Refrigerant-side Pressure Drop Research Articles

Investigation on heat transfer and pressure drop correlations of R600a air-cooled finned tube evaporator for fridge

The correlations to predict heat transfer coefficient and friction pressure drop of finned tube evaporator for fridge were developed. Twelve straight finned tube evaporators with different structural parameters were tested under different working conditions. The range of Reynolds number in air side was 750 < Rea < 9500 and that in tube side was 1000 < Rer < 15000. The models of evaporator heat transfer coefficient, air side pressure drop and tube side pressure drop were established respectively. The physical dimensions and flow parameters on both sides of finned tube evaporator were considered in the models and their effects on heat transfer and flow characteristics were revealed. The single-phase heat transfer area ratio was introduced. The correlations for household refrigerator evaporators were obtained through a nonlinear fitting method based on 210 groups of experimental data. The mean absolute percentage deviation of the new heat transfer correlation was 10.68%. The mean absolute percentage deviations of the new air side and refrigerant side pressure drop correlations were 8.8% and 10.3%, respectively. The data comparison showed that the proposed correlations were reasonable and accurate enough to be used in practical application.

Development of design guidelines for fin-and-tube heat exchangers with low-GWP refrigerants

Regulatory changes worldwide are requiring air conditioning and heat pump equipment manufacturers to switch refrigerants to alternatives with reduced global warming potential (GWP). Design of air-to-refrigerant fin-and-tube heat exchangers (FTHXs) is critical to overall performance of this equipment and is not currently well understood with low-GWP refrigerants. This paper presents a simulation study exploring the impact of design changes for a FTHX used in an indoor residential evaporator originally designed for R410A. Simulation results are used to generate design guidelines for FTHXs using HFO refrigerants R1234yf and R1234ze(E), evaluated using cooling capacity, refrigerant-side pressure drop and ratio of the two as critical metrics. When evaluated at identical operational conditions, with fixed refrigerant outlet superheat, it was found that for a fixed slab size, the FTHX was most sensitive to number of circuits, followed by tube diameter, number of tubes in the FTHX, and fin density. For the low-GWP HFOs, circuitry modifications led to maximum increase in FTHX capacity of 20%, and maximum decrease in refrigerant-side pressure drop of 88.6%. Based on simulation results, it is concluded that additional coil face area is required to match both capacity and refrigerant pressure drop, compared with R410A, this is demonstrated with two proposed FTHX designs. One design prioritizes increased FTHX capacity, and the other a reduction in refrigerant pressure drop. All these results culminated in design guidelines for HFOs that suggest it will be challenging to achieve both the same capacity and pressure drop characteristics as R410A without modifications to FTHX size.

Performance assessment of R1234ze(E) as a low GWP substitute to R410A in fin-and-tube heat exchangers

Several energy efficiency regulations worldwide are forcing air conditioning and heat pump manufacturers to replace the halogenated refrigerants currently used in these systems, such as R410A, with more climate friendly alternatives that have a lower Global Warming Potential (GWP). This study explores how the performance of R1234ze(E) compares for an R410A based fin-and-tube evaporator coil. A total of 36 experiments were completed for both refrigerants, by means of a high-fidelity experimental facility, to validate a segment-by-segment heat exchanger model. For R410A, the mean absolute percent error between the experimental and simulated capacity was found to be 0.9%, and it was 1.4% for R1234ze(E). Simulations were then carried out, using the model, with R1234ze(E), where the fin density and refrigerant circuitry of the original heat exchanger were altered independently. By increasing the fin density from 15 to 20 fins per inch (FPI), the capacity increased by 4.9%, and refrigerant side pressure drop decreased by 4.5%. For the refrigerant circuitry optimizations, a maximum increase in capacity of 5.6% was observed when compared to the baseline, but at the expense of 7.5 times higher pressure drop. Alternatively, a separate circuitry resulted in a 38% pressure drop reduction and 2.5% reduction in capacity. The results demonstrated the relative sensitivity of capacity and refrigerant side pressure drop for R1234ze(E) on only two heat exchanger geometrical parameters. This sensitivity suggests potential for additional optimizations to heat exchanger designs for low-GWP refrigerants by performing further parametric analysis on heat exchanger geometry.

