I. Introduction: The Overlooked “Gradual Failure”

After 3 to 5 years of service, industrial dehumidifiers often experience a “frog in slowly heating water”-style decline in performance—the outlet air is not cold, and condensate output decreases significantly, yet the compressor continues to run. The high- and low-pressure gauge readings, however, reveal a common abnormal characteristic: the evaporation pressure remains consistently below the design operating conditions.

This condition differs from sudden failures (such as compressor burnout, electrical leakage, or pipe rupture); it falls under the category of “sub-optimal health” in refrigeration systems. If refrigerant is added based solely on intuition or the expansion valve is adjusted blindly, repairs often make the problem worse. The correct approach is to treat “low evaporation pressure” as a systemic diagnostic entry point rather than a conclusion of a single fault.

II. Three Core Causes of Low Evaporator Pressure

From the perspective of the thermodynamic cycle, evaporator pressure directly corresponds to evaporator temperature. Low pressure indicates an excessively low evaporator temperature, leading to premature frosting or icing on the evaporator surface. This causes a sharp decline in heat exchange efficiency on the air side, ultimately resulting in a decrease rather than an increase in dehumidification capacity. The root causes of this phenomenon are overwhelmingly concentrated in the following three categories:

1. Chronic Refrigerant Leaks—The Most Common “Invisible Blood Loss”

Industrial dehumidifiers operate for extended periods in vibrating environments, and micro-leaks can occur at pipe welds, compressor suction and discharge ports, valve seals, and the connecting nuts of capillary tubes or expansion valves.

2. Clogged desiccant filter—a “blood clot” on the high-pressure side

Desiccant filters are designed to adsorb moisture and filter out impurities. However, after prolonged use, powdered molecular sieve, carbon deposits from refrigeration oil, or copper oxides may accumulate at the front of the filter screen, causing localized throttling.

3. Expansion Valve Opening Malfunction or Sensing Bulb Failure — “Control System Malfunction”

The sensing bulb in a thermal or electronic expansion valve is responsible for detecting the superheat of the return gas to regulate the refrigerant flow.

III. On-Site Rapid Troubleshooting: “Four-Step Diagnostic Method”

When faced with an air conditioner exhibiting low evaporation pressure, service technicians should systematically narrow down the cause step by step according to the following logical sequence to avoid misjudgments based on experience alone:

Step 1: Shut Down and Let the System Rest; Record Equilibrium Pressure and Compare It with Ambient Temperature

Disconnect the power supply, close the air inlet and outlet dampers (if present), and wait at least 4 hours for the system pressure to equilibrate with the ambient temperature. Measure and record the equilibrium pressure value, then compare it with the standard saturation temperature-pressure chart for the refrigerant. If the equilibrium pressure is significantly lower than the saturation pressure corresponding to the ambient temperature (deviation exceeding 5%), a system leak can generally be determined without starting the compressor.

Step 2: Start the unit and read the key “temperature difference triangle”

Start the unit and allow it to run stably for 15 minutes. Then, simultaneously measure the following three temperatures:

• Dry-bulb temperature at the evaporator inlet and outlet (the normal temperature difference should be 8–12°C);

• Compressor suction temperature and evaporator outlet pipe temperature (to calculate superheat);

• The temperature difference between the pipe walls before and after the dry filter (normally ≤2°C).

If the superheat is high (>8 K) and the temperature difference across the filter is >5°C, this suggests a blockage-related fault; if both the superheat and the filter temperature difference are normal but the evaporator pressure remains low, the primary suspicion should be a deviation in the expansion valve setpoint.

Step 3: Comprehensive Verification Using the Sight Glass and Current Measurement

For units equipped with a sight glass, observe the refrigerant flow during operation—a continuous stream of bubbles accompanied by pressure fluctuations indicates insufficient refrigerant; if the liquid flow is clear but the evaporator pressure is low, it is more likely that the throttling component is too small or the sensor bulb signal is faulty. Simultaneously, measure the operating current with a clamp meter; if the current is more than 10% below the rated value, combined with the discharge temperature (if excessively high), this further supports insufficient refrigerant supply.

Step 4: Segmented Pressure Hold Leak Detection (Final Arbitration Method)

When the above non-destructive tests cannot clearly distinguish between a leak and a blockage, the refrigerant should be recovered, and the system should be charged with nitrogen separately to perform segmented pressure holds on the high-pressure and low-pressure sides (24-hour pressure drop test). Although this step is time-consuming, it can precisely locate minute leak points, avoiding unnecessary costs caused by blindly replacing the desiccant filter or expansion valve.

IV. Routine Maintenance: Restoring Evaporator Pressure to the “Comfort Zone”

The cost of corrective repairs is far higher than that of preventive maintenance. To prevent deviations in evaporator pressure, we recommend establishing the following three routine procedures:

• Monthly filter cleaning: Clogging on the air side reduces the evaporator’s heat load, causing the refrigerant to return in a “supercooled” state, which artificially lowers the pressure set point and increases the risk of liquid hammer in the compressor. During cleaning, simultaneously inspect the evaporator fins for bent fins or accumulated dust and oil film.

• Quarterly Sight Glass and Electrical Terminal Inspection: Even in the absence of leaks, monitor color changes in the sight glass (indicating moisture) and the cumulative operating time of the desiccant filter. It is recommended to proactively replace the desiccant filter element every 2 years or after 8,000 cumulative operating hours, without waiting for a failure to occur.

• Annual system performance testing: Under standard operating conditions (e.g., supply air at 27°C/60% RH), record the evaporator pressure, condenser pressure, superheat, subcooling, and dehumidification capacity to establish a “health record” for the equipment.

V. Conclusion: Transitioning from a “Parts Replacer” to a “Diagnostician”

Low evaporator pressure in industrial dehumidifiers is by no means an anomaly in a single parameter; rather, it serves as a mirror reflecting the coordinated state of all components in the refrigeration system. The most common misconception in after-sales maintenance is equating low-pressure gauge readings directly with a “refrigerant leak.” As a result, after recharging, high-pressure surges and compressor overload occur, which actually accelerates system failure.

True professional maintenance lies in constructing a cross-validation logic using the four dimensions of temperature, pressure, current, and temperature difference. Every precise investigation into the root cause of low evaporation pressure not only extends the equipment’s lifespan but also fundamentally ensures humidity stability in the production environment. The next time you encounter an “older unit” with warm airflow and poor dehumidification performance, set aside the refrigerant charging hose for a moment and calmly follow the four-step diagnostic process—the answer is often clearer than you might imagine, and it’s safer than acting on impulse.