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experiment with oriface tubes and expansion valve

squeege on Wed September 02, 2009 10:38 AM User is offline

Year: 1988
Make: chevy
Model: suburban
Engine Size: 350 5.7
Refrigerant Type: 134a
Ambient Temp: 90
Pressure Low: 47
Pressure High: 275
Country of Origin: United States

I have a 1988 chevy suburban with dual air. This past month ive been experimenting with the expansion valve and different size oriface tubes. Before on 90 degree 50%

humidity day vent temps would be 55-60. Now I'm getting about 45-48 vent temps low side 45-47 high side 260-275. At 1500 rpm the low side 35 high side 300. To achive

this I adjusted the rear expansion valve by turning an allen screw on the inside of the valve in all the way and backing out two turns. And ended up using a brown (.047 Dia)

oriface tube for the front. On the road I'm getting as low as 35-37 at the vent and that's on high blower! I reset the low pressure cutout at 20lb. So it won't freeze up. My

only concern is that the high side pressure has gone up. Is This going to be a problem in the future? What is the max high side pressure? I originally posted "won't cool at 85

ambient" I just want to conferm my results. Just to make sure I haven't missed

anything.






HECAT on Wed September 02, 2009 5:01 PM User is offline

It would appear your "tuning" has increased the heat removal from the air passing over the evaporator.

Reducing the orifice sizes alone will increase the high side pressure, and the more heat that is now being removed from the cabin must also be released by the condenser.



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squeege on Wed September 02, 2009 9:56 PM User is offline

I realize that, but will running with the pressures i have hurt the compressor? Especially if it gets hotter say 95-100plus degrees outside? I just want to know the high side

limit on a dual air system.

TRB on Wed September 02, 2009 10:39 PM User is offlineView users profile

350 psi and above is going to shorten the life of a compressor.

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Probedude on Thu September 03, 2009 12:01 AM User is offline

Quote
adjusted the rear expansion valve by turning an allen screw on the inside of the valve in all the way and backing out two turns.
Compared to what it was before, is the net result that the screw is now turned clockwise more than before? (or counter clockwise?)

I've read that with R134A retrofit of R12 systems some people adjust the TXV, but they've posted what you wrote - "Turn all the way in until it stops, the back out 2, 2-1/2, 3 turns" (depending on who you read). What they don't say is overall is this higher spring pressure (cw) or lower spring pressure (ccw).

With R134A pressures being slightly higher I would expect that one should turn the screw CCW a bit, but it seems that the net change is that people are turning it CW.





Edited: Thu September 03, 2009 at 12:03 AM by Probedude

squeege on Thu September 03, 2009 3:35 AM User is offline

yes cw=more spring pressure ccw=less spring pressure. Overall My goal was to increase high side pressure slightly in order to lower low side pressure. Say for example on 80-

85 degree day high side would be 175-200 low would be 50. So by increasing spring pressure I increased high side (2.8)-(3.0) ambient and it lowered the low side. The

original valve took 10 turns cw to bottom out, the new one I replaced took 5 turns cw to bottom out, so at first I went in cw until it bottomed out and backed out ccw 4-1/4

turns. Ended up going cw until it bottomed and backed out ccw 2 turns. 350 and up, ok I think I'm going to be allright, I'll just have to gauge it when and if we get 95-100plus

weather. It cooled off here in Memphis, usually this is the hottest time of year w/ 95-100 degree days with 50-75% humidity. I'll see how it goes and thank you all for your

input, I really do appreciate it!




















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Probedude on Fri September 04, 2009 1:17 AM User is offline

Thanks.
FWIW, I just redid the A/C in my Geo Metro.
Replaced bad o-rings, the expansion valve with a new one, flushed, refilled oil with POE, new drier, new/bigger condenser fan, refill with R134A.
Results were lackluster. Low side pressure was ~50, high side ~230. Was getting ~ 20 degree drop out of vents. This was on a 95+ degree day in the driveway.

TXV was originally set for 3-3/4 turns cw to stop. I readjusted it to 2-1/2 turns and now my low side pressure is ~ 30psi, high side is ~220, now have 30 degree drop at vent.
My condenser and evap are tube/fin construction (low efficiency)

I had thought that with the higher vapor pressure of R134A that the spring pressure on the TXV would need to be lowered, but in my case (and yours) going up helped out by lowering the low side pressures.

I still need to figure out what my proper charge is. I've got aftermarket, dealer installed A/C and there's no sticker or information on it anywhere.


