AC system design - orifice tybe, expansion valve, and HP draw?

[mikedc]

The orifice tube design is said to draw less HP/TQ off the motor than the expansion valve design. Is it a big difference in practical terms? I assume it's just a matter of the OT design not activating the compressor as much.



Are there any experts on vehicle HVAC out there?

[Tim C]

Been a mechanic/technician whatever you want to call us for 20 years. In my opinion, orifice tube systems were a compromise from a cost or weight standpoint. O tube systems came about in the late 70s, had less bulky parts and less expensive parts than the comparable systems of the day. A 50 cent plastic o tube was way cheaper than the POA or STV systems had before. And cycling the clutch to prevent evap freeze up had the added benefit of reducing hp load and helping fuel mileage on the huge monster ac compressors of the day.

Modern expansion valves and compressors are much smaller and more efficient than the old ways.

I personally believe an expansion valve (txv) system cools better because the valve will react to the evaporator inlet and outlet pressure/temperature and adjust orifice size based on that in relation to system flow (such as high or low rpm, higher mph for better condenser air flow, etc). The result is more consistent and usually colder vent discharge temp and with the compressor usually running 100% duty cycle no hunting idle from the ac kicking on or off. The txv will open or close to prevent evap freeze up on a property sized and designed system. There's usually a backup evap temp sensor that will cycle the clutch if the txv can't keep up such as on a cooler night but it doesn't cycle as often as an o tube system.

Many manufacturers are going back to txv systems these days. My DD 2016 f250 has a txv where my 03 f250 had an o tube. Same cab basically, different hvac boxes obviously but the 03 truck barely would output 55* even new, the 16 puts out 40* at the vent.

[mikedc]

Interesting.

Have the actual expansion valves changed/improved over time? It seems like a pretty simple device.

The longer story is that I'm trying to do a custom-built HVAC setup using a lot of modern OEM parts. The factories have sold a lot of vehicles with OT systems in recent decades but the TXV design seems generally superior to me. And now they are doing variable-displacement compressors too. I'm trying to make sense of it all.

[Tim C]

Basically I think yes, the tvx valves are much smaller in general. Google 60 and 70 gm and ford POA, stv, and vir ac system pics. They were bulky, had capillary and various pressure tubes snaking around. They didn't look good and had more leak points or places to get damaged. The h block expansion valve that pretty much everyone uses replaced all that. It greatly simplified the whole thing and just bolts between both evaporator lines at the evaporator bulkhead through the firewall usually, though some are in the dash.

The o tube was just cheap to manufacture basically. Like I said above it did cycle more so the old compressors of the day didn't spin as much helping the gas mileage some at the time, but modern compressors, let alone variable are more efficient at pumping without as much hp loss. O tubes work, I've got a window ac in my shop that uses one, and I've seen home ac units that had them too, I think that's where they do great actually, in a fixed system with pretty well known unchanging parameters. The condenser and evaporator air flow are known based on fan speeds, the compressor is usually a single or just two speeds. Your car is different, it can go from 600 rpm to 6000 with the compressor along for the ride and both core air flows are variable. That's where the tvx shines as it can change orifice size to compensate. I said before my old 03 f250 o tube system barely hit 55* vent temp. Well after a line leaked and while I had it open I changed the fixed o tube to a variable one. Basically it's a standard tube with a spring loaded check ball that can seat against the outlet of the orifice. The ball seat has a small groove so it doesn't seal completely. At low flow such as idle the spring keeps it seated so the small groove is the orifice, at high rpm high flow the pressure overcomes the spring and the standard orifice size gets used. The result was a 10* drop at idle and a happy driver in NC heat and humidity. I was glad to see Ford went to a tvx when I bought my new truck and it's been great.

I have an old f150 I use rock crawling, built as a truggy. It had no ac factory so I pieced one together with junkyard parts. I used the rear unit from a lincoln navigator for the evap, fan and tvx. It's mounted behind the passenger seat and air gets ducted up along the console with shop vac hoses. This is a function over form truck mind you! The stock heat box in the dash handles heat and defrost still. I mounted a jeep cherokee compressor on a custom welded bracket and bought the biggest universal condenser and drier I could use from vintage air. I specifically chose this setup because the tvx, it made life easier with mismatch parts since it adjusts. I've thought of building my own system for my Fairlane but I'll probably just go vintage air for simplicity.

Variable compressors are cool tech too, but you need a computer to operate them as far as I know. Lots of modern stuff runs them. No clutch, they always turn, the computer varies the stroke of the piston from 0 to whatever to dial in the refrigerant flow as needed for variable conditions. At 0 stroke they pull basically no hp or they would still have a clutch because mpgs matter for manufacturers.

