The feed system of the boiler is the system where the exhausted steam will return to the boiler as feedwater. And the feed system is between the boiler and the turbine. It has four basic units that are the boiler, turbine, condenser, and feed pump. The steam which produced by the boiler, After that it will supply to the turbine. Then after it will go to the condenser as exhaust gas. In the condenser, the exhaust gas will convert to water. And later it will pump to the boiler itself.
And also the feed system of the boiler has some units like a drain tank to perform some particular tasks. It will use to collect the condensate water from the condenser and then provide a suction head for the feed pump. There is a make-up tank that helps to store the surplus feed water that comes from the drain tank. In the case of the auxiliary boiler the drain tank will exposed to the atmosphere. So this arrangement will known as the ‘open feed’ system. In some boiler like high-pressure water tube boiler, the system is called the closed feed system because it is not exposed to the atmosphere.
Open Feed System:
The open feed system will work as the exhaust steam condensate by passing through the condenser and form water. The construction of the condenser is simple in that it is circulated by the seawater at atmospheric pressure. By using the feed filter tank we can collect the condensed water that drains through the feed filter tank. For the transferring of the condensated water to the feed filter tank, we will use extraction pumps. The fuel oil heating system, oil tank heating is also connected to the feed filter system. Also, an observation tank is there to inspect the discharge and also the drain for any contamination.
The feed filter system has a baffle arrangement to facilitates the oil separation from the condensate water. After that, the condensate drain will pass through the coal to make it further clean and pure. The feed filter system will never get because of an arrangement is there. This arrangement is like an overflow pipe is connected to the feedwater system. Also, a heater will fit the feedwater system. The heater then heats the system and may sometimes help with deaeration. By doing the deaeration, we can easily remove the oxygen from the system. By removing the oxygen we can prevent corrosion. Also, the feedwater system is arranged with the feed regulator. And the feed regulator will control the pressure level of the water in it.
Closed Feed System:
The closed feed system is using for the high-pressure water tube boiler. Because it will supply the steam for the main propulsion. The working of this is, the exhaust coming from the steam turbine will going to the condenser at high vacuum. The condenser used here is the regenerative type. And this will help to condensate the steam with little temperature loss. Then with help of the extraction pipe, it can be removed, and then it will start to flow around the air ejector.
Bypassing through the air ejector the condensate water will get heated up. Also, the air ejector will remove the air from it. And allowing for further heating, It will pass through the gland steam condenser. In this, the steam is condensed, and after it will pass to the drain tank. We are using many types of heat exchangers to improve the efficiency by the recovery of the heat, feed water temperature increase also, the deaeration process.
And the dissolved gasses like oxygen will come from the feed water. There is a storage tank in the bottom part of the deaerator. It will supply the feed water to the main pump. Then the feed water will go to the feed heater having high pressure. Then it will go to the economizer then to the boiler water drain. For keeping the extra or the surplus feed water, there will be an arrangement known as an atmospheric drain tank. Also, the drain usually collects the drain water from the other systems also.
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Auxiliary Feed System:
This arrangement is for the steam recovery from auxiliaries. It may be either the open feed system or the closed feed system. For this, some kind of machinery such as deck auxiliaries will use. An auxiliary condenser that will operate at atmospheric pressure will help to condense the steam. To supply the condensate water to the air ejector an extraction pump will use. After that, it will go back to the main feed system. For the low load working, there is a reciprocating line. And also to maintain a condensate level, a level controller will be there.
Steam to steam generator arrangement is there to avoid contamination and other impurities in the steam. And then it will supply to serve for the fuel oil heating. Then the drain of it will come to the hot well of the system. The feed pump which presents there will push the feed water to the heater. The heater will also have a drain cooler. This will help to heat the steam that given to the generator. Then the feed water will pass from the heater to the steam to the steam generator. Also, there will e a packed feed system where the mountings and the units are in one base.
Components of the Feed System:
A condenser is a form of heat exchanger that will help to remove the heat from the exhaust unit. After condensing we can pump the feed water to the boiler. The condensation here done by the reduction of the condensate temperature to the level below the steam temperature. The condenser will also help to remove the gases and the vapors.
There is a circular cross-section shell attached to the end cover. This will help for the two-pass flow of the seawater through it. The steam will enter the system through the center from the top and then divide into two passes. And then it will pass through the ports in the steam inlet. The cooling surface is given to the steam by the seawater that passes through the tube of the tubes in the system. The tubes will get support by the diaphragm and the plates will get support by the stay rod. Also, there is a sump tank and a tube that will collect the condensate water. For emitting the gases out of the system, there is an arrangement called air suction.
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2. Extraction Pump:
The extraction pump in the system will use to take the water content from the condenser. Also, the pump will help to provide a sufficient amount of pressure for delivering the feed water to the deaerator system. The common extraction pump that usually uses is the centrifugal type and also has a two-stage operation. The specialty of the pump is that it needs a minimum suction head for the operation. In the first stage, The impeller will get water that boiling at high vacuum. After that, the water will discharge at positive pressure to the second stage.
Then the condenser sump level will vary or can maintain as almost dry. This will take the form of cavitation. This will happen when the suction head comes to a low value. The cavitation means the collapse of the bubble. That will cause the discharge rate to zero. usually, the cavitation will cause damage to the parts. But in the case of the low-pressure conditions, there will not be any damage.
3. Air Ejector:
The air ejector is using to remove or eject the air and also the vapors that will release from the condensing steam. In this case, if the air is not removing from the system. Then it will cause serious damage like corrosion to the system. Also, the presence of air in the condenser will also affect the process of condensing. Also, the presence of the air will cause back pressure in the system will lead to the exhaust steam pressure. this will later lead to the reduced thermal efficiency of the system.
There is a two-stage twin element air ejector. In this system, the first stage is a steam operating air ejector. And the working is like the pump will take the air in and the vapors also that present in the condenser. This will pass to the unit of the condenser that circulating by the feed water. Then the feed water will heat up and the gases will condense.
4. Heat Exchangers:
The heat exchangers are many types. These are gland steam condenser, drain cooler, and the low-pressure feed heater. But the working of these types of heat exchangers is different. But the main principle of all heat exchangers is to recover the heat from the steam by using the heater. The gland condenser will take the steam, vapors, and also the air from the system. And these elements will cool by using the feed water and then the steam will get condensed. After that, the condensate will come back to the system. Here the feed water will move through the ‘U’ Shaped tubes in the shell.
The deaerator system is using for the full removal of the air and the vapors in the system. And another function of this is it will act as a feed heater also. The function is like the feed water will heat to the point of boiling. Then at this time all the vapors and the gases will blow off the system. The feed water that coming through the spray valve of the system, then the water will spray. This will provide a large surface area for contacting the heating system.
Then the feed water later moves to the upper part of the deaerator. There it will heat up by steam coming to the system. After the feed water goes to the central passage, this will act as an ejector to remove the steam. The tank on the bottom side of the deaerator will collect the condensate and also the feed water.
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Conclusion: This is all about the types and the working of feed system of boiler on board ship. Comment your doubts and opinions below.