For many years, air pollution control systems have been treated as if they were constant, static systems, without any system dynamics. As evidence, just look at any Air Quality Permit Application, and you will see that all air pollution control systems are designed at one optimal system volume (cfm). What do you think the chances are, when actually tested, that the permitted system will actually be within even 5%, 10%, or even 20% of the permit volumetric flow target?

As most of you are aware, most air pollution control systems have an inherent dynamic portion in them; and in many modern systems, several. This inadequacy affects the employees, the cost of running the system, and the ground concentration level of pollutants on the local and not-so-local citizens.

Too Much Of A Good Thing

When new, dust collector and pollution control filter media has very little resistance to flow, and even when one precoats the filter media, oftentimes the filter media will have less than 1 in. of pressure drop. This has long been a cause for concern because the system design must include as much as 5- to 10-in. wc of resistance for the long-term requirements of the system.

What actually happens when we start a system is that the system fan doesn't see as much resistance, and we end up pulling more air than is needed in the system. Because employees seldom complain of too much ventilation at their workstations, system designers usually don't get any feedback. However, too much system airflow causes the following:

  • The design filter penetration velocity (often called air-to-media ratio) is greatly exceeded, thereby causing irrecoverable damage to the long-term filter life.
  • Significantly higher than normal motor current draw occurs because energy requirements increase at the cube of the system volume increase, sometimes tripping heaters or even causing motor damage.
  • Loss of product to a dust collector can often occur because of excessive hood capture velocities.

Incorporating Vfd's

To prevent this from happening, we can control the fan flow rate by one of three mechanisms: inlet vortex dampers, fan outlet dampers, or fan speed control. The first two are mechanical means, and most of the time, they require a manual adjustment of the damper. If, as the dust collector gains resistance due to particulate buildup, no one remembers to check the volume control dampers, the result will be diminished flow and vacuum (static pressure) at the employees' hoods. This will surely cause excessive worker exposure at some time.

The last system control mechanism, fan speed control, is automated with the use of a vfd. On 75-hp and larger motors, most companies find that the cost of a soft-start device is justifiable due to the cost of energy based on initial peak demand.

Since the mid-1990s, pricing on many of the vfds has become much more competitive with the price of a soft-start device. The vfd acts as a soft-start mechanism, but it also allows the user to operate at one volumetric flow rate through the life of the system. How can a vfd be used to control volume? There are at least two ways: continuous measurement of velocity pressure, and hood or duct static pressure.

Using System Volume Measurements for System Control

Several companies now have on the market an inexpensive, accurate air monitoring station, which, when coupled with a differential pressure sensor/transducer; will give as output a 4-20-mA signal based on the velocity pressure readings. This signal can be read by the drive, and we can control the system volume to one volumetric flow rate just by setting the drive to maintain one velocity pressure. For a few more dollars, the 4-20-mA signal can be digested logarithmically to give a digital readout in actual cfm, even in a heated airstream.

To determine the actual volumetric flow rate from a velocity pressure reading requires the use of the following formulae:

Velocity (fpm) = 4,005 x Velocity pressure2/Air density factor
Volumetric flow rate (cfm) = Velocity (fpm) x Area of duct (ft2)

What about plugging of the air monitoring Pitot tube array from particulate in the system? For most dust collector systems, the only real practical location for this station is in a long (minimum 4 times the duct diameter, preferably 8 times the diameter) section of duct between the dust collector and the induced-draft fan. However, the monitoring system can be located in a dusty or sticky-particle-carrying airstream simply by raising both the total pressure side and the static pressure side of the Pitot array equally. A "purge kit" is available with the air monitoring stations, which allows us to raise the pressure inside the Pitot tube array to about 5 psi higher than ambient conditions.

When measured with a Pitot tube, velocity pressure is a differential pressure between the duct total pressure and the duct static pressure. Sending the exact same 5-psi air pressure to be added to both sides of a differential pressure meter actually has no effect on the differential pressure. The effect is that air will continuously purge the Pitot orifices without effecting the velocity pressure measurements, since they are a differential pressure referenced only to each other. I have even used this technique successfully for more than a year on a condensable asphalt aerosol, with no evidence of plugging.

For most pollution control systems, a simple probe design that has been shown to be relatively nonfouling is the static pressure nozzle of an "S-" type Pitot tube. This is nothing more than a tube bent at 90 degrees, pointed in the direction of flow so as to not receive the incoming particulate into the orifice. This should be located at least 2 times the duct diameter downstream of the hood, to avoid a turbulent area near the hood known as the Vena Contracta. It should be removed for easy cleaning on a periodic basis and checked monthly.

Static Pressure To Succeed

If we take the industrial hygienist's viewpoint instead of the environmental engineer, a better way to judge the proper functioning of an air pollution control system is to monitor the static pressure (vacuum) level of the one of the hoods in the system. Once the system has been balanced, the volume of the entire system will vary with the hood static pressure. This is based on the following equation:

Hood static pressure = (Hood entry loss factor x Air velocity pressure) x (Acceleration loss factor of 1 x Air velocity pressure)

In other words, the hood has an inherent resistance that increases as the velocity increases. In fact, once the system has been balanced and is fixed, one specific hood static pressure will reference one and only one system volume. Indeed, this means that you can use the hood vacuum level (static pressure) to set your system volume. The only way that the hood static pressure will not give you the same system volume will be if the system has been changed. Always be sure to restrict access to air system balancing devices such as blast gates by locking them or welding them in place, once the system has been balanced.

Using the hood static pressure as the control reference, as the resistance in the media increases, the vfd will sense a drop in hood vacuum and compensate by ramping up the fan speed. The important part is that the fan will always operate at the most efficient curve for optimal performance of the system. Coincidentally, the system will also track at one volume.

The savings here comes with the installation of new filters. At that point, filter resistance (and therefore the system resistance) is low, the fan speed can be drastically reduced, and the energy usage can be a fraction of the total required to meet the design needs of the dirty filters. This is the payback needed to explain the extra cost of a vfd system. Figure 1 helps to assess the available savings for vfd-controlled systems.

After The Fact

To estimate the cost savings, take the filter maintenance interval. (Let's say one year.) Obtain fan data for the properly sized fan from a trusted vendor. Now calculate the hp difference between the system resistance with new filters vs. that pressure at changeout. Multiply that number by your company's corresponding annual $/kWh.

That number will be correct if the fan operates 24 hrs/day, 365 days/year. The actual savings will only be half that number if the filter pressure increase is somewhat linear (which most systems are). Don't be surprised if the savings pay for the vfd in the first year or two. Remember that the energy cost is based on the square of the system static pressure and on the cube of the system volume.

In the future, we will probably see a decrease in the cost of vfd's. We will also see reduced costs for remote sensors of filter pressure differential, temperature, and mass emissions monitors, dropping to a level where automatic controls will be used and compiled for hazardous air pollutants and other particulate or aerosol emissions data. A remote PC station will compile and trend this data for the environmental engineer, per the purposes of the federal client-assisted memo provisions. It's already happening at "forward-thinking" facilities. Should it be happening at yours? ES