electric process heaters

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Process Engineering Guide: GBHE-PEG-HEA-509

Electric Process Heaters Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



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 Safety 2 4.2 Environment 2 4.3 Location of Equipment 3 4.4 Low Temperature Applications 3 4.5 Cross Contamination 3 4.6 Control 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 Pipeline Immersion Heaters 5 8.2 Tank Heaters and Boilers 6 8.3 Indirect (Fluid Bath) Heaters 7 8.4 Radiant Furnaces 7 8.5 Induction Heaters 7 8.6 Hot Block Heaters 7 9 CONTROL 8

10 REFERENCES 8 FIGURES 1 ELECTRIC HEAT EXCHANGER CONSTRUCTION 5 2 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.