ELECTRIC HEAT TRACING.ppt
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Transcript of ELECTRIC HEAT TRACING.ppt
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ELECTRIC HEAT TRACING
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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
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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.
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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.
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ocoi
ap
hDhDkDD
hD
TTq
22
12
1
112
)/ln(1
Heat Loss
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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
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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
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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.
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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.
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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.
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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.
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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.
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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
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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
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EHT- The Design
30 Figure 7: Typical Config. (MI)
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EHT- The Design
31 Figure 8: Typical Config. (MI)
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EHT- The Design
32 Figure 9: Typical Config. SR Cable
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EHT- THE END
Thank you!
Questions?
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