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Material drying involves a wide range of fields that are closely related to our industrial and agricultural production and daily life. In industries that have a significant impact on the development of the national economy, such as chemical, petrochemical, pharmaceutical, food, wood, agriculture, etc., drying technology plays an indispensable role. Drying process is an essential processing process in industrial and agricultural production, and material drying is a high energy consumption operation. According to authoritative data, the energy used for drying accounts for about 15% of the total energy consumption of the national economy, and there is a large market demand.

Currently, steam drying equipment is widely used as a heat source in domestic processing and production. In small enterprises, furnace gas and direct fire flue gas drying equipment are commonly used. Steam drying equipment mainly relies on filtration, which is limited by current technology in China. The average thermal efficiency of industrial boilers in China is around 60%. At the same time, steam drying equipment loses up to 40% of steam heat energy during the intake and exhaust processes. At the same time, there are multiple factors that cause heat loss. These factors, combined, have a significant impact on the thermal efficiency of steam drying equipment, As a result, the utilization rate of Primary energy of steam drying equipment is generally below 30%. This excessive energy consumption significantly increases the drying cost, and at the same time, due to insufficient utilization of energy, a large amount of smoke and exhaust gas is generated, causing serious pollution to the atmosphere. The generated exhaust gases mainly include CO2 and NO2 gases, which exacerbate the greenhouse effect and are also the main factors causing acid rain to form and destroy the ozone layer.

The material drying process is a huge energy consuming process. According to statistics, in most developed countries, the energy consumed for drying accounts for 7% -15% of the total national energy consumption, while the thermal efficiency is only 25% -50%. Moreover, most drying processes, especially for heat sensitive materials (such as food and raw materials), will have an impact on their color, nutrition, flavor, and organization. The heat pump drying technology has the advantages of low energy consumption, low environmental pollution, high drying quality, and wide applicability. Its excellent energy-saving effect has been proven by various experimental research institutes.

In the national industrial energy-saving plan, it is explicitly required to carry out energy-saving technology transformation for industrial boilers and kilns in response to problems such as low self-control level, low average load, and outdated equipment. The State Council's "Action Plan for Air Pollution Prevention and Control", which is the first item of the ten atmospheric regulations, is the comprehensive renovation of coal-fired boilers, indicating the importance that the country attaches to the comprehensive renovation of boilers.

With the promotion of comprehensive renovation work for coal-fired boilers, clean air energy is expected to solve the urgent problem of coal combustion and open up innovative energy development paths. It is understood that the air energy industry is green, energy-saving, environmentally friendly, does not burn energy, does not emit exhaust gas, does not pollute the air, and does not cause haze disasters. It can be used in fields such as hot water, heating, and drying, and is one of the substitutes for traditional boilers.

Main application fields of drying effect

Wood industry: plywood, fiberboard pressure forming, mahogany, furniture wood drying.

Bamboo products: drying various bamboo products and supplies;

Petroleum and chemical industries: polymerization, condensation, distillation, melting, dehydration, and forced insulation.

Oil industry: fatty acid distillation, oil decomposition, concentration, esterification, vacuum deodorization.

Synthetic fiber industry: polymerization, melting, spinning, extension, drying.

Textile printing and dyeing work: heat setting, drying, and heat capacity dyeing.

Non woven industry: Non woven fabrics.

Feed industry: drying of raw materials and finished products.

Plastics and rubber industry: hot pressing, rolling, extrusion, vulcanization molding.

Paper industry: drying, corrugated paper processing.

Building materials work: drying gypsum boards, heating asphalt, and curing concrete components.

Mechanical industry: spray painting, printing and drying.

Food industry: baking, heating.

Air conditioning industry: Heating for industrial and civil buildings.

Road construction industry: asphalt melting and insulation.

Pharmaceutical industry: Raw material drying.

Light industry: producing ink and laundry detergent raw materials for drying.

Sludge drying: urban sludge, chemical electroplating sludge, and coal sludge drying.

Current Situation of Wood Drying Technology at Home and Abroad

Drying operation involves a wide range of fields and is an indispensable basic production link in many industries such as agricultural products, chemical products, and wood products. It is also a major energy consumer in China, accounting for about 15% of the total energy consumption of the national economy. Due to the fact that modern people cannot live without furniture, and the quality of furniture largely depends on the quality of wood drying, the wood drying industry has become an essential production link.

In addition, wood drying is usually an energy consuming process in wood processing enterprises, accounting for 40% -70% of the enterprise's energy consumption. Moreover, in China's wood drying industry, most of them are in a small, scattered, and backward state, with low and high energy utilization rates

About 30%.

