What causes additional pressure loss?
In this example we will explain what will happen if dust collection, or any exhaust ventilation system, pressure losses are higher compared to design values.
The pressure losses can be higher for various reasons, for example:
Because of a system effect (error), the losses are higher compared to design values.
For safety, a non-return valve was added and the valve has higher pressure losses compared to specification.
The fan that was purchased cannot generate the required pressure.
Filter bag cleaning is not working as expected.
Hood losses for some workstations such as wide belt sanders or CNC machines were underestimated.
Design system pressure losses such as the non-return valve, silencer, inlet box, dirty filter bag, etc. were incorrectly calculated.
In this example, the calculated total design air volume is 70,700 CFM, and calculated static pressure losses of the complete system are 19” w.c. If we choose a clean side direct driven fan, this air volume and pressure will require 280 BHP. If we will calculate with motor losses, the required motor power is 291 HP - a good fit is 300 HP motor.
Pictured below is operating point #1 where the system resistance curve crosses the fan curve.
What are the consequences of having high-pressure losses?
Unforeseen pressure losses in this example amounted to additional static pressure losses of 5” w.c.. As a result, the resistance of the system increased - see the new system resistance curve in red. This operating point is labeled #2. The air volume is 70,700 CFM and static pressure 24” w.c. A quick calculation will show that this would require at the same efficiency 367 HP motor power. And thus a different fan with a larger 400 HP 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 70,700 CFM/ 19” w.c. fan? The measured air volume dropped by 17,500 CFM to 53,200 CFM. This is illustrated below the operating point labeled #3. The actual fan performance curve (dashed line) is well below the design fan curve.
So what are the options to increase air volume from 53,000 CFM closer to the design value of 70,700 CFM?
The best option is to reduce static system losses: you can eliminate system effect losses, adjust non-return valve losses, install a fan outlet stack (evase) instead of silencer, etc.
Another option is to increase fan RPM. This requires a lot of checking:
Will fan manufacturers allow the use of the fan at higher RPM?
Is there a reserve in the motor power, the motor current?
Are all electrical equipment upstream of the motor capable of handling increased motor current (wires, VFD - if used, breaker, is increased power available from utility?).
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.
In conclusion, it is preferable to properly collect specifications of the pressure losses of all used equipment, in particular the ventilation system, and do a careful calculation of the pressure losses. To resolve the above situation will take a lot of time, and cost you a lot of money.
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Author: Ales Litomisky