by Ron Joseph
April, 2004
Determining VOC/HAP Flow Rate by Mass or Volume
Q. I am calculating emissions from a compressed air/siphon spray gun. I first
determined a mass flow rate through bench testing (lb/hr) and then I used this
to determine the volume flow rate of the various paints that are used (lb/hr/lb/gal=gal/hr).
I then multiply the gal/hr rate for the various paints times the VOC or Hazardous
Air Pollutant contents in each paint in lb/gal to get lb/hr.
Question: What is more accurate with such a paint gun, the volume flow rate
or mass flow rate? Thank you.
A. EPA and state regulations want to know how much VOC/HAP go into the air
on an hourly or daily basis. Since you obviously have an accurate scale to measure
the flow rate by mass there is no reason for not using it. To determine VOC
emissions you presumably used the VOC content that was provided on the MSDS
and this gave it to you in lbs/gal . If you had been able to accurately measure
the volumetric flow rate in gal/hr without having to do any conversions then
this would also have been acceptable. I'm assuming that it was easier for you
to measure flow rate by mass than by volume since you may not have had an accurately
graduated container from which to work. On the other hand if you were working
from a measuring cylinder graduated in small increments it would make no difference
if you used mass or volume.
The VOC content in lbs/gal is not 100% accurate nor is it 100% reproducible
since there are experimental errors that you encounter when you perform EPA
Method 24 . Therefore, you should not be concerned with minor differences that
you might get by measuring the flow rate either on a mass or volume basis.
Best wishes,
Ron Joseph
