Help design my new system please

[dhaslam]

That doesn't sound right. I use a calculation of about 6 watts per metre which would be about 7 watts per foot pipe run. I have 1/2 mile of pipe for a 2500 sq' bungalow. Of course being close to passive standard less than half if the circuits are used normally. South facing rooms and kitchen usually have enough passive heat.

[jeff5may]

Originally Posted by buffalobillpatrick

From another site:

"R290 and R22 have different liquid densities, and different enthalpy change in the evap. The result is that a TXV rated for R22 but used with R290 will have ~3.5 times the capacity"

Is a TXV a density controlled device? No. It is controlled by pressure. Spring pressure + evaporator pressure vs. sensing bulb pressure. Bulb pressure drops at lower temps, thus the spring wins and the valve moves closed. Outdoor temperature rises, so evaporator pressure rises also; evaporator wins and valve opens until sensing bulb pressure rises. This maintains constant superheat at the sensing bulb, assuring the compressor is not starved or flooded.

Since R22 and Propane have very similar pressure vs. temperature (PT) values, the valve acts very much the same with either. At 0 degF, they are within 0.2 psi of each other. At 100degF, R22 is at 196 psi, where R22 is at 175 psi. At 40 degF (a value in normal range), R22 wins by 4 psi. So as temperature increases, a TXV tuned for R22 but running propane will show a teensy bit of rising superheat in the evaporator. This actually works in our favor. Since propane has higher heat capacity than R22, it doesn't superheat as much in the compressor.

The author of the quote referenced probably got his refrigerants confused. R600 (butane) has around 1/3 the static vapor pressure of R290 or R22. So if you had an imaginary heat pumping thing that ran off butane and put propane in it (and didn't blow something up), it would use more energy, be less efficient, and have more capacity. Kind of like taking an R22 heat pump and filling it full of R410a. If the pipes didn't burst, and the compressor could survive the increased head pressure and compression ratio, and you could adjust the TXV enough to work, you would move more heat.

The more common and logical thing to do is to go the other way. Take a unit designed for R410a with a random problem, but a good compressor and change the gas and TXV. At reduced pressure and compression ratio, the compressor would use less power. You would lose capacity, but with oversized heat exchangers (originally designed for the higher capacity), efficiency (AKA COP, HSPF, SEER) would soar.

[jeff5may]

Originally Posted by buffalobillpatrick

interesting reads on R290:

http://www.danfoss.com/NR/rdonlyres/...ompressors.pdf

hydrocarbons21.com Recommendations for safe use of R290 in commercial refrigeration

In the first cited article, in the paragraph directly below table 1, it states R22 pressure is roughly 350% of R600a at -25 degC. Thus the statement is invalid and proves my previous rhetoric. Na na boo boo.

[NiHaoMike]

Originally Posted by jeff5may

The more common and logical thing to do is to go the other way. Take a unit designed for R410a with a random problem, but a good compressor and change the gas and TXV. At reduced pressure and compression ratio, the compressor would use less power. You would lose capacity, but with oversized heat exchangers (originally designed for the higher capacity), efficiency (AKA COP, HSPF, SEER) would soar.

I have done just that with great success. The main efficiency advantage is that the evaporating temperature can be much higher than usual. In a heat pump water heater application, getting the 140F or so needed for washing dishes would not put very much stress on the compressor.
http://ecorenovator.org/forum/geothe...ssor-r290.html
It's also a good fix for an oversized system.

[jeff5may]

So it would be good for a DHW system, especially one with a high temperature split. I was considering this for a heat pump water heater that ran off an outside air source. When it gets cold outside, the temp split could rise to above 100 degF. How much do you derate an R410a compressor to match R290/R22 capacity?

[NiHaoMike]

Something like 2/3 the original capacity, but note that it would be able to operate at higher evaporating temperature than a conventional design, at which point there would be less capacity loss.

If it might operate in below zero conditions, hot gas defrost would be more efficient than a reversing valve.

[Mikesolar]

Originally Posted by buffalobillpatrick

It looks like 2 x parallel loops of 250' of 3/4" pex,

with flow of 3.65 gpm in each loop

at Delta T of 20* F , (tank vs. water inside pex)

should transfer about 15 Tons of heat = 180K btus

At a head loss of 7.65'

HL = K x C x L x F^1.75
K = .0034, C = .933, L = 250, F = 3.65

Way more than enough HX for radiant floors.

BBP

3.65gpm is a bit high for 3/4" tubing. I would make it 3 -4 loops and drop the flow rate. Your reynolds numbers will still be good for heat transfer and the head will drop a lot.

Sorry if I am way back of the pack conversation, haha

[buffalobillpatrick]

I found this today:

35* C liquid: R290 has 43% lower viscosity & 21% better thermal conductivity than R22

-20* C vapor: R290 has 34% lower viscosity & 62% better thermal conductivity than R22

R290 will allow my 2T scroll R410a compressor to run cooler, draw less amps, & put out less pressure. Coupled with my huge HX's I should get pretty good COP ?

BTW, I need to size the High Pressure Cutout switch, should I get 360psi (just below 363psi stated capicity of 1/2" Mueller ACR soft) ?

BBP

[NiHaoMike]

You should be able to find a grade of 1/2" with a higher working pressure.

[buffalobillpatrick]

Surplus City

These gauges look pretty good for 80 cents:

2" DIAMETER HIGH PRESSURE GAUGE, RANGE: O TO 600 PSI, INCLUDES MOUNTING BRACKET

SCL # : 11199

MFG : ASHCROFT

MFG # : 733-16

Alt Part # : R5891