Re: Getting Colder, need advice
Quote:
Originally Posted by
Gary
...
Which brings us to a gadget that can limit the max TD thus safeguarding the compressor, without lengthening the run time: A crankcase pressure regulating valve (CPR). A CPR limits the pressure entering the compressor to a maximum value, and thus limits the load placed upon the compressor.
Or a MOP TEV which act for 2 purposes: protecting the compressor for a too high load and once on stable conditions, feeding more precisely the evaporator.
And it's less expensive.
Installing a much bigger condenser which can handle the big load at startup is another option.
Re: Getting Colder, need advice
Quote:
Originally Posted by
mvtop
Goober.
it sounds like the txv system would simplify the system by removing several varialbles.
what's all needed to for this?
I am limited on space with the platform that the compressor sits on. So adding several items would be tough space wise.
And more specific a MOP TEV.All you need additionally is a small receiver which can be some length of 3/4 tube in a spiral form.
Re: Getting Colder, need advice
Keep also in mind that during the off cycles with a cap tube all the refrigerant will flow to the evaporator very fast.
This will provoke liquid slugging at startup.
We always install a SV with a TEV to prevent this.
Or let it pump down each time.
So a small liquid separator - which you find in most domestic refrigerators - will be needed to protect your compressor.
You can make this yourself.
Re: Getting Colder, need advice
Quote:
Originally Posted by
Gary
We seem to be getting strange SH readings. I'm thinking the accumulator is filling with liquid during the off cycle and the SH readings are low during the on cycle until the accumulator empties.
Liquid will form in the evaporator while the system is off. If the suction line goes from the bottom of the evaporator down to the accumulator, the liquid in the evap is transported by gravity to the accumulator.
This is easily remedied. Where the suction line leaves the evap, run the suction line up to the top of the evap and then back down to the accumulator. This traps the liquid in the evap.
I think i do have the suction at the bottom of the coil.
I'll move things around to incorporate a loop so any refig. in the coil stays in coil until pulled out.
I"ll also solder the cap tube to the suction line.
any other thoughts while i have it torn down?
i'll post some new numbers when i get this done so we can see what changes it makes.
as for charging, i'll be using this "At design cutout temp, SC is 15F or less and the compressor inlet SH is 20-30F" as my target.
as allways, thank you for your feed back
Mvtop
Re: Getting Colder, need advice
Quote:
Originally Posted by
Peter_1
And more specific a MOP TEV.All you need additionally is a small receiver which can be some length of 3/4 tube in a spiral form.
thanks for your thoughts peter.
how would you spec. MOP TEV for this system?
Mvtop
Re: Getting Colder, need advice
You might want to consider a TXV/w MOP and matching condensing unit/w receiver... or the modifications to your present system that Peter_1 has suggested. Or we can save some money and continue making improvements while adjusting referigerant flow and charge along the way. The decision is yours.
But then perhaps we need to know how the finished system will be used. If you are going to frequently subject it to a full pulldown from ambient, plugging it in on the weekends just before the party, then you may be better off with a TXV system anyway.
Re: Getting Colder, need advice
Gary,
since this is the first run at this, and i started off the project planning on putting some used parts back in service, lets stay the course and adjust what we have and see how it turns out.
If it turns out good, I can always come back to the drawing board armed with better knowlege from my first run. As well as how it would be used.
So how to improve what I/we (group project) have now,
i'm thinking that i need to add some length to the cap tube to slow things down a bit. I think i'm running a 32" tube now.
I would expect to get better performance with the heat exchange once i rework the coil again. This will be done with the final version of the container. I will stay with the same size tubing, and as close to the same volume for the coil as i currently have.
Reduce refrig. charge
anything else?
thanks
Mvtop
Re: Getting Colder, need advice
I would leave the cap tube as is for now.
Re: Getting Colder, need advice
Very interesting data, the plot below shows the pulldown pressures versus mass flow rate, it is this plot that can show you where the tube is in perfect conditions and that is at the Knee of the discharge pressure. To the left of the knee i.e. at lower mass flow rates the tube will have liquid and gas entering it and your quality seems to be around x=0.1 which is fairly good and if you try to make the tube longer to gain that extra 10% of liquid and have an x=0 at the tube entrance you will certainly get higher start up pressures. 215psi is quite high but the amps seem ok so not much to be done.
The one thing is that at the exit to the tube the flow is sonic, this tends to restict an increase in flowrate when the evaporator falls in pressure. It is possible to opt for a larger tube say .050 with a length of about 9 feet - this would be about the same as your 32" of .040 but would not be sonic at the output and would be much better on a warmer day.
http://www.refrigeration-engineer.co...&pictureid=879
The only caveat about this plot is the lack of data during the critical phase from intial startup to the knee, to really nail this down it would be handy to have at least 5 (or more) points. About one reading every 1 or 2 mins for the first 10 mins.
Otherwise if you think its not worth the messing about - box it up and get cold beer.
Chef
Re: Getting Colder, need advice
Quote:
Originally Posted by
mvtop
So how to improve what I/we (group project) have now,
i'm thinking that i need to add some length to the cap tube to slow things down a bit. I think i'm running a 32" tube now.
I would expect to get better performance with the heat exchange once i rework the coil again. Reduce refrig. charge
anything else?
thanks
Mvtop
If you lengthen the tube then the following may happen:
The controlling element is the compressor and not the tube, it is the compressors suction volume and suction conditions that dictate the systems mass flow rate. The other components just adjust themselves to fit to the compressor. So if you increase the length of the tube then:-
Before the knee - i.e. at flows hight than at the knee - the condenser will begin to back up liquid and reduce its effective cooling till the pressure is high enough to force the mass flow through the tube.
