[AddMore22]

How can you charge it without checking sub-cooling if it has a TXV ? And every sight glass I've seen changes colors to indicate moisture.

 

[jpsmith1cm]

How can you charge it without checking sub-cooling if it has a TXV ? And every sight glass I've seen changes colors to indicate moisture.

On larger refrigeration systems with a receiver, subcooling is meaningless as a charging metric.

You MUST have a full sightglass proving a liquid seal in the receiver.

 

[icemeister]

ar_hvac_man, yes there is sight glass and I charged the unit until there was no bubbles. There is no indicator on sight glass.... Just one of them cheap ones that u can see liquid. My superheat was 11.. I didnt check subcooling.... I hope I did that right

Basically you did it right. Charging to a clear sightglass indicates you have attained a liquid seal at the outlet of the receiver.

How can you charge it without checking sub-cooling if it has a TXV ? And every sight glass I've seen changes colors to indicate moisture.

It's true most sightglasses have a moisture indicator, but their primary benefit is to provide a visual indication of the state of charge...if you're doing refrigeration.

On larger refrigeration systems with a receiver, subcooling is meaningless as a charging metric.

You MUST have a full sightglass proving a liquid seal in the receiver.

I agree with this wholeheartedly.

In addition...

In my experience, I have followed behind some good A/C techs who, although apparently knowledgeable about how to charge a residential A/C system with a TXV, failed to see the difference in a typical commercial refrigeration system with a TXV & receiver and how it works.

Most of these techs have been taught to ignore the sightglass and go directly to subcooling as the correct measure of system charge...because that's what's indeed required of a resi A/C with TXV. They know that when more refrigerant is added the SC will increase and vice versa. What they fail to think about is why this occurs.

When refrigerant s added to a resi A/C system with a TXV the added refrigerant starts to stack up in the condenser coil resulting in increased subcooling at the condenser coil outlet. With a refrigeration system with a receiver and TXV, this doesn't hold true.

With a refrigeration system with a receiver added refrigerant will tend to end up simply filling the receiver with no additional stacking of liquid in the
condenser coil...and so, no additional subcooling. The typical refrigeration system will only give you around 5 Deg F subcooling and that's it. Once the sightglass clears, what you see is what you get.

What would you need to do then to achieve 10-15 Deg subcooling with a refrigeration system?

You would have to charge until the receiver were 100% full at which point the refrigerant would begin to stack up in the condenser. At that point you would see an increase in subcooling.

But of course, if the system is controlled on pumpdown to cycle off you'd be called back for a system which tripped the high pressure safety.

I once had a call on 5 HP system on a walk-in at a convenience store where I had to remove about 40 lbs of R401A because the previous servicer (an A/C contractor) had noted on the unit that it must be charged to 15-20 Deg subcooling.

 

[Ben Wulkin]

A full or clear sightglass is usually a good indicator of system charge, but not always. Quite a few of the systems I work on have the sightglass downstream of the filter/drier. If it becomes plugged or restricted you will see bubbles on a fully charged system. My Dewalt cert.manual states that charging a system to a clear sightglass with a blended refrigerant such as R-410A , nearly always results in a system overcharge. At a seminar for Copeland compressors, in discussion about the blends, they said "clear the sightglass". They wanted cooling and oil return for their compressors.

 

[jpsmith1cm]

A full or clear sightglass is usually a good indicator of system charge, but not always. Quite a few of the systems I work on have the sightglass downstream of the filter/drier. If it becomes plugged or restricted you will see bubbles on a fully charged system. My Dewalt cert.manual states that charging a system to a clear sightglass with a blended refrigerant such as R-410A , nearly always results in a system overcharge. At a seminar for Copeland compressors, in discussion about the blends, they said "clear the sightglass". They wanted cooling and oil return for their compressors.

If the sight glass is after the drier, I'd say it is piped wrong.

I would agree with DeWalt, EXCEPT for systems with receivers. Once a receiver is added to the mix, you need to clear the glass. If it isn't cleared, it isn't charged right OR you have another problem.

 

[bluecool]

school me on non-condensables

The best way to find non-condensbales is to turn the unit off for 15 min... If your ref temp is within 10+/- of ambient temp your ok

I've never heard of this technique but I'm willing to here more about it. When you say ref temp? Do you mean condenser saturation temp. How can cond. liquid or vapor ever be (-) lower than ambient temp.

 

[Texas-Tech]

150psig on r22? That's pretty low. What is your out door temp

 

[bluecool]

A walk-in cooler will typically have a receiver AND a sight glass.

If the unit is outside in low ambient conditions, it will also have a headmaster or other low ambient control means.

If you do not at least charge to a full sight glass, you are undercharged. If you have the low ambient controls, there is a series of computations you must complete to be properly charged.

Throw "ambient plus 30" of any other rules of thumb you've been taught away.

15 to 20 degree above ambient might be a rule of thumb but its straight out of the text book. A minium difference between ambient and head is a good indication of charge, as you already know.

 

[jpsmith1cm]

15 to 20 degree above ambient might be a rule of thumb but its straight out of the text book. A minium difference between ambient and head is a good indication of charge, as you already know.

