Finned-tube Evaporator Research Articles

Performance improvement of vapor-injection heat pump system by employing PVT collector/evaporator for residential heating in cold climate region

The adoption of vapor injection (VI) cycle could overcome the defects of the conventional one-stage air source heat pump (ASHP) system under extremely low ambient temperature conditions. Nevertheless, the ASHP system with VI cycle still could not operate efficiently due to the low evaporating temperature in the fin-tube evaporator. For such a situation, the PVT (Photovoltaic/Thermal) collector/evaporator could reach higher evaporating temperature attributed to its physical structure, thereby improving the system COP. This study proposes the detailed mathematical model of the vapor-injection heat pump system by incorporating PVT collector/evaporator and verifies the effectiveness of the proposed system. Parametric studies have been then conducted. The proposed system can operate off-grid and its COP can reach 4.0 at the ambient temperature of −10 °C and the solar irradiation of 500 W/m2. The practical electrical efficiency of PV panels achieves at 15.1%, whereas the thermal efficiency of the system is 44.8%. A hybrid control method including three modes has been also proposed based on the results for improving the system performance. The levelized cost of heat (LCOH) of this system is 0.054 $/kWh, which is 51.5% lower than that of electric heating system (0.111 $/kWh).

Performance analysis of liquified petroleum gas (LPG) driven half-cycle air conditioning system

Liquified petroleum gas (LPG) is one of the potential refrigerants to be used in half-cycle air conditioning systems. After evaporation to get air-conditioning, LPG may be further used in combustion applications such as electric-generators, auto-motives and cooking stoves. Expansion device (or a flow control valve) is usually used before the evaporator to make sure that the refrigerant vaporizes in evaporator and provides cooling. The present study reports the investigation of an experimental study to determine the performance of three in-house designed and manufactured evaporators for an air conditioning system using LPG as refrigerant. The evaporators used are finned-tube evaporators having difference in tube arrangements, fin materials, fin spacing and tube dimensions. The thermal energy to evaporate LPG is obtained from air driven by a fan attached to one side of the evaporator. Cold air exiting from the evaporator is then supplied to a cabin having the similar dimensions of an automotive rickshaw. The performance parameters determined for a fixed time duration for different fan speeds include; the cooling effect, contact factor and rate of condensation. The investigations were conducted for different LPG flow rates and evaporative pressures. The test results show that for each flow condition; the cooling effect, contact factor and rate of condensation are also the function of air flow rate passing through the evaporator. Experimentally determined cooling effects were then analytically validated by using the Nusselt number correlations of Grimison model, modified Grimison model and Zhukauskas model for each of the evaporators used.

Evaluation of Air Side Characteristics Performance of Finned Tube Evaporator

Improvement of Refrigeration system performance should be supported by increasing efficiency of its components. Mainly, two components must have excellent performance are condenser and evaporator. In this study examined a finned tube heat exchanger for evaporator application, furthermore research goal is aimed to enhance the evaporator designs in order to get higher efficiency and effectiveness of the heat exchanger. The effects of distance of fin, shape and size of the fin and combination of tube, will be investigated deeply and the heat transfer coefficient is going to examine using U-LMTD method based on some parameters of heat. This paper is still focus on design CFD model to comply the research purpose. The results were found that geometry design and meshing already compatible to get reasonable post processing results. Analysis of the air velocity vector in the heat exchanger showed good agreement toward references. Vortex and turbulence occurs in side are of the tubes and wake or stagnant flow seems in the behind tubes. The next examine is going to concern with model validation toward experimental results and investigated heat transfer coefficient in details.

Evsel Bir Buzdolabında Kullanılan Düz Kanatlı Borulu Buharlaştırıcıların Isıl Performanslarının İncelenmesi

In this study, thermal performance of plain finned-tube evaporators in household refrigerators are investigated numerically and experimentally. The design parameters affecting on the cooling capacity are determined as air flowrate, evaporator temperature, tube alignment, number of tubes and number of fins. In the initial stage, the effects of those parameters on the cooling capacities are simulated by CoilDesigner software. The number of experiments that are necessary to obtain correlations is determined by Minitab software. The experiments are carried out in a no-frost household refrigerator in the off-mode. At the end of each experiment, air side heat transfer coefficients are calculated to generate a Nusselt (Nu) correlation as a function of the mentioned design parameters. The generated correlation of the cooling capacity can predict 95% of the experimental results within a confidence range of 15%. The correlation of the cooling capacity is converted to the Nusselt number correlation which is found to be consistent with available data in literature. The experiments reveal that the cooling capacity is dominantly affected by the evaporator temperature followed by air flow rate, tube alignment, number of tubes and the number of fins.

