Fouling Mitigation in a Heat Exchanger: High Cycles of Concentration for a Cooling-Tower Application

The purpose of the present study is to investigate the effect of a physical water treatment (PWT) technology on fouling mitigation in a simulated cooling tower operating at high cycles of concentration. Hard water was produced by evaporating pure water in a circulating open cooling tower, where dissolved calcium carbonate ions became concentrated with time. Heat transfer tests were conducted in a rectangular channel by varying the cycle of concentration (COC) from 5 to 10, and fouling resistances were measured over 270 hrs for each case with and without the PWT treatment. Another test was conducted with no blowdown case with and without the PWT treatment. The fouling resistance at 5 cycles with the PWT treatment was about 70% less than that in the case without the PWT treatment at the end of 270-hr tests. Even at 10 cycles, the PWT treatment reduced the fouling resistance by 60% from the value for the no treatment case. Thus, one can conclude that the PWT technology can help circulating cooling-tower water at relatively high COC for significant freshwater conservation, while keeping fouling resistances below industry standards.     

Heat-Treated Titanium Balls for the Mitigation of Mineral Fouling in Heat Exchangers

The present study investigated the feasibility of a physical water treatment (PWT) concept using heat-treated titanium balls for the mitigation of mineral fouling in heat exchangers. A concentric tube heat exchanger in a counter-flow set-up served as the test section, where three different velocities (i.e., 0.3, 0.5, 0.8 m/s) were used for the fouling tests. The results showed that for the PWT cases fouling resistances were 24–30% lower than those for the untreated cases, depending on the flow velocity and the electrical conductivity of water. The SEM photographs of the fouling deposits in the PWT cases revealed a soft form of scale that could easily be removed by the shear forces produced by the flow in the heat exchanger. Furthermore, the PWT case produced more particles in bulk water by approximately 35% than the untreated case, supporting the bulk precipitation hypothesis for the PWT.     

管式换热器积灰特性的实验研究

在实验室建立了气侧污垢热阻实验台，实验研究了管式换热器管内微粒污垢的积聚特性，结果表明积灰随时间呈现渐近特性，没有发现诱导期。同时还考察了流体速度，颗粒直径和浓度对渐近污垢热阻的影响．     

An Experimental Investigation of Forced Convection Heat Transfer from a Coiled Heat Exchanger Embedded in a Packed Bed

Forced convection heat transfer from a helically coiled heat exchanger embedded in a packed bed of spherical glass particles was investigated experimentally. With dry air at ambient pressure and temperature as a flowing fluid, the effect of particle size, helically coiled heat exchanger diameter, and position was studied for a wide range of Reynolds numbers. It was found that the particle diameter, the helically coiled heat exchanger diameter and position, and the air velocity are of great influence on the convective heat transfer between the helically coiled heat exchanger and air. Results indicated that the heat transfer coefficient increased with increasing the air velocity, increasing helically coiled heat exchanger diameter, and decreasing the particle size. The highest heat transfer coefficients were obtained with the packed-bed particle size of 16 mm and heat exchanger coil diameter of 9.525 mm (1/4 inch) at a Reynolds number range of 1,536 to 4,134 for all used coil positions in the conducted tests. A dimensionless correlation was proposed for Nusselt number as a function of Reynolds number, particle size, coil size, and coil position.     

超大拓展表面三维微肋管换热器的对流换热及抗积灰特性研究

工业烟气含尘的特点易导致换热器积灰,进而制约烟气余热的高效回收。本文针对一种具有超大拓展表面的三维微肋管换热器的对流换热与积灰特性进行了研究。首先,对比研究了光管与三维微肋管的对流换热特性;接着,基于所建立的积灰数值模型,探究了三维微肋管的积灰特性,并揭示了烟气流速与飞灰粒径对其积灰特性的影响规律。结果表明,相对于传统光管,三维微肋管的换热面积可增大约2.9倍;换热性能平均能提高16%;积灰后渐进污垢热阻最大能减小70%;同时,清灰周期更长,运行经济性更佳。综合而言,三维微肋管相比传统光管,在增强换热的同时,还能有效减轻积灰,因此可作为高效的抗积灰传热元件,应用于含尘烟气的余热回收场合。     

缩放管混合污垢特性的实验研究

实验室里进行了缩放管与对应光管管内混合污垢的对比实验,实验工质为加有MgO或CaSO颗粒的人工硬水.实验结果表明:在相同流速、浓度条件下,与对应光管相比,缩放管不仅有较好的阻垢性能,还有较好的传热性能;流速、颗粒浓度和颗粒粒径对混合污垢热阻有较大影响:增大流速、减小颗粒浓度、增大颗粒粒径都会使缩放管混合污垢热阻的渐近值减小.     

