Power savings with floating suction pressure, anyone with any insights on control strategies that actually work.
Plant with reciprocating compressors & VSD with massive turn down, is there any point?
Segie where are you??
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Power savings with floating suction pressure, anyone with any insights on control strategies that actually work.
Plant with reciprocating compressors & VSD with massive turn down, is there any point?
Segie where are you??
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The industrial systems I worked on did not have Variable speed control so that was not an option
but unloading cylinders was used in conjunction with a differential stat set with a logic step controller.
RT stat is below set-point so step controller starts by unloading cylinders and then shutting down comps.
Load increases so step controller starts switching on comps and loading cylinders.
The electronic controllers with Inverter technology (variable frequency drive) motors is the way forward
for full control.
I've worked on some sites where the discharge was run as low as physically possible and then the liquid
pressure was hydro statically pumped up to create the correct pressure difference over the expansion device.
this in conjunction with multiple compressors, some inverter driven and some fixed speed gave optimal
performance over a wider range of loads and increased efficiency for the system as a whole.
Regards
Rob
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Thanks Rob,
The most recent buzz word is "floating suction pressure" ,for plant efficiency.
As you say previously compressor unloading, now VSD & floating head pressure.
To me more for reasonable size plants when things are close to required temperature set points but plant not to shut down.
Suction pressure is floated up for better COP.
Imagine every plant would be unique to a certain degree.
The amount of control we can have is up to the imagination I guess & good PLC programmer.
Not in your league Guys.
But I have a site where myself and my ex colleague saved in the first year of our stewardship. Over 750,000KW.hrs of electric consumption and more again the following year.
By turning off the floating suction. Raising the Set points by a couple of degrees c. As a rule of thumb each degree raised can be as much as 10% Saving on the Electricity consumed.#
Lowering the condensing pressure sounds good but can lead to Poor Hot gas defrost. And that is a game changer if you cannot defrost efficiently. Because if the coolers are not free flowing. the suction pressure will drop and its then a case of the "tale wagging the dog".
A stable system which is not ramping up and down will out perform a floating system any day.
Just my opinion as I am no consultant but answer this!
Which uses more Fuel a steady load or one which ramps up and down?
As you rightly say Ranger it does all depend upon what you are dealing with and who wrote / setup the software controlling the plant.
Variable speed comps are brilliant and will gradually unload and bimble along with next to no load on them.
In fact the site I am talking about has screws with 50,000 run hrs on the and have not missed a beat.
Service exchange run hrs are blown out of the water because, once the surge drums are down to temp.
They just Trundle along at almost minimum speed. Who ever sized the system was spot on.
I remember Power factor correction being all the rage in the 90's. What was conveniently not said was unless the motor was above 70KW it was a waste of time.
What is the first rule when setting up a plant.
Imagine you are up to your chin in S***!
And the Engineer Says" Don't Make Waves!"
Still true today!
Grizzly
Thanks Grizzly,Quote:
Originally Posted by Grizzly
I see it as you & agree 100%, happy to be proven incorrect.
I think a consultant or engineer goes to a seminar, everyone theorizes how it will save energy with better COP, then wants it in a system, you jump through hoops, get something to work maybe, tell them what you did, then it's their idea.
If it does not work it's your fault, am I sceptical & paranoid or what, now I know what that bad taste in my mouth is!
Well, I have never heard of the term "floating suction pressure" before, so I had to educate myself...
And please correct me if I am wrong!
the way I understood how "floating suction pressure" works, it's on a single system, that instead of having a setpoint of -40 when the room is at -25 and the compressor is working from -40 - +30, the setpoint changes to -26 to be just slightly below the room temperature, to keep pulling down, but the load on the compressor is now -26 - +30, but now the lift of the compressor is 56 degrees instead of 70 degrees?
I've been reading all kinds of studies and plans, and then there is the "If the compressor is working on more than one consumer, then the suction pressure/setpoint should be determined by the consumer with the highest temperature"
Now the theory is very much correct, and I don't disagree with it... the less lift there is on the compressor, the less load.
