Research and Development of Pulp Molding Drying Equipment

(1) Direct Heating Gas Drying Line

Direct heating gas (natural gas) drying line is suitable for drying paper mold products for food packaging and paper mold products for household appliance packaging with high cleanliness requirements.

① Overall Design

Based on the drying process conditions and design requirements of paper mold products, we must first determine the structural parameters of the gas drying line. For example, the overall length of a drying line oven is 36m, the width is 2.84m, the height is 1.92m, and it is divided into 15 heating and drying sections. In the preliminary design, we require each section to have a hot air circulation system, and the chain conveyor belt should pass through the middle. The upper and lower hot air is blown to the wet paper mold on the conveyor belt through the air holes on the gas distribution plate, thereby effectively drying the product in sections. When the wet paper mold passes through different sections, we will preset the drying temperature of different sections accordingly.

In addition to installing a natural gas combustion hot air furnace on the top of the oven, an automatic gas burner is also required. The combustion chamber of the hot air furnace is welded with high-quality heat-resistant steel coils, and the inner lining is arranged with special refractory materials. The entire oven adopts a steel frame with good stability. In addition, rock wool insulation materials are installed in the oven wall, so the outer wall of the oven has a good insulation effect and the surface temperature is less than 60°C.

As for the transmission method, we use chain transmission, and through the electromagnetic speed regulating motor + gearbox + sprocket, the conveyor chain is equipped with a stainless steel mesh belt, and the conveyor chain speed is 1~3.5m/min, realizing stepless speed regulation.

② Single-Section Oven Design

The oven is equipped with hot air main pipe, upper exhaust duct, lower exhaust duct and circulating fan and other equipment. Taking a certain model as an example, we designed a single-section oven with a length of 2200mm, a width of 2840mm and a height of 1920mm.

The oven adopts a frame structure, with fixed insulation layers on the upper and lower layers, and uses insulation rock wool with a thickness of 100mm. Movable doors are set on both sides of the oven for daily maintenance. The jacket of the movable door is also equipped with thermal insulation rock wool.

The upper layer of the oven is the hot air main pipe, which is connected to the hot air furnace. The upper and lower exhaust ducts are arranged in the middle of the oven. The conveyor chain track is in the middle of the upper and lower exhaust ducts. The upper and lower exhaust ducts are connected to the circulating fan. The circulating fan is installed on the right side of the oven, with an air volume of 5500~6500m³/h and a power of 4kW. A direct current distribution plate is installed in the upper and lower exhaust ducts. Its characteristic is that air passes through it to form many air jets passing through the dried material. The gas distributor of this structure has the advantages of high drying efficiency, stable product quality, strong versatility and high adaptability.

Now we use clean energy as fuel to drive the direct heating gas drying line, which has been put into operation in some paper mold product production plants. It can fully meet the cleanliness requirements of the products and produce pulp molded packaging products with higher cleanliness, meeting the needs of the electronics industry and the food industry.

(2) Energy-Saving Steam Drying Line

This is an energy-saving steam drying line developed by a pulp molding product manufacturer based on the summary of many years of using steam drying lines.

The production capacity of the original steam drying line is 160~170kg/h, the steam consumption is 1170~1250kg/h, the total length of the drying line is 38m, the total volume of the whole machine cavity is 223m³, the total surface area of ​​the whole machine is 469.7m², and the steam volume required for drying 1kg of products is 6~7kg, and the drying cost is 1.3~1.4 yuan/kg.

With the same production capacity (160~170kg/h), the energy-saving steam drying line consumes 700~800kg/h of steam. The total length of the drying line is 29m, the total volume of the inner cavity of the whole machine is 79.78m³, the total surface area of ​​the whole machine is 220m², and 4~5kg of steam is required to dry 1kg of products. The drying cost is 0.9~1.1 yuan/kg, and the manufacturing cost of the drying line is also reduced by 35%.

The improved steam drying line adopts a leg structure, and the lower exhaust box is completely exposed. The total length of the whole machine is reduced by 9 meters, and the external heat exchanger box is cancelled, so the inner cavity volume and surface area are greatly reduced. The reduction of the inner cavity volume can reduce the heat required to maintain the inner cavity temperature; the reduction of the surface area of ​​the whole machine can reduce the heat diffusion.

The heat exchanger of the original steam drying line is designed in the insulation box on the top of the machine body. The air is first heated (160~170℃) and then sent to the exhaust box through the air duct and fan. When it is sent to the exhaust box, it is only 120~140℃. The new design eliminates the heat exchanger box on the top of the machine body and directly designs the heat exchanger at the exhaust port of the exhaust box. The air enters the exhaust box through the preheating air duct and fan on the cavity wall and is finally heated at the exhaust port. The exhaust temperature can be increased to 150~160℃.

The new energy-saving drying line reduces heat loss and increases the hot air temperature to achieve energy-saving effects, thereby reducing production costs. The new energy-saving drying line is a good choice for both drying equipment manufacturers and users.

(3) Resource Recycling Drying Line

The heat energy supply mechanism of the conventional drying tunnel drying line mostly adopts centralized heating (boiler) and medium heat transfer. The fuels are coal and diesel, and the media are steam, heat transfer oil and hot air. Heat energy will be lost efficiently when it is conducted through a medium. More importantly, the radiant heat generated by the heat source (boiler) of centralized heating will be wasted.

The direct heating drying line is a design that divides the centralized heating heater (boiler) into parts and is placed inside the drying line. The heat energy does not require any heat transfer medium and can be directly transferred to the wet paper mold for drying through three heat transfer methods (radiation, conduction and convection). Its core technology is to design the burner to effectively convert the highest temperature generated during combustion into maximum heat, because the drying process does not require the highest temperature, but requires sufficient heat, and the hot air circulation and dehumidification mechanism are designed more reasonably.

The direct heating drying line distributes several burners throughout the entire length according to the heat demand for drying. The burner is set between the working chain belt and the return chain belt inside the drying line. When a domestic paper mold factory used the direct heating drying line, it changed the original coal burning to firewood burning. Because there are many local furniture factories, the scraps from furniture production can be fully utilized, and the price is very cheap, which can reduce the drying cost of paper mold products by about 50%.

This drying line can be further improved to burn wood chaff (wood powder sawn and ground when producing density board) and crop straw powder. A newly designed powder burner is installed inside the drying line, which can make the powder burn in a suspended rotating state in the burner, with full combustion and very little dust. The dust removal and smoke exhaust device designed in conjunction with it can completely filter the dust and collect the dust in a centralized manner. The dust from burning firewood and straw is a good farm fertilizer. Using crop straw as fuel not only solves the problem of large amounts of straw being difficult to handle during wheat harvest, but also develops a new energy source and increases farmers’ income.

Resource recycling drying lines, especially drying lines that can use crop straw as fuel, can be vigorously promoted and used in paper mold manufacturers.