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科技論文及翻譯
Experiment research on grain drying process in the heat pump assisted fluidized beds
Jing Yang，Li Wang，F(xiàn)iXiang，Lige Tong，andHua Su Mechanic~Engineering School，University of Science and Technology Beijing，Beijing 100083，China
Abstract：A heat pump assisted fluidized bed grain drying experimental system was developed．Based on this system．a(chǎn) serial of experiments was performed under four kinds of air cycle conditions．According to the experimental analysis，an appropriate drying mediumair cycle for the heat pump assisted fluidized bed drying equipment was decided．which is difierent from the commonly used
heat pump assisted drying system．The experimental results concerning the drying operation perform ance of the new system show that the averaged coefi cient of perform ance fCOP1 can reach more than 2．5．The economical evaluation was perform ed and the power consumption for removing a kilogram water from grains was about 0．485 kW·h／kg(H，0)，which shows its reasonable commercialefi ciency and great application potentiality in future marke
Key words：heat pump，fluidized bed，grain，drying，air recycle
1 Introduction
Nowadays there 1s an urgent need for the development of high capacity and high quality grain drying equipment in the agriculture of China[1]．As the society and agriculture develop quickly，it is of important practical value to develop the efi cient，energy saving， non—pollution and even movable drying equipment to match the mechanization harvestry [2]．
Traditional methods of drying involve the direct combustion of fossil fuel together with controlled ventilation．Such methods are obviously ineficient，with efficiencies never exceeding 20％ ，not to mention the fact that expensive primary energy is depleted only to produce low grade heat[3]．The traditional drying methods have been prevailing domestically up to now,not only for grain drying，but also for other matedals such as wood，cement，ceramics and medicine etc．As the power supply structure improved greatly by the hydropower and nuclear power development policy of the government of China，the research on the heat pump assisted drying system is of special meanings，due to the fact that a portion of power usually can supply more than 2 portions of heat energy for drying by using a heat pump．Recently there are quite a few scholars[4—8]findin their interests in heat pump assisted drying area and the great potentiality in future for heat pump applications in drying area．However the researches[9，1 0]mostly concentrate on the low temperature heat pump with the hot medium temperature lower than 55。C．There was still a blank in the high temperature heat pump assisted drying system with temperatures above 70。C and especially for the system with grain drying and heat pump working together,which is the great motivation of the present paper．Besides the gas—solid fluidized beds have obtained widely applications in drying area with so obvious advantages such as high gas—solid contact surface，high heat and mass transferrates，easy to be mechanically operated，usually with great continuous production capability and high quality of drying due to its uniform temperature field[11，121．
This paper combined the both advantages of heat pump and fluidized bed to develop a new type of graindrying experimental system．Based on this system，aserial of experiments was perform ed under different conditions．According to the experimental analysis，an appropriate drying medium—air cycle for the new type heat pump assisted fluidized bed drying equipment was decided．which is different from the traditional heat pump assisted drying system．Th e research results also show the optimistic future and the potential market competition ability of this new equipment．
2 Experiment
In a drying system with a heat pump and a fluidized bed together．there are three material flows：the grain flow，the refrigerant flow and the dry ing medium．Air flow．This air flow relates the grain flow to be dried and the refrigerant flow of the heat pump together and become the medium transferring the moisture and heat between the both，in which the flow rate．temperature．velocitv and humidity of the air influence the working condition of the heat pump，whereas the working condition of the heat pump also afects the temperature and humi dity of the air and further influences the drying process．Here the air circulation jn the system
