Charge by sub cooling?

[gpurves]

I'm a beginner at this, though I have a degree and the liscense, I am getting the "hands on experience" on my own with no instructor.
I changed out a 3/4 hp roof top condensing unit that was on a walk in cooler last week. It was an R12 unit, and I replaced it with an R22 unit of the same type. I was having a problem overcharging it, as I was trying to charge by reading the evap. temp on the low side guage. A friend of mine said the best way to charge this unit is "10 degrees subcooling". What does he mean, and exactly what are the steps you take to do this? Is it the best way to charge, and if not, what other methods are there?

[NormChris]

The first thing you need is a solid understanding of superheat and subcooling. Then you need to learn how to charge by watching BOTH the superheat and subcooling. There are some here who will try to teach you that you charge by subcooling for one type of metering device and by superheat when you have another type of metering device.

It is always best to watch both the superheat and subcooling when charging any system.

For an understanding of superheat and subcooling go here:

http://hvac-talk.com/vbb/showthread.php?threadid=33829

Norm

[ac/dc]

Ussually walk in coolers/freezers have expansion valves
which will maintain a fairly consistant superheat when
overcharged, I agree subcooling is an excellant indication
of refrigerant charge and 10f is a good target. Of course
things change if its cold outside and a headmaster or fan
cycling is controlling head pres.
I also agree with NormChris that you cant look at
subcooling alone, sightglass,comp amps,pressures/temps,
superheat,etc should all be checked

[docholiday]

I hope you changed the indoor unit as well. Especially the TXV.

[coolwes]

If the new condensing unit has a receiver, forget subcooling. The condenser will not hold any liquid to subcool.

[hockey]

We have a difference of opinion at school. We agree that a low charge makes superheat readings higher and overcharge lowers superheat. What happens to subcoling? I say that a low charge increases subcooling (vapor condenses more quickly and the liquid has more time to reach ambient temperature). Anyone disagree?
hockey

[cde72]

Nope, not quite... Low Charge=High Superheat, Low Subcooling... Over Charge=Low Superheat, High Subcooling...

[john dalton]

Dear Hockey,

Regarding your post:

“We have a difference of opinion at school. We agree that a low charge makes superheat readings higher and overcharge lowers superheat. What happens to subcoling? I say that a low charge increases subcooling (vapor condenses more quickly and the liquid has more time to reach ambient temperature). Anyone disagree?”

Yes, I do. Like Cde72 said above, when a system is undercharged, it will have low subcooling and high superheat, and when a system is overcharges, it will have high subcooling and will have a tendency to have a lower superheat based upon whether it has a TXV or capillary tube metering device.

I’m not sure who “we” are or what school you attend, but unfortunately the “we” of the school need to hit the books again………..

Respectfully Submitted,
John J. Dalton

[hockey]

CDE72 & J Dalton, Thanks for the replies! I know I can always count on reading solicited responses AND some "extra" replies on this site. It's a great resource.
hockey

[valdelocc]

There is one thing you should always take into cosideration while reading sub-cooling or superheat and that is the boiling point of the substance,as obvious as that sounds is often overlooked by the new generation.without it you dont have an starting point.

[NormChris]

Go here and read all about it:

http://hvac-talk.com/vbb/showthread.php?threadid=49522

Norm

[gbfromsd]

Gpurves, tell us

Did you change out that TXV?? What was your final conclusion regarding superheat and subcool, and how is it running now?

[vinsfins]

The general rule for charging systems. If the system has a TXV you must charge to the sub cooling, and set the SH with an adjustable txv.
If you have a cap tube then you charge to Superheat.
10 to 15 deg of Subcooling will get you in range. An adjustable txv is very important.

I'm a lab tech for carrier corp, and i do this stuff day in and day out.

[lew2]

If memory serves me two items to keep in mind with subcooling.

Subcooling requires knowing the temperature and pressure at a given point. That is to say taking a pressure reading at one point and a temp reading at another may not be reliable.

If the unit is equipped with a receiver all subcooling will be lost unless the receiver is completly full of liquid. That is why it is important to go from the receiver back into some subcooling coil to regain subcooling.

Just the way I see it.

[hockey]

Vinsfins, sounds like a great job you have!
I haven't seen an explanation for SuperHeat vs SubCool with a low charge. Here's mine: If a system has a low charge, that means the evaporator is starved. The suction pressure will be low (so will saturation temperature be low). The refrigerant changes state early in the evap coil, allowing more time for sensible heat gain. Therefore the Superheat will be high when the charge is low (assumes cap tube).
A low charge also lowers the head pressure (and corresponding saturation temperature). The gas takes longer to condense due to extra heat. This leaves less time for subcooling at condenser exit, hence a low subcooling reading.
Sound good?

