Scotch boilers (often referred to as scotch marine boilers) derive their name from the Scottish shipyards that built boilers for marine vessels for the British Navy. Old brick set boilers used to burn through the bottom of the ships. The “Scotch Marine” design featured a cylindrical vessel enclosing a totally immersed furnace so they could not do damage to the British naval vessels.

These were the first truly “packaged boilers” since they had to be created for shipboard installation with everything essential already attached. From their use on ships, this same type of boiler caught on in the 1920’s as an alternative to the brickset boilers that were commonly constructed at that time.
Scotch design boilers are typically used for low or high pressure steam or low water heating and process applications often in industrial or institutional applications. They have medium to high space requirements. They are compatible with a variety of building management systems and are appropriate in situations calling for high efficiency.

Scotch design boilers all have an immersed internal furnace with tubes which carry the hot combustion gases through the boiler water on their way to the vent. The differences between scotch boilers produced by various manufacturers are:

• Number of Passes
• Rear Portion Construction

We reviewed the implications of the Number of Passes in the previous section. And while we have stated that the same amount of heat will be transferred by a boiler of any number of passes as compared to any other boiler if the boilers have equal heating surface...differences exist in the “quality” of the heating surfaces.

For example: Three-pass type boilers have greater primary heating surface and require fewer tubes. Four-pass type boilers have lower primary heating surface and require more tubes. Three-pass boiler types are generally accepted as the design which provides optimum economy of material in a compact vessel with acceptable draft loss.

In dealing with Rear Portion Construction, the terms “wet-back” and “dry-back” come into play. These are graphic terms which describe varying construction of a boiler at the point where the gases exit from the furnace and reverse direction to enter the first pass tubes:

• If the rear chamber is constructed so that its rear wall is water-backed, the boiler is a “wet back” boiler.
• If the rear chamber is enclosed at the rear by a removable cover containing refractory, the boiler is a “dry-back.”

The major advantage of dry-back construction is an unrestricted access to the rear ends of the flue tubes when the cover is opened.

Major Disadvantages are: that valuable radiant heating surface is sacrificed during the hottest portion of the heat exchange chain. The maintenance costs, in both time and dollars, involved in maintaining the refractory door required in “dry-back” construction is also a major drawback.