is vital not only to the working performance of the heat pump and the dry ing process but also to the final structure of the whole equipment．Hence the experi．mental system was designed as shown in figure 1．which includes mainly four parts：fluidized bed drying room，heat pump，tube connection and several valves．
This system can realize four types of air circulation by adjusting difierent valves．Type one：close valves 1，2，4，5 and open valves 3，6，7，in which the air dis．charged from the fluidized bed flows only through the evaporator of the heat pump and then the condenser where it is heated and finally into the fluidized bed．performing a close circuit circulation．Type two：close valves 2，3，6 and open valves 1，4，5，7，in which the air discharged from the fluidized bed flows directly through the condenser and then into the fluidized bed．performi ng a close circulation，and the air from cir．cumstance flows through the evaporator of the heat pump．Type 3：close valves 1，3，5 and open valves 2，
4，6，7，in which the air discharged from the fluidized bed flows completely through the evaporator and then into the ambient．whereas the ambient air is absorbed through the condenser by the ventilator and then flows into the fluidized bed．performing an open circulation．Type 4：open all the valves and adjusting their turn—down ratio．in which the air discharged from the fluidized bed splits into two parts，one mixing with some ambient air flows into the evaporator and then into the ambient，the other mi xing with the ambient air flows into the condenser and then into the fluidized bed，performing an half close and half open air circulation．
3 Parameter measurements and principles
In this experiment，the parameters need to be meas．ured are the temperatures at the inlet and the outlet of the fluidized bed and the evaporator and the condenser,the flow quantities of the air through the evaporator，the condenser and the fluidized bed respectively，the high and the low pressures of the refrigerant．All the temperature measurements adopted digital tempera．ture sensors which were calibrated by the liquid in the glass therm ometer from 0 to l 00。C．The hot spherical wind velocity meter was used to measure the flow velocity and then to get the flow quantity．The power of the compressor was calculated based on its phase current measured by a multimeter．The refrigerant pressures were measured by elastic tube manometers．
The performance parameters of the system can be calculated based on these measured parameters by following formulae．
The air flow quantity：
v=(∑u/N) A×3600,m3/h；
The refrigerating output from the evaporator：
Qe=Vc·ρa(bǔ)ir·Cρ·（toe-tie）/3600,KW ；
The heating output from the condenser：
Qe=Vc·ρa(bǔ)ir·Cρ·（toe-tie）/3600,KW；
The compressor power：
P=3×Ip×Vp=(I1+I2+I3)×220/1000,KW；
The coeficient of the performance(COP)of the
heat pump：COP=QC/P
The flow rate of refrigerant：
X=ηc·P/W,kg/s
The theoretical refrigerating output from the evaporator：
LQe=X×qe,KW；
The theoretical heating output from the condenser
LQc=x×qc, KW；
The theoretical coefi cient of the performance of
the heat pump：
LCOP= LQc/p
Here the theoretical cycle refers to the ideal thermodynamic cycle corresponding to the high and low pressures measured in the experiment．
4 Experimental results and analysis
4．1 Efects of air circulation type
