Low Temp 404A system problems

hvacdoctor · Sep 29, 2004

have a large walk in storage Freezer designed for -15F. We currently are at -8F. System has (2) 30HP Hussman SLD 0301 L4M air cooled condensing units with clean coils with matching remote electric defrost evaporators. One of the system's had R404A gas added every few months or so due the flashing sightglass. We recently leaked checked the condensing unit found no leaks, then added dye to system checked entire condensing unit had piping down to the evaporator and still no leaks. Then found 1/3 ice covered evaporator, found defrost needed repair fix it and cleared coil with hot water. Changed the defrost setting from every 6 hours @30 minutes to 45 minutes 4x per day. Is that too long for freezer defrost????

Tech thinks that iced up coil could have given us low charge indications on the sight glass. He pulled out 20Lbs or so of gas but the system sight glass is still clear.

Also performed pump down test on copeland 6DT3-300E COMPRESSOR. COULD ONLY GET SYSTEM INTO 15 PSI VACUME WHILE THE OTHER UNIT PUMPED DOWN RATHER QUICKLY DOWN TO 25PSI.

BEFORE WE OPEN UP COMPRESSOR WE WANTED TO LET THE SYSTEM RUN A FEW DAYS WITH A ICE FREE CLEAR EVAPORATOR COIL BEFORE OPENING UP THE COMPRESSOR.

ANY HINTS WITH CHARGING 404A SYSTEMS? ANY ADDITIONAL HELP OR IDEAS WOULD HELP.

[Edited by hvacdoctor on 09-29-2004 at 11:49 AM]

basser · Sep 29, 2004

What's your suction pressure? Is it low enough to achieve -25°evap (-15° box)? The pump down test really doesn't tell you alot about the compressor. The biggest thing here is after you shut the compressor off did it hold the low side pressure or did it raise?

If you want to see if the compressor is operating correctly look at the amp draw. Measure and record suction and discharge pressures, voltage and amperage. Compare this information to the proper compressor curve sheet. If you can, post the information here and we will check it for you.

rayr · Sep 29, 2004

Need to know superheat at evap. sub-cooling, td over coil etc. Can't remember whos service bulletin but i have on somewhere about running POE systems in a vacuum. Maybe Dave can help out here. I only looked it over briefly but said it was a definite no no.

westcoast refer man · Oct 1, 2004

It takes what it takes to defrost the coils. You are stuck with the limitations of this sysytem. Make sure all sources of moisture from infiltration etc have been reduced as much as possible. Is the box in good shape, or is it part of the problem?? Typically when a system has been designed properly (how often does that happen?), the equipment has been sized to allow removal of the heat load with the time alloted for refrigeration vs defrost. In other words, instead of doing say 25 tons in a 24 hr period, the capacity of the equipment is "upped" so that it can do 25 tons in an 18 hr period. This would allow for 6 hrs of defrost. Based on the changes you made to the time clock, the total amount of time spent in defrost has not changed. It may be that 3 hrs total defrost time is not enough.

Iced up coils can definetly cause a sight glass to flash. Check the reciever's liquid level as well as the sight glass to determine correct charge after the coils are clear. Assuming the reciever is not insulated, the liquid level can be determined by heating the reciever's "end bell" with your torch and then carefully touching it to see where the metal turns from hot to not hot. The transition between hot and not hot is the liquid level, but be careful not to burn yourself when doing this.. The receiver should be 1/3 to 1/2 full. If after filling to this level you still have a flashing sight glass-we need to talk again. Use compressor capacity curves to determine if compressor is pumpimg properly. While the pump down test you described is useful, it is not a definitive test.

404a at -25 or even as low as -50 is still in a positive pressure.

R12rules · Oct 1, 2004

A pump down test is practically a waste of time.
Like others have stated in many other threads, the compressor curve charts are the only way to know what capacity a compressor is operating at.

As far as the charge goes, a system will require the most amount of refrigerant once it is down to temp and about ready to shut off.
And this only counts when the whole entire set up is designed to work together for all the factors involved.

Structure, load conditions, coils/condensing units... they ALL need to be working together in perfect harmony.
Otherwise you got a mis-match. And this will cause prolonged run, at the very least!
At the worst, it will never hold the product to your desired specs. And THAT my friend, is where the nightmare begins!

I agree with West-Dude. He is right.

Once you've got the coils consistently clean and the system is fully charged... you can expect 100% refrigeration to occur.
An old rule of thumb was to desgign equipment to run 60% of the time. Then shut off.

Once your freezer drops back down near it's desired temperature, you will begin to see how much time the compressor(s) will be running.
Depending upon the size of your freezer box, this may take days before your there.
And once it is close, it may require some adjustments to get it the rest of the way.
Clear coils and properly charged condensing unit along with properly feeding TX valves will get you there.

