Open superfeed container

[Tycho]

I've been having some discussions with a fellow engineer about the use of superfeed on a screw compressor.

We have a plant with two Howden WRV 255 servicing 4 quick freezing tunnels.

refrigerant is NH3 (hence this forum )

they are fitted with a superfeed at -25C (according to the compressor supplier, these compressors should not run with a superfeed pressure above -25C, supplier is not howden)


it's an open superfeed, and the cycle goes like this:
when the pilot receiver reaches high level. the valve to the liquid separator opens and drains from the superfeed container, when the superfeed container reaches the low level it takes liquid from the pilot receiver.


The superfeed suction to the compressor has a danfoss ICS valve with a CVC, so it is keeping the pressure in the superfeed suction at a steady -25C, the pressure in the superfeed container is what it is depending on how much liquid is going through it towards the liquid separator.

Now, our discussion is weather or not the superfeed is economical, we both are "greasmonkeys" and havent done any calculations... but we both agree that a closed superfeed container would be more efficient than an open one.

the main argument is whether a closed superfeed container is efficient or not.

I say that "if it didn't give you any benefit, it wouldnt be used"

he says "is the extra effect weighing up for the extra power consumptions?"


I'm thinking along the lines that if a AC unit had a COP of 1, we could just as well have used electricity.


there has to be some savings/winnings of using a superfeed..

I have been looking around the internet for any literature on this but have turned out lot's of hit's on reciprocating compressors but nothing on screw compressors.


What are all your opinions?

[RANGER1]

Sounds primitive to me !
Why can't you fill & drain superfeed vessel at the same time & floats back up if level to low/high .
Superfeed suction with CVC also sounds to conservative at -25 deg C /65 kpa .
You are not having steady load to superfeed & are restricting it into compressor .
Usually in this application I thought a CVP back pressure regulator would be used so as superfeed vessel pressure is not to low when screw is unloaded so pressure difference is made to transfer liquid .
Of course there is a max pressure limitation which Howden would advise but doubt it would make (I guess max at 300kpa).
Nothing wrong with closed loop as long as coil inside is long enough etc to achieve max subcooling etc .
Using modulating level control tops it off as good design .

Its like each design engineer has to have their little specialties to make themselves feel good .

[mad fridgie]

Sorry for showing my ignorance.
Superfeed, are you talking about open flash and H/E enconomisers?
Is just different terms used?

[charlie n]

Open flash economizers are always more efficient than closed economizers since the liquid reaches to the side port saturated temperature/pressure (or controlled pressure) in an open economizer and is 5 degrees or more warmer than side port saturation with a closed economizer. This 5 degrees represents flash gas that must be handled by the compressor without doing any useful work.
It's called an economizer because it is economical. In almost all applications of screw compressors, some advantage in capacity and improvement of COP can be gained with an economizer. The effect is more dramatic with HFC & HCFC refrigerants. I've even used economizers on ammonia water chillers with +1 or +2 SST in large plants with high condensing pressures (40 SST). the result was to I could install a smaller compressor with higher COP to do the same work as a larger uneconomized machine.

[RANGER1]

charlie n how would you rate current control system of Tyco & how would you manage controls?

[Magoo]

Hello Tycho.
the system you have described is typical of an open flash economizer, the -25 'C limit would be to enable sufficient liquid flow rate from econo to accumulator. The PM vav and pilot would be pilot driven from compressor suction, to maintain differential pressures.
Closed flash econo only needs a stop check in econo suction and depending on how much HX area will get fairly close to partial port pressure temp., of screw. Generally for every 2 degrees sub-cooling you get 1 % increse in system performance. Turn off the economizer and watch the system performance drop off.

[RANGER1]

Magoo I think Tyco is suggesting compressor economizer line be limited to -25 downstream of PM/CVC, which is weird unless motor is on limit or something .

[charlie n]

The economizer pressure would be limited to -25 upstream of the PM valve. This is to ensure flow from the economizer to the low pressure receiver as Magoo describes. This is generally an efficient design. The control method is a little misguided. Things would be more steady & more efficient if there were two high side float valves (PMFH or Witt HR) one maintains a level and drains the pilot reciever into the economizer and another maintains a level and drains the economizer into the LP receiver. What is installed may be a fix for an undersized LP receiver.

[Magoo]

Hi Ranger1,
the down stream after PM is governed by the econo port of screw pressure/temp.,
I have have huge preference for closed flash econos. High pressure liquid feeding accumulator with a PMFL and bleeder.

magoo

[Magoo]

Where is Tycho

[Tycho]

Sounds primitive to me !
Why can't you fill & drain superfeed vessel at the same time & floats back up if level to low/high .
Superfeed suction with CVC also sounds to conservative at -25 deg C /65 kpa .
You are not having steady load to superfeed & are restricting it into compressor .
Usually in this application I thought a CVP back pressure regulator would be used so as superfeed vessel pressure is not to low when screw is unloaded so pressure difference is made to transfer liquid .
Of course there is a max pressure limitation which Howden would advise but doubt it would make (I guess max at 300kpa).
Nothing wrong with closed loop as long as coil inside is long enough etc to achieve max subcooling etc .
Using modulating level control tops it off as good design .

