Multi-tube Heat Exchanger Research Articles

Key performance indicators (KPIs): Assessing the process integration of a shell-and-tube latent heat storage unit

In this research, a multi-tube heat exchanger filled with phase change material (PCM) is constructed and investigated experimentally to evaluate its capability to serve as a heat storage unit in solar domestic water heating (SDWH) systems. Several operational conditions comprising the PCM initial temperature and the water discharge flowrates are included in the test as external factors to evaluate the performance of the proposed shell-and-tube latent-heat storage unit (STLHS) as a part of a SDWH system. The STLHS unit is assessed according to several quantitative, qualitative, and economic key performance indicators extracted from the Egyptian plumbing code (EPC). Firstly, the quantitative results show that the proposed STLHS unit increased the water temperature by a range of 7–12 °C and maintained a constant hot water supply for extended periods, 2–3 h. Secondly, from the qualitative point of view, the achieved heating levels are insufficient to fulfill the domestic needs according to the benchmark water temperature established by the EPC. Thirdly, evaluating the STLHS unit on an economic basis shows that the annual fuel saving increases by increasing the unit initial temperature, where the annual fuel consumption savings reach 130 m3 of natural gas. Finally, the acquired techno-economic performance measurements emphasize that the proposed unit has a notable possibility for several design modifications to achieve more improvements in its performance.

Numerical simulation of Multi-Tube Tank heat exchanger: optimization analysis

ABSTRACT The key-element in any heat recovery application is the heat exchanger. Many heat exchangers have been suggested for heat recovery applications among them Multi-Tube Tank (MTT). MTT is a heat exchanger that has been recently proposed especially for heat recovery processes that involve two fluids one at rest and the other one in motion. That said, MTT can be adopted for a broad spectrum of heat recovery applications and can be applied within a wide panel of geometrical configurations. Notwithstanding, operational conditions may impose thermal constraints on the outlet fluids. All things considered, to efficiently utilize MTT heat exchangers it is unavoidable to understand its thermal behavior. In the frame of this view, the present paper, examines the anatomy of MTT using a computational code. A heat recovery process is considered where exhaust gas is used to heat water. The thermal model is based on the heat equation along with different boundary conditions. Furthermore, an optimization procedure is developed in order to find the best scenario among three different configurations incorporating, respectively, one tube, three tubes, and six tubes. Three optimization criteria are considered, maximum water temperature, maximum recovered energy, and minimum gas temperature threshold. Results reflect the flexibility of the optimization assessment. To give an illustration, for a fixed interval of time, if the selection criterion is the maximum-recovered energy the configuration of six tubes is the optimal solution whereas if the criterion is a minimum gas temperature threshold then MTT with one tube is the best configuration.

The effect of external helical ribs tube on the heat transfer and pressure drop performance for multi - tube heat exchanger

The pressure drop and heat transfer characteristic for cross flow of turbulent air over helical rectangular ribs tubes heat exchanger were studied experimentally and numerically. The study includes the design and manufacturing of four models of multi-tube heat exchangers. Each model has three tubes in a line arrangement. Experiments were performed for Reynolds number range from (7200–31400) using different rib pitch to diameter ratios (P/D = 1.3, 1.5 and 1.8) with different ribs pitch angles. The height to diameter ratio of rib (h/D = 0.05) and width to diameter ratio is (w/D = 2) were fixed. The water flows with different rates inside the tubes with a range of (2, 3, 4, 5, 6 and 7) L/min. Three values for the inlet water temperatures to test tube were used. In the present numerical part, commercial software ANSYS Fluent 16.2 was employed to determine the heat transfer coefficient and pressure drop for the different models of cross flow heat exchangers. The results declared that the average Nusselt number and Pressure drop of the corrugated model has a relatively high different from that of the smooth models. In addition, the experimental and numerical were compared and gave a good agreement between them.

