Over Heat
Degree of Super Heat (OHDR)—and its Implications
In Operation of Supercritical Boiler, many important
Parameters are required to be monitored and controlled; Over Heat Degree of Super Heat (OHDR) is one of them.
Just to simplify, if in a supercritical Boiler, if the
evaporator water temperature is maintained at 405 Degc at 25 MPA , then it will
have a OHDR of 20 degc, as the critical temperature as per T-S curve for a
pressure of 25 MPA is 385 Degc
Generally Boiler Manufacturers specify OHDR around 25 Degc,
and Water to coal ratio of 6 to 7.
Higher OHDR
than 25 Degc essentially means that the Heat absorbed in the evaporator section
is more than designed as a result of which steam will be generated in in
Straight Water walls , steam being a bad conductor of Heat will not be able to
absorb heat from the Water wall Tubes, as a result of which the metal temperature
of water walls will have a tendency to shoot up. This may lead to failure of
water walls or supply tubes exposed to high heat radiant portions, but not able
to cool down.
Higher OHDR
also means that as compared to Water flow, coal consumed will be more,
displaying lower Water to Coal ratio than specified. Low water to coal ratio
will also happen in cases where GCV of coal is much lower requiring much more
coal for generating same quantity of steam.
Lower OHDR
than 25 DegC signifies that Water is not able to absorb required heat, thereby
leading to evaporation of water post evaporator, Water to coal ratio will show
higher value , establishing that water flow is higher than Coal, this will lead
to drop in Water and eventually steam temperature. Low OHDR will lead to
lowering of Steam generation and eventually MW generated.
So monitoring and maintaining OHDR and Water to Coal ratio
are significantly important aspects for stable and efficient Operation of a
Supercritical Boiler
Chetan Shrivastav
