A single "pass" is defined as the area where combustion gases travel the length of the boiler.
Generally, boiler efficiencies increase with the number of passes.
However it's time as passed, most ships nowadays use the more economical diesel burning heavy fuels.
A firetube boiler is a cylindrical vessel, with the flame in the furnace and the combustion gases inside the tubes.
The furnace and tubes are within a large vessel, which contains water and steam.
In addition, the wetback design is more prone to water side sludge buildup, because of the restricted flow areas near the turn around chamber.
Clicking on the link to Cleaver-Brooks will take you to various models and sizes of boilers.
A large quantity of tubes results in more heating surface per boiler horsepower, which greatly improves heat transfer and efficiency.
The furnace and banks of tubes are sued to transfer heat to the water.The firetube construction provides some characteristics that differentiate it from other boiler types.Because if its vessel size, the firetube contains a large amount of water, allowing it to respond to load changes with a minimum variation in steam pressure.Duties like heating cargo, fuel, and accommodations. Such as deck winches and pumps, where electrical machines would prove to be a hazard as in the oil industry.Within the boiler, fuel and air are force into the furnace by the burner. From there, the heat (flue gases) travel throughout the boiler.Firetube boilers are available for low or high pressure steam, or for hot water applications.