Kcj, Reoccurring low pressure could be caused by loss of refrigerant or restricted refrigerant flow. Vacuuming and recharging appears to have not been sufficient or it just masked original problem. Connecting two BD 50 compressors together is risky do to controlling oil level and
compressor cooling. Water
cooling is another problem. You reported a sight glass, low pressure cutoff switch and a TXV. Is there also a high pressure cut off switch? When one of these systems is equipped with a TXV refrigerant flow control there must also be a receiver to hold excess refrigerant when
freezer is at operating temperatures.
Question Is there a Refrigerant Leak?
Are you reporting high pressure of 70 psi with compressor running? This would indicate low on refrigerant. Preferred Pressure after 15 minutes running would be 95 to 120 psi depending on Seawater temperature. Best performance for compressor temperature cooling is 105 to 115 psi high pressure.
System pressure with compressor off is of little value on this type system do to its ability to store excess liquid refrigerant.
Vacuum tests on capillary flow control systems can show some results in 12 hours with a slow loss of refrigerant say one ounce per month. Once to two ounces per month would produce the refrigerant losses you might have if high pressures running was 70 psi.
Question Is there a refrigerant flow problem?
Refrigerant restrictions to flow are not common but do happen on TXV system caused by moisture freezing in TXV orifice or solid material blocking valve inlet
screen. Because you report a liquid line sight glass there will be a moisture indication inside sight glass, If not there is a simple test to check moisture freezing in TXV control valves.
Run system until low pressure is in a vacuum. Now place wet hot rag over TXV. If there is moisture freezing in TXV wet rag will temporally freeze to valve and you will hear refrigerant moving. A minute or so later rage will fall free of valve and refrigerant flow will stop again. Low pressure will also follow the freezing and thawing of moisture. Another sign of moisture freezing and melting in TXV is evaporator will be covered with 33 degree F
condensation
About the only other time a inlet
screen on TXV will clog is if solid material from filter gets into line leading to TXV. Because system’s refrigerant flow is not restricted all the time solid restriction is not likely in your case.
Recommendation
I would add refrigerant if running high pressure is less than 105 psi. Now if after 15 minutes low pressure is in a vacuum. Follow my TXV freeze test. If it is confirmed to be moisture in System a Deep dehydration process is require. Some say you cannot get moisture out of systems with Ester oil it can be done but it is easy.
If system operated normally with correct high pressure you need to develop plans for finding the refrigerant leak. Because this is a Frigoboat system it probably has sliding O ring
seals in line connections, these are reported by others to leak over time. I would order correct O rings and replace them before vacuuming system again..
If you can find some one with a electronic leak detector you can test the evaporator for leaks. Best method to test for evaporator leaks is when box is warm and closed for 12 hours. The pressure in evaporator will now be at its highest pressure box
interior will be charged with refrigerant if there is a leak. Zero leak detector before opening box. Without touching anything in the box lower detector inside to sample hopefully air without refrigerant fumes. This test will only tell if there is a leak inside box It will not find the small two month refrigerant loss you are looking for. If evaporator is leaking it needs to be replaced.
If you have not detected or confirmed a refrigerant leak there is a possibility of a micro size hole in seawater condenser. Some manufacturers units have experienced these failures where low voltage
current discharge
overboard allows water to mix with refrigerant. The test for refrigerant leaks in condenser is to remove water hoses from condenser and replace with deflated balloons. If the flat balloon opens at all after 12 hours the condenser must be replaced. Only one out of three of these failures can be saved without replacing the complete system.