Superheat

[bill1983]

i know you're not firing the question at me and i'm not saying i know it all. Nobody does, we are all here to learn. but the way i usually start if the fault is not immediately obvious, is to listen to the sound the system makes. Next i look at the condenser airflow and the evaporator, because without the correct airflow you cannot achieve the correct transfer. after that it is a question of taking temp/pressure readings for superheat/sub cooling to determine what the system is doing and as an indicator of where i shoud go next to diagnose the fault.

[bill1983]

or i could just check the fault code and follow the flow chart

[Peter_1]

Bill1983, I found your post remarkable, I enjoined reading it and I will translate it to Ducth and will show it to my students in class.

[nike123]

Bill1983,
You are right, I was quoting coolhibby1875. Your thoughts are similar to mine.

Cheers

[bill1983]

Bill1983, I found your post remarkable, I enjoined reading it and I will translate it to Ducth and will show it to my students in class.

does this mean i'm getting something right after all these years, or are you going to use my post as an example of how to ramble on and on and on
as my daughter would say lol

[chillyblue]

Hi all

i have one for you!!!
how can you have subcooling unless your reciever is full and the liquid is then backing up into the condensor??? Also if this is the case that the liquid is backed up into the condensor, what happens when the system pumps down ( no space left other than in the condenser), i believe it would trip on HP!!!
the way everybody explains subcooling is that the liquid is further cooled below it's condensation temperature, (correct) the only method i have seen of proper subcooling is by cooling the liquid exiting the reciever not before, by means of further air cooling, water cooling or refrigerant cooling. why would the liquid sit in the condensor to get further cooling (subcooling)?????

CB

[Gary]

Hi all

i have one for you!!!
how can you have subcooling unless your reciever is full and the liquid is then backing up into the condensor??? Also if this is the case that the liquid is backed up into the condensor, what happens when the system pumps down ( no space left other than in the condenser), i believe it would trip on HP!!!
the way everybody explains subcooling is that the liquid is further cooled below it's condensation temperature, (correct) the only method i have seen of proper subcooling is by cooling the liquid exiting the reciever not before, by means of further air cooling, water cooling or refrigerant cooling. why would the liquid sit in the condensor to get further cooling (subcooling)?????

CB

The static gas laws state that you can't have superheat in the presence of liquid and you can't have subcooling in the presence of vapor.

Are the static gas laws wrong? No. The key word here is "static", meaning non-energized. As soon as you hit the start button, you are working on a dynamic (energized) system and the static gas laws no longer apply.

For example, take a jug of refrigerant, place it on a block of ice and put a heating pad on top. You now have subcooled liquid on the bottom, superheated vapor on the top and saturation at the vapor/liquid interface... all in the presence of each other.

The receiver doesn't have to be full of liquid to have subcooling, so long as there is flow through the receiver.

Whatsmore, you can and will have vapor bubbles in a subcooled liquid line and liquid droplets in a superheated suction line. Depending upon velocity, the vapor bubbles in the liquid line disappear at about 10-15F subcooling and the liquid droplets in the suction line disappear at about 5-10F superheat.

[mohamed khamis]

The subcooling effect is mainly occured in the condenser if it is designed to have a portion of area to subcool the refrigerant. The liquid receiver is acted like a collector for the subcooled refrigerant and ishould be designed with a volume of 125% of refrigernat capacity to avoid the flashbacking of the liquid to the condenser.

The subcooling degree depends on the condenser design and the varitions in the operating conditions for the system regardless the recevier is full or not. And sometimes the refrigerant leaves the condenser in subcooling form but it is flashed into the recevier due to excessive pressure drop in the liquid line. That means the refrigerant in the receiver consistes of liquid+gas although it leaved the condenser in subcooling form.

Cheers

[chillyblue]

Hi Gary

correct me if i am wrong, you are saying if the system is in operation or the liquid is moving then it can be subcooled, if the liquid is not moving it can't be subcooled????
Superheat is easy as you can normally come across large superheats, but with subcooling you only normally can measure a few deg C.
I in all my years as a fridge engineer have rarely experienced subcooling, perhaps i'm doing something wrong???
I have never had the pleasure of large deg of subcooling as it always seems to be limitted to high discharge pressures.

