The radiant superheater is located in a harsh environment. The furnace exit gas temperature is one of the most difficult parameters to estimate. Several variables affect furnace performance such as fuel type,analysis,excess air,flue gas recirculation rate,burner design,presence of refractory in the furnace and furnace dimensions to mention a few. Variations of 100-200 F from measured values are not uncommon. Radiant energy varies as the fourth power of absolute temperature and hence a few degrees higher than estimated value can transfer significant amount of radiant energy to the superheater,thus increasing the tube wall and support temperatures,leading to failures.
The superheater is located in a region where it is difficult to predict the performance accurately due to the 180 degree turn to the convection bank and associated non-uniform gas temperature profile,turbulence and flow pattern.
The load versus enthalpy absorbed or steam temperature curve is shown for both superheater types. The radiant superheater absorbs more energy at lower loads,while the convective design absorbs more at higher loads. However at low loads,due to low steam and gas velocities and associated pressure drops,the flow maldistribution inside and outside the tubes will be much higher.It is likely that some tubes may even be starved.Hence with uncertain cooling inside tubes and higher radiant heat flux outside,the radiant superheater is more likely to encounter tube failures.
If fuel oils containing sodium,vanadium are fired,the radiant design is likely to be corroded at a faster rate due to the higher operating temperatures and slag/deposit formation rate.
The surface area required to transfer a given amount of energy will be lower due to the higher log-mean-temperature difference and higher heat transfer coefficient.Hence cost may be lower in spite of the better grade of materials required.
Convective superheater is located in a low gas temperature region-ranging from 300 to 1000 F lower,depending on the degree of superheat required. Since it is shielded by several rows of screen tubes,the gas is well mixed and cooled before it encounters the superheater and hence the performance can be predicted more accurately.
At lower loads,though the flow mal-distribution on gas and steam side is higher,it is associated with much cooler gas and hence failures are rare compared to the radiant design.
Depending on the degree of superheat,the superheater can be pushed further into the convection bank,thus lowering the tube wall temperatures which require lower grade tube materials. Life of the superheater is also thereby increased. Using a radiant superheater irrespective of the degree of superheat required is not sound engineering practice.A 50-200 F degree of superheat can be well handled by a convective superheater shielded by several rows of screen tubes.If the degree of superheat is say only 10-30 F,the superheater can be even located between the evaporator and economizer!
Due to the lower log-mean-temperature difference and lower
heat transfer coefficient,the surface area required will be more and hence could be more expensive than the radiant design.
The convective superheater can be designed as a two-stage unit with interstage desuperheating for steam temperature control,while it is difficult and uncommon with the radiant design. Hence steam temperatures can be much higher than predicted with the radiant superheater and tube failures are more likely.
The convective design is suitable for heavy oil fired applications as it can be shielded from hot corrosive flue gases and slagging concerns by proper location.The superheater can be located at a gas temperature region of 1300-1400 F versus 2000-2300 F in the radiant version.