[NormChris]

I always measure and monitor four things when charging a system no matter what type of metering device I have. That way I always know exactly what is happening and will know immediately if I have an undercharge, overcharge, restriction or poor airflow.

The four things are:

1) Entire low side superheat
2) Condenser subcooling
3) Air temperature drop over the evaporator
4) Air temperature rise over the condenser


Norm

[hockey]

If the receiver is at the condenser exit (should be)it will have all liquid (by design).

[NormChris]

Originally posted by hockey
Vinsfins, sounds like a great job you have!
I haven't seen an explanation for SuperHeat vs SubCool with a low charge. Here's mine: If a system has a low charge, that means the evaporator is starved. The suction pressure will be low (so will saturation temperature be low). The refrigerant changes state early in the evap coil, allowing more time for sensible heat gain. Therefore the Superheat will be high when the charge is low (assumes cap tube).
A low charge also lowers the head pressure (and corresponding saturation temperature). The gas takes longer to condense due to extra heat. This leaves less time for subcooling at condenser exit, hence a low subcooling reading.
Sound good?



You are correct!

Hocky, read this

http://hvac-talk.com/vbb/showthread.php?threadid=49522

Norm

[hockey]

Norm,
I read that thrice and I plan to reiterate it too!

[fat eddy]

Did you change the TXV or not ?

Why do you think you will lose subcooling if you have a receiver ? ( this one has me totally baffled )

If you can fill a receiver full of liquid then you have thoroughly overcharged that system.

I think your going to be visiting that store owner quite a bit in the near future.

Is there a headmaster on the new system ?

[fat eddy]

Someone needs to explain to me how having a receiver causes you to lose subcooling, and the explanation better not be because the refrigerant in the receiver is at saturation therefore it can not be subcooled.

Why are you checking subcooling at the receiver anyway subcooling is checked at the outlet of the condender doesn't much matter what happens inside the receiver it is just a storage tank and it should never be full.

Don't mistake a receiver for a condenser just because there is some heavy vapor laden gas in there does not mean that it is not a subcooled refrigerant at the bottom of the tank.

[tech cr]

Originally posted by vinsfins

The general rule for charging systems. If the system has a TXV you must charge to the sub cooling, and set the SH with an adjustable txv.
If you have a cap tube then you charge to Superheat.
10 to 15 deg of Subcooling will get you in range. An adjustable txv is very important.

I'm a lab tech for carrier corp, and i do this stuff day in and day out. No sir, I don't totally agree with this 100%. Lets say you have a split A/C with a cap tude. Your starting the system up and the return air is 80 degrees in the ahu. Your going to charge to superheat right? OK, you charge to 18/20 degree superheat. I would bet you that in order to get 18 degrees superheat with a return temp of 80 that your subcooling would be sky high and by the time the return came down to 70degrees you would now be overcharged and freezing up. Even caps tubes need to be charged with subcooling. Since tubes cannot adjust to load changes in the ahu. Does sound right to anyone. To that tech that replaced a R12 condenser with a R22 condenser,why? Now for that system to work properly you must change the evap and txv, with a matching set.

[vinsfins]

Always a good idea to have a sight glass in the system.
Also better to have a txv rather than a cap tube or a fixed orfice, the txv could save the compressor from flooding.

[lew2]

Originally posted by fat eddy
Someone needs to explain to me how having a receiver causes you to lose subcooling, and the explanation better not be because the refrigerant in the receiver is at saturation therefore it can not be subcooled.

Why are you checking subcooling at the receiver anyway subcooling is checked at the outlet of the condender doesn't much matter what happens inside the receiver it is just a storage tank and it should never be full.

Don't mistake a receiver for a condenser just because there is some heavy vapor laden gas in there does not mean that it is not a subcooled refrigerant at the bottom of the tank.



Let's try this for why the subcooling is lost going through a receiver. "Carrier Handbook of Air Conditioning System Design section 7-61 Liquid subcooling cannot be obtained if a receiver follows the subcooling coil in the refrigeration system."

If it is subcooled liquid how does the vapor exist in the receiver? The vapor should then condense back into the liquid, it would seem. If enough temperature difference is at the bottom of the tank or enough liquid pressure is increasing the pressure at the bottom of the tank then, it would seem some subcooling could exist. Are you thinking 1/2 pound per foot roughly?

