Blower Energy Saving

It depends on what valve you are using now. Gate/diaphragm etc. the flow rate change is not a straight line curve. ( see ME hand books). Flow varies in cube with rpm.

No - flow varies linearly with rpm. Head varies as rpm². Power varies as rpm³.

Cheers.........Codey

Throtteling a valve at delivary of blower is a shear waste of energy. Instead you should take feedback of temeprature and than vary the speed of blower with help of VFD.

Adjustment with valve is not an energy efficient method. Generally fans run according to variable torque characteristic. Changing the speed of a fan will change the torque demanded by the square of the speed. By changing say the speed of the fan to 80% of the nominal value the torque demand will only be 64% of the nominal value and the power consumption is according to cube law. By reducing speed the power consumption can be reduced to about 50 % of the nominal value. What is the supply & return conditions of the air

The pressure drop across the valve is a measure of the energy lost. Depending on the type and design this can vary a lot. As the valve closes the blower sees an increase in the delivey pressure and the flow decreases along the performance curve of pressure versus flow rate for that blower. When the blower is under VFD, the output of the blower itself varies because of RPM variation. There is no simple way one can relate the two unless the manufacturer gives you a curve relating rpm and power for that blower. For full analysis youwill also need RPM versus flow rate also.

I think this is the best answer. Many people, especially those selling VFDs, bring up the cube law and use that as justification for energy savings. But they fail to mention that the cube law applies to the "valve" (damper or vanes) in the exact same way!

So the ONLY extra energy that is saved by selecting a VFD over a mechanical restriction is in eliminating the losses across the restrictive device, i.e. the "valve". Mind you, that is often significant enough to warrant the VFD, but it is NOT automatic, and in fact if the amount of time the system is running under restricted flow is very low in comparison to running at full flow, the VFD may actually COST you money in the addidtional losses in the VFD itself.

Bottom line, there is no substitute for doing a proper engineering evaluation. Many many people have been disappointed in VFD energy savings for this very reason.

In my experience (fitting and advising on VFD use) if the valve is in operation there is normally a significant 'loss' at the valve which converts to heat, of course. So there is not the saving on valve operation you claim because of all the (turbulence) losses, mixing up the molecules rather than delivering in the desired direction. This is similar to throttling a pump where at no flow the water literally boils!!! as the wasted energy turns to heat and isn't moved along.

Electrically, the saving using VFD is normally very significant and where I have come across tables relating flow to damper / valve position (the question in the first place), the decrease in flow is more than linear i.e. half closed is less than half flow.

The true difference in operating cost depends on whether using electricity for heating is useful if not the most cost effective.

Many simulators take account of typical valve losses but this is non specific, so they will show reduced energy useage at lower flow on the valve and compare to VFD operation. Some of these are independent of VFD suppliers so are not presented as a sales vested interest. If a vendor is presenting the simplistic inverse cube law in all situations then you are right to query the accuracy of their claims but the opportunity for energy saving may still be there.

I had a similiar problem on a diesel generator cooling fan. What you need is a Typical daily CFM requirement for the fan based on cooling requirements. Once you determine this you can use the fan formulas to calculate the energy savings.The motor load requirements at 100 % RPM and new RPM can be used to calculate the energy savings. You may have the motor operate at several RPM' during a typical day for several hours. You can use this to estimate the savings

George