CB

[crit71]

Correct me if i'm wrong but does that mean for me to measure superheat at the evap i have 1 probe on the expansion side of the TEV and 1 probe at the coil exit in to the suction line, and is this with the evap fans on.

always eager to learn from good enginners.

regards

Wizzer

Yes Wizzer...you are correct. Forget about all the science behind it for now, aim for 6 degrees SH and your laughing.

[Gary]

Hi Gary

correct me if i am wrong, you are saying if the system is in operation or the liquid is moving then it can be subcooled, if the liquid is not moving it can't be subcooled????
Superheat is easy as you can normally come across large superheats, but with subcooling you only normally can measure a few deg C.
I in all my years as a fridge engineer have rarely experienced subcooling, perhaps i'm doing something wrong???
I have never had the pleasure of large deg of subcooling as it always seems to be limitted to high discharge pressures.

CB

Convert high side pressure to temperature. Measure liquid line temperature. The difference between the two temperatures is subcooling.

[Gary]

There always seems to be a lot of confusion about subcooling and superheat.

First to clarify:

Boiling point = boiling temperature = evaporating temperature = condensing temperature = saturation temperature. These are all different ways to say the same thing. These all refer to the temperature at which a change of state occurs, whether liquid to vapor or vapor to liquid. Let's just call it saturation temperature.

Every fluid has a different saturation temperature.

Saturation temperature (for any fluid) rises with a rise in pressure.

Pressure/Temperature charts tell you the saturation temperature for the current pressure of the fluid you are working with.

Any liquid that is below its saturation temperature is subcooled.

Any vapor which is above its saturation temperature is superheated.

[panas]

Hi!
I started in the refrigeration field on my 40 some months before. I try to work properly in spite I have no enough learning-books. I was glad to find this site and was interested this discussion.
I work at domestic fridgers which have the only capillary tube (without receiver and TXV). Could somebody explain if the described upper ( by Gary) theory of using sub cooling\super heating for diagnostic of frigers is the same for capillary tube systems? Any tips for domestic systems in this term - what is there difference for? Thank you and sorry my English.

[mohamed khamis]

Hi Chillyblue

Superheat is easy as you can normally come across large superheats, but with subcooling you only normally can measure a few deg C.

Both the superheat and subcooling has a limitations but the superheat degree is much bigger than that for subcooling, the reasons are:

1- the main target of the superheat is to fully protect the compressor from liquid floodback not to the limit to cause the decrease in compressor refrigernat capacity.

2- The more subcooling results in an enhancement in TXV capacity as a result of the increase in entering liquid density. However, the signifacnce of liquid density is rather small with decreasing in liquid temperture (more subcooling). Becus the nature of liquid density is quiet independent of its temperture not like gases. Alternatively, the more subcooling poses an enlargement into refrigerating effect. Accordingly, the refrigernat velocity inside the evaporator is relatively small results in deficiency in oil back to the compressor. So two competiting effects at a work.

The net outcome is the more subcooling leads to a slight enhancement in TXV capacity with damping in refrigerant velocity inside the evaporator.

3- the specific heat for the vapor is half the value of that for the liquid and this renders the amount of increasing in superheat degree is almost double the amount of subcooling degree for the same heat capacity portion in evaporator and condenser.

I in all my years as a fridge engineer have rarely experienced subcooling, perhaps i'm doing something wrong???
I have never had the pleasure of large deg of subcooling as it always seems to be limitted to high discharge pressures.

CB

The increase in high pressure sometimes comes from incorrect surface area for the condenser or air volume rate for the fans. So this results in boosting in HP and so less amount of subcoling due to the desuperheating takes place a large portion after the condensation with increasing in HP. I wish it could help

cheers

Mohamed

[chillyblue]

Hi

i am going to start a new thread as there are still some issues i have with the subject of subcooling and condenser draining.

thanks for the input
see you on the new thread

CB

[Peter_1]

2- The more subcooling results in an enhancement in TXV capacity as a result of the increase in entering liquid density. However, the signifacnce of liquid density is rather small with decreasing in liquid temperture (more subcooling). Becus the nature of liquid density is quiet independent of its temperture not like gases. Alternatively, the more subcooling poses an enlargement into refrigerating effect. Accordingly, the refrigernat velocity inside the evaporator is relatively small results in deficiency in oil back to the compressor. So two competiting effects at a work.

