Monitoring the thermal efficiency of fouled heat exchangers: A simplified method

Fouling is a problem whether we are aware of it or not. In an industrial plant, it is important not only to be able to measure the buildup of unwanted deposits, but also to do it in the simplest and most economically possible way. This paper addresses the question of monitoring fouling in an oil refinery plant, where the high number of heat exchanger units and the variability of the feedstock charge pose additional problems in terms of the practicability of following the energetic performance of such equipment. In this case, the flow rates and quality of the fluids flowing through the heat exchangers do not usually correspond to the design conditions, because they change with time. Therefore, to assess the fouling level of the exchangers, the day-to-day measured thermal efficiency should not be compared with the efficiency predicted in the design calculations. The latter should be recalculated by introducing whenever necessary new values of flow rates, physical properties, and so forth, and by evaluating new heat transfer coefficients. However, the procedures are too time consuming to be applied frequently. The present work describes a simplified method for following heat exchanger performances, based on the assessment of the number of transfer units and thermal efficiencies, where the effects of changing the feedstock charge, particularly the flow rates of the fluids, are taken into account. The only data that need to be collected are the four inlet/outlet temperatures of the heat exchanger unit and one of the flow rates. Several heat exchanger units in an oil refinery were successfully monitored by means of this method, and it was found that the variations in the physical properties did not significantly affect the results obtained for the particular plant under study.     

Investigation of flow and heat transfer in corrugated-undulated plate heat exchangers

 An experimental and numerical investigation of heat transfer and fluid flow was conducted for corrugated-undulated plate heat exchanger configurations under transitional and weakly turbulent conditions. For a given geometry of the corrugated plates the geometrical characteristics of the undulated plates, the angle formed by the latter with the main flow direction, and the Reynolds number were made to vary. Distributions of the local heat transfer coefficient were obtained by using liquid-crystal thermography, and surface-averaged values were computed; friction coefficients were measured by wall pressure tappings. Overall heat transfer and pressure drop correlations were derived. Three-dimensional numerical simulations were conducted by a finite-volume method using a low-Reynolds number k–ɛ model under the assumption of fully developed flow. Computed flow fields provided otherwise inaccessible information on the flow patterns and the mechanisms of heat transfer enhancement. Received on 5 February 1999     

Determination of heat transfer correlations for plate-fin-and-tube heat exchangers

This paper presents a numerical method for determining heat transfer coefficients in cross-flow heat exchangers with extended heat exchange surfaces. Coefficients in the correlations defining heat transfer on the liquid- and air-side were determined using a non-linear regression method. Correlation coefficients were determined from the condition that the sum of squared fluid temperature differences at the heat exchanger outlet, obtained by measurements and those calculated, achieved minimum. Minimum of the sum of the squares was found using the Levenberg-Marquardt method. The outlet temperature of the fluid leaving the heat exchanger was calculated using the mathematical model describing the heat transfer in the heat exchanger. Since the conditions at the liquid-side and those at the air-side are identified simultaneously, the derived correlations are valid in a wide range of flow rate changes of the air and liquid. This is especially important for partial loads of the exchanger, when the heat transfer rate is lower than the nominal load. The correlation for the average heat transfer coefficient on the air-side based on the experimental data was compared with the correlation obtained from numerical simulation of 3D fluid and heat flow, performed by means of the commercially available CFD code. The numerical predictions are in good agreement with the experimental data.     

Concepts and realization of microstructure heat exchangers for enhanced heat transfer

Microstructure heat exchangers have unique properties that make them useful for numerous scientific and industrial applications. The power transferred per unit volume is mainly a function of the distance between heat source and heat sink—the smaller this distance, the better the heat transfer. Another parameter governing for the heat transfer is the lateral characteristic dimension of the heat transfer structure; in the case of microchannels, this is the hydraulic diameter. Decreasing this characteristic dimension into the range of several 10s of micrometers leads to very high values for the heat transfer rate.

Another possible way of increasing the heat transfer rate of a heat exchanger is changing the flow regime. Microchannel devices usually operate within the laminar flow regime. By changing from microchannels to three dimensional structures, or to planar geometries with microcolumn arrays, a significant increase of the heat transfer rate can be achieved.

Microheat exchangers in the form of both microchannel devices (with different hydraulic diameters) and microcolumn array devices (with different microcolumn layouts) are presented and compared. Electrically heated microchannel devices are presented, and industrial applications are briefly described.     

