My job responsibility at the time was to maintain/troubleshoot the ammonia refrigeration system. The first shift production supervisor called over the radio for the refrigeration department to evaluate a warm room temperature. The production room was designed to be around 45-degrees Fahrenheit for this process and the room temp thermometer is reading 55-degrees Fahrenheit. This occurred during the summer months and having cooling equipment operating correctly is vital to maintaining proper room temperatures. The cooling unit was a Refrigerated Make-Up Air Unit (RMAU) located on the roof of the facility and utilized a surge drum (flooded evaporator).
This facility operates on a PLC system, and everything can be controlled using a single HMI display screen. First place I checked was that the RMAU was in refrigeration mode "cool mode" on the display screen (which it was), and the room RTD probe was in-fact reading 55-degrees Fahrenheit (this eliminates a faulty wall thermometer). Before leaving the machinery room, I knew that RMAU for that production room is equipped with a surge drum that is fed liquid refrigerant via a high-pressure liquid line (HPL). So, I verified the liquid level inside the high-pressure receiver (HPR) was within its normal range (level was correct for operation). Now it was time to head up to the roof and evaluate the actual RMAU itself.
My first check was to confirm the operation of the fan blower (all good). Next, I checked the internal louvers for "cool mode" & "sanitation mode" and those were all in the proper positions. Now, I made my way to the valve group (control station) to evaluate and found the liquid supply control valve was not energized and not calling for liquid refrigerant to re-fill the surge drum. I checked the sight glasses on the float column, and they were empty, and the float switch was calling for liquid refrigerant. I suspected an electrical fault of some sort was at hand (already had my multi-meter with me when I went to the roof).
This is where the "challenge" begins, I verified the float switch circuit was sending a 120-volt signal to the float switch and a 120-volt signal was leaving the float switch and back to the RMAU main control panel terminal block (everything was good). Now I needed to check over the electrical circuit for the liquid solenoid coil, the terminal block had 120-volts leaving the main control panel and I went to the LB box for the solenoid coil (this took some time to open because the small cover screw threads were seized from being outdoors) and I verified 120-volts going to the coil solenoid. So, at this point about 20-minutes has passed by for troubleshooting this call.
To pour salt on the wound, the production supervisor calls back on the radio to let us know the room temperature is now at 65-degrees Fahrenheit. I went ahead and turned off the RMAU because it keeps the outside air louvers partially open (15% open) for maintaining air balancing inside the facility and since the evaporator wasn't cooling this air, it was pushing this hot outside air into the production room. I made the choice to replace the solenoid coil (I should have ohmed across the coil to verify it was bad). I jumped to conclusions and went through the process of creating a work order and checking out a solenoid coil from our parts cage. I grabbed a few more hand-tools for changing the solenoid and got the new one installed. Time to test, I turn the RMAU power back on and waited for the liquid solenoid to energize....... waiting...... waiting...... and it is still not energizing to call for liquid refrigerant.
Now I'm back at square one and have made zero progress on this issue. I double check I have 120-volts coming to the solenoid coil (all good). I shut down the RMAU again and start performing continuity tests on the solenoid coil circuit and everything checks out fine on the power side and neutral side "ground side". I'm really scratching my head at this point and had to step away for a few minutes to collect my thoughts.
This is one of the rare circumstances that I might never come across again, but I decided to temporarily run some wires from the main electrical panel over to the liquid solenoid valve. After doing this, I turned the RMAU back on and the solenoid coil ENERGIZES, and liquid refrigerant is now re-filling the surge drum. I'm really confused at this point, but I'm glad the room is now cooling again, and temperatures are dropping after an hour of no cooling in that production room.
The unit is now cooling but I need to find the source of this problem. I did some more continuity tests with no bad or questionable readings being present. I decided to pull the old wiring out of the conduit run that is underneath the RMAU, and after inspecting the power wire, I found a section that had corrosion from water getting inside the conduit. When looking closely, only a few strands of the copper wire were hanging on and completing the circuit. A continuity test with a multi-meter shows nothing wrong with the circuit and when the circuit is energized it allows the voltage to flow through those few strands but doesn't allow the correct flow of current "restricts flow" and the solenoid coil does not create a magnetic field to open the refrigerant valve. If I had a megger tester available, I could have most likely found this electrical fault using that tool.
For me, the best part of about troubleshooting are the challenges I encounter and the growth that comes from overcoming these challenges. I encountered an electrical fault and wanted to share my personal experiences to help someone else be prepared for a similar scenario.
Title: Partially continuous circuit fault
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