VOV™ (Variable Orifice Valve)
versus the FOT (Fixed Orifice Tube)
Part 2 of 3-part series of
interviews with Richard C.
Part 1: The
VOV, refrigerant control of the future?
Part 1a: Refrigerant Basics
3: The Flooded Evaporator
Smart VOV Info
Appeared March/April 1998
© 2000 All Rights Reserved
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
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
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. (End of Part 2. To go to Part 3, click here.)
Editor's note: Richard C. 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.
If you have burning questions about
air conditioning, please submit them to imcooldotcom.
Dick has graciously offered to help with whatever technical assistance he