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Highlights • A numerically efficient but accurate model for frosting evaporators is developed. • The enthalpy-based reformulation and linearization method are employed • Simulation results on flat plate and finned-tube heat exchangers are provided. • It is 8 to 20 times faster than reference models but withcomparable accuracy. Abstract Growth of a frost layer on an evaporator surface due to low evaporator temperature as well as moisture contained in surrounding air deteriorates performance of a refrigeration system significantly and requires significant energy for defrost. Many studies have been performed to model the heat and mass transfer phenomena in an attempt to have insight and accurate prediction. However, many models form nonlinear algebraic differential equations which require iterative numerical solvers. Computationally efficient but accurate models are needed in order to evaluate overall system performance. The objective of this paper is to introduce a modeling approach to overcome the problem. A solution strategy based on an enthalpy-based reformulation and linearization method will be presented. Comparisons of the proposed and detailed model results for both flat plate and finned tube heat exchangers are provided. The proposed modeling approach is around 10 times faster than reference models while maintaining comparable accuracy. [ABSTRACT FROM AUTHOR]

A figure of merit, V, to characterize the value of an AFDD maintenance tool is presented. The AFDD tool is fed data representing air-conditioners operating with and without faults across a range of conditions. The calculation of V considers the probability of each scenario occurring, the tool's response, and the resulting implications. These results are summed to give a typical average value for deploying the AFDD tool, as compared to maintenance being performed without AFDD. Case studies illustrate the calculation of V. Six AFDD tools and two idealized tools are evaluated. Five of the six real tools give negative V, meaning that their use causes more harm than good. The sixth shows approximately $10 benefit per nominal ton of air-conditioner capacity per typical maintenance visit. This represents about half of the maximum potential value. These results demonstrate the importance of measuring AFDD performance and the potential monetary benefits of AFDD. [ABSTRACT FROM AUTHOR]

The primary goal of the work described in this paper was to evaluate and extend a virtual refrigerant charge sensor (VRC) for determining refrigerant charge for equipment having variable-speed compressors and fans. Based on the evaluations, the original VRC sensor (termed model I) was found to work well in estimating the refrigerant charge level for systems with a variable-speed compressor. However, for extreme test conditions such as low compressor speed, the model I needed to be improved. To overcome the limitations, the model was modified to include a term involving the inlet quality to the evaporator (termed model II). The model II gave better performance for systems with a variable-speed compressor. When the superheat of the compressor was zero, neither model II could accurately predict charge level. Therefore, a third approach (Model III) was developed that includes the discharge superheat of the compressor. [ABSTRACT FROM AUTHOR]

Abstract: A companion paper [Kim, J.-H., Braun, J.E., Groll, E.A., 2009. A hybrid method for refrigerant flow balancing in multi-circuit evaporators: upstream versus downstream control. International Journal of Refrigeration doi:10.1016/j.ijrefrig.2009.01.013 presented a hybrid approach for providing control of refrigerant flow distribution in evaporators that involves the use of small balancing valves in each circuit along with a primary expansion device to control the overall superheat from the evaporator. Furthermore, the companion paper demonstrated that the flow balancing valves should be located upstream rather than downstream of the evaporator in order to realize significant benefits. The current paper utilizes the model presented in the companion paper to more fully evaluate the effects of uneven air and refrigerant flow distributions and the benefits of upstream hybrid control in response to these effects. [Copyright &y& Elsevier]

Abstract: A hybrid method for optimizing refrigerant distribution in evaporators is presented that involves the use of small balancing valves in each circuit along with a primary expansion device to control the overall superheat from the evaporator. The flow balancing valves could be located upstream or downstream of the evaporator. This paper presents the results of a study to investigate the benefits of this hybrid scheme for both upstream and downstream flow balancing for the case of air flow mal-distribution. In order to perform this investigation, a simulation model was developed to consider evaporator flow mal-distributions for a 10.55kW residential R410A heat pump and then validated through comparisons of predicted results with measurements. Simulation results show that there are significant benefits in controlling the superheat of each circuit of evaporators through the hybrid–individual superheat control method. Furthermore, the upstream refrigerant flow control consistently outperforms the downstream refrigerant flow control, and recovers most of the loss in cooling capacity and COP due to non-uniform air flow distribution. [Copyright &y& Elsevier]

