500 m³ Dual-Temperature Cold Storage Project for Biological Reagents, Taizhou Lanxi (Cwbiotech)

1. Refrigeration System Architecture: Single Compressor Unit with Dual-Temperature Zones & Precision Cooling Supply

Rejecting the simplistic parallel dual-evaporator layout, the Xiangning engineering team developed a centralized refrigeration system featuring independent throttling via Electronic Expansion Valves (EEVs) and variable-frequency compressor regulation.

Core Compressor Unit

Equipped with Bitzer semi-hermetic piston compressors fitted with frequency converters to deliver stepless frequency modulation ranging from 30 Hz to 75 Hz, matching partial-load operating conditions. The condensing temperature is designed at 45 °C, enabling high-efficiency operation even under an ambient temperature of 32 °C.

Independent Evaporator Control

For the -20 °C cold room: Low-temperature air coolers with 8 mm fin spacing, designed evaporating temperature of -30 °C, and superheat setpoint of 6 K.

For the 4 °C cold room: High-humidity air coolers with a designed evaporating temperature of -2 °C and superheat setpoint of 8 K.

A Danfoss ETS electronic expansion valve is installed upstream of each evaporator. The PLC executes cascade PID regulation based on superheat readings and cold room temperature deviation to eliminate refrigerant flow competition between the two circuits.

Performance Verification

During commissioning, the heat load of the 4 °C cold room was artificially increased by 30%. The evaporating pressure fluctuation in the -20 °C cold room was less than 0.1 bar, with a maximum temperature deviation of merely 0.3 °C; no cross-interference occurred between the two temperature zones.

2. Air Distribution Optimization: Perforated Plenum Air Supply & Return Air Plenum Chambers

To meet the target temperature uniformity of ±1.5 °C inside the cold rooms, Xiangning adopted a perforated air supply plenum design widely used in pharmaceutical cleanrooms yet rarely applied in cold storage facilities.

Structural Design

The air outlet of each air cooler connects to a galvanized steel air supply plenum. Its bottom plate is made of aluminum sheet with an open area ratio of 4.2%, featuring holes arranged in equilateral triangular patterns. Cold air descends uniformly at a velocity of 0.25 m/s, flows through rack storage areas, enters side-wall plenum chambers via bottom return air grilles, and circulates back to the evaporators.

CFD Simulation & Field Measurement

Computational Fluid Dynamics (CFD) airflow simulation predicted a maximum temperature difference of 1.5 °C across 12 measuring points. Under full-load acceptance testing after project completion, the temperature differential at all measuring points was controlled within 1.1 °C for the 4 °C cold room and 1.4 °C for the -20 °C cold room, with zero product moisture loss or frost formation on goods.

3. Dual Buffer Airlocks & Active Humidity Interception System

The dual interlocking buffer chambers developed by Xiangning represent a core technical highlight of this project, enabling gradual transitional temperature and humidity gradients.

Operational Sequence

Materials enter the primary buffer chamber from outdoors for dehumidification, then move to the secondary buffer chamber for pre-cooling before being transferred into the main cold storage area. Field testing recorded a dew point fluctuation of only 0.6 °C inside the main cold room for a complete material inbound/outbound cycle. Compared with a scheme without buffer chambers, evaporator frosting rate is reduced by 65%, and defrost intervals are extended from 4 hours to 12 hours.

4. Defrost Strategy & Emergency Safeguard Systems

-20 °C Cold Room Defrost Logic

Electric heating defrost is implemented with a composite trigger criterion integrating time, temperature and pressure differential. Defrost cycles are not scheduled at fixed intervals; instead, defrost initiates when the pressure differential across evaporator fins doubles the initial baseline and cooling capacity drops by 15%. Defrost heaters are activated in staged power levels, and air fans shut down during defrost cycles to deliver a gentle room temperature rise curve. Full-load testing showed the peak room temperature reached only -18.2 °C, far superior to the client’s upper limit allowance of -15 °C.

Power Failure Protection

Phase sequence protection devices and automatic transfer switches are fitted. An emergency generator auto-starts within 15 seconds upon mains power loss. All cold room doors are equipped with mechanical internal emergency release handles.

5. GSP Compliance & Environmental Monitoring System

The validation protocol submitted by Xiangning fully complies with WHO TRS961 Annex 9 supplementary provisions and Appendix 5 "Validation Management" of China’s Good Supply Practice (GSP) for pharmaceutical products.

Validation Sensor Layout

Each cold room is fitted with 16 metrologically calibrated temperature sensors, deployed at high-risk locations representing worst-case operating conditions: maximum/minimum temperature zones under no-load conditions, doorways, and air return openings. Full-load validation uses heat capacity-matched simulation loads: 33% ethylene glycol aqueous solution packaged in glass bottles for the -20 °C cold room, and pure water for the 4 °C cold room, to accurately replicate the thermal mass of pharmaceutical reagents.

Continuous Monitoring System

Dual redundant PT100 temperature probes (accuracy ±0.1 °C) and standalone humidity sensors are installed in every cold room. A three-tier alarm framework is deployed:

Threshold pre-alarm (SMS notification)

Exceedance critical alarm (phone call + audible & visual light alarm)

Emergency shutdown alarm (automatic startup of standby refrigeration units)

Data is logged every minute, stored locally and synchronously uploaded to cloud servers. The system supports complete audit trails with tamper-proof immutable data records.

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