Electric Process Heaters

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-509

Electric Process Heaters

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Process Engineering Guide:

Electric Process Heaters

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

2

1

SCOPE

2

2

FIELD OF APPLICATION

2

3

DEFINITIONS

2

4

ADVANTAGES OF ELECTRIC HEATERS

2

4.1
4.2
4.3
4.4
4.5
4.6

Safety
Environment
Location of Equipment
Low Temperature Applications
Cross Contamination
Control

2
2
3
3
3
3

5

DISADVANTAGES OF ELECTRIC HEATERS

3

6

POTENTIAL APPLICATIONS FOR ELECTRIC
PROCESS HEATERS

3

7

GENERAL DESIGN AND OPERATING CONSIDERATIONS 4


8

TYPES OF PROCESS ELECTRIC HEATERS

5

8.1
8.2
8.3
8.4
8.5
8.6

Pipeline Immersion Heaters
Tank Heaters and Boilers
Indirect (Fluid Bath) Heaters
Radiant Furnaces
Induction Heaters
Hot Block Heaters

5
6
7
7
7
7

9

CONTROL

8

10

REFERENCES

8

FIGURES
1

ELECTRIC HEAT EXCHANGER CONSTRUCTION

2

5

SHEATHED HEATING ELEMENTS


0

INTRODUCTION/PURPOSE

This Guide is one of a series on Heat Transfer prepared for GBH Enterprises.
Electric heaters are used in the process industries for some duties as alternatives
to fluid heated or fired process exchangers. When specified and used properly,
electric heaters will last for many years without problems. However, there are
special features to consider in specifying and operating electric heaters, which, if
not understood, can result in damage to the equipment leading to early burn-out
of the elements or potentially hazardous equipment failure.

1

SCOPE

This Guide is intended to assist engineers in the selection and trouble free
operation of electric heaters.
This Guide describes the major types of electric process fluid heater and the
sorts of duties for which they are applicable. It gives guidelines on key points to
observe when specifying and operating electric heaters, in order to avoid
problems. It does not give detailed information on design methods; electric
heaters are generally designed by the suppliers.
Further information on electric heaters may be found in [Refs1 and 2].

2

FIELD OF APPLICATION

This Guide applies to process engineers in GBH Enterprises worldwide, who
may be involved in the specification or operation of electric heat exchangers.

3

DEFINITIONS

For the purposes of this Guide, the following definition applies:
HTFS

Heat Transfer and Fluid Flow Service. A cooperative
research organization, in the U.K., involved in research into
the fundamentals of heat transfer and two phase flow and
the production of design guides and computer programs for
the design of industrial heat exchange equipment.


With the exception of terms used as proper nouns or titles, those terms with initial
capital letters which appear in this document and are not defined above are
defined in the Glossary of Engineering Terms.

4

ADVANTAGES OF ELECTRIC HEATERS

4.1

Safety

Very high temperatures (over 1000°C, and up to 1400°C with certain designs)
can be achieved, without the potential fire and explosion hazards associated with
fired heaters. All potential fire hazards may be contained in explosion proof
terminal boxes. No fuel storage tanks or gas let-down stations, which may affect
the plant area electrical classification, are required.
4.2

Environment

There are no local pollution problems (e.g. NOx and SOx production) with electric
heaters.
4.3

Location of Equipment

An electric heater will generally be considerably lighter and more compact than a
fired heater for the same duty. There will usually be fewer restrictions on the
location of an electric heater than a fired heater, enabling it to be placed locally
within the main process structure rather than at some peripheral point; thus
saving on process pipework. No long service feed and return lines are
necessary. It can be used on locations where other forms of heating are not
available. Cost advantages are particularly great in the smaller sizes (up to 1
MW).
4.4

Low Temperature Applications

Electric heaters do not suffer from the problems associated with fluid heaters at
very low temperatures, such as freezing of condensate or viscous behavior.
4.5

Cross Contamination

There is no service fluid which could leak into the process.


4.6

Control

Very good control of power input to the process fluid can be achieved, across a
wide range, typically down to 5% of the rated maximum power. The control
response is usually quicker than with fluid heaters.

5

DISADVANTAGES OF ELECTRIC HEATERS

Electric heaters require careful selection, design, construction and operation,
otherwise premature burn-out of the heating elements may occur.
Electricity is generally a relatively expensive form of energy. However, for high
temperature applications, a fired heater often has a relatively low thermal
efficiency because of losses with the flue gases, especially if there is no suitable
low temperature duty, such as preheating or boiler feed water heating, to cool the
stack gases. Electric heaters, in conjunction with properly designed heat
insulation, can achieve local efficiencies approaching 100%.

6

POTENTIAL APPLICATIONS FOR ELECTRIC PROCESS HEATERS

(a)

Fluid heating to temperatures above 400°C up to over 1000°C for
reactors, catalyst regeneration etc.

(b)

Heating in remote locations where piping costs would be prohibitive if
heated elsewhere, or where no other heating medium is available.

(c)

Heating on offshore rigs, where the reduced size and weight of electric
heaters compared with fired heaters can substantially reduce the cost of
the platform, and the fire danger is largely removed.

