Volumetric flow rates confusion...

Normal air refers to atmospheric air at sea level that contains some moisture. It is defined in the ASME Test Code for Displacement Compressors as being at 14.696 psiA, 36% RH and weighing 0.075lb/ft3.

Standard conditions of pressure and temperature, SPT for gas volumes in compressor engineering practice are 14.696 psiA and 60 deg F.

Let me expand my problem further. How to convert Nm3/hr to m3/hr?

When it is specified that the flow rate is 4500Nm3/hr, then which temp & pressure conditions must be referred? (Is it the design temp & pressure, i.e. 8.5 barg for inst air, and ambient temp)

Similarly if I take lower, capacity equipment (dryer), and pass through it high volume of air, then the velocity of air must be very high, as the supply media is compressor. What could be its effect?

Its probably better to use normalised units here...

so 1900 cfm @ 10.5 bar referred back to a standard pressure of 1 bar gives a flow of 1900 x 10 cfm = 19000 Ncfm.

Roughly ..... if I remember correctly there are about 28 cf in a cubic metre... so the nomalised units are:

19000 Ncfm = 679 cubic metres / min = 40700 Nm^3/hour

I could have slipped up on a factor of 10 in that mental calculation but it looks close to what you want I guess...

Cue Masu to come along and prove I'm wrong

John.

As stated by others, the 'normal' rating means 1 cubic metre of air taken from the 'normal' atmosphere. Or when dealing with cubic feet we refer to 'standard' atmosphere. In round figures 4,000 Nm3 is the same as 141,258 scf. Your convert between hours and minutes to suit.

However when dealing with dryers, you need to take into account the flow rate, inlet pressure, inlet temperature and the required pressure dewpoint.

I don't know what type of dryer you have (desiccant/fridge) but if 'heat-less' desiccant your need purge air and that is deducted from the input - which if running at 20% - leaves you with about 1,900 scfm - or if a fridge dryer your inlet air could be hotter than the nominal rating of the dryer - thus reducing the flow rate it can handle.

Dryer ratings are normally quoted in 'nominal' figures and you need to convert these to your actual conditions. And since loading figures are not linear, you need loading curves of each parameter for each dryer in order to compare one make/type with another.

For your size dryer, it would make sense to get the suppliers to do these calculations for you - and in writing.

N

This can be done with a simple calculation which I have been doing whenever required. Hope this helps you.

NTP as per my understanding is considered at 0 Deg C, 100% RH and at 1 bar. Hence in order to convert the same to the site conditions the site pressure, temperature and RH is required.

Consider your site conditions as 30 Deg C and Pressure as 1 Bar, we need to calculate the actual pressure including the RH. Because in general when we mean the pressure, we generally consider the saturated pressure.

Hence the first step is to calculate the pressure actually at site.

Ps = Patm-VAPOUR PRESSURE*RH.

The VAPOUR PRESSURE can be got from the refrigeration tables for a specific temperature. In your case say it is 0.9 Bar. We need to multiply the same with the RH as the mentioned pressures will be for saturated conditions.

After calculating this it is fairly simple. We need to use the P1*V1/T1 = P2*V2/T2 for mula to find out your CFM or flow in m3/hr.

Where, P1 - Atmospheric pressure in pascals.

T1 - is 273 K (0 DegC).

V1 - is your flow of 4500 Nm3/hr.

Generally while sizing the air dryers, the following parameters are of great importance.

1. The dew point required by you.

2. The design temperatures of the dryer.

3. The design pressure of the dryers.

Generally it is safe to go in for dryers designed to work at lower pressures than at higher pressures as the dryers will perform better at high pressures due to lower velocities of the air. That is the reason why generally dryer manufactures size their dryers for 10 Bar. But if your working pressure is 7 Bar then you may have to down size the dryer. Normal design temperature of dryers are just 20 Deg C. Hence if you have to go in for a higher working temperatures, the dryers may have to be downsized further. I would always suggest a refrigerant type air dryers because they are optimally sized and will maintain the RH below 50%. Any RH below 50% will not allow corrosion as well as condensation of moisture in your air line. As a thumb rule your dryer dew point at Any point should be ambient temperature - 15 deg C to ensure a RH lower than 50%.

Hope my suggestions are inline with your needs.

Specifically for dryers it would help to use vapour pressure tables from ISO 7183 The Specifications and testing of Compressed Air Dryers. You can also refer to ISO 8778 Pneumatic Fluid Power - Standard Reference Atmosphere. (20°C, 65% RH and 1 bar).
There are other standards (depending on what business you are in) , but it does not matter too much as long as the conditions are stated.

For an input of 30°C and 100% RH the vapour pressure is only 42.46mbar - nowhere near 0.9 bar that has been suggested by the example by spv1980. (0.9 bar vapour pressure is almost boiling - 97°C to be exact) and would seriously de-rate the dryer - and if true would be a sign that your compressor systems had something seriously wrong with it.

Yes your explanation is very true. But that was just a number that I selected for the calculation and actually does not pertain to 30 Deg C. I will correct myself in case of any other examples in the future, try to be me accurate!

"The eyes see what the mind can comprehend".

There are currently so many different temperatures and pressures used as "Standard conditions" or "Normal conditions" for defining volumetric gas flow rates that there just is no universally accepted set of temperature and pressure reference conditions.