Experimental study of oil retention in microchannel type evaporators of air-source heat pump systems

In air-source heat pump systems, lubricant is often used with refrigerant compressors and, a small portion of oil, which is inevitably discharged by the compressor, circulates with the refrigerant across the systems components. Oil tends to accumulate in heat exchangers, increasing the pressure losses and reducing the heat transfer capacity. In addition to energy efficiency degradation, safe and reliable operations of the compressors can be compromised by unknown amounts of oil retained in the heat exchangers.In the present paper, the lubricant retention in two microchannel type evaporators for residential air heat pump systems was measured and the oil effects on evaporator heat transfer capacity and refrigerant-side pressure drop were investigated. Refrigerant R410A was investigated with polyolester lubricant that had a viscosity grade of 32. Saturation temperature was controlled from 0.5 °C and up to 9 °C and the impact of the degree of refrigerant vapor superheat at the evaporator outlet on the oil retention in the heat exchanger was significant. At high oil concentration, 13% of the internal volume of the evaporators was occupied by lubricant that was either retained or in circulation with the refrigerant inside the evaporators. When oil concentration was below 1 wt.%, which is typical in today's systems, the retained oil was about 10% of the internal volume, pressure losses increased up to 25% and the heat transfer capacity was reduced by 4%.

Performance Study on a Window Type Air Conditioner Condenser Using Alternative Refrigerant R407C

In this work, a performance study was achieved on air cooled condenser of an air-conditioning unit. An experimental investigation was carried out on two condenser designs aided by a controlled environmental zone, which was designed and constructed for the current study. The effect of ambient air temperature on the condenser performance was studied by varying the controlled zone air temperature from 30 to 50 oC. The first design is a four circuits 5/16 tube diameter condenser and the second is an eight circuits design with the same diameter. The experimental results showed that, an increase in the ambient air temperature has a negative effect on COP due to the decrease in the overall heat rejected, yet has a positive effect on refrigerant side pressure drop.

Experimental and numerical study of an integrated fin and micro-channel gas cooler for a CO2 automotive air-conditioning

A carbon dioxide integrated fin and micro-channel gas cooler for an automotive air-conditioning was proposed in the work. An experimental study was conducted to investigate the performance of the integrated fin and micro-channel gas cooler. Measured heat capacity for the gas cooler ranged from 1 to 6kW. Effects of various inlet air temperatures and velocities, refrigerant mass flow rates and operating pressures were studied to provide a better understanding on how these parameters affected the CO2 gas cooler performance. A segment-by-segment model was developed to simulate the integrated fin and micro-channel gas cooler. The model predicted the gas cooler heat capacity within 5% and refrigerant-side pressure drop within 8% deviated from the experimental data. The validated model was used to analyze the impact of fin geometry and air-side mal-distribution on the performance of the heat exchanger.

CFD Analysis of Nanorefrigerant through Adiabatic Capillary Tube of Vapour Compression Refrigeration System

Over time attempts have been made to understand the flow characteristics of refrigerants through capillary tubes as well as to seek more thermally efficient working fluids for refrigeration systems. This study investigated the flow of nanorefrigerants through adiabatic capillary tubes of vapour compression refrigeration systems; and afterwards creates numerical models that will account for solution of refrigerant side pressure drop and mass flow rate. Also in this study, a CFD flow analysis was carried out using a CFD simulation/solver such that the results of the simulations obtained were discussed so as to establish a distinction between the conventional and nano-refrigerants. Upon comparison of the CFD results of nanorefrigerants (CuR134a, CuR600a) and the conventional refrigerants (R134a, R600a), the conventional refrigerants were noticed to have more isothermal regions implying that heat was not being transferred quickly enough to raise the temperature of the adjoining region thus proving that the addition of nanoparticles improves the thermophysical properties of the base fluid. Also, based on the results of the study of the flow patterns of both working fluids, the density of pressure contours in the conventional refrigerants was far larger than that of the nanorefrigerant implying that more compressor work and ultimately greater power will be required. The findings from this study were validated with experimental results showing that a CFD analysis tool/method can be employed to understudy the phenomenal changes that take place in nano-refrigerant movement through capillary tubes without recourse to experimentation.

