Compressed air pipe sizing

[e024342]

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&quot

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.

 

[chappy7]

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