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Performance study of the indirect evaporative air cooler and heat recovery exchanger in air conditioning system during the summer and winter operation
... Above all, the current carbon neutrality demand is favoring sustainable solutions as recommended by Dai et al. [31]. Keeping in view the extensive applications and advantages, significant research is conducted to investigate and improve IEC system performance [32][33][34]. However, the commercial-scale deployment of ...
... In ventilation mode, COP can be improved 10%-15% by unbalancing the air flow. Moreover, in recirculation COP increases by 50% (Anisimov et al., 2015). A numerical technique is used to execute the thermal calculations of an indirect evaporative cooler (IEC) based on a model of combined parallel heat and mass exchanger with counter flow regenerative arrangement. ...
With increasing apprehensions over global warming and environmental issues, the need to develop renewable energy is becoming more critical to secure our future energy needs. Solar energy is the most abundant source of energy and is easily accessible. However, making efficient use of solar energy is not an easy task. Energy materials, especially in their micro and nanoscale, have an excellent potential for absorbing, transferring and storing solar energy when they are dispersed in an aqueous medium. The increased surface area to volume ratio of energy materials at nanoscale exhibits extraordinary characteristics. Various applications relevant to heat transfer, energy conversion, and storage have increasingly used nanoparticles due to their ability to absorb, store, and carry heat.
However, successful deployment of materials in energy harvesting and storage applications must also consider some of the very fundamental challenges, including but not limited to sedimentation, entrainment, stability, and life of these potential energy materials. Additionally, there are novel applications of newly developed specialized materials in solar energy capture, transport and storage. This topic is to circumscribe all challenges, innovative applications and numerical studies in materials for energy capture, transfer, and storage to have a safe future in terms of solar energy utilization
... In ventilation mode, COP can be improved 10%-15% by unbalancing the air flow. Moreover, in recirculation COP increases by 50% (Anisimov et al., 2015). A numerical technique is used to execute the thermal calculations of an indirect evaporative cooler (IEC) based on a model of combined parallel heat and mass exchanger with counter flow regenerative arrangement. ...
This paper presents the integrated performance of a solar-assisted desiccant dehumidifier along with Maisotsenko cycle (M-cycle) counter flow heat and mass exchanger. This system handles latent load and sensible load separately. The hybrid configuration of solar thermal collectors was analyzed for efficiency of solar collectors and solar fraction. High consumption of fossil fuels, which are already present in a limited amount, is also associated with environmental problems and climate change issues, as these increase the chances of global warming. These issues demand of us to shift towards renewable energy resources. Increase in world energy use results in a number of environmental problems, such as climate change, in addition to global warming and ozone depletion. In building services, HVAC systems are major concerns. To overcome the requirement, conventional air conditioning and vapor compression systems are mainly used for air conditioning, although these also have some environmental problems. Solar thermal applications in combination with other renewable-energy-dependent cooling practices have generated a huge interest towards sustainable solutions, keeping in view several techno-economical, environmental, and climatic advantages. The experimental investigation reveals that the maximum outlet temperature and efficiency of solar thermal collectors was 87°C and 56% respectively. The maximum cooling capacity of the system is evaluated at 4.6 kW.
... A number of similar studies performed in laboratory conditions [47][48][49] focused on different technical aspects of air handling units and heat recovery issues under selected environmental conditions. It was clear that they could not fully match real exploitation circumstances. ...
Heat recovery from ventilation air is proven technology resulting in significant energy savings in modern buildings. The article presents an energy analysis of an air handling unit with a cross-flow heat exchanger in an office building in Poland. Measurements were taken during one year of operation, from 1 August 15 to 31 July 16, covering both heating and cooling periods. Calculated annual temperature efficiency of heat and cold recovery amounted to 65.2% and 64.6%, respectively, compared to the value of 59.5% quoted by the manufacturer. Monthly efficiency of heat recovery was from 37.6% in August to 68.7% in November, with 63.9% on average compared to 59.5% declared by the manufacturer. Cold recovery was from 63.3% in April to 72.8% in September, with 68.1% annually. Calculated recovered heat and cold amounted 25.6 MWh and 0.26 MWh, respectively. Net energy savings varied from −0.46 kWh/m2 in August, when consumption by fans exceeded savings, to 5.60 kWh/m2 in January.
... Apart from experimental developments, extensive mathematical modeling and simulation have been used to study geometric and flow configurations along with heat and mass transfer analysis [16][17][18][19][20]. For different heat and mass exchanger designs based approach has also been proposed for varying inlet conditions. ...
