Danfoss bd50f, what's correct Psi?

sailboatliving

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My danfoss bd50f refrigerator compressor would only cool 2 degrees Celsius below room temperature before I added 134a refrigerant two days ago. Now it will cool to about 5.5 degrees Celsius, I set it to 4 degrees but it doesn't seem able to go that low, so I adjusted target temperature to 5.5.

What is the correct Psi?
Mine seem to be around 7-8 psi with compressor running, but it was 25 psi this morning with compressor at idle. Right now the compressor is running and the psi has gone down to 14 psi. Is it normal for idle Psi to be higher? Why has the psi increased without adding more gas?
The 134a is still connected but the valve is closed at the low-pressure gauge.

I bought the boat 7 weeks ago and yesterday was the first day with working refrigerator
 

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skedaddle

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Obviously you have a leak. The gas did not just vanish. You need to find that leak and fix it. Gassing the system and trying to reference things by static pressure while there is a existing leak is doublely a fools errand.
Read about Boyles Gas Laws. Static Pressure depends on the temperature of the entire system at the time the reading is taken.
Depending on what you find as the leak and fix. You should vacuum the system before you try to gas it. If it will hold vacuum for 24 hrs you have found the leak. If it wont hold, check for more leaks.
If the system is a standard type of Isotherm type thing. Look at the quick connect couplings. They have a rep for slow leaking with age as the sealing "O" rings harden.
Put about 80psi in the system and soap everything using a small paint brush to work up a soapy foam on the suspect area.

There are post on-line about gassing the Danfoss systems by watching the amperage draw at the battery side of the Control unit. That seems to work or at least it did for me. IIRC it is somewhere around 5 Amps on a 12v system.

Have a look on-line. There are many good articles on dealing with the Danfoss systems
 

superheat6k

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As already mentioned if the reason the thing was not working is because it has lost its gas, then simply charging more will only provide a short term fix.

So with the compressor off all will equalise. But this reflects the pressure caused by the liquid refrigerant sat in the condenser. So at idle and say ambient of 15oC your standing pressure should be ~ 3.9 Bar. You report 25 PSI = 1.8 Bar = -1oC - this is telling me you have virtually no gas in the system, and certainly no liquid present.

It is common to assume the pressure dictates the temperature function, but this is an incorrect understanding of the base principles. The suction pressure is a function of the evaporation temperature and hence the temperature of the evaporator, and hence the temperature inside the fridge box. As the temperature in the fridge reduces so will the suction pressure.

If you are trying to achieve a box temperature of 4oC then the actual Evaporation Temperature will be several degrees below this - the plate has to be colder than the space being cooled, i.e. ~ -1 to 0oC, possibly a bit lower - for R134a this equates to a suction pressure of ~ 1.7 - 1.8 Bar. This is why the plate frosts up in normal use. But as the box temperature is higher than so also will be the suction pressure. At 8oC box temperature the SET (Saturated Evaporating Temperature) should be ~ +3 for which the SP should be ~2.3 Bar. If you are not getting there then this suggests insufficient gas in the system. The photo of your gauge shows less than 1 Bar - from which I would suggest the system is very short of gas, or partially clogged (see below).

But you do not know if the system has already had air enter. Did you have nil pressure when you started ?

Purging air out is simply not adequate, especially as you likely only have one access fitting, and anyway is illegal. The only way to properly prepare the system is a pressure test with dry nitrogen - compressed air at ~ 8 bar will work, but likely introduces moisture which cause other problems. After the pressure test has passed with no leaks then evacuate with a refrigeration grade vac pump and for a couple of hours to get any moisture contaminants out. Break the vac with the refrigerant.

Assuming you have found and fixed the leak - start your search at the access point fitting schraeder valve - then the key to accurate charging on a small fridge is either to vacuum the system properly and then add the correct weight of refrigerant as stated on its data plate, or possibly, and assuming you have not had any air enter, charge until you observe sweating on the suction pipe just as it exits the evaporator plate. However, accurately reading the suction pressure against the box temperature will also tell you this.

If the fridge has had a piercing access fitting added then this will almost certainly be where the leak is, and fixing it will be very difficult.

If you can measure the suction pipe temperature accurately then you are looking for a level 6K above the SET - this is the superheat (see what I did there ?). So assuming you have almost achieved your box temp at say 5oC your SET will be ~ 0oC so your suction pipe should be at ~+6oC.

Also avoid trying to read the compound gauge scale for R134a as the detail is simply too small. instead use a refrigerant Pressure Temperature chart for R134a. Personally I am not sure why fridge gauges need the compound scales - myself and our engineers only ever use a PT chart to check SET and its opposite SCT (Saturated Condensing Temperature) - NB the latter cannot be measured on most small fridges as they generally do not have high side APs.

Be aware on small fridges it is incredibly easy to overcharge.

I also note you have obtained some R134a. Getting the stuff without oil within is not easy, and you should not be using the aerosols that include oil and are sold by Halfords et al. If you are using oiled refrigerant the the system will quickly become over filled with oil, which can clog the fine capillary tube that links the condenser to the evaporator. A blockage of oil will cause the suction pressure to drop but without any cooling. With this stuff it is easy to end up with an overcharged system and no effective flow.
 
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sailboatliving

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I added some fluorescent mixture and found the leak right away with an UV- light.
It looks like it has been brazed before.

A professional need to cut it and replace the area right? Or is there anything I can do to fix the leak without a torch gun?
 

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Daverw

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It would need to be degassed before brazing, then evacuated and recharged, not easy to do without the tools, if you try and do this without degassing the braze will just blow out when heated
 

superheat6k

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Any fridge engineer should be able to re-sweat that joint. As said the rest of the gas needs to be recovered (preferably with a recovery machine), but the quantity of gas you will retrieve will be too low to re-use as much will be lost purging the hoses and recovery unit, so you the need some fresh R134a (I presume). Ideally pressure test with dry nitrogen and to at least 10 bar, but do not go too high because that is the low side connection, then evacuate with a vac pump for at least 2 hours, prior to charging the plated weight of refrigerant.
 

Daverw

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You use this fitting, it’s on the low side but you will not have a high side connection on a small system, also you will need to take the core out when brazing as you will melt it
 
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