|
|
CO_ Performance analysis of heat pump dryer
Drying is one of the most energy consuming processes. In developed countries, drying energy consumption accounts for 7% to 15% of national industrial energy consumption. In China, the energy consumption of the general drying process accounts for about 10% of the total energy consumption of the entire processing process. Due to high energy consumption and environmental pollution, the use of traditional fuel and power consuming dryers is greatly limited. In particular, large and medium-sized dryers have complex structures and low utilization rates. With the development of rural areas in China, agricultural dryers tend to be miniaturized, while small dryers may develop in the direction of household and multi-function. Therefore, how to reduce the energy consumption of the drying process is receiving attention when the requirements for energy conservation in China are becoming increasingly high. In the late 1970s and early 1980s, heat pump drying technology gradually developed and was applied to the drying field. Due to its unique drying principle, high efficiency and energy saving, high thermal efficiency, fast dehumidification, and ability to better maintain the quality of materials, it has been valued. In recent years, heat pump drying technology has been widely used and developed in the fields of wood drying, food drying, agricultural and sideline products processing, as well as grain, medicinal materials, ceramic billets, chemicals, and light industrial products.
The characteristics of 1C2 and the principle of heat pump dryers 1.1 Application and physical characteristics of CO2 The circulation of heat pump systems cannot be separated from refrigeration working fluids. Traditional refrigerants, CFCs, have been eliminated due to their destruction of the ozone layer, and HFCs have become alternative working fluids for CFCs because they have no ability to destroy the ozone layer. However, its chemical properties are stable and can accumulate after release, which will ultimately lead to a significant greenhouse effect. Using carbon dioxide as a refrigerant substitute is different in that it not only does not harm the ozone layer (ozone layer damage potential ODP=0), but most importantly, it is a natural working medium that does not harm the environment, has good safety and chemical stability, and has a greenhouse effect potential GWP=1. Therefore, studying the C2 heat pump drying technology using natural working medium C2 as a refrigerant fully conforms to the social development trend of energy conservation and environmental protection, It has broad development prospects.
The research on the C2 transcritical refrigeration cycle was conducted by GLorentzen of the SINTEF Research Institute in Norway. Petterson et al. pioneered the use of natural refrigerant C2 as a refrigerant in the heat pump cycle. In addition to the aforementioned advantages of being a refrigerant, it also has the characteristics of large specific heat capacity, high thermal conductivity, and low dynamic viscosity, which can reduce the size of pipes and heat exchangers, making the entire system compact in structure. The heat release process of CO2 as a circulating working medium is a variable temperature process with a large temperature slip, which can achieve a good temperature match with the cooling medium. This special Lawrence cycle is very suitable for the requirements of step heat release in heat pump drying.
1.2 Principle of CO2 heat pump dryer The heat pump drying system consists of a heat pump cycle and an air cycle, as shown in. The heat pump cycle consists of a compressor, gas cooler, throttle valve, evaporator, etc; The air circulation consists of a fan, a drying chamber, and an auxiliary condenser. In the heat pump cycle, the gas is compressed by the compressor and condensed in the gas cooler for heat release. It flows into a liquid state through the expansion valve section, and the temperature decreases. Then, it absorbs heat and evaporates in the evaporator. After entering the gas-liquid separator for separation, the gas is sucked into the compressor to complete the refrigeration cycle.
In the air circulation, the high humidity and hot air entering the evaporator is condensed and dehumidified, becoming low humidity air, entering the gas cooler and being heated into dry and hot air. The air is blown into the drying chamber by a fan, taking away the moisture of the dried material, and then becoming humid and hot air, entering the evaporator to complete the air circulation.
Evaluation index and performance analysis of 2C2 heat pump dryer CO2 heat pump dryer consists of air cycle and heat pump cycle. Its advantages are mainly evaluated from the aspects of unit time water removal, unit dehumidification rate, heat pump Carnot efficiency, energy saving potential of the entire system, and drying time on the air side.
2.1 Evaluation index The Carnot efficiency of the heat pump at a certain time of water removal, and the performance of the heat pump system. The higher the Carnot efficiency of the heat pump, the closer the cycle is to the ideal cycle, that is, the better the cycle, the less irreversible thermodynamic losses.
COP is the performance coefficient of the actual refrigerator.
COP=effective energy output by the heat pump HP=electrical energy consumed by the compressor COPc is the coefficient of performance of the Carnot refrigerator.
Low temperature heat source temperature c - High temperature heat source temperature - Low temperature heat source temperature Unit dehumidification rate 5MER5MER is used to evaluate the energy utilization rate of the entire system.
The total energy consumed by the drying system. The total energy consumed here refers to the sum of the energy consumption of the fan and the compressor.