Common artificial wood drying methods

The commonly used artificial wood drying methods that have achieved industrialization so far include conventional drying, high-temperature drying, dehumidification drying, solar drying, vacuum drying, high-frequency drying, microwave drying, and flue gas drying. Among them, conventional drying refers to using atmospheric wet air as the drying medium, steam, hot water, furnace gas or hot oil as the heat source, indirectly heating the air, and the drying medium temperature is below 100 ℃. High temperature drying means that the temperature of the drying medium is above 100 ℃. The drying medium can be normal pressure Superheated steam or wet air, but most of them are normal pressure Superheated steam. Both conventional furnace gas drying and flue gas drying are equipped with combustion furnaces that burn wood waste (or other fuels), using the flue gas generated by combustion as the heat source. However, flue gas drying is the primary stage of furnace gas drying, generally referring to drying kilns constructed by local methods, where the flue gas generated by fuel combustion directly or indirectly heats and dries wood. Dehumidification drying (also known as heat pump drying) is the same drying medium as conventional drying, both of which are moist air. The difference between the two lies in the dehumidification method of air. Conventional dry air adopts an open cycle, which means that a portion of hot air with high humidity is regularly discharged from the drying room, while an equal amount of cold air is sucked in from the outside. Therefore, the heat loss from air exchange in conventional drying is relatively large. During dehumidification and drying, the wet air passes through the refrigeration system of the Dehumidifier, cools, dehumidifies, heats and returns to the drying chamber for closed air circulation. The heat released during dehumidification of wet air is recovered by refrigeration refrigerant and used to heat the air. Therefore, the energy-saving effect of dehumidification drying is relatively obvious. Compared with steam drying, its energy-saving rate is generally over 40%.

Both high-frequency and microwave drying use wet wood as the dielectric, and under the action of alternating electromagnetic fields, water molecules in the wood rotate rapidly and frequently, causing friction and heat generation between water molecules, causing the wood to be heated and dried simultaneously from the inside out. The characteristics of these two drying methods are fast drying speed, uniform temperature field inside the wood, low residual stress, and good drying quality. The difference between high-frequency and microwave drying is that the former has low frequency, long wavelength, deep Penetration depth to wood, and is suitable for drying thick wood with large section. The frequency of microwave drying is higher than that of high-frequency drying, but the wavelength is shorter. Its drying efficiency is faster than that of high-frequency drying, but the Penetration depth of wood is not as high as that of high-frequency drying.

When wood is vacuum dried, the pressure difference between the water vapor inside and outside the wood increases, accelerating the migration rate of water inside the wood. Therefore, the drying speed is significantly higher than conventional drying. At the same time, due to the low boiling point of water in vacuum state, it can reach a higher drying rate at a low drying temperature (such as around 70 ℃), with a short drying cycle, low drying stress, and good drying quality.

Solar drying of wood generally utilizes solar energy to directly heat air, relying on a fan to circulate the air between the solar collector and the drying room stack. There are generally two types: greenhouse type and collector type. The former integrates the collector and the drying chamber, while the latter adopts a separate arrangement between the collector and the drying chamber. Its capacity is larger than that of a greenhouse type and its layout is also flexible. Solar drying is often combined with furnace gas, steam, heat pumps, etc. due to climate limitations.

Due to the proportion of vacuum, high-frequency, microwave, and solar drying being less than 3%, this article focuses on introducing the current status of conventional and dehumidification drying.

Application of conventional drying

Among all wood drying methods, conventional drying using steam as the heat medium (commonly known as conventional steam drying) still plays a leading role in wood drying equipment around the world, accounting for over 80% in China due to its advantages of stable performance, mature technology, large drying capacity, good drying quality, and easy operation. Conventional drying using furnace gas as energy source accounts for a considerable proportion of small and medium-sized wood factories in non heating areas of southern China. Due to its ability to handle wood waste in factories and reduce drying costs, it is popular among some factories with relatively low drying capacity. The flue gas drying room constructed by local methods is still prevalent in areas with low environmental requirements in China and some underdeveloped countries. Conventional drying with hot water as the heat source, due to the much lower price of hot water boilers compared to steam boilers, is on the rise in some factories that do not require high-temperature drying and have a small drying capacity. Conventional drying using hot oil as a heat source currently has a few applications both domestically and internationally.

China's conventional drying equipment accounts for about 80% of China's equipment [3], of which furnace gas drying equipment is almost all domestic equipment. There are more than 20 manufacturers of conventional drying equipment with a certain scale in China, which are close to foreign products in terms of design level and technical performance. In some aspects, they also have their own characteristics and meet the needs of national conditions. The quality of domestic drying equipment can fully meet the requirements of wood drying production [3]. Moreover, due to the large price difference between domestic and foreign products, domestic manufacturers and enterprises should base their choices on domestic products when choosing drying equipment.

The main gap lies in: ① poor accuracy and reliability of detection and control systems. The main reason is that there is a large testing error in balancing moisture content, relative humidity, and wood moisture content The quality of spare parts such as steam valves and steam traps is poor, and the qualification rate is low. The qualification rate of domestic steam valves is only about 30% [3], causing serious steam leakage and high energy consumption The thermal calculation of the drying room is extensive, usually based on experience, with 3-6 m3 of heat exchange area per m3 of material. ④ The equipment is rough in processing and has an unattractive shape. ⑤ The sealing and insulation performance of the drying kiln are poor, and some drying rooms have a heat dissipation loss of up to 20% [3]. ⑥ The disorderly competition in the drying equipment market has led to a decrease in product quality by reducing prices to attract users.