Investigation of ultrasonic array defrosting method based on synergism of standing wave intermittent phase-stagger and multi-frequency for finned-tube evaporator

Frost dramatically deteriorates the operating performance of an air source heat pump unit and leads to additional energy consumption. Results of our previous study show that intermittent ultrasonic vibration can greatly improve the coefficient of performance (COP) under frosting condition. To meet the ultrasonic defrosting demands of large-scale evaporators and to eliminate the standing wave effect of ultrasonic defrosting, an optimised ultrasonic array method for defrosting is proposed based on synergism of standing wave intermittent phase-stagger and multi-frequency excitation. Firstly, the surface vibration characteristics and defrosting effect difference of copper plate with standing wave intermittent phase-stagger in random mode and regular mode are investigated using a plate model. On this basis, the surface vibration characteristics and defrosting effect of a finned-tube evaporator are analysed in both phase-stagger modes. Finally, to optimise further the defrosting effect of the ultrasonic array, the standing wave intermittent phase-stagger and multi-frequency excitation are integrated to realise chaotic motion of the standing wave on the evaporator surface. Finite element analysis results and experimental results indicate that the standing wave effect of ultrasonic array defrosting can be eliminated using random intermittent phase-stagger, and the defrosting efficiency under random intermittent phase-stagger is superior to the regular method. Moreover, when arrayed ultrasonic transducers are controlled by synergistic random intermittent phase-stagger and multi-frequency excitation, there is no dead defrosting angle on the evaporator surface, the average frost layer thickness is approximately 0.282 mm, and it is 81.1% of mode I (intermittent and multi-frequency excitation).

The effect of PM2.5 air pollution on the frosting process of outdoor finned-tube evaporator

Air source heat pumps (ASHPs) have worldwide applications due to their superior performance in energy saving and environmental friendliness. Two challenges of frosting and fouling for outdoor coils of ASHPs were of attention individually. This study investigated the effect of PM2.5 air pollution on the frosting process of the outdoor evaporator of ASHPs. The system performance and frosting characteristics of ASHPs with different environmental parameters were measured experimentally. The results showed that PM2.5 could significantly promote the frosting process when the concentration was greater than 146 μg/m3. Under the condition of severe pollution, both the experimental data and evaporator surface images reflected the shortening of the frost column generation period and frost layer growth period. On the theoretical basis of the three periods of the frosting process, the frost layer growth period was further divided into transition period and frost thickening period. At the same time, in order to quantitatively analyze the difference of frosting rate between severe pollution condition and no pollution condition, frosting rate ratio (FRR) was defined to reflect the influence of PM2.5 on frosting in different periods. Based on the analysis of the experimental results under different temperature and humidity conditions, it was considered that the frosting zone could be divided into two regions. In the Region I where the moisture content of the air was very low, the effect of PM2.5 on the frosting was weakened, and the FRR was positively correlated with the moisture content of the air. While in Region II with a relatively high moisture content of air, the FRR was positively correlated with the mass ratio of PM2.5 to moisture.

Heat exchanger optimization of a domestic refrigerator with separate cooling circuits

In this study, the effects of evaporator type and geometry, capillary tube diameter and capillary length on the cooling system performance and energy consumption in the bypass two-circuit cycle have been examined experimentally. In the experiments, wire on tube and finned tube evaporators were used in the fresh food compartment of a 500 L top mount freezer refrigerator. 0.66 mm and 0.8 mm capillary tube diameters and 3000 mm and 3500 mm capillary tube lengths were selected as other design parameters. Experiments were performed according to IEC 62552 household refrigerating appliances standard at 25 °C ambient temperature when the average temperature of the fresh food compartment is set to 5 °C and the warmest temperature of the freezer compartment is −18 °C. The Design of Experiments (DOE) technique which is a six sigma method has been used to determine simultaneously the individual and combined effects of parameters on the refrigerator performance. General linear model method, which is a useful framework for comparing how several variables affect different continuous variables, was chosen to identify the effectiveness of the parameters. According to the experimental results, it was found that the most effective parameter of the system is the evaporator type. Finned tube evaporator with 12 tube passes were found the best option for the bypass two-circuit refrigeration cycle among the 8 different options. The optimum cooling performance and the lowest energy consumption were provided with 0.66 mm capillary tube diameter and 3500 mm capillary tube length in the experiments.

