Author: Mycond Technical Department
The efficiency of an air dehumidifier is determined not only by the capacity of the equipment, but also by the accuracy of the control systems that govern its operation. An incorrectly selected air dehumidification automation system can lead to excessive energy consumption, insufficient effectiveness and even material damage due to improper humidity control. In this article, we will examine the engineering principles of selecting, configuring and operating control systems for industrial and commercial air dehumidifiers.
Why is humidity control a dynamic process?
Indoor humidity constantly changes under the influence of many factors. The moisture load varies throughout the day and season, and infiltration of humid air through open doors can bring in more moisture within minutes than a dehumidifier can remove in an hour. That is why the control system must respond dynamically to these changes.
The basic functions of dehumidifier control systems include:
- Measuring relative humidity or material moisture content
- Displaying the measured value
- Recording readings to a chart or memory
- Controlling the dehumidification equipment
It is important to understand that every additional function increases the cost of the instrument and the potential for error. For a simple warehouse where it is sufficient to keep humidity below 60%, a basic humidistat costing less than 100 pounds sterling, without display and data logging functions, is often sufficient.
Types of relative humidity sensors
The principle of operation of a humidistat depends on the type of built-in sensor. Let’s consider the main technologies:
Mechanical hygrometers use the elongation of human hair or a polymer film when absorbing moisture. Leonardo da Vinci observed that a wool ball weighs more on a humid day. Such sensors are simple but have limited accuracy and a slow response.
Electronic capacitive sensors measure the change in electrical capacitance of a polymer as it absorbs moisture. They provide better sensitivity at low humidity (below 15% RH) and are widely used in modern dehumidifier control systems.
Resistive sensors measure the resistance of a polymer with quaternary ammonium salts. They are more accurate at high humidity above 90% RH because they measure volumetric rather than surface moisture absorption.
Psychrometers use a pair of thermometers – dry and wet. The temperature difference is proportional to the evaporation rate and, accordingly, to the air humidity.
The typical accuracy of industrial humidistats is ±2% RH. It is important to understand their critical limitation: if a device is calibrated at 24°C and 65% RH, it will not be accurate at 21°C and 10% RH because the difference in moisture content is too great.
Absolute humidity sensors
For many industrial applications, the critical parameter is not relative humidity but the absolute moisture content of the air. Specialised sensors are used for this:
Condensation hygrometers (dew point sensors) cool a mirror surface until condensate appears on it. The surface temperature at this moment corresponds to the dew point of the air. The method has been used since 1751, when French naturalist Charles Le Roy added ice to a polished steel container. Modern optical condensation hygrometers have a typical accuracy of ±1.5°C of dew point and are the most accurate instruments for measuring humidity.
Aluminium oxide sensors have a typical accuracy of ±3°C of dew point and are optimised for very low humidity. They are used to measure a dew point of -40°C at air temperatures over 150°C, for example, at the outlet of adsorption dehumidifiers used for drying plastic pellets.
Lithium chloride sensors operate on the principle of heating a layer of salt until it dries. At a relative humidity of 11%, lithium chloride transitions from a liquid solution to a dry form, and the salt temperature at this moment is proportional to the absolute humidity of the air.
Accuracy and repeatability of measurements
To correctly evaluate a humidity sensor, it is important to distinguish between two key parameters:
Accuracy – the ability to display the true value of humidity.
Repeatability – the ability to return to a previous reading when the previous humidity level is restored.
Key principle: a repeatable instrument can be calibrated and made accurate, but a non-repeatable instrument will never be accurate, regardless of calibration. That is why manufacturers of quality instruments always state repeatability in the specifications, whereas cheap sensors are described only in terms of accuracy.
How to choose the type of controller
On-off positional control of a dehumidifier is sufficient where high precision is not required. For example, for a refrigerated warehouse loading dock where the main goal is to prevent floor icing, there is no point in precision control of ±1% RH. The typical swing range for condensation dehumidifiers with on-off control is ±10% RH.
Modulating control is necessary for production with tight tolerances: pharmaceuticals, semiconductors, confectionery drying. Adsorption dehumidifiers with modulation provide accuracy of ±5% RH and better.
Let’s consider three types of controllers using the example of a loading dock at 4°C:
- Relative humidity controller set to 80% RH will switch on the dehumidifier when the moisture content exceeds 4 g/kg. This is the cheapest option with ±2% RH accuracy, but it only works correctly at a constant temperature.
- Condensation controller would react only when moisture actually condenses on surfaces, but such sensors are usually too fragile for industrial conditions.
- Dew-point control set to be 1°C more precise than a humidistat and independent of fluctuations in air temperature.
Strategies for modulating dehumidifier capacity
Bypass control of a dehumidifier is used in desiccant systems: when the moisture load decreases, part of the air bypasses the desiccant rotor through a bypass, mixing with the dried stream. It is critically important to match the aerodynamic resistance of the bypass and the rotor using a fixed damper in the bypass.
Regeneration energy modulation is the most effective way to save energy. A temperature controller at the outlet of the regeneration zone reduces heater power when the temperature exceeds 49°C (for lithium chloride desiccants). Savings with such control amount to 25–50% of annual energy costs.
There are two levels of modulation:
- Reactivation load following control – reducing heater power as the moisture load decreases. This has the most cost-effective ratio of price to savings.
- Part-load control dehumidifier – using microprocessors and variable-speed drives for fans and compressors. The cost is high and justified only for large systems.
Humidity sensor placement as a critical success factor
Correct placement of sensors is often more important than their accuracy. Here is a practical example: a steel corrosion protection system did not work even though the dehumidifier was operating properly. The reason was that the humidistat was installed near the discharge of dry air, 23 metres from the storage racks. The dehumidifier kept the ductwork dry, while steel worth 50,000 pounds sterling was rusting.
The main rule: the sensor must be placed close to the object being protected, not near the dehumidifier. This is especially important at low humidity (below 10% RH), when the difference between a dew point of -29°C and -26°C is less than 0.01 g/kg.
Integration of the dehumidifier with a BMS
Modern industrial dehumidifiers are equipped with a Modbus RS485 interface for connection to building management systems (BMS). PLC controllers with touch displays allow you to programme time schedules and complex control algorithms, and remote monitoring provides a prompt response to parameter deviations.
FAQ: The most important questions about dehumidifier control systems
What is the typical control accuracy for different types of dehumidifiers?
Condensation dehumidifiers with on-off control provide ±10% RH accuracy, adsorption units with modulation ±5% RH and better. For precision applications with optical dew-point sensors, ±1–2% RH is possible.
How to choose between control by relative humidity and by dew point?
RH control is cheaper and sufficient for most comfort and storage applications. Dew-point control is necessary when air temperature varies significantly or when high accuracy at low humidity is required.
Why shouldn’t the sensor be placed near the dehumidifier outlet?
The air at the outlet is the driest in the system and does not reflect real conditions in the protected zone. The sensor must measure humidity where the result matters.
When is simple on-off control sufficient?
For long-term storage warehouses with a stable load, for spaces where a wide humidity range of 40–60% RH is acceptable, and when annual energy costs are low compared to the cost of a modulation system.
Conclusions
The choice of a dehumidifier control system is determined by the required humidity control accuracy and economic justification. For most applications with a tolerance of ±5–10% RH, a humidistat with regeneration energy modulation is sufficient. Precision production requires dew-point control and full power modulation.
It is important to remember that sensor placement often matters more than its accuracy. The most accurate instrument in the wrong place will give a worse result than a simple humidistat located near the object being protected. Integration of the dehumidifier with a BMS ensures prompt control and documentation of parameters for process validation.
