Evaporator fin density and differing TXV adjustments

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Al9
Posts: 77
Joined: Tue Mar 10, 2020 3:26 am

Evaporator fin density and differing TXV adjustments

Postby Al9 » Mon May 04, 2020 3:46 am

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
Posts: 291
Joined: Mon Apr 18, 2016 6:10 pm

Re: Evaporator fin density and differing TXV adjustments

Postby Dougflas » Mon May 04, 2020 5:49 am

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
Posts: 77
Joined: Tue Mar 10, 2020 3:26 am

Re: Evaporator fin density and differing TXV adjustments

Postby Al9 » Mon May 04, 2020 7:59 am

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
Posts: 108
Joined: Sat Apr 23, 2016 3:40 pm

Re: Evaporator fin density and differing TXV adjustments

Postby ice-n-tropics » Thu May 07, 2020 6:00 am

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
Posts: 77
Joined: Tue Mar 10, 2020 3:26 am

Re: Evaporator fin density and differing TXV adjustments

Postby Al9 » Thu May 07, 2020 7:17 am

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.

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