Super heat hunting.

[Drew]

Hi all,

I'm working on a system that runs R22 low temp for an air separation unit.

Runs at -45c suction and about 30c head.
Head pressure stable with good sub cooling.
It's an old piece of gear with recip compressors , head coolers , liquid line sub coolers etc. Been running 20 years. So much for not using r22 low temp.?
The evap load is stable.
A while ago we noticed the super heat varying from flooding back occasionally to maybe 30 degrees.

I adjusted the sporlan valve in and there was enough of improvement to prevent flood back but still had big hunting swings.

Going back to site today noticed the superheat was still varying but at a much higher level.: 18 to 35 degrees.
The sporlan power assembly had been recentley replaced ( by others) and it's seems to be correct low temp r22 etc...

i understand a valve needs to be correctly sized with regard to pressure drop etc.....

I battling to understand whether I should be opening or shutting the valve to find my 6 degrees of superheat. ?

An oversized valve can hunt causing flood back etc....but this is high super heat hunting?

The valve could obviously be faulty. It would be a nightmare to change so I'm hoping I've either shut it to much or something easier than replacing it!

Any ideas?

[RANGER1]

Drew,
can you be sure liquid line is always full of liquid.
Does it have a sight glass & how is liquid line run, up, down, how long.
Also bulb position, pipe size, any air flow over it.
Evaprator icing, any chance of that.
Liquid line filter drier been changed lately, as moisture could be I ing up. Valve.

[Glenn Moore]

Hi Drew
Rangers points are all relevant, and need to be checked. You state the discharge and evaporator duty is stable which if that is the case then I would check where the bulb is fitted, ie if its fitted on a vertical pipe it could be acting like a Liquid Level Controller which acts like an ON /OFF control which causes the superheat to swing up and down. But I would suspect the valve itself could be at fault. I spent most of my working life working for Danfoss in a technical roll but often got involved with sorting out plants with non Danfoss components fitted. One of the things I found with Sporlan expansion valves was that the expansion valve push pins stick or jam. This often caused dramatic pressure swings as the valves open and closed instead of modulated. I put this down to the body being distorted during the installation process by being overheated not enough wet rag maybe. On older valves the push pin gland seals can become hard causing the pins not to be able to slide freely again causing these pressure /superheat swings. Danfoss some years back removed their orifice packing glands and changed to an interference fit to remove this friction/stiction problem
So you may have to bite the bullet and change the valve

[RANGER1]

Drew agree with Glenn as well.
Maybe you can get a repair kit for it!
On the ones i have worked on which were on ammonia you could get a repair kit, it came with all internals & new pins.
The lift pins had to be fitted, & a special card for measuring pin height.
You then have to grind pins to length, then fit & adjust gland for free movement.
That's if it's a similar type of course.
It might be easier than a new valve in some cases.

http://sporlanonline.com/literature/10/10-10

http://sporlanonline.com/literature/10/10-9.pdf

[Drew]

Thanks guys. I'm sure I'm getting a full head of liquid to the valve as I'm sub cooled.
I also suspect it to be faulty....., but just trying to make sure?Does a valve only hunt if it oversized or to open ( if loads and hp are constant) as it overshoots and then over compensates?
can it hunt if it's to far closed? Or will it just have a high super heat.
Thanks again. Great bit of info Glenn.

[RANGER1]

Thanks guys. I'm sure I'm getting a full head of liquid to the valve as I'm sub cooled.
I also suspect it to be faulty....., but just trying to make sure?Does a valve only hunt if it oversized or to open ( if loads and hp are constant) as it overshoots and then over compensates?
can it hunt if it's to far closed? Or will it just have a high super heat.
Thanks again. Great bit of info Glenn.

Drew if it's hunting suction pressures should also change.
What about bulb position as requested, what's it's position, pipe size etc.
Also liquid line could still have flash gas in it & have subcooling.
Does it have a sight glass to observe.

You could also try increasing discharge pressure, pretty low, which can also upset valve capacity & operation.