[mikedc]

I actually looked at the rear of a Lincoln Navigator for my project. I came away thinking it wasn't big enough for a muscle car cabin. And it doesn't look like something they intended you to service in general.

I hadn't thought of the size & inefficiency of older compressors being a motive for the switch to orifice tubes. That makes perfect sense.

I'm trying to put together a muscle car setup that is easy to deal with. The issues with modern systems . . . compressors/clutches dying early, freon levels being finnicky, snap-tight connectors on the hard lines that should have been threaded nuts, the under-dashboard stuff being a PITA to access . . . I'm trying to minimize all that.

On paper I'm sure it's not worth all the effort. But it's a matter of outsmarting my own laziness. HVAC problems are not like engine or brake problems; you can't procrastinate on failing brakes. You can procrastinate on failing AC. If the repair job is big & annoying then I might spend 2 months in a hot car.

[Tim C]

The Lincoln setup was fully serviceable. You can split the case to replace evap, heater, tvx, etc. The evap was the same width and thickness as an f150 front evaporator of the same years, but 2" shorter. But the f150 front evap is the same wether it's a single, extended, or crew cab. So I figured the 2" difference wouldn't hurt on my single cab 78 f150, and it's fine. I don't know if the thing is big enough for a muscle car cabin, that's probably more cubic feet to cool with more glass greenhouse area too.

As far as compressors go if I piece a system together I'll use a sanden 508 probably. It's pretty much an industry standard. It comes with several different OEM mounting configurations but also in universal mounts too. That's what vintage and the other aftermarket guys use and they have tons of brackets for all the common engine families. They are used in everything from semis, school buses, farm tractors and OEM cars and SUVs.

[mikedc]

Do you know what year of Lincoln Navigator that HVAC box was out of?

[Tim C]

No it was back around 2012 or so. I want to say it was the 05 to 08 ish. The first gen with independent rear suspension. But I can't say 100%. I didn't use any of the electric door actuators in my application. I cut the box in half just after the evap and make a new end that I tapped for shop vac hose ducts.

[mikedc]

The Ford explorers (probably a smaller platform) went IRS by about 2002. I'm not sure when the Navs did it.

I've investigated the idea of building a whole custom HVAC dash box. It seems like the best result but it's probably more work than I will tackle.

I don't think it would be THAT difficult. Not compared to the design & effort that guys put into other aspects of muscle car projects. But it's mostly uncharted territory. There aren't exactly online tutorials for it. It's rare to see a custom-built dashboard box for the heater core alone, never mind a whole HVAC box. I haven't found a single case of anyone else even attempting it.

[Tim C]

If you do, take plenty of pics. That'd be an awesome build to follow in my opinion. I love this stuff, homemade ac, junkyard electric power steering.

[mikedc]

I see good reasons to consider a custom one.

Pros:
You could fit some BIG cores inside a 50yo car.
Use modern parts that won't become rare for a while yet.
Have the core tubes pass through the firewall where you want them to.
Make the cores & blower motor all individually removable, without taking the whole box out of the car.
Do a blend-door design rather than a heater control valve.
Put a cabin air filter inside.

Cons:
Trying to make a box that holds all that stuff correctly. It's playing Tetris in 3 dimensions.
It needs flaps/doors and those might need some kind of power assist.
My inclination is to build the box out of steel sheetmetal but that would end up being pretty heavy.
Airflow efficiency would be an unknown in general. Sorta like trying to port cylinder heads by eyeballing it and hoping for the best.

[CamaroAJ]

Anyone figure out if TXV location is really important? I'm planning on running a C7 condenser and a C6 A/C compressor on my car and need to use a TXV for a long system life. So far all the systems I've looked at has it close to the evaporator with the exception of the Chevy Volt Hybrid battery pack. I had a TXV for battery cooling and it has about 15" of line between it and the evaporator section.

[Tim C]

Tvx or orifice location isn't that critical. But the closer the better, the coldest point in 5he system is immediately after the restriction, after that the refrigerant is picking up heat. If the valve isn't right on the evaporator or if any lines between the valve and evaporator are not enclosed in a box and exposed to the in car air flow they should be insulated so they don't pick up heat. Otherwise you reduce the cooling capacity.

Most tvx are right on the evaporator for packaging. Since it needs to sense both lines may as well sandwich it between the hoses and evap for smaller packaging and convenience. I have seen o tubes mounted right at the condenser exit. Dodge and gm trucks did that but then you have to insulate the hose from the condenser to the evaporator or it'll be pulling under hood heat into the refrigerant and won't be as cold once it gets to the evaporator.

