Steam recovery systems - What is an Auxiliary Feed System?

Introduction

An open or closed feed system is used as a steam recovery system between the boiler and the turbine. For steam recovery from auxiliaries and ship services of the ship, auxiliary feed system is used. An auxiliary feed system can be an open feed system or a closed feed system or a combination of both. It can even be a part of the main feed system.

All the components of an auxiliary feed system are same as that of an open or closed feed system. The system consists of a condenser, an extraction pump, air ejector, gland steam condenser, drain cooler.

Auxiliary Feed System

If the system includes deck auxiliaries that are driven by steam, a separate auxiliary condenser operating at atmospheric pressure is used to condense the steam coming from the auxiliaries. The condensate thus formed is transferred to the air ejector with the help of extraction pumps. Any amount of dissolved oxygen is removed from the feed water and the temperature of the water is also raised. The air ejector supplies the condensate back to the system at a point somewhere between the gland steam condenser and the drains cooler.

Just as in an open or closed system, the auxiliary system is also provided with a recirculating line for low load operations. A water level controller is also provided to maintain the level of condensate in the condenser.

The main problem with an auxiliary system is that it cannot have a direct contact de-aerator as the chances of contamination are more in case of auxiliaries. For that reason,more often a steam-to steam generator is used with a separate feed system. A steam-to -steam generator is generally used for low pressure applications. The low pressure steam is used for services such as fuel oil heating. The drains of all the auxiliaries return to the hot well.

Feed Heater

The system also uses a feed heater, which is basically a heat exchanger. Feed pumps are used to supply the feed to the feed heater. The same feed heater, is used as a drains cooler, which heats the feed water and cools down the drains. The feed water after getting heated from the feed heater goes to the steam-to-steam generator.

Nowadays, even packaged feed system are available in the market. In this type, all the components of the feed system are mounted on one common base plate, which makes the system more easy to access and operate.

References

McGeorge, H.D. (1999) Marine Auxiliary Machinery. 7th edn. Oxford: Butterworth-Heinemann Publishing