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Wetback vs Dryback: How Marine Boiler Design Affects Efficiency and Costs
- 1. 1 Wetback vs Dryback Scotch Marine Boiler Design
- 2. 2 Wetback vs Dryback Goal The purpose of this presentation is to provide project decision-makers with fundamental, and critical, boiler design information.
- 3. 3 Wetback vs Dryback Two Basic Designs Dominate the Scotch Marine Boiler Market: Wetback Dryback
- 4. 4 Wetback vs Dryback Wetback vs. Dryback • Basic construction comparison • Design principles • Technical considerations • Total operating costs comparison This presentation will show you the differences
- 5. 5 Wetback vs Dryback Three-Pass Wetback
- 6. 6 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”
- 7. 7 Wetback vs Dryback Four-Pass Dryback
- 8. 8 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”
- 9. 9 Wetback vs Dryback 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.
- 10. 10 Wetback vs Dryback Rear Tube Sheets Wetback is Separate Dryback is Common
- 11. 11 Wetback vs Dryback 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.
- 12. 14 Wetback vs Dryback 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
- 13. 15 Wetback vs Dryback Doors Wetback • Front Doors are typically split-hinged, or davited • Rear Doors are lightweight (< 60#) lift-type
- 14. 16 Wetback vs Dryback Doors Wetback • Larger models typically feature hinged, or davited rear doors • Split doors maintain efficient “functional footprint”
- 15. 17 Wetback vs Dryback 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.
- 16. 18 Wetback vs Dryback 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
- 17. 19 Wetback vs Dryback Simplified design requires far fewer seals All are non-proprietary, inexpensive, and easy to install Sealing Kits Wetback
- 18. 20 Wetback vs Dryback Sealing Kits Dryback “Watch Case” design requires numerous proprietary sealing kits for each inspection, and every vessel service
- 19. 21 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
- 20. 23 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 -
- 21. 24 Wetback vs Dryback 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;
- 22. 25 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
- 23. 26 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!!
- 24. 27 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?