A couple of questions about target superheat

[Rtic]

Yes, this is another thread asking about superheat! However I assure you I have worn away that search button to try and get answers & I'm almost there!

My main source of business is in commercial refrigeration, such as kitchen refrigerators, bottle coolers, cold rooms, beer cellars and the occasional A/C systems. Over the years I try my very best to use available weights to weigh in the refrigeration, but with beer cellar systems, cold rooms & A/C system installs I need a reliable way to charge the systems. Up-to now I've been doing it by sight glass & touch, but I understand that's not the best way of doing it, so I've been looking into superheat & sub-cooling. Besides I understand It's a useful indicator when diagnosing faults in the refrigeration process.

Using most of what I found on here I'm now at a point where I'm confident how to measure actual superheat on a system. However I'm now struggling to understand how to calculate target superheat.

I was taught that superheat is the where the refrigerant has completely boiled off, occurring at the end of the evaporator, and is required so that no liquid is entering the compressor. So in my mind if you can achieve a couple of degrees above the boiling point of the refrigerant then you have adequate superheat. For example if I have an r134 system, and messure the super heat (temp measured on suction - saturation temp from the gauges) anything above -26c (Boiling Point of r134a) would be adequate.

So isn't any temperature above the boiling point of the refrigerant adequate super heating, and thus the target superheat?

I'm guessing I'm totally wrong in regards to target super heat, because on the few guides I have found wants superheat to be between 5C - 10C, which is far above the boiling point of any refrigerants, and these guides don't mention any variance in refrigerant types, so I guess I'm missing something! However I'm here to learn and I hope you guys can point me in the right direction!

Thank you kindly

Jim

[NH3LVR]

It only works that way in theory.
The reality is that the expansion valves are not perfect. They will hunt a bit as the load changes. If you have a "perfect" superheat and then someone causes a sudden load you will flood back.
Also the instruments we use to measure superheat are not perfect. If you were to set superheat at 2 degrees F you could have that much error in your measurement devices.
Remember that you can measure only positive superheat. There is no difference in readings between zero superheat and flood-back.

[mad fridgie]

Not as easy to answer as you would expect.
If your vapour temp is just above saturation, you can get liquid droplets in the suction, that do not have time vapourize.
You also have to condsider is it the superheat at the evap outlet or the super heat at the compressor inlet.
Which depending of whaas in the suction line could me a pressure drop.
So even if the actual temperature did not change, your superheat would, because your pressure at the compressor would be lower than that at the exit of the evap.
many compressor manufactures have a minimum superheat entering temperature, A as safety factor, B, to ensure refrigerant does not dilute the lubricant. C the liquid is not formed in the compression process.
practically for cap systems use manufactures charge "the SHOULD have done all the work for you"
For TEV systems without to much complicated gear on it, the shorter the pipe run the higher the evap superheat, longer pipe run, reduce superheat slightly.
To much superheat cause very high discharge temps.
Short answer

[Gary]

Static gas laws have little to do with dynamic systems.

There is a vapor/liquid mixture churning through the evaporator at high velocity, the vapor portion increasing and the liquid portion decreasing, with the vapor portion becoming superheated. There is no clear line where all of the liquid suddenly becomes vapor.

By the end of the evaporator, it is entirely possible to have liquid droplets surrounded by superheated vapor.

All of the liquid droplets disappear by the time you have 10F/5.5K superheat.

[desA]

What damage could be expected from a few liquid droplets (not flood-back) exiting evaporator?

In other words, would say 5.5K be the 'safe' point to aim for, in terms of minimising potential liquid damage to a compressor?

[Gary]

What damage could be expected from a few liquid droplets (not flood-back) exiting evaporator?

In other words, would say 5.5K be the 'safe' point to aim for, in terms of minimising potential liquid damage to a compressor?

Anything that reduces airflow (dirty filters, coil, fan blades, etc.) can drop the superheat, endangering the compressor, so keeping the compressor inlet superheat at 10F/5.5K leaves no wiggle room.

Most compressor manufacturers want a minimum of 15F/8.5K superheat at the compressor inlet. Some specify a minimum of 20F/11K.

Most compressors can take a little liquid without damage. Some more than others. Rotary compressors come with an accumulator attached, because they have zero tolerance for liquid.

[desA]

Thanks, Gary. Good points.

[Rtic]

Ok I think I get all that!

So is there a way of calculating target superheat for a system? If I stick to between 5C & 10C as a target superheat as it says in my Refrigeration Manual, is this an accepted value?

Jim

[NoNickName]

For example if I have an r134 system, and messure the super heat (temp measured on suction - saturation temp from the gauges) anything above -26c (Boiling Point of r134a) would be adequate.

There is no such a thing as a boiling point of a refrigerant, unless the corresponding pressure at which boiling happens is declared.
-26°C is the boiling point of R134a at 0 barg, 1 bara.
Given that, the rest of the sentence is wrong, because plenty of systems with R134a have a boiling point above -26°C and for which the SH is insufficient or excessive.

[Rtic]

There is no such a thing as a boiling point of a refrigerant, unless the corresponding pressure at which boiling happens is declared.
-26°C is the boiling point of R134a at 0 barg, 1 bara.
Given that, the rest of the sentence is wrong, because plenty of systems with R134a have a boiling point above -26°C and for which the SH is insufficient or excessive.

