A/C compressor cycling fast

[Bob Ayers]

So Trex, how much $$ for the switch? Oh crap, bet you gotta de-pump the whole system, right?

The last one I replaced years ago screwed onto a schrader valve, so you didn't loose the system charge when the switch was replaced.

 

[TrexMex]

That's what I'm thinking too, i.e. that the pressure sensing switch would be designed to fail-safe in the direction of triggering early, rather than late or not at all.

So Trex, how much $$ for the switch? Oh crap, bet you gotta de-pump the whole system, right?

I wanna take MAKG's advice, but then again we're guys, so we all have to overcome the urge to "mess with it", eh?

Additionally, I have manual tranny, so even at boulevard speeds (45mph) I can FEEL it cycling via "the seat of my pants"... this triggers the Sgt. Schultz voice in my head... "zer izz ZUMthing wr-r-r-rong wiss zee mechaNISM!"

lol!!!!
.............

OK, I had to control myself not to keep laughing before starting to write this... people started to look at me funny here at work...

It shouldn't cost more than 30 bucks... gotta get back to you on the de-pump.. anyone?????

I guess one of the things I love about my truck its that since day one it has given me the chance to overcome that urge you're talking about....quite often......

"There is a schrader valve built into the fitting on the receiver/dryer (accumulator) where the LOW PRESSURE CYCLING SWITCH is threaded onto. You do NOT have to discharge the system to change the switch."

Got this quote from another forum... googled it......

Did not remember because I serviced the Compressor back then as well so I took it off the truck and, of course, did the system de-pump.....

 

[CraigK]

Interesting thread.
Here's my thoughts . . .

Humid air easier to cool?
While water may "transfer heat" ??? better than air (yes - the specific heat of water is much higher than that of air), an automotive or home air conditioner condenses OUT the water vapor in air, using a tremendous amount of cooling capacity to do so (look up the definition of latent heat). This is why automotive (and home) air conditioners drip water. This condensed water can then even freeze (again, more cooling required - cooling NOT being used to cool air, but to change the state/phase of water - again, more latent heat). When the auto is shut off, the frost & ice then melts. Look for the puddles of water under cars parked in a shopping mall on a hot summers day. This is also why the MAXIMUM AC setting on autos shuts off external air and recirculates cabin air - as it dehumidifies the air and recirculates drier air, it becomes much easier to cool. (BTW - A dehumidifier is exactly the same as an air conditioner.)

When the compressor starts cycling as described by StreetRanger (10s on, 10s off) on either my two old R-12 to R-134a retrofitted A/C systems (87 BII, 89 Integra) or my one original R-134a system (98 Windstar), I know it's time to top up with R-134. Usually less than half a can, and the excessive cycling is gone. It certainly could be a pressure switch, but I'm with 92MiniMonster on this.

Try it. You'll like it.
Go to Walmart.
By a can of R-134 and a charging hose with a gauge (maybe $8 + $12???)
Add the freon.

CraigK
P.S. Adding freon to an AC system is not an example of some is good, so much more must be much better - do not overcharge!)

 

[TrexMex]

way too complicated for a thread but ok... in a nutshell..

Completely agree with you craigK... but you have to remember... an AC system is 2 parallel systems in one.... on one side you have the condenser, which works with ambient air... the lower the air temp on that side the lower temp you will get in the refrigerant. Water helps on that side because it brings down air temperature by absorbing the heat present in air (dry bulb/wet bulb). once you achieve that, refrigerant gets in contact with cabin air by means of a separate air circuit that is being cooled down with a separate heat exchanger (evap) which will "trade" heat between the refrigerant and cabin air bringing down its temp. So its just a matter of having a colder fluid absorbing more heat from the cabin... cooling is a technicism, as what you are actually doing is removing heat from the cab. And yeah what you do by lowering temp is getting rid of humidity as well because bringing down the air temp to its dew point causes water to condense. Then again we're talking about two different circuits.

The humidity principle is used on air coolers on dry enviroments where by raising relative humidity what you "feel" is wet bulb temp which is lower than dry bulb.

The magic word here is Psychrometrics... NOT psychometrics....

Geez called it for the day and still talking about work... fun!!! Hope I didn't confuse you more, though......

 

[CraigK]

TrexMex

Hmmmm . . .

"Two parallel systems"

Okay.
How about . . .
Parallel, possibly, - but not the same.

