Benson Boiler- Construction & Working Principal- Askpowerplant - ASKPOWERPLANT

Benson Boiler- Construction & Working Principal- Askpowerplant

working principal of benson boiler

Heii all my Askpowerplant readers after a long time of interval today I am coming back with a fresh topic which is related to boiler operation because now these days boiler operation becomes a trending job so everyone wants to know that how to do boiler operation in a safe manner so that as per looking into the demand of this topic we discussed today one of the familiar boilers which are Benson Boiler. today we discussed what is Benson boiler? what is the working of Bension Boiler? what are the advantages and disadvantages of the Banson Boiler? what is the working criteria of Benson Boiler? All these sections we covered in this topic so that please read the full article if you won’t really get sufficient information related to Benson Boiler.


At that time Benson boiler comes into the category of super critical steam generator . it is a high pressure, drumless, supercritical, water tube steam boiler with forced circulation. Benson boiler invented by Mark Benson in the year 1922. Benson boiler can generate high pressure steam, which is further used in production of electricity in the thermal power plant and other industrial processes like cotton indusrties milk and dairy products and also used in beverage plants . this type of boiler is water tube boiler. if we talk about the early stage water tube boiler used to generate steam at the pressure up to 10MPa, which is known as sub critical boiler. at that time two major problems occured in sub-critical boiler.

1. First problam comes into the sub critical boiler . wehen we heat up the water inside the water wall panel water form the vapor bubbles at the tube surface, which decrease heat transfer rate, hence it’s decrease the boiler efficiency.

2. The second comes into, the sub critical boiler . steam purification is required so that uses of the water steam separator inside the drum to separate steam which make it too complex and bulcky in construction and also in weight . It cannot be easily transfer from one station to another. due the


To get ride from these problems, in year 1922, Mark Benson developed a boiler which works above super critical pressure and generate steam. after these development this boiler known at the Benson boiler. In this boiler feed water is compressed to a supercritical pressure and this prevents the formation of bubbles inside water tube surface generally formed in the low pressure boiler during steam formation when it gains the latent heat from the combustion products .


The water bubbles do not form because of the supercritical pressure the density of water and steam becomes the same. Mark Benson was the first person who first proposed the idea to compress the water at supercritical pressure before heating into the boiler and due to this the latent heat of water reduces to zero. As the latent heat of water reduces to zero the water directly changes into steam without the formation of bubbles.Benson boiler works at pressure above critical pressure, at which the water suddenly convert into steam. No air bubbles generate in this boiler during steam formation process. This boiler generate a high pressure steam which is used in various industrious processes as mentioned above.


The working principle of the Benson Boiler is that when the boiler pressure was raised to critical pressure, the steam and water have the same density and therefore, the danger of bubbles formation can be eliminated. in the Benson Boiler, the pressure of the water is increased up to the supercritical pressure (i.e. above the critical pressure of 225 bar).

When the pressure of water inside the boiler is increased to the supercritical level, the latent heat of water becomes Zero and due to this, it directly changes into steam without boiling. And it’s avoiding the formation of bubbles at the inside of tube surface. forced circulation also applied in the beanson boiler but has a unique characteristic of the absence of the steam separating drums.

Also main working principle of the Benson Boiler is that at the critical pressure, the steam and water coexist at the same density and heat is zero. The water this transforms into steam, without boiling and passes quickly from liquid to vapor without changing the volume.


Benson boiler consists of six main parts which are given below. in this boiler, large tubes are assembled in which water is entered into one side and the superheated steam are comes out from another side. no extra steam drum is used for separating moisture from the steam. main parts of the Benson boiler are followings :
Benson Boiler- Construction & Working Principal

1. Feed Pump :

The boiler Feed pump is critical equipment used in a thermal power plant which supplies deaerated and LP dosed feedwater to the boiler. This centrifugal pump has to pump feedwater into the boiler drum against the boiler drum pressure So, the discharge pressure of this pump is higher than the boiler drum pressure. Generally, a boiler feed pump is a multistage pump which is driven by an electrical motor or turbine. In a large boiler, like Benson Boiler where steam generation capacity is more, more feedwater is required to be pumped. In this case, a turbo feed pump is preferred.