盘状螺旋管换热器的实验研究

介绍了一种适用于小流量大温升的新型盘状螺旋管换热器,具有结构简单、质轻,体积小的特点;利用相关的实验研究,获得其换热性能和流动阻力特性。实验结果表明,其在小流量下仍具有较高的传热系数,适合于做热泵热水器的水冷换热器。     

冷却水水质参数对板式换热器污垢特性的实验研究

实验研究了松花江水浊度、铁离子(Fe3+)、碱度、化学需氧量(COD)等水质参数对板式换热器污垢热阻的影响.结果表明:松花江水污垢热阻类型为渐近型且没有诱导期;浊度开始变化很小,后逐渐增大;碱度和铁离子随着时间逐渐增大,达到某一值后趋于平缓;COD随时间逐渐减小.所研究的水质参数之间相互影响、互利共生,共同影响着污垢沉...     

Empirical Study of an Air-to-Air Heat Pipe Heat Exchanger in Tropical Climates

Abstract

This article presents the empirical study of a heat pipe heat exchanger that was directly experiencing the ambient tropical air in its evaporator section. The performance of the heat pipe heat exchanger was monitored during two weeks of operation to determine the performance curves. The temperature of return air was controlled at about 22°C as the representative temperature of inside air, and a face velocity of 2 m/s on the heat pipe heat exchanger coil was established for this purpose. It was found that for the present investigation, the heat pipe heat exchanger can pay for itself in 1.5 years.     

Modeling Fouling Factors for Microscale Heat Exchangers

This article presents an approach to the modeling of CaCO₃ fouling and cleaning in a microscale cross-flow heat exchanger. The fouling progress was detected by thermal, fluid dynamical, and optical measures. In general, the observed fouling phenomena at microscale were comparable to those at macroscale. The detected thermal fouling resistance was between 10^? 6 and 10^? 3 m^? 2 K^? 1. As expected, crystallization fouling strongly depends on the surface temperature. In addition, the surface coverage can be a useful measure for both processes—fouling and cleaning. An extended approach to derive a reasonable fouling allowance was formulated through a fractional fouling resistance.     

Evaluation of Nusselt number and effectiveness for a vertical shell-coiled tube heat exchanger with air bubble injection into shell side

Air bubble injection was employed to increase the heat transfer rate (Nusselt number) of a vertical shell and coiled tube heat exchanger in this article. Hot and cold water flowed into the coil side and shell side of heat exchanger, respectively, and air bubbles were injected inside the shell side of heat exchanger via a memorable method. Bubbles' vertical movement due to buoyancy forcing through the heat exchanger can enhance the heat transfer rate by mixing the thermal boundary layer, increasing the turbulence level of the fluid flow and increasing the shell-side fluid Reynolds number.     

Performance of a compact,spiral coil heat exchanger

A spiral coil heat exchanger consists of a number of horizontal layers of spirally wound, finned tubes connected to vertical manifolds at the inner and outermost turns of each coil. This design has advantages in heat recovery and air-conditioning applications. Two theoretical models to predict the performance of this compact heat exchanger are presented based upon unmixed and mixed air-flow considerations. Results of experimental studies on a laboratory model of the spiral coil heat exchanger agree well with the predicted performance of the theoretical models. Charts of effectiveness vs NTU suitable for design are presented.     

析晶与颗粒混合污垢的实验研究

在实验室实验研究了CaCO3和Al2O3或MgO混合污垢特性,考察了颗粒浓度、工质流速和水浴温度的影响。结果表明:混合污垢热阻值比纯粹的析晶污垢的污垢热阻值大,流速增大,其污垢热阻渐近值减小,颗粒浓度增大,混合污垢渐近值增大,但当浓度达到某一值后再增加时,混合污垢渐近值基本维持常数,水浴温度在实验范围内变化对结垢影响不大。     

A Modified Alderman–Grant Coil makes possible an efficient cross-coil probe for high field solid-state NMR of lossy biological samples

The design, construction, and performance of a cross-coil double-resonance probe for solid-state NMR experiments on lossy biological samples at high magnetic fields are described. The outer coil is a Modified Alderman–Grant Coil (MAGC) tuned to the ¹H frequency. The inner coil consists of a multi-turn solenoid coil that produces a B₁ field orthogonal to that of the outer coil. This results in a compact nested cross-coil pair with the inner solenoid coil tuned to the low frequency detection channel. This design has several advantages over multiple-tuned solenoid coil probes, since RF heating from the ¹H channel is substantially reduced, it can be tuned for samples with a wide range of dielectric constants, and the simplified circuit design and high inductance inner coil provides excellent sensitivity. The utility of this probe is demonstrated on two electrically lossy samples of membrane proteins in phospholipid bilayers (bicelles) that are particularly difficult for conventional NMR probes. The 72-residue polypeptide embedding the transmembrane helices 3 and 4 of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) (residues 194–241) requires a high salt concentration in order to be successfully reconstituted in phospholipid bicelles. A second application is to paramagnetic relaxation enhancement applied to the membrane-bound form of Pf1 coat protein in phospholipid bicelles where the resistance to sample heating enables high duty cycle solid-state NMR experiments to be performed.     