Experimental investigation of phase change in a multitube heat exchanger

Abstract The present paper investigates melting and solidification processes in a multitube heat exchanger. The shell side is filled with paraffin (RT35) while water flows through the tube passes as the heat transfer fluid (HTF). The arrangements of one, two, three and four inner tubes cases are studied for HTF inlet temperatures of 70 °C, 75 °C and 80 °C. The thermal characteristics of the units involving melting front, heat transfer rate and Nusselt number are also analyzed. Results indicate that increasing the number of inner tubes results in a faster pace of melting and solidification. The effect is so pronounced that, when the HTF inlet temperature is 80 °C, application of 4-tube heat exchanger reduces the duration required for complete melting and solidification up to 43 and 50 percent, respectively as compared to the corresponding single-HTF-tube case. The results also show that by increasing the number of tubes, the surface-averaged Nusselt number decreases. This can be attributed to the inhibition of upper pipes against PCM melt movement. Also a correlation is proposed to calculate Nusselt number of tubes. Comparing the temperatures recorded by the thermocouples in rotational direction, it is indicated that the variation of the inner tube arrangement affects solidification process more than melting.

Nano-enhancement of phase change material in a shell and multi-PCM-tube heat exchanger

Abstract The fundamental defect of PCMs in discharging process is their low conductivity which results in long solidification time. In this study distribution of a PCM in a multi-tube heat exchanger is conducted numerically to reduce the solidification time. Therefore, the PCM mass is distributed in the inner and outer tubes between which a heat transfer fluid (HTF) passes through. Various volume fractions of copper nanoparticles are added to the PCM and the consequences of the conductivity enhancement is observed. Moreover, the variation of Stefan number (Ste) is considered to evaluate the effect of this parameter on the solidification process. In addition, it is shown that the distribution of the PCM is greatly effective on the heat transfer enhancement. Proper PCM mass distribution leads to 62% reduction in the solidification time. Results also indicates that by increasing the nanoparticles volume fractions to 4% and Ste to 0.45, the solidification time reduces 15% and 26%, respectively in the best case.

Study on the heat transfer characteristics of a shell-and-tube phase change energy storage heat exchanger

Natural convective has significant effects on the heat transfer effect and the growth of PCM’s interface shape. So the discussion of the influence on multi-tube heat exchanger by natural convective is conductive to the enhancement of heat transfer and reduction in charging time. In this paper, a CFD model on the shell-and-tube phase change energy storage heat exchanger was established. In addition, the effects of the number of inner tubes and inlet temperature on the heat transfer characteristics of the heat exchanger were studied during charging process. The results indicated that the thermal disturbance between heat pipes were improved natural convection, and significantly enhance the convection heat transfer in the charging process; both adopting smaller diameter pipe and higher inlet temperature could shorten the melting time of phase change material (PCM); the influence of temperature of heat carried fluid on the melting time was significantly weakened when it exceeded a limited value. This study provided a reference value for the structure geometrical parameter optimal design of heat exchanger.

Assessment of Heat Transfer Characteristics of a Latent Heat Thermal Energy Storage System: Multi Tube Design

In this study, a numerical investigation of heat and mass transfer characteristics of a high temperature phase change material (PCM) is presented. The phase change material used is Sodium Nitrate with a melting point of 306 °C. For a fixed PCM mass, investigations are carried out on a multi-tube heat exchanger system for examining the effects of tube size and number of tubes on overall heat transfer rate. The performance of latent heat storage system during the charging and discharging processes are discussed. Further, the results are elaborated for three multi-tube systems to optimize the number of tubes to be used with the constant heat transfer surface area. The natural convection pattern during charging is observed with the help of temperature contours and streamlines. The solidification front pattern of the PCM, during the discharging process is visualized through melt fraction contours. With the increase in the number of tubes from 13 to 25, the heat distribution throughout the storage system is very effective which lead to reduction in charging and discharging times by 20% and 48% respectively.