How does 'ambient + whatever' ensure a solid column of liquid to the TXV?

It doesn't, it's crap and it needs to be forgotten.

 

[bluecool]

How does 'ambient + whatever' ensure a solid column of liquid to the TXV?

It doesn't, it's crap and it needs to be forgotten.

Its not crap, just its a indication that heat transfer is taking place.

 

[knave]

Its not crap, just its a indication that heat transfer is taking place.

easy, killer

 

[-frozen-ocean-]

Sometimes when your discharge stem is front seated too much your high side guage will go up and down, back seat it all the way then crack it open a hair to get your reading.

Best way to tell if you have non condensables is by high subcooling reading it does work, do pump down test to confirm charge etc. If you have 40* subcooling with a flashing sightglass you got non condensables.

Not to stir the pot, but on some rack systems you do see your sightglass flash even with a full charge, all depends on load etc.. but for small single systems yeah clear the sightglass common sense.

 

[jpsmith1cm]

Its not crap, just its a indication that heat transfer is taking place.

OK, great. Heat transfer is taking place.

I don't need gauges to tell me that. My hand will typically tell me that there is a temperature difference between the discharge and the liquid line.

Still doesn't tell me what I NEED to know about the system.

That the TXV had a solid column of liquid.

Since it doesn't tell me that, it is NOT a charging metric.

 

[Ben Wulkin]

If the sight glass is after the drier, I'd say it is piped wrong.

I would agree with DeWalt, EXCEPT for systems with receivers. Once a receiver is added to the mix, you need to clear the glass. If it isn't cleared, it isn't charged right OR you have another problem.

I believe a case could be made for and against the location of the moisture indicator on the liquid line. All the racks that I work on have the SG located downstream of the drier. Basically as you have indicated, the purpose is to ensure a solid column of liquid , at that point , at least. My point is that used as as part of the diagnosis the SG is valuable.....but , to make a final decision only on a full or bubbling SG will often lead you down the wrong path.

 

[dangpgt97]

Sight glass should be installed downstream of the filter drier. You can figure out if you're short on gas or the drier is plugged it's not that difficult. Best place for the glass would be at the txv. On some racks you will get bubbles in the sight glass. You will get what looks like a half full glass sitting there. You will get rivering. You don't want to use that as a charging indicator. I think on a large vertical receiver there is turbulance going on in the bottom from the liquid being dumped in the top.

Now if you're working on a tyler equalizer rack then your sight glass is a proper charge indicator. You have to be careful.

 

[ar_hvac_man]

I've never heard of this technique but I'm willing to here more about it. When you say ref temp? Do you mean condenser saturation temp. How can cond. liquid or vapor ever be (-) lower than ambient temp.

Hes talking about temperature/pressure correlation. The machine must be idle. It must also be pumped down so your not getting a mixed pressure reading from the liquid that may still be in the liquid line inside the conditioned area. Take your pressure and temperature and find it on the chart, just like you would on a container containing an unknown refrigerant.

Ive tried it and found its still hard to tell, especially on an old r-12 or 502 cooler where a number of refrigerants are very close as far as press/temp correlation.

If the refrigerant is known as it supposedly is in this instance then non-condensibles would cause the pressure of the r-22 to not correlate with the temperature.

 

[Phase Loss]

On smaller equipment and singles compressor walk-in boxes I usually see it's order as

1) Filter/Drier
2) Liquid Solenoid Valve
3) Liquid/moisture indicator

Usually all the above items are located inside the condensing unit.

The sight glass should be full. If you see any bubble in the glass, you would want to take action and look for...

1) Short on charge
2) Rapid Liquid pressure/temperature fluctuations (fan cycling)
3) Restricted filter drier
4) Restricted liquid solenoid valve

If it's a pump down system, and you charge to a clear sight glass...you need to make sure the system can pump down without tripping the high pressure control.

Some systems receivers can't hold the pump down. You could re-locate the liquid solenoid from the condensing unit and put it closer the the evap coil. You gain some additional volume to store pump down in the liquid line.

I'm getting off track...

Whether the sight glass is installed before or after the filter, it is a charge indicator. But when installed after the filter, it becomes so much more. and can alert you of a few extra systems problems.

 

[bluecool]

easy, killer

Ok, maybe I should not have taken it so badly. It was my bedtime. However, you can't call something crap without explaining. I'm willing to listen.

 

[bluecool]

OK, great. Heat transfer is taking place.

I don't need gauges to tell me that. My hand will typically tell me that there is a temperature difference between the discharge and the liquid line.

Still doesn't tell me what I NEED to know about the system.

That the TXV had a solid column of liquid.

Since it doesn't tell me that, it is NOT a charging metric.

Lets look back. You told the guy to charge the system and move on. Did you place your hand on the liquid line? All I saying is head press is one of many observation a tech can use. Head has to referenced to something, normally that is OA. What I would be looking for is not that there is a difference, but the degree of difference between both measurments.

 

[otto8]

Phase Loss, good info... Thats what what I found on this condensing unit... From simple question. Why my head pressure was flacuating. I learned a lot more...