11.06.2003, 09:45
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The Smart VOV™ (Variable Orifice Valve), is it the refrigerant control of the future?
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 10°F 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 5-12°F 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 (3°F), and 5°F 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.
In order to understand how the VOV increases idle cooling over the FOT, a basic understanding of how a FOT functions is helpful.
Conditions. Let us assume that at 70 MPH and 100°F outdoor air temperature, the head pressure of an R-134a system is 225 PSIG, with 20°F subcooling at the condenser outlet.
At those conditions the FOT provides a flow of 7 lb./min. to the evaporator.
Now bring the vehicle to idle. The hot under hood air re-circulates into the condenser. That, plus the reduced air flow to the condenser causes the head pressure to rise to 350 PSIG. Now, with a higher pressure, you would expect the FOT to flow more refrigerant, correct? But it actually flows less. Why does this happen?
First, understand that the flow through the FOT is dependent on: 1) Head pressure, and 2) the state of the refrigerant (whether it is subcooled or has quality (contains vapor). Note that suction pressure in normal ranges has no effect on flow. While refrigerant flow increases with increased head pressure or increased subcooling, vapor at the orifice reduces the flow significantly.
Back to our slowed vehicle. Momentarily there will be much more refrigerant flowing from the condenser than is flowing in from the compressor. This causes the subcooled liquid to be flushed from the condenser, after which some uncondensed gas (vapor) flows from the condenser as well. As that vapor enters the orifice tube it slows the flow rate, which is now down from 7 to between 5 to 6 lb./min. (even though the head pressure is much higher).
The resultant is that cooling capacity is roughly one half of that at 70 MPH because along with less flow, less liquid exists in each pound of refrigerant (the percentage of vapor increased). This illustrates how a FOT is self regulating but becomes very inefficient at idle.
The orifice tube expands the refrigerant to a lower pressure and temperature and this mixture enters the evaporator. The expansion process creates flash gas which does no cooling. Only the evaporation of the liquid in this mixture does cooling. A subcooled liquid entering the orifice tube will result in a much higher liquid percentage after expansion as compared to a vapor or quality entering the orifice tube.
Now the VOV. From this it is obvious that to increase cooling performance at idle, subcooling must be increased. This is the prime purpose of the VOV. It accomplishes that feat by decreasing the orifice size to roughly one-half the 70 MPH flow area when at idle. The resultant is 10 - 30°F increased subcooling during idle (with demonstrably improved discharge temperatures…). Also, this improved cycle efficiency significantly reduces horsepower required by the compressor, which improves city traffic fuel economy and exhaust emissions.
What does a “flooded” evaporator mean to you? Has the car been under water from an El Nino flood? Of course not. First of all, the evaporator core cools the air inside the car. The term “evaporator” is used because cold liquid refrigerant is evaporated in this heat exchanger and cools the tubes and fins which actually cools the air. “Flooded” means that the evaporator core is designed to operate under conditions where some liquid leaves the evaporator and flows into the suction accumulator. The amount of flooding, or “overfeeding,” is usually five to ten percent of total refrigerant flow. This generally is equal to the “liquid bleed” from the accumulator (the explanation of which I will leave to another discussion).
While Cycling Clutch Orifice Tube (CCOT) systems generally use flooded evaporators, Thermal Expansion Valve (TXV) systems are designed to always have some superheated gas leaving the evaporator. The advantage of “flooding” an evaporator coil is increased capacity because “hot spots” (areas of reduced heat transfer) are reduced. Hot spots are caused by poor refrigerant distribution in the coil and/or superheated gases. Note however, that too much floodback can reduce system performance as it reduces compressor pumping capacity.
Advantages of smaller orifices at idle
CCOT systems have for years used Fixed Orifice Tubes (FOT). The designed size of the orifice is a compromise between idle and highway conditions. But since more time is spent at highway speeds, which requires a greater refrigerant flow, the orifice ends up being much larger than actually needed for stop and go, slow speed conditions. Here's where a “variable” orifice valve (VOV) can help system performance. By mechanically lowering the orifice size as condenser pressures increase (at idle/slow speed), the refrigerant flow is decreased and the system subcooling is enhanced (better performance).
FYI: Critical charge
Here's a little extra information that is not often told by vehicle manufacturers. If refrigerant charge is removed from a flooded system, a point is eventually reached where only gas leaves the evaporator. This is called the “critical charge.” Manufacturers add about ¼ to ½ pound of charge beyond this critical charge point. Since critical charge changes as system load changes, this overcharge is required. Plus, it provides leakage reserve.
You tell us what you want to learn
Do you have technical questions about air conditioning design and operating conditions that may require more than a phone call to answer? Would you like to hear more about Critical Charge and how to perform and achieve it in your shop, without owning a wind tunnel? How about the how's and why's of an accumulator's oil bleed hole, and how it affects liquid level and vapor-liquid separation in the accumulator? (This too affects evaporator flooding.) Please let John or I.M. know of your education needs, we'll do our best to provide the answers.
