Full-scale Heat Exchanger Research Articles

Crossflow microsand filtration in cooling tower systems to control fouling in heat exchanger devices

In the context of global warming, there is a growing need for high-performance heat, ventilation and air conditioning systems. Fouling is a challenge, affecting heat exchanger operational and energetic performance. Side stream filtration can be used to improve water quality by reducing particle concentration in the system. The aim of this study was to assess a crossflow microsand filtration unit's efficacy in reducing fouling within an 81-plate heat exchanger, in an in-situ cooling tower circuit. A pilot heat exchanger without filtration was operated under similar conditions. After 3 years of continuous operation, visual inspection of the full-scale heat exchanger plates operating with filtration showed minimal accumulation. Pairs of facing plates (7) were scraped and rinsed to collect total solids accumulated on surfaces. Samples were resuspended and analyzed for solids and metal concentration measurements. Low average values of total volatile solids (45 μg/cm2) and total solids (93 μg/cm2) were indicative of a low biofilm formation. X-ray diffraction revealed the presence of calcite associated to water hardness. Low levels of iron and copper were detected despite slightly corrosive water. In contrast, the pilot heat exchanger exhibited higher deposit accumulation of total volatile solids (90 μg/cm2) and total solids (110 μg/cm2) after only 12 weeks of operation. These findings underscore the role of side stream filtration in reducing particle deposition and biofouling potential in full-scale cooling tower systems. Crossflow microsand filtration is a promising avenue to control fouling in building cooling tower systems, which could contribute in improving the building's energetic performance.

CFD Modeling of Heat Exchanger with Small Bent Radius Coils Using Porous Media Model

The efficiency of heat transfer in air-cooled heat exchangers of various industrial facilities depends on the flow rate of the coolant, its inlet temperature and ambient temperature. These parameters are transient and depend both on the features of the technological process and on weather conditions. One option for a compact design of heat exchangers is the use of close-packed coils with a small bending radius. In this case, heat transfer in the complex geometry of the annular space cannot be described by simple one-dimensional dependencies. To solve this problem, it is necessary to consider the three-dimensional spatial structure of the heat exchange surface. Since the size of the grid elements will be several orders of magnitude less than the size of the facility, the size of the computational grids for CFD modeling full-scale heat exchangers will be billions of finite volumes, and even on powerful supercomputers, the solution time will be about a month. One way to reduce computational costs is to use reduced order models, in which the computational domain is not modeled directly; instead, simplified models, such as a porous medium model, are used to describe it. However, such models require additional closing relations and coefficients that characterize the actual channel geometry. This paper presents a technique for creating a digital twin of a heat exchanger with small bend radius coils based on a porous medium model. The values of heat transfer coefficients and hydraulic resistance depend on the speed of air movement in the space between the coils. The calculated value of the thermal power obtained using the strengthened model was 529 kW, which corresponds to the passport data of 500 kW, with less than 6% deviation for the heat exchanger under study. This confirms the correctness of the calculation with accepted simplifications. The calculation time in this case was only a few minutes when using a personal computer. The developed numerical model allows for the resolution of performance characteristics based on the temperature of the cooled medium at the inlet, air temperature, and fan speed. Analyzing the different modes of turning on the cooling fans made it possible to determine the values of the thermal power when turning off the fans or reducing the number of revolutions.

Development of a PCM-HE to harness waste greywater heat: A case study of a residential building

