YORK YMC2 — EVAPORATOR - LOW PRESSURE

The control center shows EVAPORATOR - LOW PRESSURE. The panel prints this exact text for two different trips, and the manual lists the message both as a cycling shutdown and as a safety shutdown. As a cycling shutdown, it appears when the evaporator pressure has fallen to the shutdown threshold once. As a safety shutdown, it appears when this is already the third evaporator low-pressure cycling shutdown within a 90 minute period — the machine is then locked out until the fault is cleared.

In both cases the message means that the evaporator (suction) pressure, as sensed by the evaporator transducer, has decreased to the shutdown threshold. For water cooling applications the shutdown threshold is a fixed value. For brine cooling applications the threshold varies according to the concentration of the brine solution and is programmed at the YORK factory.

What differs is what happens next. After a single trip the message is a cycling shutdown, and the chiller will restart after the evaporator pressure increases to the restart threshold. When the pressure has fallen to the shutdown threshold and caused a cycling shutdown three times in a 90 minute period, the third event initiates an EVAPORATOR-LOW PRESSURE safety shutdown: the chiller stays off until the pressure recovers and the fault is cleared at the panel.

• The evaporator pressure, as sensed by the evaporator transducer, has decreased to the shutdown threshold — this is the cause the manual gives for the cycling shutdown.
• For the safety shutdown, the same condition has occurred three times in a 90 minute period: the evaporator pressure decreased to the shutdown threshold and caused a cycling shutdown three times, and the third one initiates the safety shutdown.
• The manual lists no other cause for this message; the only stated reason for the trip is low evaporator pressure at the transducer.

• Water cooling — fixed thresholds in psig, shutdown / reset: R-134a 26.1 / 26.2; R-513A 30.2 / 30.2; R-1234ze 15.4 / 15.5; R-515B 15.2 / 15.3.
• Brine cooling — shutdown threshold as programmed, in psig: R-134a 6.0-30.0; R-513A 8.8-34.4; R-1234ze 0.4-18.4; R-515B 0.3-18.2. For each of these refrigerants the reset threshold is 0.1 psig above the shutdown threshold.
• In brine applications the threshold varies according to the concentration of the brine solution and is programmed at the YORK factory. The manual states it should not be changed by anyone other than a qualified Service Technician following instructions in the YORK YMC2 Service Manual (Form 160.78-M2).

Recovery depends on which trip the display is showing. After a cycling shutdown, no action is required at the panel: the chiller will restart after the evaporator pressure increases to the restart threshold (the reset values for each refrigerant are listed above). Note that if this shutdown occurs three times in a 90 minute period, an EVAPORATOR-LOW PRESSURE safety shutdown is initiated instead, and the automatic restart no longer applies.

After the safety shutdown, the manual's instruction is: The chiller can be started after the evaporator pressure increases to the restart threshold and the CLEAR FAULT button is pressed. In practice: let the evaporator pressure rise above the restart threshold, press the CLEAR FAULT button on the control center, then start the chiller.