Ammonia plant: rising discharge pressure on a hot afternoon
At a cold-storage warehouse in Mississauga, a two-stage ammonia plant with two 250 kW screw compressors and an evaporative condenser normally runs at about 1,050 kPa gauge discharge, which corresponds to roughly 25 °C condensing. On a July afternoon the outdoor wet-bulb is 21 °C and the discharge pressure has climbed to 1,380 kPa gauge. Both compressors are fully loaded, the freezer is holding -28 °C, and the high-pressure cut-out is set at 1,550 kPa. All condenser fans are running, but the coil looks dry over the middle third and there is a heavy algae mat on the sump strainer. The purge valve on the high side has not been opened for over a year, because the previous operator insisted that 'ammonia systems don't need purging.' The shift supervisor tells the new operator to hose down the coil and 'keep an eye on it' until Monday.
1. Using saturation data, interpret the 1,380 kPa gauge reading. What does the approach temperature tell you, and what are the credible causes?
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Read the gauge correctly first. 1,380 kPa gauge is about 1,480 kPa absolute. On an ammonia saturation table that is a condensing temperature of roughly 35–36 °C, against a normal 25 °C. Compare with the sink, not with the past. The condenser's sink is 21 °C wet-bulb air, so the approach is about 14–15 K. A healthy evaporative condenser approaches within 4–6 K. Two credible causes fit that number:
- Fouled or deficient heat-transfer surface - dry patches, algae, scale, blocked spray nozzles, a choked strainer. The approach rises because the coil cannot reject the heat at the design temperature difference.
- Non-condensible gas (air) in the high side. Air does not condense, so it blankets tubes and its partial pressure adds to the ammonia partial pressure. The gauge reads total pressure, so the saturation temperature the gauge implies is higher than the actual ammonia condensing temperature - the apparent approach blows out even though the coil may be clean.
Overcharge and excessive load also raise discharge pressure, but the load is at design and the level controls have not been touched. The argument narrows to fouling versus air.
2. How would you separate the two causes without shutting the plant down, and in what order?
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Clean first - it is fast, cheap and diagnostic. Backflush and clean the strainer, restore full water distribution over the coil, check spray nozzles, drift eliminators and fan rotation, then re-read discharge pressure and wet-bulb after a stable 30–60 minutes. If the approach drops back to 4–6 K, the cause was fouling and no purging is justified.
If the approach stays high, suspect air. Air collects where the gas is coolest and slowest - the condenser outlet and the line to the receiver. A useful cross-check: compare the high-side gauge reading with the temperature of the liquid line leaving the condenser. With air present, pressure is higher than the saturation pressure that matches that liquid temperature.
Confirm with a controlled purge. Run the plant purger, or vent a small quantity of gas from the top of the high side through a water/absorption container that dissolves the ammonia (never into the machine room), and watch discharge pressure with load and wet-bulb unchanged. A fall of 50–100 kPa that holds is the signature of non-condensibles. Do not prove it by cracking a valve into the room - that is a release, not a test.
3. What immediate actions, follow-up and records apply for the rest of the shift and beyond?
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Immediate: keep the high-pressure cut-out at its set value - do not raise it to buy time. That device is the last line of defence for the compressor and high-side equipment, and the relief capacity assumptions do not change because the coil is dirty. Clean the condenser and keep machine room ventilation running, especially if any purging is done. Watch discharge pressure, oil temperature and compressor amps; if pressure reaches about 1,450 kPa with load unchanged, reduce load or shut down deliberately rather than letting the cut-out do it.
Follow up: open a work order for a proper purge routine - the belief that an ammonia plant never needs purging is a maintenance gap; write the purge procedure and frequency into the standing instructions. Clean the condenser on a schedule tied to water quality.
Record everything: the 1,380 kPa reading, wet-bulb, approach before and after cleaning, quantities purged and times. Note the missing purge routine in the plant log so the next shift has the information, and report the finding to the supervisor in writing - 'watch it until Monday' is not a corrective action.