Reply To: Performance Monitoring

#2161
dcicero
Participant

Thank you for the additional reference. I’ve read through it and found it very helpful.

I’m struggling to reconcile the Calculation of LMTD in the Appendix A with Section 5. In Section 5 the LMTD calculation requires:

A = heat exchange surface area, m2
LMTD = log mean temperature difference, K
Q = heat duty, W
U = overall heat transfer coefficient, W/m2/K
hfg = design value of the latent heat of vaporization, J/kg
m = design value of the steam mass flow rate, kg/s
x = design value of the steam quality

In Appendix A, th same calculation requires:

A = heat exchange area, m2
hfg = latent heat of vaporization at design backpressure, J/kg
m = design steam mass flow rate, kg/s
Q = ACC heat load, W
U = overall heat transfer coefficient, W/m2-K
x = design steam quality

And then, in the example calculation…

A = heat exchange surface area (total air side), m2 = 1,150,643
hfg = latent heat of vaporization, kJ/kg = 2,390.3
m = design steam mass flow, kg/s = 234.6
Q = heat load, kW = 560,856.3
U = heat transfer coefficient, W/m2-°C = 34.49
x = exhaust steam quality, .932
LMTD = log mean temperature difference, °C = 14.13

I suspect this is a unit conversion problem, but I can’t seem to get the given equations and given values to calculate the answer of 14.13. I’ve highlighted the different units used in these calculations.

Has anyone else seen this difference between the same calculations in different parts of this document? Any ideas about how I might resolve this?

Dan

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