Inverter ASHP to WSHP conversion (on a boat), design&discussion

[jeff5may]

how about a water analogy? If you only have 20 psi of water pressure, you're going to need much larger supply pipes to feed the same gpm than if you had 50 psi of water pressure.

Yes, the laws of physics still apply, but with R410a, the increased pressure and corresponding gas density lends itself to smaller hx size and tube diameter at the same exchange capacity. Obviously, this effect is much more prevalent in the condenser side.

[doug30293]

Originally Posted by AC_Hacker

I tried to find a copy of the Prapainop and Suen paper you mentioned, but none of the links I tried were live.

Do you have a working link to the paper?

http://www.ijera.com/papers/Vol2_issue4/BV24486493.pdf

[AC_Hacker]

Originally Posted by doug30293

http://www.ijera.com/papers/Vol2_issue4/BV24486493.pdf

Thanks for posting this very useful paper.

I did look at the authors and it gave me pause, as the world of HVAC was "ruled" by US businesses after WW2 and now the crown has been clearly passed to Asia.

Great info in this paper though, it will greatly inform our discussions.

Thanks.

-AC

[AC_Hacker]

Originally Posted by jeff5may

how about a water analogy? If you only have 20 psi of water pressure, you're going to need much larger supply pipes to feed the same gpm than if you had 50 psi of water pressure.

Yes, the laws of physics still apply, but with R410a, the increased pressure and corresponding gas density lends itself to smaller hx size and tube diameter at the same exchange capacity. Obviously, this effect is much more prevalent in the condenser side.

I think that your comparison breaks down because the water analogy assumes pressure at one end of a pipe, and no pressure at the other end.

In the refrigeration circuit, all events take place within a hermetically sealed 'world', thus the pressure is balanced. Refrigerant flow is caused by the "unbalancing of pressure" due to the work of the compressor.

-AC

[doug30293]

Originally Posted by AC_Hacker

I did look at the authors and it gave me pause, as the world of HVAC was "ruled" by US businesses after WW2 and now the crown has been clearly passed to Asia.

The US may still dominate the business world but research left our shores more than 30 years ago. I was a member of the Society of Automotive Engineers in the 80's and 90's. By then nearly all the SAE papers were coming out of Japan.

[jeff5may]

Originally Posted by AC_Hacker

I think that your comparison breaks down because the water analogy assumes pressure at one end of a pipe, and no pressure at the other end.

In the refrigeration circuit, all events take place within a hermetically sealed 'world', thus the pressure is balanced. Refrigerant flow is caused by the "unbalancing of pressure" due to the work of the compressor.

-AC

Ok, lets get detaily.

In a typical R22 unit, we have 70 psig low side and 225 psig high side pressures. In a R410a unit, these pressures are 120 psig and 350 psig respectively. So we have twice as much pressure, nearly. R410a therefore has nearly twice as much gas per unit of volume compared to R22. This helps the heat capacity a lot. So much that it throws everything off balance by 40%. You need either 40% less compressor or 40% more heat exchanger to balance the load.

With home- or mini-split-sized heat pumps, the trend has been to add 40% more heat exchanger. A previously 1 ton sized compressor has now been spec'd to 1.5 tons for R410a use, and that part was easy. The heat exchangers have been altered to have more capacity per square inch of face area by doing what? Instead of a hx that has 3 circuits of 3/8 inch pipe in them, they now have 6 circuits of 3/16 inch pipe in them. The pipes are also stacked closer together.

The result is just like a new 1.5L vtech motor that has more horsepower than an old 2.8L slant six motor used to. The new breed of heat exchangers have much lower internal volumes, less weight, and higher flow rates than the old ones did. Guess what? Noone cares that they have 15% more pressure drop. The R410a works anyways. Less refrigerant charge, less copper, less weight, more capacity. TADA!

[NiHaoMike]

R410a is a 50/50 mix of R32 and R125. R32 makes a great refrigerant (competitive with R290), but R125 isn't particularly good. Look it up and you'll find that R125 has a high GWP. As it turns out, it was really put in so the fluorocarbon companies would have an excuse to phase it out when the patents expire!