[RANGER1]

Try 40 deg C condensing

[Glenn Moore]

Hi Drew
When you select an expansion valve you select it by Capacity (Kw), Evap Temperature, Condensing Temperature, Sub cooling (K) and refrigerant type.
So you then select the valve that suits the above design criteria. Almost in all cases the expansion valves will be either just the right capacity or slightly oversized. To select a valve that is to small would cause poor duties low suction conditions and poor evaporator control.
So the expansion valve should always be able to overfill the evaporator which is where 'Hunting' comes in. If the valves superheat setting or the installation isn't as it should be for what ever reason the valve can over inject then the valve closes due to low superheat , the valve then stays closed until the liquid has started to evaporate and become superheated for the valve to start to open again hence 'Hunting' occurs. So every expansion valve even though it comes 'Factory Set' must be adjusted to suit the plant its fitted to, which is why they have a superheat adjustment to increase or decrease the orifice spring tension. As you say approx. 6 Deg K superheat is a good general rule.
If the valve is undersize it can never hunt as it can never overfeed the evaporator to cause liquid to exit into the suction line.
The problem with Thermostatic valves is that they have no brain and they can be opened and closed by the change in superheat detected by the bulbs temperature/pressure, as design, but they can also be opened and closed by compressors loading and unloading, as the suction pressure changes with the compressor load patterns this can cause the TEV to over inject as the pressure in the equalisation line is a closing force for the TEV, if that pressure suddenly falls as the compressor loads up, it forces the TEV to go open , it will eventually recover its superheat control but this can cause liquid slugs etc.
TEVs are the most misunderstood control in a fridge plant yet they are one of the simplest. Kr Glenn

[Goober]

Glenn,

Nice reply, very nice indeed. Especially the last sentence. Rep point for you!

I would add, and I beat this into my staff on occasion, that if they think a TX needs to be adjusted after operating fine, then its either f*cked and needs replacing, OR there is something else going on with the system.

Its an automatic beating with the biggest stick I can find if an engineer writes on his service report, Adjusted TX Valve, and nothing else. Gggrrrrrr....

[Glenn Moore]

Hi Goober
I used to test all the Danfoss warranty returned controls, many were T2 expansion valves returned from the supermarket contractors. 99.9% were found to be within factory spec. The warranty sheet just said TEV faulty but on test NO FAULT was found. So the TEV was changed to cover the site visit call out!!!!!!!! as it was an easy option. ie Bull--it baffles brains
This is a true story :-
I was at one of the trade counters when an engineer (I use the term lightly) came in and wanted help on a cold room he was working on. He had changed the TE2 expansion valve and orifice and since then the cold room had never reached its cut out temperature. After questioning him about the valve type the installation , bulb position etc etc everything seemed OK but when I asked how much superheat he had, he said a couple of other engineers had asked the same question but he had not bought any as he could not find it in the wholesalers catalogue. If I could give him a code no he would buy some. At this point Im thinking Your taking the p-ss, but he was deadly serious. The job was only a couple of streets away so I went with him and showed him how to set the valve and measure superheat. Like many engineers he had never adjusted a TEV in his life as his old boss had told him TEVs come factory set.
The problem is many of these engineers are now fitting electronic expansion valves and all to often I am asked to attend site due to poor performance and find the controller left at factory default settings and not tuned to the plant to get the ultimate efficiency as possible. Many systems with EEVs can run stable at 4 K superheat yet the controller is running the system at 10K making the systems totally inefficient and causing other system problems such as high discharge superheats , high oil carry over, low suctions etc. But the worst thing is the customer is left with a nightmare of a plant that is costly to run costly to maintain and he gets non of the benefits that the plant and controls should give him

[Drew]

I'm fitting a carel electronic texv to another job on a freezer room.. Set by default at 11k superheat.
I understand the added benefit of a temp sensor controlling the superheat for capacity control (. At the detriment of superheat ) but just as a expansion valve with set superheat?
it took all day to fit , had to run supply etc... And by default it's no more effecient than the 20 year old, half price, bullet proof mechanical valve. Nice to have programable mop logic etc..... , but I don't see the advantages. Hard to justify them to your average store owner.

[Glenn Moore]

Hi Drew
I find the Carel valves are a bit slow in control , but you should change the superheat setting down as you know yourself that 11 k superheat is rubbish. So you must try to reset the superheat control to get the best effect from your evaporator. In future try the Danfoss ETS valve with EKD316 controller they work very well and certainly give a more stable superheat control than the Carel. Ive only seen the Carel on AC units and ive replaced a few as well.
If you fit an electronic expansion valve and then set it up with the lowest stable superheat possible, you should save your customer energy by running with a slightly higher suction condition, which should increase your compressors capacity by a few % and also if the condenser is capable , run with a lower discharge pressure again saving energy. You should also see shorter running times etc . Yes the old mechanical valve is reliable , but modern day electronic valves are just as reliable and can give your customer a far more enhanced plant.
How good it is and energy savings are down to your fridge expertise. I have achieved energy savings of up to 28% by using EEV's , invertors on the lead compressor, and the condenser etc. Any plant is only as good as the engineer who installs it and looks after it, and advises his customer on the benefits of using the latest technology.
Keep up the good work Br Glenn

[Drew]

Makes sense over time I suppose.
What sort of super heat could I set the valve to?
is their a chance of it hunting if I set it to low?
i was thinking 3 k ?. Why not even lower to 1 degree then? Most systems would react well to the gain in capacity.It's default alarm is at 4 degrees!?
Only a 2 degree gain over your average mechanical valve ?

