Paper Machine Dryer Section Ventilation System | How Closed Hood, Pocket Ventilation and Waste Heat Recovery Affect Paper Machine Energy Consumption

This article combines professional technical explanation and introductory popular science. Engineers may read the main text to understand principles, while industry newcomers can gain quick understanding through plain-language interpretation. It focuses on analyzing the application of Closed Hood and Yankee Hood in drying and ventilation systems.

In the paper machine drying process, paper web is not dried solely by heat from dryer cylinders. The ventilation system continuously discharges water vapor evaporated from the paper and supplies hot air of suitable temperature, which directly affects paper quality, equipment service life, as well as the power and heat consumption of the entire production line. Once the air extraction and supply volume of the hood become unbalanced, hot and humid air cannot be discharged smoothly, and condensation will easily form on the inner wall of equipment, causing corrosion of frames and roll sets. A well-designed ventilation solution can also recover waste heat contained in moisture vapor to reduce steam consumption of paper mills.

A complete drying ventilation system consists of the hood body, exhaust fans, hood air supply unit, pocket ventilation device, interface airflow stabilization components and waste heat recovery equipment.

There are two main types of hoods widely used in the paper industry: Closed Hood, mostly applied to Fourdrinier multi-cylinder paper machines; Yankee Hood, mostly used for single-cylinder paper machines, commonly seen in tissue paper and specialty paper production lines. The underlying ventilation logic of the two types is similar, yet obvious differences exist in structural form and air pressure control schemes.

【Plain-language Interpretation】The dryer section can be imagined as a bathroom at home. Dryer cylinders are like hot water that constantly generates large amounts of water vapor; the ventilation system works like a bathroom exhaust fan. Improper exhaust control leads to water vapor filling the space, dripping condensate at the top and rusted metal fittings. A proper exhaust scheme enables secondary utilization of heat recovered from waste gas. Closed Hood and Yankee Hood are customized thermal-insulated and sealed enclosures for this "bathroom". The difference is that the Yankee Hood fits one large single dryer cylinder, while the Closed Hood is designed for multi-cylinder paper machines.

I. Hood: Core Unit for Collecting and Discharging Drying Moisture Vapor

The core function of the hood is to collect water vapor evaporated during paper web drying, discharge it in an orderly manner to the outside of the plant, and control the air pressure inside the hood.

1. Fan selection shall match waste gas working conditions: stainless steel fans must be selected for conveying saturated hot humid vapor to resist corrosion from condensed water. Carbon steel or aluminum fans can be adopted under working conditions with unsaturated waste gas and no waste heat recovery requirement.
2. The hood air supply system consists of heating coils, fans, air ducts and nozzles. Part of the supplied hot air is delivered into the hood and may also be used for dryer fabric drying.

Zhengding Environment Closed Hood is a high-performance closed hood. Relying on insulated sandwich wall panels and built-in collecting air ducts, it coordinates vapor collection, air pressure balance and air supply-exhaust matching to reduce disordered escape of hot and humid gas. For working conditions of single-cylinder tissue paper machines, Zhengding Environment Yankee Hood adopts customized curved sealing structure fitting the contour of Yankee dryer to stably control airflow inside the hood.

【Plain-language Interpretation】The hood acts like the enclosure of a bathroom, gathering vapor for discharge by the exhaust system. Ordinary plate enclosures have poor thermal insulation and suffer vapor leakage everywhere. Closed Hood and Yankee Hood feature superior sealing and thermal insulation performance. The Yankee Hood has a curved structure specially wrapping the large Yankee dryer; the Closed Hood is of long box structure enclosing a whole row of dryers.

II. Open Hood / Medium-humidity Closed Hood / High-humidity Closed Hood

1. Open Hood: It features massive vapor leakage. Higher fan exhaust volume is required to prevent condensation, resulting in high fan power consumption and difficult waste heat recovery. It is barely adopted for newly-built high-speed paper machines.
2. Medium-humidity Closed Hood: It adopts an integral thermal-insulated and sealed shell equipped with lifting doors and sliding access doors, and air leakage at base plate gaps is strictly controlled. Operating range: 0.11-0.13kg/kg, exhaust temperature approx. 80℃. The recovered heat is used to preheat supply air.
3. High-humidity Closed Hood: It adopts insulation panels with higher sealing grade standards. The operating exhaust humidity is 0.16-0.17kg/kg and exhaust temperature approx. 85℃. It can reduce the overall ventilation air volume and further cut energy consumption, serving as the preferred solution for new Closed Hood installation and large-scale paper machine retrofits.

