Confused by the science

[buickwagon]

I'm still sifting through the old archives and learning. I'm a bit confused by one thing: misting the condenser. I understand that it improves heat exchange efficiency, ensures all the vapour is condensed to liquid and may thereby lower high-side pressures somewhat. In at least two places, I read that misting the condenser will raise vent temperatures. To me this seems counter-intuitive -- I assumed that better cooling at the condenser would lower vent temperatures if anything -- at least, down to the point where the LP switch cycles. Is what I read a typo? Or what am I missing in the relationship between vent temperatures and condenser efficiency?

[Tom Greenleaf]

If for whatever reason the condenser isn't taking compressed gas condensing it to a liquid it won't work for beans for cooling inside.

If system is operable with no wild funky problems misting a condenser should make it work better and output vent temp should improve but not meant to be a freezer.

Click on the 'Knowledge Base" above for a wealth of info and should have fundamentals of how a system should work,

T

[buickwagon]

Click on the 'Knowledge Base" above for a wealth of info and should have fundamentals of how a system should work,

That is one of the places I read that misting the condenser will raise the vent temperature. From the knowledge base article on charging:

When you get the condenser wet, vent temperature is likely to raise. This is normal. You'll also note pressure drop in both gauges.

By "vent temp should improve", I take it you mean "vent temp should get cooler"? (within the limits of what the system can do, of course). Is the knowledge base statement a typo?

[Tom Greenleaf]

That's a mistake/typo. Pls tell me where it is and I'll try to fix that. Output temps will go down with water on condenser which is all important to getting the most out of a system.

Dirt, a strange problem, tons of bugs and check in between radiator and condenser for any unseen dirt/debris messing up the show.

I can make my own vehicle fog the windows on the hottest days and prefer to work outdoors over unshaded asphalt which is much hotter than the day's forecast temp. Neat items I use are the silly in/out temp things that are usually fast and can set one on dashboard and another if needed in front of grille. Wanna spend some big bucks get an infrared thermo but the cheap ones can work but can mess you up if windy or just have an attitude

You really should feel if safe the progression of condenser working and not hot or cold spots indicating a problem. Some won't flush out and some are not that expensive but are the heart of how many BTUs of heat exchange you will get which is the name of this game.

This was never my primary anything for repair or service. Can't count on it being warm enough or laugh when we have a heat wave I can't take being in it to work under a hood!

Just go easy or get help. Mistakes can cost you a fortune,

Tom

[buickwagon]

That's a mistake/typo. Pls tell me where it is and I'll try to fix that. Output temps will go down with water on condenser which is all important to getting the most out of a system.

http://www.aircondition.com/tech/questions/95/Basic-Charging-Procedures In step 11, near the bottom of the page.

I can make my own vehicle fog the windows on the hottest days and prefer to work outdoors over unshaded asphalt which is much hotter than the day's forecast temp.

I don't think I'm going to have that opportunity until next spring. Once the gauges and vacuum pump arrive, I'll open the system, install the new condenser and accumulator, pull a vacuum and charge with coolant / oil. Fine tuning will have to wait for warm weather to return.

Neat items I use are the silly in/out temp things that are usually fast and can set one on dashboard and another if needed in front of grille. Wanna spend some big bucks get an infrared thermo but the cheap ones can work but can mess you up if windy or just have an attitude

I have an IR thermometer, but I like the indoor/outdoor thermometer idea. I have a battery operated wireless one that might fill the bill. It also records the min/max of each.

Just go easy or get help. Mistakes can cost you a fortune,

That's why I'm reading Thanks for your time and assistance.

[Tom Greenleaf]

Just a note: Those in/out thermos (wired ones) are fast cheap and accurate. Hey, site lives on selling stuff at your discretion but those dumb things work great!

Face it, if you don't have the temps the pressures are near useless.

80F is real nice. lower than 60F is real confusing at least to me. Less than 60F is near useless IMO. Done it but wasted a ton of propane to heat a car up headed south with an empty system and was literally 0 degrees outside so had to cook my shop to get a clue of performance! Never heard another word so it must have worked? Had shop at 100F but all the tools took forever to warm up.
Was and accident fix and local shops couldn't do that. Generally shops heat to as bad as 40-45F in Winter. Trust me, it suks for a while. Look at my avatar --- real! there's a car in that mess!
Smile,

Tom

[buickwagon]

I know what you mean. We have the same problem up here too: Air conditioning is nice, but not really necessary for 6 months of the year.



I didn't even get to unwrap the central air this summer -- Oh well, one less thing I have to do this fall...

[Tom Greenleaf]

Nice pic - hope all were well,

Tom

[buickwagon]

Nice pic - hope all were well,

Does cabin fever count?

[Z2TT]

Yeah looks like a typo on Step 11.

To me spraying water is a means of getting the condenser to do it's job quicker (during the charging proccess), so you can
get the system down to stable operation and have a better idea of how the system is operating? Would that be the whole reason
for spraying water during the charge process?

[buickwagon]

By implication then, the inverse must have an effect too. That is, the vent temperature is probably affected by the humidity as the system tries to cool the moisture, in addition to the air. Googling that theory seems to support it, but I can't find any information on how much affect will it have in a real world auto air conditioner. Will it even be noticeable?

[Tom Greenleaf]

The action is "change of state" of liquid to gas. Two calories per change of state. If you don't get that send the job out.

Can takes years to learn this stuff and mega thousands equipment to do it!

Pay for it if no clue,

T

[buickwagon]

Two calories per change of state.

Huh?