Edited: Fri September 04, 2009 at 1:18 AM by Probedude

squeege on Fri September 04, 2009 2:02 AM User is offline

I'd say with those numbers you would be seeing 32-35 at the vent. What are the inlet and outlet temps on the evap if they both feel cold then I would suspect a blend door

problem. Because even when you had 50lb on low side when 95+ you should have had at least 50-55 at the vent. But if the evap inlet feels cold and the outlet feels warmer

then I'd say a little low on refridgerant. If that's the case I'd try adding a little slowly until the outlet is as cold as the inlet. As far as proper charge I would ask the dealer who

installed the ac.



Edited: Fri September 04, 2009 at 6:17 AM by squeege

Probedude on Sat September 05, 2009 10:17 PM User is offline

Quote
Originally posted by: squeege
I'd say with those numbers you would be seeing 32-35 at the vent. What are the inlet and outlet temps on the evap if they both feel cold then I would suspect a blend door problem. Because even when you had 50lb on low side when 95+ you should have had at least 50-55 at the vent. But if the evap inlet feels cold and the outlet feels warmer then I'd say a little low on refridgerant. If that's the case I'd try adding a little slowly until the outlet is as cold as the inlet. As far as proper charge I would ask the dealer who installed the ac.

Not a blend door problem. Inlet to evap is warm, outlet is cold.
I think it's just not an efficient evaporator (or condenser). Both are tube-fin construction.


squeege on Sat September 05, 2009 11:06 PM User is offline

what! that doesn't make sense. Are you sure that your not backwards. If the pipe between expansion valve and the evaporator, is cold, then that is the inlet. If that pipe is

warm and the other is cold then I don't know what to tell ya.


Probedude on Sun September 06, 2009 1:58 AM User is offline

Quote
Originally posted by: squeege
what! that doesn't make sense. Are you sure that your not backwards. If the pipe between expansion valve and the evaporator, is cold, then that is the inlet. If that pipe is warm and the other is cold then I don't know what to tell ya.

I was telling you about the pipes at the firewall.
The TXV is in the evaporator valve and like many cars it cannot be accessed in operation Given it's cooling and the suction line from the evap is cold, the system is plumbed and working fine.
If I could stick a thermocouple on the evaporator itself, I'm sure my temps are going to be much better than what my vent temps are. This evap, like the condenser, is not a high efficiency type.

My only question is how much more, if any, refrigerant should I put in given the capacity of the system is not known.

Edit: Found a web page that describes how to charge based on sub-cooling method. (Has sub-cooling charts too.)

Edited: Sun September 06, 2009 at 2:22 AM by Probedude

squeege on Sun September 06, 2009 3:17 AM User is offline

Well, which one is it? I'm saying that if the pipe between the txv valve(the one you adjusted)and the evap is cold and the other line from evaporator to compressor is warmer,

then you are a little low on refridgerant. Because on 95+ day you should have 40-45lb on low side 209- 237.5 high side. I would go by the numbers on your gauges. Put in

a little at a time, if it starts to cool better at the vent, keep going until it no longer cools any better at the vent. That's what i did. On my conversion I put an aftermarket

parallel flow condenser, the sticker said w/rear air it should take 4.0 lbs of r-12. I really don't know how much 134-a I have in it because I didn't have an accurate scale to

weigh it in with. I used 30lb cylinder to fill with. I just went by the numbers on the gauges. I'm guessing it is around 3.2- 3.8lbs. All i had to weigh with was a bathroom

scale, not very accurate. As long as you don't exceed 325-350 on high side i wouldn't worry about it.















ScotY on Fri September 11, 2009 9:44 AM User is offlineView users profile

Quote
Originally posted by: Probedude
Quote

Originally posted by: squeege
Edit: Found a web page that describes how to charge based on sub-cooling method. (Has sub-cooling charts too.)

Can you post the link? Curious to read about this.

Thanks!

squeege on Fri September 11, 2009 11:16 PM User is offline

I Found this site. I just googled subcooling for automotive ac and found doc blocks. It is a real simple way to find out if you are under or overcharging the system by

subcooling method.

www.docsblocks.com/faq/E-24.pdf -

Edited: Fri September 11, 2009 at 11:21 PM by squeege

ScotY on Tue September 15, 2009 3:47 AM User is offlineView users profile

Thanks for the link! That's a very interesting read...seems simple enough that even I could do it and get it right.

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