[fat eddy]

Uh lets see if I can quote Carrier, You don't check subcooling at the receiver it is a receiver not a condenser it is not there to do any subcooling so why would you check it there. What you don't understand about the Carrier manual is that they are writing down to you because they don't think that you will understand the true physics of it. Perhaps they are right. It is not a true statement to say that besause vapor and liquid exist at the same pressure that the fluid is at a saturation state and will therefore be at a corresponding temp on a TP chart.

[hockey]

I believe there HAS to be liquid and vapor co-existing in order to call it saturated. That's the definition: When saturated, the vapor begins to condense. When the liquid is saturated, vapor begins to evaporate out. Same thing.

[bornriding]

Saturation occurs when there is "only" liquid or "only" vapor. The saturation point of the evap. is when the liquid has turned completely to vapor ( 100% vapor )and at the condenser when the vapor has changed completely to liquid ( 100% ). At saturation only, temp can change but pressure stays the same. With vapor / lquid mixture ( any amount) temp & press are still directly related. Remember, the vapor is what creates the pressure.

[jvrba]

You mean to tell me that if you have any liquid present you will not have any subcooling/superheat? That is the definition right?

[hockey]

There seems to be some misinformation regarding saturation. Let's take tea and sugar. At any given temperature, the tea can hold only so much dissolved sugar. When the tea is 100% saturated with sugar, the sugar begins to precipitate out (back to a solid). The tea is still saturated (100%). Now there is liquid AND solid (two states).
The P-T charts apply only to a saturated state (i.e. liquid and gas). Once the liquid in the evaporator becomes 100% vapor, the saturation table no longer applies because heat gain becomes sensible (temperature change, aka superheat).
Subcooling is a temperature below saturation (subcooling is all liquid; saturation is liquid and gas still).
When two states occupy the same volume there is no change in temperature (latent heat).

[hockey]

"Remember, the vapor is what creates the pressure."

If this is true, then why is it that the high side is mostly/all liquid and has the greater pressure?

I recommend a review of the basics.

[bornriding]

The high side of a system is obtained by the compressor taking the low side pressure ( created by the vapor expansion in the evaporator ) and raising the pressure by the ratio of the compressor. The compressor doesn't create the pressure ( normally ) it just 'pumps' it up.

[hockey]

Bornriding,

Alot can be lost in translating and taking liberty with definitions. Time for some tea 10% saturated with sugar.

hockey

[firstmechanical]

For one you have replaced the refrigerant, thus changing the aspects of your evap. Did you check specs on evap, exp valve etc. You cannot just replace cond unit and gas without taking into consid the evap and valve.... Eng from copeland will give you heat transfer specs on evap and valve thi is a careless move for your customer ...

[gpurves]

Originally posted by NormChris

I always measure and monitor four things when charging a system no matter what type of metering device I have. That way I always know exactly what is happening and will know immediately if I have an undercharge, overcharge, restriction or poor airflow.

The four things are:

1) Entire low side superheat
2) Condenser subcooling
3) Air temperature drop over the evaporator
4) Air temperature rise over the condenser


Norm

You have said more in this post than all the others combined. I am hearing terms that I am not familiar with, yet no one seems to recall my original question. (How do you charge by subcooling?) I undersatnd what subcooling is, but it's a term that was never discussed in school. Superheat was discussed, as well as latent heat, but not subcooling.
The unit in question was an r22 condensing unit (outside) for a walk in cooler. I was called back by the owners about two weeks after the work was done, and at that time the unit would not cool. It had a very high pressure, which I think was a result of the warmer temps outside. (The weather had taken a warm spell after the installation of this unit) I removed a portion ot the charge, and when I left it was again cooling properly. The sugestions you mention, such as air change at evap and cond. are things that I need to learn. What should these temp differences be on this unit. the exp. vlve was changed out, and it is a cap tupe controlled valve. I am working alone, and have no one to give me advice when I stumble across somthing which is new to me, such as this.
Please keep in mind that you are working with a true beginner, so you may have to be rather detailed in your explanation.

