Two-Stage Plant: High Discharge Pressure and Falling High-Stage Suction
A plant runs a two-stage ammonia system: a low-stage screw on a -35°F suction discharging to a horizontal flash intercooler, and a high-stage reciprocating machine discharging to an evaporative condenser. Since a new -35°F spiral freezer was commissioned, the high-stage machine trips on high discharge pressure (195 psig, about 103°F condensing) with 82°F ambient and all condenser fans and spray pumps running. At the same time, high-stage suction has fallen from 38 psig to 28 psig and high-stage discharge temperature has risen from 190°F to 235°F. Low-stage discharge is 150°F. The intercooler level indicator reads steady at setpoint. A mechanic suggests adding refrigerant to bring the discharge pressure down.
1. Separate the symptoms into distinct faults and give the numerical evidence for each. Could one fault explain everything?
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Fault A - condenser performance (the high discharge pressure).
Ammonia at 195 psig saturates near 103°F. With 82°F ambient and every fan and spray pump running, that is about a 21°F approach. A healthy evaporative condenser should be in the 8–15°F range, so the condenser is not rejecting heat as it should. Possible causes: scale or fouled tube bundle, poor water distribution or plugged nozzles, blocked fill, airflow problem, or noncondensables accumulated in the condenser.
Fault B - a restriction or false level at the intercooler (the falling high-stage suction).
A higher discharge pressure by itself does not pull the intercooler pressure down - if anything, reduced high-stage volumetric capacity at a higher ratio raises intercooler pressure. A 10-psi drop at the compressor suction with the intercooler reportedly holding level means the compressor is seeing a pressure lower than the intercooler shell. That is a restriction between the intercooler and the compressor inlet, or the level indicator is lying and the float is stuck (so the shell is actually starved).
Can one fault explain everything? Partly. The high ratio (about 4.9 vs about 4.0 before) explains part of the 235°F discharge temperature. But high superheat at the high-stage suction from a starved/restricted intercooler is required to get discharge temperature that high, and the suction pressure drop cannot be produced by the condenser alone. Treat these as two faults until measurement proves otherwise.
2. Evaluate the mechanic's proposal to add refrigerant to bring the discharge pressure down.
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Reject it. The proposal treats discharge pressure as a charge problem when the evidence says it is a heat-transfer problem.
- An overcharge does not improve condenser UA. It raises the condensing temperature for the same heat rejection, so head pressure rises further and compressor capacity falls.
- Adding charge also raises the risk of backing liquid into the high-stage compressor and of carrying liquid through to the low stage, which is exactly the failure mode a plant must avoid.
- There is no evidence of a shortage: the intercooler is reported at level, low-stage discharge is normal at 150°F, and nothing indicates a starved evaporator or a low receiver level. Confirm the receiver level and the condenser outlet subcooling before any charge decision.
- If noncondensables are present, the correct action is to purge, not to add refrigerant - adding charge to a condenser with air in it makes the head pressure worse.
Write the order of work as: prove the condenser problem, fix it, then re-evaluate suction pressure before touching charge.
3. List your next four checks in order and state what result would confirm or eliminate each fault.
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1. Measure pressure simultaneously at the intercooler shell and at the high-stage compressor suction. A measurable drop across the gas line confirms a restriction (valve, strainer, iced line, collapsed gasket). Nearly equal pressures eliminate Fault B as a line restriction and shift suspicion to the level indication.
2. Verify the intercooler level on the sight glass, not the indicator, and check the float valve and liquid line strainer. A glass showing low level with the indicator reading 'at setpoint' proves the level instrument is false and the high stage has been starving. A normal glass with a restricted gas line confirms check 1.
3. Check the high-stage suction stop valve position and its strainer. After a commissioning, a throttled suction valve or a strainer plugged with debris is a common and easily corrected cause. Valve wide open with no strainer delta-P eliminates it.
4. Evaluate the condenser: water distribution and nozzles, fill condition, fan amps and airflow, and check for noncondensables. Compare condenser outlet liquid temperature with the saturation temperature at the discharge pressure; a large difference plus a high approach points to air in the condenser - purge it. If the approach stays above about 15°F after cleaning and purging, the condenser is undersized for the new freezer load and that is an engineering problem, not an operating one.
Do all four before changing any setpoint or adding charge - each check either confirms or eliminates a specific hypothesis.