HRSG off-design calculations
HRSG off-design calculations
(OP)
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!
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!





RE: HRSG off-design calculations
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
RE: HRSG off-design calculations
Jack Hardie