Principle of PLC Differential Pressure Transmitters for Automatic Monitoring of Fan Filter Clogging

In supply fans, dust extraction fans, fresh air handling units, air compressor intake systems, and industrial ventilation equipment, filters are used to intercept dust, fibers, and particulate matter. As operating time increases, dust accumulation on the filter media gradually rises, increasing the resistance to airflow through the filter. If clogging conditions cannot be identified in time, the system may experience insufficient airflow, increased equipment load, and higher energy consumption. In severe cases, process stability and equipment service life may also be affected.

The core of a PLC differential pressure transmitter monitoring solution is to measure the pressure difference between the inlet and outlet sides of the filter. When the filter is relatively clean, the differential pressure across it is usually low. As dust accumulates on the filter, airflow resistance increases and the differential pressure across the filter continues to rise. By acquiring the 4-20mA, 0-10V, or RS485 communication signal output from the differential pressure transmitter, the PLC can convert differential pressure information into visualized values, alarm statuses, and maintenance instructions.

Compared with manual inspections and scheduled replacement, differential pressure monitoring can determine the appropriate maintenance time based on actual changes in filter resistance. In environments with significant fluctuations in dust concentration, this condition-based maintenance approach is more targeted. It can avoid premature replacement before the filter becomes clogged and reduce out-of-limit operation caused by prolonged lack of inspection.

A complete system typically consists of a differential pressure transmitter, pressure tapping lines, a PLC analog input module or communication module, an HMI touchscreen, an audible and visual alarm, and a host computer platform. Xi'an Shenghongchuang Instrumentation Co., Ltd. can configure suitable differential pressure measurement and automatic control solutions for industrial customers based on fan static pressure, filter type, on-site medium, and control method.

Relationship Between Filter Differential Pressure and Clogging Status

Differential Pressure Changes Reflect Filter Ventilation Resistance

Differential pressure is the difference between the upstream and downstream pressures of a filter. Common units include Pa, kPa, mbar, and mmH₂O. Under conditions where the fan is operating normally and the damper opening is generally stable, a higher differential pressure indicates greater resistance that the air must overcome to pass through the filter. For general ventilation filters, the initial resistance may range from 30Pa to 80Pa. When the differential pressure rises to the specified final resistance, cleaning or replacement of the filter element is usually required.

The same clogging threshold cannot be applied to different filters. Primary filters, medium-efficiency bag filters, high-efficiency filters, metal filter screens, and industrial dust collection cartridges differ in rated airflow, filtration grade, and structural resistance. During actual setting, the initial resistance and recommended final resistance data provided by the equipment manufacturer should be referenced first, then confirmed in combination with on-site airflow, available fan pressure, and production process requirements.

Looking only at the differential pressure at a particular moment is insufficient to comprehensively determine equipment status. The PLC can sample at intervals of 1 second, 5 seconds, or 10 seconds and calculate a moving average from 30 seconds to 300 seconds to reduce transient interference caused by fan startup, damper adjustment, and airflow pulsation. Triggering an alarm only after the average differential pressure continuously exceeds the threshold for a certain period can improve the reliability of monitoring results.

In addition to clogging, abnormally low differential pressure also deserves attention. For example, a fan not operating, disconnected pressure tapping lines, damaged filter media, an unclosed bypass damper, or sensor signal failure may all cause differential pressure to remain close to zero for a long period. Therefore, automatic monitoring logic should include both high differential pressure clogging identification and low differential pressure abnormality diagnosis.

Hardware Configuration of Differential Pressure Transmitters and PLCs

Pressure Tapping Location Determines Data Validity

During installation, pressure tapping points should be set at relatively stable duct locations before and after the filter. The high-pressure side is typically connected to the upstream face of the filter, while the low-pressure side is connected to the downstream face. Pressure taps should be kept away from elbows, fan outlets, damper edges, and local turbulence areas as much as possible. Generally, a certain length of straight duct should be maintained from disturbance-generating components to reduce measurement fluctuations caused by airflow impact.

