Rotary Regenerative Heat Exchangers Research Articles

The effect of leakage on pressure loss of rotary regenerative heat exchangers

The pressure loss of the rotary exchanger is an important parameter that affects the energy consumption. Based on the literature review, the available correlations for calculating the pressure loss of rotary regenerative heat exchangers are presented in the article. The article presents extensive experimental research of rotary regenerative exchangers with different wheel geometries. A total of nine exchangers with eight different heat wheel geometries were investigated. The experiment was divided into two tasks concerning the pressure loss of the wheel channel and the total pressure loss of the built-in exchanger. The results of both approaches show differences that are caused by the leakage of rotary exchangers, which is their natural property. Based on the results of the experiment, correlations were proposed for determining the coefficient of friction in a channel in the shape of a sinusoid and a correlation for calculating the total pressure loss of the built-in rotary exchanger. The presented correlations have been experimentally verified and can be applied with a sufficient accuracy of ±5%. The new correlations were compared with the published results. The article discusses the effect of leakage of exchangers equipped with a labyrinth seal. Based on the experimental investigation, it was found that the leakage of the examined exchangers varies in a wide range of 1 to 4% and depends on the quality of the exchanger construction.

Performance Prediction of a Model Rotary Air Preheater Through Porous Media Approach

Abstract Rotary regenerative heat exchangers or rotary regenerators are used for various heat recovery applications in order to fulfill the energy requirements. Rotary air preheater is a rotary regenerative heat exchanger which is widely used in thermal power plants to recover heat energy from exhaust flue gases. Several numerical works are being carried out to improve the performance and reduce the cost of the rotary air preheater. However, the accurate modeling of the internal structure of the matrix demands a lot of CPU power and is a big challenge for researchers. To overcome this problem, the rotary matrix of the preheater can be modeled using a porous media approach. But a thorough investigation is required to evaluate its performance. In this work, rotary air preheater simulations were carried out by applying a porous media approach. The effects of different operating conditions, such as speed of rotor, the mass flowrate of incoming fluids, material of matrix, and inlet temperature of hot fluid on the performance of rotary air preheater were investigated. Simulation results were validated with the results obtained from the experimentation performed in-house. The effect of fluid flowrates on pressure drop was also studied. It has been observed that the rotational speed of the rotor significantly affects the performance of the preheater, while the inlet temperature of hot fluid has a negligible effect on performance.

The energy saving potential of a rotary heat exchanger with bypass dampers

The article deals with the possibilities of using a bypass damper in a rotary regenerative heat exchanger for heat recovery in ventilation and air conditioning system and calculates the energy savings related to overcoming the pressure loss of the exchanger. For the purposes of the analysis, a prototype heat exchanger with a wheel diameter of 1000 mm and bypass damper with a diameter of 200 mm was designed and constructed. The exchanger was subjected to an experimental investigation in order to determine its behaviour. The pressure loss of the exchanger, depending on the air flow rate, as well as the pressure loss for three nominal air flow rates, depending on the opening of the bypass damper, were measured. The measurement showed that the new exchanger design enables a reduction in the pressure loss of the exchanger by up to 49% compared to the original value. Based on the measured values, an analysis of the potential savings of electrical energy for driving fans in various European cities was carried out. The energy savings depend on the given climatic location where the exchanger will be used and on the operating time of the air handling unit. From the analyses carried out, it is clear that, by including the bypass damper, 18 to 44% of the electrical energy of the fan can be saved, which overcomes the pressure losses of the heat exchanger. Based on the current electricity prices, the return on the investment for the bypass damper was calculated based on the price of electricity at the given time and place (Prague - Czech Republic, 2022), which is from 0.5 to 3 years, depending on the type and period of operation of the air handling unit equipment, at common air velocities.

