摘要
随着经济的发展、技术的进步和世界人口出的增长,人类对能源的需求将不断增加。干燥是一个高耗能的工艺过程,在许多工业生产中干燥耗能高达12%,占全部生产费用的60～70%。因此,干燥所面临的问题就是既要保证产品质量又减少能量的消耗。热泵技术应用于农产品物料干燥具有节约能源、产品质量高、干燥条件可调节范围宽和环境友好等显著优点。本文主要研究热泵干燥系统的工作原理及热力学性质,以胡萝卜为原料进行热泵干燥实验、热泵热风组合干燥实验及干燥产品质量的对比研究。
通过对热泵不同干燥循环特性的分析,表明不同的干燥条件应采用不同的干燥循环。在系统中增加空气回热器可提高系统的效能,但同时也增加了空气流动的阻力,增加了系统的功耗。热泵除湿干燥在温度小于60℃和相对湿度大于30%时能达到最理想的效果。
根据热力学第一定律和热力学第二定律,应用能量平衡及“(火用)”(exergy)方法分析了热泵干燥系统的各部件的(火用)损失和减少(火用)损失的途径。压缩式制冷循环热泵装置(火用)损失的途径大致可归为传热(火用)损失、节流(火用)损失和压缩机(火用)损失三个方面。为了提高压缩式制冷循环热泵装置的(火用)效率,就必须设法减少这三个方面的(火用)损失。
研制了一套综合的热泵干燥试验装置,采用变频技术对压缩机和循环风机的转速进行无级变速,大大地提高了试验装置的适应能力。试验装置既能进行单独热泵干燥或单独的热风干燥实验,也能进行热泵和热风组合干燥实验。通过调节空气流过蒸发器的流速和状态,改善湿热空气通过蒸发器的传热效率,增强了蒸发器的除湿效果。
由不同热泵干燥循环的性能试验可知,开路式热泵干燥循环运行稳定,但蒸发器析水的速率比半开路式循环低。在闭路式热泵干燥循环过程中,随着空气旁通率BAR的增加,流过蒸发器表面空气流速的降低,冷凝效果不断提高。开路式、半路开式热泵干燥循环系统的单位能耗除湿量(SMER)比闭路式高。在闭路式热泵干燥循环过程中,BAR的值在0.4到0.6的范围内,系统的单位能耗除湿量(SMER)较高,当BAR的值超过0.6时,系统的单位能耗除湿量(SMER)反而下降,说明空气旁通率(BAR)有一个合理的范围。
以胡萝卜为研究对象进行了热泵干燥试验研究。胡萝卜片的热泵干燥实验结果表明,热泵适合于高含水物料的前期干燥,由0.4至6小时为快速干燥区,物料的水分呈现明显下降,6小时以后水分下降减慢。特别是在干燥的后期,物料水分从20%降到10%,大约需要3到4个小时。热泵流化床干燥实验表明,热泵流化床干燥在9小时内,能将胡萝卜的水分从90%左右烘干到10%以下,但主循环风机的动力消耗较大,约占系统总能耗的50%左右。
采用热泵与热风组合干燥技术解决了单一热泵干燥时间长的问题,组合干燥既能节约能源又能提高产品质量。实验结果表明,当被干物料达到一定数量时,其耗能只有隧道式干燥的74.1%,网带式干燥的84.7%和真空冷冻干燥的9.4%。组合干燥产品中胡萝卜素的保持率为92%,而单一热泵干燥为80%,热风干燥仅为59%。胡萝卜产品经24小时充分复水后,组合干燥产品复水性比热泵干燥高16.6%,比热风干燥高24.5%。
Energy consumption depends on several factors such as economy, technology and population growth. It is apparent that drying itself is an energy-intensive process because of the latent heat to be supplied to the material to evaporate the moisture. It is reported that industrial dryers consume on average about 12% of the total energy used in manufacturing processes. In manufacturing processes where drying is required, the cost of drying can approach to 60%—70% of the total cost. Thus, one of the most important challenges of the drying industry is to reduce the cost of energy source for good quality dried products. Heat pump-assisted drying is an energy efficient process because the heat is recoverable. Heat pump assisted-drying provides a controllable drying environment (temperature and humidity) for better products' quality at low energy consumption (cost).This dissertation has conducted a systematic study on the performance of the heat pump drying system, on the energy and exergy analyses of the heat pump unit and the drying cabinet, and on drying mechanism and experiments of heat pump drying of the carrots.The performances of different from heat pump dryer configurations were studied. By comparing the performances of those cycles of the heat pump dryers, it was concluded that the different operating mode of the heat pump dryer depends on the drying conditions. If the air to air heat exchanger is applied to the united cycle system, the efficiency of the heat pump unit will increase, otherwise, will increase the resistance of the process air and the energy consumption of the heat pump drying system. Ideally, the heat pump drying should be carried out at temperatures <60°C and average relative humidity >30%. These are ideal conditions for the drying of many agricultural materials.Based on the first and second laws of thermodynamics and the concepts of energy (enthalpy) and exergy (stream availability), the energy and exergy analyses of the heat pump dryer are investigated. The exergy losses of the heat pump's four main components (the evaporator, the compressor, the condenser and the expansion) and the dryer were studied. The exergy losses due to heat transfer, expansion and compress have been considered. In the design of a system, the exergy method provides the useful information to choose the appropriate component design and operation procedure.The heat pump dryer used in the experiments was assembled from air conditioner parts. Frequency control is a suitable method for an AC electrical motor working at variable speeds. This method changes the discharge rate of the compressor and the fan by varying their rotation speeds to meet the requirement of variable load. The experimental system is suited to the heat pump assisted-drying, the heat pump fluidized bed drying and the combination drying. The heat pump and the dryer were connected by an adjustable air duct system which enables the HPD to be operated as a partially closed cycled or open cycled.The performances of opening air cycle, semi-opening air cycle and closed air cycle configurations were studied. When the heat pump works with the opening air cycle, the heat pump unit works at the steady state and extraction water from the process air is lower than the semi-opening air cycle. When the heat pump works with the semi-opening air cycle, the extraction water from the process air will increase by exhausted air loss. In the closed system, the bypass air rate (BAR) play a significant role in
the system performance. As BAR increases, the amount of air passing the evaporator is reduced and the BAR directly alter the HPD performance. The extraction water from the process was not sensitive to BAR values within the range from 0.4 to 0.7. To obtain the peak SMER, the system should be operated with BAR between 0.4 and 0.6. At BAR greater than 0.6, the peak values seem to be reduced. it was, therefore, concluded that the bypassing air rate (BAR) is effective at the maximum SMER.With the heat pump drying experimental sets, the carrots slices were chosen as the drying materials.
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