Refrigerant pressures are critical measurements for monitoring, control, diagnostics, and optimization of vapor compression cycle equipment. Direct refrigerant pressure measuring practices are expensive and more than often problematic. This paper describes a method, termed virtual pressure sensing, wherein refrigerant pressure values are indirectly derived from low-cost temperature sensors that can be surface-mounted. In this manner, physical pressure sensors are eliminated and pressure sensing can be achieved at a much lower cost and in a non-invasive way. Five virtual pressure sensors are developed to obtain the five most important pressures in vapor compression cycle equipment: compressor discharge line pressure, condensing pressure, liquid line pressure, evaporating pressure, and suction line pressure. The performance of the proposed virtual pressure sensors, in terms of accuracy in estimating pressures and inferring liquid line subcooling, suction superheat, compressor power consumption, and refrigerant flow rate, is evaluated extensively using laboratory data collected from four systems. These systems include air conditioners and heat pumps, split and packaged systems, refrigerants R-22 and R-410α, fixed-orifices and thermal expansion valves, and reciprocating compressors and scroll compressors. Ultimately, the virtual sensors are used as a part of a decoupling-based fault detection and diagnosis (FDD) technique to diagnose multiple simultaneous faults. The impact of the virtual pressure sensors on the FDD performance is evaluated extensively using the laboratory data collected from the four various systems. [ABSTRACT FROM AUTHOR]

Proper refrigerant charge level is critical for vapor-compression cycle refrigeration systems to operate efficiently and safely. The current practice of accurately determining charge levels is very time consuming and costly. Technicians must stop and evacuate the system, weigh the removed charge, and add the correct amount of charge to the system using a scale. This paper presents a method for obtaining accurate estimates of refrigerant charge level using noninvasive temperature measurements obtained while the system is operating. The method could be used as part of a permanently installed control or monitoring system to indicate charge level and/or to automatically detect and diagnose low or high levels of refrigerant charge. It could also be used as a stand-alone tool by technicians to determine existing charge and during the process of adjusting the refrigerant charge. The accuracy of the virtual refrigerant charge sensor is evaluated using laboratory data for seven different systems and over a wide range of operating conditions with and without the presence of other faults. The evaluation and demonstration verify that the virtual refrigerant charge level gauge has very good performance in terms of accuracy and robustness and can be easily implemented and installed in terms of both hardware and software. [ABSTRACT FROM AUTHOR]

Most of the previous diagnostics research for vapor-compression equipment has been performed for packaged air conditioners and chillers. This paper describes application of a decoupling-based diagnostic technique that was originally developed for air conditioners to equipment used in walk-in coolers and freezers. Testing was performed on a small, restaurant- style walk-in cooler and a small walk-in freezer for faulted and unfaulted conditions. The data was then used to characterize how the fault features respond to faults and to evaluate the performance of the decoupling-based diagnostic technique. [ABSTRACT FROM AUTHOR]

This paper presents the development and validation of a computer model for hermetic two-stage rotary compressors that can be used for design optimization. The model considers the effects of refrigerant leakage between the compressor chambers, heat transfer between the cylinder wall and the refrigerant gas, and heat loss by natural convection to the surroundings. The model numerically solves mass and energy balances for each of the compressor's chambers to predict the pressure and temperature variations over a crankshaft revolution. The compressor power input and refrigerant mass flow rate are calculated and used to determine the isentropic and volumetric efficiencies. The model can operate both with and without intercooling between the stages to allow for study of the effects of intercooling on compressor performance. The model results are compared to the results of external performance tests that were conducted using a prototype compressor. The intermediate temperature and pressure, discharge temperature, power consumption, and mass flow rate were recorded for 17 different operating conditions. The model predicts the compressor power consumption and mass flow rate within ±5% of the experimental results. [ABSTRACT FROM AUTHOR]