Air-conditioning load is generally composed of sensible and latent parts. Currently, various stand-alone electric and heat driven HVAC systems serve the purpose with each having performance limitations while managing cumulative load. However, integration of both electric and heat driven systems can be efficient especially if sensible and latent loads are handled separately. Here an integrated solar assisted cooling system is proposed consisting of a solid desiccant system for handling latent load and a Maisotsenko cycle (MC) based evaporative cooling system for sensible loads. The experimental setup consists of a purposely designed hybrid arrays of solar thermal collectors, a solid desiccant wheel with heat recovery and a coupled indirect MC evaporative cooler in cross flow arrangement. The integrated system is tested for the dehumidification effectiveness, dew point effectiveness, thermal COP, and cooling capacity. The resulted average cooling capacity of the system is around 3.78 kW with average COP of 0.91 at solar fraction of about 70%. The uncertainties for cooling capacity and COP are ±8.6% and ±9.3%, respectively.
... A global optimization method based on the entransy theory was proposed to be applied on the evaporative cooling systems by Yuan and Chen [28]. The optimal NTU p to achieve maximum efficiency of IEC was discussed among three commercial prototypes [29] as well as a cross-flow dew-point IEC [30]. Pandelidis and Anisimov [31] conducted an optimization study on the hole arrangement and size of the secondary and primary part in the dry channels of M-cycle IEC. ...
The indirect evaporative cooler (IEC) is a low-carbon device which cools the air with water evaporation. Its performances in dry regions have been investigated intensively. However, its application in hot and humid regions, where the IEC is used as a pre-cooling device in an air-conditioning system, is still at developing stage. The exhausted air from air-conditioned space is humidified and used as secondary air to pre-cool the fresh air. As the dew point temperature of the fresh air is high, condensation may occur in the dry channels. However, the parameter sensitivity analysis of IEC with condensation is lacking. Besides, the optimized IEC configuration may be different from that of dry regions because of distinguished air handling process. So the sensitivity analysis among seven parameters by orthogonal test was conducted based on the experimental-validated IEC model emphasizing on condensation condition. The optimization was then conducted to the most influential and engineering controllable parameters. The results indicate the channel gap and cooler height are the key influential factors on IEC thermal performance. The optimized channel gap is 2–3 mm and 3–4 mm under condensation and non-condensation state, respectively. The optimized NTUp is 4–7 and 3–5, respectively.
... A two-stage evaporative cooling system with higher cooling ability was tested and evaluated [30,31]. The performances of IEC and heat exchanger in summer and winter operation were theoretical analysis and measured [32]. ...
The indirect evaporative cooler (IEC), regarded as a low-carbon cooling device, was proposed as fresh air pre-cooling and heat recovery device in the air-conditioning system to break the region limitation of application in hot and humid areas. In this hybrid system, the exhausted air with low temperature and humidity from air-conditioned space is used as secondary air to cool the inlet fresh air. As the dew point temperature of the fresh air is high, condensation may occur in the dry channels. However, the modeling of IEC with condensation has been seldom studied and corresponding parameter study is also lacking. So the paper established a new numerical model taking the condensation from primary air into consideration. The model was validated by the published data from literature with good agreement. Nine parameters were analyzed in detail under three operation states (non-condensation, partial condensation and total condensation) using four evaluation indexes: condensation ratio, wet-bulb efficiency, enlargement coefficient and total heat transfer rate. The results show that the condensation lowers the wet-bulb efficiency of IEC but improves the total heat transfer rate due to dehumidification.
... Для ефективності збільшення повітряного охолодження запропоновано враховувати атмосферні фактори в умовах жаркого та сухого клімату з встановленням спеціального випаровуючого осушувача [2], або за умови різних значень відносної вологості та зміни вартості на гідрофільні матеріали [3]. Іншими шляхом є пошук методу для вирішення задачі математичного моделювання, в якій пропонується виконувати порівняння типового повітряного холодильника з рекуперативним, без застосування експериментальних дослідів, але після попереднього визначення переважних умов роботи [4], чисельне моделювання теплообмінних апаратів незалежно від пори роки, в якій працює відбувається робочий процес [5]. Проблему підвищення ефективності використання повітряних холодильників пропонується вирішувати також за рахунок застосування нових композитних матеріалів з удосконаленими характеристиками [6], та з обов'язковим врахуванням робочого середовища теплообмінних апаратів з допустимими технологічними параметрами [7]. ...
