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Cockpit freezer charge level
August 5, 2010
11:58 am
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Steve Pooler
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reefgeorge said:

Thanks, good info. What is the best t- stat set point for a cockpit freezer?…Can't really say George…Except you always want the compressor to cycle before it pulls down into a vacuum…What should the max compressor case temp be?…No cut & dry recommendation here either George….Like I said…Just try to get it a bit cooler (cooler than 190 is better) without frost all the way back to the compressor.


I will say that if all you are keeping in a cockpit freezer is bait...You don't need much colder than 15-20 degrees.

Steve~

August 5, 2010
11:50 am
reefgeorge
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Thanks, good info. What is the best t- stat set point for a cockpit freezer? What should the max compressor case temp be?

August 5, 2010
10:20 am
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Steve Pooler
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I should add...You may loose a bit of ambiet air temp box performance (this will depend on your box & insulation etc.) but that is the price you pay to keep your compressor happy....134a really is at it's limit down here, and you may have to set your t-stat a couple of degrees warmer to make the compressor cycle....Box temp will be what it is at 2 psi cycling...

Steve~

August 5, 2010
9:46 am
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reefgeorge said:

Thanks so far. The thermal expansion valve discussion makes sense but then how do you charge to a "full sight glass"?…You don't really…Presumably to a fixed set of conditions?…When you think about it, there really are no "fixed" set of conditions…Everything continuously changes…How much of the compressor cooling comes from the refrigerant vs. the heat exchanger?…None…No compressor cooling comes from the water…It only removes heat from the refrigerant…If you look at it closely, the only thing the water touches/interacts with IS the refrigerant…I would think that a part of the reason for the hot compressor is the long line length and relatively warm returning gas?…To some extent yes, and I'm about to finish my explanation from yesterday below.


Two things are of concern here:

  • Compressor Temp
  • Finish suction pressure at T-stat cut off

Let me first explain what I started about R-134a…It was developed as a Green replacement for the long time standard R-12, and 134a is close to R-12 in pressure vs temp at any given point, but down in the lower temp/psi range is where it becomes critical… Both R-12 & R-134a are considered medium temp refrigerants that can operate at evaporator temps of down to -10 F…Here's where it gets a bit interesting…

One of the things you always want to stay away from is allowing a system to run down into a vacuum (below "0″ psi) except with some specialized systems that are designed to run that way…It's never good for oil return to the compressor to run in a vacuum…You always want a positive pressure for that oil to return.

Typically (depending on insulation, box vs plate size, door gaskets etc.) You need a plate temp of 10 to 15 degrees below the ambient box temp you are trying to maintain…So to maintain that 0 to +5 degree ambient box temp, you need every bit of that -10 degree plate…

To attain a -10 degree plate with each refrigerant….

  • R-12 @ -10 degrees is at 4.5 psi…
  • R-134 @ -10 degrees is at 1.9 psi…A little less fudge room here to keep you out of a vacuum…

This is where refrigerant charge, or TXV superheat adjustment is critical…

I don't really advocate a DIY guy going in and adjusting superheat on his TXV valve (and you need special/accurate temp sensing of tubing to do so properly) but you can tweak the refrigerant charge a bit, and I'll try to help with that…

If you want more explanation on superheat & subcooling, you can find much better explanations by Googling either or both terms than I can provide easily here….Too much typing  Wink

Tweaking the charge…

First thing to know is…What is the low side pressure at the time of the T-Stat cycle….It's likely to be in a vacuum if no one has done this before…Most guys won't, or can't stick around to wait for this event to happen as it can take hours, and on who's nickle ?

When operating in this range we know we don't want to run in a vacuum, but to maintain maxx cooling performance with R-134…We do want to maintain 2 to 3 psi…As I said…Not much room to fudge with…

If the system is in fact running into a vacuum…You want to add small shots of 134a to see if you can get the system to maintain that 2-3 psi just before t-stat cycling…Don't over do it (wait 10 or so minutes between shots) but what should also happen is you will see frost develop on the suction line closer & closer to the compressor with each bit you add…This slight bit of extra refrigerant will tend to cool the compressor too, but try not to add so much as you get frost all the way back to the compressor can.

You may ask "Why use 134a if it's that critical"….Good question and I'd have to agree…It's not really best suited for this type of job, but it is a old "Standard" (R-12) replacement that is slightly less suited to this application than the old Standard was…But 134a works great for the automotive A/C industry at temps that don't require these lower pressures…

A much better, or better suited alternative would be R-404a (Green replacement for R-502) which is good for evap temps down to at least -20 degrees…This would give some fudge room to still easily keep pressure in the + range…And No…You can't use R-404a in a 134a system. 

Steve~  

August 5, 2010
7:28 am
reefgeorge
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Thanks so far. The thermal expansion valve discussion makes sense but then how do you charge to a "full sight glass"? Presumably to a fixed set of conditions? How much of the compressor cooling comes from the refrigerent vs. the heat exchanger? I would think that a part of the reason for the hot compressor is the long line length and relatively warm returning gas?

August 4, 2010
7:08 am
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Steve Pooler
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George~

Yes that sounds like normal operation to me…What you are seeing in the sight glass is the TXV or TEV (Thermostatic Expansion Valve) Throttling to control the amount of refrigerant in the evaporator plate…The TXV is like a carburetor jet with a needle…When your glass is low on liquid, the jet is usually wide open to fill the evaporator fully…As cold refrigerant starts to exit the evap, the TXV's sensing bulb senses this and starts to close the carb jet with the needle to restrict the flow, and also lower the pressure, which also lowers the temperature of the refrigerant, and thus makes a colder evaporator...This is when you will see the sight glass fill because it is before the jet & as the jet restricts, the refrigerant backs up, or fills the glass.

This "throttling" will happen throughout the run cycle to keep both the evap full, and at it coldest even with changing load such as opening a door…TXV systems usually have much better recovery times to full evap than a fixed orfice system might…As you might imagine…More evaporator surface area at it coldest picks up the most amount of heat…As opposed to a evap the may be cold but not quite full yet.

I'm a little concerned about your compressor temp…Though it's common for refrigeration compressors to run a bit hotter than A/C systems…I also have a little concern in your box temp and how it relates to R-134a…Don't worry, I'm not concerned enough to say you should shut it down (it's been working 7 years now) but I need to get ready for work today and don't have time to get into it right now.

I'll explain more later…

Steve~ 

August 3, 2010
4:27 pm
reefgeorge
Melbourne, Fl.
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I have a Glacier cockpit freezer (Copeland compressor) with a remote compressor in the engine room that is water cooled with a March pump. The refrigerent line length is about 20 feet.

The freezer has been working fine for about 7 years except for the ocassional water flow restrictions. This week I was cleaning out the water circuit to improve cooling water flow when I decided to watch the refrigerant flow through the hi side sight glass. I noticed some vapor at the top of the sight glass before the on cycle (off for 15 minutes). Shortly after start up the vapor bubble dissapeared and the sight glass was full of liquid. I went back and looked after about 10 minutes and the bottom half of the sight glass body was liquid and the top half was vapor. The liquid level top is at the top of the incoming liquid line. There are never any bubbles, oil streaking, or foam. The water sensor shows no contamination.

BTW, this compressor has always felt hot to me. With all of my newfound refrigeration knowledge (much of it from you), I shot the top of the can with in IR gun and the hot spot is around 190F. The returning suction line is barely colder than ambient. There is no sweating anywhere. The box does pull down to +5F. The cooling water flow output is not warm, although I did not measure it. It's 134a refrigerent.

Is this proper operation for this system?

Thanks,

George