Determination of the sensible heat effectiveness and pressure loss of a rotary regenerative heat exchanger using CFD

The paper presents a new method to determine the sensible effectiveness and pressure loss of rotary regenerative heat exchangers using computational fluid dynamics (CFD). It is based on CFD simulations of a single thermal wheel microchannel with a small cross-sectional area and thin walls, with cyclic inlet and outlet boundary conditions. Two unique measurement set-ups were designed and built for the experimental measurement of the heat exchanger characteristics. Five different types of thermal wheels were manufactured, measured, and simulated in ANSYS Fluent. All wheels achieve an effectiveness greater than 73% under certain (air flow) conditions, which is the minimum effectiveness required by Ecodesign (in the EU). For the examined exchangers, the effectiveness ranges from 66.5% to 83.3%, depending on the boundary conditions and geometric parameters of the rotors. The highest sensible effectiveness is achieved by heat exchangers with the largest heat exchange surface Ac; on the other hand, these wheels have the largest pressure loss. The paper discusses the use of a simplified ε-NTU correlation model for the HVAC systems (typically C* = 1). The correlation model and CFD results were compared and found to be different from each other. The results of the CFD simulation were compared with measurements to prove that the proposed simulation method can predict the behaviour of the real heat exchanger as a whole. It was demonstrated that the sensible effectiveness and pressure loss of the rotary heat exchanger predicted by the CFD simulations correspond well to the measured values within the measurement uncertainty ±1.3%. The proposed method can be used for the comparison of different rotary regenerative heat exchangers before their manufacturing and for verification that they meet the EU Ecodesign requirements set by the current legislation. It reduces the cost of the initial optimisation and testing of new designs.

Numerical simulation of a novel regenerative heat exchanger with combined sensible–latent heat storage matrix

A new rotary regenerative heat exchanger with combined sensible–latent heat storage matrix was proposed. A local thermal non-equilibrium model with three energy equations for fluid, sensible heat storage material, and latent heat storage material in the porous matrix was developed. Rotary regenerative air preheaters (RAPHs). The RAPHs with combined sensible–latent heat storage matrix (CRAPH), with sensible matrix (SRAPH) and with latent heat storage matrix (LRAPH) were comparatively evaluated according to their thermal and economic performances. The temperature distributions of sensible and latent heat storage materials in the matrix exhibit different characteristics. The latent heat storage material that undergoes a phase change process during charging or discharging can greatly buffer the temperature swings of fluid and sensible heat storage material, especially under lower rotating speed conditions. Even if a small amount of latent heat storage materials were added to the sensible heat storage matrix, the efficiency of RAPH can be greatly improved.

Novel rotary regenerative heat exchanger using cascaded phase change material capsules

A novel rotary regenerative heat exchanger with cascaded phase change material (PCM) capsules was proposed and proved to be a better alternative to the traditional one. The rotary regenerative air preheater used in coal-fired power plant was taken as an example to comparatively analyze the performance characteristics of the rotary regenerative heat exchanger with latent heat storage material and the one with sensible heat storage material. The latent rotary regenerative air preheater (LRAPH) can achieve more uniform outlet temperature of air at low rotating speed than the sensible one (SRAPH), which is conductive to the measurement and process control in application. PCM capsule functions as both a latent heat storage material at its phase change temperature and as a sensible heat storage material at other temperature values. At lower rotating speed, more PCM capsules in LRAPH will work at the phase change temperature resulting in small temperature swings of fluid and matrix. The heat exchanger effectiveness is comparatively analyzed for SRAPH and LRAPH with different matrix parameters. The replacement of sensible heat storage materials by the latent ones would lead to an efficiency improvement of RAPH as large as 11% especially at lower rotating speed. The LRAPH is more capable to counter the entrainment leakage and the problems induced by temperature swings than traditional SRAPH. The cascaded distribution of PCM capsules with different phase change temperatures should be reasonably set according to the specific characteristics of temperature distribution in application to make full use of advantages of cascaded PCMs.

A comprehensive study of the second law of thermodynamics for a rotary regenerative heat exchanger with two types of simple and anodized aluminum sheets: an experimental study

ABSTRACT Rotary regenerative heat exchangers are commonly used in the industry. However, the corrosion of sheets that are in contact with the operating fluid is considered as the disadvantage of these heat exchangers. This causes a reduction in the lifespan of this kind of heat exchanger. The object of the present study is to evaluate and compare the exergy loss and the second law efficiency of a rotary regenerative heat exchanger, which is made from two types of simple aluminum sheets and anodized aluminum sheets are. Anodized sheets are used to enhance the corrosion resistance of the regenerative heat exchanger. The results indicated that the lower exergy loss and the dimensionless exergy loss with anodized sheets about 14–24% and 15–20% compared to the heat exchanger with simple sheets, respectively. Besides, the effects of rotational speed of the matrix, inlet temperature of hot flow, and the flow rates of hot and cold flows on the rotary regenerative heat exchanger are evaluated. Finally, some empirical correlations have been developed based on the obtained data and curve fitting process.

