Sunrod boilers

In a Sunrod boiler a waterwall makes a gas-tight furnace from which the flue gases pass through the Sunrod pin tube elements to the smoke-box on the top of the boiler.
You may consider the Sunrod boiler as a hybrid between fire tube boilers and water tube boilers. The waterwall consists of water tubes, but the Sunrod element is a flue tube with a water-tube inside.
Svenska Maskinverken originally designed the Sunrod boiler but nowadays several boilermakers manufacture them.

Sunrod boilers,
water and steam flow diagram

Blue: the downcomers lead the water from the steam drum to the circular water drum.
Red and blue: the waterwalls lead the water and steam emulsion back to the steam drum.
Note the heavy steaming in the Sunrod tubes.
Red: Steam to the consumers.

The Sunrod boiler tubes

The Sunrod boiler tubes, with the extended surface, make it possible to provide body compactness combined with the large furnace volume.
This gives good combustion, high efficiency, low furnace load and low stress in the furnace body material combined with very quick response in steam generation.
The self-circulation in the boilers vertical tubes, with the gas side surface extended by the tube-pins, is very efficient due to the high heat transfer from the gas to the water.

If you have a circulation pump for an exhaust gas economizer connected to the lower drum then avoid starting or stopping the pump while the burner is firing, since the water flow in the waterwall might be disturbed. For the very same reason the drain valves on the waterwall headers mustn't be opened when the burner is firing. This goes for all boilers with water waterwalls.

A small size Sunrod steam boiler
0.5 to 6 tons of steam per hour.

Two Sundrod marine steam boilers just lifted onboard into a hardly half-finished ship at a shipyard.