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Old 08-08-2003, 02:11 PM
ctaylor738 ctaylor738 is offline
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Join Date: Apr 2000
Location: Falls Church, VA
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Interesting Post on AC from the Ritter/Easley Board

Date: Fri, 08 Aug 2003 12:09:28 -0400
From: Pete Schreiner
Subject: RE: [MB] airconditioning changeover/aux fan

Marshall and Richard,

Just a couple of observations on auto A/C system load capacities.

A consideration whether using R12 or R134a is that the A/C system has its
lowest cooling capacity when the car is stopped and the engine is
idling. In this condition there is abnormally low air flow across the
condensing heat exchanger (unless temp extremes have been reached and
miraculously, the viscous fan clutch has decided to cut in). Under the
stopped/idle condition, the best aid for the wimpy MB R12 and anemic R134a
converted systems is to have the auxiliary condenser/radiator fan on at
high speed. The forced air flow across the condenser allows the system to
exchange enough heat at the condenser that "normal" cooling can be achieved.

To improve the stopped/idle cooling capacity of the R12 system in a 1984
300D, I have installed an additional aux fan relay in one of the vacant fan
relay box positions on the 300D. All the necessary connections for the
relay to control the aux fan when the A/C compressor clutch is enabled are
available in this relay box. I can sit in traffic in 90F heat and have the
evaporator outlet air temperature at 42F. Try that with the standard
system and watch the radiator temperature climb as well as the temperature
in the car. Yes, the radiator temperature does still rise, but not
to >95C. And yes, the aux fan is running a lot more, however engine
temperatures and condensing temperatures are lower all the time the A/C is
on. Fan life? On many if not all front wheel drive cars, the aux fan runs
continuously when the A/C is on.

For both R12 and R134a systems, all the compressor is expected to do is
provide a continuous stream of liquid refrigerant to the expansion
valve. The heat exchange is then purely a function of how much heat is
removed at the condenser and added at the evaporator. If the heat removed
at the condenser (i.e. air flow, heat exchange efficiency, heat of
condensation of the refrigerant, and volume of refrigerant) is not adequate
for the heat load at the evaporator (i.e. air flow, heat exchange
efficiency of the evaporator, heat of evaporation of the refrigerant , max
volume of refrigerant permitted by the expansion valve), the desired
temperature differential across the evaporative cooling coil in the
interior of the car will not be achieved. This is a simplistic description
without getting into the thermodynamic properties of the refrigerant,
however it is useful for system understanding and diagnostics.

Anything we can do to take out more heat at the condenser increases the
cooling capacity of the system. We have already in the past mentioned the
ways that can be done. More air flow, continuous operation of the
compressor, a higher capacity heat exchanger, and a different refrigerant
can all be used to achieve higher cooling capacity.

Without doing a detailed walk around the pressure-enthalpy diagrams for the
two refrigerants, it is difficult to explain why there is lower cooling
capacity in an R12 A/C system converted to use R134a. In a system designed
for R134a, the heat exchanger areas for both the condenser and evaporator
need to be greater than a comparably rated R12 system. Some hints at why
there might be reduced capacity with R134a are: higher head pressure(lower
compressor volumetric efficiency), lower density (20% lower charge by
weight permitted), higher Btu/lb needed to liquify (more heat removal per
lb of refrigerant needed at the condenser.)

The end result is that, sure enough, a system that was designed for R12
does not have as high a heat load handling capacity when it is converted to
using R134a.

This is to say nothing of the issue of oils compatible with the two
refrigerants. The POE and PAG oils that are compatible with R134a are not
as friendly to humans and much more hydroscopic than the mineral oil used
with R12. This is more of a problem for people working on the systems than
those using them. The type of oil has no significant effect of the
capacity of the system of which I know. Sealing rings used in an R12 are
most likely not compatible with the PAG or POE oils used with R134a and the
desiccant used for the older R12 systems dissolves in the R134a compatible

Because of the "small" thermodynamic differences between the two
refrigerants, there is little need for much change in the expansion valve
other than that seals must be compatible with the new type of refrigerant
oil. The flow volume requirements of expansion values optimized for the
two refrigerants are close enough, as are the pressure/ temperature
characteristics, that one expansion valve design can be used successfully
for both refrigerants.

So, for those who find a properly installed R134a conversion A/C system to
have inadequate cooling capacity, the only realistic ways I know of
increase the system capacity in an existing installation are to increase
the airflow at the condenser and/or increase the condenser heat exchange

The evaporator thermal cutout is only a problem in limiting system cooling
if the set temperature is being achieved and the compressor clutch is
disabled. This temperature set point may be anywhere from 38F to
48F. There is a fault in the sensing circuit if this is limiting the
center vent exit air temperature to 56-58F. More likely the system is
being limited to 56-58F by lack of cooling capacity. It can not handle the
heat load. This can be caused by many things: under charge, air or
moisture in system, dirty heat exchanger surfaces, inadequate air flow
across the condenser, inadequate air flow across the evaporator,
misdirected air flow from the heater box, hot water flowing in the heater
core, failing compressor, etc. If when the system is handling maximum heat
load, a temperature differential across the evaporator of 20-25F is being
maintained, the system is doing all it can do and working properly. If the
car is still too warm, this means the heat load is greater than the system
capacity. If under the same conditions, the temperature differential is
less than 15F, the system may be trying to operate beyond the capacity
limit of the condenser or not working properly and a diagnostic check of
the A/C system needs to be performed.

Refrigeration is neat! I guess it's time for me to pull up the P-E
diagrams of R12 and R134a and plot the refrigeration cycles.

Pete S.
Chuck Taylor
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe
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