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This article was downloaded by: [University of Illinois at Urbana-Champaign]On: 02 April 2013, At: 13:17Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registeredoffice: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UKHVAC&R ResearchPublication details, including instructions for authors andsubscription ynamic modeling of refrigeratedtransport systems with cooling-mode/heating-mode switch operationsaabcBin Li , Neera Jain , William F. Mohs , Scott Munns , VikascbPatnaik , Jeff Berge & Andrew G. AlleyneaaDepartment of Mechanical Science and Engineering, Universityof Illinois at Urbana-Champaign, MC-244, 1206 West Green Street,Urbana, IL, 61801, USAbDepartment of R&D Engineering, Climate Control Technologies,Ingersoll Rand, Minneapolis, MN, USAcGlobal Modeling & Analysis, Climate Solutions, Ingersoll Rand, LaCrosse, WI, USAVersion of record first published: 27 Sep 2012.To cite this article: Bin Li , Neera Jain , William F. Mohs , Scott Munns , Vikas Patnaik , Jeff Berge &Andrew G. Alleyne (2012): Dynamic modeling of refrigerated transport systems with cooling-mode/heating-mode switch operations, HVAC&R Research, 18:5, 974-996To link to this article: SE SCROLL DOWN FOR ARTICLEFull terms and conditions of use: nsThis article may be used for research, teaching, and private study purposes. Anysubstantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,systematic supply, or distribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representationthat the contents will be complete or accurate or up to date. The accuracy of anyinstructions, formulae, and drug doses should be independently verified with primarysources. The publisher shall not be liable for any loss, actions, claims, proceedings,demand, or costs or damages whatsoever or howsoever caused arising directly orindirectly in connection with or arising out of the use of this material.
Downloaded by [University of Illinois at Urbana-Champaign] at 13:17 02 April 2013Dynamic modeling of refrigerated transport systems withcooling-mode/heating-mode switch operationsBin Li,1 Neera Jain,1 William F. Mohs,2 Scott Munns,3 Vikas Patnaik,3Jeff Berge,2 and Andrew G. Alleyne1, 1Department of Mechanical Science and Engineering, University of Illinois at Urbana-Champaign, MC-244, 1206West Green Street, Urbana, IL 61801, USA2Department of R&D Engineering, Climate Control Technologies, Ingersoll Rand, Minneapolis, MN, USA3Global Modeling & Analysis, Climate Solutions, Ingersoll Rand, La Crosse, WI, USA Corresponding author e-mail: alleyne@illinois.eduThis article presents dynamic modeling approaches to predict system performance characteristicsof cooling-/heating-mode switch cycling operation, a commonly used temperature regulation approach inrefrigerated transport systems. A dynamic model of a commercially available transport refrigeration systemis presented, which describes the system dynamics during the mode switch transients. The developmentof the heat exchanger and accumulator models is highlighted using the switched modeling framework.Model validation against experimental data demonstrates the capabilities of the modeling approach inrepresenting the transient behavior of the mode switch process. Simulation case studies to predict refrigerantmass distribution during transients and system performance with the influence of door-opening events arealso provided to demonstrate modeling capabilities. The presented dynamic modeling framework can serveas a valuable tool to evaluate performance with different system configurations and operating strategies intransport refrigeration applications.IntroductionRefrigerated transport systems, such as refrigerated road vehicles and refrigerated shipping containers, are widely used to distribute chilled andfrozen products throughout the world. As an essential sector in the food supply cold chain, more andmore attention is paid to food transport refrigerationbecause of increasing concerns on food safety andquality, as well as its impact on energy consumptionand the environment (Tassou et al. 2009; Akkermanet al. 2010). An important characteristic of these refrigeration systems is temperature regulation so thatthe quality of perishable foods is preserved and theshelf life is extended during transport (James et al.2006).To satisfy customer needs for shipping a widerange of cargo under tight temperature control,the transport refrigeration industry has respondedby improving temperature control techniques, providing greater cooling capacity, and offering loadflexibility, for example, from single cargo space toReceived November 22, 2011; accepted February 9, 2012Bin Li is PhD candidate. Neera Jain, Student Member ASHRAE, is PhD candidate. William F. Mohs, Member ASHRAE, isSenior Engineer. Scott Munns is Senior Engineer. Vikas Patnaik, PhD, Member ASHRAE, is Engineering Manager. Jeff Berge,Member ASHRAE, is R&D Manager. Andrew G. Alleyne, PhD, is Ralph M. and Catherine V. Fisher Professor.974C 2012 ASHRAE.HVAC&R Research, 18(5):974–996, 2012. Copyright ISSN: 1078-9669 print / 1938-5587 onlineDOI: 10.1080/10789669.2012.670685
