Analysis of the exploration of heat pump combined system

Relying on the traditional drying technology of convection and heat conduction, the heat required to evaporate the moisture must be gradually transferred from the surface of the material to the inside, so that in the later stage of drying, the outer layer of the material becomes dry, and the resistance of the internal moisture distribution to the outside is increased. This phenomenon causes a large decrease in the amount of moisture evaporation and a long drying cycle. The microwave heating and drying mechanism is obviously different from the traditional drying. The microwave energy penetrates the dielectric material and selectively heats inside. This internal heating establishes a vapor pressure gradient inside the material, and the moisture is forcibly transported to the surface of the material. This wet separation effect increases the speed of drying and improves product quality. Due to this unique advantage, microwave drying has been used in the wood, paper, textile, food and ceramic industries in recent years. However, compared with traditional drying, microwave drying has high investment and low energy efficiency (for high water content materials). The economics of microwave drying combined with energy efficient heat pump technology will increase. This paper aims to investigate the performance and practicability of a heat pump microwave combined drying system.

1 heat pump microwave combined drying test device

The test drying device consists of a vapor compression heat pump dehumidification system and a continuous belt microwave drying chamber, which are connected by a duct to form a whole, as shown in the system flow chart. The microwave energy generated by the magnetron is injected into the drying chamber through the microwave conduit to heat the dried material, and the heat and mass are transferred in two cycles (air and refrigerant cycle) in the device.

In the air circulation, a part of the hot and humid air from the drying chamber is directly cooled and dehumidified by the evaporator, and the air from the evaporator is mixed with the bypass air and heated by the condenser to enter the drying chamber, and the microwave energy is used to heat the dried material and absorb the material. Wet points to form an air circulation. In the refrigerant cycle, the refrigerant absorbs the heat of the hot humid air out of the drying chamber in the evaporator and then vaporizes, and the vaporized refrigerant is sucked by the compressor, and the compressor raises its temperature and pressure and sends it to the condenser. The refrigerant in the condenser exotherms to the air and condenses into a liquid. The high-temperature and high-pressure liquid finally passes through the expansion valve to generate a low-temperature and low-pressure gas-liquid mixture and enters the evaporator again, thereby forming a refrigeration cycle.

This study is to explore the feasibility of using a heat pump microwave combined drying system for drying Chinese herbal medicines and agricultural products, so the test drying system is designed for a maximum air temperature of 70 °C. The system uses R142b as the heat pump working fluid; the QR series fully enclosed reciprocating piston compressor is used to adjust the speed of the compressor with the frequency converter; the TQ 5 type thermoelectric expansion valve is selected to control the evaporator superheat degree within ±015 °C; 2M is selected The 137 magnetron emits microwaves, and its maximum output power and conversion efficiency at 2145 GHz are 112 kW and 70%, respectively.

In order to perform a thermodynamic analysis of the system, a large number of system parameters are measured. The air circuit includes the following parameters: total air mass flow, bypass air mass flow, and dry and wet bulb temperature of the air. The refrigerant circuit includes the following parameters: mass flow rate of the refrigerant, temperature and pressure.

In order to determine the overall performance of the heat pump microwave dryer, the input and output energy of the magnetron, the input power of the air circulation fan motor, the input power of the compressor motor, the amount of water removed by the evaporator, the moisture content of the material, and the production must also be measured.

The accuracy of the temperature meter and refrigerant flow meter used in the test is ±013 °C, and the accuracy of the air flow meter is ±2 °C. The measurement error of the input power of the motor is within ±5%, and the measurement error of the removed moisture and material weight is within ±1%. The input power of the magnetron is obtained by directly measuring the input voltage and current value, and the measurement error is ±015. Within 100% of the loss, the reflected microwave energy can be measured with a diode detector with an accuracy of ±10%.

Foam rubber was selected as the dry material of the test, which can be repeatedly absorbed and dried without loss and damage.

2 mathematical model

Prior to this study, the authors have established a mathematical model that simulates the performance of a heat pump convection drying system consisting of a heat pump submodel and a continuous convection drying chamber submodel, which are linked by mass and energy conservation laws. With the intervention of microwave heating and drying, the drying chamber model should consider the microwave energy consumed in the material to be dried. According to the definition, the microwave energy consumed in the dielectric material per unit volume is related to the intensity and frequency of the microwave electric field.

P = 2ΠfΕ0Ε"E 2(1) where f microwave frequency, Hz E electric field strength, V mΕ0 dielectric constant of free space, F mΕ material relative loss coefficient The world-recognized microwave frequency for heating and drying is 2145 GHz, therefore The microwave energy consumed in the material to be dried is only related to the relative loss coefficient of the material and the distribution of the electric field in the material.

211 microwave field distribution

The distribution and numerical value of the microwave electric field in the material to be dried is a complex function of the energy emitted by the microwave source (magnetron), the position of the magnetron in the drying chamber, the geometry of the drying chamber, and the nature and spatial position of the material. Some simple geometric shapes can be solved by finite element method and other numerical methods. However, for industrial size microwave dryers, the actual size is large and the boundary conditions are complex, which makes the numerical calculation of the microwave field distribution in the drying chamber extremely difficult. In order to analyze the industrial size microwave drying problem, some assumptions must be made. Parallel waveguides can improve the uniformity of microwave field distribution inside the application. Therefore, in this study, it is assumed that the microwave field in the drying chamber is uniformly distributed, that is, the electric field strength is constant.

