ABSTRACT

THE BASICS OF ELECTRIC PROCESS HEATING

Copyright Material IEEE Paper No. PCIC-94-14

by: Rob Bohn, Mike Bange and Joe Foreman

The application of electric heaters to heat process streams is more complicated that it appears at first glance. Not only do the thermal properties of the media and the thermal load required need to be considered, but the compatibility of materials used in the heater and vessel with thp media need to be investigated. Watt density limits need to be investigated as well as the mechanical design of the heater. Electric process heating is usually used where a close degree of control is necessary. Control of large electric process heaters is discussed.'

MTRODUCTION

Tubular electric heater technology has been around for more than 30 years, but until recently it has not been recognized as a product that can be used in many applications in the Chemical and Petrochemical industries. Improvements in product design, safety features and control schemes have given this product many advantages over other means of heat.

The basic tubular heating element consists of 80- 20 nickel-chromium or 70-30 nickel-chromium wire that provides resistance to electricity, generating heat. The Ni-Cr wire is surrounded by compacted magnesium oxide and a metal sheath. Electrical connections are made to the resistance wire by "cold pins" (metal conductors) that are terminated outside the metal sheath. Various forms of electric termination can be made to this cold pin. The MgO is sealed to prevent moisture fiom entering the MgO, which is very hydroscopic.

T U B U L A R E L E M E N T S

Where: 1 - Element Coil 2 - Element Cold Junction 3 - Magnesium Oxide (MgO) 4 - Element Sheath 5 - Element Terminal Post

94-CH345 1-2/94/0000-0121 $03.00 @ 1994 IEEE

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These tubular elements can then be welded into a flange to make an immersion heater assembly. This is also referred to as an electric heat exchanger. An immersion heater assembly consists of heating elements, blind flange or tube sheet, terminal housing, bussing for element circuits, thermocouples and thermocouple housing. The housing may be offset from the flange face to minimize the heat build-up in the terminal housing. Figure 1 and 2 show the stand-off distance of the terminal housing to the flange face versus temperature. This immersion heater assembly can then be bolted into a tank to heat a fluid or gas directly, or the immersion heater assembly can be mounted into a pressure vessel and the end product will look and function similar to a shell and tube heat exchanger.

f7J 0 0

0 0

1 2 3 4 5 6

Where: 1 - Electrical Housing 2 - Stand-off Section 3 - Heater Flange 4 - Hi-Limit Thermocouple 5 - Tubular Elements 6 - Element Supports

STAND OW HOUPND TEST TEWEPANRCPROFILE fORAlRCllYIURGASEI

Where: 1 - Electrical Housing 2 - Stand-off Section 3 - Heater Blind Flange

4 - Shell Weld-neck Flange 5 - Support Saddles 6 - Shell 7 - Vent I Drain 8 - Nameplate (standard or Code) 9 - Inlet I Outlet Connections 10 - Weld Cap

For most applications in the Petrochemical market, detailed information about the application is needed to ensure successful performance of the heater.

As a minimum the information includes:

Medium to be heated Inlet temperature Outlet temperature Operating pressure Flow rate or tank size Design temperature Design pressure Area of use (i.e.: indoor, outdoor) Hazardous location (If yes, state Class, Group, & Division) Allowable pressure drop Heat-up time InleVoutlet pipe connection sizes Orientation Voltage available Accuracy of temperature control required

Many companies use shell and tube heat exchanger specification sheets from TEMA (Tubular Exchanger Manufacturers Association) for specifying the heat requirements.

Once this information has been gathered, the engineer then starts the design process to determine the best heater for the application.

1. Determine the heat load required.

KW = (M)(C,)( AT) x SF 859.824

Where: M = kg per hour if flowing C, = Specific heat KCALkg-OC AT =Temperature use required in "C SF = Safety Factor (1.1 or 1.2)

The safety factor normally used is anywhere fkom 10-20% depending on heat looses,

voltage variation and manufacture tolerances of the heating element.

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A d d c h y d s

Tvl"lC

T-S

A-W Alr

de = hydraulic diameter in m

180 81 40 6 2 3 1 6 3 257 115 40 6 1 TlfvllUm 180 82 40 6 1 Inconel MK) 180 82 40 6 2 TlUnWm 180 81 13 36 lncaloy8W I50 6J 20 3 I 316.1s 121 50 10 I 6 316.1% 180 81 40 3 I InconclMX) 112 50 10 I6 316.1% 161 15 20 3 I 316"s 180 81 40 6 2 316.1' 180 81 23 3 6 Inmloy8W 180 81 13 3 6 304 .18 180 82 40 6 1 COPPU

1611180 7 5 8 1 23140 36/62 S ~ v I 0 4 . 1 1 I80 81 40 6 1 11611% 180 81 10 I 6 CoPF- 110 54 IO I 6 304 riri * ' - * l"colOY8W

2.

