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View Full Version : High Side Float - Internal Degassing / Degas Bypass with Thermosyphon Oil Cooling







ascofridge
07-09-2011, 05:34 AM
Hi there!

First time poster here after an explanation from the wise ones.

Am designing an NH3 system with High Side (WITT) float, priority vessel and thermosyphon oil cooling.

WITT Tech bulletin calls for blocking internal degassing line and installing a degas bypass (using solenoid valve). Solenoid open during compressor operation, closed during compressor standstill. They dont explain back ground theory...

Why is this required ?

Why dont you need to do it when you dont have thermosyphon oil cooling?

Thanks in advance fellow Fridgies !!

Magoo
07-09-2011, 06:00 AM
Nothing to do with thermosyphon, how does a closed chamber act as with a float. It will sit there looking at you. PM me as I think I know you, ChCh and Veitnam Nitro plant, ring a bell

RANGER1
07-09-2011, 10:48 AM
ascofridge,
Is it possible to show tech bulletin?

The ones I have seen used simply have liquid reciever or pilot reciever with normal t/syphon connected.

Float is fed from take off in liquid reciever which is eg 25% from bottom so as to give good level for t/syphon.

Witt float has its own internal bleed/vent to downstream of float so float does'nt get a gas lock.

Accumulator obviously holds all of system charge & has to be sized correctly, allowing for pump downs, rooms cycling off etc.

RANGER1
07-09-2011, 11:25 AM
ascofridge,
Is it possible to show tech bulletin?

The ones I have seen used simply have liquid reciever or pilot reciever with normal t/syphon connected.

Float is fed from take off in liquid reciever which is eg 25% from bottom so as to give good level for t/syphon.

Witt float has its own internal bleed/vent to downstream of float so float does'nt get a gas lock.

Accumulator obviously holds all of system charge & has to be sized correctly, allowing for pump downs, rooms cycling off etc.

OK found bulletin & its OK with above description but suggests that if plant has off priod, plant pressures will equalize through vent possibly causing low temps in t/syphon oil cooler.
Witt suggest instal solenoid valve externally with internal vent blocked. Solonoid is only energized while plant is running so off cycle plant will not equalize causing possible low oil temps.

ascofridge
08-09-2011, 03:19 AM
Thanks Ranger,

Yes, the equalizing of pressure I had considered. We typically run low temp evaporators (below -30C) so subject to system volume the priority vessel could see a significant decrease in pressure (condensing above 35) and potential cause oil viscosity issues at start/up. (I have not done the numbers to estimate the equalized system pressure.)

I also wondered if boil off in the thermosyphon circuit would also cause issues.

We are saved somewhat as we have an open flash eco with integrated side load (+5C intermediate) so the final equaliztion pressure will not be so low.

Not convinced that these are all the issues / probably something else too... but nevertheless there is appears to be enough potential problems to warrant the WITT advice !

Will call Magoo for his thoughts.

Cheers.

Magoo
08-09-2011, 04:58 AM
Hi ascofridge.
Sorry for missing your call today, up to my ears with Ice client and WRC starting tomorrow. As usual everthing left till last minute.
I am not a huge fan of open flash economizers at all, too many ditz to go wrong and the true potental of max subcooling is not acheivable, closed flash is the way to go. Ranger1 would agree.
With the thermo oil cooler at system idle the system will settle to ambient conditions and oil fairly close to that temp, start up can induce carry over dependent on load rates of compressor, so recommend a thermo reg oil controller that mixes suply from oil pump with thermo oil, for the life of me I cannot remember the name of them, something lilke Orit .
I will call you in the morning, I think we should get together soonish and look at whole system concept.
magoo

RANGER1
08-09-2011, 05:34 AM
Magoo,
I think you mean AMOT valve for controlling oil temp.