Experimental investigation on minichannel parallel flow condenser performance with R22, R410A and R407C

A performance evaluation of minichannel parallel flow (MCPF) condenser in residential/commercial refrigeration system has been carried out in calorimeter room with wind tunnel in this paper. The heat rejection and pressure drop characteristics for heat exchangers were compared using R22, R410A and R407C as working fluids. The experimental results showed that heat rejection of MCPF condenser with R410A was higher than that of R22 and R407C by 15.6~26.3% and 12.3~22.7% under full and partial load conditions, respectively. The refrigerant side pressure drop trend of R410A in MCPF condenser was smaller than that of R22 and R407C under the same mass flow rate.

Experimental analysis and modeling of lubricant effects in microchannel evaporators working with low global warming potential refrigerants

In any vapor compression system, lubricant is needed to guarantee safe and reliable compressor operation. Indeed, most compressor mechanical failures are generally due to improper oil management, such as, lack of proper lubrication inside the compressor. Since in the usual operating condition of the system a small portion of oil circulates with the refrigerant through the system components, it can be retained inside the heat exchangers leading to oil missing from the compressor. In present article, the lubricant retention characteristics of two microchannel type evaporators for residential air conditioning systems and the oil detrimental effects on evaporator capacity and refrigerant-side pressure drop were investigated. The working fluids used in this study were R410A, as reference refrigerant, and low global warming potential refrigerants R32, R1234yf, and R454B, a new low global warming potential mixture candidate to replace R410A, whereas the lubricant was VG 32 Polyolester oil. The effects of oil mass fractions and degree of superheat were experimentally investigated considering working conditions typically found in air conditioning systems. Finally, the experimental results of the present article provided data that were used to develop and validate a semi-empirical model for microchannel evaporator simulation able to account for the oil presence. The developed model was summarized in the current article and the results were compared with the experimental ones in term of oil retention, capacity, and refrigerant-side pressure drop.

Numerical model of a parallel flow minichannel evaporator with new flow boiling heat transfer correlation

In this paper, a distributed parameter (DP) numerical model with the new proposed flow boiling heat transfer correlation was established for parallel flow minichannel (PFMC) evaporator. DP model validation was made by comparing the measured values obtained on experimental studies, which were conducted under refrigerant mass flow rate range of 34.6–245.6 kg h−1 and evaporation pressure of 200–500 kPa. The effects of four different flow boiling heat transfer correlations on DP model performance were investigated. Results showed that the new correlation predicted 99% of experimental data in ± 30% error bands. Moreover, the DP model with the new correlation yielded the mean absolute error (MAE) of 1.5%, 9.1%, 18.8%, 14.2% and 19.8% in prediction of cooling capacity, outlet air temperature, refrigerant superheat, air side and refrigerant side pressure drop, respectively. The presented DP model can be implemented to evaluate the performance of PFMC evaporator, and therefore can save efforts on component and system design and optimization.

Numerical analysis of the influence of circuit arrangement on a fin-and-tube condenser performance

In the present paper a model for the steady-state simulation of fin-and-tube condenser is developed. The model is based on a finite volume approach that divides each tube into small elemental volumes where mass, momentum and energy conservation equations are solved using the effectiveness-NTU method and with appropriate correlations for void fraction, friction factor and heat transfer coefficient calculation. The model is validated against experimental data on two small condensers finding that the calculated heat transfer rate and refrigerant-side pressure drop agree within ±5% and ±21% respectively to the experimental values. The model is then used to numerically analyse the impact of different circuit arrangements on the condenser heat transfer rate, refrigerant-side pressure drop and refrigerant charge.

Modeling for multi-pass parallel flow condenser with the effect of refrigerant mal-distribution

Performance of multi-pass parallel flow condenser (MPFC) is unavoidably influenced by the refrigerant-side flow mal-distribution (MD) caused by the pressure drop in headers. Thus, a numerical MD model is proposed to predict flow distribution and performance parameters of MPFC with vertical headers and horizontal tubes. The advantage of MD model over UD model (uniform distribution model, refrigerant is assumed to be distributed into each tube evenly by ignoring the effect of header pressure drop) has been indicated by achieving better agreement with experimental data measured from variably-sized condensers A–D and data derived from a published literature. The effects of condenser aspect ratio, pass arrangement (pass number and tube number of each pass) and refrigerant mass flow rate on flow distribution, heat transfer and refrigerant-side pressure drop are investigated. The prediction results indicate that heat capacity varies much less than flow distribution and pressure drop with the change of aspect ratio or pass arrangement. Moreover, for a fixed geometry, a moderate flow rate can be searched by the MD model to obtain well flow distribution, high capacity and efficiency.

