Walkin Freezer, Bitzer compressor

[smitc2]

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As found this unit was;
SubCool 4 deg F
SuperHeat 32 deg F
Compressor amps 7.7 Amps / Rated Max is 10.3A

After adjusting TXV & adding refrigerant to compensate for bubbles in sight glass as I opened the TXV, stopped charging the instant I had a full sight glass,
the results are as follows;

R-404a Refrigerant
Walkin freezer compartment air temperature is -1.2 deg F
Liquid sight glass is full without any bubbles
Ambient air temperature thru condenser 88 deg F
Suction pressure 22 psi (-12 deg F -PT CHART)
Discharge pressure 270 psi (110 deg F -PT CHART)
Liquid line temperature 95 deg F
Compressor rated max amps 10.3 but actual is 7.7 Amps
Evaporator exit tube line temperature -2.2 deg F
Evap SuperHeat is 9.8 deg F
Cond SubCool is 15 deg F
TXV adjustment is backed out all the way open.

In my rookie wisdom, I would say I've got this unit running pretty darn good. BUT, my compressor is iced over??? What have I gotten wrong??? What shall I do? Or is this what normal should look like? I was told the SuperHeat should be 8 - 10 deg F, should I increase the super heat & how much?

I'll attach a couple photos…
Thanks for reading & helping...

[smitc2]

And to add, I know it's taboo to touch that TXV...but the company who built this unit instructed me to charge for a full sight glass then adjust the TXV for 8 to 10 deg F superheat. This unit is on a drilling rig in the Gulf of Mexico, several others have worked on this unit before me. No one has ever taken ownership of it though, until now, I want to keep these units in top performance. I am a rookie at HVAC / refrigreation. So any advice or pointers is appreciated.
Thanks, David

[mikeref]

smitc2, TX valve has no automatic control when opened fully. Ice over compressor is never any good so you cannot run it this way. Make sure evaporator is clear of ice and fans are going the right way. TX, depending on make, is 2 and 1/2 turns open from fully closed position for factory set so valve can function correct. Also check orifice size..Mike.

[Brian_UK]

Also check the oil sightglass, it looks to be very frothy which could mean liquid refrigerant in the oil due to flood back.

If you added refrigerant then it means that it must have leaked out of the system somewhere; find the leak.

Get rid of all of the ice first and then start recording readings at design conditions.

[smitc2]

Thanks Brian, you're right on the oil foaming up, I think I was carrying liquid to the compressor. What should superheat be on a typical walkin freezer or is there a generic rule of thumb for that?
I made a change see below;

Went back to find the unit shutdown on high pressure.
Reset the switch & got the unit running.
Turned the TXV in 3 turns from full open.

Discharge pressure 255 psi (107 deg F - PT CHART)
Suction pressure 15 psi (-22 deg F - PT CHART)
Evap line temperature four inches upstream from the TXV Bulb hunting from -2.8 to -4.8 deg F
Liquid line temp 3 inches upstream from sight glass 99.1 deg F
I calculate SuperHeat 17.2 to 19.2 deg F and subcool 7.9 deg F
NO ICE FORMING on the suction line or compressor

What's the deal with my super heat? What should it be? Apparently 9.8 deg F is too small.

[Sandro Baptista]

The problem is that the TEX could be hunting due to "flash gas". Despite you have the sight glass full load it doesn't mean that at the TEX inlet you have only liquid. Check the head losses due to pipe friction (small section pipe) OR maybe even more important maybe the evaporator is to high and so the outlet liquid from the receiver is losing pressure due to hydrostatic penalization. In this case it would be important to have liquid subcooling by means of an heat exchanger. HOW MUCH the liquid is being raised from the liquid receiver?

Notice that when you close a little more the TEX the suction pressure has decreased and so the compressor capacity due to lack evaporating feeding.

[smitc2]

@ Brian_UK, I agree in principle about refrigerant leaking causing the need to add, BUT, maybe not in this case. This set of units have never been "taken care of" properly. It could have been low already without a leak & I have sniffed it out with nothing found.

There are 3 or 4 of us that keep them running, but 1 is a contract HVAC / refrigeration guy that we call out when need. I am the only other one that wants to work on this stuff, and I want to get them running perfect - not just charge to a full sight glass. I have gone to a couple basic schools & bought the tools needed. I read all I can about refrigeration on the internet even on my off time. I love it. I just feel kinda dumb right now, with trying to figure out whats actually technically wrong. The other guys don't care, they just put in refrigerant until it makes cold & leave it be...I want it right.

[Sandro Baptista]

@ Brian_UK, I agree in principle about refrigerant leaking causing the need to add, BUT, maybe not in this case. This set of units have never been "taken care of" properly. It could have been low already without a leak & I have sniffed it out with nothing found.

