Before you do anything:

Do not mess with the fan system as the dust control and collection system was designed with that size motor and fan for a reason!!!!!!!!!!!! your fan is creating an air stream to collect an move the dust all the time every time so you are defeating the purpose and by slowing it down or stopping it effects its efficiency especially if the dust is hazardous to health.

The fan may not be able to pick the dust up from an at rest stateeffecting the efficiency of the dust collection system also. ■

Yes of course, Greg.

Retrofitting subpressure regulation is no big deal with most applications. I don't know your application in detail, so responsibilty is up to you.

0) Ensure your fan motor can handle VFD.

1) Generally you would first want to measure the sub-pressure a few metres before the filter (and fan, if it's in the raw gas stream), with the plant in current condition and an operating point which fits your need (f. i.: amount of open flaps and suction power is right).

2) Remember this value (probably but not necasserely in the range of 700 to 5000 Pa).

3) Install a pressure converter sensor with the correct amount of measuring range and a variable output (f.i.: 0-10VDC or 4-20mA).

The sensor should measure the subpressure against atmosphere at the same point you took the value.

4) Configure your VFD to react to this analog signal from the sensor. It should try to regulate the signal to a steady value by varying its frequency (simple PID-Loop). The steady value it should achieve would have to match your intial measurement.

For details and tuning of the system contact an expert. ■

I agree with Izaharis. If you want the system to be shared by different machines, the system and fan can be designed for that. But if you are thinking of changing an existing static system, it won't work.

According to the fan laws, flow varies directly as speed changes, pressure varies as the square of speed change. Think about it.

Michael Reid. ■

Interesting, a lot of you guys are saying not to retrofit VFD's to fans.

My company is spending a lot of money doing just this to reduce energy efficiency.

Its fitting VFD to all aggregate screening plant and coating plant fans. ■

It's just make sure that you understand what you are doing if you start messing about with fans speeds. If you are ignorant of the basic fan laws, learn quickly. ■

Sounds sensible.

By the way, I meant to improve energy efficiency of course, not to reduce it. ■

Is the complexity worth it, economically and operationally?

For dust systems, you must maintain a minimum transport velocity to avoid blockages. This means that the ducts ahead of the fan and the fan itself must operate at a constant flow, which means constant speed (or bleeding in extra exhaust air).

More importantly, the system static pressure varies as the square of the speed change. Any reduction in speed has a substantial effect on the static pressure available to move the air through the system.

If you are considering such a system, seek the advice of an expert designer as well as the VFD salesman.

Michael Reid. ■

While making systems more energy efficient sounds good, it isn't if it ruins the system performance.

I've had enough salesmen tell me how their gizmo would improve this and make that more efficient who have had no idea of my system until I explain it to them when they realise I won't get the gains they claim.

One in particular related to problems starting a fan. I was roundly condemned by a salesman (promoting his gizmo) and our electrical engineers for suggesting closing the fan outlets to get it up to speed, then opening the outlets gradually up to the limit of the motor current. Apparently ramming all that air into a closed outlet "will guarantee that the motor overloads". (No it didn't!!) ■

While making systems more energy efficient sounds good, it isn't if it ruins the system performance.

I've had enough salesmen tell me how their gizmo would improve this and make that more efficient who have had no idea of my system until I explain it to them when they realise I won't get the gains they claim.

One in particular related to problems starting a fan. I was roundly condemned by a salesman (promoting his gizmo) and our electrical engineers for suggesting closing the fan outlets to get it up to speed, then opening the outlets gradually up to the limit of the motor current. Apparently ramming all that air into a closed outlet "will guarantee that the motor overloads". (No it didn't!!) ■

A VFD can do the job of a soft starter but, why use it if you don't need it? Power station engineers have known for centuries that the best way to get large fans started is with a damper.

Designer, were the VFD salesman and your electrical engineers convinced after a practical demonstration?

Regards,

Michael Reid. ■

Sparkys really don't like being proved wrong by mechanical engineers!!!! ■

They are not impressed either when you tell them the reason a system is performing badly is that the fan is running backwards! ■