This paper presents a detailed experimental validation of a simple transient model for cooling and dehumidifying coils that is developed in a companion paper (Zhou and Braun 2007). In all cases, the model provided very accurate predictions of exit air and water states. This appears to be the first experimental validation of a transient coil model under conditions where moisture condensation occurs. [ABSTRACT FROM AUTHOR]

Fin efficiencies are used within cooling coil models to simplify heat and mass transfer calculations associated with the use of air-side fins. These models utilize relationships developed for dry fins (i.e., no condensation) in determining convective heat transfer and modified fin efficiencies for estimating combined heat and mass transfer. However, the fin temperature distribution is different for wet and dry fins and, therefore, the use of a dry fin efficiency relationship for convective heat transfer is not strictly correct under wet conditions. This paper develops a correction factor for existing fin efficiency relationships that allows a better estimate of convective heat transfer fin efficiencies under wet conditions. The improved method is validated using results obtained from a two-dimensional numerical analysis and experiments performed on an eight-row cooling coil. [ABSTRACT FROM AUTHOR]

Dynamic coil models are useful in the development and evaluation of feedback controllers and fault detection and diagnostics algorithms. Existing models in the literature have not been validated fully using experimental data and have not been evaluated in terms of trade-offs between model accuracy and computational requirements. This paper develops a computationally fast, distributed model for the transient behavior of cooling and dehumidifying coils. The model utilizes several steady-state performance indices that simplify dynamic modeling, including fin efficiency, air-side effectiveness, and water-side effectiveness. Predictions from the simplified model were compared with a detailed reference model and with other simplified models from the literature. Comparisons with detailed experimental measurements are given in a companion paper (Zhou and Braun 2007). The model provides very accurate predictions of coil transient performance and executes more than 1000 times faster than real-time on a 1.2 GHz personal computer. As a result, it is very practical for implementation within system simulation programs that execute over long time periods, such as a year. [ABSTRACT FROM AUTHOR]

A near-optimal control method was developed for charging and discharging of cool storage systems when real-time pricing (RTP) electric rates are available The algorithm requires relatively low-cost measurements (cooling load and storage state of charge), requires very little plant-specific information, is computationally simple, and ensures that building cooling requirements are always met (e.g., storage isn't prematurely depleted). The control method was evaluated for ice storage systems using a simulation tool for different combinations of cooling plants, storage sizes, buildings, locations, and RTP rates. The simplified method worked well in all cases and gave annual costs within approximately 2% of the minimum possible costs associated with optimal control. [ABSTRACT FROM AUTHOR]

Existing methods addressing automated fault detection and diagnosis (FDD) for vapor-compression air-conditioning equipment have good performance for faults that occur individually but have difficulty handling multiple simultaneous faults. In addition, these methods either require high-cost measurements or measurements over a wide range of conditions for training reference models, the development of which can be time consuming and cost prohibitive. This paper formulates model-based FDD in a generic way and demonstrates that decoupling is the key to handling multiple simultaneous faults. To eliminate a cost-prohibitive overall system model, an alternative physical decoupling methodology to mathematical decoupling is developed. During the mathematical development, a previously developed FDD method termed the statistical rule-based method is reexamined and cast within the general mathematical framework. The paper also includes an evaluation of the FDD method in terms of both sensitivity and robustness. [ABSTRACT FROM AUTHOR]

Abstract: Air conditioning is a non-critical application for fault detection and diagnosis (FDD) where decisions about servicing faults should involve the use of economics. Existing methods for evaluating impacts of faults on equipment performance only consider some individual factors such as the equipment coefficient of performance (COP) or cooling capacity. This paper develops an overall economic performance degradation index (EPDI) for air conditioning equipment that includes the combined effects of degradations in COP, cooling capacity, and sensible heat ratio (SHR). EPDI quantifies the performance degradation caused by faults based on economics so it can be used as part of the decision making process in an overall FDD system. Furthermore, EPDI can be used along with estimates of typical field performance degradations to assess the economic benefits associated with the application of automated FDD. A case study is presented where EPDI was applied to measurements for an existing unit where faults were artificially introduced. [Copyright &y& Elsevier]