[hockey]

Answering your question: To charge via subcooling, get the high side pressure (preferably at the tail end of the condenser- that's usually where the high side service valve is anyway). Convert that to a temperature by using the saturation tables (P-T charts). Now use a thermometer to measure thetemperature of the high side tube at the exit of the condenser. You are measuring the liquid temperature (if the condenser did its job). Now subtract the measured temperature from the converted temperature. The difference is your subcooling.
As mentioned earlier, subcooling should be taken after the unit has stabilized and there should be a 30-35 degree temperature rise across the condenser's cooling air.
Charge a little at a time. You should wait 20 minutes after each iteration for stabilization.
A rule of thumb: subcooling should be 15-20 degrees.
Anyone else with other thumbs?

[hockey]

High head pressure could mean a restriction in the metering device. Was the suction pressure lower than normal? The restriction could be the result of moisture in the tubing that froze at the meter device. It will freeze/melt/freeze/melt..

[valdelocc]

I' could be wrong,but I' believe that the delta T for air cooled units is more like 15-30 degrees.

[hockey]

valdelocc, you are right. The air temp across the condenser should not exceed 30 degrees (15 -30 is ok).

[fat eddy]

On the system he is on 10 to 12 subcooling id=s the number.

[valdelocc]

I' would like to also agree with bornriding he'is on target as far as I' remember from my tech schools days.
The compressor does not create the high side pressure ,Like bornriding stated "it raises the pressure by the ratio and heat of compressure.

[ac/dc]

I also think 10 to 12 degrees for low and med temp (no reciever) but 15 to 20 for air conditioning...ussually
Does this system have a sight glass? I would use it first
then check superheat,subcooling,delta T's

[cb cooling]

Is there a head master on this condenser ? If it bypasses the coil hot liquid enters the evap and no cooling

[karlheppard]

Or something like that!!!

[karlheppard]

@ john dalton. You are exactly right. Subcooling increases with overcharge, and decreases with undercharge.

Nothing can be stated in exact terms without other considerations, but this is generally what happens.

An undercharged unit, having less refrigerant, takes longer to build up pressure in the condensing unit to actually condense the vapor back into liquid. It will remain vapor longer and travel quickly through the coil. The vapor will not condense until further down the coil circuit and exit the coil more quickly having less time to cool before exiting the coil.

With an overcharged unit, the opposite is the case. Pressure in the condenser coil will build up faster, more refrigerant will turn from vapor to liquid more quickly and have more coil surface to cool the liquid before it exits. It is also a factor that liquid will not travel as briskly through the coil as vapor will. The liquid will tend to get somewhat hung-up in the coil, presenting another factor that gives more time in the coil for subcooling.

[Bocephas]

There is a term called "Stacking".It refers to the liquid staying in the condenser to long be cause of a over charged system.

[Williamsma]

If this is a walk-box with a remote condenser, and the coil txv has been changed for the new refrigrant, you set the new txv to 8-10deg low temp, 10-12deg med temp superheat. Prior to setting the txv, you need to add enough refrigerant to the system. If this condensing unit has a receiver with a headmaster, your ambient is above the temperature of 70deg, fill the system till a 40-50% receiver is acheived. Torch the reciever to find out where your liquid level is at. No temperature change means liquid, hot means gas. You should have about 20deg superheat at the compressor when set up properly.

[allan38]

Was the metering device changed to match the refrigerant used?
The correct metering device will be different between R-12 and R-22 on the same evap coil.
You will be wasting your time and potentially destroying a compressor if the wrong metering device is in use. The pressures will be way off at the correct superheat and the system won't perform as designed.

[FrostyBeer]

You all realize this thread is 6 years old right?:whistle:

[energy star]

You all realize this thread is 6 years old right?:whistle:


:det:

[PalmettoMaintPro]

The general rule for charging systems. If the system has a TXV you must charge to the sub cooling, and set the SH with an adjustable txv.
If you have a cap tube then you charge to Superheat.
10 to 15 deg of Subcooling will get you in range. An adjustable txv is very important.

I'm a lab tech for carrier corp, and i do this stuff day in and day out.

Lab tech , cool . I always say the people that build and design them know best most of the time. I was taught to charge systems as described by vinsfins . The adjustable Txv part is new . I always just determined what type of metering device it uses in cool mode if it is a heat pump and what type of device it is period if it is any thing other than a heat pump.

[Maximum19]

Yes according to what efficiency the system is. You will need the super heat and sub cooling from the manufacture. Sub cooling- the degrees f. That the liquid line is compaired to the liquid saturation temp.
Super heat- the degrees f. That the suction line is compaired to the suction saturation temp.

[daniel87829]

call the manufacturer or call the supplier that distributes the unit. and ask for tech assistance they can help