Control of cooling water loops.

[StoneCold]

I am designing a cooling water loop for a new batch reactor plant and I am wondering how others have set their loops up. I am particularly interested in what they are doing for low flow conditions.
I think there are a few options.
The first is just to have a spill back loop at the end of the cooling water pipe run that is large enough to give you slightly more than the minimum flow rate and let the pumps run backed up on their curve if no water is being used.

I could add a more sophisticated option that used a VFD on the Pump motor to maintain a constant discharge pressure at all flows and use a much smaller bypass line to keep the pumps going.

I currently believe that the pumps are going to be about 40HP single stage centrificals. Discharge pressure around 80psig. Flow rate about 800 gpm.

What types of controls are some of you using?

Thanks

 

[25362]

How did you arrive at 40 HP ?

 

[StoneCold]

25362
I guessed. But if you want to really narrow down the pump then how about a Goulds 3196 3x4x13, 3570 RPM, 8 in impeller. 75 hp. running 800 gpm and 231 ft of head.
The point is really how should I control the system.
Multiple pumps? VFD on full size pump? Just let it run wide open?
That is what I am looking for.
Sorry my first guess was so far off.

 

[MJCronin]

Stone,

I have sized a couple of these systems and we have always used some kind of control valve recirc loop.....But I believ that is all in the past.

Get a pump/motor vendor to perform an economic evaluation with a VFD driven pump. They all have economic evaluation programs to justify the purchase price.

Get your supervision to buy into a VFD based simply on the uncertainty of the cost of electricity in today's market (remember, as the price of Natural gas rises, the price of electricity will also)

Come back to this thread and let us know what you decided and the reasoning for your decision.

My opinion only.....

-MJC

 

[quark]

Controling a pump with discharge pressure can take care of the design redundancy during frictional loss calculation and can't give you any other benefit.

There is no single and correct solution for these types of application and like MJC suggested, analyze the system for benefits with and without VFD. Here is one such calculator from Danfoss.

http://www.namc.danfoss.com/hvac/energysavings.html

When you use VFDs, the optimum control scheme would be to control the speed of the pump by the differential pressure across the supply and discharge headers. Your minimum setting will correspond to either the safe minimum flow of the pump or the minimum end user requirement, which ever is higher. You should use 2 way control valves.

Unlike HVAC systems, you either require or don't require cooling to a reactor and flow rate to each reactor is more or less constant. If the load fluctuation is not very rough, I would go with multiple pumps. A switch is a better energy optimizing device.

 

[MortenA]

The economics of a VFD is faily easy:

Whats the difference in flow between your design flow rate, your normal flow rate and how often/for how long do you run at "low flow" and how low is low?

The the law of affinity states that:

Qx/Qd=nx/nd, Hx/Hd=(nx/nd)^2, Px/Pd=(nx/nd)^3

So that if you normal flow is 90% of your design flow rate (and is used 80% of the time) then the saing in eenrgy is about 20% - allthough is propably more like 15% due to reduced efficiency of the pump and cost of energy running the vfd. Then add the flow flow (eg 80% of design flow rate) for 10% of the time: about 5% or a total average energy saving of 20%. Now the savings will then depend on your cost of power.

The relieability of vfd has increased greatly in resent years and the cost has gone down a low (small pumps cost allmost the same with vfd as without) but you have to remember the added cost of including the vfd in the control loop. On the other hand you may now save a control valve and the hook-up for this valve.

Best regards

Morten

 

[StoneCold]

MortenA
The VFD does not bother me at all. We use them all the time however we have not tied the VFD into a PLC and some type of transducer to make a feedback loop to control the pump.

There are really two problems. Should we go with two or three pumps to make up the design flow rate or have two pumps that can each due the design flow rate?

Then how do you efficiently control a variable flow need without just installing a minimum flow bypass from the supply loop to the return loop and running the pump or pumps full speed?

My gut feeling is to go with one full flow pump with a backup. Then install a minum flow bypass at the end of the supply line to the return line. Then add a pressure transducer to the end of the supply line and just try to maintain a comfortable supply pressure. That should slow the pump down on low usage and speed it up when there is a lot of usage. However I have never tried this and I am wondering what some other people have done.

Thanks