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Wetback vs Dryback

Scotch Marine Boiler Design

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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?