Pressure tapping lines should use aging-resistant, corrosion-resistant hoses or rigid pipes that are not prone to collapse, and the line lengths on both sides should be as close as possible. If the air has high humidity or contains oil mist or condensate, drainage structures or condensate collection measures should be provided to prevent liquid accumulation from blocking the pressure lines. For dust collection systems with high dust concentrations, pressure tapping holes should also be checked regularly for blockage by particulate matter.

The range of the differential pressure transmitter should match the actual differential pressure of the filter. If the range is too large, resolution in the low differential pressure range decreases; if the range is too small, the measurement range may be exceeded after the filter becomes clogged. Common fan filters may use ranges such as 0-500Pa, 0-1000Pa, 0-2000Pa, or 0-5kPa. The specific selection should be based on the maximum expected resistance and safety margin.

For electrical connections, two-wire 4-20mA differential pressure transmitters powered by 24VDC are widely used. They offer strong anti-interference capability and are suitable for long-distance transmission. After the PLC analog module reads the current signal, the engineering value can be converted according to the transmitter range. When RS485 Modbus RTU communication is used, differential pressure, temperature compensation parameters, equipment status, and other information can be read directly.

4-20mA Signal Conversion Example

Assume that the differential pressure transmitter range is 0-1000Pa, corresponding to an output of 4-20mA, and that the actual current acquired by the PLC is 12mA. The differential pressure calculation formula is: differential pressure value＝(measured current－4)÷16×1000. Substitution gives 500Pa. If the PLC analog module converts 4-20mA into a digital value of 0-27648, a proportional conversion of the digital value can also be performed first before outputting the engineering value in Pa.

Upper and lower limit diagnostics should be configured for input signals in engineering applications. For example, below 3.6mA may be determined as an open circuit or power supply abnormality, while above 20.5mA may be determined as overrange or a line fault. Specific diagnostic limits should be determined in accordance with the PLC module manual and transmitter technical parameters. Providing independent alerts for fault signals can prevent the PLC from mistakenly interpreting line abnormalities as a clean filter condition.

For fans with variable-frequency speed control, it is recommended to simultaneously acquire fan operation feedback, inverter frequency, or damper opening. When the fan is stopped, clogging determination can be frozen or the status can be switched to standby. When frequency variation is significant, frequency-based thresholds, airflow compensation, or stable operating condition determination can be used to avoid false alarms caused by differential pressure differences at different airflow rates.

Xi'an Shenghongchuang Instrumentation Co., Ltd. provides industrial measurement and control products for pressure, differential pressure, level, temperature, and humidity. Based on the on-site PLC brand, control cabinet structure, signal type, and installation space, the company can provide range selection, output type matching, and non-standard customization support.

PLC Clogging Identification and Alarm Interlock Logic

Set Three-Level Thresholds for Pre-Alarm, Alarm, and Interlock

PLC programs can set multiple differential pressure thresholds rather than using only a single alarm point. Taking a filter with a final resistance of 800Pa as an example, 600Pa can be set as the pre-alarm value, 800Pa as the clogging alarm value, and 950Pa as the high-high alarm value. The pre-alarm stage alerts maintenance personnel to monitor filter status; once the clogging alarm is reached, the filter is included in the maintenance plan; and the high-high alarm can, according to process safety requirements, interlock a standby fan, reduce the operating load, or prompt a shutdown inspection.

To prevent frequent alarms caused by transient differential pressure fluctuations, the PLC should be configured with delayed confirmation and hysteresis control. For example, a clogging alarm is confirmed when the differential pressure exceeds 800Pa continuously for 60 seconds. After the alarm is generated, automatic reset is permitted only when the differential pressure falls below 760Pa continuously for 30 seconds. A 40Pa hysteresis can reduce interference caused by repeated alarms near the threshold.

When equipment is equipped with pulse-jet cleaning or online dust cleaning functions, the PLC can combine cleaning actions with differential pressure trends. If the differential pressure reaches the set value, the system can first initiate one cleaning cycle. After cleaning is completed, wait 120 seconds to 300 seconds for airflow stabilization before reevaluating the differential pressure. If the differential pressure remains above the alarm threshold, the filter media may be severely clogged, damaged, or at the end of its service life, and the process should proceed to manual maintenance.

