High Condensing Pressure on a Water-Cooled Chiller
A 180 kW R134a water-cooled chiller serves a process cooling loop. Design condenser water is 30 °C in and 35 °C out, with a design condensing temperature of 40 °C. Actual readings are: condenser water in 29.8 °C, out 34.9 °C, condensing temperature 44 °C, liquid-line subcooling 4 K, and compressor loading 95%. The design subcooling is 5 K. The cooling-tower basin strainer is partly blocked and the condenser water flow is measured at 65% of design. The chiller is maintaining process temperature but the compressor is noisy and the discharge pressure is higher than normal. The client wants R134a added because “the pressure is high and the unit must be short of gas.”
1. Calculate the actual condenser approach and compare it with design. What does the result indicate?
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Condenser approach is condensing temperature minus leaving condenser water temperature. Actual approach = 44 °C − 34.9 °C = 9.1 K. Design approach = 40 °C − 35 °C = 5 K. The actual approach is therefore about 4 K higher than design. A high approach means the condenser is not transferring heat as effectively as it should for the water temperature entering it. Because the water flow is only 65% of design and the strainer is partly blocked, the most likely cause is a water-side problem: reduced flow and possibly fouled tubes. The refrigerant-side reading does not support overcharge, because subcooling is 4 K against a design of 5 K, not excessive. The high discharge pressure is a symptom of poor heat rejection, not a shortage of refrigerant.
2. Why is adding refrigerant the wrong response? Use the subcooling and approach readings in your reasoning.
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If the chiller were short of refrigerant, subcooling would normally be low and the evaporator would show poor performance, but the immediate fault here is the condenser approach. Subcooling at 4 K is close to the 5 K design value, so there is no evidence of a large undercharge. Adding refrigerant raises the total charge and, because the condenser cannot reject heat efficiently, it will increase condensing pressure further. That raises discharge temperature, compressor load and noise, and may push the high-pressure switch. It can also reduce the compressor’s cooling and lubrication margin. The correct response is to restore condenser water flow and clean the water side, then reassess charge only when the chiller is at design flow and stable conditions.
3. Recommend corrective actions and state the measurements you would use to verify the repair.
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Isolate and clean the basin strainer, verify the cooling-tower pump and valves, and restore condenser water flow to design. Check the flow switch and water-treatment records. If the approach remains high after flow is restored, brush or chemically clean the condenser tubes and inspect for scale or fouling. Do not add refrigerant before this work. After cleaning, verify condenser water flow, entering and leaving temperatures, condensing temperature and approach at steady load. The approach should return to about 5 K or better at design flow. Check subcooling against the manufacturer’s value, discharge pressure and compressor current. If subcooling is still abnormal at correct flow and load, then investigate charge or liquid-line restrictions, but the first repair is water-side heat transfer.