[mikedc]

TXV location - yeah, that makes sense. The closer to the evaporator the better. Any pipe distance between there is heat-soaking room to reduce the accuracy of the valve's reading.


Generally speaking - I've always heard & read that the condenser is the limiting factor in most AC systems (more than evap core, the blower motor, or the compressor). Would you agree with that? As in, always try to run the biggest condenser you can fit?

[Tim C]

Definitely. You want the biggest and most efficient (more fins, serpintene vs tube style, etc) as you can fit. OEM guys are even using what's called a suction line heat exchanger now, but it's part of a matched system, the tvx has to be calibrated for it. I've seen it on ram trucks and Hyundai and Kia cars most obviously. It basically lilooks like the liquid line (from condenser to tvx) is teed into a fat aluminum section of the suction line (from evap back to compressor) but actually the liquid line is inside the suction line for the length of the fat section of line, it uses the still cold evaporator discharge to keep the refrigerant in the liquid line cold, and therefore still a liquid before it gets to the tvx. Basically it adds to the condensing action especially since it's actually colder than ambient air temp which the condenser uses. It also prevents the refrigerant from getting warm enough to flash boil from underhood heat as you want a liquid going to the tvx, the pressure drop after the tvx and change from a liquid to a gas is what makes the ac work in the first place. Any refrigerant that flashes to a gas before the valve is not cooling the car, reducing capacity.

[CamaroAJ]

Definitely. You want the biggest and most efficient (more fins, serpintene vs tube style, etc) as you can fit. OEM guys are even using what's called a suction line heat exchanger now, but it's part of a matched system, the tvx has to be calibrated for it. I've seen it on ram trucks and Hyundai and Kia cars most obviously. It basically lilooks like the liquid line (from condenser to tvx) is teed into a fat aluminum section of the suction line (from evap back to compressor) but actually the liquid line is inside the suction line for the length of the fat section of line, it uses the still cold evaporator discharge to keep the refrigerant in the liquid line cold, and therefore still a liquid before it gets to the tvx. Basically it adds to the condensing action especially since it's actually colder than ambient air temp which the condenser uses. It also prevents the refrigerant from getting warm enough to flash boil from underhood heat as you want a liquid going to the tvx, the pressure drop after the tvx and change from a liquid to a gas is what makes the ac work in the first place. Any refrigerant that flashes to a gas before the valve is not cooling the car, reducing capacity.

GM has been doing that for a few years now and refer to it as a super cooler line. I've been keeping an eye out on how the bends are in a few different cars to try and work that into my system.

[mikedc]

That sounds like quite a bit of added complexity for a minor gain in the cooling efficiency.

Is it because of the physically large engine bays in modern trucks? Their underhood lines have inevitably got a lot of distance to cover.


Could you reap some of that benefit by just insulating the liquid line more thoroughly? Or maybe pairing those two lines side-by-side inside the same insulation layer?

[Tim C]

IDK if it's a minor gain or better. I have seen a paper published that says it can reduce pumping losses by lowering the head pressure. That means less HP wasted turning the compressor. I'm sure that's a big reason the oe guys use it. Got to eek out that last .01 mpg somewhere. As far as complexity it's not really from a service standpoint. I'm sure it's more complex byo make but to service it's no different than changing any hose. Just there's 2 connections to loosen at each end.

[mikedc]

Yeah, googling up some images, the line-inside-a-line thing is not as complicated as I was initially thinking. Seems like a decent idea if there's a practical gain from it.

[Twentyover]

The main driver for tube in tube A/C line heat exchangers was CO2 credits. Prior to about 2018, when the EPA lost control of carbon emissions, they had a table of carbon emission credits vs design changes. Tube in tube, pwm blower motors, 1234yf refrigerant, among other design features would be granted reductions in CO2 emissions from the measured amount in the Federal Emission Test Procedure. The current drive by OEM's for 1234yf is EU regulation. Want to sell in the EU? Not doing that with 134a.

Regarding designing a mix plenum as suggested in posts 9-11- I think most would be surprised how much effort is put into this at an OEM level. Getting Temperature Control Curve slope positive so they don't invert (temp lever goes higher, actual outlet temperature drops), outlet airflow and temperature stratification (near equal, or specified, airflow from each outlet, and equal or specified temperature difference from outlet to outlet) is where months of development work take place. Start moving components around after the heater core, and these kind of go away. It was a problem that gave me grey hair in my 20's.