Investigation of Boiling of Refrigeration Medium Under Periodic Disturbance Conditions

Abstract

Experimental investigations of boiling in flow of a refrigeration medium under disturbances typically occurring in refrigeration units are conducted. The development and decay of boiling is stimulated by periodic changes of mass flux density in the coil tube. It is confirmed that the two-phase systems exhibit wave properties and the disturbances generated in the system propagate with finite velocities. An experiment-based correlation to describe the disturbance propagation velocity during the development and decay of boiling in the coil tube is elaborated. The investigations are carried out for an environment-friendly refrigerant R404A supplied under pressure to the heat exchanger.     

Using a cross-coil to reduce RF heating by an order of magnitude in triple-resonance multinuclear MAS at high fields

Four different coil designs for use with MAS in triple-resonance multi-nuclear experiments at high fields are compared, using a combination of finite element analysis (FEA) software and NMR experiments, with respect to RF field strength per unit power and relative sample heating, as governed by mean E/B(1) within the sample region. A commercial FEA package, Microwave Studio 5.1 by Computer Simulation Technology (CST) is shown to obtain remarkably accurate agreement with the experiments in Q(L), L, B, E, and mode frequencies in all cases. A simplified treatment of RF heating in NMR MAS samples is derived and shown to agree with the NMR experimental results within about 10% for two representative stator designs. The coil types studied include: (1) a variable-pitch solenoid outside a ceramic coilform, (2) a conventional solenoid very closely spaced to the MAS rotor, (3) a scroll coil, and (4) a segmented saddle cross coil (XC) for (1)H with an additional solenoid over it for the two lower-frequency channels. The XC/solenoid is shown to offer substantial advantages in reduced decoupler heating, improved S/N, and improved compatibility with multinuclear tuning and high-power decoupling. This seems largely because the division of labor between two orthogonal coils allows them each, and their associated circuitry, to be separately optimized for their respective regimes.     

微通道-微槽群中间换热器特性实验研究

 对用于固体激光介质冷却的组合式中间换热器的流动与传热特性进行了实验研究。实验研究结果表明：努塞尔数随雷诺数的增加而增加，总热阻随微通道侧蒸馏水流量的增加而减小，总换热量随微通道侧蒸馏水流量的增加而增加，且换热器的传热系数可以达到1.5×104 W/(m²·K)，总热阻小于0.3 K/W，能较好地解决当前固体激光介质冷却系统中间换热器所存在的问题。     

Comprehensive Study of the Effect of the Addition of Four Drag Reducing Macromolecules on the Pressure Drop and Heat Transfer Performance of Water in a Finned Tube Heat Exchanger

Abstract

In the present study the effects of the addition of four drag reducing agents (DRA), including carboxy methyl cellulose with high molecular weight (DRA1) and medium molecular weight (DRA2), polyacrylamide (DRA3) and the natural polymer, xanthan gum (DRA4), to water on the pressure drop and heat transfer performance in a finned tube-heat exchanger were compared. Laminar flow (Reynolds number (Re) <1400) was studied to transfer heat between water and air in the finned tube heat exchanger. The results showed that DRA1, with a maximum %DR of 26%, and DRA4, with a maximum %DR of 5%, were the highest and the lowest obtained results, respectively. In the case of heat transfer reduction percentage (%HTR), DRA4, having more than 34.5%, was the highest, and DRA1, with about 13.7%, was the lowest result for the concentration range of 0-100?ppm and temperature range of 40–65?°C.     

基于热量流法的超临界CO2换热器性能分析

本文基于热量流法,应用进口温差定义的换热器通用热阻,采用分段法考虑超临界CO₂的物性变化,结合能量守恒方程,构建了超临界CO₂-冷却水换热过程的整体热量流拓扑模型,结合超临界CO₂物性库和经验关联式提出了换热器性能分析的流程图,实现了综合考虑物性-结构-运行参数的超临界CO₂-冷却水换热器的性能分析,进一步结合[火积]耗散热阻的概念提出了评价物性变化时换热器换热性能的等效热阻。通过分析获得了换热量的沿程分布规律,发现当进口压力为11 MPa时,总的换热量最大,此时等效热阻最小。总之,热量流法对超临界CO₂为工质的换热器设计和系统分析是可行和有效的。     

Heat Transfer Characteristics in a Double-Pipe Heat Exchanger Equipped with Coiled Circular Wires

An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by inserting coiled wire around the outer surface of the inner tube of the double-pipe heat exchanger. Insulated wires, with a circular cross-section of 2 mm diameter, forming a coil of different pitches (p = 6, 12, and 20 mm), were used as turbulators. The investigation is performed for turbulent water flow in a double-pipe heat exchanger with cold water in the annulus space for both parallel and counter flows. The experiments were performed for Reynolds numbers ranging from 4,000 to 14,000. The experimental results reveal that the use of coiled circular wires leads to a considerable increase in heat transfer coefficients compared with a smooth wall tube for both parallel and counter water flows. The mean Nusselt number increases with Reynolds number and pitch. The convective heat transfer coefficient for a turbulent water flow increases for all coiled wire pitches, with the highest enhancement of about 450% for counter flow and 400% for the parallel flow. New correlations for mean relative Nusselt numbers at different coiled wire pitches are provided.