Dear mouadrabihsenhaji,

... the air flow in the graph is m^3/hr not kg/hr
You are correct.
I saw a straight curve and assumed kg/sec. My mistake.

Q1:why you calculate the volume ?
The volume is required to calculate the air velocity (not necessary to calculate anymore)

Q2:give the therotical equation that you use to calculcate volume ?
Volume = mass/density

Q3: why you calculate v begin and v end ..and how?
v-begin = Volume at begin / pipe area

v-end = Volume at end / pipe area

Have a nice day