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The performance merits of a mobile air conditioning (MAC) system with an orifice tube (OT) were experimentally evaluated for R1234yf relative to R134a. The effect of employing a suction/liquid line internal heat exchanger (HEX) was also investigated for R1234yf. To this end, an experimental unit employing the components of an R134a MAC system was set up, equipped with a coaxial internal HEX, and tested at various compressor speeds for both refrigerants. The air temperatures entering the condenser and evaporator were simultaneously maintained at 30, 35, and 40 °C. The R1234yf unit provided on average 22.5% lower and 6.9% higher cooling capacity than the R134a unit for the inactive and active HEX, respectively. Furthermore, the coefficient of performance (COP) for the R1234yf unit without and with HEX was on average 10.9% and 3.5% lower than that for the R134a unit, respectively. The R1234yf unit without and with HEX yielded on average 41.5% and 8.4% lower exergetic efficiency than the R134a unit, respectively. These results revealed that the use of an internal HEX in an R1234yf MAC unit with an OT compensates for the cooling capacity degradation, and the R1234yf unit has almost as high performance as the R134a unit. [ABSTRACT FROM AUTHOR]

With new low-GWP HFO refrigerants, the heat transfer performances and methods for decreasing system refrigerant inventory are receiving increasing interest. In shell-and-tube heat exchangers, refrigerant distribution via pressurized liquid spray has the potential for high heat transfer performance while reducing refrigerant charge. However, no published studies have investigated LGWP refrigerants with spray evaporation on tube bundles. A test apparatus was constructed to measure the shell-side heat transfer coefficients of R1234ze(E) on bundles of tubes with two different enhanced-surface types and in two different bundle geometries at various refrigerant saturation temperatures. The results showed strong dependence on refrigerant properties, tube heat flux, enhanced-surface type, bundle geometry, and refrigerant inlet subcooling. The bundle heat transfer coefficients of R1234ze(E) were similar to that of R134a in the same test setup, usually within ±15% for similar conditions. In both cases, they first increased with the heat flux until a local maximum value was achieved. A localized dryout of the tubes at the bottom of the bundle penalized the overall bundle heat transfer coefficient at very high heat flux. For the condensing surface, bundle heat transfer coefficients rarely exceeded 10 kW/m2-K, whereas values in excess of 30 kW/m2-K were sometimes seen for the evaporating surface. [ABSTRACT FROM AUTHOR]

In this work, the effect of flow circuitry on the performance of a household freezer with a tube-on-sheet evaporator is investigated. The evaporator used in the freezer is a tube-on-sheet type where the tubes are bent and attached underneath the shelves, as in the serpentine heat exchanger. The evaporator is composed of six shelves that are located inside the cabin with a particular distance between each. Four different refrigerant circuitries are modeled to identify the one that provides a homogeneous temperature distribution while consuming less energy, and experiments are conducted by switching the refrigerants flow route between the shelves. The uniform temperature distribution at around the target temperature and the energy consumption of the entire system while achieving this target are monitored as the major performance measures of the investigation. The tests are conducted according to IEC 62552:2015, standard clause 8. The experiments found that the change of the flow circuitry has a crucial impact on the temperature distribution's uniformity and energy consumption. The type in which the refrigerant circuitry between the shelves is 2-1-6-5-4-3 has the best performance in terms of temperature distribution and energy consumption. [ABSTRACT FROM AUTHOR]

In direct expansion ground coupled heat pumps (DX-GCHPs), the borehole heat exchanger (BHE) plays a major role, as it is the component where heat transfer between the ground coupled heat pump (GCHP) and the soil occurs. To address the current challenges of using CO2 as refrigerant in DX-GCHP systems, theoretical and experimental investigations were performed on a single U-tube vertical direct expansion BHE using CO2 as the refrigerant. First, an experimental setup of a transcritical CO2 DX-GCHP built at CanmetENERGY laboratories was used to monitor, collect, and analyze data for refrigerant flow, pressure drop, and ground temperature along the boreholes. The CO2 evaporation process in the BHE for different mass flow rates per borehole was then experimentally analyzed. Later, a previously developed theoretical model of the BHE was validated using a set of experiments collected with the current test apparatus. Finally, the model was used for the analysis of the effect of the following parameters on the performance of the BHE: (i) the refrigerant flow rate, (ii) the borehole wall temperature and average fluid temperature over the test period during the evaporation process, (iii) the pressure drop, (iv) the required power for CO2 circulation, and (v) the evaporating temperature change in the BHE. [ABSTRACT FROM AUTHOR]

The multi-functional variable refrigerant flow system enables simultaneous heating and cooling of the zones it serves by allowing the heat exchangers of both indoor units and outdoor units to operate as evaporators or condensers. To realize automatic and smooth switching between different modes, a mode switching strategy is proposed for a multi-functional variable refrigerant flow system. Whether to turn on or off an indoor unit is determined by the zone temperature and a preset hysteresis band about the temperature set-point. Based on a thermodynamic analysis, a decision variable for determining outdoor unit-heat exchangers mode switching is proposed as the outdoor unit-heat exchangers airside temperature differential normalized by the dimensionless outdoor unit fan speed. Two bumpless transfer strategies are applied to produce smooth mode switching. To evaluate the proposed strategy, a simulation study is performed with a Modelica-based dynamic simulation model of a four-zone multi-functional variable refrigerant flow system. Simulation results validate the effectiveness of the proposed strategy and the performance of the bumpless transfer strategies. [ABSTRACT FROM AUTHOR]

This article presents a nonlinear reduced-order modeling method for evaporators in vapor compression systems. This method utilizes linearized reduction techniques to replace the fast dynamic modes of a system with nonlinear functions calculated from the full-order equations of slow dynamic modes. This process creates a nonlinear reduced-order model that is capable of matching full-order nonlinear model results without suffering from linearization error and simulates over 20 times faster than the full-order model. Simulation results are presented for the application of this reduction method to both moving-boundary and finite-control-volume evaporator modeling paradigms. A wide array of case studies are performed for various model input perturbations to illustrate this method's performance advantages. Simulation time step results are presented that show the reduced-order nonlinear model's capability of numerical time steps two orders of magnitude larger than the full-order nonlinear model, resulting in drastically reduced computational times. This reduced computational time allows the use of more heavily discretized models, increasing the feasibility of using more detailed and accurate models for real-time simulation and control. [ABSTRACT FROM PUBLISHER]

The design and the performance of an innovative shell-and-tube evaporator using round copper microchannels are presented in this article. This prototype has been designed aiming at the minimization of the refrigerant charge, which can be required by safety or environmental restrictions. Experimental data of heat transfer and pressure drop are reported in the present article. The measurements have been obtained with two different evaporator inlet headers and two different working fluids (i.e., R22 and R410A) to investigate the mutual influence of the design of the distribution system and the refrigerant properties on possible maldistribution issues. A computational procedure implementing different correlations has also been developed and validated against experimental data; this procedure allows the prediction of the performance of the same evaporator with a hydrocarbon, such as propane, and comparison of the prototype to a brazed-plate heat exchanger. [ABSTRACT FROM PUBLISHER]