The only way to avoid confusion is to always state the reference temperature and pressure for a stated volumetric gas flow rate.

I would most strongly urge you to read this:

Standard_conditions_for_temperature_and_pressure

I disagree, there are only a few different temperatures used for standard conditions...

1013 mbar (760 mm Hg) and 0 *C, 20*C and I think 15*C

For most calculations the difference between 273, 293 and 288 Kelvin makes the temperature differences a second order error term, the primary error is in the absolute pressure which is well defined as 1013 mbar and 760 mm Hg...

John.

Electroman:

You obviously have not read the article I referred to:

Standard_conditions_for_temperature_and_pressure

YES I DID and found it very unhelpful and possibly confusing....

The important bits are that the pressure is standardised at 1013 mbar / 760 mm Hg and around 20 degrees celsius...

Which bit didn't you understand???

Electroman:

As mentioned in the lead-in text, and also listed in Table 1 of that article as documented by the given reference: The International Union of Pure and Applied Chemistry (IUPAC) uses 1 bar = 1000 mbar and 0 deg Celsius and has done so since 1982. Others listed in Table 1 who also use 1 bar = 1000 mbar are:

• The Compressed Air and Gas Institute (CAGI) uses 1 bar = 1000 mbar and 20 deg Celsius
• The Society of Petroleum Engineers (SPE) uses 1 bar = 1000 mbar and 15 deg Celsius
• The Standard Ambient Pressure and Temperature (SATP) is defined as 1 bar = 1000 mbar and 25 deg Celsius by the USA's National Bureau of Standards in the "Table of Chemical Thermodynamic Properties", Journal of Physics and Chemical Reference Data, 1982, Vol. 11, Supplement 2.

I would also point out that Table 1 lists the Organization of Petroleum Exporting Countries (OPEC) and the USA's Energy Information Administration (EIA) as using 14.73 psi and 60 deg Fahrenheit which translates to 1.0156 bar = 1015.6 mbar and 15.6 deg Celsius.

With all due respect, please read that article again and do it carefully.

As one who was a practicing chemical engineer for over 50 years, I know that engineers often use approximations ... but when precision is needed, there are no universally accepted standard conditions of temperature and pressure for the volumetric flow of gases. It is very important to always state the exact standard conditions being used. When writing a contract for the pipeline delivery of very large volumes of natural gas, very small differences in temperature and pressure can result in very large differences in dollars or pounds sterling.

Hello mbeychok. I liked the detailed Wikipedia summary of STP - Thanks for the link.

It would help here if you could demonstrate with an example the sort of unwitting error that arises from making assumptions about temperature and pressure - for instance in everyday circumstances.

Like measuring flow at the actual pressure and temperature on day - and them comparing it to the error when corrected to the reference conditions - with a further comment on how one can choose a suitable standard for best commercial advantage.

In the compressed air industry one could quote 'STP' for a compressor (but having chosen one that uses 0°C) - when for most of the time compressors are working at 25 to 35C at the best. Most compressor advice is based on 'stated' conditions rather than 'standard' conditions. You have to do the calcs to a common reference point for true comparison.

Here's a couple more:

ISO8778: Pneumatic Fluid Power - Standard Reference Atmosphere: calls for 20C , 100kPa, and 65%RH.

ISO8573: Compressed Air - Contaminants and Purity Classes: calls for 1 bar absolute (being 0.1MPa), Relative water vapour pressure 0.

mbeychok, why do you keep on about 1 bar = 1000 mbar???

Of course it does worldwide 1 millibar is a thousandths of a bar, so what are you on about???

The question is about the pressure used as an atmospheric pressure and that has long been 1013 mbar or 760 mm Hg no problem!!!

Its the temperature that is either 0 *C, 15.6*C or 20*C and the difference in calculations is only a few percentage points....

Usually as with compressors, you are talking of doubling the pressure or even changing pressures by a factor of 10 times, so forget temperature, its not all that important to the question!

Instead of quoting obscure figures why not be more lucid in your answers add a bit of clarity to show that you know what you're talking about?

John.

Electroman:

There is absolutely nothing obscure about the fact the International Union of Pure and Applied Chemistry (IUPAC), Compressed Air and Gas Institute (CAGI), Society of Petroleum Engineers (SPE), the USA's National Bureau of Standards, the Organization of Petroleum Exporting Countries (OPEC) and others do not use standard pressure as being 1013 mbar ... as you keep insisting we should.

Nor is there anything obscure about the article I referred to in my previous comments, namely: Standard conditions for temperature and pressure

As I also said before: There is no universally accepted set of standard temperature and pressure. Therefore, we should always explicitly state what standard conditions we are using when we state a volumetric flow rate of a gas.

I am sorry if you don't find this to be lucid enough for you ... and I will no longer respond to further comments of yours in this thread.

Horace40:

The easiest way to see the gas volume difference between two sets of temperature and pressure is to use the combined gas law:

(V₂/V₁) = (P₁/P₂) (T₂/T₁)

where:

V = volume
P = absolute pressure (i.e., not gauge presure)
T = absolute temperature (i.e., deg Rankine or deg Kelvin)

The above holds true for ideal gases. If warranted, you should add another multiplier to the right hand side of the above equation, namely (Z₂/Z₁), to correct for the gas non-ideality.

where:

Z = the gas compressibility factor