(d)

In place of fired heaters for small to medium applications or for
temperatures above 400°C in batch mode, or where extremely careful
temperature control is required.

(e)

For cryogenic duties, or where the ambient conditions could cause
condensate return lines to freeze.

(f)

Where electric power is cheap.


7

GENERAL DESIGN AND OPERATING CONSIDERATIONS

Electric process heaters are not only pieces of electrical equipment; they are also
heat exchangers. Their specification and selection should involve not only an
electrical engineer but also a process engineer with an understanding of process
heat transfer. Some of the past problems experienced with electric heaters, can
be attributed in part to a lack of process engineering input at the selection stage.
Many electrical heaters are of a very lightweight construction for domestic and
light commercial duty. Moreover, the manufacturers of such units may have only
a limited understanding of heat transfer. This type of unit is unsuitable for heavy
process duty. Use only equipment that has been specifically designed and built
for refinery or process plant duty by a competent manufacturer with a proper
understanding of process heat transfer.
The heat transferred between two fluids in a conventional heat exchanger is
limited by the surface area, the temperature difference and the overall heat
transfer coefficient. In contrast, the heat transferred in most types of electrical
heater is limited only by the power input to the heating elements. Many of the
problems associated with electric heaters arise from a failure to appreciate the
implications of this.
The power generation in an electric heater is governed by the design of the
resistance heating elements and, except for minor variations in electrical
resistance with temperature, will depend only on the applied voltage. The system
will seek to dissipate this power to the fluid regardless of either the area for heat
transfer or the heat transfer coefficient, by adjustment of the temperature of the
heating elements. Moreover, the power output is usually uniform along the
elements. Thus, if the local coefficient is low, either because of low local fluid
velocities or fouling, the local element temperature will rise to compensate. If the
process fluid is temperature sensitive it may degrade in these regions, leading to
a progressive build-up of fouling deposits. This in turn will lead to an increasing
element temperature, until the maximum safe working temperature is exceeded,
and element burn-out occurs.


The key points to remember when seeking to avoid this are:
(a)

Do not use designs which have dead zones in the heated region. For
example, segmental baffles should not be used on immersion type heaters
[see (1) below].

(b)

Heaters should not be run at below the design minimum flow rate; trip
systems to prevent this are recommended.

(c)

Tank heaters should not be operated below a minimum safe liquid level
which ensures that the elements are covered at all times. Trips may be
required to guarantee this.

(d)

Heaters should not be operated in a badly fouled condition.

Failure to understand the operating characteristics of electric heaters has lead to
several failures in plants. Two examples are given below:
(1)

An electric heater of the pipeline immersion type (see 8.1) was installed on
a European plant, to heat a heat transfer oil used to raise the temperature
of the reactor. The process operators were experiencing difficulty in
obtaining the desired reactor temperature. They incorrectly deduced that
this was simply due to the low temperature of the heat transfer oil, and to
raise this they reduced the flow rate. The heater had segmental baffles,
which are undesirable in this type of heater (see 8.1.2). Breakdown of the
oil in the dead zones behind the baffles occurred, leading to severe
coking.

(2)

An electric heater was installed on an Aromatics plant in Europe to provide
hydrogen at 200°C for start-up. In 1989, the shell of this unit failed, leading
to a fire. The Dangerous Incident Investigation [Ref 3] concluded that the
cause of failure short term overheating at pressure. The heater was
allowed to operate in this condition because the low flow protection had
been defeated, and the temperature trips were set too high.


8

TYPES OF PROCESS ELECTRIC HEATERS

8.1

Pipeline Immersion Heaters
8.1.1 General
This type of heater resembles a shell and tube heat exchanger, with the
tubes replaced by electric resistance heating elements encased inside
metal tubes. See Figure 1. The tubes may be sealed at one end and pass
through a tubesheet at the other, or be of a U-tube construction with both
ends passing through the tubesheet. The tube material depends on the
process fluid. The space between the element and the tube is packed with
an inert material, usually magnesia, at a sufficient density to provide good
thermal conductance whilst retaining electrical resistance. The tubes
containing the elements protrude beyond the tubesheet and are fastened
to a terminal box, where all the electrical connections are made. This can
be designed to be explosion proof if necessary. Figure 2 shows the main
components of a typical heating element.
Pipeline immersion heaters are available for duties up to 5 MW, for
heating liquids to about 350°C or gases to about 600°C. Typical design
heat fluxes are 40-100 kW/m2. They are available in most metals, in
working pressures up to 700bar.

