Application of a Closed-Enclosure Hood System in the Dryer Section of a Paper Machine

With the implementation of policies promoting energy conservation and consumption reduction, an increasing number of new paper machine installations and upgrades of aging production lines in China are opting for enclosed hood systems. Compared with traditional semi‑enclosed hoods, enclosed hoods equipped with automated control systems can optimize steam and electricity consumption, enhance the workshop’s working environment, and ensure stable, uninterrupted production.

Zhengding Environment possesses integrated production capabilities for enclosed hood systems, accompanying waste‑heat recovery units, and corresponding automated control systems. Its equipment is compatible with paper machines ranging from 1,880 mm to 6,600 mm in width and can be deployed on production lines for white cardboard, packaging paper, and household paper. The hood‑based waste‑heat recovery system is pre‑equipped with mounting interfaces for connecting boiler flue‑gas heat‑exchanger units.

I. Structure of the Enclosed Hood Body and Two-Stage Gas-to-Gas Heat Recovery System 

1.1 Main Structure of the Enclosed Hood 

The air hood fully encloses the drying section, isolating the hot, humid air inside from the workshop atmosphere, thereby stabilizing the internal production environment and minimizing external environmental influences on paper quality.

The hood panel features a sandwich‑type thermal insulation structure, with rust‑resistant aluminum alloy sheets on the inner and outer layers and a 100–150 mm layer of flame‑retardant insulating cotton in the core. It employs an aluminum‑profile grid‑type internal frame and is securely mounted to the paper machine’s frame. The panels are joined using a detachable labyrinth‑type interlocking design, complemented by sealing strips to minimize heat‑transfer losses.

The operating side is equipped with an electric lift-up door, and the paper‑break signal can automatically trigger the door to rise, facilitating troubleshooting. The drive side features a manually operated sliding maintenance door. Inside the hood, 36V safety‑rated lighting is installed, and fire‑safety temperature‑monitoring points are strategically positioned to meet workshop safety requirements.

1.2 Two-stage air-to-air heat recovery unit 

The high‑temperature, high‑humidity air discharged from the hood contains recoverable heat. It is paired with a modular shell-and-tube heat recovery unit, which comprises three sections: an air‑collection section, a heat‑exchange section, and an air‑return section. The heat exchange tubes are made of stainless steel or corrosion‑resistant aluminum, and the system employs a cross‑counterflow heat exchange configuration, with exhaust air flowing through the tube side and outdoor fresh air flowing through the shell side.

The heat exchange unit is equipped with a timed, automated spray‑cleaning system that removes paper fibers and dust adhering to the tube walls. The heat exchange tubes can be individually removed for maintenance, thereby mitigating the decline in heat transfer efficiency caused by dust buildup. The system recovers heat in two stages: the first‑stage hot air is used to preheat the bagging area, while the second‑stage fresh air is delivered into the workshop ceiling, helping to reduce condensation and dripping on the overhead surfaces during winter. The equipment also features a provision for an external flue connection, enabling integration with boiler exhaust‑gas heat recovery units to further enhance heat recovery.

1.3 Airflow Path of the Ventilation System 

The moist, hot air generated by the drying section is conveyed via the hood‑top plenum to a primary heat‑recovery unit for heat recovery; the exhaust gas then passes through a secondary heat exchanger for further heat exchange before being discharged. Outdoor fresh air is preheated in the primary heat exchanger, temperature‑adjusted in a steam heater, and delivered to the blow boxes in the felt zone and to the sealing blow boxes at the paper web’s entry and exit points. The fresh air, after being preheated in the secondary heat exchanger, is distributed into the workshop ceiling. All ductwork parameters—temperature, humidity, and air pressure—can be integrated into the control system for unified regulation.

II. Basic Requirements of the Closed Hood Control System for Paper Machine Production 

The entire ventilation system relies on several fans to circulate air in and out, and is equipped with steam control valves, hood lift doors, and fire‑monitoring components. Coupled with a PLC‑based automatic control system, it achieves automated operation. During production and use, the control system must meet the following basic requirements:

Real-time acquisition of temperature and humidity data from each air duct and within the hood; adjustment of fan speed based on parameter changes to maintain stable temperature and humidity inside the hood.

Adjust the steam valve opening in response to changes in chamber temperature to maintain a stable supply-air temperature, thereby accommodating production with different paper grades and machine speeds.

