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Thursday, February 14, 2008

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Recently i was supervising a young engineer in the design of Steam - Water system for an LNG plant. This kind of system can be easily simulated in any Process Simulator such as HYSYS, PRO-II, etc and generate a Heat & Material Balance (HMB). If you have several years experience, you may notice that nowadays young engineer tends to use Process simulator and ignoring the basic behind. Due to this reason, i insisted the young engineer to conduct Heat & Material Balance for Steam -water system MANUALLY using Excel spreadsheet. Without any surprise the young engineer having difficulties...

In "Useful Steam - Condensate Calculator", there is an Excel Add-in (Water97_v13.xla or Alternative download) available FREE for download, is very useful for calculating thermodynamic and transport properties of water and steam using the industrial standard IAPWS-IF97. In this post, i will elaborate a little bit on the method to conduct steam-water balance manually using above add-in.

Problem statement :
High pressure steam (HS) at 20 barg @ saturated condition mix with Boiler Feed Water (BFW) at 15 barg @ 60 degC to 1000 kg/h Low Pressure Steam (LS) at 3.5 barg @ Saturated condition. Find quantity of HS & BFW.

Material Balance :
M1 + M2 = M3
M2 = M3 - M1
Heat Balance :
M1 x h1,vs + M2 x h2,T = M3 x h3,vs
M1 x h1,vs + (M3-M1) x h2,T = M3 x h3,vs
M1 x h1,vs + M3 x h2,T - M1 x h2,T = M3 x h3,vs
M1 x h1,vs - M1 x h2,T = M3 x h3,vs - M3 x h2,T
M1 x (h1,vs - h2,T) = M3 x (h3,vs - h2,T)
M1 = M3 x (h3,vs - h2,T) / (h1,vs - h2,T)
a) For HS Steam : Get Specific enthalpy (h1,vs) for Saturated steam at pressure 20 using enthalpySatVapPW (P)
[e.g. enthalpySatVapPW(20+1.01325)]
b) For BFW : Get Specific enthalpy (h2,T) for Subcooled liquid at 15 barg @ 60 degC using enthalpyW(T,P)
[e.g. enthalpyW(60+273.15, 15+1.01325)]
c) For LS Steam : Get Specific enthalpy (h3,vs) for Saturated steam at pressure 3.5 using enthalpySatVapPW (P)
[e.g. enthalpySatVapPW(3.5+1.01325)]
d) As M3 is 1000 kg/h, M1 & M2 can be obtained by modeling it in the EXCEL sheet.
For M1 :
=1000 * (enthalpySatVapPW(3.5+1.01325) - enthalpyW(60+273.15,15+1.01325))/ (enthalpySatVapPW(20+1.01325) - enthalpyW(60+273.15,15+1.01325))
For M2 :
= 1000 - M1
Another method is using GOAL SEEK in EXCEL sheet...
M1 & M2 can be adjusted until the enthalpy difference between INLET section (e.g. M1 x h1,vs + M2 x h2,T ) and OUTLET section (e.g. M3 x h3,vs) equal to zero AND M1+M2 = M3. The adjust function can be easily setup using GOAL SEEK feature in EXCEL.

Details may refer to following image.

It is rather simple programming as seen from above and no simulator is required.

Young engineer is always encourage to conduct above calculation as least 2-3 times so that you are sure yourself understand heat & material balance in depth. By doing this kind of calculation, you will surprise your depth understanding. Later stage, you can always simulate balance with process simulator to save time.

For those who are interested in the EXCEL spreadsheet, you may drop a note to me.

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posted by Webworm, 2:10 PM


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