Oscillating Suction Line Temperature

[glaird]

Hello,
I have a 1500 BTU freezer in an Ice Cream Vending Machine that I am repairing. The unit was originally an R12 unit that I have converted to MP66. It was not working when I started the repair work and I have no idea what the correct R12 high side and low side pressures were. The unit label states that the charge should be 15 Oz of R12. I have taken 80% of this value (MP66 adjustment) leaving a 12 Oz MP66 charge. I have added a larger liquid line filter so I have then added 1 more oz to account for this--the total charge is about 13 oz.

I have had a number of problems with the unit, most of which have been fixed now. The last being a restricted cap tube. I cut off the first 2" of the cap tube and this fixed the restriction. My high side pressure is running about 125 PSI, the low side about 0 PSI. The compressor is running about 6 amps, the RLA value. I may be a little low on charge--I haven't added more for the reason below.

When I operate the freezer and the freezer cabinet is close to its set temperature (1 degree F) my suction line temperature seems to oscillate with a pretty high amplitude. I have attached a pdf file with a chart of the oscillation. I am thinking that if I add more refrigerant, the oscillations will get larger and I may return liquid back to the compressor--so I am hesitant to do this.

I have taped the cap tube to the suction line for most of the suction line length (50"). The suction line and the cap tube are inside an insulated sleeve. I did this with the idea of precooling the refrigerant before it hits the evaporator. I am wondering if this thermal connection may be causing my oscillation.

Has anyone seen this sort of thing before?

Thanks,
Greg Laird

[glaird]

My attachment didn't upload on the first post. It is here.

[royalchiller]

I know this may seem stupid, but, are you sure you fluke clamp-on isn't having problems? I've found that fluke thermistors tend to fluctuate when they get old. Just a thought.

[mikeref]

Glaird, i've not used MP66 but would you consider a different refrigerant and compare the results?

[Gary]

Assuming the suction line temp measurement is near the compressor inlet, it seems a little overcharged to me.

[razor007]

what is the ambient temprature/ air onto condensor and did u change the oil from mineral to alkylbenzene at the time of retrofit?

[after5HVAC]

Did you find the trouble? I'm having the same problem with a Goodman A/C where the suction line temp. varies from 50 to 63 deg. every 6 to 7 minutes.

Thanks

[nike123]

Did you find the trouble? I'm having the same problem with a Goodman A/C where the suction line temp. varies from 50 to 63 deg. every 6 to 7 minutes.

Thanks

Is it TXV or capilary/fixed orifice system?

[after5HVAC]

On the parts list the metering device is called a "flowrater". I think that is a fixed orifice