($/kWH)

Replacement Cost

The cost of a new heater, lost produc­tion time, removal and installation of
the new heater must be considered.
Generally, these costs are actually
much greater than expected. Heater
life must be such that replacement
can be scheduled and planned dur­ing off-peak production times to avoid
lost production.

• Removal of existing heater

• Equipment downtime cost

• Material cost—

heater, brackets, wiring

• Labor to remove and install
heating elements

• Additional purchasing costs

• Scrap products after heater failure

and during restart of process

• Frequency of burnouts

Electric Heaters

23

W A T L O W

Case 2: Shrouded and Insulated = 2.38 kW/H

Annual Energy Cost:

2080 Hours •2.38 kW/H •$0.07/kWH = $346.00

Replacement Cost:

Same as Case 1 = 340.00

Total/Year = $686.00

Case 1: Shrouded and Uninsulated = 4.06 kW/H

Annual Energy Cost:

2080 Hours •4.06 kW/H •$0.07/kWH = $591.00

Replacement Cost:

5 Heaters •$12.00 Each = 60.00
4 Hours Labor to Install •$20.00/hr = 80.00
4 Hours Lost Production Time •$50.00/hr = 200.00

Total/Year = $931.00

Application Guide

Electric Heaters

Select Heater Type, Size
and Quantity

Example: A plastic extrusion barrel is operating 40 hours per week. Five

band heaters are utilized, 1000 watts each. Energy cost $0.07/kWH. Assume
one shift operation or 2080 hours per year Actual power usage is as follows:

Here, the cost of operation is much less when insulation is used.

24

Application Guide

Electric Heaters

Reference Data

Ohm’s Law

Ref. 4

VI

WR

(Volts)

(Amps)

(Ohms)

(Watts)

WR

IR

I

2

R

VI

W

I

V

I

W

I

2

V
R

W

V

2V2

W

2V2

R

W

R

Volts

Volts = Watts x Ohms

Volts = Amperes x Ohms

Volts =

Watts

Amperes

Amperes

Amperes =

Watts

Volts

Amperes =

Watts

Ohms

Amperes =

Volts

Ohms

Watts

Watts =

Volts

2

Ohms

Watts = Amperes2 x Ohms

Watts = Volts x Amperes

Ohms

Ohms =

Volts

Amperes

Ohms =

Volts

2

Watts

Ohms =

Watts

Amperes

2

)

W2 = W1 x

V

2

2

V

1

(

3 Phase Amperes =

Total Watts

Volts x 1.732

Wattage varies directly as ratio of
voltages squared

Electric Heaters

25

W A T L O W

3-Phase Open Delta

Ref. 8

3-Phase Delta (Balanced Load)

Ref. 7

3-Phase Wye (Balanced Load)

Ref. 5

3-Phase Open Wye (No Neutral)

Ref. 6

Application Guide

Electric Heaters

Reference Data

Continued

R

2

R

1

R

3

I

L

I

L

I

P

V

P

V

L

Equations For Delta Only

IP = IL/1.73
V

P

= V

L

W

DELTA

= 3(V

L

2

)/R

W

DELTA

= 1.73 VLI

L

R

1

R

3

I

L03

I

L01

I

P01

V

P

V

L

I

P03

I

L02

Equations For Open Delta Only

VP = V

L

I

PO1

= I

PO3

= I

LO2

I

LO3

= 1.73 I

PO1

W

0DELTA

= 2 (V

L

2

/R)

V

L

I

L

R

3

R

1

R

2

I

P

V

P

Equations For Wye Only

I

P

= I

L

V

P

= V

L

/1.73

W

WYE

= V

L

2

/R = 3(V

P

2

)/R

W

WYE

= 1.73 V

LIL

V

L

I

LO

R

1

R

2

I

PO

V

PO

Equations For Open Wye Only

(No Neutral)

I

PO

= I

LO

V

PO

= V

L

/2

W

0WYE

=

1

/2 (V

L

2

/R)

W

0WYE

= 2 (V

PO

2

/R)

Typical 3-Phase Wiring Diagrams and Equations for Resistive Heaters

Definitions

For Both Wye and Delta
(Balanced Loads)

VP= Phase Voltage
VL= Line Voltage
IP= Phase Current
IL= Line Current
R= R1= R2= R3=

