When Thermal Control Is Everything

In freeze-drying, product quality (cake structure, cycle times, uniformity between vials) depends on two thermal "anchors": tray temperature and condenser temperature. Both are closely linked to the stability of the refrigeration circuit. If the evaporation pressure/temperature fluctuates, it translates into thermal waves in the trays and condenser: the sublimation rate changes, risks of melt-back/collapse arise, and the primary and secondary phases are unnecessarily lengthened.

Here it is where electronic expansion valves (EEVs) make the difference. By maintaining stable superheat and adjusting their opening within milliseconds in response to any change (vapor load, start up, defrost, condensation variations, or cascades), EEVs:

• Stabilize evaporation, giving the tray and condenser PID loops a solid thermal foundation.

• They maximize the evaporator's useful capacity without the risk of liquid carryover, protecting compressors and oil.

• They reduce hunting and, therefore, temperature fluctuations that penalize process quality and repeatability.

In industry, the difference between a stable system and a problematic one often lies in how we control superheat. This is where electronic expansion valves (EEVs) make the difference: greater reliability, more useful evaporator capacity, and real compressor protection.

How does an EEV decide how much to open?

With two very simple, but critical signals:

• Temperature probe in the suction line at the evaporator outlet (well insulated).

• Pressure transmitter at that same suction point.

The controller transforms the suction pressure into the refrigerant evaporation temperature (using the gas P-T curve) and calculates the superheat:

Superheat (SH) = T_suction – T_evap(P_suction)

The typical SH setpoint in DX is between 4 and 8 °C.

• Below ~4 °C, ↑ risk of liquid carryover into the compressor.

• Above ~8 °C, ↓ evaporator utilization, ↑ discharge temperatures, ↓ COP.

 Why does an EEV improve reliability and performance?

 • Protects the compressor: maintaining a stable SH prevents liquid slam and oil dilution.

• More usable surface area in the evaporator: less "starved evaporator" due to excessive superheat.

• Fast response to transients: variable load, defrost, startups… the stepper motor adjusts the opening in milliseconds with precision from hundreds to thousands of steps. • Control stability: Current controllers (PID + anti-hunting + ramp limits) maintain SH within ±0.5…1 ºC during operation, reducing oscillations and consumption.

Good practices (those that separate a refined system from a “nervous” one)

• Probe placement: Temperature and pressure as close as possible to the evaporator outlet and at the same hydraulic point (avoids errors due to pressure drop). Perfectly isolates the probe from T.

• Correct P-T curve: Selects the exact refrigerant in the controller.

• Filtration and timing: 1–10 s moving average and step speed limitation to prevent hunting.

• Safety limits: Max/min opening, fail-safe in case of probe failure, and start-up/pull-down mode with controlled initial opening.

• Circuit maintenance: Proper dehydration, clean filters, and absence of particles: a precise EEV is unforgiving of dirt.

• Coordination with the rest of the system: If there is a VFD on the compressor or EC fans, use adaptive SH setpoints (e.g., 4–6 ºC at full load, 6–8 ºC at high load) to maximize COP without compromising safety.

• Defrost and pump-down: dedicated sequences to set opening/setpoints during non-stationary events.

What about a thermostatic valve (TRV/TXV)?

• The EEV does not rely on a thermal capsule or mechanical inertia: greater precision, less drift, better control at low load and under changing conditions (ambient temperature, floating condensation, etc.).

• Telemetry and diagnostics: SH, opening, and alarm logs enable predictive maintenance.

In short: good superheat control (4–8 ºC) with EEV is synonymous with compressor reliability, process stability, and energy efficiency. And, in freeze-drying, this translates into stable evaporation that maintains the temperature of the trays and condenser, keeping the sublimation rate constant. This reduces thermal fluctuations, prevents melt-back/collapse, improves uniformity between vials, shortens primary and secondary cycle times, and reinforces GMP reproducibility and traceability.

The hardware matters, but the detail in the instrumentation (well-positioned suction T and P) and the fine-tuning of the loop (anti-hunting, ramps, limits) are what make the difference in product quality and cycle times.