The biggest advantage is not the increase in density but the increase in available enthalpy of the liquid in the refrigerant which will increase the evaporator capacity.
There will also less flash gas formed direct after the TEV

[mohamed khamis]

The biggest advantage is not the increase in density but the increase in available enthalpy of the liquid in the refrigerant which will increase the evaporator capacity.
There will also less flash gas formed direct after the TEV

The TXV capacity that I meant is the amount of refrigerant mass flow rate not the cooling capacity. The mass flow rate that is metered by TXV functions in the pressure drop across the TXV and refrigerant inlet density. Hence the more subcooling the more amount of flow rate metered by the TXV as a result of the increase in density and free of flashing effect as u mentioned.

If u have a unit with amaximum cooling capaicty of 20 kW at a certain conditions and u implement a mechanical subcooler to improve the subcooling effect, the question may be raised to what extent u can do subcooling if u have facilty to introduce subcooler coolant with different tempertures to the subcooler?

As it is known the more subcooling the more ample in the refrigeranting effect (outlet evaporator enthalpy - inlet evapotaor enthapy) as a result of the decrease (not increase as u mentioned) in liquid refrigerant entering evaporator. if we plot cooling capcity vs the subcooling degree at a specified conditions, i imagine that the trend will go up with increaseing in subcooling degree. Then at a certain value of the subcooling degree the cooling capcity will almost be fixed at a certain value.

It may be explained by that at the begining the more subcooling the more mass flow rate metered by the TXV plus the more ample in refrigerting effect. This accounts for an enhancement in cooling capacity upon some point. Then after that the more subcooling will result in a decrease in evaporator mass flow of refrigernat to offest the big enlargemnt in refrigerating effect to give maximum cooling capacity should it give by the unit. This a drop in mass flow can lead to decelarate in oil velocity and makes the oil to cling on the tube inner surface. So it is certainly a limit for the subcooling plus the enlargemnt in condenser area if the subcooling mainly occurs there.

Cheers

[Peter_1]

As it is known the more subcooling the more ample in the refrigeranting effect (outlet evaporator enthalpy - inlet evapotaor enthapy) as a result of the decrease (not increase as u mentioned) in liquid refrigerant entering evaporator.

This isn't what I said, I was saying that the liquid entering the TEV will have a bigger enthalpy. and this is correct.
If you have a high subcooling, then you have to take this in account because the TEV will be too big for a given nominal capacity

[Gary]

To avoid confusion, it should be stipulated that Peter_1 and mohamed khamis are discussing the benefits of adding subcooling after the receiver outlet by mechanical means, such as a suction/liquid heat exchanger.

The subcooling at the receiver outlet should not exceed 15F/8.5C, because this backs up liquid into the condenser.

[Peter_1]

To avoid confusion, it should be stipulated that Peter_1 and mohamed khamis are discussing the benefits of adding subcooling after the receiver outlet by mechanical means, such as a suction/liquid heat exchanger.

The subcooling at the receiver outlet should not exceed 15F/8.5C, because this backs up liquid into the condenser.

Gary, the last phrase needs some clarification if you want please.

[Gary]

The subcooling at the receiver outlet should not exceed 15F/8.5C, because this backs up liquid into the condenser.

To clarify:

When you are charging a system, the bubbles in the sight glass disappear between 10-15F/5.5-8.5C subcooling. Continuing to add refrigerant at this point backs liquid up into the condenser, unnecessarily raising the head pressure.

