Anyone considered orifice tube conversion? - Page 2 - PeachParts Mercedes-Benz Forum

leathermang · #16 06-15-2011, 11:58 PM

I am afraid the concept simplification has gone so far that generalizations are producing muddiness......

Specifically I perceive that some assumptions have been made that orifice type systems are automatically of the non cycling clutch variety.

From my Auto AC Book :

"
1. Cycling - clutch systems
a. With thermostatic expansion valve (TXV)
b. with orifice tube - the cycling clutch orifice tube system (CCOT)
2. Evaporator - pressure control system
a. with suction - throttling valve or pilot operated absolute
(POA) valve
b.with valves in receiver ( VIR).

... On the CCOT type it says " The accumulator has a bag of desiccant to absorb moisture from the refrigerant. There is no sight glass in the CCOT system. The accumulator takes the place of the receiver - dryer."

1980sd · #17 06-16-2011, 12:12 AM

Quote:

Originally Posted by leathermang

I am afraid the concept simplification has gone so far that generalizations are producing muddiness......

Specifically I perceive that some assumptions have been made that orifice type systems are automatically of the non cycling clutch variety.

From my Auto AC Book :

"
1. Cycling - clutch systems
a. With thermostatic expansion valve (TXV)
b. with orifice tube - the cycling clutch orifice tube system (CCOT)
2. Evaporator - pressure control system
a. with suction - throttling valve or pilot operated absolute
(POA) valve
b.with valves in receiver ( VIR).

... On the CCOT type it says " The accumulator has a bag of desiccant to absorb moisture from the refrigerant. There is no sight glass in the CCOT system. The accumulator takes the place of the receiver - dryer."

The CCOT system is what GM adopted in '77 when they abandoned the TXV. It's my understanding that cycling the compressor replaced the action of the TXV. (The R4 compressor was in service from '74 through '92. Many of them failed in the early days because the TXV put too much strain on the rotating assembly.)

I made that last part up

leathermang · #18 06-16-2011, 12:29 AM

I AM afraid you have lost sight of what you do know and what you are making up....
The TXV cycles the compressor. The CCOT also cycles the compressor.
the latter has an orifice tube.

1980sd · #19 06-16-2011, 12:45 AM

Quote:

Originally Posted by leathermang

I AM afraid you have lost sight of what you do know and what you are making up....
The TXV cycles the compressor. The CCOT also cycles the compressor.
the latter has an orifice tube.

Don't think so Mang. The TXV just changes the orifice size, compressor keeps chugging along when the AC is on. (This is in a W116 with ACC, some are probably different.)

With an CCOT a switch cycles the compressor clutch. (This is in your typical malaise era GM)

There are no electric goodies in my TXV that could disengage the clutch. There are electric goodies in a CCOT system that can engage/disengage the clutch.

I suggest you take a nap with your AC book under your pillow...

Something I found that may clear the air:

AUTO A/C TERMS - GLOSSARY
Cycling Clutch Orifice Tube - CCOT A/C System

General Motors introduced the cycling clutch orifice tube A/C system in the 1977 model year. The unique advantage to having a cycling clutch with a fixed orifice tube was that now, the only moving part on the A/C system would the A/C compressor. The fixed orifice tube, which provides a one size, fixed opening for the flow of refrigerant into the expansion valve had replaced the typical expansion valve or TXV valve that opened and closed to control the refrigerant flow into the evaporator.

The fixed orifice tube means that there is always a flow of refrigerant into the evaporator. In order to control the evaporator pressure and maintain the coldest possible temperature in the evaporator without freezing the condensation, the system was designed to cycle the compressor on and off.

The cycling clutch system meant that the typical normal pressure gauge readings of a properly operating system would have a wider range. Of course, the low side pressures would be at their lowest just before the compressor cycles off while the high side pressures would be at their highest. As soon as the compressor clutch disengages, the high pressures start to drop while the low pressures increase. That happens because the fixed orifice tube is always allowing refrigerant to flow through it, regardless of whether the compressor is on or off. As soon as low pressures began to rise causing a higher temperature in the evaporator, the compressor would cycle back on to pull the pressures and temperature down on the low, once again increasing the pressures on the high side. The cycle continues in a controlled manner that maintains mean or average pressures in the evaporator so that the A/C system can provide maximum cooling performance with the minimum number of moving components.

Understand that the CCOT or fixed orifice tube system is still slightly different than typical cycling clutch systems. In other system designs, expansion valves are still used to control the flow of refrigerant into the evaporator. Those typical expansion valves open and close to monitor refrigerant flow while the compressor cycles on and off to maintain proper evaporator pressures. The orifice tube system utilizes the fixed opening in the orifice tube which maintains a constant flow of refrigerant.

This does mention cycling with a TXV...

leathermang · #20 06-16-2011, 01:23 AM

Quote:

Originally Posted by 1980sd

Don't think so Mang. The TXV just changes the orifice size, compressor keeps chugging along when the AC is on.
There are no electric goodies in my TXV that could disengage the clutch. There are electric goodies in a CCOT system that can engage/disengage the clutch.
This does mention cycling with a TXV...

