Refrigerant Type: R134a
Country of Origin: United States
Question about superheat specs of automotive TXV's...
If you look at families of vehicles, let's say Ford Expedition rear A/C for example. Within the various generations of them, say from 1996 thru 2014, each one specs a significantly different superheat rating, like from -1 for a few years, to +5 the next few years, to -1 again a few years later. All of the aforementioned cars have similar evaporator types and sizes and installed configurations. Seemingly the only variable is different superheat ratings from year to year.
Chrysler minivans do the same thing with various superheat specs with similar evaporators over the years.
Why the different TXV superheat ratings with respective similar evaporators? And, how does a negative superheat-rated TXV work, anyway?
Negative superheat potentially allows some liquid refrigerant to leave the evaporator. Which would be OK in a SUV or minivan rear unit since with it also having front CCOT, there is an accumulator to separate liquid before it can reach the compressor.
With the rear units, yes, agree that the long distance of the suction tubing ought to ensure vaporization.
However, ALL of the example cars I used in my 1st post have TXV's only, front and rear, with no CCOT's, which means no accumulators either. Also, been doing some more digging and found multiple instances with front-mounted evaps with negative SH TXV's... several Toyotas, GM cars, Fords...
One variable that may play into this: all of the aforementioned cars have variable-displacement compressors, and from what I understand about those is, if the suction press gets too low, the compressor automatically reduces displacement.
Best to just ignore superheat on MVAC systems. We do not charge by superheat in MVAC.
If you tried to plot the operating envelope for a MVAC system, you would need paper the size of a bedsheet, and a fine point pen. From 40f to 140f ambient, idle to 6,000 rpm, ground speeds from zero to 80 mph, variable evaporator airflows..... Nothing is ever constant.
Superheat is used in buildings with constant evaporator airflow, and a fairly narrow evaporator operating temperature range with large evaporators.
In MVAC, with size limitations on evaporators & a huge delta across them, it is common to run a partially flooded evaporator to increase capacity. 40f vent temps mean that the evaporator is very close to becoming an ice block - but are necessary when you need to cool a vehicle in the sun.
Only a small percentage of vehicles have variable displacement compressors. Your Expedition does not have variable displacement, it has a scroll compressor.
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.
Bohica, that's all good info and thanks for that.
But, I wasn't asking about charging via measured superheat. I know that doesn't work in MVAC applications. I was specifically asking about TXV superheat settings or ratings, which is how TXV's are identified. What I wanted to know was, how does a TXV work which has a negative superheat rating? As background, and the way I interpret the workings of a TXV is that the superheat rating of a TXV is the temp difference between the inlet (measured indirectly via inlet pressure) and outlet of an evaporator. With a negative superheat rated TXV, you would expect the outlet of the evap to be colder than the inlet, which doesn't seem possible. Is it possible due to pressure drop in the evap maybe?
One way I could see how a -2 deg-rated TXV might successfully work is from the suction line picking up some superheat in the engine compartment on the way to the compressor.
You can see what the superheat ratings are of TXV's by looking at the 4-seasons catalog. The valves are rated by superheat and tonnage.
Finally, yes you are right, the early Expedition does not have a variable-displacement compressor. I was only using that as an example of TXV superheat rating changes through different years of production. What I'm actually doing is mixing and matching components to create a custom setup for a kit car.
And, variable-displacement compressors are not rare anymore. I have 5 cars and 4 of them have variable compressors. (Silverado, Scion xB, Mercedes E-class, BMW 3-series, and an old GTO).
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