If the microwave energy delivered to the drying chamber by all magnetrons is P m , the average electric field strength in the drying chamber can be calculated by the following formula: E 2 m = 1 2ΠfΕ0 P m∑ni= 1Ε"ivi(2) where Ε" and vi They are the loss factor and volume of the material in the i-th control volume, respectively. Therefore, the microwave energy consumed in the ith control volume can be calculated by the following formula: P i = 2 Π f Ε 0 Ε "i E 2 mvi (3) 212 material loss coefficient at a specific microwave frequency, the material loss coefficient is its physical property, temperature and The complex function of the wet quantity must be determined experimentally. However, for heat pump assisted microwave drying, the material temperature change is usually less than 30 ° C, so the material loss factor is assumed to be independent of temperature during the entire drying process at 214 GHz, so For a particular material, the loss factor is only a function of the moisture content of the material. [4> indicates that for most materials, the loss factor with the change in moisture content can be approximated by two straight lines, ie Ε"=Ε"0 + AX (X ≥ X c) Ε "0 + AX ​​(X < X c) (4) where Ε", Ε", A, A is determined from test data, and X c is the critical moisture content of the material.

In this paper, the variation of the material loss coefficient in the drying chamber is calculated by the formula (4).

By combining the simplified model of the above microwave energy distribution with the heat pump dryer model, the performance of the heat pump microwave combined drying device can be predicted.

3 results and discussion

It can be seen from the table that the test and simulation results are basically consistent, and the maximum error is less than 8%, which is within the experimental error range, thus verifying the correctness of the mathematical model.

311 system parameters

In order to further understand the characteristics of the heat pump microwave dryer, and seek the optimal design, the following analysis of the main design and operating parameters on the system performance. The performance of the dryer was evaluated by unit energy dehumidification (SM ER) and dry yield.

The air bypass rate is defined as the ratio of the mass air flow bypassed from the evaporator to the total air mass flow in the system. The effect of air bypass on SM ER and dry material yield is shown. It can be seen from the figure that the test and simulation results are basically consistent, and the optimal bypass rate is about 67% (the bypass rate is as small as possible on the performance of the dryer). It should be noted that the optimum bypass rate is greatly affected by the total air mass flow and the relative humidity of the air at the evaporator inlet. As the relative humidity of the air increases and the total air flow decreases, the optimal bypass rate decreases. In this test, when the foam rubber is dried, the relative humidity of the air at the inlet of the evaporator is between 40% and 50%. For such low relative humidity, a higher bypass rate can be used.

Both the test and the simulation results show that as the compressor speed increases, the SM ER decreases and the drying yield increases. This trend is similar to convection drying, except that it changes slowly. This phenomenon occurs mainly because approximately half of the energy input to the drying chamber is microwave energy independent of the compressor speed.

For a given design, dry yield and SM ER balance must be maintained.

The SM ER and dry yield are shown as a function of total air mass flow. From this curve it can be seen that the test agrees well with the simulation results, which show that the performance of the dryer is not sensitive to the total air mass flow within the scope of the study. This phenomenon may be caused by the fact that the input power of the ventilator is less than 10% of the total input power of the system. This is different from heat pump convection drying, where the best total air mass flow is present when the heat pump is convectively dried.

Comparison of 312 heat pump microwave drying and convection drying

The energy efficiency and yield of heat pump microwave drying and convection drying are compared below. Excerpts from the test results for drying foam rubber when different microwave energies are input. The heat pump input power is about 4 kW in all cases. It clearly shows that microwave drying can increase the yield compared to convection drying, but as the input microwave energy increases, the dryer SM ER decreases, as the input microwave energy increases from 0 to 415 kW. At the time, the SM ER of the test dryer was reduced from 213 to 112 kg (kW h).

The power generation efficiency is 30%, and the SM ER of the heat pump microwave combined dryer is converted according to the primary energy consumption, and the conversion result is shown. The traditional steam heated dryer can achieve a maximum SM ER of about 0155kg (kW h). As can be seen from the table, the primary energy consumption of the test dryer using a magnetron is comparable to the most well-designed conventional drying, while the microwave dryer is less energy efficient than the well-designed steam drying when using multiple magnetrons. Device. It should be noted that in the design of the test dryer, due to site and funding constraints, the size of the drying chamber is relatively small, severely limiting the efficiency of the test dryer. Therefore, carefully designed heat pump microwave combined drying can compete with conventional convection drying in terms of energy utilization.

313 drying of vegetables

In order to study the performance of the heat pump microwave combined dryer in drying the actual commodity, the carrot slices and the whole ginger were tested on the test dryer, and the results were extracted. The results show that the performance of the heat pump microwave combined dryer is similar to that of dry foam rubber when drying carrot slices; its performance is much lower when drying ginger, which is caused by the high hygroscopicity of ginger, and the firm skin of ginger also makes it It is more difficult to diffuse moisture to the surface.

4 Conclusion

(1) The influence of system operating parameters such as air bypass rate, compressor speed and total air mass flow on the heat pump microwave combined dryer SM ER and drying output is similar to the effect of these parameters on the heat pump convection dryer. The only difference is It varies at a lower rate than the heat pump convection dryer.

(2) Compared with heat pump drying, heat pump microwave combined drying can increase drying yield, but SM ER is reduced, and its size is proportional to the input of microwave energy. In terms of the authenticity of the test, this phenomenon is caused by the excessive standing wave of the microwave heater (applicator).

(3) Through careful design, heat pump microwave combined drying can be equivalent to traditional convection drying in terms of energy consumption.

(4) The test results of drying ginger and carrot slices showed that the SM ER of the microwave dryer was greatly affected by the characteristics of the material to be dried, and the SM ER of the high hygroscopic material like ginger decreased a lot.