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Inverters for energy savings

Bearded wanderer

I apologise for what will end up being a waffly post. 

I'm using my non-work account, but I have been a member here for many years.

Right. I have recently replaced some cooling towers and while I was at it, I had the control gear replaced. It used to be star-delta 17kW x 3 and I wanted to be able to adjust the speed so I specced an inverter per fan set, with a 0-10v ambient air temperature control, plus a load of other improvements to the plant, and I'm really pleased with it. 

I adjusted the parameters of the speed so it runs at 20% at anything below 10c and when it gets to 30c, it runs at 95% speed. 

Great, it works well, and I am getting the same cooling performance for our process. 

I then came across the first problem with it. I monitored the energy usage of the old towers, which were also 17kW x 3, and compared it to the new towers while they were running at about 50% speed, they are using one fifth of the energy of the old ones, they dont run for longer or anything, they just tick over for hardly any power. The problem with this is that I'm sure nobody will believe me, so I have kept this energy saving project to myself.

Anyway, I remain undeterred, and I have identified some plant that accounts for a significant load, two 22kW pumps, again, star-delta. now, when they were installed some years ago, one of them was pulling too much current, and we were told to throttle it back on the pressure side, and it is, it is only open 2/9ths of the way. The other pump has it's valves almost fully open. The delivery from them looks about the same - there is some restriction designed into the outlets to increase velocity. 

The easy thing for me to do is to have a panel built, inverters etc but there isnt really a reference to control the speed, so it would be set up at a fixed speed that still gave sufficient flow and velocity, maybe then having the valve open fully. 

I think I'm questioning it a bit, would it just be better to get two new pumps sized correctly? They arent cheap, one pump would be more than the inverters. I still cant quite get my head around why closing the pressure side valve reduced the current draw on that pump. Even if we only mananged to turn the pump speed down by 5% would be a decent saving. 

Finally, the pumps arent ancient, but they arent brand new, has anyone had experience of doing anything like this and what was the outcome?

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  • Hi, Sounds like the first set up is working well.  Why not allow their doubters to have their day and open your results up to their scrutiny?  In the worst case they'll flag up something that you have missed and a lesson will have been jointly learnt.  

    In terms of your results, it sounds like you have some old and new systems operating together.  Are they on the same system?  If so, can you be sure the duty is being shared equally?  If they are seperate systems, how do you know the heat rejection demand from both sets of towers is equal?  

    In relation to the scond point - closing a valve at the discharge of a pump will increase the pressure at the pump discharge unless it is inverter controlled to deliver a set pressure and the transducer is across the pump.  In this case the pump will throttle back to deliver a lower flow and thereby lower it;s pressure to the set-point.   

    The question for me in this situation is what the pump is serving?  Is it a process reuqiring a set flow rate?  in whiuch case, what is the low -flow threshold before the process at the end of it starts to think about shutting down?  Or is it a variable flow, e.g. due to varying demand, in which case, what are the high and low limits 

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  • Hi, Sounds like the first set up is working well.  Why not allow their doubters to have their day and open your results up to their scrutiny?  In the worst case they'll flag up something that you have missed and a lesson will have been jointly learnt.  

    In terms of your results, it sounds like you have some old and new systems operating together.  Are they on the same system?  If so, can you be sure the duty is being shared equally?  If they are seperate systems, how do you know the heat rejection demand from both sets of towers is equal?  

    In relation to the scond point - closing a valve at the discharge of a pump will increase the pressure at the pump discharge unless it is inverter controlled to deliver a set pressure and the transducer is across the pump.  In this case the pump will throttle back to deliver a lower flow and thereby lower it;s pressure to the set-point.   

    The question for me in this situation is what the pump is serving?  Is it a process reuqiring a set flow rate?  in whiuch case, what is the low -flow threshold before the process at the end of it starts to think about shutting down?  Or is it a variable flow, e.g. due to varying demand, in which case, what are the high and low limits 

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