The paper presents findings on the efficiency of air coolers of the equipment for primary oil refining. The research has revealed that in the process of condensation of light fractions of oil products, air coolers emit into the environment a high-temperature exhaust gas, which should be used for regenerative heating in heat exchangers. Gases with a complex hydrocarbon composition are supplied with their thermal parameters at the operating temperatures. The heat capacity that can be used to transfer heat energy from hot to cold flows reaches 20 MW. This allows using the heat of the exhaust gases and saving primary energy expended in the production process in the facility for primary processing of oil. Temperature pressure in the air coolers can be reduced by regenerative use of their thermal potential energy for district heating.
Indirect evaporative cooler (IEC) applied as an exhaust air heat recovery component for fresh air pre-cooling in air-conditioned zone, which successfully extends its application range to hot and humid areas. To explore more effective application method of the heat recovery IEC, an energy and exergy transfer model for IEC operating as a heat recovery devices was proposed in this study. The thermal performance and exergy transfer characteristics of three basic types of IECs: cross-flow, counter-flow, and parallel-flow IECs were numerically investigated. The results show that the irreversible heat transfer in the primary air channels and the irreversible mass transfer in the secondary air channels are the main causes of exergy loss. These two factors account for more than 90% of the total exergy destruction in all the three types of IECs. To alleviate this situation and improve the energy efficiency of the IEC, two two-stage modified heat recovery systems which combine IEC with sensible heat exchanger and energy recovery exchanger were proposed and numerical analyzed. The simulation results show that, for the modified systems, the exergy transfer efficiency is increased by 104.6% and 131.7%, while the heat transfer capacity is increased by 47.6% and 48.5%, respectively.
Driven by the economic outbreak and the growing demand of thermal comfort, the energy consumption of air conditioning (AC) keeps increasing promptly. Indirect evaporative cooling, as an energy-efficient and eco-friendly AC approach, attracts attention in recent years. However, this traditional technology has some drawbacks associated with its working principles. For instance, the limited output temperature constrains its application scopes. Insufficient evaporation due to the poor wettability on the wet channel surface significantly affects the cooling performance. This study provides an updated review of the research progress for solving these problems. Specifically, lower-temperature air can be produced by dew-point evaporative coolers. Innovative wicks with different materials strengthen the surface wettability as well as promote evaporation. Besides, hybrid systems and system optimizations can ensure cooling performance under hot-arid and hot-humid weather conditions. With the recent developments and foreseeable future opportunities to cope with these problems, IEC is expected to make more contributions to reducing the energy consumption of AC in buildings.
This paper presents an experimental study of a novel household ventilator with exhaust air heat pump enhanced by indirect evaporative cooling. In this study, a prototype was made according to this theory and tested on the test rig. The test was carried out to evaluate the energy performance of the system during the cooling season. During the test, the thermal performances of the prototype under four different operating modes were comprehensively studied. In the typical summer conditions, the prototype operating as an indirect evaporative cooler followed by an evaporative condenser heat pump shows the highest cooling capacity at 4.27 kW and energy efficiency ratio (EER) at 5.12, which is a great performance improvement compared with that of conventional operating methods. The results of variable conditions tests show that this system can operate efficiently under different summer ambient conditions with the EER varying from 4.16 to 5.65. Besides, a comparison with other prototypes found in existing literature was also made in this study. Give consideration to compact and energy efficiency, the novel ventilation system shows a good academic value and application prospect in energy saving air-conditioning domain.
Thermal effectiveness was performed as preliminary research in shell and coiled tube heat exchanger with addition of Al 2 O 3 nanoparticles. In the present study, the effect of small concentration of Al 2 O 3 nanoparticles analysis for steady state. Water is selected as the working fluid of cold fluid and refrigerant R-22 as hot fluid. The present work also includes the effect of nanoparticles on logarithmic temperature difference (LMTD), amount of heat absorbed by the water as a time series function. Based on the result, the effectiveness of water-Al 2 O 3 nanoparticles as cold fluid shows good agreement on heat transfer parameter enhancement. The heat exchanger effectiveness increase until 2.2% compare to that of heat exchanger without Al 2 O 3 nanoparticles. This phenomenon indicates an increases of heat transfer process inside heat exchanger. The application of water-Al 2 O 3 nanoparticles on shell and coiled tube heat exchanger enhanced the convective heat transfer passively.
