GENERAL SECTION
Mass Balance Calculations: Common Mistakes

As new air regulations are being promulgated for different industry areas, it has become more important to ensure that emissions are calculated correctly. There are several ways to calculate air emissions, each with varying levels of accuracy. For example, in general, emission factors tend to have lower accuracy and mass balance calculations tend to be more accurate. When performing mass balance calculations the data and assumptions used are critical.

OVERVIEW
One of the first steps in using mass balance calculations is determining if this method can be used in the first place. Mass balance calculations are based on the theory that amount in equals amount out. Therefore, if accurate raw material data and finished product data are not available, mass balance should probably not be used. In addition, if the actual amount of volatiles lost may be so small that the inherent margin of error in mass balance will exceed the loss itself, mass balance will not provide helpful numbers. In such cases, alternate methods of emissions estimating, such as source testing or use of emissions factors or engineering calculations can be used.

Commonly mass balance calculations are used for determining emissions from paint booths and other coating operations. There are some common pitfalls that should be avoided when using this method, especially when trying to calculate an average or maximum hourly emission for permitting purposes. Some questions that should be asked before calculating hourly emissions are:

1. Am I using the right number of hours?

Oftentimes a company will use the “department” hours to calculate hourly emissions. However, the paint booth may only be operating a fraction of that time. It takes time to get a part set up for painting, and to do any surface preparation or cleaning that is necessary. Where possible, calculate the emissions per hour for when the paint booth is actually operating and do not count in any time when no emissions are generated.

2. Am I reflecting a heavy production day or planned production increases in my numbers?

If I take an average year’s worth of production and divide it by my average hours, I will not be reflecting the production peaks in my hourly number, which may lead to permit limits that are too restrictive. Typically a permit application will ask for your potential to emit (PTE), which is the maximum usage rate at 8760 hours per year. However, somewhere between what you did on a given day and the PTE number is an hourly number that is pretty realistic for a heavy production day. This is the number that would be best to use as your actual hourly value in your air permit application. It is important that your company has a daily ceiling VOC limit that can be complied with during a heavy production day. Account for fluctuations in production and targeted growth in production when you do your calculations, and use the worst-case scenario when you permit.

3. Are my raw materials going to change?

Suppose you have been using Coating A for the past five years and it was the material you had based your last permit on, and now you are up for permit renewal. The production supervisor tells you in two months they will be switching to Coating B on the line. In such a case, or in the event that you use a multitude of coatings, use your worst-case coating for your calculations. Give yourself some flexibility to change coatings without having to repermit.

SUMMARY
As with any calculation, the assumptions and quality of data that you start with for mass balance will determine the accuracy of your final number. Reasonable, accurate numbers are the necessary foundation for an air permit your company can live with. If you would like assistance in evaluating your air emissions, please contact Emily Rynders or Tony Domanico at the Payne Firm at (513) 489-2255 or via e-mail at ear@paynefirm.com or aid@paynefirm.com.