VFD and a centrifugal pump??

[Buhsman]

Hello all,
I am an electrician in a can manufacuring plant with a question. Our requirements for cooling water used to be around 100gpm for our ironers. They have done several modifications and now think that we need to increase the flow. I agree that they do need to increase the flow but our superintendant wants to know if we can regulate the flow of a bigger pump and motor with a Variable Freq. Drive.
Supposedly we are replacing a 100gpm pump and a 5hp motor with a 200gpm pump and 10hp motor and he wants to be able to regulate flow by changing pump speed. Is this something you should do with a centrifugal pump??

Thanks,
Kyle

 

[cbarn24050]

Hi, yes there should be no problem there.

 

[jbartos]

Suggestion: It is better to have the pump Torque-speed characteristics properly aligned with the motor Torque-speed characteristics to establish appropriately integrated motor-pump set.

 

[macmil]

as jb says, check your requirements. If you need 5 hp at 100 gpm, then depending on your system requirements, it is probable you will need more than 10hp for 200 gpm. That said, because centrifugal pumps have a power / speed relationship that is cubic, the power requirements of the pump reduce dramatically as the speed is reduced. But the reduction in speed also affects your pump delivery . I would suggest that your situation needs some more work so you are not disappointed. In general, variable speed control offers excellent pump and process control. In fact, have you looked at whether your existing pump can be run a little faster ? That might give you enough extra, though nowhere near the 100% in your original post.

 

[gsimson]

In a pump either the head or the flow is more than required by which the motor power is much more than required. Please check the existing pump characteristiscs and the actual requirements. It may B possible to achieve Ur requirements with the same motor & different pump

 

[aolalde]

Precise control of pump processes is possible with VFDs. Pressure in water can be maintained to closer tolerances. A VFD is an electronic controller that adjusts the speed of an electric motor by varying frequency/voltage and then the amount of power supplied. That allows the motor to continually adjust to work just hard enough, rather than running full speed all the time.

 

[jcarrr]

Depending on the actual power requirements and a number of other issues, if the present pump is not heavily loaded, boy is this going to get a response, you might try just adding a drive and running the pump motor in excess of 60 Hz. Due to current limiting and maximum available voltage, the maximum torque will fall off as the speed increases, but you might not actually be using all 5 Hp in the present installation.

 

[jbartos]

Suggestion: When it comes to a variable speed control for the motor-pump set, the speed approval above the rated speed by the motor and speed manufacturers would be most appropriate steps.

 

[amptramp]

AFFINITY LAWS FOR CENTRIFUGAL DEVICES
W.R.T SPEED CHANGE (AS OPPOSED TO IMPELLER DIAMETER, FOR INSTANCE)

Q1/Q2 = (S1/S2)
HD1/HD2 = (S1/S2)^2
HP1/HP2 = (S1/S2)^3

S = SPEED
Q = FLOW RATE
HD = HEAD
HP = HORSEPOWER

VFD's have been used for years to control centrifugal devices such as fans, chiller compressors, and pumps. In the "early" days a marketing ploy was to sell "energy conservation" and VFD/PUMP combo.

I concur you should take a close look at the pump curves and the application. Otherwise, you are "shooting in the dark" at the process outcome. If you are unfamiliar with pump curves, system curves, etc., most drive vendors are glad to help with such applications.

 

[kassad]

well this is too late for an answer, but maybe it might help.
Using VFD's you can control the speed of the motor and hence the flow of the pump. The advantage lies in the fact that power decreases by the cube of the speed decrease. Thus a 5% (0.95) speed decrease will cause the power requirement to decrease by cube of the decrease (0.95x0.95x0.95 = 0.86). This is provided that the motor is suitably matching the pump (10HP for 200GPM @ a certain head) and also make sure that the motor you buy is an inverter rated motor.
But you have to check the economics behind this, since the power decrease in a 10HP motor would be lot less than the investment done in buying an inverter and an inverter rated motor, even if you are considering a couple of years for payback.

 

[jbartos]

Suggestion: Often, it is prudent to procure an efficiently integrated item: pump-motor-drive with a good warranty.