I just have to compliment the people that maintain this board, for keeping a nice place going here.
I have loitered around here, as a non-registered reader, for a long time.
This is my first post, and it will probably be my last post. I prefer to stay on the side. I just thought that I would make one post regarding a subject that has perplexed me for a long time. That is the lack of R12, and modern R134a central air, and other air conditioning systems out on the market. Nothing seems inherently wrong with using R134a in central air units, and in some ways, R12/R134a seems better suited for central air conditioning systems. So, why are they almost non existent?
I will go over my basses for my opinion below.
Feel free to set me straight, and point out the error in my ways.
I have done a moderate amount of thinking in regard R134A in central air conditioners.
R134a is my juice of choice. (Hence my nickname)
It’s temperament is to my liking.
No major problems with heat of compression. (Don’t really like R22)
No wacko PSI levels. (I am no fan of CO2)
There is a small penalty with regard to line size, and increased refrigerant flow rate, but that is not enough to rule out the use of R134a in air conditioning. When you consider the higher heat tolerance levels resulting from lower heat of compression, then R134a also has a few benefits too. With the relatively uncontrolled condensation temps that central AC’s are subject to, R134a should be far more tolerant. Especially on situation where the evaporator freezes up, to the point that a R22 system would burn it’s valves and carbonize it’s oil from the high pressure lift.
Even using certain existing pieces of R22 equipment, you should be able to produce a air conditioning system that is far more durable, and heat tolerant.
My ideas are aimed at discharge cooled compressors.
Rotaries, some recips, and select scrolls.
Now down to the math.
For an existing R22 system, converted over to R134A with just an oil change, and TXV.
Existing size of compressor…….. Lets say 10KBtu in a standard environment..
Standard environment (for this comparison) will be evap temp of 35F and condensing temp of 100F.
R22 heat of vaporization is 87.65Btu per pound at 35F
107.9-20.25 = 87.65
R134a heat of vaporization is 84.93Btu per pound at 35F.
108.2-23.27 = 84.93
R22 enthalpy drop from 100F to 35F is 19.25 Btu per pound
39.5-20.25 = 19.25
R134a enthalpy drop from 100F to 35F is 21.88Btu per pound
45.15-23.27 = 21.88
Net cooling effect for R22 is 68.4Btu per pound
87.65-19.25 = 68.4
Net cooling effect for R134a is around 63Btu but per pound.
84.93-21.88 = 63.05
Density of R22 at 35F is 1.396 Lb per f^3
Density of R134a at 35F is 0.9544 Lb per f^3
Compressor displacement of a 10KBtu AC is 1.75cfm
(10,000/(68.4*1.396))/60 = 1.7454.…..
Btu of a 1.75cfm compressor on R134a
63.05*0.9544*1.75*60 = 6318Btu.
Compressor pressure lift.
R22 will have a 134.4PSI lift.
210.6-76.2 = 134.4
R134A will have a 93.8PSI lift.
138.9-45.1 = 93.8
Compression ratio of R134a is a bit higher than R22.
Motor load will be about 62% of R22
Motor efficiency will not be as high with reduced load.
So power consumption will be a bit more than 63% to 64% of R22
But the boosted real world COP, from boosted evap and condenser efficiency, from the reduced capacity will make that difference up plus some.
Reduced Delta T of the evap and condenser will increase suction, and decrease discharge pressures, so the net EER effect will be a positive one.
Heat of compression is far lower with R134a, so discharge temperatures will be way lower. So no major burn out problems from poor air flow over condensers.
With a discharged cooled compressor, the compressor will think it’s driving a condensing temp of about 70F to 85F instead of 100F
Compressor discharge temps/pressures, and the temperature/pressure of the refrigerant in the compressor will be about the same as it would be on a 70F or lower day.
Vapor density around the compressor components, vapor temperature, and compressor load would be the same as if it was running R22 on a cool mid summer day, instead of 100F+. Compressor cooling will not be a problem.
You will have made just a slight improvement in efficiency, increased operating margins for the compressor, and increased reliability by reducing strain on the system.
If it is in a hot climate, where the compressor normally operates overloaded from extreme discharge pressures, then you may actually improve the efficiency of the compressor, including everything else.
And if the contractor put in a system that is twice as big as the house needed (don‘t they always?), then you will now have one that is almost right on the mark.
Suction cooled compressors may not work very well though.
With the 45PSI suction, the compressor will think it’s pulling a 5F to 10F evaporator. It would not be within it’s designed parameters. Refrigeration density around the motor may be low enough that overheating of the windings may be a problem, Even at reduced load.
Now that we have done a basic conversion, and not lost any of the EER rating, now we can do something to boost the EER even more. something that I almost never see on R22 systems, because it would put them past the edge of operability.
That is, a heat exchanger on the liquid/gas lines. Like you see on R12/R134a refrigerators. Where the suction line is soldered to the liquid line going to the evaporator. It makes a highly efficient linear heat exchanger. How many times have you seen the suction and liquid line soldered together going to the inside evaporator coil on a central air unit? Even though it would boost the EER/SEER by several points. The reason you don’t, is because the extreme superheat, combined with the heat of compression of R22, would kill the compressor quickly.
So, lets strap the liquid and condensate line together, and see what we would get with R134a.
Lets say that the superheat of the refrigerant leaving the evaporator is 10F. Or a total temp of 45F
Nix that….. The heat that it would of lost from the 10F gain after it all changed to liquid would equal the amount that it would provide to the liquid stream. So lets just say zero super heat for the comparison.
35F vapor refrigerant going from evaporator to the compressor. 100F liquid refrigerant going from the condenser to the evaporator.
Lets say that the vapor temp reaching the compressor is about 95F, which would be achievable on a moderate length line set. So lets see how many Btu the 50F shift in vapor temp will give the liquid.
You will have a Btu gain of 6.2Btu per pound
115.7-109.5 = 6.2Btu
That is 6.2Btu that isn’t lost by the temp drop from the high side to low side conversation at the TXV
So, net R134a cooling capacity per pound is now 69.25Btu.
Btu of a 1.75cfm compressor on R134a with heat exchanger is now
69.25*0.9544*1.75*60 = 6940Btu.
That is close to 70% of the cooling power, with only about 60 to 63% of the power draw.
That should boost a 14 SEER system to about 15.5 to 16 SEER.
Compressor discharge temps on discharge cooled compressors will still be within the range that you would see on a normal R22 system. So compressor damage should not occur.
As far as real world testing.
I have operated a few R22 rotary and scroll discharge cooled compressors on R134A with a oil change, in a controlled environment, and I have found that it extends the operating range greatly before destructive temperatures are reached..
Condensing temperatures can be pushed up a good deal ~140F to 150F and the compressor case (windings) temp will be running at the same point as A R22 filled compressor running at a condensing temp of 100F to 120F.
If you are in a mild climate, then changing the compressor out with a purpose built R134A unit may be advisable. A refer compressor would do a great job of pulling the outside coil down to sub freezing temps on a heat pump heating cycle.
Now I know some would call me a hack, for thinking about that type of stuff. Ow well….
I am just sharing my ideas. Do with them as you please.
I am just doing my best to think outside the box.
Things listed in the above article are only my opinion.
Procedures listed above, will void any equipment warranties.
Any damage you incur, while implementing any ideas stated, is your own problem.