Measuring your superheat and subcooling will be the best thing to do when your at that design temp.
But prior to that ... your system is running an uphill battle and those components will be maxed out unitl they settle in near design temp.

condenseddave · Oct 2, 2004

rayr said:
Need to know superheat at evap. sub-cooling, td over coil etc. Can't remember whos service bulletin but i have on somewhere about running POE systems in a vacuum. Maybe Dave can help out here. I only looked it over briefly but said it was a definite no no.

Dave will be back, (Yes, I'm going BACK to ork, again.....

) but, in the meantime, read this thread:

http://hvac-talk.com/vbb/showthread.php?postid=602537#post602537

condenseddave · Oct 2, 2004

By the way, is the product being loaded into this thing at temp, or are they attempting to pull the product temp to a final temp with it?

3956 · Oct 3, 2004

condenseddave said:
Dave will be back, (Yes, I'm going BACK to ork, again..... ) but, in the meantime, read this thread:

[/B]

AH-Haaaa! Your true identity shows itself Daavvve! your Mork, from Ork,....... say, could you introduce me to Mindy??? I allways wanted to meet her.

Sorry bout your little egg buddy on that other thread. Musta been one of your missing probes.

[Edited by TB on 10-03-2004 at 02:26 AM]

condenseddave · Oct 3, 2004

TB said:
condenseddave said:

Dave will be back, (Yes, I'm going BACK to ork, again..... ) but, in the meantime, read this thread:

Click to expand...

AH-Haaaa! Your true identity shows itself Daavvve! your Mork, from Ork,....... say, could you introduce me to Mindy??? I allways wanted to meet her.

[/B]

If I knew Mindy, I wouldn't be going back to work at that time of night.

That third person thing was in jest regarding Ray's reference that I 'd be along.

condenseddave · Oct 3, 2004

Kinda like the suspenders, though.

condenseddave · Oct 3, 2004

And I really need to fix my "W" key. It's getting annoying, now.

condenseddave · Oct 3, 2004

Scattered thought #4

The only thing I remember reading about not running anything in a vacuum is the Copeland scroll AE's. It'll immediately cause shorting across the compressor terminals, due to there being no resistance in a vacuum.

You should never run ANY compressor in a vacuum, really, for the same reason, unless that compressor is designed for it.

I know of some old dipping cases (R12) that need to run in a vacuum to maintain their case temps, but none with 404a. I mean, a 5 pound suction on 404a is a 40 below coil...

POE is a touchy oil, the lower the suction, the touchier it gets. The key is to maintain a good superheat, to prevent the discharge temps from getting out of hand.

All that said, it's not real relevant to the original question, I guess.

There is no set standard that I know of for a compressor to be able to pull against a closed SSV. Also, there are other factors that can effect the outcome of a vacuum deacy test.

BUT, if you have 2 identical compressors, one goesto -25, and the other struggles to -15, your first order of business is to take the operating suction and discharge pressures, write them down, and record the operating voltage and amperage of each phase, on each compressor, write them down, and average the readings on each. Then get your Copeland wholesaler to give you photocopies of the curve sheets, or go to the Copeland Energy calculator and see if the compressor is inefficient. I suspect that it is. If it is, pull the heads, look for broken valves, (ICED coils usually FLOOD) loose, or broken reed pins, bad gaskets, bad plates, broken pistons, excess oil on top of the piston, etc. Fix or replace the pump, and be on your merry way. Apparently, the system wasn't designed to run 100% on just one unit, but was well designed to hold well below zero with one bad one.

Here is an example of the report you get when using the Energy Calculator progrma from Copeland:

Note that I used the compressor number you gave. This does NOT mean that you should see the same amperage as the example above. You have to do your own curve based on what conditions you are seeing.

hvacdoctor · Oct 5, 2004

Thanks for the replies. ? about pump down test. Why don't you recommend the pump down test to check compressor eff? The reason is we installed a new compressor last year on the other unit it pumped down rather quickly -25psi while the unit in question took 2x as long to get down to -15psi??? Just curious about this.

coolprod · Oct 6, 2004

hvacdoctor said:
Thanks for the replies. ? about pump down test. Why don't you recommend the pump down test to check compressor eff? The reason is we installed a new compressor last year on the other unit it pumped down rather quickly -25psi while the unit in question took 2x as long to get down to -15psi??? Just curious about this.

You can use "pump down" as only PART of the diagnosis!! Yes this will tell you how well the suction valves are working, but no 2 compressors will be identical.
Use the "time" it takes to pump down. Then shut it down and watch how fast it "leaks back" and how fast it takes for it to get to "operating" pressures. Use this information with with the compressor curve (amp draw) superheat and subcooling. with all this information you should be able to determine if the compressor is efficient or not.

My 2 cents...

3956 · Oct 8, 2004

granted, this "curve sheet " thingy is new for me, but one way I proved before a leaky set of valves, instead of leaky solenoid was the temp rise of the suction line, as the hot gas leaked through the compressor valves, warmong considerably the suction line.

