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Part 1: The Smart VOV (Variable Orifice Valve), is it the refrigerant control of the future?
by I.M. Cool
From November/December 1997 Cool Profit$ Magazine
2000 All Rights Reserved
Part 1a: Refrigerant Basics
Part 2: The VOV (Variable Orifice Valve) versus the FOT (Fixed Orifice Tube)
Part 3: The Flooded Evaporator

Part 1 of 3 of series of interviews with Richard C. Kozinski, inventor of the Variable Orifice Valve

I.M.: Dick, since you are a pioneer in the field of refrigerant control devices, would you set us up with a little background.
Dick: Sure. Factory installed air conditioning became available on American built automobiles in the early 1950's. Today it is standard equipment on most autos and light duty trucks built in this country. Since its inception, automotive air conditioning has for the most part used only two types of refrigerant flow controls: the thermostatic expansion valve (TXV) and, on many vehicles since 1973, the fixed orifice tube (FOT). For reasons I'll explain later, we introduced a new control called the Variable Orifice Valve (VOV) to the aftermarket in mid 1997 as a drop-in replacement for the FOT.

I.M.: What is the VOV?
Dick: The VOV is a pressure actuated valving device which produces significantly enhanced cooling performance at low vehicle speed and at engine idle. Compressor horsepower is also reduced at these conditions compared to systems using a FOT. OEM tests confirm this. The VOV accomplishes this by simply reducing orifice size at idle and low speeds as head pressures rise. This produces subcooling and reduces liquid floodback to the compressor. Physically it is similar in size to the FOT.

I.M.: Many readers might need a refresher course in refrigeration basics. Can you be more specific with subcooling, for instance?
Dick: Sure, but if I have to explain how the VOV works I'm afraid I'm going to throw around many more technical terms. Perhaps, I can define these terms in a separate writing and fax them to you.

Editor's note: Dick did fax the definitions, and they appeared along with this part of the series. To read them, click on: Refrigerant Basics. (Note, we've also linked specific terms in this part for easy reference.)

I.M.: That sounds good, if you don't mind doing it.
Dick: It'll be my pleasure. Subcooling in this context is defined as the amount in degrees Fahrenheit that a liquid is below its boiling point. Subcooled refrigerant per pound has more cooling potential than say refrigerant at its boiling point. And much more than a mixture of liquid and vapor. Subcooling refers to refrigerant at the condenser outlet.

Floodback, in this case, is liquid from the suction accumulator flowing to the compressor. A little is O.K. - enough to get oil back. Too much and performance, durability, and horsepower are detrimentally affected.

I.M.: Doesn't the FOT system have subcooling?
Dick: Yes, at road speeds but not usually at idle. This is the major drawback of the FOT. In fact, it allows a liquid - gas mixture to leave the condenser in many idle conditions, very poor efficiency. How a FOT actually controls flow over a wide condition range is probably not in any popular textbook. (Perhaps I can discuss this in greater detail later.) The FOT is about twice the flow area at idle as it should be.

I.M.: Is the VOV a new concept?
Dick: No, the benefits of a variable orifice have been recognized for many years by air conditioning system designers. In fact, pressure actuated valve designs have been built and tested for over twenty years by the auto and commercial air conditioning companies. But until now none have been commercialized. Frankly, the VOV patents I have studied simply won't work in cars.

I.M.: Why not the VOV sooner?
Dick: R-12 systems of the past produced adequate enough performance with a FOT to keep development of a viable VOV a low priority. Now, with R-134a, smaller compressors, and poor condensing, idle performance on many vehicles is very poor. Foreign makers with good performing TXV systems are gaining market share in the hotter areas of the country. This is making auto companies seriously consider eliminating fixed orifice tubes in future designs. Also, reduced emissions due to lower compressor power will be part of the decision. Fuel economy should be improved in hot ambient city traffic. Environmental concerns loom very large today. An emission test conducted on a 1997 Minivan by Southwest Research Institute showed the VOV significantly reduced emissions. I can send you the actual results.

I.M.: Can you share some cooling test results with us?
Dick: Sure. A major California police agency has been testing the VOV for the past two years in the hottest desert areas of the country. They report a drastic reduction in compressor failures. Idle cooling performance is improved by 10F and more. Canine units, which idle for hours in the desert, report excellent comfort. Previously, dogs required cooling fans and were still uncomfortable. The agency has ordered seven hundred VOV's to upgrade their aging fleet because it made economic sense when potential savings in compressor repair costs were factored in.

Instrumented tests of numerous newer R-134a vehicles with fixed and variable displacement compressors show improvement of 5-12F in hot ambient idles. Vehicles with poor idle condensing gain the most. Big Three test results are confidential. Suffice to say I am working with all three.

I.M.: What about R-134a conversions?
Dick: A conversion from R-12 to R-134a on a 1993 Suburban showed the VOV improved idle performance significantly over the original R-12 system (3F), and 5F over the R-134a FOT at 325 psi condensing pressure. At higher heads the gain was more. Field reports show even greater gains on other R-134a conversions.

I.M.: Will the VOV give equal improvements in all sections of the country?
Dick: No, discharge air improvement in cooler more humid regions such as Florida will not be as great as in hot dry conditions. This is because some of the capacity gain is used in dehumidification. Comfort, however, is greatly improved with this dehumidification. Thousands of VOV's have been sold and reports from installers confirm these results. I have personally tested the VOV in many vehicles. You usually don't need a thermometer to see the gain. You feel it.

