[mrkelly]

Really core, a good micron gauge IS as important as your actual gauge set.
IT will tell you a multitude of things from a leaking manifold set,hose to a weak vac pump,to a leak.
My work is not often "critical" and have a YJ digital,not super-duper,but really is a "working man" tool

 

[mrkelly]

Measure your LL temp AT the Evap.
All you need to know.

 

[penderway]

penderway, KindaSorta, on a 50' LL, going up 30' ,5' horizontal outside, and 15' H in the attic, where does the "not full LL begin" ? Not a joke, I don't know.

Terry, your logic is sound. The ideal situation if for all metering devices is to be fed a full column of liquid. This way the operation is very predictable.

But, with fixed orifice systems (omitting refrigeration units with liquid receivers) the charge is considered critical.

Meaning that the amount of refrigerant keeps the compressor cool enough but without ever flooding it. It needs to do this over the entire range of conditions the compressor is designed to operate in.

The Carrier fixed orifice superheat chart is used to ensure that this is achieved. Proper superheat when the humidity is high assures the proper superheat when the humidity is low. Compressor always stays cool enough, and without ever flooding.

I have observed that under conditions where the superheat is required to be low, the evaporator is pretty full but the condenser not so much... so no subcooling happens. The refrigerant leaves in a saturated state. Without any means of further cooling... the fixed orifice receives bubbles.

This is by design and considered a fair trade off for a simple, less expense metering device with increased reliability.

Plenty of FO systems have had compressor failures due to charging it to a full sight glass instead of the target superheat.

 

[jtrammel]

penderway, KindaSorta, on a 50' LL, going up 30' ,5' horizontal outside, and 15' H in the attic, where does the "not full LL begin" ? Not a joke, I don't know.

It's more about when the indoor humidity is low that you'll have low/no subcooling with a fixed orifice and you'll see bubbles in the glass.

Pressure drop in the liquid line should be accounted for with the additional charge added for the lineset. You can use up all your subcooling by going through a good ways through a hot attic. In that case you'd want to insulate your liquid line.

It would begin at the point that all subcooling was used up.

 

[penderway]

It's more about when the indoor humidity is low that you'll have low/no subcooling with a fixed orifice and you'll see bubbles in the glass.

Pressure drop in the liquid line should be accounted for with the additional charge added for the lineset. You can use up all your subcooling by going through a good ways through a hot attic. In that case you'd want to insulate your liquid line.

It would begin at the point that all subcooling was used up.

But you charge Fixed orifice systems by target superheating right?

The LL is un-insulated in order to give off heat. Insulating it (when needed) from gaining heat when probable is a good idea,

Thanks for that tip.

 

[jtrammel]

But you charge Fixed orifice systems by target superheating right?

Yes, but I always check SC too regardless of MD.

 

[penderway]

Yes, but I always check SC too regardless of MD.

Excellent, but like me... have you encountered situations with 0 subcooling on fixed orifice systems when the target superheat was correct?

 

[TechmanTerry]

I've had "encounters" w/ systems like that. But something is wrong somewhere. I have systems that "break the rules" and I settle for good/better/best when I do have an encounter. But, I NEVER FORGET AND I ALWAYS WONDER ,always! Its a curse, and its a gift.

 

[ckartson]

Thank you Jtrammel. Evacuation is not a form of leak checking. Pressure tests are. Evac leak checks are only good to 14.7 psi but pressure tests are good to whatever pressure is supplied. System is also being contaminated during an evac pressure test if there is a leak. Blank off of system with a micron gage on tells us how much moisture is left. This is done after one is sure there are no leaks through pressure testing procedures.

 

[TechmanTerry]

Hay, ckartson, I think that is a negative 14.7 psi. So the deepest vac is just 14.7 psig away. Just/only.

 

[jtrammel]

Excellent, but like me... have you encountered situations with 0 subcooling on fixed orifice systems when the target superheat was correct?

Yes. Even on txv systems you'll see bubble sometimes. Not a heavy foam of bubbles but the occasional flash. I assume it's from the txv hunting.

 

[mrkelly]

Fixed orifice systems are a "happy medium" type gig.period.
You win some,you lose some.As long as your in the window,it's good to go.Low or high ambient,indoor,You outside the window.Never saw one with a sight glass.THAT would be a BIG problem with the 'glass chargers.:grin2:
All the fixed have just a SH chart and the TXV's have a SC chart.
About spells IT out IMO.

Evacuation is not a form of leak checking.

Yes it is.You can do an extended Pressure check with Nitrogen and pass.You do a vac check and can fail.
Welds,tubing,"stuff" CAN change and the Factory even uses Helium(smaller molecule)which leaks from a "smaller hole" but doing critical work in the sub 500 rating is a necessity.
Just Saying.