= Resistance of each branch

W = Wattage

Wye and Delta Equivalents

W

DELTA

= 3 W

WYE

W

ODELTA

= 2⁄3 W

DELTA

W

OWYE

= 1⁄2 W

WYE

26

26

Application Guide

Electric Heaters

Heat Loss Factors
and Graphs

Heat Losses at 70°F Ambient

How to use the graph for more
accurate calculations

Ref. 9—Convection curve
correction factors:

For losses from Multiply convection
top surfaces or curve value by
from horizontal 1.29
pipes

For side surfaces Use convection
and vertical curve directly
pipes

For bottom Multiply
surfaces convection curve

value by 0.63

Radiation Curve Correction Factors

The radiation curve shows losses
from a perfect blackbody and are not
dependent upon position. Commonly
used block materials lose less heat by
radiation than a blackbody, so correc­tion factors are applied. These cor­rections are the emissivity (e) values
listed to the right:

Total Heat Losses =

Radiation losses (curve value times e)
+ Convection losses (top)
+ Convection losses (sides)
+ Convection losses (bottom)
= Conduction losses

(where applicable)

Ref. 9

Temperature of Surface—°F

Losses—W/in

2

2000

1800

1600

1400

1200

1000

800

600

400

200

0.1 0.2 0.3 0.4

0.6 0.8 1 2 3 4 5 6 7 8910 20 30 405060 80100

Convection—Vertical Surface
Combined Radiation and Convection—

Oxidized Aluminum
Radiation—Black Body
Combined Radiation and Convection—

Oxidized Steel

Heat Losses from Uninsulated Surfaces

Electric Heaters

27

W A T L O W

Asbestos 0.25-0.2
Asphalt 0.40-0.2
Brickwork 0.22-0.2
Carbon 0.20-0.2 Most non-metals:
Glass 0.20-0.2 0.90

Paper 0.45-0.2
Plastic 0.2-0.5.-..2
Rubber 0.40-0.2
Silicon Carbide 0.20-0.230.-..
Textiles —0.2
Wood, Oak 0.57-0.2

Additional information on emissivities is available
from Watlow.

Specific

Material Heat Emissivity

Btu/lb-°F

Some Material Emissivities/Non-Metals—Ref. 11

Application Guide

Electric Data

Heat Loss Factors
and Graphs

Continued
Helpful Hint: The graphs for losses

from uninsulated and insulated
surfaces are hard to read at low
temperatures close to ambient. Here
are two easy-to-use calculations that
are only rule-of-thumb approximations
when used within the limits noted.

Rule #1: Losses from an uninsulated
surface (with an emissivity close
to 1.0): (This applies only to temper­atures between ambient and about
250°F)

Losses (W/in2) =

∆T (°F) rise above ambient

200

Rule #2: Losses from an insulated
surface: (This insulation is assumed
to be one inch thick and have a
K-value of about 0.5 Btu-in/hr - ft2-°F.
Use only for surfaces less than 800°F.)

Losses (W/in2) =

∆T (°F) rise above ambient

950

Blackbody 0.75 1.00
Aluminum 0.24 0.09 0.11 0.22
Brass 0.10 0.04 0.35 0.60
Copper 0.10 0.04 0.03 0.65
Incoloy®800 0.12 0.20 0.60 0.92

Inconel®600 0.11 0.20 0.60 0.92
Iron, Cast 0.12 — 0.80 0.85
Lead, solid 0.03 — 0.28 —
Magnesium 0.23 ———
Nickel 200 0.11 ———

Nichrome, 80-20 0.11 ———
Solder, 50-50 0.04 ———
Steel

mild 0.12 0.10 0.75 0.85
stainless 304 0.11 0.17 0.57 0.85
stainless 430 0.11 0.17 0.57 0.85

Tin 0.056 ———
Zinc 0.10 — 0.25 —

Some Material Emissivities/Metals—Ref. 10

Specific Emissivity

Material Heat Polished Medium Heavy

Btu/lb-°F Surface Oxide Oxide

Incoloy®and Inconel®are registered
trademarks of the Special Metals Corporation.