Heating, ventilating and air conditioning systems (HVAC) are essential for providing a comfortable and healthy indoor environment for human beings and required conditions for manufacturing and operations in industrial sectors. Maximizing energy efficiency and minimizing the environmental impact of HVAC systems has become more and more important to address the issues of sustainability and environmental protection. With the recent climate warming, the demand for air conditioning is expected to hit a new high. The dilemma of high demand while simultaneously reducing energy consumption requires a full-scale campaign to develop new technologies for HVAC systems.
Recovering wasted energy is one way to reduce the energy consumption in HVAC systems. In a modern building, the energy lost through ventilation can be more than 50% of the total thermal losses. In this study, two advanced technologies were investigated, the air-to-air heat recovery ventilation (HRV) and the indirect evaporative cooler (IEC). The energy wasted by the exhaust air was recovered by pre-cooling the supply air when the heat was transferred from the fresh air to the exhaust air in an HVAC system.
A test rig was designed, manufactured, modified and calibrated to meet the special aims of this research. It is a cost-effective and inventive test facility with the flexibility to be used for both dry (HRV) and wet (IEC) operation modes.
The experimental study of the HRV was conducted by testing two polymer heat exchangers with two different plate geometries, one with a flat plate and the other with a dimpled surface plate. The key aims were to understand the effect of the surface geometry of the plates on the performance of the air-to-air heat exchanger. Regarding the performance of the HRV, the sensible efficiency of the heat exchanger with the dimpled surface was 50% to 60% higher than that of the flat surface plate heat exchanger at lower air velocity and higher air initial temperatures. The highest COP of the heat exchanger with dimpled surface heat was 6.6; achieved under primary air operating temperature of 32.6 °C.
In the investigation of the IEC system, the main aim was to find the effect water spray arrangement on the performance. Experiments were conducted with three water spray modes: (1) external spray, (2) internal spray and (3) mixed internal and external sprays. An ANSI/ASHRAE Standard 143-2015 evaluation indices with changing the primary air condition parameter (Primary air inlet temperature and velocity) were applied to evaluate the system performance with water spraying variation. The results show that the internal spraying mode performs better than the external spraying mode does in terms of the wet-bulb efficiency, cooling capacity and the COP of IEC. The mixed-mode improves the performance further but increases the water evaporation rate. The innovative internal spray system benefits the sensible heat transfer in the IEC system. The cooling load capacity increased by 12.5% with the internal spray mode and 25% with the mixed mode. The COP varies in a wide range under different water spraying modes and operating conditions with the highest of 19.12 achieved in the mixed-mode.
CFD modelling was also performed to further investigate the IEC system. An Eulerian-Lagrangian 3-D numerical model was developed which was capable of simulating a representative IEC system with realistic nozzles in the mixed-mode operation. A solid-cone spray representation in Eulerian-Lagrangian numerical models was applied in order to replicate real nozzles characteristics in the simulation. The model was verified by experimental results. The verified model was employed to study the effects of primary air temperature, humidity ratio and velocity on the performance of the indirect evaporative heat exchanger (IEHX). Operating under variable inlet air temperature and humidity conditions, simulated results showed that the wet-bulb effectiveness for the mixed mode ranged from 68%% to 80% with a primary air supply temperature near dew-point temperature at primary air temperature lower than 28 °C and velocity less than 1.5 m/s.
Indirect evaporative cooling is recognized as an alternative air-cooling solution with low carbon potential and considerable energy efficiency. An indirect evaporative cooler (IEC) can handle both of the sensible and latent cooling loads because of possible condensation when it is used as a precooling unit in an air-conditioning system in hot and humid regions. Cross flow and counter flow, as two basic flow configurations of an IEC, differ in condensation behavior that affects their cooling performance. In this paper, a novel 2-D model of cross flow IEC considering condensation is established and validated. The performance of the cross flow and counter flow IEC is thoroughly compared under the same configuration. The channel gap and height to length ratio (H/L) are optimized to provide references for the design and operation of the IEC under condensation conditions. Results show that under the same operating conditions, the condensation ratio of counter flow IEC is 2–15% higher than that of the cross flow IEC, leading to 2–7% decrease of wet-bulb effectiveness. The difference in the total heat transfer rate between the two configurations is less than 5% when the number of transfer units (NTUp) is lower than 3.1. For cross flow IEC, there is an optimal value in H/L among 0.4–0.8 considering the cooling capacity and energy consumption.