In order to get the effect from different air circulations on the drying process，we conducted four types of air circulation experiments．keeping all the other conditions the same，such as the air flow quantity and the inlet temperatures etc．a(chǎn)nd found that the temperature at the inlet of the fluidized bed is the highest in type 2，that in type l is the lowest and that in type 3 is slightly higher than that in type l，that in type 4 is between that in types 2 and 3．This is because in type 2，the air circulates only through the condenser and the heat wasted is comparatively small resulting in the high temperature at the outlet of the condenser．The humidity of the air at the inlet of the fluidized bed was also higher，since the air discharged from the fluidized bed did not remove its moisture．However,in fact the connection tube was not closely sealed，therefore there was some fresh air from ambient flowed into the condenser．That is like such a circulation：close valves 3．6 and open all the other valves．in which the air discharged from the fluidized bed，mi xing with the ambient air flowed through the condenser and then into the fluidized bed，meanwhile some air from the fluidized bed flowed into the ambient through the interspace
between the fluidized bed and its blastcap．In this experiment system．1 0％一20％ fresh air was estimated mi xing into the flow in air circulation between the condenser and the fluidized bed．Hence type 2 with a certain fresh air ratio can be chosen as the best air circulation used in the following experiments．
4．2 Efects of air flow quantity through the evaporator
Two experiments were performed．In one experiment．1 00 k2 grains were put into the fluidized bed with a moisture of about 25％ fits denominator contains no water)，and then the system was started and the airflow through the evaporator was kept relatively low at about 500 m ／h．The performances of the heat pump are shown in figures 2-4．In the other experiment，the system was started firstly and the airflow through the evaporator was kept high at about 2500 m3／h． after the temperature at the outlet of the condenser reached about 70。C． 100 kg grains with a moisture of 27％ were put into the fluidized bed．The heat pump performances are shown in figures 5-7．The air flow through the condenser is about the same
of 1 650 m3／h for both experiments．
Figure 2 Pressure variation of the refrigerant in the dry ing process at v~=soo ma／h．
Figure 3 Performance of the heat pump in the drying process at v~=5oo ma／h．
Figure 4 COPoftheheatpump atv~=5ooIll3，l1
Figure 5 Pressure variation of the refrigerant in the drying process at v~=25oo ma／h．
In figures 3，4 and 6，7，the theoretical results are all calculated from the ideal refrigeration circulation respectively coresponding to the high and low pres。sures of the refrigerant in figures 2 and 5．The comparison between figures 3 and 6 shows that the heat output from the condenser greatly increases by in creasing the air flow through the evaporator and the COP is much closer to the idea1 value in figure 7 than that in figure 4．Thus the airflow quantity through the evaporator is very important to the performance of the heat pump．In fact，the growing of the air flow quantity through the evaporator greatly enhances the heat transfer rate between the air and the surface of the evaporator resulting the worldng medium in it absorb more heat at the same temperature difference．Th e theoretical analysis also show that as the airflow increases further,the heat absorbed by the evaporator will reach its peak and then level off．
Figure 6 Performance of the heat pump in the drying process atv,=25oom3／h．
Figure 7 COP of the heat pump at v~=2soo m3／h．
4．3 Drying process of the heat pump assisted fluidized bed grain dryer
Figure 8 shows the temperature variations at the inlet and the outlet of the fluidized bed during the drying process under the same condition to the experiment at Ve=2500 m ／h．It shows that both the temperatures rise gradually with time．At such temperature variation，the wheat drying process is shown in figure 9，which displays the wet moisture(the fraction of the water quantity contained in the grains to its total quantity)and dry moisture(the fraction of the water quantity contained in the grains to its absolute dry quantity)variations of wheat wifh drying time．W e can see that it takes about 60 min for the wheat to drv from a wet moisture of 21-3％ to 13％ ．
Figure 8 Temperature variations at the inlet and outlet of the fluidized bed during the drying process at v,=2500m3／h．
Figure 9 W heat moisture variations during the dry ingprocess at v,=2500 m3／h
4．4 Economical evaluation
