Failure Mechanisms of Cooling Loss in Thermal Shock Test Chambers ——A Systematic Analysis Based on the Reverse Carnot Cycle

Thermal shock test chambers are indispensable in reliability qualification for electronics, aerospace, and automotive industries. Once the “no-cooling” fault occurs, the test sequence is immediately interrupted and secondary damage to the specimen may follow. Using the reverse Carnot cycle as the theoretical backbone and integrating years of field-maintenance data, this paper systematically reviews the macroscopic manifestations, microscopic mechanisms and discriminating methods of cooling loss, and puts forward actionable preventive-maintenance strategies. The findings provide laboratory operators with rapid fault-location and handling guidelines, and also offer equipment manufacturers a reference for reliability-oriented design.

1?Introduction
By rapidly shuttling specimens between high- and low-temperature zones, thermal shock chambers expose latent defects through extreme thermal gradients [1]. Continuous low-temperature holding relies on the correct operation of the reverse-Carnot refrigeration cycle. When this cycle is disturbed, cooling capacity is lost. Although manufacturers perform multiple verifications before shipment, long-term operation under grid disturbances, mechanical wear and refrigerant ageing can still trigger sudden cooling failures. Clarifying the failure mechanisms and establishing a standardized troubleshooting workflow are therefore essential for ensuring test accuracy and minimizing downtime.
2?The Reverse Carnot Cycle and System Architecture
2.1?Cycle Theory
The reverse Carnot cycle comprises two isothermal and two adiabatic processes [2]. In a test chamber the cycle is decomposed into four stages:
(1) Adiabatic compression: low-pressure refrigerant vapour is compressed to high pressure and temperature;
(2) Isobaric heat rejection: superheated gas condenses in the condenser, transferring heat to the coolant medium (air or water);
(3) Adiabatic expansion: liquid refrigerant passes through a throttling device (capillary or electronic expansion valve) and experiences a sharp pressure and temperature drop;
(4) Isobaric heat absorption: low-pressure two-phase refrigerant evaporates in the evaporator, removing heat from the specimen and chamber walls before returning to the compressor.
2.2?System Configuration
A typical three-zone chamber consists of a hot zone, a cold zone and a specimen transfer basket. The refrigeration system is usually a two-stage cascade:
(1) High-temperature stage: R404A or R507 for precooling and medium-temperature holding;
(2) Low-temperature stage: R23 or R508B for deep cooling below ?55 °C;
(3) Switching devices: hot-gas-bypass solenoid valve, intermediate heat exchanger and check valves for inter-stage coupling and load matching.
3?Macroscopic Symptoms of Cooling Failure
3.1?Temperature Anomaly
When the set point is ?40 °C but the chamber remains above ?20 °C after 30 min and the cooling rate is <1 °C·min?1, insufficient capacity is diagnosed.
3.2?Pressure Anomaly
High-side pressure <1.0 MPa or negative low-side pressure indicates cycle imbalance. 3.3?Compressor Behaviour Motor current drops >20 % below rated value or the protector trips repeatedly.
4?Systematic Analysis of Failure Mechanisms
4.1?Compressor Faults
4.1.1?Electrical Factors
Voltage sags or harmonic distortion can erode contactor contacts and prevent coil pull-in; phase loss raises winding temperature and triggers the internal thermal protector.
4.1.2?Mechanical Factors
Wear of scroll tip seals, broken piston rings or increased crankshaft eccentricity reduce volumetric efficiency. Discharge temperature decreases while suction temperature increases—opposite to normal behaviour.
4.1.3?Lubrication Failure
Carbonized or emulsified refrigerant oil destroys the oil film; metal-to-metal contact leads to seizure. Oil level and colour observed through the sight glass provide early warning.
4.2?Refrigerant Anomalies
4.2.1?Leakage
Micro-cracks in welds, aged gaskets or cracked valve stems (especially of the hot-gas-bypass solenoid) cause gradual loss. When the charge falls below 80 % of design, evaporator superheat rises sharply and suction pressure collapses.
4.2.2?Ice and Dirt Blockage
Moisture >50 ppm forms ice crystals at the expansion orifice; particulate debris causes oil slugs. Both manifest as a sudden evaporator-pressure drop and frequent compressor cycling.
4.2.3?Non-condensables
Inadequate evacuation leaves residual air, raising condensing pressure and compressor power while lowering cooling rate.
4.3?Control System Faults
4.3.1?Sensor Drift
Ageing temperature or pressure sensors yield erroneous feedback, causing the PID algorithm to issue wrong commands.
4.3.2?Program Logic Error
If the hot-gas-bypass valve remains open during the low-temperature dwell, evaporating temperature rises and the set point cannot be maintained.
5?Diagnostic and Localization Procedure
5.1?Preliminary Checks
(1) Power: three-phase unbalance <2 %, no phase loss; (2) Display: log alarm codes and compressor run time; (3) Sight glass: continuous bubbles >5 s·min?1 indicate undercharge.
5.2?Combined Pressure–Temperature Test
Digital manifold gauges measure high- and low-side pressures. With ambient dry-bulb temperature, calculate subcooling (normal 3–5 K) and superheat (normal 6–8 K). Subcooling <2 K plus superheat >15 K indicates refrigerant shortage or expansion-valve misadjustment.
5.3?Infrared Thermography
Scan compressor shell, condenser outlet and evaporator inlet; abnormal temperature gradients reveal potential leaks or blockages.
5.4?Vacuum–Pressure Leak Test
After refrigerant recovery, pressurize with nitrogen to 1.8 MPa; pressure drop <0.03 MPa in 24 h is acceptable. If exceeded, locate leaks with an electronic halogen detector.
6?Preventive-Maintenance Strategy
6.1?Refrigerant Management
(1) Metered charging: refill to nameplate ±5 g using an electronic scale in closed-loop control;
(2) Moisture control: replace drier filters every 1000 h; target moisture <20 ppm. 6.2?Compressor Maintenance (1) Every 2000 h measure winding insulation with a 500 V megger (target >100 MΩ);
(2) Every 4000 h analyse oil; replace if acid number >0.05 mgKOH·g?1;
(3) Every 8000 h renew synthetic oil of identical grade as per OEM specification.
6.3?Valves and Piping Maintenance
(1) Replace the entire valve body when the solenoid stem shows cracks—welding repair is prohibited;
(2) Perform annual penetrant testing (PT) on stainless-steel brazed joints; repair cracks and re-solution-treat.
6.4?Control-Program Optimization
Implement “pressure–temperature dual-variable redundancy” in the PLC: if both temperature and pressure sensors indicate anomalies for >30 s, the system shuts down and outputs a fault code, avoiding false trips from single-sensor drift.
Cooling loss in thermal shock test chambers is usually the result of coupled compressor degradation, refrigerant-cycle anomalies and control-system faults. Using the reverse Carnot cycle as a theoretical framework, a three-dimensional fault tree (electrical–mechanical–refrigerant) reduces fault-location time to within 30 minutes. Standardized leak detection, metered charging, preventive replacement of critical parts and logic upgrades can raise the system MTBF from 4000 h to over 7000 h, providing sustained technical assurance for environmental reliability testing.