Ales Litomisky, the co-founder of Ecogate energy saving technology, recently published an article looking at the causes of increased pressure loss in dust extraction systems, and some of the solutions that can be implemented if this is an issue:
What causes additional pressure loss?
"In this example we will explain what will happen if a dust collection system or any exhaust ventilation system is subject to pressure losses that are higher than the design figures.
To begin, eventual pressure losses can be higher for various reasons, for example:
- Because of a system fault.
- For safety, a non-return valve was added to the ductwork system and the valve has a higher pressure loss than anticipated.
- The fan that was purchased cannot generate the required pressure.
- Filter unit cleaning system is not working as expected.
- Hood losses for dust capture on LEV systems were incorrectly calculated.
- System design pressure losses such as ductwork, silencers, filter differential pressure, etc. were incorrectly calculated.
In this example, the total design air volume is 120,120 m3/hr, and calculated static pressure losses of the complete system are 4.73 kPa. If we choose a clean side direct driven fan, this air volume and pressure will require around 209 kw absorbed power. If we will calculate to include motor losses, the required motor power is 217 kw therefore a good fit would be a 250 kw motor.
Pictured below is operating point #1 where the system resistance curve crosses the fan curve.
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What are the consequences of having higher pressure losses?
Unforeseen pressure losses in this example amounted to additional static pressure losses of 1.25 kPa. As a result, the resistance of the system increased - see the new system resistance curve in red, the operating point is labelled #2. The air volume is 120,120 m3/hr and static pressure 5.98 kPa. A quick calculation will show that at the same efficiency this would require a fan with 274 kw absorbed power and as such, a different fan with a larger 300kw motor would be necessary.
Since we don’t have a new larger fan and larger motor, what in reality will be the system airflow with the existing 120,120 m3/hr / 4.73 kPa fan? The measured air volume dropped by 29,733 m3/hr to 90,387 m3/hr. This is illustrated above with the operating point labelled #3. The actual fan performance curve (dashed line) is well below the design fan curve.
Solutions
So, what are the options to increase air volume from 90,387 m3/hr closer to the design value of 120,200 m3/hr on an existing system?
The best solution is to try and reduce static system losses: i.e. improve capture hood design, optimise ductwork sizes and air velocities, install a low-pressure fan outlet stack instead of a silencer, etc. however the effectiveness of this will depend on how far out the system pressures are.
Another option is to increase fan speed, however this requires careful consideration:
- Will the fan manufacturer allow the use of the fan at a higher speed?
- Is there any reserve in the motor power and the motor current?
- Is all electrical equipment upstream of the motor capable of handling any increased motor current i.e. cabling, VSD (if used), breakers, mains electrical supply?
Based on fan law #1, with increased fan RPM the air volume should also increase linearly. However, if increased static pressure resistance is caused by system effect (error) and the airflow is turbulent, the air volume will not increase linearly because turbulent airflow will again increase the status pressure losses.
Conclusion
In conclusion, it is of course preferable to properly and accurately calculate the pressure losses of the complete system from hood entry loss, through the ductwork systems, filter plant and outlet losses at design stage. To resolve the above situation on an existing plant can take a lot of time and require significant additional cost."
If you'd like to speak to Filtermist's experienced Contracts Team to discuss how we can ensure your dust extraction system is correctly specified, please call 01952 290500 or email sales@filtermist.com.