Thermal performance enhancement of energy storage systems via phase change materials utilising an innovative webbed tube heat exchanger

In this study, the phase change materials (PCM) thermal energy storage system utilizing a novel webbed tube heat exchanger was designed and numerically investigated. The obtained simulation results were then compared with three types of conventional heat exchangers. These included: shell and tube heat exchanger, triplex tube heat exchanger and multi-tube heat exchanger. The proposed innovative webbed tube heat exchanger consisted of four horizontal inner heat transfer fluid tubes contained in a shell. The inner heat transfer fluid tubes were connected together by using metal plates welded to these tubes. The additional plates were designed to increase heat transfer surface area. Two-dimensional numerical models were developed. The process of melting (charging) was monitored during the simulation. The acceleration in melting process in the new design was observed owing to improved thermal conductivness.

Study on performance of horizontal single pass shell and multi-tube heat exchanger with baffles by air bubble injection

ABSTRACTThe single pass shell and multi-tube heat exchanger with double segmental baffles'’ performance improvement were experimentally investigated by two methods of air injection into shell side when an increase in air bubble created an effect in the corresponding values of shell and tube heat exchanger such as the overall heat transfer coefficient, effectiveness, NTU and the pressure loss. In the first method, air bubble injection into shell side was parallel to the cold water flow; also in the second method, air bubble injection into shell side was cross flow to the cold water with different air flow rates to calculate approximately the most favourable performance conditions. 1–6 LPM of air flow rates and 10–20 LPM shell side water flow rates were transformed with constant tube side hot water flow rate.

Melting and solidification of PCM embedded in porous metal foam in horizontal multi-tube heat storage system

In this paper, melting and solidification processes of phase change material embedded in metallic porous foam in a multi-tube heat exchanger are investigated numerically under local non-equilibrium thermal condition. RT 35 is used as phase change material in middle shell of 3D multi-tube heat exchanger. Also, water flows across inner tube/tubes and outer tube as a heat transfer fluid (HTF). The effects of number of inner tubes, their arrangement as well as porosity of metallic foam on thermal characteristics of heat storage unit are studied. Results show that increasing number of inner tubes and adding metallic foam enhance melting and solidification rates significantly. A composite of phase change material/metallic foam with porosity of ε=0.7 engenders shorter melting and solidification time comparing to pure PCM. Arrangement of inner tubes has no effect on melting rate of metal foam/PCM composite. By inserting metallic foam with porosities ε=0.9and ε=0.7, melting time is decreased by 14% and 55%, respectively. Highest melting rate is for case with four inner tubes for all porosities of metallic foam. Regarding solidification process in pure PCM, case with four tubes shows shorter solidification time. Also, highest solidification rate amid composites of phase change material/metallic foam is seen for case with three tubes. Consequently, inserting metallic foam is more efficient in solidification process rather than the melting one. Moreover, increasing number of inner tubes has more influence on phase change rates in metal foam/PCM composites compared to pure PCM.

Investigation of PCM charging for the energy saving of domestic hot water system

In this paper an investigation on the charging process of a phase change material (PCM) integrated in a domestic hot water system is presented. In order to increase the amount of the stored energy in a limited portion of time during maximum solar radiation it is preferred to reduce the melting time of the PCM used in the heat exchanger. Therefore the effect of mass distribution of a PCM in a multi-tube heat exchanger is studied. Paraffin RT50, is used as the PCM and water as the heat transfer fluid (HTF) carries the energy gained in solar flat plate collector. Enthalpy porosity method is applied for modeling the phase change process and an experimental result is used to validate the model. There are six different cases in order to examine the effect of mass distribution on the melting time to reach the minimum value. Temperature and heat flux versus time as well as liquid fraction contours and behavior of the charging process are described. The results show that by distributing the PCM mass in a specific arrangement in which 40% of PCM mass is in inner tube, 52% reduction in melting time is achieved hence solar energy can be used more efficiently in the time of maximum solar radiation.

