Troubleshooting and Subcooling

[iamlakota]

this is my second post. i'm a student studying HVAC. currently we are working on compression cycle charting/calculations. i wondered if measuring subcooling could be used to help troubleshoot a system but was surprised when my instructor said no when i raised the issue in class. i can't help but want to challenge him because in the past, he has presented material that flew in the face of logic, or contradicted information from other sources (here included). is he right? if not, can members explain/describe situations where subcooling could be used as a method(s) for cycle troubleshooting? thanks in advance

matt

[bdclark]

Yes it can be very helpful...

Example...

bdclark
Professional Member

Registered: Sep 2006
Posts: 109
Had this bugging me for a while. Its not something I can go back and do anything about cause I no longer work for the company who services this customer. These numbers are old so Im not sure they are 100% accurate, but very close.

Called out on a "not cooling."
LGA156HS1G

OA= 93* / DP= 215 SP= 48 / Suc. Temp. 72.5 / Sat.= 23 /
SH=39.5 / Return=76 / Supply = 66/67 // Cant read own writing for air in/out of cond.

This is after 5 mins operation/ Then suction pressure drops to 20*, all other reading stay about the same.
No kinked piping. Cleaned out TXV to make sure it wasnt sticking. Replaced LL drier. Suction pressure drops to 10, then slowly into a vacuum. I immediately opened the disconnect. Before I got much further was called off of roof to another job. What was going on? My first impression was that the TXV was bad.
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10-04-2006 06:18 PM IP: Logged

mark beiser
Professional Member

Registered: Aug 2004
Posts: 3524
Subcooling?
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You're not finished learning until you're dead.

http://www.rses.org


10-04-2006 06:31 PM IP: Logged

bdclark
Professional Member

Registered: Sep 2006
Posts: 109
I know. I know. that would help but I cant read all my notes because the page got wet at one point, somewhere in time and its all smeared.

I do remember that the discharge temp was so extremely hot that I was worried about burning up the rubber seal on the gauges...tap the compressor and it left a welp! BTW, does anyone know the part number of the TXV in this unit?


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Always remember, those who hate you don't win, unless you hate them. And then, you destroy yourself. -Richard Nixon-

[iamlakota]

hi bdclark... i'm not sure i understand your post, other then the fact that no subcooling reading was taken. could you please explain further? is subcooling predictable/standardized? i imagine it could depend on the selection/type/condition of the condenser, air/water volume used to remove heat and refrigerant charge itself. what could high subcooling be indicating? what could low subcooling be indicating? i really appreciate your time helping me understand this better.

by the way... my first rses meeting is this coming wednesday here in central new jersey

[muchofreeo]

High subcooling would let you know if you were overcharged. Low subcooling would let you know if you were undercharged. However it depends on the superheat, TD across the evap/cond etc. It can be a helpful tool, but is only part of the equation. But yes it helps you troubleshoot. Ask your instructor how long he has been in the field. Not all instructors are qualified, just take what they teach and if you have questions ask them. If he won't answer, ask another instructor. A good instructor will always take the time to explain.

[bdclark]

Ive heard some guys say that you can usee sub-cooling along with superheat to determine correct charge. I also had a guy from Texas say that you can use subcooling to set a TXV, but he is incorrect. TXVs are set using superheat, for it is superheat that the TXV is controlling. Too little subcooling might also let you know if you had too little air flow across the cond. coil.

[muchofreeo]

Correctamundo, like I said subcooling is only part of your troubleshooting. But it can and does help diagnose a problem. This post can be pages long if we list all the symptoms for superheat/subcooling.

[gpwasr10]

I treat a subcooling reading as a measurement that tells me how efficient the condenser is. If the reading is too cool then the condenser is to efficient, if it is to hot then it is not efficient enough. It can tell you if the condenser is to dirty, if the unit has an improper charge or in one case it lead me to a fan being installed backwards. Subcooling should be around 15 deg. under full load.

[bdclark]

gpwasr10= A Romanian semi-auto, usually 10 round, can be thiry round, imported version of the Russian AK-47...cool name.

I was going to mention that subcooling could help in determining proper operation of Condensor fan motors, but most likely if one is not working or running backwards you will notice with visual inspection. On a unit similar to a Trane Voyager, what you might not notice is if the fan is sitting too high on the mounts and is not drawing enough air thus not performing to capacity. Subcooling could help in that instance (which goes along with saying not enough air across the cond. coil)

[hands]

Yes, subcooling can be very important in performing A/C check-outs. To check for the proper charge in a system that has a TXV, subcooling is more important than superheat because the TXV will open up to try to maintain the set subcooling when a system is low on refrigerant. It is always a good idea to check subcooling, superheat, evap. & cond. delta-t and sight glass for flash gas. Ask your instructor how he would check the charge on a system with a TXV.

[ericnyc]

you can perform a condenser approach on water cooled units using subcooling temps to determine a fouled condenser.

[bdclark]

I am thinking that maybe he is saying this for now so that you guys dont get too far ahead of yourselfs, but as much as that might be his reason, it is a sorry excuse.

[gpwasr10]

Originally posted by bdclark
gpwasr10= A Romanian semi-auto, usually 10 round, can be thiry round, imported version of the Russian AK-47...cool name.

I was going to mention that subcooling could help in determining proper operation of Condensor fan motors, but most likely if one is not working or running backwards you will notice with visual inspection. On a unit similar to a Trane Voyager, what you might not notice is if the fan is sitting too high on the mounts and is not drawing enough air thus not performing to capacity. Subcooling could help in that instance (which goes along with saying not enough air across the cond. coil)

Yeah, they are actually really great AKM's (ak-47's are full auto and have milled receivers as opposed to stamped) if you get one that was built with a de-milled kit instead of one with a single stack trunnion. They have a com-block sight rail, chrome bores, bayo lug, threaded barrel. All for around $350.00. Their a little ruff around the edges but clean up nice. Always inspect before you buy though and never buy one "sight unseen". Your one of the few people who have ever got my screen name right. You a shooter?

Yeah you would think that a visual inspection would reveal a wrong fan blade (Clock wise operation as opposed to counter clockwise). But when you look at hundreds of units and never have had it happen, you don’t even think to look at it. All you can see is that the fan is spinning, it is spinning fast, the coil is not clogged, yet the high press. still trips..... Then, after you stand over the fan as it spins while spending a bit of time taking all of your readings to determine your subcooling... you realize something odd... your hair is not moving one bit. And think .... ****.... what asshat did this?

[bdclark]

Definitely a shooter. I own one of these guys. People will say that these AKMs are cheap and a bad choice. But an AK is pretty much an AK. Ive shot Russian AKs and I noticed no difference in accuracy, recoil, etc. An AK is naturally inaccurate after about 200 meters anyways. My reason for buying an WASR-10 was the cost. You cant shoot the gun out! I bought one instead of an AR-15 simply because I have no long range purpose with an assault rifle, and within a hundred yards or so the AK will do a little more damage. And after owning this AK, I think before I would own an AR, I would probably buy a AK-74. Similar damage to an AR, Distance of an AR, but the look and feel of the AK-47 or AKM.
By the way, I have done some research, and my AKM still has a few fully automatic parts in it. The trigger assembly included. Only thing missing is the fully auto seer setup and the fire selector, of course, Im not stupid enough to try to convert it...Jail time...no can do.