28

Application Guide

Electric Heaters

Heat Loss Factors
and Graphs

Continued

1. Based upon combined natural convection
and radiation losses into 70°F environment.

2. Insulation characteristics

k = 0.67 @ 200°F
k = 0.83 @ 1000°F.

3. For molded ceramic fiber products and
packed or tightly packed insulation, losses
will be lower than values shown.

For 2 or 3 inches Insulation multiply by 0.84
For 4 or 5 inches Insulation multiply by 0.81
For 6 inches Insulation multiply by 0.79

Losses—W/in

2

123456789

10 11

12 13 14 15

16 17

18 19 20

Temperature of Surface—°F

1400

1300

1200

110 0

1000

900

800

700

600

500

Heat Losses from Horizontal Metal Surfaces

0

Losses—W/ft

2

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Temperature of Surface—°F

210

19 0

17 0

15 0

13 0

110

90

70

60% Humidity

40% Humidity

Combined Convection and Radiation—Losses from Water Surfaces

0

0.1 0.2

0.3

0.4

0.5

0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

1. 6

1.7 1. 8

Losses—W/in

2

2600

2400
2200

2000

1800

1600
1400

1200
1000

800
600
400

200

Temperature of Insulated Surface—°F

6" Insulation

5" Insulation

4" Insulation

3" Insulation

2" Insulation

1" Insulation

0" Insulation

Heat Losses from Vertical Insulated Surfaces

0

Ref. 14

Ref. 13

Ref. 12

* For losses of molten metal surfaces,

use the curve e=0.40.

0

2

.

0

=

e

n

o

i

t

a

i

n

o

i

t

c

e

v

n

o

C

l

a

r

u

t

a

N

n

i

b

m

o

C

t

c

e

v

n

o

C

d

e

n

i

b

m

o

C

o

C

d

a

R

d

n

a

n

o

i

n

o

C

d

e

e

n

i

b

m

n

a

n

o

i

t

c

e

v

t

c

e

v

n

o

C

d

o

i

t

a

i

d

a

R

d

R

d

n

a

n

o

i

*

0

4

.

0

=

e

n

0

.

1

=

e

n

o

i

t

a

i

d

a

Electric Heaters

29

W A T L O W

Application Guide

Electric Heaters

Heat Loss Factors
and Graphs

Continued

Losses—W/ft

2

100 200 300 400 500 600 700 800 900 1000 1100 1200 1300

Temperature of Surface—°F

600

500

400

300

200

100

Combined Convection and Radiation—Losses from Oil or Paraffin Surfaces

0

0

Wind Velocity Correction Factor

3.5

3.0

2.5

2.0

1.5

1.0
100 200 300 400 500 600 700

Wind Velocity—MPH

2.5

5

10

15

20

25

30

35

Wind Velocity Effects on Exposed, Bare and Insulated Surfaces

Temperature Difference Between Exposed Surface and Air—°F (∆T)

Ref. 15

Ref. 16

How to Use:

1. Calculate surface heat losses at still air
conditions (ref. Equation #3, page 17)

2. Multiply result by proper wind correction
factor from the curves below to determine
total heat losses.

30

For Steel: Use table or metric equation.

kW = Kilograms x Temperature Rise (°C)

5040 x Heat-up Time (hrs.)

kW = Pounds x Temperature Rise (°F)

20,000 x Heat-up Time (hrs.)

Application Guide

Electric Heaters

Quick Estimates of
Wattage Requirements

The following tables can be used
to make quick estimates of wattage
requirements.

25 0.06 0.12 0.25 .37 0.50 0.65 0.75

50 0.12 0.25 0.50 .75 1.00 1.25 1.50
100 0.25 0.50 1.00 1.50 2.00 2.50 3.00
150 0.37 0.75 1.50 2.25 3.00 3.75 4.50
200 0.50 1.00 2.00 3.00 4.00 5.00 6.00

250 0.65 1.25 2.50 3.75 5.00 6.25 7.50
300 0.75 1.50 3.00 4.50 6.00 7.50 9.00
400 1.00 2.00 4.00 6.00 8.00 10.00 12.00
500 1.25 2.50 5.00 7.50 10.00 12.50 15.00
600 1.50 3.00 6.00 9.00 12.00 15.00 18.00

700 1.75 3.50 7.00 10.50 14.00 17.50 21.00
800 2.00 4.00 8.00 12.00 16.00 20.00 24.00
900 2.25 4.50 9.00 13.50 18.00 22.50 27.00

1000 2.50 5.00 10.00 15.00 20.00 25.00 30.00

Amount

Temperature Rise °F

of Steel

50° 100° 200° 300° 400° 500° 600°

(lb.)

