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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 oils. 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 efficiency. 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.
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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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A member by the name of JimF has provided a way of running the fans continuosly on certain vehicles. Here's the URL:
http://pages.prodigy.net/jforgione/MB_CTS.html |
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This article points out the importance high air flow has on
the cooling effeciency when the car is at low speed/stopped/low rpm ... This is where the systems thermal load is strained to the max., and the reasoning for my mention in prevoius post of jumpering the a/c low fan pre-resistor...if there is any time that these systems need high fan/air flow .. this is it.... The rest of the system is there , [ high pressure sensor, aux. fans, relay, etc] , so... simply getting rid of the dropping resistor gives us HIGH fan when we need it the most...with a simple mod of moving a wire ...[ rather than jumper] The system will still have the protection of the high side sw and controls , but the benefit of air volume when called for ... and as the load decreases from car movement and passive air condensor cooling, the high sw will cut the fan when not called for....[ie- low demand of system and adequate ground speed air cooling] One might also think that a negative of this this would be a larger draw with the high speed vs. low , but the opposite is true .. the low fan and series resistor circuit actually draw more than the no resistor high speed. [ notice the pre-resistor fusing is larger than high aux fan circuit fuse] The only negative I have been able to come up with is the noise of the high fan.....but who is listening when you are nice and coooooll..???? |
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Interesting point Arthur...
Did you do this mod to your car? How is it working?
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J.H. '86 300E |
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Re: Interesting point Arthur...
Quote:
hahaha.. No, I live in New England, so it is not needed...but I recommend it in southern climates [ as I do a lower thermostat, in some cases] and I have had some people do it that live in Texas and Fl, and they both report good success... |
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