best way to defrost?

A couple clarifications:

1) A TXV (Thermal Expansion Valve) does not reverse refrigerant flow, a "Reversing Valve" does. The TXV only reacts to the suction line temperature (changes in load) to regulate the amount of refrigerant entering the evaporator. As opposed to a "Fixed Restrictor" which relies on system pressure to regulate the volumn of flow. It enters the evaporator as 80% liquid and 20% vapor. The flashing of the liquid to vapor helps drop the liquid temperature lower than the subcooling temperature. This facilitates the heat pick up in the evaporator.

2) When the refrigerant leaves the compressor it is a high temperature, high pressure vapor. The "change of state" occurs when the ambient air being forced through the condenser causes the vapor to start condensing back to a liquid. This change of state occurs throughout the condenser becoming a solid column of liquid as it exits a certain point, this is where subcooling comes into play. The change of state is stopped prematurely so that "sensible heat" can be removed from the liquid to aide in vaporization when it re-enters the evaporator to start its next cycle. The only time any work is being done (heat pick-up or heat rejection) is when the refrigerant is changing state from a liquid to a vapor, and from a vapor to a liquid. This is refereed to as "Latent Heat of Vaporization, and Latent Heat of Condensation".

3) If there is no humidity in the winter air how could it freeze? Water freezes, not dry air. The colder it gets outside the lower the humidity levels become but even your respiration adds to the cabin humidity levels.

 

1. yes, you are correct, it is a switching or reversing valve, however in SOME common circles it is classified as a TXV (thought you are right, it technically is not a TXV). I was thinking in a less broad terminology, but I should have clarified things more. I agree that as it passes the TXV or orifice, depending on the system, it is more liquid than vapor, but my training has shown that it has achieved vapor state before it passes into the core of the evaporator.


2. Looking back, yes, you are right, the R134a does remain in a partially gaseous form as it enters the condenser. I stand corrected, and admit my error on that point. Technically though, it is not the ambient air passing the condenser that causes the change to liquid state. That simply cools the R134a that has been pressurized.

Of course we have hijacked this thread away from it's original direction. I apologize for that. I was attempting to explain why a vehicle should have it's AC compressor running when it is in defrost mode. Kinda took it off track a bit.

 

1) The metering device is just at the inlet of the evaporator so it can not start evaporating until it leaves the orifice and the pressure drop causes it to start flashing.

2) No offense but you are wrong. The reason for compressing it to a high temperature vapor is so the difference between the ambient air and the condensing gas temperature is great enough to cause it to reject its heat and start to condense.

3) Yes, we went a little off track but understanding the refrigerant cycle will help to understand what is going on when they run their a/c. And since it is a dehumidifier by design, drying the air absorbs the moisture from the windshield quicker.

Sorry for the off topic direction, been an HVAC tech too long I guess.

 

1) Not all metering devices are located right at the inlet. Many are several inches up to a foot from the actual "core" itself.

2) A gas will condense into a liquid at it's specific pressure point. Cooling is not required to get a gas to condense into a liquid, though I agree it can help.

3) We both agreed on this.

I also agree we both have been HVAC techs. Been doing it for over 25 years in the automotive world myself. You may have been doing it longer from what it shows your age to be in your profile.

 

Yes, 35 years and two Master's Cards. I am a commercial tech and have worked on units up to 450 tons (Centrifugal).

 

Wow good thread and lots of info, i should be ASE certified for reading this but still confusing a bit

 

Takes 2 years of experience, and passing a pretty decently challenging exam to get it, but well worth it if you are a career wrench turner.

Failure, congrats on the certifications. I am ASE Master Certified myself doing nothing but automotive AC work. I just wish that the government would do in the automotive field the same thing they do everywhere else, and REQUIRE an auto tech to be licensed. I mean you wouldn't let an unlicensed contractor work on your home, why would you let one work on your car? Those in the industry have been working on it for years, but not getting much headway.

 

Ok, is my truck different from everyone elses'? Because while the A/C does kick on automatically when switching over to defrost (windshield), I can simply hit the a/c button to turn it back off again. Am I missing something?

Back to the OP, one little trick I learned (that seems to work) is on really cold mornings when you have significant frost on the windshield, put your sun visors down while running the defrost fan on high.

 

I cant really concentrate on anything in this thread other than....

 

yupp^^^ and that causes a fogged glass problem.

 

just throwing this out there...


your windshield defrosts in half the time if you put your sun visors down...keeps the warm air circulating on the windshield instead of blowing through out the truck/car

 

Wow bamatoy. My head asploded and now I get to re-learn what I thought I knew all over again... don't you just love days like that.

Thanks for putting up the good info and correcting my false assumptions.

I've only worked with 'dumb' Honda systems - solely regulated by either a thermistor or thermal switch + low pressure failsafe. You obviously have FAR more experience.

 

I'm just here for Bill's sig.

 

You and about 200 other people. If rep were based on sig alone I would be top of the heap by now...lmao. And to think I went through about 10 other sigs before this one! (including 2 sigs that moderators notified me were just a bit too much....hehehe

 

Thank you. I wish I had your knowledge of vehicles, I can work on them but compared to you I am just a rookie.

 

After reading through this thread I checked my 2012 last night...for what it's worth, the A/C doesn't come on automatically when I switch over to defrost on the fan setting.

It does, however, come on automatically when turn the the temp knob all the down to lowest setting (now labeled "Max A/C").

In my previous 2005 Tacoma, the a/c did automatically switch on when using a defrost fan setting, however I was able to manually hit the a/c button and turn it off.

 

That is also a standard feature you will find on pretty much every vehicle on the road because "MAX" AC is just that: Maximum cooling.

 

The light may not come on when you go to defrost mode, but the a/c is on. At least that is how it is on my 2012.

 

Well, as there are obviously several folks on this thread that know their stuff regarding a/c (and to be honest, I don't), here is a simple question, albeit one that may have already been answered in one of the technical posts I was unable to understand fully:

If I were to run my defroster, with the heat turned on in cold weather, do I run the risk of doing damage to the A/C unit if it is indeed switched on with the defrost setting?

This is a topic that interests me because in my last Tacoma my A/C unit completely blew up on my at about 110,000 miles. I'm not saying that running the defroster is what did it...I have no idea; but I would like to be able to keep my A/C running in top condition for as long as possible!

 

heat



or there is always this thing that heats your wiper fluid