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. (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
can provide.
If you mean the condenser-radiator fan is not coming on, either the refrigerant temperature is not high enough to engage the condenser fan, or, the sensor could have failed.
If the A/C system is putting out cold air there must be enough cooling air passing through the condenser. If the condenser temperature rises and the fan does not turn on, the A/C system will put out warm-hot air.
michael – montserrat w.i
February 10, 2008 - 16:41
Subject: aircondition problem
i have a problem with my nissan sunny car
the a/c is cooling but the a/c fan is not comeing on at all
can somebody help me here please
Reply to michael
CPMJohn – Petaluma, CA
February 10, 2008 - 20:59
Subject: Nissan Sunny?
Not sure what a "Sunny" is.
However, you say the A/C is cooling but the fan is not coming on. Is it the condenser-radiator fan that is not coming on or the cabin fan is not blowing?
If the A/C system is cooling okay, and you're talking about the condenser-radiator fan not coming on, maybe the air to the condenser is cold enough not to need the fan to turn on. Could you be more specific about the system's problem please?
ben – louisiana
July 29, 2007 - 22:05
Subject: auto a/c
i have two ford trucks that i have recently change over from r12 to r134. one all i had to do was change refig. the problem i havewith this one is that my low side is around 70 and highside around 150 at idel. when accelerated it runs up to normal pressure. could my orfes tube be causing this. (1992 f150 v8)
Reply to ben
michael – montserrat
February 10, 2008 - 16:45
Subject: auto a/c
when your a/c is cooling and your a/c fan is not comeing on what can cause that
CPMJohn
– Petaluma, CA