Effects of thermal dispersion on heat transfer in cross-flow tubular heat exchangers

Effects of thermal dispersion on heat transfer and temperature field within cross-flow tubular heat exchangers are investigated both analytically and numerically, exploiting the volume averaging theory in porous media. Thermal dispersion caused by fluid mixing due to the presence of the obstacles plays an important role in enhancing heat transfer. Therefore, it must be taken into account for accurate estimations of the exit temperature and total heat transfer rate. It is shown that the thermal dispersion coefficient is inversely proportional to the interstitial heat transfer coefficient. The present analysis reveals that conventional estimations without consideration of the thermal dispersion result in errors in the fluid temperature development and underestimation of the total heat transfer rate.     

Heat transfer augmentation and pumping power in double-pipe heat exchangers

One of the criteria for evaluating the performance of a heat exchanger with extended surfaces is the pumping power required for a specified heat duty. The results of an experimental project to relate the pumping power to heat transfer augmentation in a double-pipe heat exchanger are reported. The inner, electrically heated pipe was provided with external, rectangular, axial extended surfaces with interruptions. Heat transfer augmentation and friction factors were determined for different configurations with air as the fluid. Starting with continuous fins, cuts were introduced in the fins to give four ratios of the finssegment length to the gap between the segments, and finally all the fins were removed, which resulted in smooth pipes. Five different mass flow rates in two different inner pipes were employed. Lengths, surface areas, and pumping powers for finned pipes are compared with those for smooth pipes. The average heat transfer coefficient increases with an increase in the frequency of the interruptions. For equal heat transfer rates a significant reduction in the lengths can be achieved by interrupted fins. In many cases the reduction in the length is also accompanied by a reduction in the pumping power.     

Heat transfer by plate spacers in plate-fin heat exchangers

This paper is concerned with heat transfer by plate spacers in plate-fin heat exchangers. The spacers are considered as fins attached to the plates and are exposed to certain boundary conditions. The thermal resistances at the contact surfaces between the plates and the fins are assumed to be different from one another. Based on this model, equations for heat fluxes at contact surfaces are derived. Calculations from these equations reveal a considerable influence of the thermal resistances at the contact surfaces on the heat transfer.In der Arbeit wird der Wärmetransport durch Abstandshalter in Platten-Rippen-Wärmeübertragern behandelt. Die Abstandshalter werden wie Rippen betrachtet, die an den Platten befestigt und bestimmten Randbedingungen ausgesetzt sind. An den Kontaktstellen zwischen Platten und Rippen werden unterschiedliche Wärmewiderstände angenommen. Aufgrund dieses Modells lassen sich Gleichungen zur Berechnung der Wärmeströme an den Kontaktflächen herleiten. Die nach diesen Gleichungen vorgenommenen Berechnungen zeigen einen deutlichen Einfluß des thermischen Widerstandes an den Kontaktflächen auf den Wärmetransport in den Abstandshaltern.     

Three-dimensional numerical investigation of heat transfer for plate fin heat exchangers

In the present study, the potential of rectangular fins with 30° and 90° angle and 10 mm offset from the horizontal direction for heat transfer enhancement in a plate fin heat exchanger is numerically evaluated with conjugated heat transfer approach. The rectangular fins are mounted on the flat plate channel. The numerical computations are performed by solving a steady, three-dimensional Navier–Stokes equation and an energy equation by using Fluent software program. Air is taken as working fluid. The study is carried out at Re = 400 and inlet temperatures, velocities of cold and hot air are fixed as 300, 600 K and 1.338, 0.69 m/s, respectively. Colburn factor j versus Re design data is presented by using Fluent. The results show that the heat transfer is increased by 10 % at the exit of channel with fin angle of 30° when compared to channel without fin for counter flow. The heat transfer enhancement with fins of 30° and 90° for different values of Reynolds number with 300, 500 and 800 and for varying fin heights, fin intervals and also temperature distributions of fluids on the top and bottom surface of the channel are investigated for parallel and counter flow.     

Heat transfer coefficients of shell and coiled tube heat exchangers

In the present study, the heat transfer coefficients of shell and helically coiled tube heat exchangers were investigated experimentally. Three heat exchangers with different coil pitches were selected as test section for both parallel-flow and counter-flow configurations. All the required parameters like inlet and outlet temperatures of tube-side and shell-side fluids, flow rate of fluids, etc. were measured using appropriate instruments. Totally, 75 test runs were performed from which the tube-side and shell-side heat transfer coefficients were calculated. Empirical correlations were proposed for shell-side and tube-side. The calculated heat transfer coefficients of tube-side were also compared to the existing correlations for other boundary conditions and a reasonable agreement was observed.     

Lateral heat transfer in conducting and mutually irradiating plates

Lateral heat transfer effect in conducting and mutually irradiating parallel plates has been investigated. The effect of reflection in the diffuse-specular regime has been included. The governing equation of this problem is a complicated integro-differential equation, and this has been solved using the accurate Gauss-Jacobi orthogonal collocation method. The effective thermal conductivity along the lateral direction increases with decreasing conduction-radiation number, increasing emittance of the plates and increasing spacing. Specular reflection effects are insignificant.     