Downloaded by [University of Illinois at Urbana-Champaign] at 13:17 02 April 2013HVAC&R RESEARCHmulti-space systems, as noted in Vaclavek et al.(2003). Compared with stationary systems, transport refrigeration systems are required to performreliably over a variety of operating conditions,such as broad temperature ranges of transportedfood products and wide variations in climaticconditions. Additionally, the refrigeration systemsneed to be designed to be energy efficient withoutcompromising the temperature control of the products. To investigate and improve the refrigerationsystem performance, good knowledge of the systembehavior is required and can be obtained eitherfrom modeling and simulation tools or throughexperimental studies. As mentioned in Koury et al.(2001), the use of well-verified numerical modelscan facilitate the understanding of system dynamicbehavior, serve as a tool to evaluate alternativesystem designs and operating strategies, and minimize the time and expense of test-cell experiments.However, considering refrigerated transport isa complex interacting system, complete understanding of dynamic models to predict the systemthermodynamic behavior is still lacking, and manyefforts have been dedicated to improve predictivecapabilities (James et al. 2006; Jolly et al. 2000).This present study develops a specific modelingapproach to simulate common operations in refrigerated transport applications: cooling-/heatingmode switch cycling operations (Tso et al. 2001;Repice and Stumpf 2007). Different from cyclingthe refrigeration systems on and off for temperatureregulation (Li et al. 2010), in cooling-/heating-modeswitch operation, the product temperature is maintained at a set-point below the ambient conditionsby continuously running the system and cyclingbetween cooling and heating modes. One reasonto drive the refrigeration equipment to switch fromcooling to heating mode is the need for defrostingthe heat exchangers (Hoffenbecker et al. 2005;Dopazo et al. 2010). Another reason is to maintain acontinuous supply of air moving over the transportedfood product, which is a requirement for freshproduce (e.g., strawberries). The cooling-/heatingmode switch allows for temperature regulationwhile maintaining continuous air circulation.It is challenging to simulate the mode switchcycling operation since it is a complex and highlytransient process involving component functionvariations as well as many indeterminate variables(Krakow et al. 1993). From the open literature inheat pump applications, there have been extensiveexperimental investigations of system performance975during the switch between normal operating modeand defrosting mode. These include reverse-cycledefrosting processes (Miller 1987; O’Neal et al.1989; Qu et al. 2012) and hot-gas bypass defrostingmethods (Cho et al. 2005; Byun et al. 2008). Nevertheless, there are few studies on the development ofsimulation models to predict the system dynamicsunder normal-/defrosting-mode switch cyclingoperation. Krakow et al. (1993) developed ananalytical reverse-cycle defrosting model where themelting process on the coil surface was idealizedby subdividing it into different stages. Based on theabove modeling theory, Liu et al. (2003) presenteda validated reverse-cycle defrosting model for anair-source heat pump system. While the defrostprocess is important, this article focuses on thedevelopment of capabilities to simulate the modeswitch cycle operation for temperature regulationin transport refrigeration, rather than detailingthe frost melting mass and heat transfer processinvolved in the defrosting mechanism.To investigate the dynamic behavior of refrigeration systems, heat exchangers are usually treatedwith transient models. Two common heat exchangermodeling approaches, finite-volume and movingboundary methods, have been reported in theliterature (He et al. 1997; Jensen and Tummescheit2002; Bendapudi and Braun 2002; Rasmussenand Alleyne 2004; Bendapudi et al. 2005; Ebornet al. 2005; Limperich et al. 2005). Liu et al.(2003) used distributed-parameter finite-volumemodels for the condenser and evaporator during thedefrosting cycle. Bendapudi et al. (2008) comparedthe two approaches in predicting system start-upand load change transients in a centrifugal chillerapplication, while Kapadia et al. (2009) appliedthe finite-volume approach to analyze the start-upperformance of a split air-conditioning system. Aswitched moving-boundary framework was demonstrated in Li and Alleyne (2010) to simulate systemshut-down and start-up performance. In this framework, the heat exchangers were developed withdifferent model representations to accommodate thetransitions of dynamic states during transients. Jain(2009) developed cooling-mode and heating-modeoperation dynamic models for a commercial transport refrigeration unit, where the moving-boundarymodeling approach was applied. On the basis ofthe switched modeling framework (Li and Alleyne2010) and the transport refrigeration models (Jain2009), this article models the single cargo spacetransport refrigeration system, validates the system
