Four Standard Methods of Controlling Desiccant Dehumidifiers

Precise humidity control is critical in industrial environments ranging from pharmaceutical and food processing to water treatment and aerospace manufacturing. Desiccant dehumidifiers provide dependable moisture removal through adsorption and reactivation, but how a system is controlled makes a significant difference in performance, efficiency, and operating cost.

There are four typical methods used to control desiccant dehumidifiers. Each method has unique advantages and tradeoffs, and the right choice depends entirely on the application and the required humidity tolerance.

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1. On/Off Control of the Dehumidifier

This is the simplest control strategy and is most often applied to smaller, standard units like IAT’s Compact Series dehumidifiers, or in environments where humidity precision is not critical.

How It Works

The dehumidifier is powered on when the process air relative humidity (RH) or dew point exceeds a set threshold and shuts off once the condition is satisfied. The entire system — including fans, heaters, and wheel rotation — is cycled on and off.

Advantages

• Low complexity and cost: Requires minimal controls and sensors.

• Ideal for intermittent operation: Suited for smaller or stand-alone systems.

Limitations

• Limited precision: Humidity typically fluctuates several points around setpoint, typically +/- 5-10% RH.

• Increased wear: Frequent cycling of motors and heaters can reduce component life.

Best Use Case

• Small-scale or non-critical applications.

• General drying where RH tolerance is not critical.

2. On/Off Control of Reactivation Heat

This method improves efficiency by controlling only the reactivation heat source, while allowing fans and the desiccant wheel to continue running.

How It Works

When the target RH is reached, the control system turns off the reactivation heater. The desiccant wheel continues to rotate, but with no heat energy to regenerate it, the unit’s drying capacity decreases until humidity rises again and the heater is re-energized.

Advantages

• Energy saving: Reduces heater run time during periods of low moisture load.

• Smooth restart: Wheel remains in motion, avoiding full system cooldown.

Limitations

• Moderate precision: Typical control tolerance is about ±5% RH, typically.

• Best for steady-state loads: Performance may fluctuate under rapidly changing conditions.

Common Applications

This method is often used in water treatment facilities or other environments where humidity control is important but tight tolerances are not required.

3. Modulation of Reactivation Heat

This is the most common and effective method used by IAT. Instead of simply turning the reactivation heat on or off, the system modulates heat input proportionally to the humidity load.

How It Works

Reactivation energy — electric, gas, or steam — is continuously adjusted through modulating valves or SCR controls. The control system monitors process RH and modulates reactivation temperature in real time to maintain the desired humidity level.

Advantages

• High efficiency: Energy input matches actual load conditions, minimizing waste.

• Precise control: Maintains humidity within ±1-2% RH, typically.

• Smooth operation: Eliminates the on/off cycling associated with simpler systems.

Limitations

• More complex control hardware: Requires PID tuning, sensors, and actuators.

• Higher initial cost: Offset by significant energy savings over time.

Common Applications

Ideal for industrial, manufacturing, or process-critical environments where humidity must stay tightly controlled but energy efficiency is also a priority.

4. Face and Bypass Control

The face and bypass control method provides the tightest humidity control of all available options, often used in high-precision environments where extremely stable conditions are required.

How It Works

A modulating damper assembly divides the process air stream — a portion passes through the desiccant wheel (“face” air), while another portion bypasses it entirely. The two airstreams mix downstream to achieve the exact target RH. The desiccant dehumidifier itself operates at maximum capacity continuously, while the bypass proportion provides fine control.

Advantages

• Exceptional precision: Achieves humidity stability within ±0.5% RH.

• Immediate response: Adjusts humidity by airflow mixing rather than temperature lag.

• Highly stable: Ideal for environments sensitive to even minor RH fluctuations.

Limitations

• High energy usage: The dehumidifier runs at full load continuously.

• System complexity: Requires well-balanced airflow and robust control integration.

Common Applications

Used in critical manufacturing, aerospace, lithium-ion battery production, or any process where even slight RH variations can impact quality or safety.

Choosing the Right Control Method

Selecting the best control strategy depends on three key factors:

1. Humidity tolerance — how tight the RH or dew point must be maintained.

2. Energy efficiency requirements.

3. System complexity and budget.

Conclusion

There’s no single “best” control method — only the most appropriate method for your specific application. Smaller, simpler systems may benefit from on/off or reactivation control, while high-precision or continuous processes demand modulation or face/bypass control.

At IAT, we work closely with engineers, plant operators, and system designers to determine the ideal control strategy for each desiccant dehumidifier we build — ensuring the best balance of precision, energy efficiency, and reliability.