I.M.: O.K., you have been comparing the VOV against the very inefficient (as you say) FOT. How does it stack up against a TXV?
Dick: We have compared the TXV against the VOV both in the lab and in an OEM wind tunnel. The VOV outperforms the saturated cycle TXV and competes very favorably with a subcooled TXV system. (More definitions.) The TXV control has a history of failing with dire consequences. Failing open will slug a compressor into submission, while closed it will starve it to death.

The VOV has the advantage of a suction accumulator protecting the compressor against slugging, and it should not fail shut since there is always a designed-in flow path. Evaporator coil refrigerant distribution is better with a VOV since the coil runs flooded (another fax). Compressors run significantly cooler with the VOV due to some floodback. The VOV differs from the TXV in that most of the time VOV parts remain stationary, moving only if head pressures rise high enough. The TXV internal components however, are constantly moving when the system is cooling. Common sense dictates which is more reliable. The VOV is not competing with the TXV in the aftermarket and this is perhaps beyond the scope of your question, so I'll stop.

I.M.: Why not just a smaller FOT?
Dick: The VOV at high ambient idle in our tests doubles the gain of an 0.057" diameter orifice versus an 0.072 FOT which is used on most General Motors cars. The 0.057 raises road speed head pressures exceeding some manufacturer's engineering guidelines. A small orifice at road speeds increases compressor horsepower (I won't go into the reasons now). With a small FOT, airflow on Outside air mode is usually reduced to limit evaporator load so as to manage these higher head pressures. This forces occupants to run on Re-circulation at elevated ambients to maintain comfort. A relatively few factory systems use this size and it is controversial. May I expand on this subject a little more?

Installing an orifice significantly smaller than production intent is risky for the repair shop without knowing condensing capability at high speed under controlled high load conditions, compressor discharge temperature, and charge quantity tolerance. Shops simply don't have the facilities to run controlled conditions like the OEM have. An 0.057" FOT has about 38% less flow area than a 0.072. At equal refrigerant liquid temperature a sizable increase in head pressure (and horsepower) is required to flow enough to still flood the evaporator. Many accumulators will not return adequate oil if no liquid refrigerant is present and compressor failure will result due to lack of lubrication. Running the system on re-circulation will fool the installer into thinking everything is O.K., but problems surface at high humidity outside air operation, especially at extreme car speeds like 80 to 90 MPH. Remember, the OEM's know the benefits of a smaller orifice but in general have stayed between 0.062 and 0.072.

I.M.: What sizes of orifice are in the VOV?
Dick: Significantly larger than 0.057 at full open and significantly smaller at idle. This is all I can say at this time. Something I have failed to mention in your previous question on orifice size is that a smaller FOT requires significantly more refrigerant charge. Environmentally this is bad since more refrigerant will be eventually released into the atmosphere. This orifice size is an oxymoron. I am saying a small orifice reduces compressor idle horsepower but increases road speed horsepower. In a later writing this will become more clear. There are also many subtle factors such as compressor discharge temperature, hose life, clutch cycle rate, etc., which play a part in orifice size decisions.

I.M.: What do you see down the road for future A/C designs?
Dick: In the near term the VOV is going to fight it out with the TXV. I feel the FOT will become obsolete as manufacturers strive for the most efficient, most reliable alternative driven by environmental and competitive concerns.

We will probably see more manufacturers introducing variable displacement or variable speed compressors. There is a move to combine components by the auto manufacturers such as condenser and receiver in one assembly. Desiccants will be replaceable.
In the longer term, air conditioning systems may be hermetically sealed packaged units with more efficient electrically driven high speed centrifugal compressors possessing great reliability. Electric and hybrid vehicles will accelerate these developments. Much of this is just my opinion, of course.

I.M.: Final question. How does the VOV impact repair shops?
Dick: The VOV is an opportunity to increase sales as the potential audience is going to increase. For example, FOT vehicles which cool poorly but had no remedy. Police fleets and taxi cabs are prime targets. People should now not balk at R-134a conversions since cooling will be increased not decreased. There may be less compressor call backs if the experience of the major California police organization is representative. Customers shocked at repair bills will be less hostile if performance is demonstrably improved. A reduction in emissions will make public officials happy. The potential fuel savings is in the right direction for fleet managers.

I.M.: This has been very interesting. I'm really looking forward to getting your definitions and technical explanations.
Dick: Thank you for the opportunity to discuss the VOV. Thirty years ago when I developed the FOT it was done in secrecy. Dissemination of this type of information is very helpful to speed up acceptance of a new product if people understand it. (End of Part 1, to go to Part 2, click here.)

Richard C. Kozinski Editor's note: Mr. Kozinski is an automotive HVAC engineer with over 35 years experience, including over 25 years in commercial HVAC. His masters thesis in 1967 covered the fixed orifice tube system. He wrote this while working for Chrysler Corporation. He co-invented the system with Mr. Ed Bottum, owner of Refrigeration Research. In 1969 he and Ward Atkinson spearheaded the FOT development while at General Motors. He later helped develop the system at GM's Harrison Radiator Division. He is currently the owner of a mechanical contracting firm and is also a consultant to several companies involved in HVAC component development.




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