Dehumidification drying has become the second drying technology after conventional drying [3] [4] due to its remarkable energy-saving effect, good drying quality, no environmental pollution caused by using electricity as energy, and relatively mature technology. It has accounted for a considerable proportion of wood drying equipment in Canada, Japan, Italy, the United States, Britain, France and other countries. Since 1980, China began to introduce foreign Dehumidifier. In the mid-1980s, China began to imitate foreign products and gradually began to design and manufacture Dehumidifier independently. After entering the 1990s, the manufacturing and application of Dehumidifier in China has made great progress, and its popularity in the south is better than that in the north. At present, there are 8-10 Dehumidifier manufacturers in China. According to rough estimates, the total drying capacity of the Dehumidifier being used in China accounts for about 1/10 of the total drying capacity of the country [6]. The domestic equipment of the dehumidifier dryer accounts for about 70%. The Dehumidifier imported from China mainly comes from Italy, Canada, the Netherlands, Japan, the United Kingdom and other countries.

There are several reasons that affect the promotion of dehumidification and drying:

① The dehumidification drying temperature is low and the drying cycle is long. Currently, the commonly used refrigeration refrigerants are R22 and R142b. The corresponding drying chamber temperatures for both are below 50 ℃ and 65 ℃, respectively, while the working temperature for conventional drying is generally between 80 ℃ and 90 ℃. Dehumidification drying is 1/3 to 1/2 longer than conventional drying cycles.

② Simple dehumidification drying generally uses electric heating as an auxiliary heat source, and the Dehumidifier can only recover the heat from the drying room exhaust. The heating of the drying room relies on an electric heater, which consumes high electricity. Therefore, dehumidification and drying can save energy and money in areas with high electricity prices due to power shortages.

③ Most wood processing enterprises have poor energy conservation awareness and do not have a good understanding of the principles of dehumidification and drying for energy conservation.

Clean air energy source

With the promotion of comprehensive renovation work for coal-fired boilers, clean air energy is expected to solve the urgent problem of coal combustion and open up innovative energy development paths. It is understood that the air energy industry is green, energy-saving, environmentally friendly, does not burn energy, does not emit exhaust gas, does not pollute the air, and does not cause haze disasters. It can be applied in fields such as hot water, heating, and drying, and is indeed one of the substitutes for traditional boilers.

Heat pump energy-saving drying unit

The air energy heat pump uses the reverse Carnot principle. The gas Freon is pressurized by the compressor to become high-temperature and high-pressure gas, which enters the heat exchanger at the indoor side. The refrigerant condenses and liquefies to release high-temperature heat to heat the air in the drying room. The materials in the drying room are vaporized and evaporated by hot air, and the evaporated water vapor is discharged by the dehumidification system to achieve the purpose of drying the materials. After condensation and heat release, the refrigerant passes through the throttle valve and becomes a low-temperature and low-pressure liquid. Due to a sudden decrease in pressure, the liquid refrigerant entering the evaporator will absorb the low-level heat energy of the surrounding air and quickly evaporate into a gaseous state. The refrigerant that has absorbed a certain amount of energy will return to the compressor and enter the next cycle. In this way, the continuous circulation of refrigerant can transfer the heat in the air to the drying room Negage.

Using the working principle of heat pumps for heating, which is opposite to air conditioning refrigeration - the national refrigeration standard is 1000 watts, while electric refrigeration is 2800 watts. According to the principle of Thermal equilibrium, 2800 watts of heat is generated at the same time. In addition to 1000 watts of electricity input, the actual heat generated is 3000 to 4000 watts. When these heat are delivered to the insulation water tank, the power consumption is only a quarter of that of the electric water heater (even if the thermal efficiency of the electric water heater is 100%, 1000 watts of electricity input is only 1000 watts of heat).

Under the situation where the country vigorously advocates clean energy and green buildings, the air energy industry will also face more development opportunities. The domestic dryer technology has always been in a stable state of maintenance, without significant development changes.

However, due to the fact that dryers are also major energy consumers, with the increasing energy shortage in recent years, new energy-saving dryers such as air energy high-temperature heat pump dryers, automatic adjustment of baking time, and intelligent dryer control types have been rapidly promoted and popularized, which is also an inevitable trend for the development of the drying industry in the future.

The material drying process is a huge energy consuming process. According to statistics, in most developed countries, the energy consumed for drying accounts for 7% -15% of the total national energy consumption, while the thermal efficiency is only 25% -50%. Moreover, most drying processes, especially for heat sensitive materials such as food and biological materials, can have an impact on their color, nutrition, flavor, and organization. The heat pump drying technology has the advantages of low energy consumption, low environmental pollution, high drying quality, and wide applicability. Its energy-saving effect has been proven by various experimental research institutes.

It is reported that the main principle of using a high-temperature heat pump dryer in the drying process is to use the heat pump evaporator to absorb heat energy from the external air, or to recover the waste heat from the exhaust during the drying process. After the compressor works, the energy is transported (transferred) to the drying box. The hot air in the drying box is repeatedly circulated and heated, absorbing moisture from the material, cooling and humidifying itself, and passing through the process of hot air dehumidification or condensation to remove water, Drain the moisture from the material