Maybe we need to split the liquid line on the enthalpy chart ? One line going into the sub cooled region and another stopping in the 2 phase area?
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Maybe we need to split the liquid line on the enthalpy chart ? One line going into the sub cooled region and another stopping in the 2 phase area?
Drew,
you are confusing yourself to be polite. Check system balance charts in the book you have not read yet. Sorry for being blunt.
magoo
The system is balanced at - 27 c sst which is better than the original -32 c sst. Clearly the added refrigerant has helped and the system is better fo it. Confusion is my middle name Magoo, as you can see by my signature, but to understand how we can have flash gas in a sub cooled liquid line is fundamental to understanding refrigeration . Of course I could walk away and say its all ok, which it is , which I have, is easy, but I am curious ,as you can see from the above, how this can happen? It changes the way I've been charging systems without sight glasses as I've always looked for a certain amount of sub cooling to ensure we have a full charge. So it's not really about this specific site, it's a fundamental question. And If you could provide me with the book I haven't read yet Ill give it a go.
Very criptic advice Ranger1.
You can not rely on a sight glass as a means of ensuring full charge and sub cooling is not a sure shot way either, you must look at the system as a whole: super heat, discharge temp, and sub cooling.
Super heat, we normally want to see 20 degrees for cap tube systems, 10 degrees for TXV, Discharge temp will be set by the oil and refrigerant type cooler the better for the compressor valves with in reason. Sub cooling at the condenser as technically once we hit the receiver there can technically be no such thing!
There will all ways be bubbles in the sight glass most so small we do not see them usually by the time it hits the txv it will be a solid column of liquid (Which where we'd want our sight glas any ways! just sadly not so practical)
The sight glass serves as a quick check of moisture and to small extent gas charge but your T/P relationships will tell you fare more then the sight glass ever will!
So on your system what is the discharge temp measured from within 6 inches of the compressor, what is the SH at the compressor suction side within 6" (SH is what determines to some extent compressor loading and discharge temp we need enough to keep it cool but not so much as to over load it) This is what will tell you if the gas charge is good!
So are you saying that with a 16 c compressor superheat ( long runs ,old insulation ) -27C c sst, 38 c act, 8 c liquid line sub cooling, (not sure of comp discharge temp and not sure that it's that relevant) AND the flashing sight glass the system is as good as it'll be considering the room runs at -20 c ?
Strange how something as simple as bubbles in a sight glass could attract such varied approaches.
Drew just to confuse you even more, there is a method by charging to theoretical compressor discharge temperature, system pressures, compressor amps etc etc as well!
Discharge temperature is also important as it is influenced predominantly by compressor suction superheat.
Different refrigerants have different temps at same conditions.
Ammonia needs water cooled heads & oil coolers on recip compressors.
If it gets to hot it could break down oil & compressor valves break.
There are a number of ways to to the same thing, at the same time you can use several of them at once to confirm whats happening in system.
Well if the insulation lagging is old there is your first thing to attend to, get that right, then start again!
On a TXV I personally would tune it till I saw about 10K super heat at the compressor (This ensures the evap is well flooded as it should be, after ensuring the pipe lagging was as sound as I could get it.
All ways start simple, is the evap clean? Are the fans running well? it all starts there, as your suction line pressure and density ultimately control mass flow and compressor loading, not enough we get an under performing system, too much we cook the compressor.
If the lagging on the suction line is poor then we are getting to much super heat where we do not want it, this increases discharge temps, lowers compressor loading, and reduces mass flow.
Start with the simple but not so obvious things. 16K is good on a cap tube system, not so good for a TXV system!
So check the pipe lagging then ensure all evaps are running correctly and clean. check the TXV's sense bulb, is it making good contact and mounted at the top of the pipe? The truest saying I have ever heard is: The devil is in the details! non are too small.
Once the evaps and TXV's are dealt with then we can look on the condensing side of the equation.
The discharge temp can help tell us how our charge and system are behaving, to dense suction gass, or to much super heat raises our discharge, it will all so tell us the ruff quality of our oil in the system tell us if we are burning it or damaging the valves, it is often over looked as your statement shows us.
Discharge temp is another valuable measure that tells us allot about what is going on inside the compressor
that is indeed correct between 3am to 9pm on the pipe would be a more percice one I supose
Attachment 10154Drew tis picture may satisfy you in regards to sub cooled liquid & liquid/vapour on P/E diagram
There is no concept in refrigeration that confuses more people than the static gas law which says you can't have superheat in the presence of liquid nor subcooling in the presence of vapor. The key word here is static, meaning non-energized. As soon as energy is applied, it all changes. It is only true when the system is off.
In a functioning system, it is entirely possible to have superheated vapor bubbles surrounded by subcooled liquid and subcooled liquid droplets surrounded by superheated vapor... and this is in fact common in portions of the liquid and suction lines.
Velocity carries the bubbles and droplets far past the points where you might expect them to disappear. That's why you need substantial amounts of subcooling to clear a sight glass and substantial amounts of superheat to avoid flooding the compressor. How much you need depends on the velocity.
It varies, but essentially you need 10-15F SC (5.5-8.5K SC) to clear the sightglass... and sometimes a little more. If any of the TXV's are flooding the coil, it should be fixed before you try to clear the sight glass.
You might find this thread helpful:
http://www.refrigeration-engineer.co...rigeration-101
Thanks Ranger1 and Gary. I like the sound of a static gas law as opposed to the other type.
ill see further what the inter web has to say.