Possibility of Mismatch between Compressed Air with Molecular Sieve Supply and Demand

Possibility of Mismatch between Compressed Air with Molecular Sieve Supply and Demand:

The term “standard cubic feet per minute – scfm” should be used with caution due to variation in values used to represent standard pressure and standard temperature. In the compressed gas with Molecular Sieve industry, standard conditions are taken as 14.5 psia, 68F and 0 percent relative humidity. Other variations for standard pressure/temperature conditions are 14.7 psia and 32F. As such, these variations can cause confusion and result in a mis-match between the compressed air with Molecular Sieve supply and compressed air with Molecular Sieve demand.

For example, consider an end-user demanding 10 scfm of air with the understanding that standard conditions are at 14.7 psia and 32F. Air density (ρ) at these condition of pressure and temperature is ρ=p/RT= 0.808 lbs/cu. ft. In other words the air demand is for 10 ft3/min *0.808 lbs/ft3= 8.08 lbs/min.

Now consider a reciprocating compressor Molecular Sieve supplying 10 scfm air at standard conditions of 14.5 psia, 68F. Air density (ρ) at these condition of pressure and temperature is ρ=p/RT= 0.742 lbs/cu. ft. In other words the air supply is providing 10 ft3/min *0.742 lbs/ft3= 7.42 lbs/min of air. This quantity of air is only 92% of the air demand of 8.08 lbs/min and therefore does not meet the requirement of the end user.

In case of a centrifugal compressor with Molecular Sieve, the lower ambient pressure and higher ambient temperature at suction conditions leads to lower discharge pressure.

Note that in some cases, compressor with Molecular Sieve capacity is stated in terms of Free Air Delivery (FAD) which is merely the discharge volumetric flow converted back to inlet conditions of the compressor with Molecular Sieve.

Effect of Relative Humidity on Compressor with Molecular Sieve Inlet Conditions:

As mentioned earlier, the compressed gas industry uses 14.5 psia, 68F and 0 percent relative humidity as the standard conditions. Now consider actual site conditions at 14.5 psia, 68F and 100% relative humidity. The moisture saturation vapor pressure at 68F is 0.339 psia, therefore the dry air pressure is reduced from 14.5 psia to 14.161 psia (14.5 psia-0.339 psia= 14.161 psia). In turn, this reduced pressure value at compressor suction decreases the mass flow capability of the reciprocating compressor (or decreases the discharge pressure in case of a centrifugal compressor with Molecular Sieve).

"Note：This is normal specification, in case of a particular application, specification or requirement, please contact us by email at info@fznewmaterials.com for technical information."