Steam gets produced only when there is an effective circulation of water within the boiler. For tank-type boilers, circulation is not distinct and the water circulates within the tank of the boiler itself.
For water tube boilers, circulation is necessary within each and every tube and header. This happens naturally if both the steam and water drums are connected with tubes. But there is some basic design reason which has to be emphasized.
Natural Circulation within a Water Tube Boiler
The steam drum and water drum may or may not be separated depending on the boiler design. To understand the boiler water circulation principle, we will assume the following components to be separate.
– A steam drum at the top is connected to the water drum at the bottom through downcomers outside the boiler shell and risers inside the boiler shell.
– Coldwater is fed to the top drum/steam drum, by the water feed pump through a screw-down non-return valve and a check valve. As we all know, hot water stays on top and relatively cold water (being denser) travels to the bottom of the drum.
– The steam/water drum has downcomers connecting to the water ring at the bottom of the boiler.
– The downcomers are located outside the boiler shell and are large in diameter when compared to the water tubes.
– As the cold water from the downcomers reaches the water ring at the bottom, the circulation starts within the tubes and drums.
– As the water particles enter the water tubes, which are inside the boiler furnace, they start to heat up and become wet steam with some bubbles.
– As they are less dense, they immediately rise up to the steam drum and thus are continuously being replaced by relatively cold water from downcomers.
Thus boiler water circulation happens naturally inside a water tube boiler.