The factors afecting the commercial eficiency of the drying system are the drying time，the COP and power consumption of the heat pump．However the drying time mainly depends on the air temperature at the inlet of the fluidized bed．In order to analyze the com ercial efficiency of the system．we assume that the wheat is continuously dried bv the system as it was done by the most dryers in industry，the temperature at the inlet of the fluidized bed is about 70。C which was
obtained in the experiment．In such assumptions，we can obtain that the drying time needed for wheat to dry from its wet moisture of 20％ t0 13％ is about 35
min，which is deduced from another experiment[2]and will not be presented here．According to the known parameters mentioned above and the heat
pump power consumption in figure 6，including the blower power consumption in the system，the averaged power consumption of the system is gotten (about 5．5 kW)during the drying process．Considering the capability of the fluidized bed is about 100 kg，we can conclude its totally power consumption per unit grain output is about 0．0321 kW ·h／kg or that per unitwater removed from the grain is 0．458 kW ·h／kg(H20)
5 Conclusions
(1)Th e appropriate air cycle for drying grain is that the air discharged from the fluidized bed directly flows into the condenser of the heat pump with 10％ 一20％ flesh air where it is heated and then flows into the fluidized bed to form a circulation
(2)The airflow through the evaporator is very important to the perform ance of the heat pump．The higher the flow quantity，the beaer the perform ance of
the heat pump．
(3)Th e economical evaluation shows that if the system working at continuous state，its power consumption for removing a kilogram water from the grains is about 0．485 kW·h／kg(H2O)and shows great potentiality in the future market．
Nomenclature
V,Ve,Vc:Th e flow quantities for the fluidized bed，the evaporator and the condenser,respectively；
u:The air velocity at the measured point．m／s；
N:Th e number of the measured points；
A：Th e tube across area，m ；
Qe,Qc:Th e refrigerating and heating outputs of the heat pump，kW ；
r
ρa(bǔ)ir:Th e air density，kg／m ；
Cρ:Th e air specific heat capacity，kJ／(kg‘。c)；
toe,tie:Th e temperatures at the inlet and the outlet of the evaporator，。C：
toc,tic:Th e temperatures at the inlet and the outlet of the condenser,。C：
Ip:Th e averaged phase current of the compressor,A;
Vp:Th e phase voltage，V；
X：Th e mass flow rate of refrigerant．kg／s；
ηc：Th e compressor efficiency，about 0．9 here；
W：Th e theoretical compressor work per unit mass．
kJ／kg；
LQe,LQc: Th e theoretical refrigerating and heating output of the heat pump，kW ；
qe,qc:Th e refrigerating and heating outputs from theoretical cycle per unit mass refrigerant，kJ／kg；
COP,LCOP:Th e practical and theoretical coeffi—
cients of the perform ance of the heat pump．
References
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關(guān)于用帶有熱泵輔助流化床在谷粒烘干過程中的實(shí)驗(yàn)研究
Jing Yang，Li Wang，F(xiàn)iXiang，Lige Tong，and Hua Su
摘要：一種熱泵輔助流動(dòng)床谷物干燥實(shí)驗(yàn)系統(tǒng)已經(jīng)開發(fā)?；谶@系統(tǒng)，在四種不同的空氣周期條件下進(jìn)行了一系列的實(shí)驗(yàn)。根據(jù)實(shí)驗(yàn)分析，為熱泵輔助流動(dòng)床烘干設(shè)備制定了一個(gè)適當(dāng)?shù)母稍锟諝饨橘|(zhì)循環(huán)，它完全不同于普通使用的帶有烘干系統(tǒng)的熱泵。關(guān)于新系統(tǒng)的烘干操作系統(tǒng)性能的實(shí)驗(yàn)結(jié)果表明：平均性能系數(shù)超過2.5。進(jìn)行經(jīng)濟(jì)評(píng)估，從谷粒中去除一千克水的消耗能量大約是0.485kw.h/kg，它顯示合理的經(jīng)濟(jì)效益和在在未來的市場(chǎng)上有著巨大應(yīng)用潛力。
關(guān)鍵字：熱泵，流化床，谷物，烘干，空氣循環(huán)
1.引言
現(xiàn)在，在中國(guó)農(nóng)業(yè)里急需發(fā)展高容量和高質(zhì)量的谷粒烘干設(shè)備[1].隨著社會(huì)和農(nóng)業(yè)快速地發(fā)展，為了匹配機(jī)械化收割，提高效率、節(jié)能、不污染和甚至可移動(dòng)的烘干設(shè)備有著重要的實(shí)際價(jià)值[2]。傳統(tǒng)的烘干方法涉及到連同控制空氣流通的直接石化燃?xì)獾南?。這種方法的效率明顯不高，效率從來不超過20%，更不用說：昂貴的基本能量大大減少，它僅僅來生產(chǎn)低階段的熱量[3]。
到目前為止，傳統(tǒng)的烘干方法在國(guó)內(nèi)很流行，不僅為谷粒烘干而且為其它材料例如木材、水泥、陶瓷和藥等等。因?yàn)橹袊?guó)政府氫能和核能的發(fā)展政策大大地提高了電源結(jié)構(gòu)，關(guān)于帶有熱系統(tǒng)的熱泵研究具有特殊的意義，由于一部分動(dòng)力通常能提供大于2%由熱泵烘干的熱能量。近來相當(dāng)多的學(xué)者[4-8]對(duì)促進(jìn)干旱地區(qū)的熱泵有興趣并且發(fā)現(xiàn)：在干旱地區(qū)熱泵在未來具有很大的應(yīng)用潛力。然而，研究者[9,10]大部分關(guān)注低于55℃適中的溫度的低溫?zé)岜谩?duì)帶有溫度超過70℃烘干系統(tǒng)的熱泵仍然是一片空白，尤其是帶有谷物烘干和熱泵一起工作的系統(tǒng)，它對(duì)目前的論文具有很大的動(dòng)力。除此之外，由于它具有明顯地優(yōu)勢(shì)例如數(shù)量大地氣固接觸面、高的熱量、質(zhì)量轉(zhuǎn)移率、容易機(jī)械操作、由于它有一致的溫度通常有著連續(xù)產(chǎn)量能力和高的烘干質(zhì)量，氣固流化床在干旱地區(qū)獲得廣泛地應(yīng)用[11，12].
