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Closed Feed System for Marine Auxiliary Boiler

The feed system completes the cycle between boiler and turbine to enable the exhausted steam to return to the boiler as feedwater. The feed system is made up of four basic items: the boiler, the turbine, the condenser and the feed pump. The boiler produces steam which is supplied to the turbine and finally exhausted as low-energy steam to the condenser. The condenser condenses the steam to water (condensate) which is then pumped into the boiler by the feed pump.

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Other items are incorporated into all practical feed systems, such as a drain tank to collect the condensate from the condenser and provide a suction head for the feed pump. A make-up feed tank will provide additional feedwater to supplement losses or store surplus feed from the drain tank.

In a system associated with an auxiliary boiler, as on a motor ship, the drain tank or hotwell will be open to the atmosphere. Such a feed system is therefore referred to as 'open feed'. In high-pressure watertube boiler installations no part of the feed system is open to the atmosphere and it is known as 'closed feed'.

Closed feed system

A closed feed system for a high pressure watertube boiler supplying a main propulsion steam turbine is shown in Figure above. The steam turbine will exhaust into the condenser which will be at a high vacuum.

A regenerative type of condenser will be used which allows condensing of the steam with the minimum drop in temperature. The condensate is removed by an extraction pump and circulates through an air ejector.

The condensate is heated in passing through the air ejector. The ejector removes air from the condenser using steam-ope rated ejectors. The condensate is now circulated through a gland steam condenser where it is further heated. In this heat exchanger the turbine gland steam is condensed and drains to the atmospheric drain tank. The condensate is now passed through a low-pressure heater which is supplied with bled steam from the turbine. All these various heat exchangers improve the plant efficiency by recovering heat, and the increased feedwater temperature assists in the de-aeration process. The de-aerator is a direct contact feed heater, i.e. the feedwater and the heating steam actually mix.


Closed feed system

Fig: Energy conversion in a Closed feed system

In addition to heating, any dissolved gases, particularly oxygen, are released from the feedwater. The lower part of the de-aerator is a storage tank which supplies feedwater to the main feed pumps, one of which will supply the boiler's requirements. The feedwater passes to a high-pressure feed heater and then to the economiser and the boiler water drum. An atmospheric drain tank and a feed tank are present in the system to store surplus feedwater and supply it when required. The drain tank collects the many drains in the system such as gland steam, air ejector steam, etc. A recircuiating feed line is provided for low load and manoeuvring operation to ensure an adequate flow of feedwater through the air ejector and gland steam condenser.

The system described is only typical and variations to meet particular conditions will no doubt be found.


steam turbine

Fig: Energy conversion in a steam turbine

What is a steam turbine

The steam turbine is a device for obtaining mechanical work from the energy stored in steam. Steam enters the turbine with a high energy content and leaves after giving up most of it. The high-pressure steam from the boiler is expanded in nozzles to create a high-velocity jet of steam. The nozzle acts to convert heat energy in the steam into kinetic energy. This jet is directed into blades mounted on the periphery of a wheel or disc (Figure above).





Related Info:

Open feed system for an auxiliary boiler - how it works

Auxiliary Feed systems for boilers and steam turbines

Boiler turbo feed pump - working procedure

Heat exchangers in a boiler feed system / steam turbines

Boiler feed operation and maintenance

Open feed system for an auxiliary boiler - how it works

Auxiliary Feed systems for boilers and steam turbines

Boiler turbo feed pump - working procedure

Feed extraction pump - working procedure

How an air ejector works in a boiler feed system

Function of condenser in a feed system for boiler and steam turbine

How De-aerator works in a boiler feed system

Heat exchangers in a boiler feed system / steam turbines

Boiler feed operation and maintenance

Function of High pressure feed heater



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