Design and operational condition optimization of a rotary regenerative heat exchanger

In this study, comprehensive optimization studies of rotary regenerative heat exchangers (RHEX) used in the flue gas desulphurization system are conducted. In this context, two optimization studies are taken into consideration; the first one is the optimization for the existing design by replacing the matrix elements with corrugated – undulated (CU) plates and the second one is the design and operational condition optimization to design a RHEX for the same boundary conditions utilizing CU plates. Sinusoidal pitch, inner height and the undulation angle of the undulated plate are selected as the decision variables where effectiveness, entropy generation number, and cost are treated as the objectives in the first case. For the second case, RHEX outer diameter and length, as well as the angular velocity, are added to the design variables and RHEX volume is considered to be an additional objective. Case-specific heat transfer coefficient and friction factor based on the experimental data are adapted through the thermal–hydraulic modelling of the RHEX with the effectiveness and number of transfer unit method. NSGA-II is implemented for multi-objective optimization. It is concluded that thanks to optimized CU and RHEX geometry as well as the angular velocity, RHEX effectiveness can be augmented by 92.06% with a penalty of 66.66% increase in the total cost by only replacing the existing matrix profile with CU elements. Also, minimizing the cost by 46.11%, an increase of effectiveness by 32.02% and 16.60% decrease of entropy generation number can be obtained. A new design with the same budget can be obtained by improving the effectiveness by 53.19% and reducing the entropy generation number by 18.95% in a volume which is 8.90% smaller. Similarly, a RHEX design to have approximately the same volume with the existing RHEX is possible with 50.64% increased effectiveness, 20.65% decreased entropy generation number and 23.35% reduced cost.

Reducing the water usage of post-combustion capture systems: The role of water condensation/evaporation in rotary regenerative gas/gas heat exchangers

Water usage is expected to greatly increase when CO2 capture is added to thermal power plants. A major contribution is the reduction of flue gases temperatures from 100 to 150 °C to 30–50 °C. The majority of studies to date propose the use of direct contact cooling, combining a cold water loop with water cooling. This article expands on a previous study of the same authors (Herraiz et al., 2015) proposing dry air-cooled options with rotary regenerative gas/gas heat exchangers, relying on ambient air as the cooling fluid, to eliminate the use of process and cooling water prior to the carbon capture system. It proposes, for the first time, a new stand-alone model of a bi-sector air/gas rotary heat exchanger, which includes the contribution to heat transfer of condensation/evaporation when flue gases are cooled below the dew point. It shows that water condensation from the flue gases in one sector of the heat exchanger, enhances the total heat transfer rate, due to the diffusion of water through the non-condensable gases boundary layer. In order to maintain the cooling capacity of these rotary regenerative heat exchangers, initially designed to operate without condensation, this article shows that they should be designed with surface properties, gas velocities and heat transfer channel geometries with the aim of allowing water condensate to remain on the metal elements surface, and then evaporate into the air stream when the metal elements have rotated to the air side. The model also predicts the location of water condensation, so that enamelled elements can be incorporated to the cold-end tiers of metal elements and mitigate any possible long-term corrosion problems.

A computational fluid dynamics model of a rotary regenerative heat exchanger in a flue gas desulfurization system

Rotary regenerative heat exchangers (RHEX) are commonly used in flue gas desulfurization (FGD) systems to improve the dispersion of pollutants, reduce the visible plume, avoid liquid droplet rainout from the stack and avoid corrosion problems on the system materials. In this study, transient behaviors of heating and cooling cycles of a rotary regenerative heat exchanger in an FGD system were investigated via a 3-D computational fluid dynamics (CFD) model. For this purpose, the rotary regenerative heat exchanger was modelled using porous media approach for the heat transfer surfaces (i.e. matrix) inside the heat exchanger. A standalone channel model as used to obtain porous media parameters and heat transfer coefficient. Numerical results confirmed by published literature. Effects of different operating conditions on the performance of the heat exchanger have been investigated. In conclusion treated gas outlet temperature increases with increasing angular velocity and treated gas inlet temperature while it decreases with decreasing load. Consequently, untreated gas outlet temperature decreases with increasing angular velocity and decreases with the decreasing treated gas inlet temperature and load. By means of overall system performance it is observed that overall system performance increases with decreasing angular velocity and treated gas inlet temperature while it increases with decreasing load.