On the AC system side you have a condenser/evaporator system.
(Freon circulating while alternately evaporating and condensing).
Cooling, or yes, removing heat, from the evaporator side.
On the cabin side of the auto, you have a simple ambient air cooling system by mean of a heat pump/exchanger (air blowing through the AC evaporator fins).

Unless you have water condensing on the cabin side - like from humidity.
Then you have two similar systems not so much working in parallel, but working AGAINST each other.
Cooling generated by the AC side by freon evaporation is simply absorbed by water condensing on the cabin side.
Cooling - removing heat, used to condense water rather than cool air.
(BTW - the latent heat of vaporization of R-134a is about 1/10th that of water, i.e. you must evaporate about 10X amount of R-134 to condense 1X amount of water).

"the lower the air temp on that side the lower temp you will get in the refrigerant"

Yes. The Carnot efficiency of heat transfer is INVERSELY proportional to the temperature differential, but in °K. (And why counter-current heat exchangers are most efficient.) But why would the temperature necessarily be lower with water? Any even if it was, if the nominal increase in efficiency (remember, a difference in K° is relatively small) is used up condensing water (lots of cooling required), where is the gain?

All this physicsbabble is moot unless the AC system in question is working at maximum capacity. When freon is being recirculated at the maximum rate, and the compressor is on 100% of the time, cooling capacity is being maximized. If the ability to cool air to the desired temperature requires maximum functioning of the AC system (as on my retrofitted little Integra on a hot day!), diversion of cooling capacity to condense water vapor into liquid water and/or frost/ice only results in reduced cooling capacity of the system. If the heat absorbed (or "cool created") by the maximum evaporation of freon is being used to condense water, there is simply less available to cool the air.

The wet bulb of a psychrometer gives a lower reading than the dry bulb because - the cooling effect of water evaporation!, not the opposite - water condensation. And cooling fans inject water mist into the air stream to cool, because the water evaporates (absorbs heat& cools, not condenses). AC systems remove the water vapor.

Both my Integra and BII AC systems struggle on hot muggy days relative to equally hot, but less humid days.

Back to you . . .
I'm sleepy.

CraigK

 

[fixizin]

That's great design... Schrader valve isolation... gauge is only $12, and that'll tell me if I need the $8 can of r134a+oil+additives, or the new switch, only $12... sweet.

That frequent cycling can't be good, plus if it's low on refrig, it might be low on oil too... bad for o-rings, compressor, etc.

 

[MAKG]

Frequent cycling on an automotive A/C system can be completely normal, especially if it isn't all that hot out.

And NO additives. Bad, bad, bad.

As for the thermo argument, it's entirely true that the air conditioner capacity is used to condense interior air water vapor. But the working temperature of the refrigerant -- which determines how "hard" the compressor is working -- doesn't care (much). It cares that the condenser is being cooled more effectively to ambient.

Retrofit R-12 systems are very often running at capacity all the time. The condenser was sized for R-12, but it's being used for R-134a.

 

[TrexMex]

"Two parallel systems"

Okay.
How about . . .
Parallel, possibly, - but not the same.

Granted, I should have said two "separate" systems using the same transport fluid circuit... (R-12 or R-134)

Cooling generated by the AC side by freon evaporation is simply absorbed by water condensing on the cabin side.

Cooling - removing heat, used to condense water rather than cool air.

I have to disagree with this. Water condensation is a result of getting the Humid air to its Dew Point, given by it's relative humidity and pressure (we're talking about air at ambient pressure here, so one less variable to throw into the mix). Yes, on the "heat pump" side of the system Relative Humidity will work in favor of the system because water "stuck" on the condenser tubes/fins will get cooled by the air "forced" through by the cooling fan. This is a convection/conduction phenomena that I'm pretty sure you know about just by reading at you explanation (Which is very nice, BTW).

Yes. The Carnot efficiency of heat transfer is INVERSELY proportional to the temperature differential, but in °K. (And why counter-current heat exchangers are most efficient.) But why would the temperature necessarily be lower with water? Any even if it was, if the nominal increase in efficiency (remember, a difference in K° is relatively small) is used up condensing water (lots of cooling required), where is the gain?

OK let's see...

Q = W*Cp*deltaT

Given this, the bigger the temperature difference (granted flow rate and Cp is constant), the bigger the Heat given/substracted from the system, right? this means that deltaT will actually be DIRECTLY proportional to Heat transfer. Heat Transfer Efficiency (i.e. "effectiveness") on the other hand is actually just an indication on how much energy (entropy) you are loosing in the whole process. This would be different for the Carnot Cycle and the evaporator alone... it all depends on your frame of reference (control volume).