Among them, SEChp SECc+SECf refers to the energy consumption of the input compressor, SECF refers to the energy consumption of the fan, and the energy consumption of other auxiliary facilities is omitted. SECdh is the energy consumption of direct electric heating. This value is obtained when electric heating drying and heat pump drying have the same dehumidification rate. In an electric drying system, if the fan does not change and the energy consumption of the fan is the same as that of the heat pump dryer, then the SECF is the same here, and the energy saving potential is: here, SECh represents the energy consumption in the electric dryer except for the fan. The energy saving potential of this article is expressed using ESP.
The feasibility of CO2 heat pump drying was analyzed and discussed through an example and performance analysis of a CO2 heat pump dryer. Based on the characteristics of the heat pump and the operation results of the dryer, they analyzed and studied the energy saving potential of the CO2 heat pump dryer, and finally obtained the relationship between the energy saving potential and the Carnot efficiency of the heat pump, as shown in.
The greater the Carnot efficiency of the heat pump, the greater the energy saving potential. When the Carnot efficiency of the heat pump is 0.5, the energy saving potential is 48%; When the Carnot efficiency of the heat pump is 1, the energy saving potential can reach 70%. The relationship between the Carnot efficiency of the heat pump and the Carnot efficiency of the heat pump. They also compared traditional electric dryers with CO2 dryers of different types of compressors, as shown in Table 1.
Table 1 Structural Parameters of Compressors Manufacturer Bock GermanyDorin Italy Model fkxco2CD 4.017S Open semi closed cylinder number 22 stroke (mm) 4911 cylinder diameter (mm) 2834 volumetric displacement (m3/h) 1.8-5.4 Up to 1.7. The characteristics of different types of dryers and CO2 heat pump dryers with different compressors were analyzed.
Table 2 Experimental results and performance parameters The air inlet temperature of the electric heating dryer is 130C, while the inlet temperature of the CO2 heat pump dryer is 60C and 50C. For some items that require low-temperature drying, in order to consider their quality requirements, the CO2 heat pump dryer has more advantages than the electric heating dryer.
The unit water removal SMER of the electric heating dryer is 0.72kg/kWh, and the SMER of the BOCK compressor used in the CO2 heat pump dryer is 2.2 and 2.8 times that of the electric heating dryer, respectively. It can be seen that the CO2 heat pump dryer is significantly superior to the electric dryer.
2.2.3 Low energy consumption and high energy saving potential The energy saving potential of heat pump dryers is 0.65 kWh/kg and 0.49 kWh/kg, which are 47% and 35% of the former, while the energy saving potential of C2 dryers is 53% and 65% 2.2.4 Short drying time In the experiment, it was also found that with the extension of drying time, the inlet temperature of the dryer 1 gradually increased and then stabilized. After stabilization, the temperature was about 52C, and the difference between the outlet temperature and the inlet temperature gradually decreased. After dehumidification, the air temperature before heating slowly tended to about 17C. Using a Bock compressor, if the drying time is long enough, the COP can reach 5.5, and the Carnot efficiency of the heat pump can reach 0.6; If the drying time of the Dorm compressor is long enough, the COP can reach 6.5, and the Carnot efficiency of the heat pump can reach 0.65. If a certain dehumidification rate is to be achieved, the energy saving potential can only reach 53%, but the drying time is still shorter than the electric drying time. It can be seen that the CO2 heat pump dryer is still superior to the electric dryer in terms of time.
Optimization of 3C02 heat pump dryer 3.1 Using an expander instead of a throttle valve. Through research, it has been found that during the heat pump cycle, the loss at the throttle valve is the greatest, and optimization at the throttle valve will achieve good energy-saving results. The expander is coaxial connected with the compressor to recover part of the expansion work and reduce the power consumption of the compressor. Due to the use of an expander to recover some expansion work, when the isentropic efficiency of the expander is 60%, the COP of the same CO2 heat pump drying system can be increased by about 12%.
3.2 Selecting an appropriate air flow rate The impact of the evaporator air flow rate on system performance depends on the relative humidity of the air at the evaporator inlet and outlet. In general, the increase in evaporator air flow rate is limited by the increase in evaporator fan power and the normal operating range of the compressor. When the relative humidity decreases (less than 50%), the maximum value of dehumidification per unit energy consumption (5MEA) gradually increases as a function of the evaporator air flow rate. When the speed of air entering the evaporator is 1.6 m/s, the actual heat pump performance coefficient COP has a maximum value. Therefore, air flow rate is also one of the important factors to be considered in optimizing heat pump drying units.
Conclusion Compared with electric dryers, CO2 heat pump dryers have advantages in inlet air temperature, unit water removal, energy saving potential, and drying time, and are worth further research and promotion.
For optimizing C2 heat pump dryers, an expander can be used instead of a throttle valve to recover expansion work; Select an appropriate air flow rate and increase the heat pump coefficient CQP. For carbon dioxide heat pump drying units, there are different optimization measures in different aspects, and further research and discussion are needed to ensure that C2 heat pump dryers can be applied in practical industrial and agricultural fields and achieve good energy-saving effects.
|
| 〖Close Page〗 |
|
1688 shop 
Weibo 
Technical Info