Experimental investigation of air injection effect on the performance of horizontal shell and multi-tube heat exchanger with baffles

This paper presents a laboratory experiments for investigation of air injection into the shell side of shell-and-multi-tube heat exchanger aims to augment the thermal performance. The air has been injected inside the heat exchanger shell with two methods (cross injection from shell wall and parallel injection from the shell front side) and different air flow rates to estimate the optimum performance conditions. The air and shell side water flow rates were changed between 1 and 5 LPM and 12–21 LPM respectively with constant tube side water flow rate. Also, pressure loss between the shell side outlet and inlet caused by air bubble injection was measured to know the power loss in heat exchangers under enhancement technique. The results presented that the injected air flow rate and injection method have a significant impact on the heat exchanger performance enhancement. And also presented that, increase of air flow rate increases the overall heat transfer coefficient (U). The effect of air bubble injection on effectiveness (ε), U and NTU of heat exchanger in first method flow was more than the second method for all test conditions. The shell side pressure loss for the cross injection was higher than in the case of parallel injection from. The augmentation on the U caused by cross air injection into the heat exchanger shell was 131–176% depending on air and shell side water flow rates.

Numerical investigation of heat transfer enhancement in a multitube thermal energy storage heat exchanger using fins

ABSTRACTThe application of a phase change material (PCM) as thermal energy storage observed unprecedented growth due to its large latent heat storage capacity at a constant temperature. However, the design of an energy storage heat exchanger is a challenging task because of the poor thermal conductivity of PCMs. In an effort to improve the heat exchanger design, this paper presents a numerical performance investigation of a PCM-based multitube heat exchanger incorporated with two new fin configurations. The analysis of the results shows that the placement of fins is one of the important aspects, which needs to be cogitated in the design of heat exchangers.

GETSLOCK-Advanced Front Closure Design for Multi-Tube Hairpin Heat-Exchanger

GETS lock is a modified front closure for hairpin heat exchanger with newly introduced characteristics like full ring, split locking ring and the slope on the tube sleeve.

Heat transfer enhancement of a single row of tube

A rod was positioned upstream of a circular cylinder to enhance its heat transfer and reduce its drag in air stream. The results are increasing the overall heat transfer by 40% over and reducing the drag by 30% the value for a single circular cylinder. In the present, this technique applied to a single row of tubes. Heat transfer enhancement and drag reduction are important factor for multi-tube heat exchanger. The present study investigated the heat transfer and surface pressure characteristics of each tube for single row of tubes under various numbers of tubes, pitch and Reynolds number. The tube diameter, D, was 20 mm, and the rod diameter was 5 mm. The distance between the center axes of the rod and the tube was 30mm. The Reynolds number based on D ranged from 5.3×103 to 2.1×104 . The result is that this technique is effective for heat transfer enhancement and drag reduction of row of tube.

Numerical study on geometrical specifications and operational parameters of multi-tube heat storage systems

In this paper, a numerical investigation as well as a comparison among multi tube heat exchangers is carried out for which variations of both geometrical and operational parameters are studied. The internal part of the shell is filled with RT35 as the phase change material which is heated by the heat transfer fluid (HTF) carrying inner tubes. This paper studies the effect of number of the tubes as well as their arrangement as geometrical parameters on liquid fraction, phase front and melting time. The details of the process progression are presented via liquid fraction contours and streamlines. The effective operational parameters considered are inlet temperature and flow rate of the HTF. Results show that conduction is the dominant heat transfer mechanism at initial stages of the melting process which is proceeded by a convection dominated interval. As the number of tubes increase, the contact surface between PCM and HTF tube increases, thus the convective heat transfer accelerates the melting process which reduces melting time. An increase in both operational parameters shortens the melting time for which the inlet temperature more effectively reduces the time. The results also show that among the arrangements, the ones whose tubes are located within the upper half of the shell are candidates for the longest melting time. Generally, for same inner tube numbers, at cases whose tubes occupy the bottom section of the system, a faster melting rate is obtained.