This paper numerically investigates the performance of the three highly efficient, advanced indirect evaporative air coolers: the “classical” cross-flow Maisotsenko cycle (M-Cycle) heat and mass exchangers and two novel combined M-Cycle air coolers proposed by authors. The novel heat and mass exchangers are based on a combination of parallel and counter-flow and cross and counter-flow schemes. An original mathematical models were developed and validated against experimental data.
Analysis of the results allowed establishing appropriate operational parameters (i.e. airflow ratios and initial sections relative length) for the new heat exchangers. The novel units were compared with cross-flow M-Cycle heat and mass exchanger. The main conclusion is that proposed solutions are characterized by higher cooling efficiency than the cross-flow M-Cycle unit. Combined cross-regenerative counter flow heat and mass exchanger obtained highest overall performance.
Results are presented from previous research on direct evaporative coolers for residential use in arid regions of Israel. These results emphasize both the economic superiority of a desert cooler over the air-conditioner, and the limited amount of comfort that it is capable of providing. As consequence, a feasibility study is presented, concerning possible utilization of a direct-indirect evaporative cooler in residences. It is shown that such a system can provide a significantly higher level of thermal comfort. Furthermore, it can also provide cooling relief in more humid regions. The thermo-economic model developed for the desert cooler is used to estimate the value of such a system as an alternative to the air-conditioner.
In this paper heat transfer of air-water-vapour mixtures in plastic crossflow heat exchangers is studied theoretically and experimentally. First, a model for heat transfer without condensation is derived, resulting in a set of classical differential equations. Subsequently, heat transfer with wall condensation and fog formation are considered in some detail. Separate attention is paid to the heat transfer and condensation of pure steam in the heat exchanger. Finally, the experiments performed are reported and the results compared with the models presented. From this comparison it can be learnt that the models are well able to predict the rate of heat transfer and phenomena such as condensation and fog formation.
More and more air handling units are equipped with heat recovery systems, with the aim of decreasing the energy use in buildings for heating and cooling. The efficiency of the heat recovery system is often used to calculate the energy saving. However, air-handling units do not always function as planned. In particular, parasitic shortcuts and leakage may decrease dramatically the efficiency of ventilation and heat recovery. In addition, these units need electrical energy for fans, which may be more precious than saved heat. Measurements, using tracer gas dilution technique have detected various malfunctions in several units.This paper addresses real energy recovery with air handling units from a theoretical point of view and presents results of measurements on 13 units. In the best three cases, the real, global heat recovery efficiency was between 60 and 70% for units having a 80% nominal efficiency. In the three worst cases, the global efficiency was less than 10%. For these cases, the heat recovery system uses more energy than it saves.
The Maisotsenko Cooling cycle combines the thermodynamic processes of heat exchange and evaporative cooling in a unique indirect evaporative cooler resulting in product temperatures that approach the dew point temperature (not the wet bulb temperature) of the working gas. This cycle utilizes the enthalpy difference of a gas, such as air, at its dew point temperature and the same gas saturated at a higher temperature. This enthalpy difference or potential energy is used to reject the heat from the product. Consider the cooling gas to be air and the liquid to be water; the Maisotsenko Cycle allows the product fluid to be cooled in temperature ideally to the dew point temperature of the incoming air. This is due to the precooling of the air before passing it into the heatrejection stream where water is evaporated. For purposes of this paper, the product fluid is air. At no time is water evaporated into the product airstream. When exhausted, the heat rejection airstream or exhaust air is saturated and has a temperature less than the incoming air, but greater then the wet bulb temperature. This cycle is realized in a single apparatus with a much higher heat flux and lower pressure drop than has been realizable in the past due to its efficient design.
Liquid desiccant indirect evaporative cooling is an ideal alternative system for conventional vapor compression systems to meet new economic, environmental, and regulatory challenges. This alternative system consists of two air-handling processes: moisture removal in the dehumidifier and sensible heat removal in the M-cycle indirect evaporative cooler. The performance of the first stage influences the cooling capacity of the second stage. SHR (sensible load divided by total load), dew point effectiveness, moisture reduction, and temperature reduction were adopted as indices to describe the heat and mass transfer performance of the integrated liquid desiccant and the M-cycle indirect evaporative cooler. The effects of air and desiccant inlet parameters, as well as the working air ratio, on the performance of the hybrid were experimentally investigated. The results showed that the variation of dehumidification capacity in the first stage directly affected the cooling capacity in the second stage when increasing the inlet parameters of the air or desiccant. The energy balance in both the dehumidifier and the M-cycle indirect evaporative cooler were in the range of ±20% for all the experiment runs. To achieve performance in the second stage, the supplied water flow rate to the wick surface had to be approximately five times that of the evaporative water.