論文結(jié)合熱泵和流化床兩者優(yōu)勢(shì)發(fā)展了一種新的烘干實(shí)驗(yàn)系統(tǒng)?；谶@種系統(tǒng)，在不同的條件下執(zhí)行了一系列的實(shí)驗(yàn)。根據(jù)實(shí)驗(yàn)分析，執(zhí)行了為帶有流化床的烘干設(shè)備的一種適中的烘干的中等空氣周期，它完全不同于帶有烘干系統(tǒng)的傳統(tǒng)熱泵。研究結(jié)果也表明：未來很樂觀并且這種新設(shè)備具有市場(chǎng)競(jìng)爭(zhēng)潛力。
2．實(shí)驗(yàn)
在有熱泵和流化床的烘干系統(tǒng)里，有三種材料流動(dòng)：谷物流動(dòng)，制冷流動(dòng)，烘干適中空氣流動(dòng)?？諝饬鲃?dòng)把谷物流動(dòng)和熱泵的制冷流動(dòng)聯(lián)系在一起并且變成兩者之間轉(zhuǎn)移濕度和熱度的工具，流速、溫度和空氣的速度和濕度影響熱泵的工作條件，然而熱泵的工作條件也影響空氣的速度和濕度并且進(jìn)一步影響烘干過程。在這一點(diǎn)上系統(tǒng)的空氣流通是至關(guān)重要的，不僅對(duì)熱泵的工作性能和烘干有影響而且對(duì)整個(gè)設(shè)備最終結(jié)構(gòu)有影響。因此按照?qǐng)D一設(shè)計(jì)了實(shí)驗(yàn)系統(tǒng)，它主要包括了四部分：流化床烘干室、熱泵、管聯(lián)接和幾個(gè)閥門。
圖一：帶有熱泵輔助流化床谷物烘干實(shí)驗(yàn)系統(tǒng)的電路原理圖
這系統(tǒng)通過調(diào)整不同的閥門能實(shí)現(xiàn)四種類型的空氣流通。類型一：關(guān)閉閥門1，2，4，5同時(shí)打開閥門3，6，7，從流化床釋放的空氣僅僅通過熱泵的蒸發(fā)器然后通過它被加熱的冷凝器最終流入流化床，它叫閉合的電路循環(huán)。類型二：關(guān)閉閥門2，3，6同時(shí)打開閥門1，4，5，7，從流化床釋放的空氣直接通過蒸發(fā)器然后流向流化床。它叫閉合循環(huán)。類型三：關(guān)閉1，3，5同時(shí)打開2，4，6，7，從流化床釋放的空氣完全通過蒸發(fā)器然后流進(jìn)周圍的空氣，然而通過通風(fēng)設(shè)備周圍空氣被冷凝器吸收然后流入流化床。它叫開放循環(huán)。類型四：打開所有的閥門并且調(diào)整降低的比率，從流化床釋放的空氣分成兩部分，混合著周圍空氣的一部分流入蒸發(fā)器然后流入周圍空氣；混合周圍空氣的另一部分流入冷凝器然后流入流化床，它叫半閉半開的空氣循環(huán)。
3．參數(shù)測(cè)量和公式
在這個(gè)實(shí)驗(yàn)中，必須測(cè)量的參數(shù)：流化床、蒸發(fā)器和冷凝器的出口和進(jìn)口的溫度，通過蒸發(fā)器、冷凝器和流化床各自的流動(dòng)空氣的數(shù)量，制冷劑的高壓和低壓。所有溫度測(cè)量采納數(shù)字溫度傳感器，它是從0℃到100℃通過液體用玻璃溫度計(jì)來校準(zhǔn)的。熱的球形風(fēng)速儀表被用來測(cè)量流速然后得到流數(shù)量?；谕庥帽頊y(cè)量的階段氣流來測(cè)量壓氣機(jī)的動(dòng)力。用彈性管壓力計(jì)測(cè)量制冷壓力。