Experimental study of a direct expansion solar-assisted heat pump (DX-SAHP) with finned-tube evaporator and comparison with conventional DX-SAHP

Direct expansion solar-assisted heat pump system (DX-SAHP) is proven to be an effective energy-saving application. Due to the fact that the traditional bare-plate type direct expansion solar-assisted heat pump occupies large footprint and is inefficient under low solar irradiation, a modified direct expansion solar-assisted heat pump system with finned-tube evaporator is designed and tested in the enthalpy difference lab with a solar simulator. The effects of ambient temperature, relative humidity, solar irradiation, and wind speed on the performance of the finned-tube type system are investigated. Furthermore, the performance of finned-tube type system and bare-plate type system is compared and analyzed under identical experimental conditions. It is revealed that raising ambient temperature, solar irradiation and wind speed can effectively improve the heating performance of the finned-tube type system. In comparative analysis, frosting enhances the overall performance of bare-plate type system, while the performance of finned-tube type system significantly declines due to the heat transfer deterioration caused by the frosting. The COP of finned-tube type system decreases from 1.72 to 1.54, when the relative humidity rises from 50% to 90%. For bare-plate type system, with the relative humidity rising from 70% to 90%, COP increases by 16.3%.

Air-side heat transfer and pressure drop characteristics of microchannel evaporators for household refrigerators

Microchannels are likely to be the next heat transfer technology for household refrigerating applications, especially due to their compact design and high heat transfer rate per unit of volume. In contrast to the conventional tube-fin evaporators, the available heat transfer and pressure drop correlations are not well-stablished yet. This study introduces a new microchannel evaporator design to be used in ‘no-frost’ household refrigerators. The performance of sixteen evaporators prototypes with distinct geometric characteristics were evaluated experimentally in an open-loop wind-tunnel calorimeter facility coming up with data for the overall thermal conductance and the air-side pressure drop data as a function of the air flow rate. Empirical correlations for Colburn j-factor and friction f-factor were devised in terms of the Reynolds number and key compact heat exchanger parameters, being able to predict 90% of the experimental counterparts within ±10% and ±20% error bands, respectively. Furthermore, comparisons against a typical no-frost tube-fin evaporator was carried out with respect to air-side pressure drop and overall thermal conductance, which indicated a promising potentiality of application in household refrigeration appliances.

Experimental study of an air humidity absorption cycle based on the MHI

An office drinking water cooler is converted to a humidity absorption device. The reservoir-type evaporator of the water cooler is separated, and a finned-tube evaporator is installed. A channel is installed at the inlet of the evaporator. A fan, a cool-mist humidifier, and a heater are installed inside this channel, where the amount of humidity and heat production can be adjusted. Several pressure gauges are installed at different locations of the cycle and monitor cycle performance while working. Pressure variations in different locations of the cycle are measured at various inlet air conditions. The MHI is defined as the ratio of condensation enthalpy to the total given heat. Changes of this index are evaluated by changing the input conditions. Results show that with increasing the air temperature, the condenser and evaporator pressure increases. Results of absorbed water in various MHIs show that with increasing this index, the amount of absorbed water increases. The graph of the absorbed water based on the MHI can be used to estimate the amount of water collected from this device under different climatic conditions. The amounts of collected water from this device for several different cities of Iran are presented. Results show that in high MHIs for a device with a quarter horsepower, the water production rate can reach to 250 g h−1. Also, if the device is working continuously in these conditions, it can produce about 4 kg of water per day.

Performance analysis of a novel air source hybrid solar assisted heat pump

Aiming to enhance the performance of heat pump under adverse working conditions by utilizing solar energy and air source simultaneously, a novel air source hybrid solar assisted heat pump (AS-SAHP) system with finned tube evaporator and collector evaporator connected in series has been proposed. A mathematical model is established to characterize the behavior of the proposed system. Then the influence of key factors such as ambient temperature, solar irradiation, and evaporating area has been analyzed. The results indicate that the AS-SAHP system can operate efficiently under a range of working conditions, with the average COP of 2.71 at the solar irradiation of 100 W/m2 and ambient temperature of 10 °C. Due to the heat transfer characteristics of finned tube evaporator and collector evaporator existing striking difference, the hybrid heat source in AS-SAHP system can play the complementary role at various operating conditions. To assess the irreversibility of the AS-SAHP system, the exergy loss ratio of each component has been analyzed. The results illustrate that most of the irreversibility is yielded in compressor and condenser. At last, the performance of AS-SAHP has been compared with that of conventional ASHP and DX-SAHP, and it is proven that the AS-SAHP system shows the optimal performance.

