View Full Version : onder

27-02-2008, 07:36 PM
Hi !

I'm working in ice cream process.I want to make some improvements on my system.I have two nh3 pumps (one stand-by) on -45 pump vessel.Our cooling demand always changing
Can we use frequency inverter for nh3 liquid pump.For examle ;If I put a pressure transmitter with PLC on the end of the liquid line, can i run this pump according to line pressure ?

I'm sorry for bad english:( !


04-03-2008, 01:04 PM
Hi Onder,
my understanding of a liquid pump operation is it should have a reasonably constant pressure but the flow rate can change.If it gets out of its design range it would most likely cavitate.Depending on type/brand of pump it may have maximum and minimum flow orifice plate to keep pump within its limits (PUMP CURVE)
To keep it simple the pump should be able to handle max demand and be able to bypass enough liqiud through bypass on minimum demand to avoid cavitation.
You could use a manual expansion valve in bypass line back into accumulator or suggest in your case a pressure automatic regulating valve so you will not effectively waste a lot of liquid on maximum demand possibly then requiring a larger liquid pump to keep up.
I hope this is of some help

US Iceman
04-03-2008, 03:53 PM
Most of the refrigerant pump curves I have seen are very flat in the area where you need to operate at. This is normally the left side of the pump curve and also where the NPSHR (net positive suction head required) is the lowest.

In the left side area of the pump curves the VFD will not provide hardly any control because of the flat pump curve.

If the pump operates on the right side of the pump curve then you usually see very high flow rates, which require very high NPSHR.

The amount of energy you can potentially save on a refrigerant pump is minimal at best. On the other hand, if you want to use a VFD as a soft start for the pump motor then it works quite well. And, since the systems are usually over designed with too much flow the VFD can be used to slow down the motor and flow rate slightly, which also lowers the NPSHR.:cool:

Oregon Jim
05-03-2008, 05:07 AM
I believe that it would be a mistake to use a VFD on a liquid pump, and have a strong suspicion that your pump simply would quit pumping. You would be miles ahead to used a VFD on the compressors.

I am curious as to why your process is operating at such a low temperature. I am ASSUMING that you are probably using some form of scraped surface heat exchanger to freeze your ice cream. Is this correct? Is there some reason that you must operate your system in such a deep vacuum (low temp)?