The HMI interface is recommended to display the current differential pressure, alarm thresholds, filter status, fan operating status, most recent maintenance time, and differential pressure trend curves. By recording the daily maximum differential pressure, average differential pressure, and cumulative duration of limit exceedance, managers can observe the rate at which filter resistance increases and obtain quantitative data for spare parts procurement, maintenance scheduling, and energy consumption analysis.

Selection and Maintenance Considerations for On-Site Applications

Key Details for Improving Monitoring Stability

Before selecting a differential pressure transmitter, it should be confirmed whether the measured medium contains corrosive gases, high-temperature steam, oil mist, or high concentrations of dust. Conventional low differential pressure products can be used for ordinary clean air applications. For chemical, metallurgical, coating, and high-temperature operating conditions, wetted materials, temperature resistance range, protection rating, and isolation measures require attention. In locations with strong electromagnetic interference, signal methods with better anti-interference performance should be prioritized, and shield grounding should be properly implemented.

Before commissioning, record the baseline differential pressure when the filter is clean or newly installed, and use it as a reference for subsequent trend analysis. If the initial differential pressure of a new filter is 55Pa and rises to 420Pa after several weeks of operation, although the final resistance has not yet been reached, the rate of increase can still be used to determine whether the dust load is abnormal. A rapid short-term rise in differential pressure often indicates changes in on-site dust concentration, abnormal filter installation, or insufficient filtration area.

Maintenance personnel should regularly inspect pressure tapping holes, pressure lines, fitting seals, and transmitter zero status. It is recommended to perform one zero check during the equipment maintenance cycle. If pressure lines on either side are found to be loose, cracked, filled with water, or blocked, line issues should be eliminated first before assessing the actual filter resistance. After replacing the filter, verify under stable fan operating conditions whether the differential pressure has returned close to the initial level.

For plants requiring centralized management, PLC data can be uploaded to configuration software, MES systems, or industrial Internet of Things platforms to achieve centralized monitoring of multiple fans and multi-stage filters. The system can generate alarm records and maintenance reports by equipment number, shifting filter management from manual experience-based judgment to closed-loop management based on real-time measurement data.

Building a Reliable Intelligent Monitoring Solution for Fan Filters

Automatic monitoring of fan filter clogging is not simply the installation of a sensor; it is a control system jointly composed of pressure tapping location, differential pressure range, PLC algorithms, alarm strategies, and maintenance procedures. Accurate differential pressure data enables managers to promptly understand changes in filtration resistance, reduce airflow attenuation and ineffective energy consumption, and maintain stable operation of ventilation, dust collection, and air purification systems.

In actual projects, it is recommended to first confirm the filter's initial resistance, recommended final resistance, rated airflow, and fan control method, then determine the differential pressure transmitter range and PLC thresholds. For equipment with variable-frequency control, multi-stage filtration, or pulse-jet cleaning functions, fan operating conditions and differential pressure signals should be evaluated in conjunction to obtain clogging conclusions that more closely reflect actual operating conditions.

Xi'an Shenghongchuang Instrumentation Co., Ltd. and its production base, Shaanxi Qinkong Sensor Technology Co., Ltd., provide differential pressure transmitters, pressure transmitters, intelligent digital display control instruments, and supporting measurement and control solutions for applications including machinery manufacturing, chemical energy, metallurgy and building materials, environmental monitoring, intelligent equipment, and automated production lines. Products can be configured with output options such as 4-20mA, 0-10V, and RS485 according to on-site requirements and support integrated applications in industrial automation systems.

To configure a PLC differential pressure clogging monitoring system for fan filters, provide the filter type, current differential pressure range, fan parameters, installation environment, PLC model, and required output signal. Professional technical personnel can assist with range selection, installation recommendations, alarm threshold settings, and control solution configuration.