Saturday, 9 September 2017

Boiling Curve and Types of Boiling

Today we will discuss about boiling, boiling curve and types of boiling. Boiling is a major concern for any thermal industry. It is a phenomenon in which water is converted into steam or we can say it is a phase change process from liquid state to gases state. When liquid is heated to its saturation temperature (a temperature at which the liquid starts to convert into vapor) its vapor pressure becomes higher than the surrounding pressure of atmosphere. Due to this pressure difference the liquid particles start to move into atmosphere. This process is known as boiling.

What is boiling?

Boiling can be easily understood by the following experiment.

Let’s takes water into an open container at room temperature.  The container is open to atmosphere so the pressure exerted by the surrounding to the water is atmospheric pressure. Now put the container on fire which added heat and rises the temperature as well as energy of water. Due to this energy rises vapor pressure of water also increases. After continuous heating we reach a point at which the vapor pressure of water becomes equal to atmospheric pressure. This point is known as boiling point. Further increase in temperature causes to increase in vapor pressure and it becomes more than atmospheric pressure which causes movement of water particle into atmosphere. Further rise in temperature converts whole water into steam. This phenomenon is known as boiling.

The temperature at which the boiling starts is known as saturation temperature at constant pressure. The saturation temperature of water at atmospheric pressure is 100 degree centigrade. If the pressure increases the saturation temperature is also increases.

Boiling Curve and Types of Boiling:

Let’s take a look of the curve given below.

Boiling Curve and Types of Boiling

This curve is known as boiling curve. This curve is divided into four regions according to heat transfer phenomenon. When we continuously supply heat into water the phase change process takes place. This process depends upon the surface temperature. These regions sometimes refer as types of boiling.  

Natural Convection Boiling:

AB is the first most regions in which the heat transfer process takes place by free convection. The water particles associated with the heating surface take heat from it and start moving at the top. Now the other particles occupied blank space and start heated from heating surface. This process continuously run until whole water is converted into steam.

Nucleate Boiling:

The next region BC is known as Nucleate region or Nucleate type of boiling. This is the most desirable part of boiling which is used by many industries. In this region, bubbles formation take place. The water particles associated with the heating surface vaporized and converted into bubbles. These bubbles move at top of the liquid and collapse. This region can be further divided into two regions.

The first one is shown by BB’ in which bubbles formation take place and the bubbles start to move at top side. These bubble passes form water which is below its saturation temperature so the bubbles transfer its heat to surrounded water particle and collapse into water.

The other region is B’C in which the whole water converted at saturation temperature and the bubbles start to pass from surrounded water. They do not transfer its heat into surrounding and they collapse at the top of the liquid and release steam into surrounding. This phenomenon of boiling is known as nucleate boiling. This region provides maximum heat transfer rate.  The top most point at boiling curve in nucleate boiling region is known as critical heat flux point.

Transition Boiling:

The third region is shown by CD which is known as transition region. This region is nothing but the mediator region of nucleate boiling and film boiling. This is very undesirable part of boiling curve due to its poor heat transfer rate. In this region the water particles associated with the heating surface converted into steam and forms a vapor blanket. This blanket has poor heat transfer rate thus other liquid particles find hard to take heat from source. This is the reason behind the low heat transfer rate and the undesirability. The lowest point of transition region is known as Leidenfrost Point.

Film Boiling:

The last but the least part of boiling curve or types of boiling is known as film boiling. This region has huge heat transfer rate. It is shown by DE in boiling curve. This region has highest temperature difference between water and the heat source. When we increase this difference further the heat transfer rate increases continuously. In this region the most of heat is transferred by radiation. When we further increase the temperature of heat source after transition region the density of vapor blanket start decrease and form a radiation heat transfer region between liquid particles and heat source. That’s why this region shows significant increase in heat transfer rate.

These are the most common boiling region of boiling curve or types of boiling. If you have any query regarding this article, ask by commenting. If you like this article, don’t forget to share it on social networks. Subscribe our website for more informative articles.


Post a Comment