It is absolutely right the way you explained the question about superheat.
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The subcooling tells you if there is not enough refrigerant (low subcooling) or too much refrigerant (high subcooling) in the high side of the system.
The superheat tells you if there is not enough refrigerant (high superheat) or too much refrigerant (low superheat) in the low side of the system.
If there is not enough refrigerant in the high side (low subcooling) and not enough refrigerant in the low side (high superheat), then there is not enough refrigerant in the system (undercharged).
If there is too much in the high side (high subcooling) and just enough or too much in the low side (normal to low superheat), then the system is overcharged.
If there is not enough or just enough in the high side (normal to low subcooling) and too much in the low side (low superheat), then the metering device is overfeeding or the compressor is inefficient.
If there is just enough or too much in the high side (normal to high subcooling) and not enough in the low side (high superheat), then the system is restricted/underfeeding.
None of the above is accurate unless you have sufficient airflow through both the evaporator and the condenser, so always check the airflow first.
Indeed, if I were teaching a course on system trouble shooting, I would not allow my students to use gauges until somewhere near the end of the course.
They would become airflow experts first.
They would be taught how to measure saturation temperatures without gauges.
And then they would be taught to measure saturation temperatures with gauges.
And heaven help the student who tells me what the pressures are. I want to know what the saturation temperatures are. Trouble shooting is about temperatures, not pressures.
My first approach to system is "face in wind and hands on pipes", than "sniffing, hearing, touching" and than gauges.:D:D
When we modify some of terms, we could say "approach to system as you would approach to adored woman". First some foreplay, and than penetration! With right foreplay, maybe you don't need penetration at all.:D:D
The general rule is: TXV's almost never go out of adjustment. Assuming it was adjusted properly to begin with:
If the superheat is high then:
a. The evap air in temp is high or...
b. The refrigerant charge is low or...
c. The inlet screen is plugged or...
d. The TXV is undersized or...
e. Somebody has been screwing around with the adjustment.
If the superheat is low then:
a. The airflow through the coil is insufficient or...
b. The system is grossly overcharged or...
c. The compressor is inefficient (broken valves) or...
d. The TXV is oversized or...
e. Somebody has been screwing around with the adjustment.
Sadly, the most common reason I have found for adjusting TXV's is "e".
hi guys please do not shoot me down in flames but i have been a member off this forum now for best part off 2 years and every week it seems that the question off superheat appears.its almost that people think that superheat is the be all and end all off refrigeration i have been in the trade 18 years 14 as an engineer and have never once worrried about superheat now i am not having a go at anyone for asking the question but it seems people worry about superheat ask the forum then get bamboozeled with the science when people try to explain. not trying to upset any one but dont worry about it
Waw, what a long sentence.
i have been in the trade for 25 years but didn't start thinking about superheat until 9 or 10 years ago. i don't know how i managed to fix anything and make the repair last.:eek: In commercial refrigeration the emphasis wasn't(and still isnt) on the scientific side of the business, but on how many calls you can do in a day. When i was younger i used to pride myself on doing as many calls as i could. Now i tend to concentrate on properly fixing the 3 or 4 calls i now attend each day.
What made me change?
Well gather round and i'll tell you. :D
It was a chance meeting with a retired engineer who was shopping in the store where i was working. I was having a problem setting a tev in a freezer case. Despite working through a 5 year apprenticeship i couldn't recall ever using eithe a guage or thermometer to set up a tev. I was always tought how to do it with the frost test; that is if the suction line is frosting back to the compressor, then shut the valve in until it stops frosting, then you would be in the right area. and this teaching came from a well respected engineer. The retired engineer (i never found out his name) told me how to measure superheat properly (and what is more impportant why):cool:;); when i saw how simple and obvious it was i was amazed, and began to start questioning all i had been tought. i got as far as i thought i could with logical thought then started buying books. eventually i found this forum and others like it.When you see the kind of questions posted and the depth of knowledge and experience in some of the answers, it must make a lot of engineers realise how little a lot of us know. My advice to any members of this forum is not just to use it to ask questions for your own benefit, but every once in a while flick through the forums to see if anything catched your eye. If it doesn't, select a question at random you never know, something you think you know all about may just be being questioned, and it might just make you think. For those who like me thought they knew enough to do a good job try it somebody elses way for a change; you never know, it might just be better than the way you've been doing it for years.
oh and the question is, if you've never bothered about superheat then how can you tell if the system is efficient, if you have a full refrigerant charge, if the tev is correctly set, if the compressor is receiving the correct amount of superheat to maintain it's cooling, if there is no liquid returning to the compressor, if the evaporator is defrosting correctly, if the controls are correctly set for the systems best and most efficient operation. in these times of energy efficiency drives, anybody who doesn't understand superheat really should ask the question: what kind of engineer do i want to be?
i'm not shooting you down coolhibby because i used to be just like you, but it is time now to start thinking for yourself, use this forum and any other sources that are available to you to begin to become an engineer.
sorry about the length everybody but i thought the point needed to be made
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.:D
or i could just check the fault code and follow the flow chart
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.
Bill1983,
You are right, I was quoting coolhibby1875. Your thoughts are similar to mine.
Cheers
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.
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
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
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.
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.
Hi Chillyblue
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.
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
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
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
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.
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
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
I am talking about subcooling at the receiver outlet, not subcooling at the TXV inlet.
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.
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.
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.
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. :)
We do seem to have slightly different terminology, at least the equipment stays the same:D 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.
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
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