January 2, 2020

Troubleshooting a low-pressure alarm in industrial chillers requires a systematic approach to identify the root cause without overlooking critical factors. Here’s a step-by-step guide:

1. Immediate Safety & Preliminary Checks

Lockout/Tagout: Secure the chiller before inspection.
Check Secondary Alarms: Verify if other alarms (e.g., low temp, flow) are active.
Review BAS History: Identify when the alarm triggered relative to load changes or startups.

2. Common Causes & Diagnostic Steps

A. Refrigerant-Side Issues

Cause	Diagnostic Action	Key Evidence
Refrigerant Undercharge	Check sight glass for bubbles; compare suction pressure to design specs.	Low superheat; subcooling >10°F below normal.
Leak in Evaporator	Inspect tubes for corrosion/oil stains; use UV dye or electronic detector.	Oil traces in water; pressure drops during isolation test.
Restriction	Check ΔP across filter drier/expansion valve.	Frost on TXV/Liq. line; high subcooling.
Purge Unit Overworking	Monitor purge cycle frequency; check for non-condensables.	High discharge pressure; air in refrigerant sample.

B. Water-Side Issues

Cause	Diagnostic Action	Key Evidence
Low Chilled Water Flow	Verify pump operation; check strainers for clogs; measure ΔP across evaporator.	ΔT >12°F; flow meter readings < design.
Setpoint Error	Confirm chilled water temp setpoint hasn’t drifted.	BAS shows illogically low target temp (e.g., <38°F).
Load Drop	Review process demand; check valve positions.	Actual load < 30% capacity; low return water temp.

C. Mechanical/Control Failures

Cause	Diagnostic Action	Key Evidence
Faulty Sensor	Calibrate pressure transducer; compare with gauge readings.	Sensor reading vs. manual gauge mismatch.
Expansion Valve Fault	Check valve signal; listen for hunting/chattering.	No response to manual override; erratic superheat.
Compressor Wear	Inspect oil for metal flakes; check bearing clearance.	High vibration; amperage fluctuations.

3. Step-by-Step Troubleshooting Procedure

Isolate the Section
- Close service valves to separate evaporator from the circuit. Monitor pressure for drops.
Test Water Flow
- Measure evaporator ΔP:
  - Normal: 8–12 PSI
  - Low flow: ΔP < 5 PSI → Check pumps/strainers.
Check Refrigerant Charge
- Record subcooling/superheat:
  - Low charge: Low suction pressure + high superheat (>15°F).
  - Restriction: Low suction pressure + high subcooling (>20°F).
Inspect Purge Unit
- If cycling >5 times/hour: Likely air ingress or leak.
Validate Controls
- Bypass BAS temporarily; manually command valves/sensors.

4. Critical Red Flags Requiring Shutdown

Evaporator Freezing (risk of tube rupture):
→ Symptoms: Suction line frost; water outlet temp < 36°F.
Oil Foaming (compressor damage):
→ Symptoms: Oil sump level rising; erratic pressure.
Non-Condensables >2%:
→ Symptoms: Head pressure spike after restart.

5. Solutions & Repairs

Low Flow: Clean strainers; balance water system.
Leak: Repair tubes (epoxy/plug) or replace bundle.
Undercharge: Recover, fix leak, evacuate, recharge.
Faulty TXV: Replace valve/strainer.
Sensor Error: Recalibrate or replace.

6. Prevention Best Practices

Annual Leak Checks: Helium testing (sensitive to 0.1 oz/year leaks).
Water Treatment: Prevent tube fouling/corrosion.
Real-Time Monitoring: Install pressure transducers with cloud alerts.
Training: Teach operators to spot early signs (e.g., gradual pressure drop over days).

When to Call a Professional

If suction pressure remains low after basic fixes.
For brazing repairs on pressurized vessels.
If oil analysis shows contamination (acid/metals).