ELECTRIC HEAT TRACING

Agenda

• Why Heat Tracing?• Types of Heat Tracing Systems• Design Using Vendor Software

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Why Heat Tracing?• To compensate for heat losses and to maintain a

minimum temperature (i.e. to prevent freezing.• The most common heat tracing applications

include:- Freeze protection & Temperature maintenance- Heat-up

3

4

C

-12 C

Insulation

Heat loss

Heat

Heater

Freeze Protection & Temperature Maintenance Every pipe or vessel is subject to heat loss when its temperature is greater than ambient temperature. Thermal insulation reduces the rate of heat loss but does not eliminate it. Trace heating is used to replace the heat that is lost to atmosphere. If the heat replaced matches the heat lost, temperature will be maintained. Normally, a thermostat is used to energise when it measures temperature falling below a set temperature value - usually between 3°C and 5°C and often referred to as the 'setpoint'. The thermostat will de-energise the trace heating when it measures temperature rising past another set temperature value - usually 2°C higher than the setpoint value.

Heat-upThis type of application is required where a

critical or emergency condition is foreseen Heat-up of the fluid or pipe / vessel may be required during start-up or in order to re-start the system after a shutdown of the system.

5

6

ocoi

ap

hDhDkDD

hD

TTq

22

12

1

112

)/ln(1

Heat Loss

As per IEEE 515, neglecting the hi, hco and ho parameters for conservative high heat loss, we get

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– Types of Heat Tracing

Hot water jacketing

Steam Heat Tracing / Jacketing

Thermic Fluid Heat Tracing / Jacketing

Electric Heat Tracing

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• Selecting the method: Considerations while choosing heat tracing system

1. Economics2. Type of pipeline – Long or Short3. Cost of steam Vs electricity4. Availability of free steam from exothermic reactions.5. Maintenance temperature required.6. Temperature control accuracy required.7. Hazardous area classification.8. Type of power available during plant shut down.

• Steam heat tracing involves laying of steam supply and return pipelines. Steam tracing requires investment in a steam generation boiler with attendant issues of operation and maintenance.

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EHT- Circuits and Cables

• Types of Electric Heat Tracing CablesSeries cables

Mineral-insulated (MI) cableSkin-effect current tracing (SECT)

Parallel CablesParallel / Zone Heater Cables.Self-regulating (SR) cable

Impedance – Direct Joule Effect Heating10

Parallel / Zone Heater Cables

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Features: 1. Maximum Maintenance Temperature upto – 66⁰C2. Maximum Continuous Exposure Temperature -

204⁰C3. Watt Density upto – 33W/m.4. Circuit lengths upto 200 mtrs.

Zone

Node

(0.5 m to 2 m)

Resistance wire

Bus wires

Dielectric insulation

connection Node

Nichrome heating element (38-41 AWG)Bus wire

Advantages: 1. Cut to length feature.2. Not subjected to inrush of current as compared

to self regulating cables.3. Flexible4. Usually inexpensive5. If one small element fails then the rest of the

system will continue to operate.

Disadvantages: 1. Requires infre-red scanning to detect faults.2. Cannot be overlapped.3. Poor impact resistance.

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Series Cables

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Resistancewire

Plasticdielectricinsulation

Braid Outer plastic jacket

Features: 1. Maximum Maintenance Temperature upto – 204⁰C2. Maximum Continuous Exposure Temperature -

260⁰C3. Circuit lengths upto – 3658 mtrs.

Advantages: 1. Long circuit lengths2. Easy to monitor3. Flexible4. Usually inexpensive

Disadvantages: 1. Not suitable for short lengths.2. Comes in pre-fabricated lengths.3. Cannot be overlapped.4. Single failure shuts down entire circuit.5. Poor impact resistance.6. High Replacement cost.

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Self Regulating Cables

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Bus wire

Heating element

Dielectric insulation

Braid

Outer plastic jacket

Re

sist

an

ce

Temperature Temperature

Po

we

r

+

-

Features: 1. Maximum Maintenance Temperature upto – 150⁰C2. Maximum Continuous Exposure Temperature - 250⁰C3. Circuit lengths upto – 300 mtrs.4. Watt Density – 66 W/m

• Suitable for Low Maintenance and Design Temperature Application.

• Parallel resistance constant wattage cable.

Disadvantages: 1. Compared to MI cables these are suitable for Lower

Maintenance Temp. & Lower exposure temperature

2. Lower Watt Densities upto 66 W/m.

3. Higher start-up / inrush current.

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Advantages: 1. Higher reliability – cannot burnout even if overlapped.

2. Easier to design - Parallel circuit, cut to length. Unconditional T-ratings, Compensates for variations.

3. Lower installation and operating costs.

MINERAL INSULATED (MI) CABLE

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Dielectric Insulation

Resistance wire

Magnesium Oxide Metal Sheath

Features: 1. Maximum Maintenance Temperature upto – 500⁰C2. Maximum Continuous Exposure Temperature -

600⁰C3. Circuit lengths upto – 1200 mtrs.4. Watt Density – 262 W/m.

