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How does this evaporator work (vertical flow)?

digidocs on Mon February 11, 2013 2:10 AM User is offline

Hi guys,

I was looking for an evaporator for a custom AC system and found this evaporator which meets most of my needs, except that it seems to have a strange vertical flow arrangement:

This comes from a ~99 Isuzu Rodeo. After studying the service manual and several other pictures online, I'm pretty sure it is installed in the orientation shown in the picture.

My question for the gurus is---how does the flow work in this evaporator? Among other things it seems like all of the oil would get trapped at the bottom? Almost all 134a evaporators I've seen seem to have horizontal orientation.


NickD on Mon February 11, 2013 8:31 AM User is offline

One rule is the lubricant has to be miscible with the refrigerant used, kind of like an attraction between the two. Then shear velocity.

Could wonder the same thing in my older vehicles where the compressor, that is the only component that requires lubrication was mounted way up high compared to the other components, but it worked. Gravity apparently is not an issue with these systems.

HECAT on Mon February 11, 2013 3:16 PM User is offline

I have cut them open. You are looking at a parallel flow evap. There are separations in the top and bottom chambers that have no external identification. I will guess it is a 4 pass circuit. The incoming flow will rise thru 4 or 5 of those vertical runs, travel over to the next 4 or 5 and then down to the bottom, over to the next 4 or 5 and up, then over and down thru the last set. This is designed for maximum heat removal with good oil movement. The pooling of oil beyond the manufacturers design does occur as the charge leaks out or if the system is overcharged. The exact charge by weight ensures the oil movement as the OE engineers tested.


HECAT: You support the Forum when you consider for your a/c parts.


Gerald K4NHN on Tue February 12, 2013 12:41 PM User is offlineView users profile

I have an earlier Isuzu and the evaporator looks to be about the same as this one. There's a line from the expansion valve that goes to the fitting with the yellow cap . Is this a bypass line or what is the purpose of this line.

Gerald K4NHN
Cayce, SC

mk378 on Tue February 12, 2013 3:38 PM User is offline

That is the compensator line. It lets the valve sense the pressure at the outlet of the evaporator for more accurate expansion control. The TXV is trying to control conditions at the outlet of the evaporator, so measuring both pressure and temperature directly there is best.

Many TXVs do not have this feature, so they have to use the pressure they see at the inlet of the evaporator (the outlet port of the valve). That pressure is going to be slightly higher since flow through any passive component incurs a pressure drop. The "internally compensated" (no pressure sense line) scheme works for most applications though.

Edited: Tue February 12, 2013 at 3:38 PM by mk378

Gerald K4NHN on Tue February 12, 2013 4:30 PM User is offlineView users profile

Thanks, always wondered about that.

Gerald K4NHN
Cayce, SC

digidocs on Tue February 12, 2013 4:53 PM User is offline

Thanks to everyone for the help. I appreciate it.

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