Plate Heat Exchangers: Calculation Methods for Singleand Two-Phase Flow

Plate heat exchangers were first developed about 100 years ago but have won increasing interest during the last two decades, primarily due to the development of methods of manufacturing brazed plate heat exchangers. This type of heat exchanger offers very good heat transfer performance in single-phase flow as well as in evaporation and condensation. Part of the reason is the small hydraulic diameters, typically being less than 5 mm. Other advantages of plate heat exchangers are the extremely compact design and the efficient use of the construction material. In spite of their long use, the calculation methods for predicting heat transfer and pressure drop are not widely known. It is the purpose of this article to present such calculation methods for single-phase flow and for flow boiling and to discuss some of the specifics of this type of heat exchangers.     

Gas/Liquid Flow in Plate-and-Frame Heat Exchangers - Part II: Two-Phase Multiplier and Flow Pattern Analysis

Pressure drop data measured during adiabatic two-phase flow in a plate-and-frame heat exchanger (PHE) are normalized with respect to the single-phase liquid pressure drop to give two-phase multipliers. A curve-fitted equation defines this relationship, which is a strong function of the Lockhart-Martinelli parameter. C coefficients are shown to be strong functions of both the Lockhart-Martinelli parameter and liquid viscosity, making this correlation unsuited to predictions of pressure drop in PHEs. Interfacial structure, observed during air-water downflow in replica channels (dC = 3 mm), is categorized into five flow patterns. These have a number of similarities with structures reported for circular and rectangular channels of similarly low hydraulic diameter. The transition boundaries between the patterns are shown to be a function of the chevron angle.     

Two-Phase Frictional Pressure Drop and Flow Boiling Heat Transfer for R245fa in a 2.32-mm Tube

Experimental two-phase frictional pressure drop and flow boiling heat transfer results are presented for a horizontal 2.32-mm ID stainless-steel tube using R245fa as working fluid. The frictional pressure drop data was obtained under adiabatic and diabatic conditions. Experiments were performed for mass velocities ranging from 100 to 700 kg m^?2 s^?1, heat flux from 0 to 55 kW m^?2, exit saturation temperatures of 31 and 41°C, and vapor qualities from 0.10 to 0.99. Pressures drop gradients and heat transfer coefficients ranging from 1 to 70 kPa m^?1 and from 1 to 7 kW m^?2 K^?1 were measured. It was found that the heat transfer coefficient is a strong function of the heat flux, mass velocity, and vapor quality. Five frictional pressure drop predictive methods were compared against the experimental database. The Cioncolini et al. (2009) method was found to work the best. Six flow boiling heat transfer predictive methods were also compared against the present database. Liu and Winterton (1991), Zhang et al. (2004), and Saitoh et al. (2007) were ranked as the best methods. They predicted the experimental flow boiling heat transfer data with an average error around 19%.     

Experimental Investigation of Frictional Pressure Drop for Two-Phase Flow Inside Spirally Fluted Tubes

This article reports the results of an experimental study of frictional pressure drop for two-phase flow (air and water) inside horizontal spirally fluted tubes. Nine spirally fluted tubes, representing a wide range of geometric parameters, were tested adiabatically with various flow rates of air and water. Average pressure drop enhancement levels (over predicted smooth tube values) in the range of 1,4 to 3.9 were observed. A design correlation was developed based on modifications to an existing smooth-tube model. The proposed correlation predicts 84% of the database (581 experiment data points) within ±20%.     

Two-Phase Flow Visualization in Chevron and Bumpy Style Flat Plate Heat Exchangers

Adiabatic flow visualization in a chevron plate, a 1:1 aspect ratio bumpy plate, and a 2:1 aspect ratio bumpy plate heat exchangers were investigated for vertical upward flow with R134a. Qualities ranging from 5% to 90% and mass fluxes of 60, 90, and 125 kg/m²-s were investigated. The flow visualization experiments were conducted at a 10°C inlet temperature. Four flow regimes were observed for the flat plate geometries investigated: bubbly flow, rough annular flow, smooth annular flow, and mist flow. The four flow regimes are mapped out on a mass flux versus quality basis for each geometry. The chevron geometry was seen to undergo flow transitions at lower qualities and mass fluxes than the bumpy plate geometries, and the 2:1 aspect ratio bumpy plate geometry was seen to undergo flow transitions at lower qualities and mass fluxes than the 1:1 aspect ratio bumpy plate geometry.     

