Adequate Area of Economizer for High Pressure Water-Tube Boiler

[ghamsa]

During detailed design, how can I assure adequate area of an economizer for high pressure water-tube boiler?
The boiler will produce 625 psig super-heated steams.

Many thanks

 

[davefitz]

You will ned to develop a program or spreadsheet that calculates the heat transfer duty of all of the boiler components simultaneously to ensure the design requirements are met, at several different loads. Assuming a coal fired unit:
Working backwards from the required boiler efficiency, you can determine the stack gas temperature ( for a given assumed boiler excess air and air heater leakage rate). Then move back toward the required flue gas temperature entering the air heater, assuming an "X ratio" ( = WgCpg/ WaCpa) of 1.2:1, and air heater heat transfer effectiveness of 0.9 = (Tao- Tai)/(Tgi, Tai) and WgCPg ( Tgi-Tgo) = WaCpa(Tao-Tai) ( corrected for leakage), and correcting the Tai for the use of a steam coil air heater. Tao must be high enough to dry the worst coal. The upper end of allowable Tgi is about 750 F to ensure the flues can be fabtricated with carbon steel.
The boiler feedwater temp entering the economizer is obtained from the steam turbine cycle heat balance, equal to the water temp leaving the highest pressure feedwater heater.
An overall heat and mass balance for the boiler envelope will yield the neccesary temperature entering the economizer, assuming it is downstream of the reheater in a series pass configuration of the boiler's back pass. The only degree of flexibility you have is selecting the preferred economizer outlet water temperature- this usually should be 40 F below drum saturation temperature for drum type boilers to provide some subcooling as the feedwater is distributed over the water that is discharged from the drum steam-water centrifugal separators- this subcooling reduces the tendency to entrain steam down the downcomers.

A check on the proper design is to confirm the economizer NTU's are in the range of 1.5-2.5.