Kilowatts to Heat in One Hour

* Read across in table from nearest amount

in pounds of steel to desired temperature rise
column and note kilowatts to heat in one hour.

Includes a 40 percent safety factor to
compensate for high heat losses and/or low
power voltage.

Kilowatt-Hours to Heat Steel*—Ref. 17

0.5 3.74 0.3 0.5 1 2 2 3

1.0 7.48 0.5 1.0 2 3 4 6

2.0 14.96 1.0 1.0 2 4 6 11

3.0 22.25 2.0 3.0 6 9 12 16

4.0 29.9 2.0 4.0 8 12 16 22

5.0 37.4 3.0 4.0 9 15 20 25

10.0 74.8 5.0 9.0 18 29 40 52

15.0 112.5 7.0 14.0 28 44 60 77

20.0 149.6 9.0 18.0 37 58 80 102

25.0 187 11.0 22.0 46 72 100 127

30.0 222.5 13.0 27.0 56 86 120 151

35.0 252 16.0 31.0 65 100 139 176

40.0 299 18.0 36.0 74 115 158 201

45.0 336.5 20.0 40.0 84 129 178 226

50.0 374 22.0 45.0 93 144 197 252

55.0 412 25.0 49.0 102 158 217 276

60.0 449 27.0 54.0 112 172 236 302

65.0 486 29.0 58.0 121 186 255 326

70.0 524 32.0 62.0 130 200 275 350

75.0 562 34.0 67.0 140 215 294 375

Kilowatt-Hours to Heat Oil*—Ref. 18

Temperature Rise °F

Amount of Oil

50°

Cubic

Feet

Gallons

100°

200°

300°

400°

500°

Add 5 percent for uninsulated tanks.* Read across in table from nearest amount
in gallons of liquids to desired temperature rise
column and note kilowatts to heat in one hour.

For Oil:

Use equation or table.

kW = Gallons x Temperature Rise (°F)

800 x Heat-up time (hrs.)
OR
kW = Liters x Temperature Rise (°C)

1680 x Heat-up time (hrs.)

1 cu. ft. = 7.49 gallons

Electric Heaters

31

W A T L O W

Application Guide

Electric Heaters

Quick Estimates of
Wattage Requirements

Continued

0.66 5 0.3 0.5 0.8 1.1 1.3 1.6 1.9

1.3 10 0.5 1.1 1.6 2.1 2.7 3.2 3.7

2.0 13 0.8 1.6 2.4 3.2 4.0 4.8 5.6

2.7 20 1.1 2.2 3.2 4.3 5.3 6.4 7.5

3.3 25 1.3 2.7 4.0 5.3 6.7 8.0 9.3

4.0 30 1.6 3.2 4.8 6.4 8.0 9.6 12.0

5.3 40 2.1 4.0 6.4 8.5 11.0 13.0 15.0

6.7 50 2.7 5.4 8.0 10.7 13.0 16.0 19.0

8.0 60 3.3 6.4 9.6 12.8 16.0 19.0 22.0

9.4 70 3.7 7.5 11.2 15.0 19.0 22.0 26.0

10.7 80 4.3 8.5 13.0 17.0 21.0 26.0 30.0

12.0 90 5.0 10.0 14.5 19.0 24.0 29.0 34.0

13.4 100 5.5 11.0 16.0 21.0 27.0 32.0 37.0

16.7 125 7.0 13.0 20.0 27.0 33.0 40.0 47.0

20.0 150 8.0 16.0 24.0 32.0 40.0 48.0 56.0

23.4 175 9.0 18.0 28.0 37.0 47.0 56.0 65.0

26.7 200 11.0 21.0 32.0 43.0 53.0 64.0 75.0

33.7 250 13.0 27.0 40.0 53.0 67.0 80.0 93.0

40.0 300 16.0 32.0 47.0 64.0 80.0 96.0 112.0

53.4 400 21.0 43.0 64.0 85.0 107.0 128.0 149.0

66.8 500 27.0 53.0 80.0 107.0 133.0 160.0 187.0

* Read across in table from nearest amount in

gallons of liquid to desired temperature rise
column and note kilowatts to heat in one hour.