CFD Simulation of Heat Transfer and Pressure Drop in Compact Brazed Plate Heat Exchangers

In this paper, the thermal and hydraulic characteristics of corrugated fluid channels of compact brazed plate heat exchangers (BPHE) are investigated by computational fluid dynamics (CFD) simulations using the commercial CFD software ANSYS CFX 14.0. The influence of geometry parameters of the corrugated pattern such as chevron angle and corrugation pitch on the BPHE performance is investigated on small fluid section geometries. The influence of various types of wall heat transfer boundary conditions on the simulation results is also studied. An entire fluid channel is simulated using various turbulence models in the Reynolds number range of 300 to 3000. The CFD predictions are also validated using data obtained from laboratory experiments. The simulations of the entire fluid channel underpredict heat transfer and pressure drop by 20–30% and 10–35%, respectively. The results from the small fluid sections suggest that the CFD simulations can be used as a reasonably effective tool in determining the relative performance variation of various plate patterns.     

Single-Phase Flow Pressure Drop Analysis in a Plate Heat Exchanger

Pressure drop characteristics of a gasketed commercial plate heat exchanger configured for single-phase water-to-water flow application are presented. Isothermal pressure drop data are provided for two symmetric 30°/30°, 60°/60° and a nonsymmetric 30°/60° (mixed) chevron plate configuration in the plate heat exchanger. Reynolds number was varied from 500 to 2,500. The experimental data are found to be a strong function of chevron angle and Reynolds number. Experimental results show that mixed plate configuration can be a choice in optimizing the plate heat exchanger design for improved performance. Based on experimental data, correlations are presented for estimation of friction factor.     

Two-Phase Flow Distribution to Tubes of Parallel Flow Air-Cooled Heat Exchangers

Abstract

This paper addresses two-phase flow distribution phenomena in multiple header–tube junctions used in heat exchangers. Because of phase separation, it is very difficult to obtain uniform two-phase flow distribution to the branch tubes. The flow distribution is strongly influenced by the header orientation (horizontal or vertical) and the number of branch tubes. Other factors that influence the flow distribution are the flow direction in the header (upflow or downflow), the header shape and tube end projection into the header, and the location and orientation of the inlet and exit connections. The source of maldistribution is the flow in the dividing headers. Work performed by the authors and others (including patents) are discussed. The possibilities for eliminating two-phase flow maldistribution are identified and discussed. This investigation shows that solutions, which provide uniform flow distribution, are very design-specific. Change of the geometry or operating parameters will require modification of the design.     

Modeling for Shell-Side Heat Transfer Coefficient and Pressure Drop of Helical Baffle Heat Exchangers

This study has suitably modified the existing heat transfer and pressure drop correction factors of the modified Bell–Delaware method used for heat exchangers with segmental baffles, taking into consideration the helical baffle geometry. These correction factors are presented in parametric formulas based on the Taborek presented procedure for segmental baffles. These formulas are functions of the geometrical and physical parameters of discontinuous helical baffles. In addition, the parametric formulas are presented graphically based on the Stehlik method. The correction design method proposed by Stehlik for the helical baffle is presented in detail and a theoretical model for shell-side heat transfer and pressure drop is developed. In general, the results show that the present model matches more closely with the graphically proposed correction factors of Stehlik. In order to calculate the shell-side heat transfer coefficient and pressure drop using the present method, a computational code has been developed by the authors. In addition, in order to examine the validity, the results of the code for a case study are compared with the results obtained from EXPRESS software and experimental formulas presented by Zhang. The results of comparison show that the proposed method is accurate and can be used by designers confidently.     

内螺纹水冷壁管内两相流动的摩擦压降特性

在压力为9～35 MPa,质量流速为600～1800 kg/(m2·s),干度为0～1的工况范围内,对φ28.6 × 5.8(mm)的四头内螺纹水冷壁管中单相及两相流体在绝热和受热条件下的摩擦压降特性进行了试验研究.结果表明:在受热和绝热两种条件下内螺纹管的阻力特性不同,受热管的单相摩擦压降系数f比绝热管的小;受热管的两相摩擦倍率φ2l0比绝热管的大.无论是受热,还是绝热情况下,压力对φ2l0的影响很大,φ2l0随压力增大而减小;质量流速的影响很小.随蒸汽干度增加,φ2l0先增加,随后增幅减小.提出了由试验获得的单相水摩擦压降系数以及汽水两相流体摩擦压降的计算式.     

Frictional Pressure Drop for Gas Flows in a Microchannel Between Two Parallel Plates

In this study, air flow through a microchannel between two parallel plates of height ranging from 100 to 710 μm was investigated experimentally. Each channel was made of Plexiglas and had a large cross-sectional aspect ratio to supply the microplaneduct geometry. The flow rate and pressure drop across the microchannel were measured at steady state to obtain the friction factor. The Reynolds number ranged from 30 to 2300. The experimental friction factor values were found in good agreement with an existing analytical solution for an incompressible, fully developed, laminar flow under no-slip boundary conditions.     