Downloaded by [University of Illinois at Urbana-Champaign] at 13:17 02 April 2013976VOLUME 18, NUMBER 5, OCTOBER 2012Figure 1. (a) A typical single cargo space refrigerated transport system and (b) schematic of cooling-/heating-mode switch temperature control system.dynamic characteristics during cooling-/heatingmode switch cycles, and predicts refrigerant massdistribution among the system components intransients.The rest of this article is organized as follows.The “Experimental refrigeration system” sectiondescribes the switching operation between coolingand heating mode in the experimental transport refrigeration system as well as the process throughwhich data is acquired for the system model validation. The “System modeling” section introduces themodeling of individual components that are activeduring either the cooling or the heating mode of operation. Using the switched modeling framework inLi and Alleyne (2010), the heat exchanger components are modeled to capture the transients duringcooling-/heating-mode switch cycles. Model validation results are presented in the section entitled“Model validation” to demonstrate the capabilitiesof the developed model in predicting system dynamics during mode switch cycling. Finally, a sim-ulation case study for temperature regulation in atransport refrigeration unit involving door-openingevents is given in the “Case study” section. A conclusion section summarizes the main points of thearticle.Experimental refrigeration systemAs indicated in Figure 1a, a typical refrigeratedtransport system consists of a refrigeration unit anda refrigerated cargo space. The refrigeration unit isinteracting with the cargo space to meet its temperature requirements and satisfy the refrigerationdemands over a wide range of operating conditions(Tassou et al. 2009). Under a cooling-/heating-modeswitch operation scheme for temperature regulation(see Figure 1b), the system operation mode (cooling or heating) is driven by the difference betweenmeasured cargo space temperature and the temperature set-point. The cargo space is coupled to therefrigeration unit such that the cargo space outputs,
Downloaded by [University of Illinois at Urbana-Champaign] at 13:17 02 April 2013HVAC&R RESEARCHe.g., return air temperature (normally considered asthe cargo space temperature), are the inputs to therefrigeration unit, e.g., evaporator air inlet temperature. Simultaneously, the refrigeration unit outputs,e.g., evaporator air outlet temperature, are acting asthe cargo space inputs, e.g., supply air temperature.As mentioned earlier, the heat exchanger fans arecontinuously running during mode switch cycling.The dual-mode refrigeration system studied inthis article is a commercially available TS-500transport refrigeration unit manufactured byThermo King Corporation. The refrigeration system is charged with refrigerant R404A. The primarymode of operation is the cooling cycle in which theunit extracts heat from the refrigerated cargo spaceand transfers it to the external ambient environment.In the second mode of operation, the heating cycle,the refrigeration unit delivers heat to the cargospace. Figure 2 shows a schematic of the systemconfiguration where components are interconnectedto form a vapor compression cycle (VCC) refrigeration system. The switch from cooling mode toheating mode is completed using a three-way valvethat directs the path of the refrigerant exiting theFigure 2. Schematic of the refrigeration unit in operation.977compressor toward the evaporator through the discharge pressure regulator (DPR) valve rather thanto the condenser coil, as can be seen in Figure 2.In heating-mode operation, the refrigerant betweenthe condenser inlet and the thermostatic expansionvalve (TXV) outlet is trapped if the valve bleedport effect is not considered. The refrigerant flowsthrough the hot gas line to the evaporator coil. Theevaporator now functions as a condenser, taking superheated vapor in and condensing it into two-phasefluid while heating the cargo space. After separationin the accumulator, the saturated refrigerant vaporpasses through the throttle valve and finally returnsto the compressor. Once the measured cargo spacetemperature exceeds the upper limit of the cargospace temperature set-point, the unit switches fromheating- to cooling-mode operation, where the superheated refrigerant vapor exiting the compressoris redirected by the three-way valve to the condensercoil. A representative pressure-enthalpy (P-h) diagram for cooling- and heating-mode operationis plotted in Figure 3, where refrigerant pressuresalong the heat exchanger coils are assumed to beuniform.
