Supa:
In the true, engineering sense there is no pro or con (or competition) between the water tube boiler design and the fire tube boiler design. Each fits an application or niche that the engineer specifies or identifies. In the general scheme of things you would never apply a fire tube design (“Scotch Marine”) to a high pressure steam requirement. The reason? Very simply, the cost of applying a high pressure to a large diameter vessel goes up because the thickness of the vessel wall goes up – as well as the tube sheets and the involved welding. As any engineer will appreciate, the water tube design is “tailored” to high pressures because of the nature of the smaller diameter tubes being more accommodating to withstanding the high pressures with less wall thickness. Additionally, as the capacity needs increase, so does the diameter of the Scotch Marine vessel and the wall thickness requirements really skyrocket when compounded with high pressure steam requirements. The water tube design is much more flexible because all you have to do is add more tubes for capacity and it adapts very well to superheating and efficiency applications. It does, however, take relatively more room.
On the other hand, at the relatively small capacities and low pressures (50 – 150 psig) you will find that the cost and compactness of the Scotch Marine design is very acceptable.
The cost (and sometimes the size or “foot print”) of the ultimate solution really determine the design to apply to the application. That is why Scotch Marine designs find acceptance in commercial and small capacities such as laundries, breweries, dedicated services, etc.
My advice is: don’t try to pit the Scotch Marine against the water tube design. Each has its niche and does quite well there.
Art Montemayor
Spring, TX
