Actual vs. Standard flows

Have you ever noticed that when a flow rate like SCFM, SLPM, or NM3/hr is reported, there is a pressure associated with it? There is a reason for this. The “S” in SCFM and SLPM, is for Standard, and the “N” in NM3/hr is for Normal. It is the amount of air being used at atmospheric pressure and temperature. To further explain it, if you take a segment of air out of a compressed air line and place it next to you at ambient temperature and pressure, it will expand to a larger volume. Think of it like an air-filled balloon floating on top of the water. This would be the “Standard” or “Normal” condition. As you take the balloon into deeper water, the more pressure is applied to the balloon, and the volume decreases. This is because air is compressible. The balloon still has the same amount of air by weight (as the volume decreases, the density increases). If you return back to the surface, the balloon will expand back to the original size.

Pressure
Pressure

Being that the flow rate for nozzles, knives, etc., are rated at a standard or normal condition, why do we require a pressure rating? It should be at the atmospheric pressure and temperature. Well, this is where it gets tricky. Just like the air-filled balloon, the deeper you go (higher pressures), the less volume is in the balloon. So, when we have different pressures, we are trying to find the actual volume of air being used because that is what you are paying for. Also, this is where the term ACFM (lpm or M3/hr) comes into play. The “A” in ACFM is for Actual (the volume of air at the actual pressure and temperature). Pneumatic devices use this type of flow (ACFM), but for the ease of understanding, they convert it to a SCFM, SLPM, or NM^3/hr at a pressure. If we assume ambient temperatures because most of our products are used there, then the correlation between Actual and Standard is Qa = Qs * Pa/(P +Pa) .

Imperial Units                                    SI units

Qa          Actual flow (ACFM)                         Actual flow (M^3/hr)

Qs           Standard flow (SCFM)                    Normal flow (NM^3/hr)

P             Gage Pressure (psig)                      Gage Pressure (barg)

Pa           Absolute Pressure (psia)                Absolute Pressure (bara)

 

The reason for this explanation is because some competitors like to use a lower pressure to rate their products. As an example, two air nozzles are rated for 70 SCFM (119 NM^3/hr). One nozzle is cataloged at 60 psig (4.1 barg) and the other is cataloged at 80 psig (5.5 barg). By comparison, they look like they use the same amount of compressed air, but actually they do not. Under the actual condition (using the formula above), we have the following:

Imperial Units                                                                    SI Units

@60 psig                                                                              @4.1 barg

Qa = 70 SCFM * 14.7 psia/(60 psig + 14.7 psia)     Qa = 119 NM^3/hr * 1 bara/(4.1 barg + 1 bara)

= 13.8 ACFM (actual amount of air used)                 = 23.3 M^3/hr (actual amount of air used)

 

@80 psig                                                                              @5.5barg

Qa = 70 SCFM * 14.7 psia/(80 psig + 14.7 psia)     Qa = 119 NM^3/hr * 1 bara/(5.5 barg + 1 bara)

= 10.9 ACFM (actual amount of air used)                = 18.3 M^3/hr (actual amount of air used)

 

Even though it seems like they use the same amount of compressed air, you are actually using 27% more air with the nozzle reported at 60 psig than the one that was reported at 80 psig. Always remember that if you want to compare air usage, always do it at the same pressure and temperature. If you need help, you can always contact our application engineers here at EXAIR.

John Ball
Application Engineer
Email: johnball@exair.com
Twitter: @EXAIR_jb

 

Image courtesy of UpUpa4me. Creative Comment License

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