Study on thermal performance of micro-channel separate heat pipe for telecommunication stations: Experiment and simulation

A micro-channel separate heat pipe (MCSHP) as a cooling device for telecommunication stations (TSs) was experimentally investigated in this work. A steady-state mathematical model was built and validated with experimental data. The cooling capacity, inlet and outlet of refrigerant temperatures and refrigerant pressure of evaporator section were measured in an enthalpy difference laboratory (EDL). The average relative errors between simulation and experiment were less than 10%. The effects of geometrical design and environment conditions on thermal performance were analyzed using the validated model. As a result of the simulations, the refrigerant side pressure drop decreased by 96.61% at the flat tube height 3.0 mm compared to 1.4 mm. And the air side pressure drop decreased by 94.49%, when fin height ranged from 4 mm to 20 mm. The cooling capacity increased by 50.65% at the fin pitch 3.0 mm compared to 1.0 mm. These factors were of practical engineering importance in optimum design of MCSHP.

Periodical reverse flow and boiling fluctuations in a microchannel evaporator of an air-conditioning system

This paper presents the phenomenon of periodic reverse flow and associated boiling fluctuation found in experiments with a parallel microchannel evaporator used in an R134a air conditioning system. A simultaneous flow visualizations and measurements confirmed the periodic flow reversal. It caused synchronized oscillations of the evaporator inlet pressure and the pressure drop. The magnitude and frequency of oscillations increased with heat flux. Three potential impacts of flow reversal on evaporator performance are identified: 1) moderate liquid maldistribution; 2) reduced heat transfer coefficient; 3) increased refrigerant side pressure drop. Finally, to mitigate impacts of periodic reverse flow, a solution is proposed: to vent and bypass backflow vapor accumulated in the inlet header.

Experimental study on evaporator performance in mobile air conditioning system using HFO-1234yf as working fluid

The widely used evaporators with two different type were experimentally studied in psychrometric calorimeter room using HFO-1234yf as working fluids. The evaporators types are laminated plate and microchannel parallel flow (PF) type. The experimental results compared with R134a system show that cooling capacity has different features in laminated plate evaporator and microchannel PF evaporator under different refrigerant pressure at expansion valve inlet. In general, cooling capacity of laminated plate evaporator of HFO-1234yf is reduced up to 8.0%. But in microchannel PF evaporator, cooling capacity is comparable and/or larger than that of R134a up to 6.5%. The HFO-1234yf evaporator air off temperature deviation is larger than that of R134a evaporator. The air side pressure drop is very close for both evaporators and refrigerants. HFO-1234yf refrigerant side pressure drop is larger than that of R134a for both evaporators.

Investigating the thermal-hydraulic performance of new refrigerant mixtures through numerical simulation of minichannel and microchannel evaporators

In this study, a finite volume, steady-state evaporator model that includes rectangular minichannel and microchannel tubes with louvered fins was developed and validated. The model was then used to compare the thermal-hydraulic performance of experimental ternary refrigerant mixtures to more traditional refrigerants — namely, R-125/R-143a/R-161 (45%/40%/15%) versus R-404a and R-125/R-32/R-161 (34%/15%/51%) versus R-22. R-125/R-143a/R-161 (45%/40%/15%) exhibited significantly higher heat transfer per unit area than R-404a with only a small accompanying increase in the refrigerant-side pressure drop for the cases studied. Similarly, R-125/R-32/R-161 (34%/15%/51%) exhibited significantly higher heat transfer per unit area than R-22 with a minimal difference in refrigerant-side pressure drop. Both refrigerant mixtures also possess lower global warming potentials (GWPs). Based on these criteria (i.e. increased thermal performance and reduced GWPs), both mixtures would serve as suitable replacements for R-404a and R-22 in applications where the slight flammability of these blends was not a concern.