The electric lift door supports both manual and automatic control modes, ensuring optimal airtight sealing.

Real-time temperature monitoring at fire‑fighting locations: when the temperature reaches the preset threshold, the system triggers an alarm and simultaneously closes the steam valve while activating the fire‑fighting valve.

The human–machine interface can display information such as fan speed, air duct temperature and humidity, equipment operating status, and fault alerts, and supports seamless switching between manual and automatic modes.

III. Hardware Configuration of the Control System (Mature, In-Use Configuration; No Additional Expansion or Uncommissioned Hardware) 

Control core: such as the Siemens S7‑1500 PLC, which is resistant to high temperatures, dust, and electromagnetic interference; its modular I/O points can be expanded as needed, enabling unified control of hood fans and adjustment components for heat recovery equipment.

Fan drive: For example, the ABB ACS510 inverter provides open-loop speed control, adjusting the fan’s operating frequency based on the controller’s signal.

Actuating element: electric steam control valve, with smooth response, suitable for hot-air temperature regulation applications.

Human–Machine Interaction: Equipped with an industrial touch screen, it displays operating parameters locally, allows adjustment of operating settings, and presents fault information.

Sensor components: temperature and humidity transmitter, pressure sensor, fire‑alarm temperature detector, and door‑position feedback switch, continuously collecting on-site operating‑condition data.

IV. Control System Software Operating Logic 

The PLC program employs a modular subroutine design, cyclically acquires on-site sensor data, and utilizes fuzzy adaptive PID control logic, organized into four major operational modules:

4.1 System Initialization Module

After the equipment is started, it completes the reset of inverter and steam valve parameters, performs a self‑diagnosis of sensor signals, and supports no‑load trial operation during maintenance.

4.2 Analog Data Acquisition Module

Continuously collect temperature, humidity, and wind pressure data from various locations on the hood and heat recovery equipment, and complete data processing and analysis.

4.3 Fan and Temperature Control Module

The system generates control signals based on the acquired temperature, humidity, and pressure readings: when temperature and humidity fall outside the set range, it adjusts the fan speed to balance airflow; if the temperature drops below the set range, it gradually opens the steam valve to raise the temperature; conversely, if the temperature exceeds the set range, it reduces the valve opening, employing a staged adjustment strategy to minimize significant temperature fluctuations.

4.4 Security Alarm Interlock Module

When the equipment detects fan overload, abnormal temperature, or other conditions, the corresponding alert is displayed on the interface. If the temperature at a fire‑protection point exceeds the threshold, the system triggers an audible and visual alarm and simultaneously activates interlocked actions, such as shutting off the steam supply and opening the fire‑fighting valves.

V. Actual Application Performance of the System 

Energy Optimization: A sealed hood paired with a two-stage waste heat recovery system can capture exhaust‑air waste heat, reducing steam and fan‑motor energy consumption; over the long term, this helps lower production‑related energy costs.

Stabilized production conditions: The air hood isolates the interior from external air, reducing fluctuations in temperature and humidity and minimizing issues such as uneven paper moisture and surface defects.

Reduce manual workload: The system automatically adjusts the fans and steam valves, thereby decreasing the frequency of manual adjustments by operators.

Suitable for a wide range of renovation and new‑build projects: the complete set of hood systems, heat recovery units, and automated control equipment is manufactured in‑house, enabling one‑stop services that include on‑site surveys, system design, equipment fabrication, and installation and commissioning. The solutions have been deployed and are operating reliably at numerous paper‑making facilities.

Zhengding Environment has been deeply engaged in the fields of papermaking ventilation and drying, as well as dust control, for many years. With its own 10,000-square-meter production and manufacturing base, its core product portfolio includes enclosed hoods, Yankee hood systems, paper machine dust collection systems, waste‑heat recovery heat exchangers, and integrated intelligent control systems. The company offers comprehensive turnkey solutions for new production lines and holistic energy‑saving retrofit packages for aging paper machines, serving a wide range of paper manufacturers producing white cardboard, test linerboard, household paper, specialty paper, and more. From on‑site condition assessments and airflow‑system design to sheet‑metal equipment fabrication and field installation and commissioning, Zhengding delivers end‑to‑end, in‑house execution. Tailored to specific paper machine speeds, web widths, and paper grades, its custom‑designed solutions help papermakers optimize drying processes and achieve efficient control of production energy consumption.