variable speed pumps 2

[cimmeron]

I recently talked to a salesman and he said that the variable speeds units that control the electic power were very effective and didn't cost that much. The last variable speed pump I designed was in 1988 and it was a diaster. Could I have comments from other engineers with their opinions on the efffectiveness and desirablily of variable speed units?

 

[CB2]

North American variable speed drive market was $1061.6 million in 2002. The "AC variable frequency drive" that I believe you are refering to is a growing majority of that market. Under about 20 Hp is a commodity. Like any other piece of equipment, there are application issues to pay attention to. Today's VFD is not what you saw in 1988.

 

[RGCook]

I agree with the previous response. VFD's are commonplace on the smaller motors. If you are talking positive displacement, they are really a good option. If you are talking centrifugal pumps, its really easier just to throttle flow with perhaps a spillback. For big stuff, that is the way it is usually done, e.g., 350HP and above.

 

[smile1]

VFD's are cheap now, as the others said, and reliable. I like Toshiba. Keep in mind that they can't go as low with the same torque as a DC drive though.

 

[VER]

I had used VFD in my projects for three years now and never had a problem. They are very handy to control either the discharge flow rates or the discharge pressures of centrifugal pumps. I like Danfoss and Toshiba, they are reasonably priced. Installations above 150 Hp. may need harmonic filters.

Ver...

 

[sewerguy]

I am looking to use VFDs for two new blowers (maybe 8 to 10" water delta p max) downstream of a large evaporator, filter system, and condenser, discharging into a vent gas scrubber header. Inerts flow comes from dust collector cleaning with n2 and leakage on gasketing of evaporator (under slight vacuum) and associated conveyors. The fans will vary speed to meet the slight negative pressure in the evaporator.

The use of VFDs was proposed by a vendor, I had originaly done a more typical inlet throttling (control) valve approach.

Uncerainties include the system pressure drop clean/dirty, and I'm using much engineering judgement to estimate flows (first of a kind grass roots installation). Also the filter back cleaning cycle pulses will causes variations in flow maybe 5 to 10 times a minute.

My estimate of flow is certainly +- 100%, so it is quite possible fans will always operate at 50% turndown. I will have a bunch of process tricks to help out such as adjusting cleaning cycle, also I figure the level of vacuum is pretty self correcting with flow, finally I can always do a little manual inlet throttling.

I would be interested in any thoughts or comments from folks out there on this vfd application, fan types etc.

Thanks

SG

 

[chip1998]

cimmeron,- the most critical aspect on selection of VSDs is the engineer's judgement on applicability on the situation at head. For example, high static head and small friction head situations will not work as a slight rammping down of the speed will move out of the pump curve suitable to the system characteristics.

 

[EricC]

Sewerguy,

regarding VFD use with blowers...there have been problems with vibration of piping and silencers when VFDs are used with positive displacement blowers. The problems normally only occur a specific speeds, but this should be a major concern and be addressed in the spec. It can work as long as these issues are taken into about by the manufacturers of the blower and VFD.

 

[BobPE]

cimmeron:

I am just working on a project that involves salesperson VFD applications. I calculated with respect to my project that the VFD's would add a cost of approximately 10,000 dollars per year to the evergy bills of the facility. This adds up to a PW of over 135,000 dollars based on the life of the facility.

The moral to my story at least is that there is a big difference in talking about VFD's and understanding their use through engineering. The salesperson swore I was wrong, but could not defend against my engineering. I see a lot of this out there.

VFD's serve as great tools when understood for the specific application. They are just a poor investment if no engineering is done in support of their application.

BobPE

 

[johnd43]

A rather late contribution, but it may be helpful to someone. It will be interesting to see the specialists' reaction to a simple civil engineer's opinion.

Don't forget the option of mechanical variation. They can be supplied to set up manually, or can have a little servo to provide logic controlled speed variation.

One of the advantages I see in this method is that you will definitely buy the whole package from the pump supplier and won't have interface issues between sources if there is a performance problem. Pump suppliers are fully competent and working on home territory in the area of the mechanical gear box, and motor selection is no more critical than the FSD case.

In contrast, you Variable Frequency Drive would require supply of very specific equipment from at least two sources, and possibly more: pump manufacturer, VFD manufacturer, Panel builder and, possibly, motor manufacturer to allow for VFD requirements. Your pump-man may claim to be happy to do the full package, but he is only doing it by subbing out and therefore putting more levels of interface between the buyer and the true source.

It does depend on the application and even I have accepted VFD for large cenrifugal pumps for potable water distribution, but such things as sludge pumps around a sewage treatment works may well be candidates for a mechanical solution.

As ever, there is no universal solution and if all goes well, thats fine. Unfortunately, my experience of problems arising where there is an extended contractual string has been to discover row upon row of perfectly matched sloping shoulders. And if there is any IT involvement... Oh don't let me start on that - the web-site authority would have to bar my access!