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Next, determine watt density or heat flux of the heaters. Watt density is typically defined as the watts per square inch of surface area of the heated portion of the heating element. For most immersion applications, reference charts can be used to determine watt density based on industry standards and guidelines (see chart below). In applications where a gas stream is flowing over the elements, more detailed calculations are required. Some trial and error may be required to ensure that the proper watt density is selected for the application. To determine the proper watt density (heat flux) for a flowing application, the following calculations are used.

2 W 91 10 I 6 COPPU 211 IW 40 6 1 Stcsl

Aluminum Asctltc AluminumPwvivmSvlfuc 116.1s ran- I22 50 10 I 6

211 IM 40 6 2 hmmon,. Gu Ammonium A-te

A. Calculate the mass flow rate G

--rr-. ~~

sled . . . . 167 75 13 3 6 Incoloy 8W

G = M A

Where: M = flow rate in kg/hr A = net fiee area in m2

An assumption must be made on what size vessel and number of heating elements will work.

B. Determine Reynolds Number

Re=- P

Where: G = mass flow rate in kgihr-m

p = Dynamic viscosity in kg/hr-m

The Reynolds number is a unitless value that tells us if the flow is laminar, turbulent, or in transition.

T"".ULLNT -.o"No."Y

L..x.. ~ . o U N o . ~ * - ! - - T ~ . N * r T ~ o I .

L I I I " 1 _ , &.*re l C D I 0 U

" S L O C ~ I " B O Y N O l R "

C. Next the convective heat transfer coefficient is determined based on the Reynolds number.

If flow is laminar the following equation is used:

hc=E:*Nu Do

Where: hc = Convective Heat Transfer Coefficient in KCAL/m2- Hr-T

Nu = Nusselt Number

DO = Element diameter in meters K = Thermal Conductivity

If flow is turbulent then the following equation should be used:

hc = .023(G)'8Cp.33K,67

Where: hc = Convective heat transfer coefficient in KCAL/m -hr- "C

G = Mass flow rate kgim2-hr Cp = Specific heat KCALkg-

"C K = Thermal conductivity

KCALkg-m-"C De = Hydraulic diameter J.I = Dynamic viscosity

D. Next the watt density of the heating element is calculated using the following:

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E.

WSI = hc<Ts - TO) .8598 x 10000

Where: WSI = watts/cm2

coefficient hc = convective heat transfer

Ts = sheath temperature in O C To = outlet temperature of

process in " C

Note: For a tubular heater, Ts should not exceed 875" C, to prevent premature heater failure. Lower sheath temperatures may be required depending on the medium to be heated. Example: Heat transfer fluids have maximum film temperatures usually not exceeding 375" C.

If sheath temperature requirements can be satisfied then the next step is to calculate the heated length of the element:

HL= KW (n)(element dia.)(# of elements)(WSI)

Where: HL = heated leyth WSI = Watts/cm

F.

G.

H.

NOTE: Different heater manufacturers have different capabilities regarding the length of heaters they are able to manufacture. Also, size limitations for the location of the heater assembly need to be taken into consideration.

Calculate resistance wire size to ensure a good resistance wire design can be manufactured.

If the sheath temperature is too high, the heater is too long or an improper coil design, the calculation must be started over. In this case, the engineer may choose to use baffles, multiple heaters in series, or simply a lower watt density to achieve the best heater design.

Material selection: The proper sheath material and vessel material must then be seledted for the application taking the following into consideration:

Design temperature Sheath temperature Design pressure Corrosive nature of medium to be heated

As a general rule the following matrix may be used:

As a reference the NACE (National Association of Corrosion Engineers) guide can help make final determination on the best material to use.

Basis

Typical designs try to keep the vessel diameter as small as possible, with the highest number of elements. Keeping the net free area small improves the heat transfer coefficient, allowing higher sheath watt densities. This minimizes the total number of heating elements required, and allows smaller vessel size and wall thickness. Both help keep the cost of the system lower.

At times in applications, the designer can come up with a design that will not work. As vessel diameter increases, the net free area increases, thus decreasing the mass flow rate. The watt density must then be lowered hrther, causing the number of elements to increase, raising the cost of the overall system.