And there are enough control philosophies explained from companies that want to sell this control philosophy as the new "gold rush in refrigeration"
What I don't quite see is how this floating suction pressure can be so revolutionary... lets say you have 10 consumers, that are cooled down by 1 compressor... ideally it all works fine... then there is an issue with one of the consumers, 9 consumers are cooled down, but the compressor is now acting by the highest consumer in the chain and adjusting it's suction pressure (how, I don't know), but from what I read, it's all about the compressor running at full speed or maybe with load control.
But the whole thing with the floating suction pressure, where is it controlled? it has to be at each individual consumer/evaporator.
And the compressor is completely outside of this equation, because the compressor has been calculated by the supplier to take the maximum load of the plant, and the minimum/part load of the plant.
And the compressor is step up or step down capacity loaded...
so what we see at these plants is that a compressor goes like this:
brrrrBRRRRRRbrrrrBRRRRRbrrrrrBRRRR as it is switching between 75% and 100%
Even worse on ac plants where the supplier has overdimensioned the compressor, so it starts, runs for 5 minutes, stops, then starts again, going BRRR.... BRRR... BRRR
first we install a danfoss AK-CC-550 to control the cooling, and prevent the compressor from breaking down from wet suction.
Now we can control the temperature in the ac from 13-17 degrees, because the compressor goes BRRRRRRRRRRRRRRR, cools the air down to 13, then stops... and I have entered that it should have a 15 minute break, and in that time the AC temperature rises to 17-18.
Now... what is the worst you can do with a refrigeration plant?
to have it stop :)
Now imagine if I had mounted a VFD on that compressor that sensed the suction pressure, now that compressor could run fully loaded and I could control everything with the VFD, and it wouldn't have to stop and start every 10 minutes.
It's kind of how we are running our RSW systems with 250KW motors.
Start the water circulation, start the nh3 pump (spray chillers), start the compressor.
the compressor starts unloaded at low speed... usually it takes 5-10 minutes before the compressor has gone from 0%-100% mechanical load.
after that another 10 minutes before the motor is running at 100% speed...
And this is the pinnacle of RSW, on systems like this, the cooling curve goes pretty much at a 45 degree angle from +10 down to -0,5 before it starts flattening out.
on the older systems with star delta, it would reach maybe +1 and then the compressor would stop on low suction... then you had to wait 15 minutes before the compressor started at full speed again and would run for another 30 minutes etc etc... the start and stop of the plants, where they would consume vast amounts of fuel was always a discussion... one ship said, they didn't have enough power to run the RSW while running the hydraulics to pump the fish on board... This ship replaced one of the star delta starters with a VFD... and when they came back to Norway the chief engineer were so over eager to tell me "oy oy, you know, we started the system when we came out of the port and took on clean water! and it has been running since!" the water in the tanks are -1,4C and it's still running!"
Silly bugger dragged me on board and told me to have the honor of shutting down the system :)
I'm not an engineer with a fancy degree, but I think that as long as you have full mechanical load on a compressor and then control the speed, that sounds way better than floating suction pressure any day :)
Ok, you can adjust the load on the compressor, higher suction pressure is less lift of the pressure...
But c'mon at any system there always comes a time when you have to pull it down :)
VFD forever :D
Tycho,Quote:
Originally Posted by Tycho
Thanks for reply.
Yes VSD on the end, this is the next step to increase COP of compressor/s.
Probably can use in abattoir, cold storage or when suction can be controlled at higher pressure/temperature.
Depending on power bill structure different suction set points & if rooms pretty close to set point maybe float suction pressure a bit higher.
Probably lots of scenarios that it could be used.
Just back from vacation. Production plants. Some production processes require lower suction pressures than others. Sometimes suction pressure can be increased when only higher suction pressure is required. Holding freezers. At steady refrigeration load increase of suction pressure will require to run additional evaporators and additional evaporator fans. These fans use energy and release this energy into refrigeration room as parasitic refrigeration load. Additional compressors energy required to remove this load. At optimum suction pressure plant(compressors + evaporator fans) energy use per net refrigeration unit will be minimal.