This paper presents a numerical study of heat and mass transfer in indirect evaporative air coolers with four air flow patterns: parallel-flow, counter-flow, cross-flow and regenerative. The numerical simulation was performed on the basis of original two-dimensional heat and mass transfer models (in the case of cross-flow heat exchanger the model was 3D). The mathematical models developed were validated against published experimental data presented in Appendix A. It was established, that heat and mass transfer processes in the wet channels of counter-flow, cross-flow and regenerative indirect evaporative coolers are characterized by creation of two particular heat and mass transfer zones. Detail analysis of the temperature and humidity ratio distributions and boundary conditions, characterizing the coupled heat and mass transfer process in each of these determined zones, revels the violation of the Lewis relation unity under a certain inlet and operation conditions. A theoretical method for estimating the Lewis factor was proposed. Using the developed models the thermal performances of the conventional designs of indirect evaporative air coolers were analyzed numerically and preferable climatic zones for considered heat exchanger were established. Four novel heat exchangers utilizing the advantages of the basic cycles heat exchangers were presented.
This paper investigates a mathematical simulation and optimization of the heat and mass transfer processes in the indirect evaporative air cooler. The heat exchanger uses a novel combined parallel and regenerative counter-flow arrangement. A two-dimensional heat and mass transfer model is developed to perform the thermal calculations of the indirect evaporative cooling process. Mathematical model was validated against existing experimental data. The results obtained from the simulation reveal the high effectiveness of the presented unit. The exchanger was compared with the conventional regenerative unit. The results of comparison show, that the presented unit is characterized by higher effectiveness. The impact of the selected unitless operating factors on the performance of the investigated heat exchanger was established. The conducted multi-criteria optimization allowed to establish Pareto-optimal operating conditions and preferable climatic zones for the presented heat exchanger.
This paper investigates a mathematical simulation of heat and mass transfer in eight different types of the Maisotsenko Cycle (M-Cycle) heat and mass exchangers (HMXs) used for indirect evaporative air cooling. A two-dimensional heat and mass transfer model is developed to perform the thermal calculations of the indirect evaporative cooling process and quantifying the overall performance. The mathematical model was validated against experimental data. A numerical simulation reveals many unique features of the considered HMXs, enabling an accurate prediction of their performance. Results of the model allow for comparison of the analyzed devices in order to improve the performance of the original HMX.
The evaporation of droplets in a turbulent two-phase flow is of importance in many engineering applications. Water droplet evaporation in spray systems, for example, is increasingly used in public spaces and near building surfaces to achieve immediate cooling and enhance the thermal comfort in indoor and outdoor environments. The complex two-phase flow in such a system is influenced by many parameters such as continuous phase velocity, temperature and relative humidity, drop size distribution, velocity and temperature of the droplets and continuous phase-droplet and droplet-droplet interactions. Most of these parameters are not easily varied independently. To gain insight into the performance of the system, however, detailed knowledge of the impact of every parameter is important. Computational Fluid Dynamics (CFD) is a useful tool for performing such parametric analyses. To the best of our knowledge, a detailed analysis of the cooling performance of a water spray system under different physical conditions has not yet been performed. This paper provides a systematic parametric analysis of the evaporative cooling provided by a water spray system with a hollow-cone nozzle configuration. The analysis is based on grid-sensitivity analysis and validation with wind-tunnel measurements. The impact of several physical parameters is investigated: inlet air temperature, inlet air humidity ratio, inlet air velocity, inlet water temperature and inlet droplet size distribution. The results
show that for a given value of inlet water temperature (35.2 C), as the temperature difference between
the inlet air and the inlet water droplets increases from 0 C to 8 C, the sensible cooling capacity of the
system improves by more than 40%. In addition, injecting water droplets with a temperature higher than
the dry-bulb temperature of the air can still provide cooling, although the amount of cooling reduces
considerably compared to the case with water at lower temperatures. It is also shown that as D, the mean
of the Rosin-Rammler distribution, is reduced from 430 to 310 μm, the cooling performance of the system
is improved by more than 110%. For a given value of D , the cooling is enhanced for wider drop-size
distributions.