Gruss
Friedel
The Smart VOV™ (Variable Orifice Valve), is it the refrigerant control of the future?
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 10°F 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 5-12°F 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 (3°F), and 5°F 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.
In order to understand how the VOV increases idle cooling over the FOT, a basic understanding of how a FOT functions is helpful.
Conditions. Let us assume that at 70 MPH and 100°F outdoor air temperature, the head pressure of an R-134a system is 225 PSIG, with 20°F subcooling at the condenser outlet.
At those conditions the FOT provides a flow of 7 lb./min. to the evaporator.
Now bring the vehicle to idle. The hot under hood air re-circulates into the condenser. That, plus the reduced air flow to the condenser causes the head pressure to rise to 350 PSIG. Now, with a higher pressure, you would expect the FOT to flow more refrigerant, correct? But it actually flows less. Why does this happen?
First, understand that the flow through the FOT is dependent on: 1) Head pressure, and 2) the state of the refrigerant (whether it is subcooled or has quality (contains vapor). Note that suction pressure in normal ranges has no effect on flow. While refrigerant flow increases with increased head pressure or increased subcooling, vapor at the orifice reduces the flow significantly.
Back to our slowed vehicle. Momentarily there will be much more refrigerant flowing from the condenser than is flowing in from the compressor. This causes the subcooled liquid to be flushed from the condenser, after which some uncondensed gas (vapor) flows from the condenser as well. As that vapor enters the orifice tube it slows the flow rate, which is now down from 7 to between 5 to 6 lb./min. (even though the head pressure is much higher).
The resultant is that cooling capacity is roughly one half of that at 70 MPH because along with less flow, less liquid exists in each pound of refrigerant (the percentage of vapor increased). This illustrates how a FOT is self regulating but becomes very inefficient at idle.
The orifice tube expands the refrigerant to a lower pressure and temperature and this mixture enters the evaporator. The expansion process creates flash gas which does no cooling. Only the evaporation of the liquid in this mixture does cooling. A subcooled liquid entering the orifice tube will result in a much higher liquid percentage after expansion as compared to a vapor or quality entering the orifice tube.
Now the VOV. From this it is obvious that to increase cooling performance at idle, subcooling must be increased. This is the prime purpose of the VOV. It accomplishes that feat by decreasing the orifice size to roughly one-half the 70 MPH flow area when at idle. The resultant is 10 - 30°F increased subcooling during idle (with demonstrably improved discharge temperatures…). Also, this improved cycle efficiency significantly reduces horsepower required by the compressor, which improves city traffic fuel economy and exhaust emissions.
What does a “flooded” evaporator mean to you? Has the car been under water from an El Nino flood? Of course not. First of all, the evaporator core cools the air inside the car. The term “evaporator” is used because cold liquid refrigerant is evaporated in this heat exchanger and cools the tubes and fins which actually cools the air. “Flooded” means that the evaporator core is designed to operate under conditions where some liquid leaves the evaporator and flows into the suction accumulator. The amount of flooding, or “overfeeding,” is usually five to ten percent of total refrigerant flow. This generally is equal to the “liquid bleed” from the accumulator (the explanation of which I will leave to another discussion).
While Cycling Clutch Orifice Tube (CCOT) systems generally use flooded evaporators, Thermal Expansion Valve (TXV) systems are designed to always have some superheated gas leaving the evaporator. The advantage of “flooding” an evaporator coil is increased capacity because “hot spots” (areas of reduced heat transfer) are reduced. Hot spots are caused by poor refrigerant distribution in the coil and/or superheated gases. Note however, that too much floodback can reduce system performance as it reduces compressor pumping capacity.
Advantages of smaller orifices at idle
CCOT systems have for years used Fixed Orifice Tubes (FOT). The designed size of the orifice is a compromise between idle and highway conditions. But since more time is spent at highway speeds, which requires a greater refrigerant flow, the orifice ends up being much larger than actually needed for stop and go, slow speed conditions. Here's where a “variable” orifice valve (VOV) can help system performance. By mechanically lowering the orifice size as condenser pressures increase (at idle/slow speed), the refrigerant flow is decreased and the system subcooling is enhanced (better performance).
FYI: Critical charge
Here's a little extra information that is not often told by vehicle manufacturers. If refrigerant charge is removed from a flooded system, a point is eventually reached where only gas leaves the evaporator. This is called the “critical charge.” Manufacturers add about ¼ to ½ pound of charge beyond this critical charge point. Since critical charge changes as system load changes, this overcharge is required. Plus, it provides leakage reserve.
You tell us what you want to learn
Do you have technical questions about air conditioning design and operating conditions that may require more than a phone call to answer? Would you like to hear more about Critical Charge and how to perform and achieve it in your shop, without owning a wind tunnel? How about the how's and why's of an accumulator's oil bleed hole, and how it affects liquid level and vapor-liquid separation in the accumulator? (This too affects evaporator flooding.) Please let John or I.M. know of your education needs, we'll do our best to provide the answers.
Gruss
Friedel
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