[RANGER1]

Makes sense over time I suppose.
What sort of super heat could I set the valve to?
is their a chance of it hunting if I set it to low?
i was thinking 3 k ?. Why not even lower to 1 degree then? Most systems would react well to the gain in capacity.It's default alarm is at 4 degrees!?
Only a 2 degree gain over your average mechanical valve ?

Even with 3-4 deg C superheat you can get possibility of liquid returning to compressor.
You need insurance for compressor & system reliability.
When EEV senses superheat to low it starts to get inefficient anyway, hunting etc.
Its part of the original design to have "X" superheat.
HEad pressure has to be very stable for good control.
Still need full liquid line.
Still need all the basics to help it run reliably.
I dont think anyone here suggested EEV as standard TX valves OK anyway.
Only bonus is good turn down on low loads & you don't have to pull your gauges out to check things all the time if problems , or you just want to see.

Did you pull Sporlan apart to see if pushrods difficult to move.

[Drew]

Did you pull Sporlan apart to see if pushrods difficult to move.

im fitting the carel to another freezer room job and I'm planning to replace sporlan with another sporlan on the air separation unit. Sorry I didn't make it clear these are 2different jobs. Maybe I will replace the sporlan with an electronic valve. As you said it is nice to check out how it's running.
Still haven't stripped the sporlan out yet. waiting for the plant to shut down so I can melt the ice off the heat exchanger.

[RANGER1]

im fitting the carel to another freezer room job and I'm planning to replace sporlan with another sporlan on the air separation unit. Sorry I didn't make it clear these are 2different jobs. Maybe I will replace the sporlan with an electronic valve. As you said it is nice to check out how it's running.
Still haven't stripped the sporlan out yet. waiting for the plant to shut down so I can melt the ice off the heat exchanger.

Drew can you explain the system & evaporator function in detail, as the "ice" on it caught my attention.

[Drew]

This low temp system is the last of three stages on the air separation system at BOC's plant here in Hobart. It's aging and soon will be replaced.
It is an insulated shell and tube type evap , expansion valve and suction at the one end. Compressed air enters the shell an tube and is further cooled in the low temp stage.
The plant runs continuously, icing up the valve until it is covered in ice unless we have a shut down where the ice melts off exposing the valve.
Ideally they look for a -40c off coil but we are up to -36c which is not ideal.
ive put it down to age and the valve not maintaining superheat.

[Glenn Moore]