Optimal design of fan-supplied tube-fin evaporators subjected to frosting conditions aiming at minimum energy consumption

This work is focused on the optimal sizing of fan-supplied tube-fin evaporators for light commercial refrigerators operating under frosting conditions. Quasi-steady-state algebraic models were put forward for the refrigeration cycle and its components for computing not only the cooling capacity depletion due to the evaporator frosting over time, but also the changes experienced by the evaporating temperature and its effect on the compression power. Transient algebraic models were also advanced to account for the temperature and humidity variations inside the refrigerated compartment, and so the compressor runtime. Both component-level and system-level optimization exercises were performed for sizing the evaporator. The former was focused on the local entropy generation minimization, considering the irreversibilities due to heat transfer, mass transfer and pressure drop that take place at the evaporator, whereas the latter was aimed at the overall energy consumption of the refrigerator, including not only the compression power, but also the heat dissipation within the refrigerated compartment by the defrost heater and the evaporator fan. Notwithstanding the time-averaged cooling capacity and the evaporator envelope have been held constrained, the component-level and the system-level optimization procedures led to different evaporator designs.

Determination of defrosting start time in an air-to-air heat pump system by frost volume calculation method

Naturally, frost forms at fin-tube evaporator surface when refrigerant below 0 °C flows inside the heat exchanger. Frost tends to block the evaporator, hinders efficient heat exchange and degrades the performance of the system. The determination of the start time for defrosting is one of the essential issues that most of air heat pumps are confronted with because it is directly connected to energy consumption. Many types of sensors can be installed for frost detection but they are usually not cost-effective or related logic is not so physically presented. In this study, a novel frost volume calculation method is suggested in order to determine the start time for defrosting in an air-to-air heat pump system. The detailed explanation of the method is presented and verified with experimental data. With this method, the start timing of defrosting can be appropriately judged. At the time of defrosting, the heating capacity decreased from 16.2 to 39.3% and the COP reduced from 12.3 to 26.4% compared with maximum values. The blocking ratio from image processing at the time was between 0.83 and 0.97.

Theoretical study on the heat transfer characteristics of a plain fin in the finned-tube evaporator assisted by solar energy

Finned-tube evaporators have been widely used in air source heat pumps. Coating a solar selective absorbing coating on the fin surface is proved an effective approach to improve the heat transfer effects of finned-tube evaporators using solar energy. However, currently, most attentions were paid to the performance improvement of heat pump systems assisted by solar energy, scarce of attentions were paid to the heat transfer characteristics of this kind of fin under solar radiation. Therefore, the heat transfer characteristics of this kind of fin under solar radiation were studied using the heat transfer theory. Based on the heat transfer models, the theoretical solution of temperature field was deduced, and its accuracy was validated by numerical simulation. The effects of various factors on the heat transfer characteristics of a plain fin were theoretically analyzed. The results showed that the solar radiation would lessen the convective heat transfer capacity and fin efficiency, and even make part of solar energy to release into the environment. The maximum loss of solar energy accounts for about 12.7% of the total solar energy, and twice the fin height only generates 60% increase of the total heat transfer capacity. In the condition of larger fin height and solar radiation, the smaller convective heat transfer coefficient will produce more total heat transfer capacity. This study is helpful to analyze this kind of heat transfer problem and guide the optimization design of finned-tube evaporators assisted by solar energy.

Optimal defrost cycle for air coolers revisited: A study of fan-supplied tube-fin evaporators

The performance of fan-supplied tube-fin evaporators is assessed in the present paper aiming at identifying the key parameters which affect the cooling capacity under frosting conditions and defrost cycle performance. The analysis was carried out by means of a mathematical model comprised of a frost formation submodel, which was developed by the authors in a prior publication, together with an improved defrosting submodel. A performance evaluation criterion to account for the periodic defrosting, the so-called cooling cycle efficiency, was employed as an objective function for the defrost cycle optimization. The results pointed out that there does exist an optimal runtime which maximizes the evaporator performance even for variable air flow rate conditions. Moreover, it was observed that the optimal runtime rises either by increasing the heat transfer surface (while keeping the fin spacing) or the coil surface temperature, and diminishes by either increasing the power supplied to the heater, or the relative humidity of the inlet air.

Experimental investigation of the effect of air velocity on a unit cooler under frosting condition: a case study

Finned tube evaporators are used in a wide range of applications such as commercial and industrial cold/freezed storage rooms with high traffic loading under frosting conditions. In this case study, an evaporator with an integrated fan was manufactured and tested under frosting conditions by only changing the air flow rate in an ambient balanced type test laboratory compared to testing in a wind tunnel with a more uniform flow distribution in order to detect the effect of air flow rate on frosting. During the test, operation was performed separately based on three different air flow rates. The parameters concerning test operation such as the changes of air temperature, air relative humidity, surface temperature, air-side pressure drop and refrigerant side capacity etc. were followed in detail for each air flow rate. At the same time, digital images were captured in front of the evaporator; thus, frost thicknesses and blockage ratios at the course of fan stall were determined by using an image-processing technique. Consequently, the test and visual results showed that the trendline of air-side pressure drop increased slowly at the first stage of test operations, then increased linearly up to a top point and then the linearity was disrupted instantly. This point speculated the beginning of defrost operation for each case. In addition, despite detecting a velocity that needs to be avoided, a test applied at minimum air velocity is superior to providing minimum capacity in terms of loss of capacity during test operations.

