Anyone considered orifice tube conversion? - PeachParts Mercedes-Benz Forum

1980sd · #1 06-15-2011, 01:25 PM

Most US cars I'm familiar with use an OT. I realize that it's not as sophisticated as an expansion valve but it sure would make future servicing easier. There would be the accumulator to plumb, plus some electrical stuff for cycling the compressor...

Just a thought...

A guy on another forum says he did it with good results although not with an R4 compressor and he didn't say if he was using 134 or r12.

Most AC folks I know seem to be more familiar with the OT systems because US cars used them for many years.

Here's a pic of his setup:

Maybe use one of these:

http://www.acsource.com/smartvov-highperformanceorificetubes.aspx

Looks a lot easier to service than the valve in the dash...

leathermang · #2 06-15-2011, 01:46 PM

Yes, they have mentioned it and I beat them about the head and shoulders until they moved on to a different subject....

1980sd · #3 06-15-2011, 02:56 PM

Well, the R4 came out in '74 and by '77 GM had swapped to systems which used OT's. My MB is an '80 so they were still using a system (Expansion valve) that GM abandoned 3 years earlier... Looks like the R4 held in there until '94.

I think most folks would agree that US cars of the period had much better AC systems than ze Germans.

Remember how swell the AC was in '81 Chevettes? Legendary man...

compu_85 · #4 06-15-2011, 03:40 PM

Quote:

Originally Posted by leathermang

Yes, they have mentioned it and I beat them about the head and shoulders until they moved on to a different subject....

For those of us looking to learn more about HVAC, compare and contract the two systems

1980sd · #5 06-15-2011, 04:37 PM

Expansion valve:

Orifice tube:

The expansion valve has little springs and valves in it which can fail, get clogged up, etc. The Orifice tube is just a fixed hole in a tube. It'll still stop up but it has no moving parts.

An Orifice tube system also needs a "accumulator drier" which is kind of a reservoir for the refrigerant so it doesn't get "backed up" in the system due to a non adjustable orifice. The receiver/drier on an expansion valve system is also a type of reservoir but the expansion valve keeps pressures from building up too high.

This is the way I understand it. Someone correct me if I'm wrong or fill in the blanks.

An expansion valve system also has the compressor running all the time when cooling with the valve preventing the evaporator from freezing and maintaining proper pressures.

An orifice tube system cycles the compressor on/off via an electric switch to contend with freezing and maintaining pressures.

Someone else who actually knows something want to weigh in?

My car's expansion valve has an "equalization tube" in addition to the bulb:

Not sure what it's function is but most expansion valves don't seem to have one...

Orifice tube:

leathermang · #6 06-15-2011, 04:49 PM

Quote:

Originally Posted by 1980sd

Someone else who actually knows something want to weigh in?

What am I ?... chopped Liver ?

1980sd · #7 06-15-2011, 04:57 PM

Quote:

Originally Posted by leathermang

What am I ?... chopped Liver ?

Well, is my expansion valve vs orifice tube assessment correct or is I confused

I keep saying I'm going to the garage to dig the expansion valve out... I need to take it out to at least look at the screen...

leathermang · #8 06-15-2011, 04:19 PM

I will do that better after I review my ' Automotive Air Conditioning ' by Crouse and Anglin...
but off the top of my head... you would need an accumulator much larger than what functions as that ..the Rec-dryer on our cars..
Typically they are positioned high near the evap access on the firewall on the passenger side.... any conflicts we can think of with our cars on that point ? Battery ? Turbo ? Access to Starter ?

But I would suggest one item having to do with GM switching over... sometimes... just sometimes.... car manufacturers make changes based on the cost of production instead of the quality of the system..

But I pm'd 1980sd and suggested that he GO FOR IT !!!!
Take pictures and keep tally of the costs...
and the time involved in reengineering the car...

leathermang · #9 06-15-2011, 04:52 PM

That is not what the TxValve looks like on most of our early 1980's MB's.. Ours is a ' block valve '.. .which has the txvalve and the siphoning valve in one ' block'....

leathermang · #10 06-15-2011, 05:00 PM

WHAT ASSESSMENT ? You just described how each worked....
not how one would be of advantage over the other....
What advantage for your car do you see in the orifice tube type AC ?
What disadvantages ?
How much do you think the changeover would cost in money alone ?
How much labor do you think it would take ?

LOL... you have never even SEEN our Txvalve and you already hate it ?

1980sd · #11 06-15-2011, 05:51 PM

Quote:

Originally Posted by leathermang

WHAT ASSESSMENT ? You just described how each worked....
not how one would be of advantage over the other....
What advantage for your car do you see in the orifice tube type AC ?
What disadvantages ?
How much do you think the changeover would cost in money alone ?
How much labor do you think it would take ?

LOL... you have never even SEEN our Txvalve and you already hate it ?

I have seen it. It is in a tiny dark cave behind my instrument cluster clothed in some sort of sticky ghillie suit which helps it remain hidden, and alone... It does not want to be bothered but I shall bother it. I would not want to battle it more than once... Better to banish it and move it's replacement out in the open for future conflicts which do not require special weapons...

I think I covered the moving parts vs fixed parts... part

Labor, well I'm at a point where I'm disassembling the system. Assembly wouldn't be much different/difficult.

Money? Parts would run about the same as a new expansion valve and drier. It would cost more to have the OT holder and accumulator plumbed into the existing lines or have new ones made.

vstech · #12 06-15-2011, 10:21 PM

bottom line. TXV balances load for the compressor, and a OT flows same all the time.
if we had HUGE evaporators and condensers, and a decent accumulator, and pressure switches to cycle the compressor, it could work... I'd prefer to keep the txv and fix the issues with the dirty parts.
FYI. accumulator is to keep from flooding the compressor, receiver is to manage flow of condensed liquid into the txv so it always has perfect solid column of liquid.

1980sd · #13 06-15-2011, 11:49 PM

Quote:

Originally Posted by vstech

bottom line. TXV balances load for the compressor, and a OT flows same all the time.
if we had HUGE evaporators and condensers, and a decent accumulator, and pressure switches to cycle the compressor, it could work... I'd prefer to keep the txv and fix the issues with the dirty parts.
FYI. accumulator is to keep from flooding the compressor, receiver is to manage flow of condensed liquid into the txv so it always has perfect solid column of liquid.

Thanks for the explanation!

I just thought that this may be a good route for someone who has had a compressor failure. An OT coupled with a suction filter would make a "do over" more bearable. (I believe) Seems like there are stories of folks whose R4's ***** the bed and caused a lot of grief. This would at least allow servicing without going deep into the dash board.

After reading about the materials used in the internal parts of an R4 though, it may be "game over" no matter what you do. Seems that the block that the connecting rods are fixed to is made of sintered (or powdered) metal

There's also a counter weight in there that's made of the same stuff

This is mainly a cost cutting process that eliminates some machining processes. Had they used a different metal I don't think they would spew so much fine particulate through the system when parts failed...

Nothing wrong with sintered metal parts. But when they fail they can easily be pounded into dust which is bad for AC internals.

Quirky Mercy · #14 06-15-2011, 10:38 PM

X2

layback40 · #15 06-15-2011, 10:59 PM

X3
A good explanation John.
With due respect to leathermang, he is right. We dont need to try & reinvent wheels. We cannot get inside the heads of the original design engineers.
We have a 57 yo fridge in the shed, still works great. I have no intention of trying to improve its workings.