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the home of .TT pocketEngineer softDesign - where pocketEngineer software lives...
 Compressed Air Pipe Sizing Designer (CompAir v3.0)

compressed air piping designer - 3 model equations to find flowrate, pipe size & pressure drop for your compressed air system



To take a glance at all pocketEngineer software and OS requirements, click Overview.


CompAir: Compressed Air Pipe Sizing Designer 

Traditionally, tables or charts are used for manual calculations. With CompAir program designed for mobility, you can now do pipe sizing at anytime, anywhere. CompAir program is engineered for ultimate flexibility with 3 built-in model equations for solving flowrate, pipe size and pressure drop.


The 3 model equations adopted in CompAir program are

(1) General Compressible Flow Equation.

(2) IoP (Institute of Plumbing) / CAGI (Compressed Air and Gas Institute) model.

(3) BCAS (British Compressed Air Society) model.

Aim: creating a mobile design environment (
ShowMe!) for the practising engineers & designers in today's mobile world.

Results: Instant solutions at your fingertips.



- a small Windows PC program for piping design of Compressed Air system.

- 3 model equations available for pipe sizing with various options.

- 4 options available to solve pipe sizing problems: find flowrate, find pressure drop, find pipe size (PD method) and find pipe size (Vel method).

- built-in data, guides, etc.

- "save to file" function for printing.

- individually selectable SI-IP units.


 The selectable SI-IP units available are:


 Parametersselectable SI-IP units
 pressurebar, kPa, psi, in.wg
 pipe diameter mm, in
 pipe lengthm, ft 
 velocitym/s, ft/s
 flowratel/s, cmh, cfh, cfm 



Design Explained 1: General Compressible Flow Equation 

Calculating pressure loss with the General Compressible Flow equation allows selection of material roughness, thus allowing Moody friction factor to be calculated for any flow regime. With computer program doing the work for you, this method is feasible and practical now. The General Flow equation gives the most accurate results.


For Air at STP conditions, the General Compressible Flow equation can be reduced to the following form with Specific gravity = 1 and Compressibility factor = 1 approx. (see Note 1 below):





Note 1: For engineering calculation, we can expect that the behaviour of air within pressure and temperature ranges in compressed air application can be approximated as an ideal gas with reasonable accuracy. For example, at 300 oK, the compressibility factor ranges from 0.9987 (at 5 bar) to 0.9950 (at 20 bar).



Did you know ? . . .

In Roman numeral system, one M refers to one Thousand (i.e., M = 1000; MM = 1000 x 1000).

In Metric system, one K refers to one Thousand (i.e., K = 1000).



Design Explained 2: IoP/CAGI & BCAS models  

As solving the General Flow equation is tedious, various forms of simplified equation are available for manual calculations. Most of the simplified equations are derived from the General Flow equation with assumptions for steel pipe application.


The IoP uses the following form of simplified equation:


PD = K L Q2 / (CR d5.3


where  Q = free air flow rate (l/s)

           d = inside diameter of pipe (mm)

         PD = pressure drop (bar)

         CR = compression ratio

          L = pipe length (m)

          K = constant (800)


It is noted that "Compressed Air and Gas Handbook" published by CAGI uses the same formula for its tables.


Notably, publications by BCAS generally adopts the following form of simplified equation:


PD = K L Q1.85 / (P d5)


where  Q = free air flow rate (m3/s)

           d = inside diameter of pipe (mm)

         PD = pressure drop (bar)

           P = pipeline pressure (bar absolute)

           L = pipe length (m)

          K = constant (1.6 x 108)


Comparison of Results:  


Pipeline pressure = 97 psi (gauge), Pressure drop = 0.26 psi, Flowrate (free air) = 560 cfm, Pipe length = 375 ft.


 Model Equation

Calculated Pipe Diameter (in)

General Flow







Compressed Air Pipe Sizing Example 1: using General Flow Equation 


A quantity of 560 cfm (free air) is flowing in a 375-ft, 4-in, Schedule 40 pipe with a 97-psig inlet pressure. Determine the pressure loss. 


The following results are computed by CompAir program:


Compressed Air Pipe Sizing Calculations
Find Pressure Drop
(General Flow Equation)
Pipe MaterialSteel
Friction factor, f0.0184
Relative roughness 0.000450
Re number217,195
Pipeline temperature70 °F (21.1 °C)
Pipeline gauge pressure, P197 psi
Pressure drop, PD0.2623 psi
Pipe length, L375 ft
Pipe dia, d4 in
Flow rate (free air), Qfa560 cfm
Compressed air flow, Qca 73.70 cfm
Velocity, Vel14.08 ft/s
Compression ratio, CR 7.60



Compressed Air Pipe Sizing Example 2: using BCAS Model 



A flowrate of 100 l/s has to be supplied over a distance of 59m at a pressure of 7 bar gauge. The pipe is 53mm diameter. Find the pressure drop. 


The following results are computed by CompAir program:


Compressed Air Pipe Sizing Calculations
Find Pressure Drop
(BCAS model)
Pipeline gauge pressure, P7 bar
Pressure drop, PD0.0398 bar
Pipe length, L 59 m
Pipe dia, D53 mm
Flowrate (free air), Qfa100 l/s
Compressed air flow, Qca12.64 l/s
Velocity, Vel5.73 m/s
Compression ratio, CR7.91





For Compressed Air Ring Main pressure drop calculation, see  CA Loop .




 For Gas Pipe Sizing, see  pocketGAS .




Price: US $9.90





OS requirements: Windows.


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in selectable SI-IP units 



 CompAir explained





General Flow Equation model 



IoP/CAGI model 



BCAS model