• Suitable for High Maintenance and Design Temperature Application.

• Series resistance constant wattage cable.

--

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Advantages: 1. Compared to SR cables these have:-High maintenance temperature-Suitability for High exposure temperature-High Watt Densities

2. Easy to monitor.3. Rugged.4. Uniform Power along entire length.

Disadvantages: 1. Pre-fabricated lengths.2. Difficult to install due to non-flexibility.3. Cannot be overlapped.4. MgO Dielectric Extremely Sensitive to Moisture.5. Not suitable for short lengths.

SKIN-EFFECT CURRENT TRACING (SECT)

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..0

HighvoltageAC

Insulated conductor

Pipe

Weld Heat tube

Insulatedconductor

Advantages: 1. Suitable for Very long circuit lengths (1.5-30) km.2. High watt density up to 60 w/m per tube.3. High temperature capability up to 205°C.4. Low installation, operation and maintenance

costs.5. Rugged.

Disadvantages: 1. Not suitable for complex piping2. High voltage (500-5000 volts)

3. Special transformer required.

4. Custom design for each circuit

5. Must be welded to process pipe

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Impedance – Direct Joule Effect Heating

Advantages: 1. Low Voltage Operation – less than 30V.2. Uniform Heating – since entire pipe acts as heating element.3. Low installation, operation and maintenance

costs.4. No hot-spots and burn-outs.5. Wide temperature range from below freezing to

850°C

Disadvantages: 1. Usually operates at very high currents.2. Special transformer required.

3. Isolating flanges required.

4. Terminals must be welded to process pipe.

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Comparison between different types of cables

Sr. No

Description Series Cable

Parallel Cable MI Cable Self Regulating Cable

1 Power outputUniform & length

dependantUniform & length

IndependentUniform & length

IndependentNot Uniform

2 Start up currents High

3Variation in output to match heat losses with variation in ambient temperature

Not possible Not possible Not possible possible

4 Site modification / cutting & field repairs Cumbersome Possible Difficult to install over complicated shapes.

Possible

5 Burnout possibilityPossible design

DependentPossible design

DependentCan tolerate brief period

of overheatingBurnout proof

6 Effect of circuit failure Large Only Faulty portion Large Only faulty portion

7 Watt density - W/meter 25 33 262 66

8 Suitability for following criterias:

8.1 Short length -Pipe lines (Upto 300 meters) Can be used Can be used Can be used Can be used

8.2 Long length Pipelines (Above 301 Meters) Upto 3~4 kms - Upto 1.2 kms. -

8.3 Maintenance Temperature (Deg. C) 204 66 500 150

8.4 Exposure / Design Temperature (Deg. C) 260 204 600 250

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EHT- The DesignINPUT DATA• Line List (Process & Piping)

-Maintenance Tempertaure-Design / Exposure Temperature-Pipe Line Material, Length and Diameter-Insulation Type and thickness-Pipe Fluid / Material

• P&ID• Piping Isometrics• Area Classification – Safe / Hazardous• Application – Maintenance / Heat-up• Instrument Hook-up drawings

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OUTPUT DELIVERABLES / DOCUMENTS• Design & Calculation Sheet• Circuit Schedule• EHT Isometrics• SLD• Cable Schedule• BOQ

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Sample Calculation– q = 2K(Tp - Ta)/ln(D2 / D1)

• Pipe Size – 4” with Outer Diameter – 114.31 mm• K : Cellular glass insulation thermal conductivity – 0.0494 W/m°C• Insulation Thickness – 40 mm• D2 : Outside Diameter of the Insulation – 0.1143 mtrs.

• D1 : Inside Diameter of the Insulation – 0.1943 mtrs.

• Tp : Temperature to be maintained - +80°C

• Ta : Minimum Ambient Temperature - +5°C

Using the above formula, we arrive at Heat loss Q = 43.85 W/mSame can be verified using the vendor design software.

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Standards And Codes

• IEEE 515 - Testing, Design, Installation, and Maintenance of Electrical Resistance Heat Tracing for Industrial Applications.

• NFPA 70 – Article 427• IEC 60800 - Heating cables• IEEE 844 - Practice for induction and skin effect

heating• BS 6351 - Electric surface heating

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TYPICAL HEAT TRACING PANEL WITH OFFLINE THERMOSTATS

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TYPICAL HEAT TRACING PANEL WITH ONLINE THERMOSTATS

EHT- The Design

30 Figure 7: Typical Config. (MI)

EHT- The Design

31 Figure 8: Typical Config. (MI)

EHT- The Design

32 Figure 9: Typical Config. SR Cable

EHT- THE END

Thank you!

Questions?

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