Kilowatt-Hours to Heat Water*—Ref. 19

Amount of Liquid

Temperature Rise °F

ft

3

Gallons

20°

40°

60°

80°

100°

120°

140°

Kilowatts to Heat in One Hour

For Heating Flowing Water:

kW = GPM x Temperature Rise (°F) x 0.16
OR
kW = Liters/min. x Temperature Rise (°C) x 0.076

For Heating Water in Tanks: Use equation or table.

kW = Gallons x Temperature Rise (°F)

375 x Heat-up Time (hrs)
OR
kW = Liters x Temperature Rise (°C)

790 x Heat-up Time (hrs)

1 cu. ft. = 7.49 gallons

Kilowatt-Hours to Superheat Steam
Ref. 20

1. Plot points on lines P, Q and S.
P represents the inlet temperature

(and saturation pressure) of the
system.

Q represents the liquid content of
the water vapor.

S indicates the outlet temperature
minus the saturated temperature.

W indicates the heat content of the
water vapor.

2. Draw a straight line from P through
Q to W. Read W1.

3. Draw a straight line from P through
S to W. Read W2.

4. Required watts = Weight (lbs.) of
steam/hour x (W2-W1)

Watt density is critical. Review tem­perature and velocity prior to heater
selection.

Reference is 80 percent quality at
20 psig.

Saturated Temperature—°F

Gauge Pressure—lbs/in

2

Heat Content—Watt–hour/lb

0
10
20
30
40
50
60
70
80
90
10 0
110
12 0
13 0
14 0
15 0
16 0
17 0
18 0
19 0
200

400
300

15 0

10 0

50

25

10

0

450

400

350

300

250

210

85

90

95

10 0

Quality

—% R.h.

P

W

Q

10 0

200

300

400

500

600

700

800

900

Degrees of Superheat

—°

F

S

32

Application Guide

Electric Heaters

Quick Estimates of
Wattage Requirements

Continued

100 1.7 3.3 5.0 6.7 8.3 10.0 11.7 13.3 15.0 16.7 20.0
200 3.3 6.7 10.0 13.3 16.7 20.0 23.3 26.7 30.0 33.3 40.0
300 5.0 10.0 15.0 20.0 25.0 30.0 35.0 40.0 45.0 50.0 60.0
400 6.7 13.3 20.0 26.7 33.3 40.0 46.7 53.3 60.0 66.7 80.0
500 8.3 16.7 25.0 33.3 41.7 50.0 58.3 66.7 75.0 83.3 100.0

600 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 100.0 120.0
700 11.7 23.3 35.0 46.7 58.3 70.0 81.7 93.3 105.0 116.7 140.0
800 13.3 26.7 40.0 53.3 66.7 80.0 93.3 106.7 120.0 133.3 160.0
900 15.0 30.0 45.0 60.0 75.0 90.0 105.0 120.0 135.0 150.0 180.0

1000 16.7 33.3 50.0 66.7 83.3 100.0 116.7 133.3 150.0 166.7 200.0
1100 18.3 36.7 55.0 73.3 91.7 110.0 128.3 146.7 165.0 183.3 220.0

1200 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0 180.0 200.0 240.0

Kilowatt-Hours to Heat Air—Ref. 21

Temperature Rise °F

Amt.

of

Air

CFM

50°

100°

150°

200°

250°

300°

350°

400°

450°

500°

600°

Use the maximum anticipated airflow. This equation assumes insulated duct (negligible heat loss).
70°F inlet air and 14.7 psia.

For Air:

Use equation or table.

kW = CFM* x Temperature Rise (°F)

3000
OR
kW = Cubic Meters/Min* x Temperature Rise (°C)

47

For Compressed Air:

kW = CFM** x Density** x Temperature Rise (°F)

228
OR
kW = Cubic Meters/Min** x Temperature Rise (°C) x Density (kg/m3)**

57.5

**Measured at normal temperature and pressure.

**Measured at heater system inlet temperature and pressure.