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Old 12-31-13, 04:10 PM   #21
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I believe you made a good choice going with R134a as the working refrigerant in your rig. It has been done before. I am riveted by your dialog concerning your selection process. I know R134a has a very wide "acceptable" pressure range, making it useful in both refrigeration and air conditioning. But I did not know that you could push R22 or R290 that high without destroying something.

Obviously, R410a was designed to work at high pressure levels at moderate temps, not so much at low or high extremes. Can you elaborate on the envelopes of these refrigerants? Assuming a system (as yours) originally designed for R410a, where the compressor and plumbing will handle the high pressure, how hard can you actually push these alternative gases before they won't work? What goes wrong when a limit is exceeded?

The reason I ask is because so many of these R410a systems are hitting the scrapyards. I would love to experiment with propane or propylene in these units, but without an understanding of the temps/pressures that are achievable at safe margins, I am apprehensive.

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Old 12-31-13, 04:50 PM   #22
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It has to stay within the high pressure limit and the discharge temperature limit. A R410a compressor is going to be rated for a very high pressure (400PSI is considered a "normal" high side pressure) and the recommended discharge temperature for any common compressor is less than 190F. In practice, because R290 and mixtures based on it don't superheat as much on compression, 140F and above condensing temperatures are easily achieved.
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Old 12-31-13, 05:11 PM   #23
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Now, Im absolutely not an expert on this stuff, and should be fully verified before you use it to make any decisions. There are several things that must be considered when selecting a gas, type of compressor, oil viscosity and type, and worst case scenario.

I have a chart that lists boil at atmo and whats known as Critical Temp.
Crit.Temp is a very important thing to know if you are trying to make heat. Wikipedia explains it very well with "Above the critical temperature, a liquid cannot be formed by an increase in pressure, even though a solid may be formed under sufficient pressure." That said, you dont ever want to go near this region. To name a few, R-12 233*F, R-22 204*F, R-134 213*F, R-290 206*F, R-410 161*F. So as youll notice, 410 is by far the lowest, but 161*F equates to 688psi! Not a good gas for heating beyond say.. 110*F. (and the reason why desuperheater reclaimers dont work as well these days)

You also need to consider your Evaporating temperatures, and your condensing temperature. This can be a hard number to come up with sometimes as there are tons of variables. But for example, your making heat. You want to produce 115*F. Lets consider 2-3 common goals of 5, 10, and 15* of delta temperature. It would not be unreasonable to have 120, 125, or 130* condensing gas for this condition if your condenser is closely sized. Next youll turn to a pressure/temperature chart and find one or more gasses that are within your acceptable pressure range. For medium and high temperature stuff 200-300psi is a good place to be as most pipe work, fittings, and the alike can handle this easily. At this point, you would be eyeballing R-22, R-134, R-290 for example. R-410 is way out of the picture now since you would have to upgrade your stuff to handle 475PSI operating pressures!

Now things can get a little tricky, as the heat moving ability of these gasses vary, as does the low-side. This will effect how many BTU your system moves, given a constant of compressor capacity. The higher the spread of your pressures, the higher the compression ratio will be. The higher the compression ratio, the less "mass flow" the system will have, which means the less heat it can move. But the CR also has a lot to do with how much heat you "make". Things heat up when compressed, so a higher CR would be ideal to make heat. Its a balancing act that might end up being steered by what acceptable, available gases are at your disposal. You must also consider your compressor. Compressors come in all varieties of motor size and pump capacities, and are usually built to match a purpose and a gas.

A system expecting high heads, and a middle lowside will have more mass flow, and a lot of pressure to push it against. This will have a significantly larger motor section than a system with medium head, and a low suction, since it wont be moving much gas at all into a so-so head pressure. Thus the designations of LBP, MBP, and HBP (low, medium, high back pressure). LBP being the weakest, and HBP being the strongest motors. The pump capacity has a lot to do with expected pressure, and the intended mass flow. R-410 has a higher pressure, but less mass flow, so it will have a much smaller pump than a r-22 system with lower pressures, but more flow, even if it has the same size electric motor! Thats why if you use a 410 compressor with 22, 134, or 290, you might expect to get half as many BTU out of your unit (with consideration you changed your metering device to match your gas), but at a reduced running amperage as well. While it wont be as efficient at the given job, youll have a hard time overloading it. If you tried to use a 22 compressor with 410, it would be pulling very high currents, and likely have a very short life as its trying to pull off almost twice the work it was made for.

The same can be said about using a fridge/freezer compressor to build a baby window a/c. The motor is too small for the given load, unless you reduced the capacity with a lower CR gas. But wait, more to consider? Lets say you have a window a/c compressor, and you want to build a bad arse freezer. Your condensing temperature might only be 95*F, but your evaporating at -20*F. At this level, your mass flow has dropped to only a trickle because so little gas is evaporating. Most compressors rely on suction gas to cool the motor section. Now you have a compressor with a huge, very unmatched motor, with a little kid blowing through a straw trying to cool it down. Again, a very short life. Using a HBP in a MBP is ok. using a MBP in a LBP may be acceptable. You really need to reference your comps spec sheet to see its design limits. But trying to mix extremes will not be in your or the poor comps favor.