Hi Drew
Every evaporator has an MSS (Minimum Stable Signal (Superheat control point)) this maybe 4k ,maybe 14K it all depends on the evaporator design, distributor pressure drop, air flow etc etc.
A Thermostatic valve can never work with a 1K superheat as the superheat spring would have little or no effect and the orifice would simply bounce open and close with no control at all.
Three forces make the TEV work , Bulb Pressure(Opening Force) Evaporator Pressure force and Spring force combined (Closing forces). The spring tension is the only thing on a TEV that you can adjust to change the S/heat control point. When you find the MSS point of the evaporator the TEV setting is at its Optimum setting. This setting will all depend on the plant set up and the evaporator design typically a good evaporator can work typically between 4 -7K.
Some years ago I introduced the Danfoss TQ electronic expansion valve to the UK , and York fitted them to their Gold Standard Chiller. York had several great tech guys and we decided to carry out deep R&D work on the chiller to see what Valve suited the chiller best. We ran tests with an ALCO tev , then a Sporlan hydraulic barrier TEV , and then the Danfoss TQ electronic valve.
The Chiller barrel had reinforced glass end cover plates plus 3inch thick Perspex covers to support the glass then a steel ring to hold the special end covers on to the barrel. This allowed us to watch the refrigerant being injected right the tubes through the 4 passes of the chiller.
The 2 Thermostatic valves worked well with the compressor on full load , but when the machine unloaded you could see the chiller start to dry out of liquid and the duty fall off was dramatic. With the TQ electronic valve the full load duty with the valve set at 5K superheat was slightly better than the TEV's but the part load difference was dramatic , even at part load the chiller was still fully flooded up with no liquid carry over at all.
We then started looking at the liquid distribution through the chiller, and you could see large swirling venturi like liquid hitting the end covers, but you also see large tube areas of pure gaseous vapour near the swirling liquid. To try to help mix the liquid and vapour back up, in each end cover in the 2nd ,3rd and 4th pass of the chiller a heavy metal gauze was fitted. When we ran the plant the gauze acted like an oil separator and you could see oil becoming entrained in the chiller, so the gauze was removed. So then the gas carrying tubes were blocked off, some 17% of the tubes, and the plant re run . The chiller still did its design duty but now we actually ran it at 1k superheat. It was run fully loaded then unloaded at 50%, 25% 75% up and down at 1K and there was never any signs of liquid or foam in the compressor oil . The control was spot on at all capacity levels with the TQ valve. The pressure drop through the chiller was slightly higher but with 17% of the tubes the chiller still performed perfectly.
Air coolers are notoriously of poor design ie a square coil block with round fans is a bad start, but only the small commercial coolers go through any degree of R&D , whereas with larger bespoke coolers the only R&D done is by guys like you & me when it doesn't work properly. Many of the largest installations for cooling have the totally wrong air flow design ductwork . These design guys think are turns right angles , well believe me it doesn't.
Electronic valves will always beat a TEV but as the old saying goes You cant make a silk purse from a Sows ear.
So I have proved Electronic valves can work at 1K superheat, but the chiller was slightly supped up from the normal design, but the TEVs failed badly at this point.
As I said in the previous mail you need to know how to best set up an electronic valve, otherwise you are wasting your time and your customers money,. So get your Carel or Danfoss rep to give you some in depth training and then you will see the benefits and so will your customers. I ran a training course on electronic controls just before I retired in Preston , 2 of the engineers were using the ETS version valves and when we started to go through the parameters , the only ones they changed were the refrigerant type and the valve size , everything else was left at factory settings . When we discussed the valves speed , its superheat settings and mode of control they didn't have a clue what changing the settings could do. So their company was using the latest technology but they might as well have stayed with a hand regulator.
This chiller is now in a university in Scotland, there were many videos of the chiller taken but they were never released , but they were a fantastic training aid , and helped to give me an in depth knowledge of evaporators and liquid injection its a fascinating subject .

[Drew]

Wow, what a great read. Thank you!
i would love to see in the inner workings of an evap as the refrigerant evaporates.
That would make teaching about superheat a pleasure.
Must have been great to experiment with that sort of gear.

'square coil with round fans' , never thought of that, but now will every time I see one.
The solution is clearly obvious : square fans.

I have since set up my carel valve and over an afternoon of lying on my belly on the freezer room roof and playing with the parameters have grown quite fond of the graphics and ease of operation.
i did have to run a second cable back to the plant room after realizing I had to get a dry contact of the compressor back to the valve to enable the valve once the compressor runs. As it goes into standby mode with compressor off. If you don't tell the valve the compressors off once the compressor cycles the superheat drops and the valve goes into low superheat alarm and then takes a while to automatically recover.

With regard to a mechanical thermostatic expansion valve would it be correct in saying that if we had a wide ranging tex valve working correctly under stable conditions that if I shut it down fully the superheat would be constant but high, as we open the valve the superheat would decrease , if we could open the valve further the valve would effectively become oversized and would start hunting? And this is why we shouldn't try and achieve too low a superheat as the tex valve would hunt due to it becoming effectively oversized?

Thanks again for a great reply.

[Glenn Moore]

Hi Drew
If the valve has been sized correctly , then yes you are correct.
You could do a test on a TEV, by winding the superheat spring in to raise the superheat to max that the valve can manage. Fit a thermometer next to the expansion valve bulb tight onto the pipe. The thermometer should be nice and steady, then open the valve a couple of turns and let it stabilise , keep doing this until the thermometer starts to swing up and down. At this point wind the spring back a couple of turns until the thermometer is stable again , now you have the MSS point of the evaporator and the ideal setting for the valve. Kr Glenn

[Drew]

Hi Drew
If the valve has been sized correctly , then yes you are correct.
You could do a test on a TEV, by winding the superheat spring in to raise the superheat to max that the valve can manage. Fit a thermometer next to the expansion valve bulb tight onto the pipe. The thermometer should be nice and steady, then open the valve a couple of turns and let it stabilise , keep doing this until the thermometer starts to swing up and down. At this point wind the spring back a couple of turns until the thermometer is stable again , now you have the MSS point of the evaporator and the ideal setting for the valve. Kr Glenn

Again great reply ,thank you.
i will be trying this on the sporlan valve before I condem it.
I normally just aimed for 6 degrees without realizing I could maybe open up more to the point of hunting and then wind in a bit to stabilize to maximise capacity.
i didn't realise evaps had these mms points . I thought we just aimed for as low as is safe.( which I suppose is the same thing.)
all the pieces of the puzzle are falling into place.