Discussion on the importance of heating methods for multi-layer hot press machines

The heating system of the hot press is the most important part of the hot press system. Steam heating uses the latent heat of vaporization of steam to heat the hot press plate, while superheated water uses the sensible heat of superheated water to heat the hot press plate. After its heat is used up, superheated water can be repeatedly used by circulating and reheating. As hot water is circulated, the flow rate is relatively slow and the thermal efficiency is relatively high. Compared with steam, using superheated water as the heating medium is better, and the comprehensive utilization rate of thermal energy in the superheated water heating system is 40% -50%. In terms of temperature control, besides controlling the flow rate, superheated water heating can also adjust the temperature by changing the temperature of the circulating superheated water. Compared with steam heating, superheated water heating can more strictly control the temperature.

Due to the need to use high-pressure saturated steam to heat low-temperature water for superheated water heating, in addition to configuring a high-pressure hot water container, a high-pressure boiler is also required.

On the one hand, the equipment is complex; On the other hand, the system pressure is still relatively high, and there are also high requirements for the sealing performance of the heating system; And hot water is prone to sedimentation and scaling, which has a corrosive effect on the heating system. In order to prevent scale and corrosion in the tank, pipeline, and hot press plate, it is best to use condensed water when heating with superheated water, at least soft water should be used.

Due to the use of superheated water and saturated steam as heat media, the heating of thermal oil requires operation under high pressure conditions, which increases the difficulty of equipment sealing and reduces heating stability. Therefore, seeking a high boiling point substance as a heat medium that can generate high temperatures under normal pressure or low pressure, has a large heat capacity, and is safe to use has always been a concern for people. In the 1930s, such a heating medium was found, which is high-temperature organic heat carrier, commonly known as thermal oil or hot oil. China began researching heat transfer oil heating technology in the early 1970s and was listed as a national energy-saving promotion project during the Eighth Five Year Plan period.

In the 1980s, heat transfer oil with the characteristics of high temperature and low pressure heat transfer was gradually applied in China's wood-based panel industry. Now this heating method has been widely used in such types of hot presses as medium density fiberboard hot press, particleboard hot press, low pressure short cycle veneer hot press, and particleboard molded product hot press. For low-pressure short cycle veneers, thermal oil is almost the only ideal heating medium for hot press machines; In the production of medium density fiberboard, thermal oil has gradually replaced hot water and saturated steam.

Progressiveness of heat transfer oil heating: for wood-based panel hot press, the heating temperature of hot pressing plate is generally about 200 ℃. If steam heating is used, it is required that the pressure of saturated steam be at least 1.6 MPa or above. And thermal oil has the characteristics of high temperature and low pressure. Thermal oil can be heated to 300 ℃, and the working pressure of the system is only the circulating pressure that the hot oil pump needs to overcome pipeline resistance. Usually, heat transfer oil can reach a high boiling point at a low pressure of 0.4-0.6MPa. The heat transfer oil heating adopts a closed heating cycle system, which has ideal heat transfer, uniform heat transfer, small temperature changes during heating, and can accurately control the operating temperature. The temperature difference on the heating equipment can be controlled within ± 2 ℃, and even up to ± 1 ℃. For a hot press machine, the temperature difference between adjacent pressure plates can not exceed 3 ℃, and the temperature difference between the same hot pressure plate can not exceed 1.5 ℃. Heating with thermal oil can greatly reduce the temperature difference between the inlet and outlet of the hot press (usually 10-30 ℃), which changes the disadvantage of large temperature difference between the inlet and outlet of the hot press in the steam heating system.

Energy saving performance of heat transfer oil heating: Heat transfer oil is used in a closed circulation system. It transfers heat through continuous forced liquid phase circulation without hydrophobic emissions or other heat losses. The heat energy can be recycled and there is no dripping phenomenon, effectively avoiding heat loss and achieving significant energy-saving effects with high thermal efficiency. The comprehensive utilization rate of thermal energy in the thermal oil heating system is around 60%.

Economic efficiency of heat transfer oil heating: Due to its good thermal stability and regular inspection, heat transfer oil has no corrosion or scaling phenomenon, does not corrode heating systems and heating devices, and can be used for a long time within the specified temperature range. The operating and maintenance costs of the thermal oil heating system are much lower than those of superheated water and steam heating, which can greatly reduce production costs. When using thermal oil in production, attention should be paid to the following aspects: (1) The selected thermal oil should have a high thermal conductivity, specific heat, and flash point

The self ignition point should be high.

(2) When heat transfer oil operates at high temperatures, its chemical bonds are easily broken and oxidized to produce carbon, so it must be used below the rated working temperature specified by the heat transfer oil grade.

(3) The flow velocity of the heat transfer oil in the hot press plate should be above 2m/s. The lower the flow velocity, the higher the oil film temperature, which is more likely to cause coking of the hot oil.

(4) Moisture and other low boiling volatile substances should not be mixed into the thermal oil.

(5) Mixing different types of heat transfer oil is not allowed.

(6) It is prohibited for the heat transfer oil to come into long-term contact with air at high temperatures in the system, otherwise it will accelerate the oxidation of the heat transfer oil and shorten its service life.

(7) The hot oil circulation pump can only stop working when the temperature of the hot press plate drops below 80 ℃.

(8) According to regulations, the pipelines and valves used for thermal oil must be manufactured or selected at 1.6 times the working pressure, and the motors used must be explosion-proof.

3. Other heating methods

Hot press steam spraying heating belongs to direct heating, which is a high thermal efficiency hot pressing method that first sprays high-temperature steam into the slab during the hot pressing process, and then evacuates it. This method can make the slab uniformly heated in the thickness direction and is suitable for the production of thick plates. At present, the steam spraying heating technology is not mature enough, and it is difficult to solve the gas leakage around the slab during spraying. Moreover, the equipment structure is complex and has few practical applications.

Mixed heating refers to the mixing of high-frequency heating and contact heating with a hot press plate, or the mixing of steam spray heating and contact heating with a hot press plate. The mixed heating method combines the advantages of a single heating method, compensates for the shortcomings of a single method, and can achieve high-quality, high-yield, low consumption, and low-cost results. At present, this heating method is widely used in the production of medium density fiberboard using continuous presses, but it is rarely applied in multi-layer hot presses.