Chiller Problems – Microprocessor Low Pressure error

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This article describes how to fix a chiller system that, after startup, has a stopped compressor and a low pressure error.

Low pressure (Lo1 or lo2) error message on microprocessor controller. After power up and compressor start, compressor stops before reaching the SP1 or SP2 set point and the micro processor indicated a �Lo 1� or “Lo 2” in the top screen, the trip pressure in the bottom screen and the low pressure indicator light on the micro is lit. .

Upon compressor start, the micro processes monitors the Suction or Low-Side pressure of the refrigeration system. If at any time during a run cycle the micro sees low-side pressure below the LP set point for more than 90 seconds, the micro will lock out the chillers compressor until the micro is reset. The micro will then display the above faults and lock out the chillers cooling system(s).

********* Important Update ********
On 12-31-15, PSG Controls (makers of chiller controls from 2000 to 2012 for both Legacy and Drake brand chillers) went out of business. As a result, replacement parts for this process chiller control system are expected to run out in 2016.

As of 1-1-2013, Legacy Chiller Systems has developed a retrofit package for the PSG process chiller control system. For more information CLICK HERE

A micro reset can be achieved in two ways. First, by pressing the power button for 2-3 seconds. If that does not work, turn main supply power to the chiller OFF for one minute and then back on. Low-pressure trips should not happen provided the system is started up properly therefore further investigation is unusually needed.

Check the following items BEFORE resetting from a Low Pressure trip:

1. Make sure your pump is turning in the correct direction. You can check for pump direction by CAREFULLY sliding a nylon wire tie through the air vent at the end of the pump housing. When your wire tie makes contact with the plastic cooling fan the end will be forced up or down depending on the direction of spin. If the wire to end goes UP the pump is spinning CW and if DOWN CCW. Located at the end of the pump there is an arrow embossed into the SS head of the pump. This arrow will indicate the design rotation. You pump MUST be spinning the same direction. On THREE PHASE systems only, if your pump is spinning the wrong direction, STOP turn the main power OFF to the chiller and reverse any two legs of power on the main buss. Turn power back on and recheck rotation. On Single-phase chillers, if your pump is running the wrong direction, please call the factory for further instruction.

2. Clean ALL filters and strainers: Restrictions to flow through the chillers cooling system WILL cause Lo 1 trips. On PACT and EST systems with integrated water storage tanks a common problem is a plugged strainer located just above the tank recirculation pump discharge. Before processing with any other steps, turn the power to the chiller off and clean the strainer. On all chiller models, the factory recommends a filter or strainer on the FROM PROCESS loop line to prevent debris from entering the chiller. If you have an ES,PAC system such filters and strainers must be cleaned, as they WILL cause low flow errors.

*3. Make sure your LP micro trip point is set at 54 PSI.

*4. Make sure your SP micro setting is no lower than 45F.

* If your process requirements require settings lower that above, glycol freeze protection must be used to prevent Lo 1 trips and provide freeze protection.

5. If your chiller is equipped with a flooded condenser option, make sure both liquid refrigerant king valves entering and leaving the receiver are fully back seated � OPEN. These valves are shipped in the closed position from the factory.

6. Make sure ALL wire connections on the back of the microprocessor are tight. If there is any excess wire shielding on the gray transducer cables showing, tape the area with black electrical tape. Make sure that NO stray wires from the micro are touching the case or any other wire connection on the back of the micro.

7. Is the refrigeration system low on charge? A quick why to determine a low charge is to install a manifold gauge set on the high and low side of the refrigeration system. After resetting the chiller and compressor starts, carefully watch your gauges. A ball park for design high side pressure is ambient +30 converted to PSIG. A ball park for design low side pressure is chiller leaving fluid temp � 5F converted to PSIG. Example: An R-22 system at 60F ambient 50F chiller leaving temperature will run a high side pressure of about 175 PSI and a low side pressure of about 60 PSI. If both pressures are low, a leak inspection may be advisable.

Note: A PDF version of a TP chart for most common refrigerants has been linked to this article below.

If 1-7 above does not solve the low-pressure trip issue it is recommended that you call the factory for further support at 877-988-5464.

Recommendations in this article should be preformed by QUALIFED service personnel who have extensive experience with high voltage/control electrical and plumbing systems. Individuals who lack such experience can cause damage to this equipment that would not be covered under warranty. More importantly, injury and even death can occur.

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