two pumps in series with one vfd for one pump

[naifmbo]

hi everybody,
can i use two pumps in series : one with constant speeed and the other with vfd?why?

 

[BobPE]

sure, but what do you want to accomplish is the more important question.....

 

[naifmbo]

it is a cooling tower pumps

 

[jraef]

What it is mounted to is important but does not explain what you are trying to accomplish on that application. I think BobPE meant why do you want to vary the speed of one pump but not the other?

"Venditori de oleum-vipera non vigere excordis populi"

 

[smckennz]

Hi Naifmbo

yes this is done in certain circumstances, normally to avoid the cost of two VFDs but have some measure of flow control without throttling. It can make a lot of sense on a system where around 50% of the head is static lift and 50% is friction.
The normal area of debate is which pump should have the VFD. I tend to favour the first stage pump for ease of priming and less likelihood of accidentally producing a partial vacuum.

Cheers

Steve

 

[naifmbo]

hi every body,
i have already replaced my old 1 from 2 cooling tower pumps with pump has 20% higher head,so i want to keep this pump without trimming and install another pump with the same data and fvd.

thanks.

 

[jraef]

If you are NOT planning to vary the pump speed for some process control benefits, it is not a good idea to use a VFD set for a continuous speed. The VFD is not 100% efficient so using it just to reduce to a fized speed will be at the cost of extra losses. If however you have something to gain by speed modulation to gain some additional control or savings then by all means do it.

"Venditori de oleum-vipera non vigere excordis populi"

 

[quark]

You can do it as you will be operating your pump at a constant speed with a VFD(huh...oxymoron).

Much cheaper options are,

1. Purchasing a new impeller
2. Controlling the discharge valve

Regards,