HRSG off-design calculations 1

[Guido]

I am working on an off-design calculation for heat transfer over the HRSG's Economizer (ECO) & Superheating (SH) tube banks. Specifically on the (UA)-calculation in off-design for a generic case; meaning without any geometric data.

By simplifying the actual cross-flow of the flue gas to either parallel or counter for the ECO- or SH- tube banks for design conditions, one can calculate the (UA)-value using Q=(UA)*dTlog.(=no flow correction factor used) Here, the dTlog depends on the flow direction; parallel- or counter-flow.

Is this same (UA) calculation allowed for off-design conditions, or does the (UA) change too much for it to remain valid? Do I need a correction factor for the flow direction? Somebody told me, the (UA) changes in off-design for changing flue gas velocities. But how does this affect the calculation?

Any answers to these questions or any information (web-sites) on HRSG off-design behaviour are greatly appreciated.

Many thanks!

 

[MortenA]

one question: off-design as in out of design specifications? If yes then:

Assuming that you want more flow then if you assume that UA remains constant this would be conservative because U depends on 3 things: inside film resistance, heat transfer through pipe wall outside film resistance. The film resistance will drop when the velocity is increased and the overall U will thus drop.

However: Usually most of the resistance is on either inside or outside film, so unless you change both flowrates U might not change much. Also if you lower one velocity and increase another U might drop.

Best Regards

Morten

 

[jehar]

There is a site dedicated to answering detailed thermal design questions on HRSG and Wast Heat Boilers. The site is at

http://www.HRSGdesign.com.

Thsi site has detailed methods for computation of heat transfer coefficients that will allow you to determine what effect flue gas temperature or flow will have on the UA value.
Jack Hardie