Development and validation of a microchannel evaporator model for a CO 2 air-conditioning system

Abstract This paper presents an analysis/computer model to predict the performance of an evaporator for a CO 2 mobile air-conditioning system. Based on the finite volume method, the model has been developed using mass and energy conservation equations, with emphasis placed on the refrigerant-side heat transfer and pressure drop characteristics. Moreover, the pressure losses both in the headers and at the port inlet of the microchannel tube were also considered. The in-tube refrigerant flow was divided into two-phase region and superheated region, and both dry and wet conditions were considered. Using the newly developed refrigerant-side heat transfer and pressure drop correlations, the model can predict the performance of evaporator with a reasonable accuracy compared with the experimental data. The errors of RMS for cooling capacities and refrigerant-side pressure drops were ±1.9% and ±12.3%, respectively. Furthermore, the effects selecting different refrigerant-side heat transfer and pressure drop models on the performance of evaporator were discussed. The program can be used to analyze and design a CO 2 microchannel evaporator.

Investigating performance of new mini-channel evaporators

In this paper, a new type mini-channel evaporator is proposed and studied experimentally. The evaporator is made of two parallel flow mini-channel heat exchangers connected with a return tank. The experiments are performed on psychrometric calorimeter test bench and the results are compared with test data of the conventional laminated evaporators and used to verify a simulation software. The comparisons show that the new mini-channel evaporator has advantages on volume (17.2% smaller), weight (2.8% lighter) and heat transfer (4.3% higher), but a little penalty on air and refrigerant side pressure drop. The test data have a good agreement with the simulation results using academic software. The effect of louver fin shape on performance is studied and the results show the louver fin with a flat top reduces the air side pressure drop while maintaining the same heat transfer rate as compared to mini-channel evaporator with the corrugated louver fin. The effect of refrigerant side arrangement on the evaporator performance is researched experimentally and the experimental results show that the two-pass flow arrangement is better for heat transfer and refrigerant side pressure drop reduction.

An Extension of a Steady-State Model for Fin-and-Tube Heat Exchangers to Include Those Using Capillary Tubes for Flow Control

A multipath fin-and-tube heat exchanger may use capillary tubes to control the refrigerant distribution among the paths in order to improve the performance of the heat exchanger. The existing heat exchanger models are not developed for those using capillary tubes for flow control, and the existing algorithms for solving the refrigerant distribution among the paths cannot be directly extended to those using capillary tubes for flow control due to the choked characteristics of capillary tubes. In order to extend the existing models for heat exchangers to those using capillary tubes for flow control, two auxiliary equations are introduced for solving the refrigerant distribution among the paths. A share-distribution method is applied for the case that one or several refrigerant flows in capillary tubes are choked during the iteration process for solving the refrigerant distribution among the paths, and a standard-path-based method is applied for the case that all capillary tubes are choked during the iteration process. Experiments are performed for evaluating the extended model and algorithm. The evaluation results show that the difference between the calculated cooling capacity and the experimented one is less than ±5%, while the difference between the calculated refrigerant side pressure drop and experimented one is less than ±15%.

The commercial feasibility of the use of water vapor as a refrigerant

This paper investigates the economic feasibility of a water-based vapor compression chiller with a nominal capacity of 3520 kW (1000 ton). Simplified models of potential cycle configurations are developed and used as a screening tool to identify a baseline cycle, the most attractive configuration for a water-based refrigeration machine. More detailed component-level models are developed to accurately size equipment and predict both the performance and cost of the baseline chiller. These component models address issues that are particularly crucial when water is used in refrigeration cycles, such as compression ratio, compressor discharge superheat and refrigerant-side pressure drop. Where possible, these component models are verified through comparison against the current state-of-the-art technology for large chillers that use R-134a as the refrigerant. The capital cost and expected operating costs are determined in order to quantify the payback and life-cycle costs associated with using water as a refrigerant, relative to traditional halocarbon refrigerants currently in use. Other issues that may have an economic impact on the feasibility of water as a viable alternative to traditional synthetic refrigerants are discussed, including purging and condensation within the compressor. The results show that water-based vapor compression refrigeration systems will not be economically attractive without substantial and successful efforts to develop low-cost, high capacity compressors. The paper provides an indication of the cost targets that must be met in order to make water vapor refrigeration systems practical.