Counterflow, crossflow and cocurrent flow heat transfer in heat exchangers: analytical solution based on transfer units

By means of analysis equations for heat transfer performance based on number of heat transfer units were found, that allow to solve in a simple way single-pass and multipass heat exchanger problems when there are counterflow, crossflow and cocurrent modes of flow in any combination. There is no need to use external information such as the effectiveness concept or the correction factor F. The analysis gives new results which are at variance with traditional heat exchanger analysis when crossflow or cocurrent flow is involved.     

Flow boiling heat transfer of R134a in the multiport minichannel heat exchangers

The flow boiling heat transfer characteristics of R134a in the multiport minichannel heat exchangers are presented. The heat exchanger was designed as the counter flow tube-in-tube heat exchanger with refrigerant flowing in the inner tube and hot water in the gap between the outer and inner tubes. Two inner tubes were made from extruded multiport aluminium with the internal hydraulic diameter of 1.1 mm for 14 numbers of channels and 1.2 mm for eight numbers of channels. The outer surface areas of two inner test sections are 5979 mm² and 6171 m², while the inner surface areas are 13,545 mm² and 8856 mm² for 14 and eight numbers of channels, respectively. The outer tube of heat exchanger was made from circular acrylic tube with an internal hydraulic diameter of 25.4 mm. The experiments were performed at the heat fluxes between 15 and 65 kW/m², mass flux of refrigerant between 300 and 800 kg/m² s and saturation pressure ranging from 4 to 6 bar. For instance the boiling curve, average heat transfer coefficients are discussed. The comparison results of two test sections with different the number of channels are investigated. The results are also compared with nine existing correlations. The new correlation for predicting the heat transfer coefficient was also proposed.     

Numerical estimation of mixed convection heat transfer in vertical helically coiled tube heat exchangers

A numerical investigation of the mixed convection heat transfer from vertical helically coiled tubes in a cylindrical shell at various Reynolds and Rayleigh numbers, various coil‐to‐tube diameter ratios and non‐dimensional coil pitches was carried out. The particular difference in this study compared with other similar studies is the boundary conditions for the helical coil. Most studies focus on constant wall temperature or constant heat flux, whereas in this study it was a fluid‐to‐fluid heat exchanger. The purpose of this article is to assess the influence of the tube diameter, coil pitch and shell‐side mass flow rate on shell‐side heat transfer coefficient of the heat exchanger. Different characteristic lengths were used in the Nusselt number calculations to determine which length best fits the data and finally it has been shown that the normalized length of the shell‐side of the heat exchanger reasonably demonstrates the desired relation. Copyright © 2010 John Wiley & Sons, Ltd.     

Comparisons of the heat transfer and pressure drop of the microchannel and minichannel heat exchangers

The present study investigated the comparisons of the heat transfer and pressure drop of the microchannel and minichannel heat exchangers, both numerically and experimentally. The results obtained from this study indicated that the heat transfer rate obtained from microchannel heat exchanger was higher than those obtained from the minichannel heat exchangers; however, the pressure drops obtained from the microchannel heat exchanger were also higher than those obtained from the minichannel heat exchangers. As a result, the microchannel heat exchanger should be selected for the systems where high heat transfer rates are needed. In addition, at the same average velocity of water in the channels used in this study, the effectiveness obtained from the microchannel heat exchanger was 1.2–1.53 times of that obtained from the minichannel heat exchanger. Furthermore, the results obtained from the experiments were in good agreement with those obtained from the design theory and the numerical analyses.     

A new numerical method for determining heat transfer and packing distribution in particle heat exchangers for concentrated solar power

The complex nature of the physics of solid-gas interactions in concentrated solar particle heat exchangers signifies the need to develop new and cutting-edge numerical models to understand these interactions with the overarching goal of optimizing industrial solar thermal processes. To this end, a coupled computational fluid dynamics and discrete element method is developed to unravel near-wall particle flow physics of solar industrial heat exchangers. In addition, advanced post-processing functions are developed to provide a high-end data visualization and quantitative assessment of the packing distribution of solar particle heat exchangers. The validated numerical model shows that the particle temperature varies considerably throughout the entire fluid filled packed particle bed and it is shown that thermal radiation contribution becomes more profound at higher operating temperatures, namely 1073–1173 K. Also, the temperatures and solid volume fractions of the near-wall particles differ greatly compared to the bulk particles. The methods presented herein can be implemented by engineers and scientists to evaluate near-wall packing distributions and thermal characteristics, which would be useful for optimizing the geometric morphology of solar industrial heat exchangers.