Waste greywater (GW) from non-industrial buildings has considerable exergy that must be exploited for a sustainable future. Scavenging this low-grade heat from GW into cold water (CW) before storage in a phase change material (PCM) is a novel approach to decouple demand and supply along with integrating storage and transfer in a single heat exchanger (HE). A methodology to optimally select an appropriate PCM along with the heat transfer enhancement procedures both internal and external to the flow pipes is presented. This procedure can be extended to reduce the hot water heating demand for applications in both residential and larger commercial buildings. Furthermore, a design technique to numerically assess the performance of a full-scale HE based on empirical formulations of a unit-volume are also put forward. As a case study, the performance of a counter-flow PCM-HE with vertically cascaded GW and CW finned-corrugated pipes is assessed for heat recovery in household appliances of a residential building. Such HE with 9 m piping length can increment the incoming CW temperature by 9.5 K with complete phase change of 30 kg of PCM in 900 s. A transfer of about 5,100 kJ of heat for PCM melting with GW outflow and freezing with CW inflow occurs within this time frame. Furthermore, to fully exploit the temperature differences between the fluids a three-cascaded PCM arrangement enhances the CW outlet temperature by 64%. The installation of this PCM-HE in a four-member UK household, can save 4,687 kWh of energy annually with a payback time of 4.44 years.

Prediction of two-phase flow distribution in microchannel heat exchangers using artificial neural network

Due to the intrinsic complexity of two-phase flow distribution and the limited mathematical flexibility of conventional formulations of the phenomenon, previous attempts generally fall short in the accuracy and applicability of their prediction. To address these issues, this study focuses on methods with higher mathematical flexibility. Specifically, the construction and training of Artificial Neural Network (ANN) is presented for the identification of this complex phenomenon. The interaction of the numerous physical phenomena, occurring at different scales, is thus represented by the network structure, offering a formulation capable of achieving higher accuracy. Experimental data from a full-scale heat exchanger of an air-conditioning system operating over a wide range of conditions are used to train and test the ANN. The network optimisation with Bayesian regularisation against experimental data leads to a structure featuring 4 inputs, 3 hidden layers, and 3 neurons for each layer, which demonstrates deviations on the single output mostly lower than ± 10% and a correlation index higher than 98%, when the whole data set is used for training the ANN. The analysis of the network optimisation for different shares of data used for the network testing, shows higher training and testing accuracy as the number of training data increases, along with no apparent overfitting.

Scalable Green Synthesis and Full-Scale Test of the Metal-Organic Framework CAU-10-H for Use in Adsorption-Driven Chillers.

The demand for cooling devices has increased during the last years and this trend will continue. Adsorption-driven chillers (ADCs) using water as the working fluid and low temperature waste energy for regeneration are an environmentally friendly alternative to currently employed cooling devices and can concurrently help to dramatically decrease energy consumption. Due to the ideal water sorption behavior and proven lifetime stability of [Al(OH)(m-BDC)] ∙ x H2 O (m-BDC2- = 1,3-benzenedicarboxylate), also denoted CAU-10-H, a green very robust synthesis process under reflux, with high yields up to 95% is developed and scaled up to 12 kg-scale. Shaping of the adsorbent is demonstrated, which is important for an application. Thus monoliths and coatings of CAU-10-H are produced using a water-based binder. The composites are thoroughly characterized toward their mechanical stability and water sorption behavior. Finally a full-scale heat exchanger is coated and tested under ADC working conditions. Fast adsorption dynamic leads to a high power output and a good power density. A low regeneration temperature of only 70 °C is demonstrated, allowing the use of low temperature sources like waste heat and solar thermal collectors.

A Functional Full-Scale Heat Exchanger Coated with Aluminum Fumarate Metal–Organic Framework for Adsorption Heat Transformation

Metal–organic frameworks represent a class of microporous adsorbents with high application potential for adsorption heat transformation. Here, we present a functional, full-scale heat exchanger coated with the microporous aluminum fumarate MOF Basolite A520 using a polysiloxane-based binding agent. The function of the heat exchanger was evaluated resulting in a gross cooling power of 2900 W (at the beginning of the adsorption cycle) or, respectively an average cooling power of 690 W (up to a limit of 90% equilibrium loading in 7 min) under the working conditions of a realistic adsorption chiller of 90 °C – 30 °C – 18 °C (temperature level of heat source, heat rejection/condenser, and evaporator).

