Boiler Classification based on Pressure

Pressure is a very important parameter in boilers.  The boiler and all the connected equipment are designed to withstand the pressure developed by the steam.  Pressure, is an important criterion to classify boilers.

These boilers have an operating pressure of less than 10 bar.  Natural circulation is sufficient for these boilers.  Typical application are in industries.

High pressure boilers

High pressure boilers have an operating pressure of 10 to 14 bar.  They have forced circulation.

Super high pressure boilers

Super high pressure boiler are also used for utility applications.  The operating pressure is above high pressure boilers but generally lesser than 17 bar. 

Super critical boilers

Supercritical boilers have an operation pressure higher than 22.5 bar

Miniature Boilers

These are boilers with very small capacity with a pressure less than 6.8 atmospheres and a gross volume less than 0.1415 cubic metres.

The Critical pressure in boiling a liquid is that pressure above which there is no clear change of state between the liquid and the vapour phases.  Simply put, water turns into vapour without boiling.  Above a pressure of 22.1 MPa, water reaches this state.  

In supercritical boilers, water is boiled at a very high pressure.  At that high pressure, there is no clear distinction between the water and vapour phases .  The fluid can  no longer be called liquid or vapour.  It becomes what is known as a super-critical fluid.

Supercritical Boilers are generally used in Turbine systems.  When the supercritical fluid drives the turbine, it loses pressure.  As the pressure drops below the critical point, the supercritical fluid becomes a mixture of water and steam which then passes through the condenser.

Supercritical Boilers are boilers in which the working fluid is above the critical pressure.  At this pressure, water changes into steam without boiling.  This intermediate state is known as a super critical liquid.  
Supercritical boilers are used in Turbine systems.  

Advantages of Supercritical Boilers

The advantages of supercritical boilers over sub critical boilers are


Supercritical boilers are more efficient that sub critical boilers.  They consume less fuel.  The efficiency rating of supercritical boilers is in the range of 32 - 38 % while that of ordinary boilers is in the range of 32% - 38%.

Reduced Operating Costs

As the efficiency increases, there is a natural reduction in fuel costs which translates into reduced operating costs

Lower Emissions

Due to less fuel being burnt, there are lower emissions.

Higher Initial Costs

The downside is that super-critical boilers have higher initial costs as the boiler and the systems have to be designed to withstand higher pressures.  

Advanced Water Chemistry

Supercritical boilers require very pure water.  Even small levels of impurities can cause deposits on the turbine blades.