Evaporator fin density and differing TXV adjustments

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Al9
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Evaporator fin density and differing TXV adjustments

Post by Al9 »

Hi, another intriguing subject.

Let's take as an example a Scion Xa evaporator and a Scion Xd evaporator. The first one has about 19 refrigerant passages on each side (however, there's an application using the same expansion valve that employs an even smaller 11 passage evaporator, and another one using a slightly larger 15 passage evaporator), while the other one has 29 passages. There are many more fins on the latter one. So heat exchange surface looks definitely increased, compared to the first one. That is, the latter is able to exchange more heat. That's how all recent evaporators look.

Each evaporator has a different TXV.

How could the control characteristics really differ between each valve? Could a TXV meant for the smaller evaporator be designed to feed less refrigerant on average across the whole temperature/pressure operating range? That is, higher superheats settings so that the less capable evaporator won't get flooded (unless at low loads) and still be able to pull pressure down fast enough?

On the other hand, could a TXV meant for the larger one be designed to take advantage of the larger heat exchange surface? That is, feed it with more refrigerant and as a result lower superheat on average?

Thank you
Dougflas
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Re: Evaporator fin density and differing TXV adjustments

Post by Dougflas »

Superheat is not used in MACV because the compressor does not spin at the constant speeds. The TXV's are different according to how much refrigerant is required to load each evaporator. Smaller capacity evaps call for less refrigerant than larger evaps but you will see less capacity in smaller evaps. Smaller loads require less capacity. TVX's have preset superheat. That is why it is important to weigh the charge into a system. You want to achieve a full column of refrigerant to feed the TXV. This is how I was taught a billion years ago.
Al9
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Re: Evaporator fin density and differing TXV adjustments

Post by Al9 »

The way i understand it, the modern automotive H-block TXV is nearly always designed to have a high superheat - as high as 10F - at heavy heat loads (high evaporator pressure, temperature) and a low MOP so that suction pressure is pulled down fast enough, and a lower superheat, even 0F, at low heat loads, so that no hunting happens and so that oil return is optimal (critical in auto MVAC since compressors are seldom equipped with oil sumps). Basically a gas cross charge. Those thin metal shafts now used in place of the larger cylindrical resin-coated shafts featured in older valves make it even more clear that hunting is to be avoided at all costs. I think that the TXVs designed for the newest evaporators basically just have a little more non-condensables (such as nitrogen, helium, argon) in their charge, and as a result the entire curve shifts to lower superheats, flowing more refrigerant into the evaporator.
ice-n-tropics
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Re: Evaporator fin density and differing TXV adjustments

Post by ice-n-tropics »

Some of Your techanical comments are basically applicable to variable displacement compressor optimization and have little to do with fixed displacement comps or evaporator capacity.
Evaporator fin density is a function of evaporator air velocity, suck verses blow through evap, , heat transfer between design of primary and secondary surfaces, and water shedding characteristics of fin louver/shape design. Fins which drain water easily can be spaced closer together.
hotrodac
Al9
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Location: Southern Europe

Re: Evaporator fin density and differing TXV adjustments

Post by Al9 »

Thanks for this informative answer, i understand this basically means that different fin density won't really affect correct TXV operation as long as the TR rating is proper.
About the comments, the real thing is, i've noticed different makers are using the same TXV both with variable displacement compressor systems and fixed displacement piston compressor systems, so i guessed that the cross charge (a variable superheat setting basically) had some benefits also with fixed displacement systems.
Al9
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Re: Evaporator fin density and differing TXV adjustments

Post by Al9 »

I'd like to update this thread with new findings.

In my quest to find a suitable (and possibly even better) replacement TXV for my car's VDC system (whose OEM TXV is now an NLA part), i tested two TXVs used with both Scion xB evaporator styles. I've tested a Denso 447500-2390 (Toyota PN 88515-52040, used with the old style evap featured on Scion xBs up to MY 2006) and a Denso 447500-3070 (Toyota PN 88515-26070, used with xBs from MY 2008 onwards, the ones having a modern evaporator that features plenty of narrower passages). Both xBs use a clutchless Denso variable displacement comp (obviously externally controlled, though i know about an internally controlled Sanden SD6V12 application that's using the -3070).

The former plainly snaps shut when you get the disc ice cold with freeze spray. The latter throttles closed a bit and that's it, exactly like my car's (old style 1st gen xB evap with larger passages) TGK made factory installed one does. I'm talking -50F disc temps and lower.

What i now believe is that the former has a lower MOP than the latter (so more restriction is taking place). Judging by the facts that the first gen xBs have a smaller, 90cc maximum (vs 140cc) comp, and a smaller engine too, and that the former valve is also employed with even smaller vane and scroll compressors (which, just like fixed displacement swashplate comps, are always running at maximum displacement too), perhaps it's just meant to put less pull-down strain on the comp (and on a small engine, a peculiarity of Kei cars, also) in hot days, and hence the different TXV. A big enough externally controlled variable comp can always be commanded to "ramp up" gently in the presence of particularly high heat loads, and if you can do that, there's no purpose to a larger evap cooling capacity (better evap design) if it gets starved too much when things are hot. Perhaps Denso manages to dampen hunting and ensure oil return in some other, more efficient ways than by forcing a large valve opening at low heat loads (what possibly shows as a TXV that never snaps shut during the freeze spray test). Unsure about how much a different MOP will affect cooling.

(Another thing worth of mention is that the latter valve is actually a tiny bit shorter and therefore lighter, a matter of 1/2 oz, than the former, likely to put less load upon the evaporator to firewall secondary pipes featured in the modern evaporator design; second reason for a different part).

I might be in for some interesting tests, should the second hand factory TXV i'm currently running (and perfectly fine with, btw) ever fail or get leaky.
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