[mohamed khamis]

This isn't what I said, I was saying that the liquid entering the TEV will have a bigger enthalpy. and this is correct.

ok simply to solve the problem the example is a good evidence

If the condensation temperture is of 50C and u have subcooling of 5K and 10K of R134a the liquid enthalpy entering or exiting the TEV in both cases are

at T = 50C ====sat.table==== P = 13.17 bar

case1 P = 13.17 bar & T = 45C (5K subcooling) ====== enthalpy = 112.7 kJ/kg

case2 P = 13.17 bar & T = 40C (10K subcooling) ====== enthalpy = 105.4 kJ/kg

So as it may be concluded that the more subcooling the liquid entering the TEV will have a smaller enthalpy

If you have a high subcooling, then you have to take this in account because the TEV will be too big for a given nominal capacity

Of course, the more subcooling the more TEV capacity over the nominal cooling capacity but this comes from the increase in refrigernat flow passing thru TEV not the difference in enthalpy.

becus the designer of the evaporator will take in his account the difference in enthaply due to the subcooling and calculate the refrigerant mass flow rate from dividing the nominal cooling capacity over the difference in enthalpy. However, the mass flow rate metered by TEV is bigger than calculated by evaporator design. This may result in the comprossor to be floodback in case of excessive subcooling becus the TEV meters signifcantly higher mass flow than it is required by the Evaporator.


Gary, could u give me a reason to why the refrigernat liquid backs to the condenser when there is excessive subcooling as u mentioned over 8.5C

Becus the clarification that u mentioned this comes from the system is overchargging problem not excessive subcooling. And the head pressure is ampled due to the overchargging effect not excessive subcooling.

Could u tell me please on a basis of the abovementioned example of two different subcooling of 5K and 10 K what is happened to head pressure and liquid backing to the condenser

Cheers

[Gary]

Gary, could u give me a reason to why the refrigernat liquid backs to the condenser when there is excessive subcooling as u mentioned over 8.5C

I am talking about subcooling at the receiver outlet, not subcooling at the TXV inlet.

Becus the clarification that u mentioned this comes from the system is overchargging problem not excessive subcooling. And the head pressure is ampled due to the overchargging effect not excessive subcooling.

Yes, I am talking about overcharging, which is evidenced by excessive subcooling at the receiver outlet. Overcharging raises the condensing temperature and therefore the receiver outlet subcooling.

[Saturated]

Keep in mind that high subcooling can also be caused by liquid binding, that is, restriction in the liquid line (which can include TEV wound in too far and capillary line blockage). This is usually found when the TD is over 8K and when the TD is taken from the outlet of the condensor.

[Gary]

Keep in mind that high subcooling can also be caused by liquid binding, that is, restriction in the liquid line (which can include TEV wound in too far and capillary line blockage). This is usually found when the TD is over 8K and when the TD is taken from the outlet of the condensor.

The condenser TD is the difference between entering air temp and refrigerant saturation temp and generally runs much higher than 8K... or did you mean something else?

I have never seen a liquid line restriction cause high receiver outlet subcooling, although I would not say that it is impossible... perhaps with an undersized receiver?

Excessive receiver outlet subcooling is almost always caused by overcharge or noncondensables, with overcharge being by far the most common cause.

[Saturated]

Sorry Gary, I should have specified. Most of my work is on A/C's and don't have recievers, and I use quite a few different temperature difference calculations to diagnose and refer to them as TD's. The TD I was referring to was condensor subcooling (SCT- liquid leaving Condensor temp), though I don't see why it would be too much different for a system with a reciever, (perhaps the reciever absorbs the extra refrigerant without causing a bank up at the condensor like a hydraulic accumulator).

In high ambient areas with high discharge temps it's common to find partial to full blockage of capillary lines, high superheat and high subcooling are the results. I've also heard from a guy working on a supermarket system who picked up a partial blockage in a liquid line buried under concrete from a poor TEV feed and high subcooling. He found the blockage was a poorly welded 90 deg bend that had folded over with the flow over time.

Definately agree with you on the most common cause though, I use both superheat and subcooling TD's when charging an older critical charge system where no refrigerant weight can be found.

[Gary]

Sorry Gary, I should have specified. Most of my work is on A/C's and don't have recievers, and I use quite a few different temperature difference calculations to diagnose and refer to them as TD's.

Perhaps some definitions would help to improve communications:

A change in temperature of a single substance or flow of substance is called a delta-T or dT.

A comparison of the temperatures of two different substances or flows of substances is called a TD.

The difference between the condenser entering air and leaving air is the condenser delta-T as it quantifies the change in temperature of a single flow of substance.

