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The Flooded Evaporator
Part 3 of 3-part series of
interviews with Richard C. Kozinski
Part 1: The
VOV, refrigerant control of the future?
Part 1a: Refrigerant Basics
Part 2: The VOV (Variable
Orifice Valve) versus the FOT (Fixed Orifice Tube)
Smart VOV Info
Appeared March/April 1998
Cool
Profit$ Magazine
© 2000
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. (End of Part 3, to go back
to Part 1, 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.
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