Yes, you will have a difference because of latent heat effects, though it will be modified by the fact that you have both a phase changing fluid and air in the mix on one side and refrigerant changing phases on the other (where you could start getting fancy or just assume ALL the refrigerant goes through phase change, which is close to true).

On the other topic, °K is just °C + 273.15 to take it to absolute scale, meaning that deltaT in °C = deltaT in °K. So a difference in K is not relatively small compared to difference in C, it's exactly the same difference. Same applies to F and R; depends on wether you're doing US or SI units.

Counter-Current heat exchangers are the most efficient, yes. Still in most of the cases the thermodynamic improvement between a crossflow and a counterflow hx is so small that having a counter becomes a little bit too impractical, given that most of the time -with the exception of idling or very low speeds- your cooling air comes from air ramming the front of your truck/car.

I agree with you on the fact that the AC works on it peak efficiency when working at max. capacity, but you don't need that all the time. I would imagine the Pressure switches (and thermostats on the new, fancy Car A/C systems) are there for energy saving purposes, and as failsafe devices. You don't want a line to burst because of overpressure and you don't want to burn more gas than necessary to get a comfortable enviroment inside your truck/car.

I have to recognize car AC systems are not my area of expertise, but I do design and analyze heat exchangers as part of my job, and I can tell you water injection in industrial gas turbines and in aviation is a very common practice to increase system efficiencies (and save fuel).

On your last statement, you're right. Water evaporation will substract heat from the air, taking it elsewere. as long as the Air is not stagnant, the enviroment will be cooler up to the point where the water/air deltaT allows it to be.

There... Thanks!! I'm really enjoying this discussion.

Trex

 

[TrexMex]

As for the thermo argument, it's entirely true that the air conditioner capacity is used to condense interior air water vapor. But the working temperature of the refrigerant -- which determines how "hard" the compressor is working -- doesn't care (much). It cares that the condenser is being cooled more effectively to ambient.

And THAT's how you explain all my babbling in a nutshell.....

But again air water vapor condensation is a side effect of air cooling, not the original intent.

 

[fixizin]

And NO additives. Bad, bad, bad.

Then why do all the DIY r-134a recharge canisters seem to include "additives"?

 

[MAKG]

Then why do all the DIY r-134a recharge canisters seem to include "additives"?

You considered buying them, right?

That's why.

People think they can actually repair a leak on an air conditioner without opening it. The only way that actually works without making circulating junk is to replace the leaking O-ring (or whatever).

"Recharge cannisters" are also a VERY bad idea, as it is next to impossible to avoid overcharging.

 

[fixizin]

I'm talking about the "kits", with cheapie gauge. Besides, if a small amount of Freon has leaked out in 9 years, probably a small amount of oil has gone with it, even if not visually apparent.

Guess I need to get out my articulated mechanic's mirror and go take a careful tour of the loop...

But hey, I resisted messing with it... one day at a time... lol.

 

[bens]

I just replaced the condenser on my 02 ranger, It also cycles on and off frequently at idle while in park. Work fine at higher rpm's, I thought it was one or both of the pressure switches not working properly, but it wasn't.
Don't just go and replace the switches, each switch can be bypassed with a small piece of wire, this will tell you if its the switch or not. I also know its not because of insufficent R134A, mine is charged properly. Does anyone know if this is normal at idle? or if there is a switch in the evaporator to prevent it from freezing up that might be malfunctioning?

 

[MAKG]

It's a function of ambient heat.

If it's hot as F**K, it's not at all unusual for the compressor to be on continuously at idle.

And a jumper wire will only tell you if the switch is cutting the power. It's by FAR the most likely to, under both normal and abnormal circumstances. It's only bad if it's WRONG. You can only tell this with a low-side gauge. Even if it switches at the wrong pressure, it can be adjusted, as long as the pressure is constant.

If you really do have low refrigerant and you jumper the switch, you can kill your compressor quite quickly. Do this only BRIEFLY, preferably AFTER measuring the pressure.

 

[jagar147]

My a/c compressor seems to be cycling very quickly. It kicks on for about 5-10 seconds and then turns off for about the same amount of times. The air blows cold and seems fine but this cycling seems a little abnormal to me. I just don't want to ignore it and it be an issue so I figured I'd post and see what ya'll thought.

Sounds like it's low on freon or oil,could also be a leaking hose