 

[Core_d]

Yes. Even on txv systems you'll see bubble sometimes. Not a heavy foam of bubbles but the occasional flash. I assume it's from the txv hunting.

I forget the memo but from memory 404 & 410A it is normal to see a lazy river of flow. Ie not a full column. This is more likely in high indoor temps or low outdoor ambient.

 

[Core_d]

I forget the memo but from memory 404 & 410A it is normal to see a lazy river of flow. Ie not a full column. This is more likely in high indoor temps or low outdoor ambient.

To add on that.. With a txv if its too cold outside you overcharge if its too hot inside you overcharge. I dont pay much attention to the subcooling out side specs if i cant wait for it to pull down... Check everything else and tell em to CALL BACK if need be.

 

[Core_d]

Really core, a good micron gauge IS as important as your actual gauge set.
IT will tell you a multitude of things from a leaking manifold set,hose to a weak vac pump,to a leak.
My work is not often "critical" and have a YJ digital,not super-duper,but really is a "working man" tool

Sorry, no excuse for bad practices. Charging a unit to 150 with nitro and waiting a certain time then pulling a vacuum while buttoning up and putting aways is not good practice. I knew that. I will get a micron gauge. Ive had a crt for service purposes for some time.

 

[Core_d]

Starting out in the trade you need hand tools and what not. Ive took the stance if it needs borrowed it needs baught. I try everyday and run into issues from every direction. The day dont hold much time for shopping

 

[jebes]

My boss is a die hard believer of a good vacuum and so am I. He let me buy steel hoses from Yellow jacket because even the vacuum rated black hoses leak under a deep enough evac. I used the black ones for about 6 months before I could get my hands on 3/8" steel hoses and a 1/2" steel hose from my guages to the vac pump. I've gotten them down to holding overnight at 30 microns off the pump, though just by themselves and not hooked up to a system.

Those of you who use Nitrogen in between vacuum pump sessions, do you think it makes the whole process faster or better? Both? It seems like a waste to pull the unit down and then go back to square one with a nitrogen purge. I don't see how that helps removing water stuck in the oil.

*edit*
BTW, the last nitrogen tank I used was full of about a quart of water. It was supposed to be dry, but I didn't flip it upside down and check until it was almost empty. I'm glad I
just used it for brazing and not for breaking a vacuum.

 

[adam_s05]

This thread sure is refreshing, I'm a member of Hvac Pro Talk on Facebook and over the last couple of days there's been a long thread regarding pulling a vacuum. I seem to be fighting an uphill battle against guys that are very adamant about just listening to a vacuum pump. Some of the guys said they only brought out a micron gauge if the system was small. I tried explaining why a micron gauge was required, they seemed to be pretty proud to not use one. When I started out I never used a micron gauge because I couldn't afford one, I always knew it was wrong tho.

Sorry for the rant, I had to vent a bit. It's kind of interesting how the group is called Hvac Pro Talk, not enough pros there. If you ever want to have some fun join the group and enter the conversation.

 

[jtrammel]

My boss is a die hard believer of a good vacuum and so am I. He let me buy steel hoses from Yellow jacket because even the vacuum rated black hoses leak under a deep enough evac. I used the black ones for about 6 months before I could get my hands on 3/8" steel hoses and a 1/2" steel hose from my guages to the vac pump. I've gotten them down to holding overnight at 30 microns off the pump, though just by themselves and not hooked up to a system.

Those of you who use Nitrogen in between vacuum pump sessions, do you think it makes the whole process faster or better? Both? It seems like a waste to pull the unit down and then go back to square one with a nitrogen purge. I don't see how that helps removing water stuck in the oil.

*edit*
BTW, the last nitrogen tank I used was full of about a quart of water. It was supposed to be dry, but I didn't flip it upside down and check until it was almost empty. I'm glad I
just used it for brazing and not for breaking a vacuum.

Once you break with Nitro it only takes about 2 minutes to get back to where you were before and it gets it down lower much faster afterwards. I've had guys argue with me on here about it but I do it all the time and it does it everytime, greatly increasing my vac speed on systems that are contaminated. On a new lineset and coil i can pull down deep, quickly without breaking with nitro but on a system that was in service with a compressor full of oil it helps tremendously.

 

[TechmanTerry]

jebes,KindaSorta, two identical systems, both contaminated w/ 2 drops of water, and its a 80%RH day. Both systems start the evac at 12:00 noon. The "nitro break" system will reach 500 microns faster than the "continuous run " vac system. The "triple evac" method works. At the end of the 1st evac, the vac is broken w/ DRY nitro, the DRY nitro mixes w/ the remaining non condensibles which dilutes the remaining non-cond AND the DRY nitro also picks up some of the water. Now on a small tonnage close coupled system the "triple evac method" probably doesn't matter much. On a system w/ 1 1/8 LL & 2 1/8 SL it makes all the diff in the world.