In these situations, baffles can be used to create cross flow rather than parallel flow over the elements. Baffles will add to the cost of the heat exchanger and cause additional pressure drop, but will allow the engineer to develop a sound design.

The equation used is as follows:

Where: Do = Outside diameter of element in m

Cp = Specific Heat G = Mass flow rate

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K = Thermal conductivity p = Dynamic Viscosity

The net free area is determined by the distance between the baffles minus the area taken by the heating elements.

ASME

Once the elements, watt density, the number and length of heating elements and materials have been selected, the next step is to design the pressure vessel. Guidelines for ASME Section VI11 Division 1 are recommended. The main calculations to be considered include:

Vessel wall thickness 0 Head thickness

Flange rating Nozzle pipe wall thickness Nozzle flange rating

Pressure drop of the entire vessel assembly should then be calculated to ensure the pressure drop through the electric heat exchanger will not exceed customer specifications. Pressure drop can be an extremely important design criteria. Many times, pumps or compressors are sized based on the total allowable pressure drop of the system. Keeping pressure drop to a minimum can be a very effective cost saving measure. Pressure drop is calculated in the following steps:

1. Determine velocity through the inlet nozzle, main vessel and outlet nozzle.

V = M 3600 s e c h (A0 (p)

Where M = mass flow rate in kg/hr. Af = net free area p = density @ temperature and

pressure

2. AP= V2P (9.815 m/secz)(lOOOO cmz/mL)

Where p = density @temperature and pressure

3. CAP

Calculate the sum of pressure drops at inlet, vessel, baffles, and outlet.

NOTE: Pressure drop can be reduced by increasing the size of the inlet and outlet nozzles or increasing the spacing between baffles.

Safety Devices for electric heaters can include the following:

-: These are welded directly to the element sheath and wired in conjunction with a latching limit controller. The sensor can either be a thermocouple or RTD, welded or attached directly to the element sheath. The sensor would then be wired to a latching control that would consist of either a fixed or adjustable set point and a relay mechanism. The relay will turn power off to the electric heater in the event of a limit condition. It is recommended that latching devices be used, so that the operator must physically acknowledge the alarm and reset the control after the limit condition has been corrected.

. .

f Valve: These can be installed either in the pressure vessel itself or in the piping immediately down stream fiom the electric heat exchanger. This will prevent an over pressure condition in the system which could possibly be generated by having the heater on and valves closed.

Flow Swi&: A flow switch can be used to sense a blockedreduced flow in the system. This should be considered when heating extremely sensitive fluids. It can also be used to prevent a dry fire condition.

Proof Tr&-: For hazardous locations, an explosion proof housing is recommended to enclose the heating element connection to the power wiring supplied by the customer. There are no arching devices under normal conditions ixide the electric heater enclosure.

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One of the main advantages of electric heat is that with the proper control scheme, operating temperatures can be controlled within the f 1 " C range, depending on design conditions. There are four basic types of control schemes that can be used. Below is a matrix referencing the different types of controls, and advantages and disadvantages of each.

Sensor ControlTvDeAccuracv

Bulb & Capillary OdOff

Thermocouple Proportional f 5" C

f 15" C Thermocouple OdOf f f 10°C

Thermocouple PID f 3" c Thermocouple PID f l o c

or RTD

Mechanical Contactor Lowest Mercury Relay Medium Solid State Relay Medium SCR High

One of the main advantages of SCR control is extended heater life. The zero cross fired SCR regulates voltage cycle rates to the heater, resulting in stable temperature of the sheath and resistance coil. This also allows the resistance coil to operate at low temperatures during low heat requirements. This combination reduces thermal stress on the coil, and may extend heater life by a factor of 10, over more stresshl ordoff control systems. Replacement cost, process down time, and accuracy required should be major factors in deciding what type of control is most important for the application.

MeattrFailure

The failure of electric heating equipment to perform satisfactorily may be caused by one or more factors, such as:

Excessive Fouling Operating conditions differing fiom design conditions Maldistribution of flow in the unit

e Improperly functioning control system Improper thermal design.

The user's best assurance of satisfactory performance lies in dependence upon manufacturers competent in the design and fabecation of electric heating equipment.

e of F l e c m

1. Inspection of Unit:

At regular intervals, and as frequently as experience indicates, an examination should be made of the interior and exterior condition of the unit. Neglect in keeping heating elements reasonably clean may result in excessive sheath temperatures, causing premature failure.

