239 posts
TimePosted 11/06/2012 09:18:29
xxxx says

WHR Boiler Efficiency calculation

Dear All

I am getting eff. more than 100 % ? Why?

Pl. correct me



Steam  Temp.

330 330 330 330
Steam  Pressure 18 18 18 18
Steam Flow 10.1 12.94 9.1 20.6
Feed water Temp. 160 160 160 160
Feed water press. 25 to 26 kg/cm²
Enthalpy of Steam (kcal/kg) 741.0191 741.0191 741.019 741.0191
Enthalpy of Feed water (kcal/kg) 161 161 161 161
Flue gas flow( M³/hr) 246520.1 330205.1282 259407 584249.1
flue gas density 0.52 0.52 0.54 0.52
Sp. Heat of flue gas  0.24 0.24 0.24 0.24
Flue gas inlet  temp. °C 400 385 365 365
Flue gas outlet  temp. °C 240 241 240 240
Total heat input in boiler(kcal/hr)  4922514 5934182 4202387 9114286
heat out from boiler  5858193 7505447.154 5278174 11948393
boiler effiency 119 126 126 131


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16 posts
TimePosted 12/06/2012 06:15:18

re WHR Boiler Efficiency calculation


I don't know how exactly did you measure flows; because this is the major source of error (you can see flow specifications, on how to measure flow exactly), howver attached file is for assesing energy performance of boiler. (sorry it isn't attaching)

I hope it'll help.


138 posts
TimePosted 12/06/2012 09:02:51
lalbatros says

re WHR Boiler Efficiency calculation


You need to comment your calculations in more details.
By just doing this you might well discover by yourself many possible source of errors.
First of all, the units are missing for 9 lines of the table.
Second, your table does not show clearly how you did the calcualtions.
You could explain one column in full detail, this would be easier to check.
(a drawing would be even clearer)

As Ex-FlSmidth-Designer said, the errors on flow evaluation could in itself create big errors.
There are still many other possible source of errors:

- steam flow: what are the units? t/h maybe?
- flue gas flow: is it in actual m³ or in "normal" m³?
- flue gas flow: is it measured at the input or at the output of the boiler?
- flue gas density: is it evaluated at 400°C or at 240°C(inlet or outlet)?
- flue gas density: for a calculation I did, it was higher
- flue gas density: do you take gas composition properly into account?
- flue gas specific heat: units ???
- flue gas specific heat: how did you evaluate it?
- flue gas specific heat: did you take the composition into account?
- flue gas specific heat: is it the average heat value between 400°C and 240°C
                                   or is this the specific heat value at 400°C ?
- flue gas specific heat: why is it exactly the same for the three scenarios?
- flue gas specific heat: for a calculation I did, it was 0.28 kcal/kg/°C
- flue gas specific heat: do you take water vapor into account?
- flue gas specific heat: ...  
- heat input: I see how you made the calculation, should be ok
                  however "heat input" is a bad name, 
                  you might call it "heat exchanged" (flue gas side or steam side)

If you explain your calculations in more detail, there is really a good chance that you find some approximations and some source of errors and that you improve your balance. You could also add an uncertainty for each data and do some error bar calculation!.

Don't hesitate to put all you data and calculations in an excel worksheet and to attach it to your post. If you do so, do not forget to include flue gas composition.


239 posts
TimePosted 13/06/2012 09:29:01
xxxx says

re WHR Boiler Efficiency calculation

see the attached file .

also cp does not make much change & flows are correct