How to tune a cap tube system?

[pythagoras]

This thread is to get some discussion together on the best way to tune a cap tube system. I'll start with my thoughts and people can shoot me down as they feel free :D

Cap tube systems charge is very critical and is a function of the cap tube. That means when you alter the cap tube length the charge required needs to change.

I wont go into an initial estimate of the cap tube length, I dont have the knowledge for that :mad: , yet ;) But there are threads on here for that.

Once you have an initial cap tube length you want to tune for lowest temperatures for your load.

My thoughts are:

1. Install a sight glass on high side- This way you can check, visually that you have enough refrigerant.

2. Install a sight glass on suction side(not as critical as 1. but does let you see if too much liquid refrigerant is returning to the compressor.


Scenerio one:

System cant hold the desired load at the desired temperatures. So evaporator is starved. If you have a sight glass and you see that it isnt full. Bubbles will appear. You need to add refrigerant.

If the sight glass is full then the cap tube needs to be shortened. Cut cap tube, run system and charge again until you get a full site glass. Repeat until satisfied with the result.

Scenerio two.

System temps are too high because you have either too much refrigerant or cap tube to short.

If sight glass is full under these conditions, lengthen captube. Remove refrigerant until you can see bubbles in sight glass then add more until its clear. Repeat until desired operating conditions are reached.

These are just MY conclusions and I hope people will correct were necessary.

Regards

John.

[wdrzal]

one note, the liquid line sight glass "may" never appear "totally" free from bubbles when using zeotrope refrigerants(400 series) Fractionation may cause very tiny bubbles to be present even when the charge is correct. This is distingusable from a undercharged system,where larger bubbles appear. the very tiny bubbles are evenly disbursed within the liquid.

[Unknown_road]

cappilair systems shouldn't have a sight glass!! at full load you just want a liquid seal on your cap tube for maximum efficiency! You can all check this with superheat and subcooling calculations. You don't need sight glasses.

[pythagoras]

Why shouldnt a sight glass be used with a cap tube system? Would they above method not work? Can you list your method for tuning charge and cap length using superheat and subcooling calculations?

Regards

John.

[Jimi...]

Where would one get a sight glass in the UK?

[gclg2000]

It's a standard HVAC part. You should be able to get it from a HVAC supplier (assuming that's where you get the rest of your parts).

[wdrzal]

A sight glass not only would give you a visual indication of the charge, most have moisture indicators built in. those who don't pull deep vacuums or use fuel grade propane this would be especcially usefull. "work safe" walt

[Unknown_road]

Why shouldnt a sight glass be used with a cap tube system? Would they above method not work? Can you list your method for tuning charge and cap length using superheat and subcooling calculations?

Regards

John.

a sight glass like you suggest acts like a reservoir. cap tube systems shouldn't have reservoirs because a perfect tuned system only has a few droplets of liquid at the cap tube entrance under full load.

If superheat is high, your evap doesn't get enough refrigerant. 2 possible causes for that either the cap tube is too long or the system is undercharged. If your cap tube is too long their will be liquid at the cap tube entrance, liquid at the entrance means discharge pressure should correspond with the temp at the cap tube entrance. if the pressure is too low to correspond with the temperature their is no liquid at the cap tube entrance and thus the system is undercharged

if superheat is very low it is likely you have flood back to the compressor which means overcharged or too short cap tube.

[pythagoras]

I think to ensure a liquid seal at the cap tube entrance you would need to measure the temp and pressure at the cap tube entrance to ensure it matched the pt chart for the particular refrigerant, I could be wrong on this point however.

Even so how does a sight glass full of refrigerant effect the efficiency? I would have thought that liquid refrigerant backing up into the condensor would effect efficiency.

Regards

John.

[wdrzal]

a sight glass like you suggest acts like a reservoir. cap tube systems shouldn't have reservoirs because a perfect tuned system only has a few droplets of liquid at the cap tube entrance under full load.


This is not true,if there is only a few droplets present,where is the rest of the charge?

[bh2k]

From what I've found sight glasses can act as mini recievers in a captube system.

[Russell_hq]

Yeah, but the question still stands, why not have a reciever in a cap tube system?

[Unknown_road]

@walt: discharge and suction pressure go up, pretty amount of volume that is compared to the amount of gas that is in these small systems. look at a compressor shell for instance.

sorry guys but I can't explain it much better than I have but i read it from gary, chilly and bowman I think. maybe chilly can tell you exactly what's going on.

[kayl]

Yeah, but the question still stands, why not have a reciever in a cap tube system?

dont need one really.
actually we have one already if ya think about it the drier.

some ppl even say donnt need a reciever with cascades (when using txvs and pevs)??
i would use one though personally, i have only ever used capillary line though.





guys that havnt seen a slight glass heres one.
some also tell ya wether the system has moisture in them.
http://www.xtremesystems.org/forums/attachment.php?attachmentid=31095&stc=1

[Russell_hq]

Yup, you dont really need one, but I wouldn't say that having one is going to affect system performance.

[Unknown_road]

I wish I had one installed on my autocascade that's for sure (kayl and russel know what I'm talking about :D )

[runmc]

Here are some good answers from some guys you may have heard of.

Do you need a Sight Glass on a captube System (http://teampuss.com/forums/viewtopic.php?t=651)

[Unknown_road]

Here are some good answers from some guys you may have heard of.