2. Indications of Fouling:

Heaters subject to fouling or scaling should be cleaned periodically. A light sludge or scale coating on the element greatly reduces its efficiency. A marked reduction in performance usually indicates cleaning is necessary. Since the difficulty of cleaning increases rapidly as the scale thickness or deposit increases, the intervals between cleanings should not be excessive. Before disassembly, the user must assure himself that the unit has been de-pressurized, vented and drained, neutralized andor purged of hazardous material. To avoid possible damage during removal of a heater bundle fiom a shell, a pulling device should be attached to eyebolts screwed into the tube sheet. The bundle should be supported on the baffles, supports or tube sheet, to prevent damage to the heating elements.

--DISCONNECT ELECTRICITY-

1. A periodic check (maximum 6 months) of the electrical connections should be done. The following should be looked for:

(a) Loosening of feeder line connections. Re-tighten as required.

(b) Oxidizing or corroding of feeder wire. Usually caused from' extreme ambient temperatures or loose connections. Remove harmed feeder wire.

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(c) Check for dust, dirt or corrosion caused by atmospheric conditions surrounding the heater. Remove with stainless steel wire brush. Remove loose particles with vacuum. Consult factory if replacement parts are required.

Low

At high processlsheath temperatures, a silicone seal is typically used on the heating elements as a barrier against moisture. The life of this seal is 4-6 months depending on the environqent. This seal is used because magnesium oxide is extremely hydroscopic and will draw moisture into the element in high humidity environments. The purpose of this seal is to provide a temporary barrier only. In high temperature applications, it is important that the tubular electric heating element "breathe". During heat- up, the element will draw in oxygen and the nickel-chromium resistance wire will form chromium oxide on its surface, which will provide an oxide protection, extending heating element life.

In highly humid environments, more permanent epoxy seals are being used, if the sheath temperatures will not consistently exceed 700°C. This works well to eliminate problems associated with moisture. These lower temperatures do not create heavy "blackening" of the MgO, that occurs in high temperature sealed elements. Also, even higher temperature applications can be considered, if the heaters are only periodically used, since the degree of "blackening" is a function of time at temperature.

If some elements have a low megohm, the following options may be used to bake-out moisture (drive moisture out of the heating elements). This should be qonsidered when individual element readings are below (1) megohm.

1. Remove heater bundle from vessel and bake- out for 12-24 hours in an oven set at approximately 110" C.

2. Remove heater bundle and keep in warm, dry, low humidity area. This process will remove moisture from elements but may take several days.

3. If practical, and process temperature is high enough, the process may be used to dry out heater if process inlet is 170" C or greater. If the heater has a stand-off section, this part of the heater needs to be insulated to drive the moisture completely out of the element. Failure to do so will have a tendency to concentrate moisture in the unheated section and decrease the megohm reading.

4. Typically, 120V low voltage is applied to low megohm elements. This will drive moisture to unheated section of elements.

5 . Use silicon rubber heaters to wrap around stand-off section and electrical enclosure to drive moisture out of cold section of elements. Silicon rubber heaters are available for bake-out. The supplier should be consulted for more information.

6. Post weld heat blankets may be used instead of silicon rubber heaters if time is critical to get megohm value to an acceptable value.

Summarv

Electric heaters can be used for a wide variety of applications. These range from heating various gases to extremely high temperatures (as high as 1000" C), heat transfer fluids, steam superheating, fuel oils, or corrosive solutions. Electric heater technology has come a long way in safety, design standards, reliability and controllability. The key to proper performance is good engineering practices and knowing as much as possible about the application to ensure a good design. Electric heating can provide a low installation and maintenance solution to many process applications where gas or steam may have been used in the past. Electric heat provides a stable cost regardless of temperature range, and runs at high efficiencies without emission concerns (NO, or SO,). The physical size of an equivalent duty electric heater will generally be more compact than a gas fired system, and can be located in all types of plant locations. With existing sensing and control system, electric heat can provide precise temperature control, that can not be accomplished by traditional methods. Consider all these advantages when specifying process heatng needs.

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References

1. Standards of the Tubular Exchanger Manufacturers Association, Seventh Edition, 25 North Broadway, Tarrytom, NY 10591, Richard C. Byme, secretary.

2. Corrosion Data Survey, Sixth Edition, National Association of Corrosion Engineers, 1440 South Creek Dr., Houston, TX 77084.

3. The American SocieQ of Mechanical Engineers, Section VIII, Division Pressure Vessels, 1992 Edition, 1993 Addenda, United Engineering Center, 345 East 47th St., New York, NY 10017.