High Ammonia Discharge Pressure at High Ambient
You are the refrigeration engineer on an ammonia plant. A screw chiller with an evaporative condenser has run all summer at 150 psig discharge. On a 78°F wet-bulb day, discharge pressure is 180 psig, condenser approach is 12°F against a 5°F design, and liquid subcooling is 7°F. The evaporator is at 25 psig with 12°F suction superheat, and the load is normal. The condenser spray pump is running, all fans are on, and the water flow appears normal at the standpipe. The liquid line sight glass is full. The system has a manual purger that has not been used this season. The high-pressure cutout is set at 185 psig and has tripped twice in the past week. Maintenance wants to raise the cutout to 195 psig to avoid trips, and production wants the chiller kept online.
1. Determine whether the high discharge pressure is ambient-driven, a condenser heat-transfer fault, or noncondensable gas, and explain your reasoning.
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At a 78°F wet bulb, the theoretical minimum condensing temperature is roughly 83–85°F, which for ammonia corresponds to about 150–155 psig. The system is at 180 psig, near 94°F, which is about 16°F above wet bulb and well above design. That is not ambient-driven. The approach of 12°F against a 5°F design confirms degraded condenser heat transfer. Subcooling of 7°F does not clear the condenser; it only shows some liquid is being subcooled downstream.
The likely causes are scale or fouling on the coil, poor water distribution or plugged spray nozzles, reduced fan airflow, or noncondensable gas. The unused purger makes noncondensables a strong candidate, but do not assume it: check spray coverage and nozzle pattern first, then fan condition, then scale. A combination is possible. The normal evaporator readings show the problem is on the high side, not an evaporator load issue.
2. What actions would you take, and why is raising the high-pressure cutout unacceptable?
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Raising the cutout only removes the protective trip while the underlying fault continues. Higher condensing pressure raises the compression ratio and discharge temperature, which stresses the screw compressor, degrades oil, and increases power per ton. It also reduces the margin to the relief valve setting and can lead to an uncontrolled high-pressure event. The cutout should stay at its design setting until the condenser is restored.
Immediate actions: verify spray nozzle coverage and water distribution, inspect fans and airflow, check water chemistry and scale, and purge noncondensables using the permanent purger or a proper procedure. Clean the condenser if fouled. If pressure cannot be brought down, reduce load or shut down rather than defeat the cutout. Any purging must follow the ammonia safety procedure with PPE, ventilation, gas detection, and logging; never vent ammonia casually in an occupied area.
3. If purging is required, what safety and operating controls apply?
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Use the permanent purger if available; if manual purging is necessary, follow a written procedure. Wear ammonia-rated PPE including face and respiratory protection as required, work with a second person, and use a gas detector. Vent the purge to a safe location, not into an occupied space. Confirm wind direction and area isolation. Keep ignition sources controlled because ammonia can burn at high concentrations, and remember that it is toxic at much lower concentrations.
Operationally, purge in short controlled increments while watching discharge pressure, condenser approach, and liquid level. Log the time, pressure before and after, and any odor or detector alarm. After purging, check for air ingress sources such as low-pressure operation or poor maintenance. If pressure remains high after purging and cleaning, the condenser may need mechanical cleaning or tube replacement.