Performance of ground-source heat exchangers using short residential foundation piles

Experiments on the performance of ground-source heat exchangers using short residential foundation piles were performed in Saga, Japan. U-tube, double-tube, and multitube heat exchangers were installed in the ground to depths of 20 m. Water was used as the working fluid. Temperatures were measured on pipe walls, pile walls, and in the ground. From the measured data, thermal resistances of pipes, grout, and soil were calculated. We also investigated the effect of the ground surface temperature on heat transfer rate. Total thermal resistances of the double-tube, multitube, and U-tube exchangers were 0.231 (m K)/W, 0.295 (m K)/W, and 0.356 (m K)/W, respectively. Of these total resistances, soil, grout, and pipe thermal resistances accounted for 89%, 0%, and 11%, respectively, for the double-tube heat exchanger; 50%, 27%, and 23%, respectively, for the multitube heat exchanger; and 62%, 21%, and 17%, respectively, for the U-tube heat exchanger. Heat transfer rates of the tested heat exchangers could be predicted within 8.6% by using a measured value for the surface temperature coefficient. For atmospheric temperatures around 30 °C, the effect of the surface ground temperature on heat transfer rates was approximately 10%–15%.

Phase change in multi-tube heat exchangers

In this paper, melting of a phase change material (PCM) in a multi-tube heat exchanger (MTHX) is investigated. Water, as the heat transfer fluid (HTF), flows through the inner tube/tubes and the outer one while RT35 as the PCM fills the middle. The aim of this study is to investigate the effect of number of inner tubes as a geometrical parameter during charging process. Also consequences of increasing operational parameters including the HTF mass flow rate and inlet temperature are studied. In order to understand the effects of the proposed configurations, a comparison between double pipe and simple MTHX is carried out. Results show that as the inlet temperature increases melting process accelerates and complete melting time reduces, whereas, similar mass flow rate increase doesn't reduce the melting time to such an extent. By increasing the number of inner tubes from 1 to 4 in the shell side of the MTHX, melt region enlarges and its including vortices strengthens which leads to a dominated convective heat transfer and thus a higher melting rate. This increase in number of tubes leads to 29% reduction in melting time.

Air-Side Heat Transfer Performance of Louver Fin and Multitube Heat Exchanger for Direct Methanol Fuel Cell Cooling Application

The present work is performed to evaluate the heat transfer performance of a heat exchanger used in a direct methanol fuel cell. Because of material constraints and performance requirements, a louver fin heat exchanger is modified for use with conventional microchannel tubes and also with multiple small-diameter tubes (called multitubes). Prototype heat exchangers are tested, and the air-side heat transfer, pressure drop, and fan power are measured in a wind tunnel and simulated using a commercial code. The air-side pressure drop and heat transfer coefficient of the multitubes show similar trends to those of the flat-tube heat exchanger if the contact resistance is negligible. The tube spacing of the prototype multitube heat exchangers has a small effect on the pressure drop and heat transfer, but it has a profound effect on the air-side heat transfer performance because of the contact resistance between the tubes and louver fins. The air-side pressure drop agrees well with an empirical correlation for flat tubes.

A Co-axial Multi-tube Heat Exchanger Applicable for a Geothermal ORC Power Plant

The study proposes a Co-axial multi-tube heat exchanger (CMTHE) applicable to geothermal heat extraction. The heat exchanger is integrated with a 50kW geothermal ORC power plant having a working fluid of R-245fa. Two field tests were performed to examine the system response of the ORC system subject to change of CMTHE. In case 1 where the flow rate in the shell-side of CMTHE is maintained, the pressure variation in the shell-side of CMTHE casts minor variations on heat extraction, ORC power generation, and ORC efficiency during the transient. Moreover, the effect of pressure has barely any influence of the final states of heat extraction, ORC power generation, and ORC efficiency. In case 2 where the pressure is preserved in the CMTHE, it is found that a decrease of flow rate in the CMTHE results in degradation of heat extraction, ORC power generation and ORC system efficiency. On the contrary, increasing the flow rate in the CMTHE leads to a rise of heat extraction, ORC power generation and ORC system efficiency. Unlike that in case 1, the effect of flow rate has a detectable effect on the final states of heat extraction, ORC power generation, and ORC efficiency.