THE BASICS Air Conditioning Theory

Air Conditioning Theory
Installing a climate control system in your vehicle may seem like a daunting
challenge. But like many projects we car guys get into, when taken one step
at a time, the component installation process is easy to manage by an experienced auto enthusiast. This introductory chapter will provide an overview of
the basics of performance air conditioning and factors affecting the selection
of the main components needed to air condition any vehicle. You may choose
to contract with a professional shop for your installation, but it is always good
to have an understanding of the functions and components of these systems.
If you really want more in-depth knowledge about the theories and technology
involved in a modern climate control system we recommend ordering the book
“How To Air Condition Your Hot Rod” written by our own Jack Chisenhall and
Norm Davis, available directly from Vintage Air. See below right.
Air Conditioning Systems
Simplified: The Basic Functions
We’ll begin under the hood with the enginemounted compressor. It pumps refrigerant, in
gas form, into the high pressure gas discharge line. This gas is loaded with heat it has
absorbed from air blown by the system’s fan
over the evaporator coil inside the vehicle’s
cabin. (It is important to remember that
an air conditioning system doesn’t
“make cool air”, it just removes ambient
heat from the space you want to be
cooled). Next, as the heated high pressure gas flows on through the condenser, the heat is carried off by air flowing
through the condenser, normally mounted in front of the vehicle’s radiator. The
refrigerant then condenses into a liquid
which becomes heavier, requiring less
space. The more efficiently the refrigerant is condensed, the less room it uses
in the system. This allows the refrigerant to lower the pressure in the high
pressure side of the system. The refrigerant, now gas and liquid, then flows into the
receiver/drier where the liquid falls to the bottom of the drier tank. The pick-up tube in the
drier almost reaches the bottom of the tank.
The open end of the tube is always below the
liquid level in the receiver/drier if
the system is fully charged. This
provides pure liquid refrigerant to
the liquid line (between the drier
pick-up tube and the expansion
valve). The expansion valve is an
orifice that contracts or expands
in size according to the temperature of the evaporator coil. By
changing size, the expansion valve meters the
refrigerant according to the demands of the
evaporator. The orifice is largest when warm
and gets smaller as the evaporator becomes
colder. This orifice provides a pressure drop
with the resulting drop in refrigerant temperature. The high pressure liquid pours into the
larger opening of the evaporator and the low
pressure liquid droplets begin to pick up heat
which expands the refrigerant to a low pressure
(super heated) gas which then returns back to
the compressor to start the cycle all over again.
(See drawing above)
Basic Components: Selecting A Condenser
The most difficult challenge in building an
efficient system is getting a condenser large
enough, in a place where the air is cool
enough, and can flow across it well enough, to
remove the heat and condense the refrigerant
adequately. The condenser has to provide a
low resistance path for the condensed liquid to
flow so pressure build-up on the system’s
refrigerant can be avoided. (Excessive pressure here tends to reduce heat loss which
tries to reduce condensation.)
The old vertical vs. horizontal tubes confusion in the condenser comes into play here. On
a tube and fin condenser the tubes must run
horizontally. On a parallel flow condenser the
tanks run vertically but tubes must run horizontal as well. Why? Because lubrication oil
flows with the refrigerant in the system and
will settle in the lower loops of the condenser,
thereby obstructing the flow of liquified refrigerant. We have seen this single factor
increase the internal pressure of the high
pressure part of the air conditioner by 50%,
reducing its ability to work properly. See fig. 2
With the conversion from R-12 to HFC134a refrigerants in automotive air conditioning systems (see page 6 for more on this), we
would have had to increase the surface contact area of conventional
tube and fin condensers by
about 20% to maintain
proper efficiency, but
unfortunately many classic car and truck applications just won’t allow
that size increase. A new
type condenser was the
correct solution and
Vintage Air pioneered that
technology for the performance aftermarket.
Our new design
SuperFlow TM condensers
(see next page) introduced
back in 1991, provided that increase in capacity without additional external size! By using flat
tubes manifolded together so that the refrigerant flows through multiple tubes on each pass,
we get virtually 100% contact of the refrigerant with the condenser tube walls. This design
also offers very low restriction in the pathway
through the condenser. The SuperFlow condensers actually deliver up to 40% more
efficiency than a comparable sized conventional copper tube and fin type condenser.
This means we can get more capacity with
less space - a good thing for vintage cars
and trucks!
Testing and developing better products is what keeps Vintage Air
at the leading edge of climate control technology. Yes, we test our
systems on the street in actual driving situations. But sometimes
you just can’t test the extreme limits with accurately measured
results without such high-tech equipment as this extreme environment computerized wind tunnel.