Causes and Solutions for Compressor Frosting in Constant Temperature and Humidity Precision Air Conditioners

I. Root Cause Analysis of Frosting

1. Heat Exchange Imbalance Principle

• Frosting occurs when evaporator heat absorption < refrigerant evaporation rate, causing incomplete vaporization and liquid refrigerant return to the compressor

• Critical indicator: Suction superheat should be maintained at 5-8°C; values below 2°C indicate frosting risk

2. Fault Tree Diagram

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   Compressor Frosting

   Refrigerant Cycle Abnormality

   Airside Malfunction

   Control System Failure

   Refrigerant Shortage

   Expansion Valve Failure

   Filter Clogging

   Fan Malfunction

   Sensor Drift

   PLC Logic Error

II. Six Specific Causes and Diagnostic Methods

III. Step-by-Step Solutions

Step 1: Emergency Shutdown

• Cut power immediately to prevent liquid slugging damage

• Apply warm towels to frosted areas (NEVER pour hot water directly)

Step 2: Systematic Troubleshooting

1. Refrigerant System Check

• Normal range (R407C): High 1.5-2.0MPa, Low 0.4-0.6MPa

• Use manifold gauges:

• Refrigerant purity test: Replace if moisture >100ppm

2. Expansion Valve Adjustment

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   Theoretical Opening = (Current SH ÷ Design SH) × Factory Setting

• Example: Design SH 5°C, actual 2°C, factory 4 turns → Adjust to (5/2)×4=10 turns

3. Airside Maintenance

• Replace if bearing clearance >0.15mm

• Replace capacitor if capacity deviation >±10%

• Use pH7-8 neutral detergent

• Water pressure <0.3MPa to prevent fin damage

• Clean evaporator:

• Fan inspection:

Step 3: Verification Test

• Step-load test:

  Load Stage	Duration	Permissible Frosting
  25%	30min	Light condensation at base
  50%	1h	No frost on suction line
  100%	2h	Stable superheat at 5±1°C

IV. Preventive Maintenance Strategy

1. Smart Monitoring

• Primary (SH <3°C): Auto-increase valve opening 10%

• Critical (SH <1°C): Forced shutdown

• Install real-time superheat monitor (e.g., Danfoss AK-CC550)

• Set two-tier alerts:

2. Optimized Maintenance Intervals

   Component	Standard Cycle	High-Dust Cycle
   Air Filter	1 month	15 days
   Evaporator Clean	3 months	1 month
   Refrigerant Moisture Test	2 years	1 year

3. Retrofit Recommendations

• Install suction accumulator (intercepts >98% liquid refrigerant)

• Upgrade to electronic expansion valve (e.g., Emerson EX5 series, ±0.5°C accuracy)

V. Special Case Study

Case: Cyclic Frosting in Variable-Speed Compressor

• Symptom:

• Frosting at low frequency (<30Hz), resolves at high frequency

• Root Cause:

• Oil return design flaw during low-speed operation

• Solution:

1. Switch to higher viscosity oil (POE→PVE)

2. Install oil level balancer

3. Adjust parameters: Minimum speed ≥35Hz

VI. Safety Warnings

1. Prohibited Actions

• Starting frosted compressor (valve plate fracture risk)

• Open-flame thawing (may ignite insulation)

2. Essential PPE

• Safety goggles and cryogenic gloves for refrigerant handling

• Confirm capacitor discharge (<50V) before electrical work

Note: For R32 flammable refrigerant systems, explosion-proof equipment and adequate ventilation are mandatory.