Morphing Air Conditioner into Autocascade System

[mytekcontrols]

From the photo I gather that it's just a filter/drier, where the liquid will flow down to the captube and the gas will exit via the pipe at the top?

Actually it's just a "strainer" with nothing more then a screen inside. And yes the basic idea is that the larger internal volume of the strainer as compared to the 3/16"od tubing feeding into it, will allow the velocity to decrease, thereby letting the heavier liquid drop away from the gas. Same principle of any phase separator, this one just happens to be rather small (and cheap).

Of course an even better phase separator has the refrigerant entering from the side, taking advantage of centrifugal force (not just gravity) to separate the liquid from the gas.

The cheap phase sep probably works very well too, since if you have more liquid then gas, itll fill the bottom and act a little like a receiver I'd imagine. (bit theoretical in my head of course).

Yes I would definitely have to agree with you, but then any phase separator should, and does do this (unless the cap tube flow rate exactly matches the amount of liquid generated).

Mytek, perhaps you could design a nice unit for us... with a 1/2hp rotary and coldest temp, because I wouldn't know what lenghts of captube to use for instance.

I would love to tell you that I design systems by carefully calculating the various parameters (CFM, flow, heat transfer, ect.), but I don't. In fact it's been so long ago that I even did it that way that I have completely forgotten how. Don't laugh too hard, or judge me an idiot, but I just do this stuff by intuition. However I have found that in autocascade systems the length (flow rate) of the cap tubes is not as critical as some would have you believe.Which is good, since it would be very difficult to accurately calculate the proper size for cap tubes in the final stages anyway. Reason being; you won't have a constant liquid seal on the entrance of the cap tube especially when under load (standard practice and calculations in sizing cap tubes usually assumes you do have a constant liquid seal).

I'm not saying that it makes no difference at all, but it certainly isn't anywhere near as critical as the type, quantity, and number of refrigerants to be charged into the system in order to meet a given spec.

But just so you are at least in the ball park (and also to prevent wasted refrigerant from having to make multiple trials), I do believe I saw somewhere on this forum a calculation that will get you close (sorry I don't remember where).

Another method that works quite well when developing a brand new design, is to run several very small cap tubes in parallel. These can then be pinched off one at a time while testing under load to determine the best size for your new system. It is also a tremendous help, especially when dealing with a system using several cascades, to thermocouple the In's and Out's of all the heat exchangers and plot them in real-time. This is how you can develop a "feel" for what happens when changing flow, and/or adding refrigerants. If you choose to only monitor the evaporator, you will end up with a very incomplete picture.

What was the point of this project, if I may ask? Condensing alcohol? For what? Moonshine?

No it wasn't a fancy alcohol still

The purpose was to protect a vacuum pump when pumping down a refrigeration system that had previously been flushed of contaminates by using alcohol as a solvent.

At the service facility I contract to, we sometimes have to restore a large autocascade that had previously suffered a compressor burn-out, and gone acidity. Alcohol is cheap, has a zero ODP, and is a fairly good solvent. The problem is that it can really foul up the vacuum pump oil when we later evacuate the system for charging (hard to drain it out of all the nooks and crannies).

So as it turns out, -9C will condense it out of the gas stream even when pumped down to 10mmHg. -30C is even better. So the project is a "Cold Trap" for alcohol.