The consequences are a higher head pressure, a starved evaporator and more power consumption.
After the knee - i.e. at mass flows less than at the knee - the increase in the tube will make the quality move towards zero and the pressures will remain almost the same as now. You will see an increase in cooling effect, there will be more liquid in the evaporator because the the charge will need to be a little bit more. The corrollary is that more liquid is then in the system and during startup even more is built up in the condensor and the discharge pressures are higher.
Its a sort of win at one end and loose at the other.
If you are thinking about redoing the evap coil have you considered putting the coil inlet at the bottom and outlet at the top, it seems more common to do it that way - in all the systems I have seen anyway.
Chef
Re: Getting Colder, need advice
Hi,
I've got the latest version up and running. It seems to pull it done to temp fairly fast.
Although, when it gets about 85 deg. in the shop it seems to struggle / runs longer to keep it at temp.
Would this be a condensor issue, as not large enough or needs more air flow?
i got my new/final container for the coil that i'm working on, i'll pulling the system apart again to get it installed.
hopefully the last time.
thanks for your help with all the feed back.
i'll post more as it gets closer to finished.
thanks.
Mvtop
Re: Getting Colder, need advice
Mvtop - we did'nt really get down to the cause of the very low SX on your last set of data i.e. 1psi instead of 7 psi as noted earlier - maybe a gas leak? Also what mods are you making with the coil etc.
From your earlier data the condenser may be a little small for a hot day but that will probably be on a startup issue only - have to wait to see the latest data to see what you have done.
Keep going.
Chef
Re: Getting Colder, need advice
Quote:
Originally Posted by
Chef
Mvtop - we did'nt really get down to the cause of the very low SX on your last set of data i.e. 1psi instead of 7 psi as noted earlier - maybe a gas leak? Also what mods are you making with the coil etc.
From your earlier data the condenser may be a little small for a hot day but that will probably be on a startup issue only - have to wait to see the latest data to see what you have done.
Keep going.
Chef
Hey Chef,
I do think there could of been a small leak, since the numbers were so different from where they started.
And I had a few flare fittings installed to make dismanteling easier. They can be tricky to keep the leaks away.
as to what mods, i did add the loop to the coil so gravitiy wouldn't drain any ref. into the compressor during off times, as Gary mentioned. And moved the return to the top of the coil and the supply to the bottom as recomended. Also, i did soft solder and wrap the cap. tube to the coil return and insulated well.
for the mods of the new coil, it will be made of stainless steel, not as good as the aluminum i have now, but it's work better overall for my design.
I plan to make a tighter wrap this time and remove all gaps between container and coil, solder/braze the coil to the container to help with heat transfer.
It will hold and cool 2 gallons instead of 3 gallons. I"m not sure how much smaller the coil will be in terms of volume compared to what i have now. i'll know in a day or so as i get closer to finishing the coil. i was going to try to stay fairly close to the volume i have now so we can stick with the same length cap tube. I have a 39" .040 cap tube. I remeasured it when i had it apart again. I think I said I was using a 32".
Is there any way to estimate the length of cap tube I might need if the volume of the coil is reduced by x%?
I was using trail and error before. I don't think i can lengthen the cap. tube by x% the same as what i reduced the size of the coil.
I really won't know what the volume of the new coil will be until i'm done with it. It might be close enough to what we have now. that's what i'm shooting for.
I'll have some pics as I progress with this.
Again, thank you all for your comments / feed back.
Mvtop
Re: Getting Colder, need advice
Quote:
Originally Posted by
mvtop
Hey Chef,
Is there any way to estimate the length of cap tube I might need if the volume of the coil is reduced by x%?
I was using trail and error before. I don't think i can lengthen the cap. tube by x% the same as what i reduced the size of the coil.
Mvtop
The length of the cap tube is related to the compressor mass flow rate and the condensing and evaporating temperatures (and hence pressures) - so as these have not been altered (much) then the tube length will stay the same.
However if you change the coil size by a lot then evap temps and pressures will change a little. In your case it is not so critical as you are running your cap tube with a sonic output and so changes in the evap pressure wont affect the flow rate in the tube.
It will affect the suction conditions though and as you have a smaller coil size the pressure will drop a little so the mass flow will drop a little too and you will have less charge in the system.
It all depends on how much you reduce the coil size, the smaller the coil the smaller the heat transefer area and so to get the same heat flow the temperature differance will need to greater, this in turn forces the suction pressure down. As you are using a HBP compressure you dont really want to push it down too far?
Chef
Re: Getting Colder, need advice
Hey,
Happy Holidays.
It's been awhile since I’ve posted on this project, mainly becasue it's working fairly well and i've been off on other tangents.
I thought about putting this as a separate post, but i think there’s some history here that's relevant to the story line and instead of covering it all again, I’m just adding to the already long post.
Gary,
I would like to improve the performance of the cool down time. And as you mentioned, cap tube systems are designed for pulldown speed.
Quote:
Originally Posted by
Gary
Let's keep in mind that cap tube systems are not designed for pulldown speed. We plug them in, they pull down to the desired temp and remain very near that temp for the remainder of their useful lives. If we are designing for pulldown, we need a TXV.
So I’m taking your suggestion Gary about using a TXV in my system. As you know, this is not your typical out of the box project. I'm ready to put this project back on the workbench again.
So my questions relate to how to go about sizing / set up a TXV for my setup and what would be needed in addition to what i have. Which is basically a compressor, accumulator, condenser, and homemade evap.
Again, thank you all for your posts and suggestions.
mvtop