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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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