The present paper focuses on the development and experimental validation of a model of air-to-air heat exchanger dedicated to domestic mechanical heat recovery ventilation. The proposed model describes dry and partially wet regimes.The first part of the paper presents a semi-empirical model based on the physical characteristics of the heat recovery device and relying on empirical correlations available in the literature for the convective heat transfer coefficients. In the case of partially wet regime, a moving boundary model is applied in order to predict sensible and latent heat transfer rates. A model developed with friction factor coefficients estimated by correlations from the literature is also presented in order to predict the hydraulic performance in dry conditions.The second part of the paper describes the experimental investigation conducted on an off-the-shelf heat exchanger.Experimental data are used to tune correlations for the determination of the convective heat transfer coefficient and validate the proposed simulation model of the ventilation heat recovery exchanger in partially wet conditions. The model developed to determine the hydraulic performance with existing correlations for the friction factor coefficient does not require a calibration.Finally, examples of use of the developed model are presented, which includes coupling the model with a building simulation model, a study of the influence of the humidity on the evolution of the latent and sensible heat transfer rates and strategies to avoid freezing in the heat exchanger.
In the current work, two different types of evaporative systems are shown. A returning air recovery system is used. The indirect systems have two independent airflows, the primary airstream is used to refrigerate and the secondary flow is in direct contact with water in order to improve heat and mass transfer. The first equipment (the indirect evaporative refrigerator) works like a flat interchanger made of aluminium and there is only heat transfer in the primary airflow.The second equipment (the semi-indirect evaporative refrigerator) is made of solid porous ceramic pipes which separate the two airstreams, thus allowing that, in the primary airflow (apart from the heat transfer), there is also a mass transfer. It should also be mentioned that this system is free of legionella, because the pipes perform the role of a filter material, making it impossible for the bacterium to enter premises. This system has been named a semi-indirect evaporative system due to the permeability of the porous pipes which allow a higher or lower water diffusion and therefore a mass transfer depending on the specific humidity of the primary airstream.
In recent years, the attention of researchers has been focused on energy conservation demands due to the environmental impact of energy consumption throughout the built environment and global warming issue. Heat or energy recovery is one of the main energy-efficient systems that has been approved to overcome this problem. However, in conventional heat or energy recovery for building applications, only sensible energy has been recovered and neglecting the latent energy. In this work, enthalpy recovery system has been developed and the performances of sensible and latent energy have been investigated experimentally. The efficiency of close to 66% has been achieved for sensible energy and the latent energy efficiency was nearly 59% gained. Comparison of efficiency with effectiveness-NTU method showed both were in good agreement. Recovered energy was achieved up to 167W at 3.0m/s air velocity with 4.3°C temperature difference.
The exergy efficiency of a counterflow hot moist air/ambient air heat exchanger with vapour condensation as a function of the temperature and humidity of the hot moist air, the ratio of ambient air flow rate to hot moist air flow rate and the minimum temperature differences between the flows is determined. The ranges 20 to 120°C and 0.01 to 1.0 kg·kg-1 for the temperature and humidity ratio of hot moist air respectively are considered. Constant ambient air conditions of 20°C and 0.01 kg·kg-1 were assumed.
A short-cut method for calculating the effectiveness of wet surface crossflow plate heat exchangers is developed. It introduces a correction for the effectiveness estimated according to the method of Maclaine-cross and Banks. For this purpose a new model with a water film that flows down, corresponding to the real conditions in these heat exchangers, is developed. The method of Maclaine-cross and Banks is improved by an approach to determine the mean water surface temperature and by derivation of an equation for calculating the ratio of total to sensible heat taking into account the barometric pressure. Performance predictions by this method for wet surface crossflow plate heat exchangers are found to agree with the finite difference solutions.
There has been a continuing effort to advance the understanding and modeling of frost formation on refrigerated surfaces during the last two decades for better design of air-to-refrigerant heat transfer equipment and effective and energy-saving control of defrosting processes. A review and comparative analysis of the available literature concerning frost properties, correlations, and mathematical models are presented in this study to provide an overview of the analytical tools for researchers, product developers, and designers. The frost research can be divided into two general groups—experimental correlations and mathematical models. In general, the properties correlated are the frost thermal conductivity, the frost average density, and air-frost heat transfer coefficient (Nusselt number). A limited operational range of these relations is observed. The mathematical models include both differential and integral approaches, which are, in general, solved numerically. These models are classified based on the geometrical configuration of cold surface. A comprehensive comparison of the models is given to assist the reader in making their decisions for design analysis. The existing gaps in the frost research are identified and recommendations are made.
