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Scotch Marine Boiler Design
Wetback vs Dryback
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Goal
The purpose of this presentation is to
provide project decision-makers with
fundamental, and critical, boiler design
information.
Wetback vs Dryback
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Two Basic Designs
Dominate the Scotch Marine
Boiler Market:
Wetback
Dryback
Wetback vs Dryback
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Wetback vs. Dryback
This presentation will show you the differences
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Basic construction comparison
Design principles
Technical considerations
Total operating costs comparison
Wetback vs Dryback
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Three-Pass Wetback
Wetback vs Dryback
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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”
Wetback vs Dryback
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Four-Pass Dryback
Wetback vs Dryback
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Dryback Design Principles
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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”
Wetback vs Dryback
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Tube Sheets
Wetback
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.
Wetback vs Dryback
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Rear Tube Sheets
Wetback is Separate
Dryback is Common
Wetback vs Dryback
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Rear Turnaround
Wetback
• 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.
Wetback vs Dryback
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Rear Refractory Replacement
Wetback
• 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
Wetback vs Dryback
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Doors
Wetback
• Front Doors are typically split-hinged, or davited
• Rear Doors are lightweight (< 60#) lift-type
Wetback vs Dryback
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Doors
Wetback
• Larger models typically feature hinged, or davited
rear doors
• Split doors maintain efficient “functional footprint”
Wetback vs Dryback
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Doors
Dryback
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.
Wetback vs Dryback
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Doors
Dryback
• Large, heavy, single
front door offers
complicated multisectioned design
• Additional costs for
seal kits and labor can
significantly impact
annual operating
expenses
Wetback vs Dryback
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Sealing Kits
Wetback
Simplified design requires far fewer seals
All are non-proprietary, inexpensive, and easy to install
Wetback vs Dryback
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Sealing Kits
Dryback
“Watch Case” design requires numerous proprietary sealing
kits for each inspection, and every vessel service
Wetback vs Dryback
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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
Wetback vs Dryback
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The Wetback Advantage:
Summary
Wetback Boilers • 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: Maintenancefree water backed turnaround provides better heat
absorption at the most critical heat transfer point.
Wetback vs Dryback
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Maintenance Costs Comparison
Bottom 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;
Wetback vs Dryback
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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
Wetback vs Dryback
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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!!
Wetback vs Dryback
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Add In Consideration to
Sustainable Efficiency
Improvement.
Don’t You Think Someone
Should Know That Before Making
an Equipment Decision?
Any Questions?
Wetback vs Dryback