Technical Article

The Role of Sensors in Airflow Systems

December 08, 2020 by Muhammad Asim Niazi

Learn the role and characteristics of sensors within larger airflow systems both in manufacturing and HVAC systems. Then, read about airflow sensors’ role in a pharmaceutical production example.

Sensors in airflow systems help measure the amount of air flowing at any given time. They are an essential part of processes where air is used in production or any type of processes that involve HVAC systems. As the air passes through the sensors, the sensors generate an output that represents the airflow’s physical value in electronic form. Any increase or decrease in the airflow directly affects the electronic signal’s value.

Different controllers use these electronic signals to perform many functions. These controllers or PLCs perform specific actions that depend on the value of the output generated from these flow monitors. In addition, some manufacturers also have a display on the sensor or monitor to easily see the reading from the sensor.


Airflow Sensor Specifications

Depending on the type of sensor and application of use, airflow sensors can have different specifications. Commonly, the specifications include:

  • Type of connection: This indicates where the sensor installs on the equipment or machine. For example, if the sensor is for pipe then is it threaded or not? Size of thread etc. etc. Some sensors are for duct, and in this case, the sensor provides further detail.
  • Media: What is the media you want to measure? Some sensors are for air, and others are for different types of gasses. 
  • Power Supply: Power supply is the supply voltage for powering up the sensor. The common rating is the 20 – 36 VDC supply.
  • Range of Input: Range represents the input signal’s range, through which the sensor can measure.
  • Output: There are two types of output — PNP and NPN. The output type depends on the input type that a controller or PLC accepts. In the case of the transmitter type, there are three types. One is 0 to 10 VDC, the second one is 0 to 20mA, and the third one is 4 to 20mA.
  • Electrical Connection: This is the type of connection through which the sensor connects to the power source and output signals go out. Common ratings are M8 and M12 connectors with 8mm and 12mm locking thread, respectively.
  • Display: Some sensors have a display on their body that allows easy reading for convenient reference.
  • Material: This is the type of material of the sensor’s outer body. It becomes a critical factor when the sensor is used in the product or process critical areas such as sterile environments for biological and life sciences. In these industries, there are strict sterile requirements, and no material except stainless steel is permitted.


How Does an Airflow Sensor Work?

There is more than one working principle for an airflow sensor and these principles depend on the sensor’s type and application. Airflow sensors commonly use the principle of thermal anemometers (also known as hot wire anemometers).


Figure 1. The hot wire element’s temperature is controlled by a sensor. Image courtesy of Omega


In a hot wire anemometer, there is a heating element that uses electrical energy to heat. When airflow touches the heating element, it cools down. These anemometers also contain a temperature sensor that measures the heating element’s present temperature. As the heating element cools down, the sensor measure this difference and again heats up to its set value. 

The amount of electrical power required to maintain the heating element’s temperature (as a result of cool down by the airflow) represents the airflow rate. The output is a proportional voltage in response to the airflow rate. The measured data is then available in the form of voltage or current depending upon the type of sensor and user requirements.


Industrial Example: Tablet Coating Pan

Let’s look at an example of airflow sensors used in pharmaceutical manufacturing. Tablet coating pans are necessary and critical equipment in the pharmaceutical industry. Their function is to coat a tablet with the necessary material. A thin layer of material is deposited on the tablet surface using the spray method. 

After the spraying process, the tablet drying process takes place. In this process, hot air passes through the bed of tablets. This hot air removes the solvent into which material dissolves and, as a result, the material is left behind on the tablet’s surface.


Figure 2. Airflow sensors are used in multiple areas of pharmaceutical manufacturing.


Airflow Rate in Tablet Coating

Airflow sensors measure the amount of hot air that passes through the tablet bed. It is demanding and essential information as excessive flow rate damages the tablets, and a low flow rate creates a subpar product.


Figure 3. Diagram of the tablet coating process.Image courtesy of Eurotherm.


The product development team selects the airflow depending on the characteristics of the product. The particular value of the airflow remains fixed in the recipe for every product. Blowers that run through variable frequency drives (VFD) maintain the flow rate. 

The values from the airflow sensors are continuously monitored with the help of the main controller or PLC. Airflow sensors immediately sense any changes in the airflow. As per the algorithm, the PLC or main controller increases or decreases the blower (operated through VFD) speed until the airflow sensors sense the correct airflow.