Need diagnostic ideas/help on Danfoss BD35 refrigeration system

[TampaBaySailor]

I need a little help diagnosing a non-working Frigoboat refrigeration system. My system has a Danfoss BD35 compressor with R134a refrigerant and air-cooling, a filter/dryer and a capillary-tube based evaporator. The system is 12 years old and ran fine until a couple years ago when I had moisture-blockages form in the evaporator. After figuring the problem out, I added the filter-dryer which seemed to solve it.


Fast forward to a month ago when the unit stopped cooling the cold-box. The compressor was running continuously but the evaporator had absolutely no sweating, frost or any cold-temperature to the touch. Thought it might be a moisture-blockage again, so tried multiple times to warm the capillary input to the evaporator. Nothing helped.


Started considering other options: There was no sign of refrigerant oil leaking from any fittings or anywhere else. Compressor seems fine – it draws the expected amps at low and high speed. However, I can run it for 12+ hours and see absolutely NO cooling on the evaporator. However, there is a faint noise coming from the evaporator while compressor runs. It is continuous and hard to describe.


So I purchased a cheap set of R134a refrigeration gauges, figuring I had nothing to lose. Watched a bunch of youtube. I understand the working principles, but have no hands-on experience. I connected the low-pressure and high-pressure gauges to my compressor. The service ports have Schrader-valves and the refrigeration fittings appear to have the right hardware to depress the valve-stem. Connected everything up with the compresser cold and OFF. I expected to get the same refrigerant pressure on both gauges, somewhere around 50-100 psi. Instead, I got just a few psi on both gauges. Puzzling, maybe the Schrader-valves didn’t open?


So, then I turned on the compressor. At this point the low-gauge dropped slowly to 22 in Hg VACUUM, while the high-side gauge read +15 psi. I didn’t see anyway to have gotten a vacuum on the low-side if the Schrader valve was closed.


But, when I look up diagnostic gauge readings on the internet, no-one gives any example that corresponds to this observation. Refrigerant too high or too low both gives some positive pressure on the low-side. But I did find one example showing a vacuum on the low-side with a capillary blockage – but the high-side was still much higher than 15 psi.


Could I have lost essentially all the refrigerant? Or does a vacuum on the low-side perhaps indicate some sort of capillary blockage? Any help or ideas appreciated to understand what is wrong.


Related question: How much R134a refrigerant would I need to completely fill an empty system, at least approximately? All the examples I find online suggest using 16-oz cans of refrigerant, but no-one gives even a round-number for whether you need 1 or 10 cans. The Frigoboat people also don’t want to give me an estimate for some reason…


Thank you!

[wsmurdoch]

Your pressures on the high and low sides of the system with the compressor off and the system settled should be the same excepting gauge error. From your write up that sounds true.

Without knowing the system temperature(s), it is hard to guess the expected pressure, But, if there were liquid present in the system as well as gas, knowing the temperature(s) you could look at a vapor pressure chart and see what the expected vapor pressure should be. If less, there is no liquid present and some refrigerant has escaped. If more, there is something other than the refrigerant in your system.

Best practice is to find the leak, repair it, replace the filter dryer, vacuum the system, and recharge the refrigerant. If the system is still under pressure and you did not introduce air or water into the system in your investigations, you may get away with just repairing the leak and recharging, but you would have to be lucky.

From my experience I'd guess your leak is a bad cap on a Schrader valve. The offending cap was ether loose or the rubber disk in the cap was bad. Others might think otherwise. 50:50 Joy and water foamed over fittings with a small short bristle paint brush with the system under pressure should find it. But, there are lots of other places that a leak could occur. It will take time and patience to find it.

Not knowing your system, I'd guess the charge is measured in small numbers of ounces. Recharging a capillary system is a long and slow process of adding a little refrigerant, waiting, evaluating frosting on the evaporator, and if necessary adding more.

[TampaBaySailor]

Thanks for the advice!


An update.



I did manage to re-charge the system using less than 16 oz of R134a (a single can). The system pressure was just a few psi above 0 before I started. After charging and carefully watching the frost-line on the evaporator, I had a system pressure of around 50 psi with the compressor off. With the compressor on, the low side pressure was around 6 - 12 psi, with the high side about 150 psi. The exact values depending on the compressor speed which I can manually control.


System must have been almost empty of refrigerant. It started cooling once I got enough R134a back in. Not sure yet why gas was so low.



Once I disconnect the refrigeration gauges, I can do some leak checking with a halogen detector.


Thanks again!