Thanks, I've never been taught that before. So the change in pressure changes the boiling point of the refrigerant.

However I'm still no closer to finding out how to work out the target superheat! Any help?

Is there a rule of thumb, or a method to calculate target superheat?

Jim

[taz24]

Ok I think I get all that!

So is there a way of calculating target superheat for a system? If I stick to between 5C & 10C as a target superheat as it says in my Refrigeration Manual, is this an accepted value?

Jim

Target superheat is a manufacturing aim and not one that
you realy need to worry about.
Most TEV (TXV for our friends over the pond) manufactures
work to about 6 or 8 degs of real superheat added to the refrigerant
at the outlet of the evap. But actual superheat down the suction line
will increase towards the comp.

Valve manufacturers set the superheat to the levels they
do purly as a safety thing.

If the superheat setting was too low and a comp was flooded with liquid
people would sue the valve manufacturer for the cost of a new comp..

All the best

taz

.

[NoNickName]

There is no such a thing like target superheat. It very much depends on the type of unit and usage.
In a/c 2 to 7K is quite common, while in refrigeration it's not uncommon to stumble upon units with 15 to 25K suction superheat.
Right or wrong we don't know. Only the designer knows.

[coolhibby1875]

when working with electronic controllers the parameter is normally called TARGET SUPERHEAT this is usualy always set for 5k sometimes 6k, so when the superheat goes higher (positive) than the TARGET setpoint the valve opens more thus feeding in more liquid to try and get the superheat back to the TARGET temperature, when the superheat goes below the TARGET setpoint (negitive) the valve will close.
please fellow users save me the lessons about using the word negitive when refering to superheats please

[NoNickName]

Ok. I see. The fact that some supergeek firmware designer decided to change the name of SETPOINT to TARGET, doesn't necessarily imply that a TARGET SUPERHEAT exists.
If you own an a/c and set it a 26°C, that's your target temperature. But in general a target temperature does not exist. It depends on what you prefer for your room.

[coolhibby1875]

nothing like a good old superheat post to get the pulses racing on some folk

[jpsmith1cm]

Thanks, I've never been taught that before. So the change in pressure changes the boiling point of the refrigerant.

However I'm still no closer to finding out how to work out the target superheat! Any help?

Is there a rule of thumb, or a method to calculate target superheat?

Jim

If you don't understand that a refrigerant's boiling point changes as the pressure changes, they you really don't have much business attempting to measure superheat, let alone, determining what a target superheat for a system is.


Maybe I'm misinterpreting your posts, but you sound a bit clueless and in need of some good, solid education before you start messing with refrigerants.


In answer to you question, I have seen a 'best-fit' type formula that I use for a very specific application (piston metered AC units).

With the different types of metering devices, and vast array of different designs used in refrigeration and AC work, you NEED to consult the manufacturer's specifications before messing around with superheat.

Too high and the system will not perform correctly, too low and you can pop a compressor.

[bill1983]

Ok but what is too high and what is too low????? Surely the idea behind a controlled or target superheat value is to ensure that the liquid refrigerant has boiled off at the exit of the evaporator, creating the highest efficiency. There is no point of liquid continuing to boil out of the refrigerated space. The 5.5k measurement is to ensure that no liquid enters the suction line outside the space, with the potential to damage the compressor and wasting energy in the process. Any value over that quoted by the compressor manufacturer is to ensure the complete impossibility of liquid being present at the compressor inlet. i think that maximum superheat figures quoted by the manufacturer are to ensure at least some cooling from the returning refrigerant to prevent high discharge temperatures.

[NoNickName]

Yes, bill. You've got the point. That's why SH shall not be lower than ... or greater than ... [put your numbers there]

[Scramjetman]

Back in the good old days the Carrier 30GB series chillers used to run a ... dare I say it- TARGET - superheat of about 2.5 degC. The only reason they could get away with it is because they used Electronic Expansion Valves and they wrote a whole stack of software that detected if the superheat was falling below this point ( due to a change in load or similar) and would wind the valves shut to get the superheat temperature back up to the setpoint. TEX's can't manage this without the danger of liquid slugging because of their mechanical nature so the setpoint needs to be higher to compensate for any over-run. But TEX's are cheaper.

The object of the exercise with TEX's or EXV's metering to the evaporator is to keep the evaporator as full of liquid as possible, in order to maximise contact between tube surface and refrigerant without liquid exiting the outlet. It's vapour only, thanks Gentlemen or the compressor goes to heaven.

I have a 800kw Screw chiller at one of our sites that has EXV's which run at 2.8 deg C superheat. They run fully open to fully shut in 5 seconds. The energy savings that come from doing this are substantial. Mind you, there is are pages of software in there approaching the size of an abridged version of War & Peace simply to ensure that the valves don't overshoot.

Low superheats are possible and do create considerable running cost savings but you have to be properly set up for it

It can be a bugger though, if you get great running cost savings then the machine throws a leg out of the bed because you went down too close to the wire. It's a fine line we tread.

[jakabus]

I figure that an anthalpy chart would come in useful

[Magoo]

www.kueba.de will explain all, go to the section about superheats of evaporators