Similarly, the air in/air out difference across the evaporator is the evaporator delta-T.

The condenser TD is the difference between entering air temperature and saturated condensing temperature (SCT). This is a TD in that it quantifies the temperature difference between two different substances.

Similarly, the evaporator TD is the difference between entering air temperature and saturated suction temperature (SST).

Subcooling is a specific type of delta-T, namely the difference between SCT and liquid line temperature.

Similarly, superheat is a specific type of delta-T, namely the difference between SST and suction line temperature.

Condenser approach is a specific type of TD, namely the difference between leaving air temperature and liquid line temperature.

Similarly, evaporator approach is a specific type of TD, namely the difference between the leaving air temperature and the suction line temperature.

Hope this helps.

[Saturated]

We do seem to have slightly different terminology, at least the equipment stays the same I have the same problem discussing fishing!

What method do you use to take a pipe temperature? When I'm getting serious about setting TEV's I'll attach a thermocouple to the pipe with a nylon zip tie and place insulation around it covering about 200mm along the pipe to avoid air temperature affecting the pipe and the TD. Do you know a more effective and fast method of getting an accurate refrigerant temperature through a pipe? I'm not real keen on the lasers or touch probes as the pipework picks up at least a couple of degrees from the air temp around it.

[mohamed khamis]

Hi there,

There is misunderstanding... the subcooling degree is the difference between two tempertures almost have the same pressure (slightly pressure drop).

However, the restrication in liquid pipeline or blockage the outlet of them is not additional subcooling although its temperture is very low compared with the outlet from the condenser. This is due to the restrication acts like TEV it throttles the flow and hence drops the pressure and temperture signifcantly. This throttling process lies approximetly on a constant enthalpy typically like TEV process. And so the outlet flow from restrication can go to saturation condition with lower saturation temperture and pressure than that for condenser. And causes an improper operation of TEV.

Cheers

[mohamed khamis]

The main reason of increasing the subcooling degree in case of overcharge or noncondensable gases is the increase in the head pressure. The more increase in head pressure the more decrease in the condesation latent heat so the other processes (desuperheating and subcooling) take place the part of condenser surface area which has been left by the decrease in latent heat surface area. So the mount of desuperheat and subcooling is increased and noticed by additional subcooling outlet the liquid receiver.

Howver, this is not favourable method to enhance the system subcooling becus what u gain by right hand is lost by left one. Definitely the increase in head pressure burdens on the compressor power and its charcteristices negating the improvement occuring in refrigerting effect.

Cheers

Mohamed

[Springbok]

Hey guys,it seems that every engineer has their own opinion on the superheat theory.Guess it depends if you're in the domestic, commercial or industrial side of refrigeration too.I did my trade test back in South Africa and it is regarded as one of the toughest tests around.(old school)Probably one of the most important tasks/tests of the entire trade test was based on superheat.Not checking your superheat on your evaporator is like not checking the oil level of thecompressor or the liquid level of the system.All components of the system needs to work and maintained to work properly.....Anyway...just a quick word....

[coolblade]

Can someone give me a good guide on how to measure suereat and and subcooling when commissioning?

Hello.
Superheat, this is equal to the actual temperature of the gas minus the saturated vapour temperature of the gas.
First you need to know the pressure at the point you want to measure the superheat.
Once you have got this pressure you need to find the saturated vapour temperature of the refrigerant at the pressure measured. This can be obtained off guages or a comparitor
Next you need to measure the temperature of the vapour. This is not always easy to obtain accurately as discussed by others. I personally feel the best way is to use a thermocouple and tape it directly onto the pipe or vessel, then cover it with insulation and tape over the top. The thermocouple should be left in position for at leat 20 minutes prior to taking any reading. Once you have an accurate temperature reading superheat = temperature measured minus saturated temperature

[collo]

Message to Gary, My very real thanks Gary for your measured and knowlegible reply's. Being self and experience taught in much of my refrigeration career, I have been so very careful to know and hopefully understand the problems before sticking my fingers into the works,so to speak. Your answers have been a real help in confirming my own understanding the refrigeration process. I was amazed this past week when helping out a fridgie who had just installed a new air-con. He did not have a clue about superheat and was only interested in "what sort of pressures should I have"? My area was experiencing ambients of 22-24c. while he was experiencing 28-30c. My suggestion was, pressures will alter according to temperature, in a capilliary system he would only have SH or SC to check his gas charge against. What do you say?