판형 열교환기 Full-scale 해석을 위한 1차원 유동 네트워크 모델 및 ε-NTU 모델의 수치적 연구

Since a typical plate heat exchanger is made up of a huge number of unitary cells, it may be impossible to predict the aero-thermal performance of the full scale heat exchanger through three-dimensional numerical simulation due to the enormous amount of computing resources and time required. In the present study, a simple flow-network model using the friction factor correlation and a thermal-network model based on the effectiveness-number of transfer units (<TEX>${\varepsilon}$</TEX>-NTU) method has been developed. The complicated flow pattern inside the cross-corrugated heat exchanger has been modeled into flow and thermal networks. Using this model, the heat transfer between neighboring streams can be considered, and the pressure drop and the heat transfer rate of full-scale heat exchanger matrix are calculated. In the calculation, the aero-thermal performance of each unitary cell of the heat exchanger matrix was evaluated using correlations of the Fanning friction factor f and the Nusselt number Nu, which were calculated by unitary-cell CFD model.

Effect of a micro-grooved fin surface design on the air-side thermal-hydraulic performance of a plain fin-and-tube heat exchanger

In this study, the effectiveness of plain fin-and-tube heat exchangers constructed using anisotropic, micro-patterned aluminum fins has been explored. These fins which can more completely drain the condensate that forms on the heat transfer surface during normal operation were selected with the aim of improving the thermal-hydraulic performance of the heat exchanger. This study presents and critically evaluates the efficacy of four full-scale heat exchangers by measuring and comparing dry/wet air-side pressure drop and dry/wet air-side heat transfer data. The prototype fin surfaces were shown to reduce the wet air-side pressure drop from 9.3% to 53%, while at the same time having a negligible effect on the sensible heat transfer coefficient under both dry and wet conditions. That is to say, this novel fin surface design has shown the ability, through improved condensate management, to enhance the performance of the heat exchanger. Data pertaining to the durability of the alkyl silane coating used in this work are also presented.

Experimental validation of a theoretical model: Uncertainty propagation analysis to a PCM-air thermal energy storage unit

This paper analyzes the uncertainties propagation of the theoretical model developed to simulate the thermal performance of PCM-air heat exchanger. Numerical results are compared with experimental results obtained with a full-scale heat exchanger prototype. In the model, the energy equation is considered in terms of enthalpy, as the governing equation can be applied at any stage, determining the temperature at each point and therefore assessing the value of the PCM thermophysical properties. Due to the uncertainties of the input variables, this analysis provides a band of uncertainty associated with the solution that is useful and more realistic when comparing the experimental and simulated results. In this sense, for most of the process, there has been observed a high degree of overlapping between the two different heat rate curves. For this type of heat exchangers, the uncertainty propagation analysis also quantifies the consequences that imply making the effort of improving the determination of a parameter or enhancing the measurement of a variable. In this study case, it is not worth to invest efforts in determining the average phase change temperature of a PCM with an uncertainty better than ±0.25 °C when the main response is the average heat rate.

Design of a novel, intensified heat exchanger for reduced fouling rates

This paper describes an integrated approach into the design and evaluation of a novel tube bundle heat exchanger that achieves higher heat transfer levels at lower levels of pressure drop, while remaining less susceptible to gas-side fouling. The approach combines laboratory scale experiments with industrial observations and numerical simulations of full-scale heat exchangers to study the thermal, hydraulic and fouling characteristics of tube bundle heat exchangers. Three arrangements are compared and the advantages of the proposed novel arrangement are demonstrated. Enhanced heat transfer rates are combined with reduced pressure drop and gas-side fouling rates through careful design of the shape of the tube cross-section and reduced transverse spacing.

Flow Losses Through an Array of A-Frame Heat Exchangers

The isothermal flow losses through an array of A-frame finned tube heat exchangers are determined experimentally. Correlations that describe these losses are derived from scale-model tests conducted on different V-bundle configurations, each simulating a section of a typical A-frame array. The experimental data are then compared with test data obtained from a full-scale heat exchanger installation. It is found that the downstream flow losses behind a V-bundle section are significantly influenced by the semiapex angle, the process stream duct diameter, and the distance between the A-frames.