How do I calculate the z factor automatically
 

How do I calculate the z factor automatically by using Hall-Yarborogh equation without changing (Y) every time to reach F=0 ?
if any body know .please answer me.





INPUT DATA:
T 68 oF
p 1000 psia
SGFG 0.5 air=1

CALCULATE CRITICAL AND REDUCED TEMPERATURE AND PRESSU
Tpc=169.0+314.0*SGFG Ppc=708.75-57.5*SGFG
Tpr=(T+460.0)/Tpc t=1/Tpr
Ppr=p/Ppc


CALCULATE TEMPERATURE DEPENDENT TERMS:
A=0.06125*t*EXP(-1.2*(1.-t**2)
B=t*(14.76-9.76*t+4.58*t*t)
C=t*(90.7-242.2*t+42.4*t*t)
D=2.18+2.82*t


CALCULATE REDUCED DENSITY Y
Y = ASSUMED 0.324006985
F=-A*Ppr+(Y+Y*Y+Y**3-Y**4)/(1.-Y)**3-B*Y*Y+C*Y**D
USE goal seek MACRO SOLUTION: goal seek (F) = 0 by changing (Y)

CALCULATE Z-FACTOR:

Z=A*Ppr/Y

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