Rotary reverse flow reactor vs. adiabatic reactor with regenerative preheating - Design and comparison

The autothermal catalytic-combustion systems are commonly used for the purification of waste air streams contaminated with low concentrations of volatile organic compounds (VOC). Within this type of devices, the reverse flow reactors (RFR) are known to be more efficient than systems employing recuperative (surface) heat exchangers to preheat the waste air stream with the lean air effluent from the catalytic incinerator. The advantage of the RFR is basically due to the regenerative heat-exchange mechanism, provided by the inert and catalytic solids inside the unit.As an alternative, the regenerative mechanism of preheating can be achieved by an independent heat exchanger, which coupled to a catalytic reactor could be expected to produce similar performance as the RFR.In this context, this contribution is devoted to analyse comparatively the performances of a rotary reverse flow reactor (RRFR) and a system comprising a rotary regenerative heat-exchanger and a catalytic reactor (RHE-SR system) for the treatment of a waste air stream contaminated with ethanol and ethyl acetate, by means of mathematical simulation. Both alternatives are assumed to be composed of monoliths with square channels. A strategy of design for both systems suitable for their comparison is proposed, attending to a range of VOC concentration in the waste stream. Both alternatives can be regarded as being suitable options to carry out the target. However, the resulting designs show clear advantages in favour to the RRFR, as this alternative requires a significantly more compact equipment than the RHE-SR does and, besides, it allows to be operated under a wider range of the rotational speed, which is the main control variable once the systems are operating.

On the Failure Analysis of an Air Preheater in a Steam Power Plant

The air heater is a rotary regenerative heat exchanger which recovers heat from the outgoing hot gases of the boiler and transforms it to the incoming air needed for combustion process. This system is composed of large numbers of heat transfer elements made from corrosion-resistant material (Corten) or enamel coated in order to resist acid dew-point corrosion. In this study, a root cause failure analysis of heating elements was carried out. Visual inspection, chemical analysis, and microscopic examinations of the heating elements, enamel coating, and depositions were conducted. Sulfuric acid dew-point temperature (ADPT) was determined to be 138–142 °C based on the sulfur dioxide of the flue gases. Moreover, temperature distribution across the air heater matrix was simulated using Fluent software. The failures of the heating elements were mainly due to sulfuric acid dew-point corrosion and under-deposit pitting corrosion. The corrosion products were mainly iron sulfate, iron oxide, and iron sulfide. In the case of enamel-coated elements, the failure was attributed to the existence of a large number of cracks and big bubbles in the coating, allowing the penetration of corrosive elements into and beneath the surface of coating and thus the detachment of coating.

Reducing water usage with rotary regenerative gas/gas heat exchangers in natural gas-fired power plants with post-combustion carbon capture

It is possible to greatly mitigate the increase of water usage associated with the addition of carbon capture to fossil fuel power generation. This article presents a first-of-a-kind feasibility study of a series of technology options with rotary regenerative gas/gas heat exchangers for the management of the water balance around post-combustion carbon capture process integrated with CCGT (Combined Cycle Gas Turbine) plants with and without EGR (exhaust gas recirculation). Hybrid cooling configurations with a gas/gas heat exchanger upstream of the direct contact cooler reduce cooling and process water demand by 67% and 35% respectively compared to a wet system where the flue gas is primarily cooled prior to the absorber in larger direct contact coolers. The CO2-depleted gas stream is then reheated above 70 °C with enough buoyancy to rise through the stack. Dry air-cooled configurations, relying on ambient air as the cooling medium, eliminate the use of process and cooling water prior to the absorber and the temperature of the flue gas entering the absorber is unchanged. Rotary regenerative heat exchangers do not introduce significant additional pressure drop and gas leakage from a high CO2 concentration stream to a stream with lower concentration can be minimized to acceptable levels with available strategies using a purge and a scavenging slipstream from the higher pressure flow.