A calorie is the amount of heat required to raise the temperature of 1 gram of water 1°C (or the amount of heat released when 1 gram of water is cooled 1°C, if you prefer). The change of state for water (aka the latent heat of vapourization) is about 32,524.6 calories per gram. So obviously it takes a hell of a lot more energy to condense a gram of water from vapour to liquid than it does to cool a gram of liquid water another degree. I get that. But to calculate the precise figure, one would have to know the precise airflow, absolute humidity, radiant surface area of the evaporator, cooling capacity of the system, etc. which I don't think any auto AC tech is likely to worry about.

As I read the archives, I see many references to vent temps, underscoring the importance of recording the vent temp at the centre outlets as part of the troubleshooting process, and of course the issue of misting the condensor as a troubleshooting technique. So, does the humidity seriously skew the vent temperatures or is it not enough to worry about, in the considered and considerable experience of those professionals who are willing to help me learn this stuff. In other words, is there a rough, seat-of-the-pants fudge factor that you might use -- eg: "it's very humid out today so adjust the expected vent temp up 5°", or "I'm working in the Arizona desert, so I adjust the expected vent temp down 10°"?

[Tom Greenleaf]

Change of state: Water for example is both or either liquid or ice at 32F! To change from either to the other without changing the temp takes TWO calories then it's one to one. A/C is capturing the same idea from liquid to gas which also take two calories.

Think: When you sweat in hotter temps than you body that change of state is keeping you cool at normal body temps all within reason.

Understand dry AZ temps. Been there and Vegas at 115F!

Said: You about need a college degree to understand all or enough about A/C. Then there's insane costs of tools to do it!

Best learning is hands on and always was. Great as the web is it's NOT going to solve everything and if temps are high enough that it's dangerous get help,

T

[buickwagon]

Change of state: Water for example is both or either liquid or ice at 32F! To change from either to the other without changing the temp takes TWO calories then it's one to one.

Yup, water can be either at the freezing point. But the change of state is even worse than 2:1. The latent heat of fusion for water is 79.2 calories per gram -- almost 80:1.

At 100% relative humidity, 30°C air contains 30g of water vapour. Cooling that to 5°C condenses 23.2g per cubic meter, releasing about 754570 calories in the process -- call it 3,000 btu for round numbers. By comparison, at 50% relative humidity that same cubic meter of 30°C air contains 8.4g of water. Cooling it to 5° air at 100% RH releases 273786 calories or about 1085 btu.

So the math seems pretty straight forward so far.

However, even I now know the compressor doesn't run 100% of the time, it's controlled by temperature/pressure rather than BTU or caloric output so it cycles off from time to time. There must be a lag of some sort in the convection of the evaporator. Is that lag sufficient to affect the net vent temperature significantly when comparing humid air to dry air?

[Tom Greenleaf]

In short: Evaporation of a liquid to gas takes away heat which is what A/C is all about,

T

[buickwagon]

Yes, I absolutely understand that. I'm starting to wonder if we're talking apples and oranges here.

Here is my understanding: The evaporation of the refrigerant in the evaporator requires the input of heat energy, which it draws from the air blowing over the fins and tubes. But at the same time, the condensation of water on the outside of those same fins and tubes releases heat energy. Only the difference between the energy absorbed inside the evaporator and the energy released outside the evaporator is available to draw heat from the air itself -- as measured by the difference between the vent and ambient air temperatures -- and therefore the efficiency of the system.

Since the troubleshooting process is concerned with establishing and tweaking the efficiency of the system and since the measurement of the vent temperature is used as an indicator in that process, does it follow that an allowance should be made for that external condensation process (due to humidity) when considering the vent temperature as an indicator? Or is there sufficient surplus cooling capacity in the system that the added inefficiency is irrelevant?

The question is not entirely academic. Many posts in the archives seem to run along the lines of "the air coming out of the vent was not very cold, so I added a can of Freon", and I have to admit that before I found this forum, my knowledge was barely above that level too. Now I know that if the pressures are correct, vent temperature can be an indicator of a low charge; ie. the evaporator is only partially filled with liquid. It could also be an indicator that the temperature door is not sealing properly or that the air flow over the condenser is obstructed or that there is too much oil in the system. Is ambient humidity another factor or can I ignore it?

[Tom Greenleaf]

What you want is the exact right amount of refrigerant. Too little can burn out compressors. Too much is not good for anything either.

When not known amounts:

Look for low side in the high 20sF to low 30sF. High side about 2.2 X the temp of air coming in thru grill. Best measurements are mid day at 80F but there are formulas for higher temps. Below 60F forget it unless you have the eqipment. Mistakes cost 10 times doing it right - know that.

Humidity: Yes it can change effectiveness but in my own don't notice. Not that dry here so always have condensate and if vents aren't to expected temps usually low refrigerant and DON'T USE IT! No oil to compressor and it and the system is junk!

Leaks, easy or hard are #1 cause of problems but could be anything.

Concise as I could be with this post,

T

[Z2TT]

Yes it's a fact humidity put's a much higher load on the system, since your trying to cool down moisture (well an elevated amount) and air aswell that is passing through the evaporator. This moisture is a parasite so to say. Once this moisture is cooled down it drains out of the evaporator tube onto the road and does not contribute to making the cabin cooler, only the air that reaches your face does. So on noticeably more humid days you would expect vent temps do be higher somewhat but not to the point that the system does not "cool"

[Tom Greenleaf]

Well put "Z"

It gets humid here too but sure doesn't feel like some places in deep mid South of USA or other places. Condensate is normal with an operating system. Never been anywhere so dry that I didn't see it and be normal. As said, like sweating off skin if it can't do drier air it's not going to work as well,

Tom