通過下列公式這些測(cè)量參數(shù)可以計(jì)算系統(tǒng)的性能參數(shù)。
空氣流數(shù)： v=(∑u/N) A×3600,m3/h
來自蒸發(fā)器的制冷輸出功率： Qe=Vc·ρa(bǔ)ir·Cρ·（toe-tie）/3600,KW
來自冷凝器的熱輸出功率： Qc= Vc·ρa(bǔ)ir·Cρ·（toc-tic）/3600,KW
壓縮機(jī)功率：P=3×Ip×Vp=(I1+I2+I3)×220/1000,KW
熱泵的性能指數(shù)：COP=QC/P;
制冷劑的流量：X=ηc·P/W,kg/s
來自蒸發(fā)器的理論制冷輸出功率：LQe=X×qe,KW
來自冷凝器的理論熱輸出功率：LQc=x×qc, KW
熱泵的理論性能指數(shù)：LCOP= LQc/p;
在此，理論周期指的是理想的熱力學(xué)周期，它相應(yīng)于在試驗(yàn)中測(cè)量的高壓和低壓。
4.實(shí)驗(yàn)結(jié)果和分析
4.1空氣流通類型的效應(yīng)
為了獲得從在干燥過程中不同空氣循環(huán)的效應(yīng)，我們進(jìn)行了四種類型的空氣流通實(shí)驗(yàn).保持所有其他條件相同，如空氣流動(dòng)量和入口溫度等。并發(fā)現(xiàn)，型號(hào)2在流化床入口的溫度是最高的，型號(hào)1中是最低的，而且，型號(hào)3略高于在型號(hào)1，型號(hào)4在型號(hào)2和型號(hào)3之間。這是因?yàn)椋?在型號(hào)2中空氣流通只能通過冷凝器并且余熱比較少地造成在冷凝器出口處的高溫?？諝庀鄬?duì)濕度在流化床入口也較高，因?yàn)榭諝鈴牧骰才懦鰶]有去掉其水分。然而事實(shí)上，連接管并不完全密封，因此，有一些新鮮的空氣從環(huán)境中流入冷凝器。這就是像這樣一個(gè)循環(huán)：關(guān)閉閥門3,6和開放的所有其他閥門.其中的空氣排出流化床，與流經(jīng)冷凝器的空氣混合，然后進(jìn)入到流化床，同時(shí)一些流化床里的空氣通過流化床及其風(fēng)帽之間空隙流入周圍環(huán)境。在這個(gè)實(shí)驗(yàn)系統(tǒng)，在冷凝器和流化床之間估計(jì)有1 0 ％一20 ％的新鮮空氣混合到流動(dòng)的空氣流通之中。因此，具有一定的新鮮空氣的比例的型號(hào)2可以作為最佳的空氣流通在下面的實(shí)驗(yàn)中使用。
4.2通過蒸發(fā)器的空氣流量效應(yīng)
我們進(jìn)行了兩個(gè)實(shí)驗(yàn)，在一項(xiàng)實(shí)驗(yàn)中，含水分約25 ％的1 00千克谷物放入到流化床（其母體不包含水分）。然后系統(tǒng)啟動(dòng)和通過蒸發(fā)器氣流保持相對(duì)較低的速度約以500米/小時(shí)。熱泵的工作情況是顯示數(shù)字2-4。在令一個(gè)實(shí)驗(yàn)中，該系統(tǒng)首先啟動(dòng)和氣流保持以高約2500 立方米/小時(shí)的速度通過蒸發(fā)器。在冷凝器出口處的溫度達(dá)到約70度后將水分含量為27 ％的100公斤放入到流化床。熱泵的工作情況是顯示數(shù)字5-7 。這兩個(gè)實(shí)驗(yàn)中，氣流都是以相同的1 650立方米/小時(shí)的速度通過冷凝器。
圖2制冷劑在以500立方米/