These turbo feed pumps feed the water at supercritical pressure 222 kg/cm2. Also, the turbo feed pump is having another merit. Whenever plant power goes off in that situation, the turbo feed pump can pump feedwater into the boiler, by utilizing steam of the boiler. So, starvation can be avoided. During the cold startup of a boiler, a motor-driven feed pump is started and after steam generation at the boiler in case of supercritical boiler, the feed pump is switched over to the turbo-driven one.

2. Air-preheater :

APH air preheaters are also an important part of the boiler circuit. If we talk about its construction Air Preheaters (APH) is the Shell and tube type. This type of Heat Exchangers using for preheating the air which is fed to the boiler or furnaces/kilns for the combustion of fuels. In the boiler circuit, APH Air Pre heater’s primary objective for its installation is to extract the waste heat from the flue gases leaving which is generating after the combustion of fuel into the boiler furnace. Air Preheaters have a shell and tube type construction where exhaust heat from the flue gases is transferred to the air when they come in indirect contact with each other.

If we talk about the arrangements of Air Preheaters it consists of a shell where tubes are arranged in a square or triangular pitch and are welded to a tube plate. Depending upon the type of operation and ease of cleaning, flue gases & air can be allowed to travel either shell side or tube side interchangeably. Common air preheaters are two passes according to the flue gas diversion.

3. Economizer :

Economizer is used to utilizing the fuel economically or make the boiler economical. Hot exhaust flue gas from the boiler which would have gone to the atmosphere is used to increase the efficiency of the boiler. The economizer is a heat exchanger in which flue gas flows in a shell arranged with water tubes. The heat of flue gas is utilized to increase the temperature of the water so as to increase the sensible heat of the water.

It is found that by decreasing the exhaust flue gas temperature by 16 °C, boiler efficiency increases by 1%. Also by increasing the feedwater temperature by 6 °C, boiler efficiency increases by 1%. Economizer also helps the reduced exhaust flue gas temperature and feedwater temperature can be increased

4. Radiant Evaporator

in the Radiant evaporator, all the parallel tube of it’s located at the radiation zone of the boiler. It was placed directly in the combustion chamber. Radiant evaporator tubes are exposed to the flame of the furnace. Heat transfer is done mainly by the radiation method produced after the burning of fuel. In the radiant evaporator, most feed water gets superheated which comes from the economizer.

5. Convective Evaporator :

The remaining water which comes from the radiant evaporator is again evaporated in the convection evaporator section in the Benson boiler which is shown in the figure, here convective evaporator absorbing the heat from the hot gases by convection method. Thus the saturated high-pressure steam at a pressure of 210 kg/cm2 is produced. In the convective evaporates the superheated water from the radiant evaporator gets converted into steam. It takes place by convection mode of heat transfer to the water from the hot flue gases.

6. convection superheater

 convection superheater: Superheater is a set of tubes Normally, a number of parallel tubes are connected between the two headers to minimize the pressure drop. One header is connected to the evaporative superheater to receive saturated steam and another is connected to the outgoing pipe through the main steam stop valve (MSSV). after passing through the convection superheater steam gets superheated is gains a temperature of approximately 645 to 655 Deg.C . convection superheater is placed at the convection zone of the boiler. Heat transfer takes place in the convection superheater mostly by the convective method. radiant evaporator, the convection evaporator, and the convection superheater are all arranged in the path of the flue gases In the Benson Boiler . which all play an important role in getting superheated steam.


1. Boiler efficiency of Benson boiler up to 95%.

2. this boiler has not to steam drum so it is light in weight so its transportation from one place to another is very easy so it does not have any shifting issues.

3. at the supercritical pressure the density of water and steam almost the same so that water behaves like steam so that bubble formation not happening in the tube in waterside as like in low-pressure boilers with natural circulation.’s starting time is very less as compared to other boilers it’s started within 15 minutes and other boilers started in 2-3 hours if we talk about the cold startup.

5. in this type of boiler chance of the explosion is very negligible because in this boiler the diameter of the tubes is very less.

6. during the erection time it occupies a very small space as compared to other boilers.

7. if we talk about its weight this type of boiler contains 20 % less weight as compared with other types of boilers.


1. Benson boilers are operating at a supercritical pressure so extra care should be taken for avoiding the chances of a boiler explosion.

2. At fluctuating load it’s controlling is very difficult.


Benson boilers are widely used for generating steam for generating electricity in the power industries.

2. this type of boiler is working on the supercritical pressure or temperature so that the operating pressure, temperature range, and capacity of Benson boiler is 650 degree Celsius, 250 bar, and 135 tonnes/h.

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