We are talking about a TxValve SYSTEM ... that does not mean the electrical control over the clutch has to be INSIDE the txvalve... You are comparing a CCOT System to a Block Txvalve ... system to item... orange to apple.
Where does the wire which connects to the compressor clutch on a Txvalve system attach TO ?

lutzTD · #21 06-16-2011, 07:40 AM

I am now pricing home HVAC systems and there is an additional cost of adding a transfer valve to the system to make it more efficient and they show the efficiency differences in the systems in their literature.

compu_85 · #22 06-16-2011, 09:10 AM

Thanks for the info guys. So, why do our Benzes still cycle the clutch so often? Shouldn't the valve help prevent that?

-J

leathermang · #23 06-16-2011, 09:17 AM

There is nothing wrong with the clutch cycling... what home AC's do it have very small units ( also hermetically sealed ) which run most of the time... that is because they are dealing with the same air run through the system over and over again... and they consider dropping the temp 6-8 degrees each time it goes through just fine...and it is... with the added effect of dropping out the moisture so you feel comfortable..
A system for a car has to be much more powerful... able to deal with the car sitting in the sun... having windows all around.. not insulated very well compared to a house, and the doors opened and closed regularly.. so they have a much stronger system... to cool you down fast and well .. hopefully ... when you get into the car... but as you travel down the road... hopefully with the doors closed... it can relax some and when it is producing more than the inside of the car needs then it cycles off... which is good for fuel mileage too... ( later systems typically use compressors which take less total hp to run )...
So, the on and off situation is just built into the design and is not a problem...

pmckechnie · #24 06-16-2011, 10:24 AM

First, the R4 compressors were fine as long as everything was perfect. If the system got low on freon, there was not enough freon flow to supply oil. The R4 didn't have an oil sump so had to have good freon flow at all times. Thus, the cycling switch. If the expansion valve closed for a long time, the compressor was history. The older TVX system used the A6 cpmpressor which had an oil sump and could run for long periods of time with little flow. In fact, on the 1972 Oldsmobile, once the compressor was turned on for any reason, it stayed on until the ignition was turned off. Now they use variable displacment compressors that run all the time if the A/C system is on,on some cars.
One thing I can't explaine about the OT systems is they moved the OT to the outlet of the condensor or half way between the condensor and the evaporator. Any one have any idea why they would do that?

Paul

1980sd · #25 06-16-2011, 10:53 AM

Quote:

Originally Posted by pmckechnie

One thing I can't explaine about the OT systems is they moved the OT to the outlet of the condensor or half way between the condensor and the evaporator. Any one have any idea why they would do that?

Paul

You can hear them "hissing" under the dash so maybe it was to keep the cabin quiet?

leathermang · #26 06-16-2011, 11:00 AM

Quote:

Originally Posted by pmckechnie

First, the R4 compressors were fine as long as everything was perfect. If the system got low on freon, there was not enough freon flow to supply oil. The R4 didn't have an oil sump so had to have good freon flow at all times. Thus, the cycling switch.

Paul

Actually your description should read " Thus the low pressure cutout switch " because the cycling switch is different and works just fine with correct amounts of refrigerant in the system.

leathermang · #27 06-16-2011, 11:11 AM

Quote:

Originally Posted by pmckechnie

....One thing I can't explaine about the OT systems is they moved the OT to the outlet of the condensor or half way between the condensor and the evaporator. Any one have any idea why they would do that?
Paul

I can make some guesses based on what is and is not required in a type of system... Our BLOCK TXvalve system is very smart in that the place where the liquid is changed to vapor in order to absorb heat from the air passing over the fins of the evaporator is right at the same place to most accurately measure the temperature of the refrigerant coming out of that same area where it is important that it NOT get below freezing so as to not put frost on the fins and stop the passengers from getting cold air...
Very efficient.. it is two valves in one and with added input from temperature and barometric pressure... to tune it as to the freezing point..which varies from 32 degrees...
But you do not have to have the change from liquid to vapor at the input of the evaporator... since it is only the interaction of the warm air flowing across the fins of the evaporator with the vapor in it that produces the needed result..
Thus, once you add another method to keep the evaporator from freezing ( a separate suction throttling valve or electrical pressure or temp sensors ) ... there may be an advantage to placing and servicing of that long tube by locating it somewhere in between the condensor and the evaporator..

compu_85 · #28 06-16-2011, 11:20 AM

Quote:

Originally Posted by pmckechnie

Now they use variable displacment compressors that run all the time if the A/C system is on,on some cars.

My VW has this. Newer (2005+) VWs take the variable displacement compressor one step further... there is no clutch. The climate control system varies the compressor duty cycle down to 0% when AC isn't called for, and anything up to 100% to keep the evaporator at the right temp.

-J

leathermang · #29 06-16-2011, 11:23 AM

Quote:

Originally Posted by compu_85

My VW has this. Newer (2005+) VWs take the variable displacement compressor one step further... there is no clutch. The climate control system varies the compressor duty cycle down to 0% when AC isn't called for, and anything up to 100% to keep the evaporator at the right temp. -J

Is the compressor run by electricity ? As compared to by the engine ?

compu_85 · #30 06-16-2011, 11:35 AM

Quote:

Originally Posted by leathermang

Is the compressor run by electricity ? As compared to by the engine ?

No the compressor driven by the serp belt. The duty cycle is controlled electrically.

In my car the duty cycle is controlled by the pressure of the refrigerant.

-J