Downloaded by [University of Illinois at Urbana-Champaign] at 13:17 02 April 2013978VOLUME 18, NUMBER 5, OCTOBER 2012Figure 3. P-h diagram for cooling and heating mode.All experimental data presented in this articlewere collected at the Thermo King Corporation testfacility located in Minneapolis, Minnesota, USA.The experimental refrigeration system as shownin Figure 4 was instrumented with type-T thermocouples and pressure transducers. Air temperaturesentering and leaving the heat exchangers weremeasured with thermocouple grids placed near thecoils. Immersion thermocouples were used to monitor the refrigerant temperature at different locationsin the refrigeration unit. Four thermocouple stands(see Figure 4b) were used for measuring the airtemperature profile inside the cargo space. Table 1presents the accuracy of sensors in measurements.Along with the status of each solenoid valve in therefrigeration system, each temperature sensor andpressure transducer was connected to an Agilent34970A data acquisition system to observe the system behavior during testing. The temperature andpressure measurements were collected every 10 sec.The experimental scenario involved a temperature pull-down and control test for the enclosedcargo space. The test procedure can be describedas a “high speed pull-down of the cargo spacetemperature from ambient temperature to a givenset-point (fresh or frozen) using the refrigerationunit; then continuously run cooling-/heating-modeswitch cycle operation with low speed to maintainthe space temperature.” The experimental resultsare used to validate the dynamic model discussed inthe following sections. The reader is encouraged torefer to Jain (2009) for more information regardingFigure 4. (a) Photograph of the experimental system and (b) instrumentation inside the cargo space.
979HVAC&R RESEARCHTable 1. Locations and accuracy of sensors.SensorAccuracyDownloaded by [University of Illinois at Urbana-Champaign] at 13:17 02 April 2013Type-T thermocouples for air and refrigerant temperaturePressure transducer at compressor discharge sidePressure transducer at compressor suction sidePressure transducer at receiver tank outletPressure transducer at evaporator outletcooling- and heating-mode operation as well asinstrumentation of the refrigeration unit.System modelingThis section is divided into three parts. The refrigeration unit model (see Figure 2) is presentedfirst, where the main component models are introduced. The evaporator and accumulator models arethe key components operating in both cooling andheating modes. They are presented here to capture the mode switch transient performance usingthe switched modeling framework (Li and Alleyne2010). Second, a dynamic refrigerated cargo spacemodel is given based on the heat balance processing method (Li et al. 2010). Combining the unit andcargo space models, the overall transport refrigeration system can be represented for the simulationand validation studies discussed in the “Model validation” and “Case study” sections. The third part ofthis section illustrates the simulation environment.Refrigeration unit model developmentAs discussed in Jain (2009), the refrigerationunit system depicted in Figure 2 includes staticcomponents and dynamic components. The staticcomponents (i.e., compressor, regulation valves,and suction line heat exchanger [SLHX]) aremodeled using steady-state equations under theassumption that the dynamics of these componentsare generally an order of magnitude faster thanthose of the dynamic components (i.e., heatexchangers, receiver tank, and accumulator) and,therefore, less dominant in the overall system. Ascan be seen in Figure 2, the VCC refrigerationsystem is subdivided into three component portionsto represent the variations in mode operations,and each portion is discussed in what follows.More descriptions about the individual componentmodels of the refrigeration unit operating in coolingand heating mode can be found in Jain (2009). 0.5 C (0.9 F) 10 kPa (1.45 psi) 10 kPa (1.45 psi) 10 kPa (1.45 psi) 10 kPa (1.45 psi)Common components in both modesEvaporator: When the system switches fromcooling- to heating-mode operation, the evaporatoracts like a condenser. Specifically, superheated vapor enters the evaporator coil and exits as two-phasefluid. When the system switches back to coolingmode operation, the evaporator starts to extract heatagain from the cargo space. There have been manystudies investigating switched evaporator model development (Li and Alleyne 2010; Pettit et al. 1998;Zhang and Zhang 2006; Shao and Zhang 2007;Liang et al. 2010; Cecchinato and Mancini 2011).They focused on performance prediction with theappearance and disappearance of the superheatedzone at the evaporator outlet upon varying conditions, and the refrigerant ent
c Global Modeling & Analysis, Climate Solutions, Ingersoll Rand, La Crosse, WI, USA Version of record first published: 27 Sep 2012. To cite this article: Bin Li , Neera Jain , William F. Mohs , Scott Munns , Vikas Patnaik , Jeff Berge & Andrew G. Alleyne (2012): Dynamic mode
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