1. Scotch Marine Boiler Design

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

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

4. 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

5. Three-Pass Wetback

6. 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. Four-Pass Dryback

8. 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. 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.

10. Rear Tube Sheets
Wetback is Separate
Dryback is Common

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

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

14. 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

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

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

17. 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.

18. 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

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

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

21. 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

22. 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

23. 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.

24. 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;

25. 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

26. 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!!

27. 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?

28. 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?