Refrigerator low-pressure protection fault
Thread Content
Our company’s utility system is equipped with a 88.2 kW refrigeration unit that operates at -10°C. 30% ethylene glycol is used as the cooling water, and the unit has been functioning properly; it can start and stop automatically. It has been in use for over 1 year and 2 months； In the past few days, failures have been reported one after another: the machine stopped operating due to a \"low pressure protection\" issue, and after resetting it and restarting it for about 3 minutes, the same fault occurred again. Upon inspection, the freezer shows no leaks, no damaged electronic components, and the temperature control system is functioning properly. After communicating with the manufacturer, the set temperature was increased to -8°C for operation, and as of the time of writing, no fault reports have been received. Seeing severe frosting on the compressor area, the manufacturer immediately concluded that the concentration of chilled water was too low. What is the underlying mechanism behind this? Thank you for the guidance! The attached figure shows the condition of the oil sight glass during an alarm
What is the high-pressure value? What is the low-pressure value? Possible reasons for low-pressure protection: 1: Short circuit in the low-pressure switch (e.g., the low-pressure switch box being damp, etc.). 2: Leakage of refrigerant, resulting in a low return pressure in the system. (If there is a leak in the refrigerant, it can usually be detected, as some oil will also leak out.) 3: Clogged pipes (From what you say, it seems like they haven’t been maintained for over a year?) There may be some debris or particles blocked in the dryer filter or expansion valve, requiring maintenance. 4: The expansion valve or throttle valve (I’m not sure if your system has an expansion valve) is clogged or malfunctioning, which results in less refrigerant reaching the expansion valve or throttle valve, and thus the return gas pressure drops. 5: The high-pressure side outlet is not reaching the expansion valve, and the solenoid valve at the front of the throttle valve is not open. As a result, almost no refrigerant passes through the expansion valve or throttle valve to undergo expansion cooling; consequently, less and less refrigerant returns, and the pressure decreases. The above are my personal experiences; I hope they will be helpful to you.
The chilled water pipe broke, causing the refrigerant water to flow through the compressor into the oil separator. It is recommended to disconnect the inlet and outlet valves of the evaporator in order to check whether there are any damages to the copper tubes inside the evaporator ； Evacuate the system and fill it with nitrogen to remove moisture ； After the pressure holding is complete, proceed with refueling, vacuuming, and adding refrigerant as per the normal procedure before turning the machine on. It is recommended to regularly monitor the level of ethylene glycol in the refrigerant water, as well as the pH value ； Once a month.
It’s already more than sufficient; that’s the correct answer – use elimination to rule out the other options
The chilled water system is an independent closed system; the pressure of the chilled water supplied to the workshop is 0.4 MPa, while the internal circulation pressure in the chiller and the chilled water storage tank is 0.2 MPa. There are no signs of any external leaks. If there were internal leaks, they could only occur in components such as the chiller compressor, evaporator, and heat exchangers.
It was put into operation at the beginning of September last year; at that time, the high pressure of the refrigerant was 1.3 MPa, and the low pressure was 0.2 MPa； On the day of the alarm (20181126), the high pressure of the refrigerant was 0.9 MPa and the low pressure was 0.1 MPa; there was slight frosting at the bottom of the motor side in the automatic shutdown state. The operating set temperature was then adjusted to -8°C (it had been set at -10°C for over the past year), and as of today, December 03, 2018, it is operating normally without any further alarms.
Looking at the picture, it doesn’t seem like the evaporator has frozen.
Firstly, if the ethylene glycol concentration is insufficient, freezing will occur in the evaporator tubes; ice formation leads to expansion, and in severe cases, this can cause the copper tubes to crack. At this point, the ethylene glycol has frozen, resulting in very low load. The control system regulates the unloading on the slide valve based on the temperature of the ethylene glycol outlet water. Since the temperature sensor for the ethylene glycol outlet water shows that the temperature has not yet reached the set value, the compressor continues to operate under load. With a low load but high compressor load, liquid can end up in the intake air, leading to low intake pressure. It is also recommended to look for deals on evaporators