[Gary]

Contrary to what most people believe, subcooling and superheat are important indicators in both cap tube and TXV systems.

[lexscripta]

Very interesting conversation on a topic I am having a real problem with.

Q. Can I take an ambient temp around the condenser and then measure the air leaving the condenser for a useful measurement?

Q. What does the discharge temp (at the discharge port after PT conversion) tell me in relation to the ambient temperature?

Q. Someone asked about the temp drop across an evap coil - that is simply taking temp readings at the inlet and oulet - no pressure readings. What would I expect to see in say, a freezer?

Q. I assume subcooling to be the difference of PT chart pressure to temperature at the discharge port of the compressor and sensible heat measurement at the outlet pipe of the condenser (Liquid line).

Q.) Would a suction line accumulator figure into any of these measurements?

I am presently having a problem with a 3hp Copeland lala 031a tac, being converted to oneshot (ICOR product)

The ambient room temp fluctuates 90 -105. 6 compressors in a room with an exhaust.

I am running 260-280 PSI in this environment, and 14psi suction. Superheat is rediculous - over 30, subcooling is low - appears to be 2-4 degrees, as measured at the receiver inlet.

I know, those readings sound like classic low refrigerant charge, but with that discharge pressure, makes me leery to put more refrigerant in there. Seems like I have to fudge controls in that hot room so things don't kick off on HP.

Thats the scoop on how things were set up, and I know that the first thing they ought to be doing is getting that ambient down - which in the summer gets even higher, but the owner has had guys out there working on this stuff for years, and their attitude is that if its cooling, it must be working.

What can I do with that attitude?

Anyhow - suggestions? Answers?

LexScripta

[Gary]

Condenser air out minus air in (cond dT) is an indicator of airflow problems. As the airflow decreases, the dT rises.

Condenser pressure converted to temperature on a P/T chart is called saturated condensing temperature or SCT. As a rough rule of thumb, you might expect the SCT to be about 20-35F above ambient (20-35F condenser TD).

Similar to condenser dT, evap dT (air in minus air out) is an indicator of airflow problems. As the airflow decreases, the dT rises. In a freezer, you might expect the evap dT to be less than 20F.

Subcooling is the difference between SCT and liquid line temp at the receiver outlet. If there is no receiver, then at the condenser outlet.

An accumulator catches any refrigerant liquid overflow from the evaporator, so that liquid does not enter the compressor.

Higher ambient = higher SCT = higher operating costs and shorter lifespan.

[Apel]

Hi all

I see a simple question about superheat, when I start reading it to understand better, I found it extremely informative -truely it is not about only the question asked but many others answers also.Now I feel better thinking HVAC engineer without having too much experience.

Thanks

[majo]

95% of every single refr. engineer are not even interested on how all this stuff works precisely and if you talk to them they all have a different explanation and what you are saying is bulls***!

I've worked for 4 yrs in a company.
The oldest technician over there (15yrs) had built a chamber with a bitzer hexagon who was runnig 3/4 full of ice at -20° chamber temp.
I've told him: isn't SH a bit low?
I've got the answer: silly boy, this is very normal at -20°. Comp is getting suction temps of -30° and off course at -30° you got ice.
Chamber didn't cool down as fast as it was supposed to be and didn't reach setpoint of -30° as supposed.
After looking and looking he then added 2 probes to the and started logging SH as I showed him together with chamber's cooling rate.
Adjusted SH a bit higher and whoops... Ice away just until service valve and cooling speed of the chamber whopping fast down... comp was getting liquid.
And suddenly I wasn't as silly as before...

That was when aproxx...8months at labor after school

[nike123]

whopping fast down... comp was getting liquid.

Does compressor getting or doesn't getting liquid? This is question of survival!