Yankee Hood also adopts a highly sealed thermal insulation structure. Its shape is adapted to a single Yankee dryer, focusing on optimizing the airflow for web drying against the dryer surface. It serves as the core equipment for energy-saving retrofits of tissue paper production lines. The high-humidity Closed Hood has strict requirements for thermal insulation of spliced wall panels, sealing of access doors and internal air pressure zero control system, and it is also a key working condition difficulty tackled by Hunan Zhengding Environment.

【Plain-language Interpretation】 Open Hood: No enclosure for the bathroom, and vapor spreads all over the room; Medium-humidity Closed Hood: An enclosure is fitted, enabling partial heat recovery; High-humidity Closed Hood: The enclosure has maximum sealing and thermal insulation performance, recovering more heat with less exhaust air volume.

The Closed Hood is a long box structure adapted to multiple dryers; the Yankee Hood is an arched hood wrapping a single large dryer. Both belong to highly sealed closed hoods.

III. Pocket Ventilation: Mitigates Paper Web Flutter and Optimizes Cross-direction Moisture Uniformity of Paper

The interspace between dryers and where the paper passes through dryer fabric rolls is known in the industry as the pocket.

Note: Pocket ventilation is mostly applied to multi-cylinder paper machines equipped with Closed Hood. Single-cylinder paper machines fitted with Yankee Hood have no multi-dryer pockets; the drying uniformity of the paper web is regulated by the internal air blowing structure of the hood.

When the paper machine runs at high speed, high-pressure and low-pressure zones form spontaneously within the pocket, causing turbulent airflow. This leads to two typical problems: paper web flutter or even web breakage; uneven cross-direction moisture distribution of paper with large local wet-dry differences, which affects finished product quality.

Pocket ventilation delivers controlled hot air directionally into the pocket to achieve four major objectives:

1. The air supply volume matches the paper machine speed and dryer fabric permeability. Higher machine speed demands higher control precision of air supply volume.
2. Maintain low and evenly distributed humidity inside the pocket.
3. Capable of correcting cross-direction moisture deviation and improving moisture consistency across the whole paper web.
4. The hot air source preferentially reuses recovered hot air from the hood to reduce fresh steam consumption.

Low-speed paper machines can meet requirements by natural air suction. High-speed paper machines equipped with Closed Hood must be fitted with a pocket ventilation nozzle system to actively intervene in pocket airflow conditions.

There are three mainstream implementation forms: side blowing via pocket nozzles, air supply through upper and lower nozzles of ventilation boxes, and penetrating air supply by hot air rolls.

【Plain-language Interpretation】The gap between two dryers is the pocket. At high operating speeds, turbulent airflow occurs within the gap, causing paper shaking and inconsistent local moisture. Closed Hood is equipped with pocket ventilation to regularize the airflow in the gap. The single large dryer enclosed by Yankee Hood has no such gap, and its built-in air blowing device ensures uniform drying of tissue paper.

IV. Three Types of Heat Recovery Solutions for the Drying Section

The exhaust gas discharged from the drying section contains abundant waste heat. Recovering this heat to preheat fresh process air is the core direction for energy-saving retrofits of paper machines. There are three mainstream solutions:

1. Direct water injection type: Atomized water is sprayed into the exhausted vapor to transfer heat to the water, and the hot water is supplied for workshop production.
2. Vapor-air heat exchange: Waste heat from exhaust gas is recovered to heat fresh air, raising the hood supply air from ambient temperature to 60℃.
3. Vapor-liquid heat exchange: Heat is transferred to water or ethylene glycol solution. Paper mills in northern China often adopt this system for workshop heating in winter.

Both Closed Hood and Yankee Hood projects can be equipped with waste heat recovery systems. The hood serves as the carrier for collecting hot and humid exhaust gas. If the hood loses sealing and a large amount of hot gas leaks out, the energy-saving effect will be greatly compromised even with high-performance waste heat recovery equipment.

【Plain-language Interpretation】Instead of directly venting high-temperature exhaust gas, heat is extracted via heat exchangers to warm cold air or produce hot water, cutting steam costs. The sealing performance of Yankee Hood and Closed Hood directly determines whether the waste heat recovery system can achieve the designed energy-saving targets.