Its like each design engineer has to have their little specialties to make themselves feel good .

the original installation was with a CVP, but the superfeed valve is only activated once the compressor is in 100% capacity.
this caused the economizer/superfeed drum to be at condenser pressure when the valve opened and caused a massive spike in amps.

Also, howden said it should not have a superfeed pressure higher than -25

[Tycho]

Magoo I think Tyco is suggesting compressor economizer line be limited to -25 downstream of PM/CVC, which is weird unless motor is on limit or something .

I was not suggesting it just saying how it is now.
when it was installed it had a CVP to keep the superfeed drum at -25, but as I said that caused a massive spike in amps and caused the compressor to roll off capacity to decrease the load.
then when the cap. increased again it would happen again and again, and depending on how much liquid passed through the superfeed drum it would eventually stabilize.

[Tycho]

The economizer pressure would be limited to -25 upstream of the PM valve. This is to ensure flow from the economizer to the low pressure receiver as Magoo describes. This is generally an efficient design. The control method is a little misguided. Things would be more steady & more efficient if there were two high side float valves (PMFH or Witt HR) one maintains a level and drains the pilot reciever into the economizer and another maintains a level and drains the economizer into the LP receiver. What is installed may be a fix for an undersized LP receiver.

We are using danfoss AKS level transmitters and EVRA valves.
the reason for the design, I found out was because if for some reason we had to close off the superfeed and bypass it, we would not get liquid to the LP receiver unless we rewired the solenoids during the period the superfeed was out of commission.
so to ensure that the machinists running the system could close off the superfeed without having to mess around in the control cabinet, it was done this way.

also, the LP receiver is more than big enough

[Tycho]

also, when both compressors were running, we found that the superfeed drum would empty itself and the pilot receiver would fill up.

My first thought was that the AKVA valves were probably to small.

and after a day of checking filters and scratching their heads, they did the calculations and found that the valve feeding the superfeed was too small to feed enough liquid at those operating pressures.

yay

[charlie n]

This type of design needs a small suction pipe gunning from open economizer to the low pressure receiver. this pipe should be fitted with a PM1+ CVP and set to maintain inlet (economizer) pressure of about -10 saturated. This keeps the economizer pressure reasonable during start up and low load conditions when the compressor economizer port is closed off to the vessel by a solenoid.

[RANGER1]

Why does economizer have to limited when compressor is below 100% ?
If its on all the time with original CVP it will work itself out & be more stable.
It will be more efficient at 100% load of course , but if unloaded its inefficient to a certain degree anyway .

[charlie n]

If the compressor side port pressure is close to suction pressure there is no advantage to economizing. This is a common situation in most screw compressors. There's also the practical problem that the control valve is selected for a certain pressure difference. at part load, the flow through the CVP or PM1 is decreased and the pressure difference is increased. The valve can end up being too big & then it hunts or in the worst case chatters itself to death.

[sterl]

Well: The old floating pressure open economizer argument takes a new twist...

The flash arrangement is more efficient than a closed cycle heat exchanger set up but the real difficulty lies in the basic compromises of running more than one compressor on any thing like a "shared" side port load. Somebody is just not going to be happy.

My own opinion is: There is way too much effort in most of these case to establish a real low economizer pressure and yet most side port arrangements only operate when a compressor is (near) fully loaded....Which for a single compressor system, is also to say when the liquid flow is greatest.

So all the valves and controls are sized for the Max load, min pressure condition.

With modern makeup arrangements at the LPR: a closed system does gain some virtues it never used to have in the days of cycling solenoid valves.

I believe: a designer should size the side port valves for coniderably more pressure difference than than the "normal" means; size the check valve smaller still; and bypass the whole train when the machine suction starts to rise at 100% slide valve...Because when the machine is overloaded is the only time a small pressure difference across these valves is doing you any good. That also means those same valves will tolerate the lower flow-larger pressure drop when the machine suction decreases or the machine unloads: Which is to say you can take advantage of the side port down to 60 or 70% of slide valve, biased sliightly for the actual suction pressure. The screw compressor packagers are all promoting Economizers by comparing them with a 2-stage compression system...And for an application that runs the exact same suction pressure; with the machine near 100% load ALL THE TIME they are comparable..>Course there are very few applications like that. In reality: as they are normally arranged, economizers are an exercise in avoided investment, not in operational efficiency, because operations don't run either on or off: loads are always changing, even in high volume process lines.

[Brian_UK]

Thread reopened as requested by OP Tycho.

[Tycho]

Gonna start wit a big thank you to Brian for opening this up again for me

[Tycho]

I'm going to start somewhat at the top again here:

In the beginning there was an open economizer (I may switch to calling it a superfeed, since that was what Kvaerner, howden and SES called it back in the days, but I will try to stick wit Economizer) container on the plant, the economizer was supplied by the HP receiver and the LP receiver was supplied by the MP receiver (economizer/medium pressure receiver). there were PM valves with EVM's and CVPP's mounted on the compressors economizer suction to make sure they were not pulling a too low pressure on the economizer container...

Long story cut short, the pressure difference between the economizer and the LP receiver was too small, so in all cases the pressure difference between the economizer tank and the LP receiver were so low as we were not able to move liquid from the economizer tank onto the LP receiver.

The Economizer was replaced with a closed type, and the system is running as never before....

in this case, all the machinery is placed on the same floor, so it's all a case of the office not listening to the guys on site