[RB855]

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!