Mechanical Engineering: Boiler economisers

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Purpose

The purpose of the economiser is to increase plant efficiency by removing heat that would otherwise be lost in the flue gas and use it to indirectly feed heat the water. By heating the feed water it is also helping to prevent thermal shock as the water enters the steam drum.

Description

The flow of water is general counterflow. The exception to this is in the radiant heat boiler where the economiser is mounted immediately above the superheaters. The water flow in these so called 'Steaming economisers' is then parallel flow to decrease the tendency for the economiser to steam excessively above the design limits and lead to steam blockage , this is why these economisers are bare tube with no extended heating surface. Vent and drains are fitted to header where isolating valves are fitted a safety valve must also be added.
The modern design involves the fabrication of the header and stub tubes which can then be heat treated. The tubes are then site welded to the stubs.
The previous use of expanded joints has now fallen out of failure due to the requirement of a multitude of hand hole doors with associated joints and hence possible area of leakage. The materials used are governed the materials susceptibility to cold end corrosion (see later notes), any metal having a surface temperature below the dew point will tend to have acidic deposits forming on its surface caused by the water absorbing sulphur trioxide and dioxide from the flue gases. Some metals are more resistant to this form of corrosion at lower temperatures, choice of material will initially depend on the minimum metal surface temperature and is calculated as the feed water temperature that is passing through the economiser plus 5^oC. For temperatures greater than 138^oC solid drawn mild steel tubing is used. Fitted to these are welded on extended surface steel fins or studs.
For temperatures between 115^oC to 138^oC shrunk on or cast iron gills must be used.
The temperature should not be allowed to fall below 115oC as this can lead to heavy fouling as well as corrosive attack.
Efficient sootblowing is absolutely essential to ensure that surface are kept clear of combustion products which can not only lead to heavy corrosion and a drop in efficiency , but also to the possibility of an economiser fire with potentially disastrous consequences. With this in mind it is not unusual to find provision for water washing, something which is carried out on a very regular basis on a Motor ship with waste heat recovery.
If due to failure it is required to run the economiser dry then the maximum gas inlet temperature should be limited to about 370^oC, vents and drains should be left open to ensure that there is no build up of pressure from any water that may be still located in the tubes.