how do you determine what the superheat should be on a system

[frosty74]

when working on a system and you want to check the superheat i now you use a touch probe etc but how do you determine what is the correct temp
example if working on a remote display case on r404a

how do i check the superheat correctly?

[nike123]

You have two things to consider:


enough total superheat recommended by compressor manufacturer to ensure that compressor is safe from liquid flood back in all possible/predictable working conditions and
as little as possible to ensure low enough compressor working temperature (low discharge temperature) and higher evaporator efficiency.

[lowcool]

if the probe reads warmer at the outlet than the suction pressure it is a good place to start,given humidity & design factors will also influence your settings.fully flooded coil(narrow superheat setting) gives higher humidity than a starving coil(very wide superheat) gives a dehydrating effect.if all else fails take some no doze and go back to school hee hee.narrow superheat loads up compressor and gives good coolling in suction line.wide superheat,little coolling in suction line,compressor load not so great but higher discharge temperatures thus system design (component matching)is always important.hope this helps

[Goober]

Aaaah, the old superheat conundrum appears again....

Now to me I've always been a tad confused/unclear as to what we are trying to achieve. Are we trying to maximise efficiancy of evaporator or provide cooling for compressor/s or more likely a balance of both.
Setting superheats is not an exact science (do correct me if I'm wrong). for example I was taught that the "rule of thumb" superheats should be set as follows, Low temp 2k, Medium temp 4k, Air con 10k.

Take one of the above, and try and set, say low temp at 2k superheat, what I find is that when the TX valve closes on sensing low temp, the liquid in the evap then boils off more violently causing the thermometer temperature to drop dramatically with some drop in pressure to point where pressure/temperature and thermometer temperature are same. And prior to TX valve opening the thermometer temperature can be quite high, say 10k from converted pressure/temp ......so do I accept that somewhere in between the two figures lies the elusive 2k super heat. Or.................do I try and set the valve so that on the low swing it never drops below 2k and accept that the high swing is just that, a high swing and the normal opertaion of the valve. While thats happening the suction pressure at evap outlet will also be rising and dropping a few PSI too, just to confussss things further

Or.................Have I never had the patience to breath and sit and measure and not adjust a valve and that there are systems out there that maintain a continuous accurate 2k, 4k and 10k superheat.

And to go full circle....evap efficiancy oor compressor protection? Hope I've explained myself????

[bill1983]

hi frosty,
ideally to check the superheat correctly you will need a guage port at the evaporator outlet. if you do, you can read the pressure and temperature in the same place to get the most accurate readings. if you don't, use the nearest accessible guage port but remember to adjust your readings to take pressure difference into account.

the easiest way to remember how to measure superheat is to realise that it is the difference between the evaporating pressure and the temperature at the evaporator outlet. ie convert the pressure to temperature and calculate the difference between that theoretical temperature and the ACTUAL temperature.
the other important thing to remember is that you have to let the case reach normal operating temperature before you determine any accurate valve adjustment.
the other important side to superheat, which is the part most engineers forget is that superheat measured at the compressor.
everybody knows that the superheat must be high enough to prevent liquid from entering the compressor, but a large number of engineers mistakenly think that frost on the suction line is a clear indicator of liquid inside the suction line. it is not, all it indicates is that the suction linen itself is colder than freezing point, ie 0 c.
as well as measuring superheat at he evaporator outlet, you should also measure it at the compressor inlet, or more realistically 6" away from the compressor, as the mass of the compressor body can lead to false temperature readings.
as regards what the temperatures should be, it all depends on the system application and manufacturers recommended evaporating temperature.
for example a low temp system evaporating at -35c, with a superheat of 4k, would give you a temperature at the tev bulb of -31c.
simerlally, a high temp system evaporating at -8 with a recommended superheat of 7k should give a temperature at the tev bulb of -1c.
as a guide, compressor superheat should be no more than 20k, in the low temp case therefore, the suction line at the compressor should be no more than -15c, in the high temp case no more than 12c.
hope this helps.

[750 Valve]

Aaaah, the old superheat conundrum appears again....

Now to me I've always been a tad confused/unclear as to what we are trying to achieve. Are we trying to maximise efficiancy of evaporator or provide cooling for compressor/s or more likely a balance of both.
Setting superheats is not an exact science (do correct me if I'm wrong). for example I was taught that the "rule of thumb" superheats should be set as follows, Low temp 2k, Medium temp 4k, Air con 10k.

Take one of the above, and try and set, say low temp at 2k superheat, what I find is that when the TX valve closes on sensing low temp, the liquid in the evap then boils off more violently causing the thermometer temperature to drop dramatically with some drop in pressure to point where pressure/temperature and thermometer temperature are same. And prior to TX valve opening the thermometer temperature can be quite high, say 10k from converted pressure/temp ......so do I accept that somewhere in between the two figures lies the elusive 2k super heat. Or.................do I try and set the valve so that on the low swing it never drops below 2k and accept that the high swing is just that, a high swing and the normal opertaion of the valve. While thats happening the suction pressure at evap outlet will also be rising and dropping a few PSI too, just to confussss things further

Or.................Have I never had the patience to breath and sit and measure and not adjust a valve and that there are systems out there that maintain a continuous accurate 2k, 4k and 10k superheat.

And to go full circle....evap efficiancy oor compressor protection? Hope I've explained myself????

I hear you goob, I have been commissioning supermarkets for 8 or 9 years now so you can imagine the number of valves I've set, the only time they sit at a stable superheat is when they are starving and are matching the exact evap load (that is valve is open as far as it can go with undersized orifice) which is a far from ideal scenario, they will always swing a little. If you see 10K swing then you might want to look at tightening it up a bit.

Some manufacturers valves are better than others too, I find sporlan SBF and SQ valves to have a larger swing than say a Danfoss TUA or even TE valve, Alco's I found to be pretty good. You also need to be mindful of changes in liquid pressure. One thing I used to find useful was to listen to when the valve actually shut and open, it may take listening to a screwdriver held on the body, this way you could lidten to when the valve reacted and determine the over and undershoot really easily. The over and undershoot can also be a factor evap design (distributor, copper tubing - standard or rifle bore) just to make it a little harder.

[Goober]

Cheers 750, good to have some agreement, it is definately not an exact science and does require patience. Something I'm not known for! Now how do I reward you a rep point??

[750 Valve]

simply click on the scales above the date in the top right hand corner of my post

I once had an older bloke from an AC company (who thought his sh!te didn't stink) tell me every valve he has ever set in his life was bang on 6K, no swing. I laughed and offered him a crack at a Dairy case in the store we were standing in, he politely declined and left rather quickly :D

[Goober]

Already rep'd ya.

[750 Valve]

you da man!!! :D

[Goober]

No! no! You da Man!

[oroma]

I have a set of rules for the evaporator as follows:
4F degrees for low temp operation
6-8F medium temp
10-12 for Air Conditioning.
20F at the compressor.

[frosty74]

cheers for the advice guys very much apreciated

[Gary]

When the refrigerated space is at design temperature, then...


4F degrees for low temp operation
6-8F medium temp
10-12 for Air Conditioning.
20F at the compressor.

When the refrigerated space temp is higher than design temp, the superheat should be higher.

When the refrigerated space temp is lower than design temp, the superheat should be lower.