Making new R134a system in place of old R12 system

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HVACLearner
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Making new R134a system in place of old R12 system

Postby HVACLearner » Wed Feb 13, 2019 6:53 am

Hi guys, I'm looking for advice on a custom auto a/c installation using R134a.
I'm not in HVAC, just a mechanic who does the occasional A/C repairs (rebuild compressor, change clutch pulley/parts/etc).

My 1980s project car is engine-swapped to a newer motor; so different a/c compressor and no longer R12 obviously.

I've picked up the most efficient and largest CC displacement compressor that the engine was ever equipped with.

I've also picked up a larger and more efficient evaporator off a comparable model with same CC displacement compressor – but not the exact same model compressor as I have. It uses that style TXV that bolts to the inlet/outlet flange of the evaporator with just the power head on top, there is no capillary tube/bulb. The OEM just sells it's TVXs by parts number and without specs; so I can't look into sizing it properly really. Higher airflow and static pressure fan being used.

I have two questions at the moment:
1) Is it a relatively safe bet to purchase the OEM TXV that was equipped with my new compressor? Or should I expect problems? (Given that there is a different/larger evaporator. along with differences in the rest of the system)... Maybe when I make the hard and soft lines up I should look into using an orifice tube instead of a TXV so I can have a bit of room for tweaking it?

2) Google searches haven't helped me much with understanding the physics equations... is there a simple/rough way of determining what size my condenser will have to be to match this new evaporator?

Unless I F this up massively, the system should be better than the original system, so I'm not overly concerned about making sure it's precisely engineered - but if someone has the knowledge and patience for me I'd love to learn.

Cheers
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bohica2xo
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Re: Making new R134a system in place of old R12 system

Postby bohica2xo » Wed Feb 13, 2019 1:21 pm

In MVAC the condenser equation is simple.

" What is the largest parallel flow condenser I can cram in to the space available in front of the radiator?"

Solve for X

A TXV is a the best plan.

What compressor / condenser parts are you using?
HVACLearner
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Re: Making new R134a system in place of old R12 system

Postby HVACLearner » Wed Feb 13, 2019 3:11 pm

Hi and thank you for your response.

Re parts being used: naturally, everything is new in the box and genuine Denso.
The compressor I've selected is a 10S20C; specifically 447260-8871. It's a clutched and reciprocating swash plate type.
The evaporator I've selected is 476-0045; yet to measure the true dimensions, but the core is ~275mm x ~260mm x ~105mm.

Re old parts:
At a guess, my original compressor would have been lucky to displace 150cc.
The original evaporator is no longer available new, serpentine design and measured 245mm x 245mm x 110mm.
My original condenser is also no longer available new, serpentine design and measures 600mm x 350 x 22mm.

Re parts being looked at; I can only give a Toyota part number:
TXV equipped to the vehicle that used my new compressor: 88515-60190.
TXV equipped to the vehicle that used my new evaporator: 88515-20100.

I was planning on using a parallel condenser that was larger than original and to avoid reliance on an auxiliary electric fan for a/c.
The radiator is over-sized but uses a pusher fan, albeit that fan is very rarely working hard.

However, after reading on the internet, it seems that if high-side temperatures are too low this can be a problem for the compressor?
Perhaps you could correct or clarify.

I've also read that incorrect sizing of TXV and/or relative sizing of evap/cond can cause hunting (+ increased clutch cycling) - which is why I have posted here. Thoughts?

I just want longevity as I plan on driving this car daily for the next 20-30 years.
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bohica2xo
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Re: Making new R134a system in place of old R12 system

Postby bohica2xo » Wed Feb 13, 2019 4:27 pm

Within the ambient temperature range that you would want to use your A/C, there is not a problem. Even if you could get the liquid refrigerant to ambient air temperature on a 75f day, so what? That just means less load on the compressor.

Excessive cycling is the trademark of the CCOT system. With a TXV, the receiver dryer & the response time of the TXV damp out any sort of resonance.

MVAC systems are very dynamic in nature. Variable compressor speeds, variable condenser airflow and air temperature - as well as varying heat loads and fan speeds on the evaporator. You either need to cycle the clutch or run a variable displacement compressor.

The TXV system will cycle off on low side pressure, and the refrigerant in the receiver will keep it off for a bit. The TXV will continue to keep some refrigerant flowing if the heat load is low, extending the "off" time for the clutch.

The CCOT system runs wide open, blasting refrigerant through the evaporator at low heat loads. The excess liquid sits in a catch can under the hood cooling the engine compartment until the system cycles on again. Like holding your foot on the floor & driving with an ignition kill switch.

There is nothing "modern" or good about a CCOT, it was just a cheap solution. In industry we call it a "Bang-Bang" control.
HVACLearner
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Re: Making new R134a system in place of old R12 system

Postby HVACLearner » Wed Feb 13, 2019 8:12 pm

Okay then, so I just need parallel condenser with as much increased surface area over my original as possible. Easy.

I was assuming there would have been differences internally in the TXV or in the powerhead that would have affected its operation.

If there are no serious considerations to be made about TXV selection I'm going with the one equipped with my evaporator - just to ensure that there are no fitment issues. (After comparing online stock photos, the other TXV appears to have one smaller port on the evaporator-side - perhaps this is the only difference; explaining the different part numbers.)
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bohica2xo
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Re: Making new R134a system in place of old R12 system

Postby bohica2xo » Wed Feb 13, 2019 11:41 pm

Yes, use the TXV that was used with the evaporator you plan to use. They were meant to be used together.

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