High-Pressure Trips on One of Two Identical Chillers
Two identical air-cooled chillers sit side by side on a roof deck, each with two R-410A circuits (11 kg per circuit) and three vertical-discharge EC condenser fans. A 3 m acoustic screen surrounds the plant and stands 800 mm from the discharge face of chiller 1; a 2.5 m parapet completes the enclosure. In July, chiller 1 trips on its high-pressure cut-out (27.5 barg) whenever ambient exceeds 30 °C and both circuits run; chiller 2 has never tripped. At 31 °C ambient and equal load the engineer records: chiller 1 circuit A at 27.0 barg with 13 K subcooling; chiller 2 at 24.8 barg with 7 K subcooling. Coils on both machines were cleaned last month, all fans run at 100%, suction pressures match, and chilled water flow and leaving temperature are identical.
1. What is the most likely cause, and what single measurement would confirm or eliminate it?
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Most likely cause: hot air recirculation from chiller 1's own discharge, trapped by the acoustic screen and parapet. The screen sits 800 mm from the discharge face - far less than the manufacturer's clearance, which is normally expressed in fan diameters and is usually two or more - so the discharge plume curls back over the parapet and re-enters the coil.
The confirming measurement is air-on temperature at the condenser coil face, taken at several points across each machine at the same moment.
- If chiller 1 reads 38–42 °C while ambient is 31 °C and chiller 2 reads close to ambient, recirculation is proven.
- If both read ambient and chiller 1 still condenses 2 bar higher, recirculation is eliminated and I move to overcharge, non-condensables or a coil/fan fault on that machine.
Supporting checks: a smoke or anemometer traverse across the discharge to see the plume, and a repeat reading after temporarily removing a screen panel. That test costs an hour and settles the argument before anyone spends money on refrigerant or parts. Note the pattern already fits: it only happens above 30 °C and only when both circuits run, because that is when the heat load on the enclosure is greatest.
2. Explain why the clean coils, the fans at 100% and the high subcooling are all consistent with recirculation - and how you would distinguish it from overcharge and from non-condensables.
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Clean coil and full fan speed are exactly what you would expect - the machine's heat-transfer capability is intact. It is being fed air that is already warm, so it behaves like a condenser operating in a hotter climate. Cleaning and fan checks were reasonable first steps but they only ever eliminate half the problem; the air side includes what the air is, not just how much of it moves.
High subcooling (13 K against 7 K) fits. With the condensing pressure pushed up, more liquid stacks in the bottom of the condenser; on a fixed charging method the subcooling rises with it. It is a symptom of the condenser being unable to reject, not evidence of too much refrigerant.
Distinguishing the three:
- Recirculation - effect scales with ambient and load, affects the machine nearest the obstruction far more than its neighbour, and the air-on temperature is elevated.
- Overcharge - present at all ambients and loads, usually on both machines if they were charged the same way, and air-on is normal; the subcooling stays high even at 20 °C ambient.
- Non-condensables - condensing pressure high even at low ambient, condenser approach roughly normal, subcooling high, and the pressure does not track ambient; a standing pressure test against temperature after a long off-period confirms it.
The differentiator in every case is air-on temperature plus how the fault varies with ambient.
3. What would you do about the root cause, what interim measure would you take, and who carries the responsibility?
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Root cause is an installation/design defect, not a refrigeration fault. The acoustic screen was installed to a clearance the chiller manufacturer does not permit. That responsibility sits with whoever designed and installed the screen - and my evidence is the manufacturer's clearance drawing against the 800 mm as-built.
Permanent fix, options in order:
- Relocate the screen to the manufacturer's minimum clearance and re-check noise compliance - the screen exists for a reason, so this needs the acoustic consultant.
- Fit a discharge plenum or deflector to lift the plume clear of the parapet.
- Reduce the screen height so it does not cap the discharge.
Only after the air-on temperature is back near ambient do I revisit refrigerant charge - and then only to confirm the original charge is correct, not to add more.
Interim: run a single circuit where the load allows, add a temporary deflector panel, and set a lower condensing setpoint with the client's agreement, logging the readings daily. Make it clear in writing that this is a temporary measure with a cost and a risk.
Responsibility: raise it formally to the principal contractor, with photographs, the air-on readings and the clearance drawing, before any of my own labour goes into it.