Do you need a Sight Glass on a captube System (http://teampuss.com/forums/viewtopic.php?t=651)

that's what I meant :banana:

[wdrzal]

that's what I meant :banana:

don't get too excited some of that info is wrong. first,a sight that is sized for the line does not hold much more liquid than the line itself,they have a very thick glass,the internal volume appears more than it acutuly is.Any idea why the line from condensor to cap tube is called"liquid line"? while there is a complex balancing act their is more liquid in system than you guys think.the bottom of the condensor as well as the liquid line is full of refrigerant.at least 2 degrees of subcooling is needed when refrigerant leaves condensor so the refrigerant doesn't flash in the liquid line.the cap tube always needs a liquid head,the refrigerant moves about 2/3 of its length before the suction pressure begins to act on it. while you can charge by superheat,a sight glass has no ill effect.they are not used in manufactured systems because the excact charge is known.but would greatly benifit noobs in determining mimimum charge. finally answer me this? how much refrigerant does one of these systems hold.?

[wdrzal]

forgot to mention look at sight glass kyle posted,the center indicator is pink,that means moisture is present(wet). That glass had the factory seals removed. If they were intact the indicator would be blue,the legend is on the outer ring.

[Unknown_road]

don't get too excited some of that info is wrong. first,a sight that is sized for the line does not hold much more liquid than the line itself,they have a very thick glass,the internal volume appears more than it acutuly is.Any idea why the line from condensor to cap tube is called"liquid line"? while there is a complex balancing act their is more liquid in system than you guys think.the bottom of the condensor as well as the liquid line is full of refrigerant.at least 2 degrees of subcooling is needed when refrigerant leaves condensor so the refrigerant doesn't flash in the liquid line.the cap tube always needs a liquid head,the refrigerant moves about 2/3 of its length before the suction pressure begins to act on it. while you can charge by superheat,a sight glass has no ill effect.they are not used in manufactured systems because the excact charge is known.but would greatly benifit noobs in determining mimimum charge. finally answer me this? how much refrigerant does one of these systems hold.?

these little systems contain about 60grams of refrigerant (also depens on what refrigerant offcourse but it is a nice estimate)

Have you even read all of it?? seems to me you didn't seeing this reply. some info might not apply to systems your used to be working on but it does to these systems.

[wdrzal]

what is the difference other than you are replacing a air/over evap with a direct die one.most guys are using a/c units and changing the evap.they are simple loops

after a noob saying,this is my first post but I've been reading.......they ask?is this compressor any good......then, how long cap tube do i use...............then, what pressure do i charge my system too.......then, everyone tells them pressure doesn't matter..... a sight glass is just a simple visual tool,while not essential,will help a noob in their learning curve of what goes on inside these systems and to make sure there is enough charge for a liquid head on cap tube. There are points in the loop where liquid must be present,and points where there should be no liquid. a sight glass is simply a visual indication of this.

I use a electronic sight glass all the time, I dont even need too attach gauges or measure temps to check a charge.just clamp on the sensors. just knowing where liquid should be and where it shouldn't.tells me if system is under or over charged

[ellsworth]

the volume of the sight glass isn't going to affect anything in the slightest...
i add them to my systems just as a moisture indicator.

adding a sight glass to the suction line wouldn't be wise since frost indicates where liquid is boiling off... you can SEE where the liquid is just based on that.

adding one to the liquid line isn't important either simply because it is blatently obvious when the system is overcharged or the cap is too short or the opposite. you can see everything you need based on load temps, suction line frosting and pressures. a sight glass only should be added if you want to see if your system is properly vacuumed (which i add always just to be certain).

[wdrzal]

frosting on the suction line only indicates the tempature is below freezing, now to get a suction line to frost,usually means liquid boiling off there but not allways ,vapor colder than freezing will do the same.Besides a well insulated line does not frost nor could you see it if it did.but even that does not indicate how much liquid is entering the suction line . a perfectly tuned evap would just have a occasional drop or two leaving it.more that that will increase suction pressure.Sure a experencied person can make these determanations without a glass,but again,noobs would benifit.as many don't want to buy expensive equiptment. actully one could install sight glasses and not even need a manifold or temp meter to get system operating correctly.just a hose to fill system.

a sight glass does not indicate a properly "vacuumed" system. only that moisture is present, they do not indicate if air orpurging gas is present,while I conceed air does contain moisture, the humidty level will affect indication. they also do not react fast enough to use for that reason.only by using a micron gauge or a gauge or meter that measures very low pressures accuratly can you be sure system is vacuumed corectly.

[Russell_hq]

Of all the sight glasses I have bought, they all had pink moisture indicators and were all factory sealed. I think over time that moisture does work its way in through the seal and turns the dot pink.

Also, I wouldn't say you need a sight glass but it definatly helps if you are new to the game and are a bit unsure. They help 2-fold as mentioned before, they will show the presence of liquid and how much and they will also indicate if moisture is present. The latter one is usefull in determining if your vacuum was sucessfull in removing the moisture from the system. i think this is specially important if you are not using a standard vacuum pump to do your evacuations.

[Him]

i think this is specially important if you are not using a standard vacuum pump to do your evacuations.

What would be considered standard and non-standard?

[Russell_hq]

Standard would be a vacuum pump manufactured and aimed at the refrigeration/air con market. Non-standard would be 2 fridge compressors in series :D

[wdrzal]

A moisture indicator in a sight glass should be blue if the seal wasn never broken ,if it is pink either over time the factory seal leaked or someone removed the ends and replaced them.

[Him]

Oh, ok than. I thought it was somethign technical. It's either a real compressor, or a ghetto rigged compressor, right? ;)