FIGURE 1

ELECTRIC HEAT EXCHANGER CONSTRUCTION


FIGURE 2

SHEATHED HEATING ELEMENTS

8.1.2 Design Points
The heating elements should be welded to the tubesheet. Designs which
use compression fittings to seal the element to the tubesheet develop
leaks over a period of time due to temperature cycling.
The terminal box should be provided with an adequate stand-off from the
tubesheet. This ensures no fluid leakage into the terminal box, and also
keeps the box cool. The electrical wiring in this part of the tube is designed
with a low electrical resistance to avoid heating.
Avoid dead spots and zones of low flow. Do not use segmentally cut
baffles. Baffling to provide element support and improve heat transfer
should be by means of rod baffles or similar. The inlet zone by the
tubesheet will inevitably have dead spots; the elements should be
designed to be unheated in the entrance zone.
The shell of an immersion heater runs hotter than the fluid, particularly if a
gas is being heated, because of radiation from the elements. Remember
that these may have been designed to operate at temperatures
considerably above normal fluid temperatures. The shell should be
designed for a temperature calculated allowing for radiation from the
bundle. Shell skin temperature alarms or even trips may be desirable. It is
possible that the shell may become hot enough to be a source of ignition
for gases in the atmosphere even when the process temperatures are
below the ignition temperature.


Low flow trips are essential. It is common practice to provide high
temperature alarms/trips, usually in the form of thermocouples attached to
the outside of selected tubes. Remember that these will not give warning
of local problems, and rely on the assumption that conditions are
uniform throughout the bundle.
The magnesium oxide insulation round the elements has to be sealed
from the atmosphere to prevent moisture ingress. This is usually done with
a seal of cured silicone rubber. Although this should give a good seal, it is
possible that during periods of prolonged shut-down, moisture can get into
the magnesia. If the heater is subsequently turned on at full power, a short
may occur which could result in element burn-out. A check on the
electrical resistance should always be made before bringing a heater on
line after a prolonged shut-down, or if there is any reason to suspect
moisture ingress. Generally, the elements can be restored to their proper
condition by operating for several hours at a low voltage, until the
resistance is restored to its correct value.

8.2

Tank Heaters and Boilers

Tank heaters use similar heating elements to the pipeline heaters, but the
bundles of elements are positioned in the lower part of storage vessels to
maintain fluid temperature. The tubes may be either bare or finned on the
outside. Some designs allow for removal of the heating elements from an outer
sheath which is in contact with the process fluid. This enables replacement of the
elements without the need to drain the tank.
Bundles of heating elements in tubes may also be used for boiling liquids,
producing a design which is superficially like a fluid heated kettle boiler.
Unlike fluid heated systems, for an electrically heated reboiler or tank heater, it is
essential to provide controls to cut off the electricity in the event of the liquid level
falling sufficiently to uncover any of the tubes.


8.3

Indirect (Fluid Bath) Heaters

These consist of a pressurized shell containing a suitable heat transfer fluid with
an electric heating coil in the lower part and a fluid heating coil, usually a U-tube
bundle, in the upper part. The heat transfer fluid may heat the process coil either
by convection in the liquid phase, or may boil on the electric elements and
condense on the process coil. The heat transfer fluid is chosen to have the right
combination of properties over the operating conditions. Typical fluids are water,
ammonia, methanol or heat transfer oils such as "Thermex", "Dowtherm",
"Santotherm" etc.
Fluid bath heaters can be economic for heating corrosive fluids, since only the
process fluid coil need be fabricated from corrosion resistant alloys. They may
also be less costly than pipeline immersion heaters for high pressure operation.

8.4

Radiant Furnaces

These consist of a heating coil to contain the fluid being heated, surrounded by
radiant electric heating elements. The elements are backed by an insulated steel
shell, ceramic fibre generally being used for insulation. The radiant elements may
be divided into zones, to give a controlled pattern of heating. Temperatures up to
1300°C can be achieved.
Electric radiant heaters are an alternative to fired heaters. They have a high
thermal efficiency as there is no stack loss. For batch processes, the operating
cost of electricity may be less than that of fuel for a fired heater.
Radiant heaters require proper design of the heating elements and fluid coil,
ensuring good view factors etc.

8.5

Induction Heaters

In these, the process fluid flows in a helical coil which acts as the secondary
winding to a transformer. Very high currents at low voltage are induced in the
coil, generating heat by resistance. These are used for special applications and
are designed on a one-off basis.


8.6

Hot Block Heaters

One potential problem with the pipeline immersion heater is burn-out of the
heater elements, resulting from a failure in the process flow. This is avoided in
the hot block heater. This uses a cast block, generally of aluminium, in which
both electric heating elements and coils carrying the process fluids are cast. The
temperature of the block is monitored by thermocouples in tubes in the block,
which are used to control the power input to the heating elements. The elements
are generally removable cartridge heaters.
9

CONTROL

Very precise and programmable heating control, with full proportional control, can
be achieved with electric heaters. Electronic controls are usually employed. The
preferred form uses Silicon Controlled Rectifiers (SCRs) operating with zero
voltage switching. These operate by energizing the heater for some of the cycles
in the supply voltage, and cutting off the power for others, the switching taking
place at the zero voltage points. The heater can be energized for as little as one
cycle per second up to the full 50 cycles.
Other forms of power control, such as phase angle control, where the current is
cut off for part of each cycle, but not at the zero voltage condition, can result in
radio frequency interference, which may affect other electronic equipment in the
area.
The SCRs generate some parasitic heat, which requires the control panel to be
cooled to keep the temperature below 50°C.
For further information on the control problems, consult a Control/Electrical
Engineer.


Electric Process Heaters

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