holdover refrigeration

Got called to a little Yacht:

Unit description and application: Refrigerant R12 low temp freezer box with two holdover paltes

There are two refrigeration circuit for this unit. two separate condensers, one water cooled and the other air cooled. two different compressors one engine driven and the other compressor sites on a 1/3 hp hermetic compressor commercial condensing Tecumseh unit . two hold over plates are held in the freezer section. there is two TXV's on there respective line sets. from the txv refrigerant then passes to the first holdover then through the exit of the first hold over to the second hold over. I did not see any type of oil return between compressor (over course they are very different) i did not see liquid line solenoid.

When they have 115 volts on shore power then they would run the Tecumseh. (for what i think is one circuit of r12)
When the boat is off shore they use one of the two diesel engines to turn a belt/engine driven compressor (for the what i think is the second r12 circuit)

Interesting fact Tecumseh unit is controlled by a low pressure switch. But the other unit just has a high pressure differential switch and nothing else? i not sure how the second refrigerant circuit is maintaining box temp when out at see. any idea?

Question #1 are the two refrigeration circuits connected? (I did not see any evdnce that lead me to belive that but maybe im missing something)

Question #2. The Tecumseh condensing unit needs to be replace (small receiver leak and bad compressor)
The unit is R12.....so is the TXV. i really want to avoid touching the txv as it seems i will have to remove part of the counter in order to replace it. THAT BEING SAID I AGREE THAT I SHOULD CHANGE THE VALVE OVER TO THE NEXT REFRIGERANT. But i would like to know if i can use a R134a condensing unit and let it run on MP66. MP66 can work on many different oils. what are the problems might i run into if i do this?

try to keep in mind there is a very very limited amount of space around that txv thats why i ask. i know if i buy something r134a then i should really only put r134a.

Just posting for more information. At this point i will replace existing r12 with 404a and change the txv.

Thanks for any info

R404a, why would you replace R12 with that.
Compare it on PT chart to see what I mean, totally different.

ok, i looked at PT chart in regards to R404a, i do understand that you must mean its higher pressure at the same temp (-10) when compared to R12 (almost 20 psi)
R134a will have less pressure at the same temp as R12.

I understand that some compressors will run into a vacuum when they get close to design temp for low temp applications. I get the basic temp and pressure relationship but not how its able to work fully.

So should i take my thinking back to replacing that R12 unit and replace with R134a?

what else should i look into for my situation?

I would source a drop in for r12,134a retrofit will need all the oil removing and replacing with lubricant that suits.

ok...so MP66??? i can not buy an R12 unit...MP66 works with POE oil...can i get an r134a condesing unit and charge with mp66? if not why?

Im understand i can use R401B, R409A, R414B, R417C, are replacements for R12 low temp application.
the thing is i have little experience with temp glide...

Short answer is yes if you are changing the unit,the problem will be if there is any old mineral oil in the evaporator etc, remember most drop ins are charged liquid not vapour 134a is vapour charge so be very very carefully how you charge with liquid you could damage the compressor.

Thanks again for your reply.
another question is can i just use my nitrogen to blow out the old oil? should i use RX11 for just the evap section and then flush with nitrogen? will that help at all to remove the old mineral oil?

Yes but you will need good evacuation,via a vacuum pump to remove any moisture etc,be carefully to make sure the oxygen free nitrogen has been vented,then fit a new filter drier .

sounds good. thanks for your help

No problem,keep us updated on your progress,happy safe sailing take care.

you got it bud.
its not a sail boat, its a motor yacht.

But i hear you "wear my life jacket"

Head on over to Long Beach CA, would love to buy you a beer ;)