Things also to consider, the higher the difference between your evaporator temp and your condenser temp, produces a pretty nasty drop curve in your energy returns. It takes a lot of energy to move heat against its will. You can move a lot of heat a little distance, or a little heat a long distance at the same price. The closer you can keep the difference, the less its going to cost you on your power bill. Bigger coils, and thoughtful of what your trying to make go a long ways. To really simplify it, If you dont NEED 130* for your purpose, but 110* is your limit... maybe 115*F quicker will satisfy your need. This might mean finding a way to deliver your heat in a faster manner. Higher flow fans, larger capacity pumps, what ever it is you have to do to get the heat there faster with a lower delta. Instead of using lots of delta to drive the heat through your medium.

Please know, there is a lot more to this field of knowledge out there. This would be considered "beginners intro". So many things to consider to make a properly efficient system, that wont suddenly let the smoke out. It is often easier to research existing products from large experienced companies that have millions to blow on R&D, and extract knowledge for your own personal gain. Go look up a a/c unit of your desired BTU. Find out what gas it takes, go find a parts list and look up the compressor PN. Go google that PN and find a spec sheet. This will tell you the pump capacity and rated power. Now you have a gas, and a pump spec to build around to make a a/c. If you want to make a heatpump, research that. A Freezer, research that.

I dont have the knowledge to tell you how to do the math to properly size a compressor, or what capacity your compressor will have. Google it, there are dozens of papers out there that are pages long, with math that makes my brain itch, GL!

Last edited by RB855; 12-31-13 at 05:44 PM..
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Old 12-31-13, 06:32 PM   #24
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Thank you for the highly informative explanation! This has thrown dynamite into some of the conceptions I had concerning the limitations of the various refrigerants and compressors, as well as their proper and "fringe" uses. My level of understanding has just been broadened.
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Old 01-01-14, 10:13 AM   #25
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That was a very nice and concise little overview. It is hard, with a subject as wide as refrigeration, to get something like this.
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Old 01-01-14, 11:33 AM   #26
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In my learning, there wasn't really a basics page. I dove into refrigeration with operational knowledge and nothing else. I didn't know there were so many different compressor configurations, how much gases varied, such and so forth. I had visions of using freezer compressors to make baby a/c units and such. Now I know why my mini fridge has trouble starting on the same weak circuit a small window a/c doesnt (motor size/starting torque) While perhaps a bit off my original topic, it may still help shed some light for future creative minds. I had no idea the post was so long winded until after I submitted it lol
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Old 01-01-14, 03:34 PM   #27
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Hi all,
Been lurking here for a while learning lots. Thanks to all of you.

RB, thanks for that explanation. Perhaps you could help me fill in some blanks in my understanding.

"Above the critical temperature, a liquid cannot be formed by an increase in pressure, even though a solid may be formed under sufficient pressure." That said, you dont ever want to go near this region. To name a few, R-12 233*F, R-22 204*F, R-134 213*F, R-290 206*F, R-410 161*F. So as youll notice, 410 is by far the lowest, but 161*F equates to 688psi! Not a good gas for heating beyond say.. 110*F. (and the reason why desuperheater reclaimers dont work as well these days)"

Having your refrigerant becoming supercritical would be a problem if your condenser became overwhelmed and couldn't reject enough heat to condense it before cap tube / txv. Or if it were to exceed the pressure rating of the machine and potentially rupture something. But what else would happen?

If the fluid failed to condense it would whistle through a cap tube as vapour and equalize pressure between the high side and low side (basically turning your evaporator into another condensor). I'm thinking a txv would do the same because the vapour would be superheated entering the evaporator already. So basically pressure in the evaporator would render it incapable of absorbing more heat. No pressure diff = no phase change= no heat pump.

If the pressure of the fluid in the supercritical condition didn't exceed design spec, would it not just sit there blowing heat until the condensor started to condense again? Or would a critical failure result?

I haven't even started trying to think how efficiency would be affected.

I'm a marine engineer not an hvac guy. My experience is mainly with steam, so I could be making some incorrect assumptions or poor analogies.
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Old 01-01-14, 04:23 PM   #28
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CO2 systems generally operate in the supercritical region by design, although the pressures involved are high enough to make DIY impractical. At the other extreme, water vapor operates at too low of a pressure (and chemical incompatibility issues) to easily build practical systems with, even though it is a very efficient refrigerant when properly used.

As a general guideline, higher critical temperatures and lower operating pressures (within reason) correlate with higher efficiency.
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Old 01-01-14, 05:13 PM   #29
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Consider this. you have 1 lb of liquid. now turn it to gas. Its going to do 1 of 2 things. Its going to take up a lot of space, or its going to make a lot of pressure. I get the feeling you could very quickly experience thermal runaway. I never actually researched what happens over crit temp, I just know its well out of the design range for almost anything Id ever need to do lol. It would probably also displace liquid in your condenser, reducing its capacity. That, and that poor compressor would have to be way out of its discharge tmax lol.

Funny you bring up water, Mike. I was doing some research into heatpipe heat sinks. Thought it was very interesting that many many use water under a vacuum to get it to boil and condense at such low temperatures. Iv started fooling with heatpipes as pre-stage in a HRV tinker toy.

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Old 01-01-14, 06:32 PM   #30
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Water has a very high critical temperature and very low (vacuum) operating pressures. The latter presents many problems in making a practical system. Even small pressure drops are significant, so split systems using water vapor are impractical. (In fact, even package units are difficult to engineer. Most usual is a chiller.) Also, the compressor would ordinarily have to be impractically large to get a useful capacity, at least until Brittany Benzaia figured out how to use a switched reluctance motor in a centrifugal compressor that runs at over 100k RPM, thereby getting a high flow rate in a small package. (In contrast, ordinary compressors run at about 3600 RPM or 3000 RPM.)

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