Add R290. The sight glass must be clear.
Printable View
Add R290. The sight glass must be clear.
Maybe its just sucking the R23 and leaving the R290 to just sit there? The Suction line near the compressor, and after the R23/R290 junction is definatly sub zero.
The whole purpose of shortening the cap tube is to push the R290 through the evap. If there isn't solid liquid at the cap tube entrance, then this is far less likely to happen. We need that sightglass to be clear. Add More R290 until it is clear.
The R23 is doing exactly what it is supposed to do. it is flooding its evap, absorbing the heat load, and is superheated at the exit, meaning it is entirely vapor at that point.
If only the R290 evap were working that well. But we are not going to flood that evaporator with liquid by feeding it vapor. The sight glass must be clear.
Hoping you are keeping up with my chronic editing... LOL
Another point on clearing the sight glass. If the sight glass shows 85% liquid and 15% vapor, I would assume that the 15% is R23, which robs the R23 evap of part of its refrigerant, while starving the R290 evap.
R290 cannot condense at this point, therefore if the sight glass is clear, we know that the separation is complete.
Im adding more R290 but its hurting my cpu temperatures as the R23 SCT is rising due to LP rising. I'm still adding the R290. Do i keep going till there are absolutely no bubbles. I normally have a stream of bubbles rushing from the inlet pipe to the outlet pipe when I look in the sightglass, even when the sightglass appears full. The flow enters from the bottom of the sight glass if that makes any difference.
Is the exit temperature dropping?
The less bubbles, the more complete the separation of the refrigerants.
The more we get into this, the better I like the parallel condenser/separator (C/S) idea, and the suction line heat exchanger Chilly has added I think will greatly enhance this. There should be a sightglass at the C/S exit, and preferable a TXV. I am still undecided on the value of the EPR.
http://www.blairwing.com/images/Autocascade14.jpg
BTW, I am thoroughly enjoying the learning opportunity here. Thanks, Russell. :toast:
The captube is suitable in this application, although a pev at the inlet to the R290 and a captube on the block with a epr on the outlet of the block would let us play with the balance. I am checking my sources for the condenser today and when I find something I'll post it.
As you can probably tell I like using controls to balance a system..
I'm thinking an EPR on the outlet of the R290 evap would make more sense.
Strictly from an experimental point of view, I would like to start with a stripped down version, just the bare essentials, i.e. compressor, C/S, evaps, and (adjustable) metering devices.
Then control the variables, and add gadgets.
The R290 evap exit temp is increasing.
What are all the numbers?
The reason I think an epr on the R23 evap is that the evaporating P/T that has the same enthalpy as R290 is higher by about .8 bar. I suguessted this to balance the heat gains within the system. I'm interested enough to build a system, been incapacitated for a week and a half with a resperatory infection so I still gotta wait....
I left it running for a good 25minutes on full load, these are the numbers.
R23
Evap Exit: -39.1ºC
SST: -53ºC (3.2 barg)
SCT: -2ºC (22.4 barg)
R290
Evap Exit: 25.6ºC
SST: -4ºC (3.2 barg)
SCT: 65ºC (22.4 barg)
Air in: 21.9ºC
Air out: 30.8C
CPU: 5ºC
Your delta is increasing, it's removing more heat, how hot did the compressor get on this run?
Not only has the condenser delta-T increased, but the difference between the R290 SST and the R23 SCT has dropped. This indicates much better heat transfer in the heat exchanger. I think we are getting close. I like it. :D
The compressor top is hot, but I can leave my hand on it. The bottom is mild/cool. So i'd say its not overheating.
Whats the next step?
The next step is to chop another foot off the R290 cap tube.
Once we get the R290 gushing through, as you put it, am I going to see lower R23 evap temperatures? Or will my suction pressure get too high? One more thing, if we get the cap tubes balanced will a bigger compressor then bring down the the suction pressure also will the cap tubes need to be rebalanced with a larger compressor?
Note that despite the large increase in R23 SST, there was little increase in CPU temp. Once the R290 cap tube is right, I am thinking we could decrease the R23 evap temp by lengthening the R23 cap tube, or possibly just by increasing the amount of R23.
But let's cross that bridge when we come to it.
And yes, once we have the cap tubes right, a bigger compressor would contribute directly to the bottom line.
Took another 12 inches off the R290 cap tube. Left it running on full load, here are the results.
R23
Evap Exit: -37.4ºC
SST: -52ºC (3.4 barg)
SCT: -2ºC (22.5 barg)
R290
Evap Exit: 26.5ºC
SST: -2ºC (3.4 barg)
SCT: 65ºC (22.5 barg)
Air in: 22.8ºC
Air out: 30.5C
CPU: 6ºC
Hmmmmm... we have now totally eliminated the difference between R290 SST and R23 SCT, and it still isn't enough. I would guestimate that we are flowing enough R290 to handle almost 300 watts of load. What would you estimate the CPU load to be?
The sight glass is clear, right?
Is the bottom of the compressor cold yet?
Please tell me to butt out if this is the wrong time to step in here;) I wont be offended:cool:
I teach 11 to 16 year old's maths and I find that I really understand high end maths, much more, after I have reduced a difficult topic into something the kids will understand. And I also see things there that I had missed before.
Now whilst I understand classic cascades quite well, this autocascades has me really confused. There seems to be at least 3 variables here:
1. cap tube length for r23.
2. cap tube length for r290.
3. charge of r23 and r290.
Is the ratio of the r23 and r290 important, shouldnt the ratio be the same and only the quantity altered i.e. shouldnt you calculate an efficient ratio first lets say its 2 to 1 than increase the charge in that ratio?
If you can explain this in very basic terms to me you may be able to see something that you are missing. Sorry if this isnt appropriate at this time, but if it isnt just ignore the post as I said at the beginning.
regards
John.