FAD ACFM Nm3/hr

[shreyds9]

Hi everybody,

This may have become monotonous discussion but I am not clear about the same so posting it.

My understanding is for compressors, capacity is specified in Nm3/hr say 500Nm3/hr. This capacity is defined at discharge of compressor package.

to size the compressor, 500 Nm3/hr wud be converted to ACFM (i.e at site atmospheric conditions for eg: 0.998 bara,45 Deg C, 65% Rh)

Now in catalogues of compressor manufacturers, capacity is defined in FAD m3/min at conditions. This conditions are at
- dry air
- absolute inlet pressure 1 bar(a)
- cooling and air intake temperature 20 °C
Refer following URL

http://www.atlascopco.sg/images/2935 0438 16_zr-zt55-90vsd-ff_en_tcm795-1691250.pdf

Should FAD capacity be converted to site conditions to check the suitability of model to be sure about suitability of model?

 

[rotw]

Actual flow (e.g ACM/H) refers to thermodynamic conditions at compressor inlet [Inlet pressure, Temperature (not necessarily atmospheric like you mentioned except if the intake is atmospheric such for process air applications) and actual Molecular weight] and this actual flow is not constant (it decreases) along the compression process.

Constant flow through the compressor (assuming no extractions/injection at inter-stage) can be specified in terms of mass flow or as volume flow either at standard conditions or normal conditions (e.g. Stdm3/h or Nm3/h) although internal leakages through the machine impact the real mass flow seen by the impellers.

Even Nm3/h normal flow is still specified at compressor inlet - in general. In some cases, Nm3/h normal flow can be defined at compressor discharge (last flange) but that happens for some process applications (e.g. CO2) where for instance water is extracted at inter-stages phases via process coolers/separators; so you do have dry and wet Nm3/h flows through the compression process and client may be interested only to know the net resulting dry flow capacity at discharge.

The common practice is still to specify the mass flow or standard/normal volume flow conditions at inlet together with pressure and temperature conditions and Mol. weight at inlet and discharge pressure, actual flow ACM/H is then calculated (generally) by manufacturer based on appropriately chosen equation of state. That's generally how you do specify the data for sizing a compressor.

 

[shreyds9]

Rotaryw,

I suppose FAD concept is applicable only to air compressors and not process compressors handling CO2 etc.

ACFM would be specified at site inlet temp. and inlet pressure (inlet pressure wud be atmospheric pressure at site).

How are FAD and ACFM related to each other?

 

[rotw]

I don't have experience with FAD terminology and this seems to be more relevant to positive displacement type compressors but I can tentatively interpret based on the above referred atlas copco website.

Start by figuring out the conditions required at the consumer (i.e. downstream the compressor) in terms of discharge volume flow (I will get back to that later). By now lets call that the consumer conditions. Then lets look at two scenario:

(1) -> At atm. conditions corresponding to sea level, you have a certain actual flow at compressor inlet (FAD@sea_level) that must be compressed to consumer conditions.

(2) -> At atm. conditions corresponding to 3500 m (hypothetical) altitude, you have another actual flow at compressor inlet (FAD@3500m) that must be compressed to the "same" consumer conditions. Because the air is less dense at 3500 m, you basically start the compression process from some more stringent conditions at inlet to get to the "same" final state.

So FAD (2) is higher than FAD (1) in comparative terms. By the way ACFM (2) is also higher than ACFM (1) not even sure if this is by the same prorata or not but does it matter?.
In Absolute terms, the reference for FAD is 0 degree C and 1.013 bar as per the reference website.

Back to the discharge volume flow of the consumer. The website says it has to specified in terms of normal state (Nm3/h). Anyway when the change of temperature at discharge, due to the change of the site conditions, is minor and discharge pressure being fixed, reasoning with ACFM or Nm3/h at discharge seems to me equivalent.

 

[shreyds9]

One correction in my post first... Nm3/hr is SCFM since we are using US units. In SI it would be Nm3/hr.

Altitude aspect is adding to complexity.

With all explanation how do I check if vendor proposed model which is based on FAD suitable for my requirements which are specified in SCFM (or Nm3/hr).

 

[rotw]

select the compressor which has a FAD (Nm3/h) = Nm3/h x(T_In / T_ref) x (P_ref / P_In)
P_In and T_in are your actual inlet conditions at site (downstream the filer if you use one and inclusive the piping losses)
P_ref = 1.13 bar , T_ref = 273.15 K as per my previous post. Source: [URL unfurl="true"]http://www.atlascopco.com/images/cam_05_calculation_tcm48-705084.pdf[/url]

PS: Change in altitude is not that complex as basically it can be translated into a change of the intake pressure.