Transient temperature response of crossflow heat exchangers having finite wall capacitance with both fluids unmixed is investigated numerically for perturbations provided in both temperature and flow. Results are presented for step and ramp change in flow rate of hot and cold fluids, and step, ramp, exponential and sinusoidal variation in hot fluid inlet temperature.
Dew point evaporative cooling system is an alternative to vapor compression air conditioning system for sensible cooling of ventilation air. This paper presents the theoretical performance of a novel dew point evaporative cooling system operating under various inlet air conditions (covering dry, moderate and humid climate) and influence of major operating parameters (namely, velocity, system dimension and the ratio of working air to intake air). A model of the dew point evaporative cooling system has been developed to simulate the heat and mass transfer processes. The outlet air conditions and system effectiveness predicted by the model using numerical method for known inlet parameters have been validated with experimental findings and with recent literature. The model was used to optimize the system parameters and to investigate the system effectiveness operating under various inlet air conditions.
The rapidly growing world energy use has already raised concerns over supply difficulties, exhaustion of energy resources and heavy environmental impacts (ozone layer depletion, global warming, climate change, etc.). The global contribution from buildings towards energy consumption, both residential and commercial, has steadily increased reaching figures between 20% and 40% in developed countries, and has exceeded the other major sectors: industrial and transportation. Growth in population, increasing demand for building services and comfort levels, together with the rise in time spent inside buildings, assure the upward trend in energy demand will continue in the future. For this reason, energy efficiency in buildings is today a prime objective for energy policy at regional, national and international levels. Among building services, the growth in HVAC systems energy use is particularly significant (50% of building consumption and 20% of total consumption in the USA). This paper analyses available information concerning energy consumption in buildings, and particularly related to HVAC systems. Many questions arise: Is the necessary information available? Which are the main building types? What end uses should be considered in the breakdown? Comparisons between different countries are presented specially for commercial buildings. The case of offices is analysed in deeper detail.
In this paper the thermal performance of parallel flow microchannel heat exchangers subjected to constant external heat transfer has been theoretically analyzed. Equations for predicting the axial temperatures as well as the effectiveness of the fluids of a microchannel heat exchanger operating under laminar flow conditions have been developed. In addition, an equation for determining the heat transfer between the fluids has also been formulated. Irrespective of the heat capacity ratio, for a specific NTU, external heating always decreases and increases the effectiveness of the hot and cold fluid, respectively. The opposite trend in the effectiveness of the fluids is observed when they are subjected to external cooling. Moreover, under unbalanced flow conditions (heat capacities of two fluids are not equal) the effectiveness of the fluids depended on the fluid with the lowest heat capacity. Among the two unbalanced flow conditions (heat capacities of the two fluids are not equal) the effectiveness of the fluids is greatest when the hot fluid has the lowest heat capacity. At a given NTU, reduction in heat capacity ratio improved the effectiveness of the fluids. Under certain operating conditions temperature cross over was observed in the heat exchanger.
Wymiana ciepła w wymiennikach krzy _ zowych w systemach wentylacyjnych i klimatyzacyjnych z odzyskiem ciepła z powietrza wywiewanego/heat transfer in cross flow heat exchangers for heat recovery from exhaust air in ventilation and air conditioning systems
- A Jedlikowski
Jedlikowski A. Wymiana ciepła w wymiennikach krzy _
zowych w systemach
wentylacyjnych i klimatyzacyjnych z odzyskiem ciepła z powietrza wywiewanego/heat transfer in cross flow heat exchangers for heat recovery from
exhaust air in ventilation and air conditioning systems [Praca Doktorska/PhD
Dissertation, Promotor/Supervisor: Professor Sergey Anisimov]. Politechnika
Wrocławska/Wrocław University of Technology, Wydział In _
zynierii
Srodowiska/Department of Environmental Engineering; 2012 [in Polish].
Frost formation in an air to air heat exchanger
- Mrl Bantle
Bantle MRL. Frost formation in an air to air heat exchanger [MSc thesis].
A.2. Safe working conditions as function of temperature effectiveness
- Fig
Fig. A.2. Safe working conditions as function of temperature effectiveness.
- L P Erez-Lombard
- J Ortiz
- C Pout
P erez-Lombard L, Ortiz J, Pout C. A review on buildings energy consumption
information. Energy Build 2008;40:394e8.
- S Anisimov
S. Anisimov et al. / Energy 89 (2015) 205e225