3956 · Oct 8, 2004

westcoast refer man said:
Iced up coils can definetly cause a sight glass to flash.

can you explain this?? seems to me that if the coil is frosted over the txv will be shut tight to keep SH up in a coil not absorbing heat due to low air flow. If its a fixed orfice, pressures will drop creating lower flow, ...unless...the lower temp in the system lowers in turn the refrigerant volume enough thatit mimics a low charge condition???

not arguing, just not understanding the physics

westcoast refer man said:
Check the reciever's liquid level as well as the sight glass to determine correct charge after the coils are clear. Assuming the reciever is not insulated, the liquid level can be determined by heating the reciever's "end bell" with your torch and then carefully touching it to see where the metal turns from hot to not hot.

just slap your hands on the hot reciever and the red will indicate refrigerant level like an upside down thermometer

Another thought: you got headmasters on the systems that could be leaking or open. Txv sticking open?

thehumid1 · Oct 8, 2004

Originally posted by westcoast refer man
Check the reciever's liquid level as well as the sight glass to determine correct charge after the coils are clear. Assuming the reciever is not insulated, the liquid level can be determined by heating the reciever's "end bell" with your torch and then carefully touching it to see where the metal turns from hot to not hot.

well I just learned something I was gonna suggest remove all the charge pull a vacuum and recharge it. If you have been adding freon for awhile because of a low sight glass the system could still be overcharged even after what has been taken out by your "tech" which may be why it takes longer to pump down. If the system worked fine before on a 1/2 hour defrost every 6 hrs why not try to locate the real problem instead of create more by playing with defrost cycles. Someone mentioned door seals/humidity level, another cause of icing of the coil could be gas backfeeding thru a solenoid valve if that valve is bad, bad txv, inefficient compressor(which is the hardest to diagnose for me) without a sure sign of compressor inefficiency(high amp draw,bad pressure readings, I try everything else first. Unfortunately by adding refrigerant as your cure it makes the diagnostic starting point more difficult as it just added to the problem and who knows may have damaged the compressor compounding one problem with another.

[Edited by thehumid1 on 10-08-2004 at 04:16 PM]

westcoast refer man · Oct 8, 2004

TB said:
westcoast refer man said:

Iced up coils can definetly cause a sight glass to flash.

Click to expand...

can you explain this?? seems to me that if the coil is frosted over the txv will be shut tight to keep SH up in a coil not absorbing heat due to low air flow. If its a fixed orfice, pressures will drop creating lower flow, ...unless...the lower temp in the system lowers in turn the refrigerant volume enough thatit mimics a low charge condition???

not arguing, just not understanding the physics

TB, no I can't explain the physics on this one, but in my many years of experience I have found this to be true. Perhaps I should have made that statement differently, which is the problem with a message board; people can pretty much take you only in the literary sense. Let me rephrase my comment- I've seen many an evaporator iced up with the sight glass flashing. In those situations, I always get the evap clear and clean before I consider the charge. Plently of times, but not every time, I've found that the system was fully charged after having defrosted. In other words, don't assume you're short on gas if your evap is iced.

3956 · Oct 9, 2004

OK then, I'll float my boat along side you there, I agree with that, and have seen that myself too.

Also seen a chiller compressor puke cause they added more refrigerant while it was allready flooding the compressor, just because the sight glass was flashing.

I guess the jist of these posts is, just because the sight glass bubbles a bit don't guarantee a low charge, just guarantees a need to look into why its bubbling.

degenetron · Oct 10, 2004

You make mention that box is utilized as a storage freezer.Assumming product is frozen when loaded into box,and system was designed,and installed properly,ie; proper refrigeration line sizing,piping practices, dehydration,excessive infiltration etc.,and product does not interfere with air patterns in box.Would be helpful to know suction and discharge pressures, liquid line temp.,suction temp.at txv bulb,at inlet to compressor,and what type of head pressure control.If system has been losing refrigerant, R404a is a blend,and will fractionate.If you do not know how much freon has been lost,adding freon may be futile.You would be better off replacing entire charge.As far as charging correct amounts into system,your equipment manufactures data will tell you how much refrigerant evaps. take,and cond. units take with 80%of reciever filled.Estimate piping lengths,and refer to charts on how much freon they hold.(Bohn offers this info. in thier installation manual).This will get you started,fine tuning now can be done when box pulls down to design temp..You say that your defrosts are set for 30,or45 min..Keep in mind that these are fail safe times,your defrost/termination are the controls you want determing adequate defrosting,not failsafe time.You may have to replace non-adjustable thermo-disc with adjustable type,depending on your frost loads.

Low Temp 404A system problems

hvacdoctor

basser

rayr

westcoast refer man

R12rules

condenseddave

condenseddave

3956

condenseddave

condenseddave

condenseddave

condenseddave

hvacdoctor

coolprod

3956

3956

thehumid1

westcoast refer man

3956

degenetron

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