Current air-conditioning system models seek to balance the level of code complexity with the speed of execution. Ideally, the simplest code possible that can accurately predict system performance is desired. To further understand this issue, two methods of modeling unitary air-conditioning systems are compared: a detailed, local approach and a simplified, global approach. These methods have been applied to two commercial unitary air conditioners for which extensive experimental data were obtained. The global approach predicted absolute system performance as well as or better than the local approach. In a separate analysis, the effects of two-phase modeling parameters on system model predictions were considered. The two-phase friction factors, heat transfer coefficients, and void fraction models were independently varied. The resulting system performance predictions were sensitive to variations in the air-side heat transfer coefficients and the void fraction model and not as sensitive to variations in the friction factor. Both the void fraction model and the condenser parameters mainly affect the compressor discharge pressure and the compressor power consumption. The evaporator parameters mainly affect the refrigerant mass flow rate and the heat transfer rates in both heat exchangers. [ABSTRACT FROM AUTHOR]

Abstract: In this work, a residential air conditioning compressor designed for vapor injection has been modified in order to inject large quantities of oil into the working chamber in order to approach an isothermal compression process. The compressor was tested with oil injection mass flow fractions of up to 45%. At an evaporating temperature of −10 °C and condensing temperature of 30 °C, the overall isentropic efficiency was up to 70% at the highest oil injection rate. Overall, over the testing envelope investigated, there are no significantly negative effects experienced for the compressor and the compressor isentropic efficiency and refrigerant mass flow rate improve monotonically as the oil injection rate is increased. [Copyright &y& Elsevier]

Abstract: In two companion papers, simulation models for the working processes of liquid-flooded scroll compressors and expanders have been developed and validated against experimental data. In this study, analytic models are presented for the modes of irreversibility generation in the liquid-flooded scroll compressor including built-in volume ratio maladjustment, pressure drop and leakage. A thermodynamic model is used to derive the ideal volume ratio for a liquid-flooded compressor, which is higher than that of dry compression. An optimum set of built-in volume ratio and scroll base circle radius is found which maximizes the overall isentropic efficiency by minimizing the leakage and pressure drop irreversibilities. The irreversibility generation models are used to optimize a scroll compressor for the Liquid-Flooded Ericsson cycle application. The model predicts scroll compressor overall isentropic efficiency of over 80% (based on the shaft power) at an oil mass fraction of 88%. [Copyright &y& Elsevier]

Abstract: A detailed mechanistic model has been developed for scroll compressors and scroll expanders with liquid flooding for application to the Liquid Flooded Ericsson Cycle. This model is based on the integration of a set of differential equations that arise from the conservation laws. The impact of the flooding liquid on the working process is also included through mixture properties as well as treatment of two-phase flow pressure drop. With the working process known, it is then possible to solve for the compressor power, total mass flow rate and other model outputs. The fine level of detail in the model allows for the investigation of the impact of design changes to the scroll machines. This model can also be readily extended to allow for the analysis of liquid flooded vapor compression refrigeration. [Copyright &y& Elsevier]

Abstract: The use of liquid-flooding in the compression and expansion of non-condensable gas in scroll compressors and expanders enables the possibility of quasi-isothermal working processes. Liquid-flooded scroll machines were installed in a fully-instrumented Liquid-Flooded Ericsson Cycle test rig to conduct entire cycle performance tests. In addition, detailed compressor and expander performance data was obtained. Oil mass fractions of up to 92% and 76% were added to the gas entering the scroll compressor and expander respectively. The overall isentropic efficiency of the scroll compressor based on the shaft power with flooding was up to 73% and the volumetric efficiency was above 92%. For the expander, the best overall isentropic and volumetric efficiencies achieved were 66% and 105% respectively. The mechanistic model presented in the companion paper was validated against the experimental data for both the compressor and the scroll expander with good agreement, though the agreement is better for the scroll compressor. [Copyright &y& Elsevier]