Experimental Investigation on Port-to-Channel Flow Maldistribution in Plate Heat Exchangers

Experiments have been conducted to analyze the flow and pressure distribution in a plate heat exchanger by measuring local port pressure distribution in a commercial plate heat exchanger. Flow rate in channel and channel pressure drops are evaluated by measuring the pressure inside the inlet and exit ports at different locations for different port dimensions. In these experiments, the measurement of pressure is done without disturbing the fluid flow inside the port. This technique also offers the option of manipulating port size without changing the plate characteristics. Direct experimental measurement provides the scope for eliminating other effects, such as gasket, end losses, and improper wetting of channels from the flow maldistribution effect. The measurements indicate the existence of non-uniform flow distribution that increases with flow rate and decreases with port diameter. Results clearly show that it is important to consider the flow maldistribution for better design of plate heat exchangers.     

Calculation of Two-Phase Convective Heat Transfer Coefficients of Cryogenic Nitrogen Flow in Plate-Fin Type Heat Exchangers

The two-phase convective heat transfer coefficients for nitrogen inside the flow path of plate-fin type heat exchangers operating at cryogenic temperatures are calculated using CFX Release 13.0. Using a homogeneous two-phase model, the governing equations are solved to find pressure, velocity, and enthalpy distributions for three types of fin geometries: plain, wavy, and serrated. The results are further processed to evaluate the wall shear stress and heat flux, which in turn yield the friction coefficients and convective heat transfer coefficients. The coefficients are presented as functions of system pressure, flow rate, and local quality. The results can be used for the design of plate-fin type exchangers with the same fin configurations and operating conditions as the calculation.     

泄流槽式螺旋折流板换热器传热系数与压降实验研究

以螺旋角25°为例,研究了泄流槽式螺旋折流板换热器传热系数与压降的变化,并给出了泄流槽螺式旋折流板换热器的传热系数与压降相关的计算公式。该结果可为系统仿真和泄流槽式螺旋折流板换热器的设计计算提供借鉴。     

An Evaluation of the Effect of Twisted-Tape Swirl Generators in Two-Phase Flow Heat Exchangers

Correlations covering all modes of single- and two-phase heat transfer with twisted-tape swirl generators have been incorporated into a computer program that predicts the effect of twisted tapes on the heat duty in fluid-to-fluid once-through vapor generators. These Results are compared with those from axial flow once-through vapor generators operating at the same inlet conditions. Pumping power and pressure drop with and without swirl generators are compared. The correlations used satisfactorily predict the performance of both types of heat exchangers. Significant area, pressure drop, and pumping power reductions are possible using twisted-tape swirl generators; almost the entire area reduction is attributable to the suppression of the critical heat flux condition.     

Hydraulic Characteristics of Flow Boiling of Hydrocarbon Fluids: Two-Phase Pressure Drop

Two-phase hydraulic characteristics, in terms of pressure drop and void fraction data, are reported for boiling of single-component hydrocarbon fluids in vertical upflow. These data were obtained simultaneously with the boiling heat transfer measurements. The systematic trends of the measured pressure gradient with respect to vapor quality, mass flux, and pressure are examined. This provides useful information in terms of the relative importance of the constituent parts of the two-phase pressure gradients and confirms the internal consistency of the measured data. These two-phase pressure-drop data under flow boiling conditions are then compared with various correlations from the open literature and also with a proprietary correlation used in commercial heat exchanger design and simulation software. Typical results of these comparisons are presented. It is noted that in the near-zero vapor quality region the measured pressure-gradient data may be lower than expected because of the effect of subcooled boiling.     

Pressure Drop Analysis of Steam Condensation in a Plate Heat Exchanger

In cooperation with Alfa Laval Thermal AB Company in Sweden, a steam condensation test rig of a plate heat exchanger (PHE) was set up. The heat transfer and pressure drop characteristics of two kinds of typical operation condition in a PHE were obtained: complete condensation and partial condensation. This article introduces the test rig, various sensors, and the data acquisition system used in the measurements. The process of steam condensation in a PHE is analyzed in detail. The two-phase frictional pressure drop along this wavy channel was obtained from the measured steam condensation pressure drop. The Lockhart-Martinelli model was extended to predict the pressure drop of steam condensation in a PHE and was verified by the experimental results. Based on the data processing of 109 experimental points, a correlation of frictional pressure drop of steam condensation in a PHE is suggested. This correlation is recommended for calculation of the steam condensation pressure drop in a PHE.