Pump Running Costs 2

[iken]

Hi all,
This is more for my general interest, and yes also specific situation.

I am curious on the running costs of an oversized pump that is on a VSD, as opposed to a correctly sized pump that is funning at full speed.

For example, assume a pump is acheiving 9m3/hr against 8.5m head, all at 23hZ (yes this low). I know for a fact, that the installed pump cannot be selected for these given duties. If I where to select a much smaller pump on site conditions, and assume this ran at or close to 50Hz. Would the power consumptin be the same, or close enough to ignore.

I know there is higher costs in installation for the larger pump (pump itself, wiring, MCB's etc), but I am more focused on the running costs for now.

If anyone knows of a case study or the like taht I can read up on etc, that would be great.

Cheers.

 

[stanier]

Check out this presentation. Be aware that the affinity laws need to be carefully considered. There is a great deal of myth and legend about the savings of a VFD. Its right up there with global warming and the convenient lies.

 

[Artisi]

Without getting too hung up on the finer points at this stage you need to consider the pump operating efficiency at this duty / speed which is probably well away from BEP plus you need to factor in the inefficient operation of the motor being controlled by a vfd.
Having said that, it is such a small unit - about 0.2 Kw it is probably even worth bothering to calculate

 

[iken]

Thanks stanier, the prersentation is well worth the read (and links within the presentation).

 

[BigInch]

In the hydraulic power calculation, speed makes no difference in power consumption. Power required is flowrate * density * head /eff.

It is likely that adding a vsd to that oversized unit could reduce your present operating costs, but you will also be adding at least a 5% loss with that vsd, probably more, if the percent of rated load you are running at now is low. A vsd running at frequency less than its rated output frequency will likey introduce more inefficiency.

The most important thing to remember is that you will not do any better than having a well built properly sized pump running at BEP flow and rated speed, unless the efficiency of that pump is extremely poor when compared to the larger alternative running at reduced speed and with its motor and vsd at reduced load. I would expect that, as long as you buy a reasonably efficient smaller pump, you will do much better with that correctly sized smaller unit. If that is one of your options, take it.

"I am sure it can be done. I've seen it on the internet." BigInch's favorite client.

"Being GREEN isn't easy." Kermit

http://www.youtube.com/watch?v=hpiIWMWWVco

http://virtualpipeline.spaces.live.com

 

[ccfowler]

iken,

This is a subject that "has been beaten to death multiple times" here. With a bit of searching of the numerous postings, many points of view can be found along with many well stated examples and much excellent advice.

VSD's can provide dramatic energy savings in applications where the flow varies substantially--particularly when these criteria are satisfied:

1- A significant portion of the duty cycle is at greatly reduced flow rates
2- The pump and drive have both been prudently selected
3- The duty is essentially a circulation type of service where the required head varies fairly closely with the square of the flow rate.

If the duty is for a high head application such as a boiler feed pump, energy savings compared to a simple induction motor drive and a control valve are likely to be trivial or worse.

As a general rule, the use of a VSD with an ill-suited pump only makes everything worse, not better.

If the flow rate does not vary substantially, then a VSD will impose an energy consumption burden of 5% or more even with a well selected pump. For an application such as you seem to be describing, the situation would amount to excessive initial costs compuonded by needlessly great energy consumption and likely excessive maintenance needs.

You don't need a case study. You need to study the specific application that is the subject of your posting. All pumps are always an integral and interactive part of their complete connected system. While many systems may be very similar, the detailed, specific different items and needs may dictate very different pump, drive, and control choices.

If your situation is one of already having a pump that may "almost work" to save on initial cost and using that savings to buy a VSD to magically make the ill-suited pump work, then you are likely to find that elevated operating and maintenance costs will soon exceed any savings may have seemed attractive.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.