[handy andy]

just to say this is my frist time on this forum and suberheat and subcool is needed to get the best efficiancy from any refrigeration system in my view and as we engineer are ment to save companys running cost then we need to do this more often. the one thing that all way got me is enthalpy charts

[El Padre]

Ciao,

Actual suction temperature (measured at compressor inlet) - Saturated suction temperature(read from gauges) = Superheat (system superheat). Saturated condensing temperature(read from gauges) - liquid line temperature = Subcooling. Super and Sub derive from the Latin for above and below.
I wish that somebody would have told me a few years ago not to be preoccupied with pressures and just look at temperatures!!

Cheers

[Gary]

I've been preaching this stuff since the early 1980's, and yet...

The industry still has this obsession with pressures, and the schools just don't get it.

Sometimes I wonder if anyone is listening.

[El Padre]

I've been preaching this stuff since the early 1980's, and yet...

The industry still has this obsession with pressures, and the schools just don't get it.

Sometimes I wonder if anyone is listening.

I have been mate, I have overlooked water flow rates through chillers especially in the past, your comments on air/water flows have fine tuned my thinking!!

Cheers

[cavalieri85]

well u charge a piston by superheat and a txy by subcooling its very simple to do your gauges should have your saturation temperature that corresponds to the pressure with subcooling your liquid line temp minus your ligid saturation temp gives you subcooling and suction line temp minus suction line saturation temp is your superheat this is very important stuff to know you will not get to far in this trade without mastering this skill to a t for example when you have low suction there are alot of possible problems but low suction psig high superheat and good subcool its almost definitely a bad txv i hope this can help i am a terrible teacher and can show youbut cant explain it very well

[chillyblue]

Reguarding evaporator superheat.

I normally set my superheat exiting the evap at about 10degC i.e. gauge reading -10deg c, evap leaving temp 0 degc. thought this was fine have been doing this for years, the other day i thought if i have a evap td of 8deg, i would need to set my superheat lower than 8 deg c to know that i am fully utilizing the evap. If the evap td is less than 10 deg your superheat must be as well.
is my findings correct????

CB

[Gary]

Reguarding evaporator superheat.

I normally set my superheat exiting the evap at about 10degC i.e. gauge reading -10deg c, evap leaving temp 0 degc. thought this was fine have been doing this for years, the other day i thought if i have a evap td of 8deg, i would need to set my superheat lower than 8 deg c to know that i am fully utilizing the evap. If the evap td is less than 10 deg your superheat must be as well.
is my findings correct????

CB

Let's say your refrigerated space temp is -2C and your SST is -10C. That's 8K TD.

Can you have 10K superheat? No... because the suction line is in a space that is -2C.

There is no way for the suction line temp to rise above -2C (the refrigerated space temp) to 0C, which is what it would have to get to for a superheat of 10K.

And you cannot accurately set the superheat to 8K, either. The superheat must be less than the TD.

[chillyblue]

Let's say your refrigerated space temp is -2C and your SST is -10C. That's 8K TD.

Can you have 10K superheat? No... because the suction line is in a space that is -2C.

There is no way for the suction line temp to rise above -2C (the refrigerated space temp) to 0C, which is what it would have to get to for a superheat of 10K.

And you cannot accurately set the superheat to 8K, either. The superheat must be less than the TD.

Thanks thats as i thought, so if you have a td of 6, your superheat must be less than 6. That a low superheat?? I know in theory there should not be any liquid there but it dosen't leave much room for error.
2ndly why do all TEV manufacturers not mention superheat when setting the valves but only mention hunting, surely a system may not hunt but have a dangerously low superheat (or none at all)

CB

[Tony]

Hi Chillyblue,

2ndly why do all TEV manufacturers not mention superheat when setting the valves

Have you never read the data sheet or information on the box that your TEV comes in?

Its all there.

[chillyblue]

Hi Chillyblue,



Have you never read the data sheet or information on the box that your TEV comes in?

Its all there.

Yes i've seen that, but if you read Danfoss's fitters notes they say to get the system to hunt then shut down by 1/2 a turn, how does that set the superheat???

CB

[Tony]

Have just read them and yes, you are correct in what you say. They state that procedure will ensure that all of the evaporator will be utilised. It seems they are not after a set figure for the superheat at the end of the evaporator.