This article deals with the famous ESD III type water tube marine boilers available in the market today. ESD III type boilers substituted ESD II type for the same reason for which ESD II substituted ESD I, i.e. to accommodate a higher superheated steam temperature range. Moreover, the ESD II type are more technically advanced and have better efficiency. They are the most widely used boilers in the industry today.
The ESD III type boilers have burners located on top of the furnace roofs. This type of arrangement facilitated a provision for long flame path along with heat transfer through out the furnace. Many of these types of boilers are made without using any kind of refractory material.Instead of a refractory material the boiler has a monowall construction, which is made up of finned tubes welded together to form gas tight casing.
The monowall construction also forms the furnace floor , side walls and roof tubes. All these tubes are welded into the steam and water drum. Upper and lower headers are connected to the front and rear walls of the furnace on either sides. Moreover, the steam drum is connected to the lower headers through external downcomers, while riser tubes are used to connect the steam drum to upper headers.
The gases, before entering the secondary superheater passes, through a stack of screen tubes. These screen tubes permits the flow of steam through the passages between them. This feature results in higher volume of heat transfer to take place. The gases from the secondary superheater flow to primary superheater and economizer before going to the exhaust. The steam is procured from the steam drum with the help of a dry pipe located in the steam drum and is then transferred to the primary and secondary superheater. The temperature of steam during all these processes is controlled by the attemperator.
The main drawback of this type of boiler is that the carbon particles that results from the combustion of fuel on the fire side of the boiler sticks to the outer surface of the water tubes. This deposited layer reduces the transfer rate of heat to the water. It is for this reason that soot blowing is carried out on ships as a routine exercise. Another method for removing this carbon is by inserting a long rod shaped tool in the fire side of the boiler.This results in a blowing effect of the steam, which results in removal of carbon deposits on the outside of the water tubes.
References & Image Credits
Introduction of Marine Engineering 2nd Edition by D.A Taylor