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Compressed air pipe sizing

Compressed air pipe sizing

Compressed air pipe sizing

(OP)
Hello,
 
I have a compressed air application.  
 
Air flow  = m             -    360 lb/min = 21,600 lb/hr
Air Pressure = P        - 750 PSIG = 764.1 PSIA = 110,030 lb/ft2
Air Temperature = T  - 1400 Deg. F = 1860 Deg. R
Molar Mass of air = M = 28.97
Universal Gas Constant = Ru = 1545
Equivalent length of Pipe = 100 Feet
Maximum air velocity in pipe = 3,500 ft/min (A standard I made an Assumption)
 
Problem:  Determine the pipe inner diameter from the above operating conditions
 
1.    I calculate the specific volume from the given conditions
        
       v=RuT/PM = (1545)(1860)/(110,030)(28.97) = 0.902 ft3/lb
 
2.    I multiply the specific volume by the mass flow rate to obtain CFM
 
       CFM = 0.902 ft3/lb x 360 lb/min = 325 ft3/min
 
3.    I calculate pipe area knowing the specific volume and CFM rate
 
       Pipe Area = A = CFM/FPM
                        
                          = 325 ft3/min / 3,500 ft/min = 0.093 ft2
 
4.    The pipe diameter can be calculated as follows
 
        Dia = d = (4A/3.145)**0.5
 
                   = ((4 * 0.093)/3.145)**0.5 = 0.344 ft = 4.12 in.  (Use 4" ID pipe, Incoloy 800HT)
 
5.    I calculate the pressure drop as follows
 
    Delta P per 100 foot of pipe = 0.000336fm**2v/d**5  (f is friction factor which is 0.017 from moody chart)  (Friction formula from Crane "Flow of Fluids")
 
    Delta P per 100 foot of pipe = (0.000336)(0.016)(21,600**2)(0.902)/(4**5) = 2.21 psig/100 ft of equivalent pipe
 
    Delta P = 2.21 psig/100ft * 100ft = 2.21 psig
 
Question #1:  Are my calculations correct?
 
Question #2:  My Research Engineering group at the plant uses a Mach 0.3 as a standard and they figure a 1.9 pipe ID.  Based on my experiences with compressed air and the fact I'm not sure how they calculated this inner pipe diameter, I think the ID is too small.  Are they correct?  
 
Please respond with your questions or comments.  I look forward to hearing from you.
 

RE: Compressed air pipe sizing

Your calculation is correct! their calculation is also correct!
The question here is: what pressure drop is acceptable in your application.
Their pressure drop is about 98 psig and a velocity of 253 ft/sec.
Your pressure drop is 2.21 psig and your velocity is low. Does it need to be so low (Pressure drop and velocity?)
You should not exceed 50% of sonic speed, 30% is very very safe!
In this case 50% of sonic speed is 1035 ft/sec.
Their velocity is about 253ft/sec. and yours is about 58 ft/sec.
These figures are based on 2 inch schedule 80 pipe and 4 inch schedule 40 pipe.
I have used a compressibility of .96

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