There are 3 or 4 of us that keep them running, but 1 is a contract HVAC / refrigeration guy that we call out when need. I am the only other one that wants to work on this stuff, and I want to get them running perfect - not just charge to a full sight glass. I have gone to a couple basic schools & bought the tools needed. I read all I can about refrigeration on the internet even on my off time. I love it. I just feel kinda dumb right now, with trying to figure out whats actually technically wrong. The other guys don't care, they just put in refrigerant until it makes cold & leave it be...I want it right.

smitc2,

Can you answer to my post, please?

[smitc2]

@ Sandro Baptista,
I haven't been out to the unit today to take exact elevation measurements but, I would "guess" the lines rise about 6 feet and travel about 15 feet to the evaporator from the condenser. So I would say the total length to be about 21 feet. Liquid line size is 1/2 inch, Suction line is 1-1/8 inch.

I have thought about what you said & it makes good sense. So can I just add refrigerant to push the solid liquid point to my TXV? Must I have a heat exchanger to get liquid at the TXV? This freezer compartment is supposed to run about -8 deg F.

My outdoor return line insulation is in terrible condition, drips condensate, no insulation on my liquid line...would insulating the liquid line help? Maybe reinsulating the return line would help?

[smitc2]

I got to thinking about it...so I had a 9.8 deg SH, the purpose of SH is to ensure the vapor is above the the boiling to keep the compressor healthy, right? So 10 deg SH comes out of the evap but has go thru about 6 or 8 feet of sub zero temperature pipe which begins to eliminate the SH, which starts to condense the refrigerant in the pipe as it travels that 6 or 8 feet plus the 15 or so feet out and down 6 feet to the compressor. By the time it gets to the compressor it's starting to partially condense into liquid - causing it to ice up my compressor. By increasing my SH to 19 I now have sufficient heat to keep it vapor by the time it gets to the compressor.

Is this a fair assessment?

[smitc2]

@ Sandro Baptista,
The lines rise 87 inches from the outlet/inlet of the condenser unit cabinet, then they drop 22 inches from the bulkhead penetration and connect to the evaporator unit.

Do I understand correctly, you want me to connect the gauges at the evaporator & check the pressure there???

[Sandro Baptista]

87 inches-22 inches...The hydrostatic penalization due to the liquid rise is not very significant...however everything helps.

1st Do you have the sign glass upstream or downstream the filter dryer?...this one could be a little blocked.

2nd The best would be put the sign glass just upstream the TEX so you would view if vapor exists at TEX inlet.

3rd Is the TEX frozen? Even if not clean is internal filter any way.

4th Install a good coaxial liquid-vapor heat exchanger so you have subcooled liquid and higher superheat, without reduce the liquid mass flow on the evaporator (less capacity at evaporator)...you will profit in means of compressor capacity.

[Sandro Baptista]

Don't insulate the liquid line unless you have a environment temperature higher than the condensing temperature where the liquid lines passes on.

[Magoo]

Hi smitc2.
TXV settings from factory are generally set mid way from fully closed to fully open.
Evaporator superheat settings....., should be 60 > 70% of evaporator TD., TD is the difference between air on to evap coil and the evaprating temp inside the coil. calculated from the pressure in coil PT conversion.
So if you have -33'C at compressor estimate pressure in coil at approx -30'C. Air on at -23'C [ room temp -8.0 F ] TD = 10'C x 0.6 superheat = 6'C . Check temperature of suction pipe at the TXV bulb position and it should read -30'c less +6.0'c = -24 'C .
Sorry I work in SI metric but same applies to imperial degrees F.
Any adjustments to TXV settings wait 15 minutes for valve to stabilize.

The enthalpy of low temp vapour going back to compressor wont frost suction at compressor, lots of sweating though.
Hope this helps.
If you added refrigerant to system to a clear sight glass with TXV cranked wide open, then adjusted TXV again, system is probably overcharged so the HP fault.

[mikeref]

Magoo, ( now you are just showing off).You forgot to tell him where to place the TX bulb, position on pipe?? Also, setting and checking superheat is best done when room is near setpoint, allowing time for valve to "cycle" through it's auto range .. Mike.

[Magoo]

Hi Mikeref.
thanks for comment, OK the TX buld should in the evaporator enclosure, on the suction pipe leaving the coil header, personally I like t wrap with insulation., if on the horizontal, position at 4 oclock position, if on the vertical place on outside flow aspect after bend or P trap. On both place so to tail is at the top and in direction of flow [ a sporlan thing that has stuck in my mind for decades ].
The method I described is good for pull down or at design. The Kueba method that I found years ago and works a treat if TXV has been selected correctly initially, now that is a totally new sudject.
Evap super heat settings and checking has had truck loads of scribe on this site, a quick seach will pull up huge discussions and comments.

[smitc2]

Thanks, Magoo...will go look at that. Currently I have trouble with another unit. Think it had a moisture ice plug in it. Recovered, evac, nitrogen, evac now. I have more work to do than I can accomplish by myself. I'll get'er done though...

[Magoo]

Hi again smitc2.

Good on ya. What do they say..., a problem shared is a problem halved, tell all and we can help.