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1
Wetback vs Dryback
Scotch Marine Boiler Design
2
Wetback vs Dryback
Goal
The purpose of this presentation is to provide project decision-makers with
fundamental, and critical, boiler design information.
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Wetback vs Dryback
Two Basic DesignsDominate the Scotch Marine
Boiler Market:
Wetback
Dryback
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Wetback vs Dryback
Wetback vs. Dryback
• Basic construction comparison
• Design principles
• Technical considerations• Total operating costs comparison
This presentation will show you the differences
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Wetback vs Dryback
Three-Pass Wetback
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Wetback vs Dryback
Wetback Design Principles
• Separate tube sheets between all major temperature changes (between tube passes)
• Rear turnaround is totally surrounded by water • No expensive refractory to maintain• Rear doors are either lightweight lift off type or
split-hinged • Sealing materials are inexpensive, non-proprietary• Efficient “functional footprint”
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Wetback vs Dryback
Four-Pass Dryback
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Wetback vs Dryback
Dryback Design Principles
• Common rear tube sheet between passes
• Rear turnaround is a refractory wall
• Door refractory is a maintenance item
• Rear door is vessel-sized in diameter, extremely heavy, and hinged or davited
• Sealing materials are typically proprietary
• Large “functional footprint”
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Wetback vs Dryback
Tube SheetsWetback
Separate rear tube sheets
• Separate tube sheets are free to expand and contract at their own rate in response to the 1300 – 1600 F temp. differential between passes.
Dryback Common rear tube
sheet
• Common tube sheet experiences extreme thermal stress in response to temp. differentials, increasing the likelihood of leaks.
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Wetback vs Dryback
Rear Tube Sheets
Wetback is Separate Dryback is Common
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Wetback vs Dryback
Rear TurnaroundWetback
• Surrounding water absorbs burner heat, improving efficiency by 1 to 3%.
• Efficiency is sustainable, as turnaround does not deteriorate over time.
Dryback• Rear refractory wall
reflects burner heat, promoting greater exterior radiation losses.
• Hot flue gases erode refractory baffle resulting in “short circuiting,” and loss of efficiency.
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Wetback vs Dryback
Rear Refractory ReplacementWetback
• NO expensive refractory to maintain
• Significant maintenance cost savings over the life of ownership
Dryback • Refractory must be
inspected regularly and replaced periodically
• Replacement costs are burdensome, involving proprietary sealing kits, special rigging and down time
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Wetback vs Dryback
DoorsWetback
• Front Doors are typically split-hinged, or davited• Rear Doors are lightweight (< 60#) lift-type
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Wetback vs Dryback
DoorsWetback
• Larger models typically feature hinged, or davited rear doors
• Split doors maintain efficient “functional footprint”
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Wetback vs Dryback
DoorsDryback
Annual inspections are typically more costly for the dryback, requiring proprietary door sealing kits, special tools, and considerable manpower in “muscling” massive, and typically sagging, doors into “bolt-thru” alignment.
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Wetback vs Dryback
DoorsDryback
• Large, heavy, single front door offers complicated multi-sectioned design
• Additional costs for seal kits and labor can significantly impact annual operating expenses
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Wetback vs Dryback
Simplified design requires far fewer seals
All are non-proprietary, inexpensive, and easy to install
Sealing KitsWetback
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Wetback vs Dryback
Sealing KitsDryback
“Watch Case” design requires numerous proprietary sealing kits for each inspection, and every vessel service
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Wetback vs Dryback
Functional Footprint
Dryback
Wetback
• Dryback: Vessel-diameter door means a larger functional footprint, demanding additional floorspace
•Wetback: Lift-type or split-hinged doors have minimal impact on floorspace requirements
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Wetback vs Dryback
The Wetback Advantage:Summary
• Offer far fewer maintenance concerns: - No rear door refractory to repair- No refractory baffling to burn-out - Far less thermal stress on tube sheets, and tube ends
• Don’t require proprietary parts
• Offer maximum sustainable efficiency: Maintenance-free water backed turnaround provides better heat absorption at the most critical heat transfer point.
Wetback Boilers -
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Wetback vs Dryback
Maintenance Costs ComparisonBottom Line
We surveyed a few of our service reps who perform repair/maintenance work on boilers and specifically asked
them to share dryback expenses.We averaged them together and came up with the following
maintenance report;
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Wetback vs Dryback
Based on repairs costs of a 300 HP boiler with a life span of 25 years
• Average cost to replace refractory rear door; $6,000 each time
• Average cost to replace proprietary door gaskets; $500 each time
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Wetback vs Dryback
The rear door needs to be replaced every 3 years, or 8 times. The gaskets need replaced 2-3 times per year.
• Refractory door; $6,000 x 8 times = $48,000• Door gaskets; $500 x 2 times/year x 25 = $25,000• Wetback gaskets; $30 x 1/year x 25 years = $750 Total maintenance costs for 25 years $72,250
They could have bought a new boiler and burner!!
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Wetback vs Dryback
Add In Consideration to Sustainable Efficiency
Improvement.
Don’t You Think Someone Should Know That Before Making
an Equipment Decision?
Any Questions?