Study on the performance of small tube diameter R290 fin-tube evaporator

Reducing the diameter of copper tube is beneficial to reduce the amount of metal material used in heat exchanger. The change of the diameter of the copper tube can not only reduce the volume of the heat exchanger, but also the filling amount of the refrigerant can be reduced. In this paper, the R22 fin-tube evaporator with 7mm tube diameter is improved. The diameter of the tube is reduced to 5mm, and the environment protection refrigerant R290 is used. Diameter reduction may cause the pressure drop increases, and the new branch number is 5 by calculating. Using the simulation software EVAP-COND to simulate the heat transfer performance and compare the refrigerant flow, pressure drop, copper usage with the evaporator before improved. Result show that the heat transfer effect of R290 evaporator is better than that of R22 system, and the heat transfer effect of system is the best. Under the premise of no increasing pressure drop, the heat transfer of the R290 fin-tube evaporator with a small tube diameter increased by 34.62%, refrigerant flow rate, pressure drop, copper and aluminum consumption decreased by 28.7%, 43.1%, 28.6%, 30%.

Effect of frost morphology on the thermal-hydraulic performance of fan-supplied tube-fin evaporators

In refrigeration applications, frost build-up on the evaporator surfaces depletes the thermal-hydraulic performance due to an increase in the thermal resistance and the blockage of the air flow. These negative effects depend not only on the total amount of frost, but also on the frost morphology that affects the density and, therefore, the thermal conductivity of the frost layer. In this paper, the studies of frost accretion on fan-supplied tube-fin evaporators carried out in previous works are advanced to investigate the effects of the frost morphology on the thermal-hydraulic performance of fan-supplied evaporators. A visual inspection of the frost deposition was carried out to characterize the frost morphologies under different operating conditions. Moreover, a mathematical model was used to figure out how the operating conditions and the geometric parameters affect the cooling capacity and the pressure drop. The visualization confirmed that the operating conditions have a strong effect on the in-situ frost morphology. In addition, the numerical results not only illustrate the impact of the frost morphology on the evaporator performance, but also point out that the number of defrost operations may be reduced if a proper heat exchanger design is adopted.

Working fluid charge reduction, part 1: CO2 fin-tube evaporator designed for light commercial appliances utilizing flow pattern based phenomenological prediction models

The present study focused on improving the thermal performance of CO2 evaporators while reducing their volume and thus significantly reducing the working fluid charge. To achieve this, the leading two-phase flow pattern and physically-based prediction models for flow boiling of CO2 and void fraction were selected from the literature, together with the leading air-side flow and heat transfer methods, and finally those for modelling oil effects on CO2 flow boiling. The effects of tube size, shape and wettability for significant performance improvements, miscible oil concentration, etc., were addressed while staying within the beverage industry's fabrication and operational limits. Compared to an actual existing design, the volume and refrigerant charge of the CO2 evaporator were reduced by 50% and 58.7%, respectively. The adverse influence of oil was found to reduce the mean CO2 side heat transfer coefficient and to increase the CO2 pressure drop by up to 11% and 94%.

Modeling of Finned-Tube Evaporator Using Neural Network and Response Surface Methodology

A new response surface methodology (RSM) based neural network (NN) modeling method is proposed for finned-tube evaporator performance evaluation under dry and wet conditions. Two RSM designs, Box–Behnken design (BBD) and central composite design (CCD), are applied to collect a small but well-designed dataset for NN training, respectively. Compared with additional 7000 sets of test data, for all the evaporator performance including total cooling capacity, sensible heat ratio and pressure drops on both refrigerant and air sides, the standard deviation (SD) and coefficient of determination of trained NNs are less than 2% and higher than 0.998, respectively, under dry conditions while those are less than 4% and greater than 0.974, respectively, under wet conditions. Classic quadratic polynomial response surface models were also included for reference. By comparison, the proposed model achieves higher accuracy. Finally, parametric study based on the trained NNs is conducted. This new method can remarkably downsize the training dataset and mitigate the over-fitting risk of NN.