Scotch Marine Boiler Design

Goal The purpose of this presentation is to provide project decision-makers with fundamental, and critical, boiler design information.

Two Basic Designs Dominate the Scotch Marine Boiler Market: Wetback Dryback

Wetback vs. Dryback Basic construction comparison Design principles This presentation will show you the differences Basic construction comparison Design principles Technical considerations Total operating costs comparison

Three-Pass Wetback

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”

Four-Pass 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”

Separate rear 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.

Rear Tube Sheets Wetback is Separate Dryback is Common

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.

Rear Turnaround Wetback Better Heat Transfer at the Turnaround Means Reduced Maintenance Concerns for the Tube Ends

Rear Turnaround Dryback Higher Turnaround Temperatures -Up to 300F Higher- Mean a Tougher (and Often Shorter) Life for Tube Ends

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

Doors Wetback Front Doors are typically split-hinged, or davited Rear Doors are lightweight (< 60#) lift-type

Doors Wetback Larger models typically feature hinged, or davited rear doors Split doors maintain efficient “functional footprint”

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.

Doors Dryback Large, heavy, single front door offers complicated multi-sectioned design Additional costs for seal kits and labor can significantly impact annual operating expenses

Sealing Kits Wetback Simplified design requires far fewer seals All are non-proprietary, inexpensive, and easy to install

Sealing Kits Dryback “Watch Case” design requires numerous proprietary sealing kits for each inspection, and every vessel service

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

Maintenance Cost Comparison The True Cost of Dryback Ownership Annual Inspection In some cases (Ohio units, for example) dryback requires proprietary tubes, adding as much as a 50% premium to tube replacement costs Larger (increased vessel pressure drop) proprietary motors can add as much as a 350% premium to motor replacement costs Proprietary sealing kits and more labor to open & close the boiler amount to a 100% premium over costs for a comparable wetback Dryback utilizes proprietary brick throat tile. Specialty labor adds a 25 – 40% premium to replacement costs.Wetback instead uses a one-piece casting. No refractory costs for a wetback vs. at least $5,000 per repair for a comparable dryback boiler Rear Door Refractory Throat Tile Tube Replacement Blower Motor

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: Maintenance-free water backed turnaround provides better heat absorption at the most critical heat transfer point.

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;

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

They could have bought a new boiler and burner!! 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!!

Add In Consideration to Sustainable Efficiency Improvement Add In Consideration to Sustainable Efficiency Improvement. Don’t You Think Someone Should Know That Before Making an Equipment Decision? Any Questions?

After all, isn’t that the name of the game? Bottom Line: Wetback boilers deliver substantial maintenance and fuel cost savings over the life of equipment ownership, with minimal equipment downtime. Any Questions? After all, isn’t that the name of the game?