My employer is the equipment owner. I found this site while troubleshooting my chamber and have learned a lot from reading the threads on here. At this time I do not have any of the specialized tools required to truly troubleshoot refrigeration equipment, so all I can do is guess.
Some basic facts:
Tenney BTC with 5 cubic foot work space.
Air cooled high stage condenser.
For the intents and purposes of this discussion the work space is empty.
This chamber has been refurbished.
From what I can tell, the machine is about 20 years old (the only background I have for guesstimating a machine's age based on its patina is from working on old welders).
No way to know for sure because the refurb co. stripped the original nameplate that had the s/n.
During refurbishing, new 1 HP Copeland hermetic compressors were installed
-New Watlow F4 controller installed
-High stage refrigerant is R404A (same as original according to TPS who now owns Tenney), 36 OZ charge
-Low stage refrigerant is R508B (aka Suva 95), static charge 75 psi at 75°F (originally was R23)
There is a large condenser exposed to ambient on the back of the machine. There are two evaporators (one finned assembly with two separate coils) inside the work space -- one is the high stage and one is for the low stage. There is also a dehumidify coil in the work space that is part of the high stage. The intercooler (or evaporator/condensor) is tucked away in an insulated space behind the work space. The manual describes a system used to provide time proportioned cooling power that diverts refrigerant back to the compressor before it goes to the evaporator (I believe you guys call it a hot gas bypass).
Let me know if I need to describe more.
The problem: the chamber cools slowly and cannot reach as low of a temperature as it should.
I have performed several "validation" runs with the work space empty, full cooling power on (I program a -77°C set point in the controller), and I manually record the time and the temperature as measured by the controller, to create a characteristic curve of this chamber's cooling capacity. Before doing the validation run, I make sure the chamber has been warm for several hours to make sure the evaporator isn't iced.
My chamber changes from room temperature to -40 in about 70 minutes. It's supposed to do it in 15 minutes. It goes from room temperature to -20°C in about 20 minutes.
My chamber's lowest temperature is -53.6°C. It's supposed to be -73°C. It reached that temperature after running for three hours. When left to run for an hour and a half more, it began slowly warming back up (-52°C).
I have already eliminated all the low-hanging fruit -- ambient condenser fan works, adequate clearance from walls, the circulation fan in the work space works, controller is sending the "on" signal to the refrigeration system, pressure safety switches are not opening...let me know if I left anything else out.
When the refrigeration system was running as described above, the high stage compressor had frost on the housing surrounding the suction line. The low stage compressor was hot (about 170°F) and did not even have condensation near the suction line. This makes me think that the low stage is not working properly. I know if could be any number of things:
-hot gas bypass stuck open
-oil accumulated in the evaporator
-restriction in the cap tube
-leaky valves in the compressor
-loss of refrigerant charge
Can anybody comment on this situation? Am I on the right track in assuming that problem lies with the low stage? I would expect the suction line of the low stage to be even colder than that of the high stage.