A Versatile Computer Simulation Model for Rotary Regenerative Heat Exchangers

A simulation technique for predicting the thermal performance of rotary regenerative heat exchangers, in particular those used for heating the intake air to power station boilers, has been developed and verified by means of site measurements. Various geometries of both rotating-hood and rotating-matrix types of air heaters can be accommodated in the simulation model, including packings of corrugated steel plates of various specified profiles and any given thicknesses and lengths in the flow direction. Rotational speed, leakage, blockage, and non-uniform inlet flow distribution may be taken into account as input variables. The heat transfer and pressure drop correlations for the various plates considered, which also form part of the input data, were determined experimentally using a single-blow transient test facility, constructed as part of this research program. The effect of erosion of the plates by fly ash particles carried in the outlet flue gas on heat transfer performance is also considered, and experimental results show that erosion has little effect on the thermal performance (up to the point that structural integrity is about to be compromised), but also that the pressure drop is reduced.

A model to predict erosion on mild steel surfaces impacted by boiler fly ash particles

Fly ash particles entrained in the flue gas from boiler furnaces in coal-fired power stations can cause serious erosive wear on steel surfaces along the flow path. Such erosion can, as a particular example, reduce significantly the operational life of the mild steel heat transfer plates that are used in rotary regenerative heat exchangers (‘air heaters’) that extract heat from the flue gas and transfer it to the incoming boiler combustion air. This paper describes research into fly ash impingement erosion on such surfaces. The effect of the ash particle impact velocity and impact angle on the erosive wear of mild steel surfaces, using three different power station ash types, was determined through experimental investigations. The experimental data were used to calibrate a fundamentally-derived model for the prediction of erosion rates. The model incorporates the properties of the ash particles and the target metal surface, as well as the characteristics of the ash particle motion in the form of the impingement velocity and the impingement angle. When tested using the three different types of ash, the experimentally-calibrated general model yielded results that generally differed by less than 15% from the values that had been measured experimentally.

Rotary regenerative heat exchanger: U.S. Patent 5,740,856. Apr. 21, 1998 K.E. Ritter, assignor to ABB Air Preheater Inc., Wellsville, N.Y.

Rotary regenerative heat exchanger: U.S. Patent 5,740,856. Apr. 21, 1998 K.E. Ritter, assignor to ABB Air Preheater Inc., Wellsville, N.Y.

98/01461 Rotary regenerative heat exchanger: application to atmospheric fluidized-bed combustion boilers: Gilkey, C. H. et al. Proc. Int. Conf. Fluid. Bed Combust., 1995, 13, (1), 405–418

98/01461 Rotary regenerative heat exchanger: application to atmospheric fluidized-bed combustion boilers: Gilkey, C. H. et al. Proc. Int. Conf. Fluid. Bed Combust., 1995, 13, (1), 405–418

Performance of Rotary Heat and Mass Exchangers

Rotary regenerative heat and mass exchangers (enthalpy exchangers) can reduce air-conditioning costs in ventilated buildings by recovering energy from the exhaust air and transferring it to the supply air stream. In this study the adsorption isotherms of a desiccant used in a commercially available heat and mass exchanger are measured. The isotherms and other property data are incorporated into the program MOSHMX which numerically solves the governing equations for combined heat and mass transfer. The numerical results are then used to develop a computationally simple model for determining the performance of a specific enthalpy exchanger as a function of the air inlet conditions and the matrix rotation speed. The numerical results agree with the catalog information provided by the manufacturer. The enthalpy exchanger model is used in the transient simulation program TRNSYS to estimate the annual performance. Integrated energy savings (heating and cooling) are determined for a commercial application (a 200...

Advanced sealing technology for rotary regenerative heat exchangers

Advanced sealing technology for rotary regenerative heat exchangers

A Correlation for the Heat Conduction Effects in Counterflow Rotary Regenerative Heat Exchangers

Equations that predict the dependence of regenerator effectiveness on heat conduction in the matrix both parallel to and perpendicular to the fluid flow are derived from numerical simulations. The equations developed use Biot numbers parallel to the direction of fluid flow (Bix) and perpendicular to the fluid flow direction (Biy), the ratio of heat capacity rates (C*), the heat capacity rate ratio (C*Γ), and the overall number of transfer units (NTUo) to characterize the regenerator performance. Comparison of numerical predictions with those obtained using the equations developed in this paper show excellent agreement. These equations enable designers to accurately account for two-dimensional conduction effects when regenerators are designed.