pumps and frequency inverters.

Hi all,

We are going to install frequency inverters on all our pumps in a production line.

When there are nog products on the chain, then, the pumps may stop. That's the main concept.

But, I want to lower the frequency a little, just to save a little more energy...

3 Hz lower the 50 Hz on a 22kW pump, can it be calculated how much we will save by lowering 3Hz ? A simulation would be great!!

Here is an Excel pump simulator from ABB in Excel. Let the macro run. Lots more on goolge. Put a frequency inverter is already giving the motor more protection and there is the definite savings already.

pump.xls

Pump Afinity Laws show that the change in power will be a cube of the change in speed. So (47/50)^3 = 83% So you will be using ~83% of the power you are using now.

..

But the flow of water will be also a lot less... ?

Brownhat said:

Pump Afinity Laws show that the change in power will be a cube of the change in speed. So (47/50)^3 = 83% So you will be using ~83% of the power you are using now.

Click to expand...

Brownhat said:

Pump Afinity Laws show that the change in power will be a cube of the change in speed. So (47/50)^3 = 83% So you will be using ~83% of the power you are using now.

Click to expand...

Not quite. First of all, the affinity laws only apply if you are using centrifugal pumps. With a positive displacement pump like a gear or piston pump your savings will be linear with speed if the discharge pressure is constant.

Second, in your math the power is 83% so the savings are 17%.

Third, you have to remember that as the speed drops so does the flow. You have to make sure the flow delivered meets the needs of the process.

Finally, the pump power is a function of the flow rate and the pressure. Even with a centrifugal pump you have to plot the system curve and superimpose the pump curves at reduced speed. The intersection defines the operating point, from which you can then calculate the power.

Combo talks about chains, so I am going to assume that these are positive displacement gear pumps. As the pump speed drops so does the hydraulic motor speed and the conveyor speed. A complete system analysis is required to determine actual power savings potential.

Not so fast. Spend some time here:

http://www.pumped101.com/

hmm

So yo need 12% Power for 25Hz ??? Don't believe this math

Tom Jenkins said:

Not quite. First of all, the affinity laws only apply if you are using centrifugal pumps. With a positive displacement pump like a gear or piston pump your savings will be linear with speed if the discharge pressure is constant.

Second, in your math the power is 83% so the savings are 17%.

Third, you have to remember that as the speed drops so does the flow. You have to make sure the flow delivered meets the needs of the process.

Finally, the pump power is a function of the flow rate and the pressure. Even with a centrifugal pump you have to plot the system curve and superimpose the pump curves at reduced speed. The intersection defines the operating point, from which you can then calculate the power.

Combo talks about chains, so I am going to assume that these are positive displacement gear pumps. As the pump speed drops so does the hydraulic motor speed and the conveyor speed. A complete system analysis is required to determine actual power savings potential.

Click to expand...

Tom Jenkins said:

First of all, the affinity laws only apply if you are using centrifugal pumps.

Click to expand...

Agreed. I posted the link to show the basis of my answer, and this includes the assumption that Combo is using centrifugal pumps.

Combo said:

But the flow of water will be also a lot less... ?

Click to expand...

If you look at the links provided, you'll see that the volume flow is proportional to the pump speed, so your volume flow will be at (47/50)=94%

Affinity laws for centrifugal pumps and blowers

flow directly proportional to speed
Head (discharge pressure) is proportional to speed squared
Power proportional to speed cubed.

So for pump requiring 100 kw at 100 psig discharge pumping 100 gpm at 3600 rpm (60 hz)
drop speed to 1800
flow now 50 gpm
head now 25
power now 12.5
CAUTION if system has a head loss greater than 25 pump is essentially stalled and no cooling - good way to wipe out a pump.

READ YOUR PUMP CURVES AND KNOW YOUR SYSTEM

If your overriding concern is saving on electricity cost
-- everyone leave the plant and pull the main disconnect.
Downside is not much production and loss of paychecks.

You can save a lot of money with VFDs. However as pointed out you have to look at the big picture ie the whole machine and process and interconnected systems ie chillers, cooling towers etc etc. What you save on cost of driving a centrifugal pump you may quickly lose by not keeping adequate flow thru a chiller and burn it out.

Dan Bentler

Combo said:

So yo need 12% Power for 25Hz ??? Don't believe this math

Click to expand...

Running the pump twice as fast requires 8 times the power. Running the pump half as fast requires 1/8th the power.

Combo said:

Hi all,

We are going to install frequency inverters on all our pumps in a production line.

When there are nog products on the chain, then, the pumps may stop. That's the main concept.

But, I want to lower the frequency a little, just to save a little more energy...

3 Hz lower the 50 Hz on a 22kW pump, can it be calculated how much we will save by lowering 3Hz ? A simulation would be great!!

Click to expand...

Combo

Do you have the pump curves for your pumps from the manufacturer ?

One problem with pump curves, they are usually drawn for a fixed speed. You need the curves for the pump at various speeds. Those are available but you usually have to ask specifically for them.

But, if a centrifugal pump operating within its normal pumping speed range, you can pretty much go with the affinity laws mentioned above.

And, yes, the savings is dramatic for a rather small change in speed and flow rate. Just make sure you still have enough pressure available at the slower speed.

One other benefit that is often overlooked when considering the use of VFD's on pumps and fans----you can often run the pump or fan overspeed to maximize the flow thru the pump under high demand conditions. You reach the overspeed limit when the pump hp equals the motor hp.

It depends on the pump character and on the load it has to drive.
like pressure and the product.
it is not all simple and the cost of a vfd is also rather high.
fot 3 percent the savings are only 6 percent max so too low to earn it back.

Combo said:

So yo need 12% Power for 25Hz ??? Don't believe this math

Click to expand...

Maybe.

Power is a function of flow x pressure. The affinity laws don't predict performance, they predict the change in location of each point on the pump curve. At 1/2 speed you would theoretically shift the original curve point to 1/2 the flow at 1/4 the head. The power would theoretically be 1/8 the original at 1/2 the flow of the original point. Note that these same laws apply when changing the impeller diameter.

As DickDV points out, you need to use caution in interpreting these calculations. First of all, the accuracy isn't complete with very large speed changes. A 50% speed change may be pushing the accuracy of the calculations because some changes occur in efficiency.

Second, the pump curve alone doesn't identify the operation. In a pure friction loss system pressure drop changes with the square of flow. If you have any static head you have to add this constant pressure to the friction loss. The net result is ths system curve. The intersectio of this curve with the new pump curve calculated at the new speed is the new operating point.

euhm

There are 14 pumps. The chain is the production line at 9 m/min. At 9 m/min the products going in a building where these 14 pumps are pumping a product on them.

The 14 pumps are pumping a product too sprayers. Lowering the speed a little is something we can do for sure, because now with the star-triangle system, these are running at 50 Hz and the sprayers are spraying way too much then necessary.

When no products are detected at the input of the building, then the pumps must ramp down sequentially. Stopping the pumps when the line runs at no products are in this building, is the best cost saver we have. But I'm sure we can pull down the speed, my boss is sure we can lower to 40Hz.

I don't know what kinda pumps these are, I should ask the maintenance departement.

My boss calculated it like this: lowering to 45 hz = 10 lower speed, so the power needed is also 10% lower, so will the costs be, but I remember from pump curves that this isn't true. Anyone who can show a curve ?