René Hofmann

Experimental and Numerical Investigation of the Gas Side Heat Transfer and Pressure Drop of Finned Tubes—Part I: Experimental Analysis

In this study, a heat transfer and pressure drop correlation are determined for helically I- and U-shaped finned tubes as well as for solid I-finned tubes at constant transverse and longitudinal spacing. In the heat transfer correlation, the influence of the number of tube rows arranged in flow direction is taken into consideration. A detailed description of the test rig and the data reduction procedure is presented. A thorough uncertainty analysis was performed to validate the results. The investigation has shown that the influence of the fin geometry on the heat transfer of the helically segmented I- and U-shaped tubes can be disregarded. The heat transfer correlation, which is valid for the helically segmented I- and U-shaped tubes in a staggered arrangement, can describe 90% of all measurement data within ±15%. All measurements are performed for constant transverse and longitudinal spacing. For the pressure drop coefficient, two new correlations, which are only valid for helically segmented U shaped finned tubes in a staggered arrangement, show an average deviation of approximately ±13% for 90% of all measurement results. All new correlations are compared with correlations from open and established literature for industrial boiler applications. The new heat transfer and pressure drop correlations show a relative deviation of ±20% in comparison with correlations in open literature. The new pressure drop correlations show the same characteristic as most correlations in the open literature.

Durability of a fin-tube latent heat storage using high density polyethylene as PCM

Polymers have rarely been used as storage materials in latent heat storages up to now. Thus, we systematically screened all polymers available on a large-scale, selected promising ones based on their theoretical properties and experimentally tested more than 50 candidates. We found that polyethylene, polyoxymethylene and polyamides are promising even as recycled material. Especially high density polyethylene (HDPE) turned out to be suitable as was shown by detailed thermophysical characterization including more than 1000 heating and cooling cycles for INEOS Rigidex HD6070EA. We built a storage with 170 kg HDPE and a total mass of 600 kg based on a fin-tube heat exchanger and characterized its energy capacity, power characteristics and temperature profiles using a thermal oil test rig. In total we performed 30 melting and crystallization cycles where the whole storage was above 100 °C for more than 140 hours. After usage we examined the interior of the storage by cutting it into various pieces. A thin layer of degradation was observed on the surfaces of the PCM which is most likely related to thermo-oxidative degeneration of HDPE. However, the bulk of the PCM is still intact as well as the heat exchanger itself.

Comparison Between Numerical and Experimental Gas Side Heat Transfer and Pressure Drop of a Tube Bank With Solid and Segmented Circular I-Fins

In the present work, a comparison between numerical and experimental gas side heat transfer and pressure drop for a tube bundle with solid and segmented circular finned tubes in a staggered arrangement is investigated. For the numerical simulations a three dimensional model of the finned tube are applied. Renormalization group theory (RNG) based k–e turbulence model was used to calculate the turbulent flow. Experiments have been carried out to validate the numerical predictions. The numerical results for the Nu-number and pressure drop coefficient show a good agreement with the data from measurement. A comparison between solid and segmented finned tubes from the global calculation of the Nu-numbers within the analyzed Re-range shows an enhancement by applying segmented finned tubes rather than finned tubes with solid fins.© 2012 ASME

Extensions for Multi-Period MINLP Superstructure Formulation for Integration of Thermal Energy Storages in Industrial Processes

Abstract In this paper, extensions to the MINLP superstructure formulation for multi-period heat exchanger network synthesis (HENS) proposed by Zhang (2006) which is based on the work by Yee and Grossmann (1990) are introduced. These modifications allow for simultaneous HENS including storage-subsystems with different characteristics. Besides stratified tanks and two-tank systems models for latent heat thermal storages (LHTS) and sensible storages are presented. Also, a sequential targeting procedure for the integration of storages is presented which combines insights from pinch analysis methods and a MINLP-formulation to calculate an approximation for optimal storage size and melting temperature. The results of the proposed integration procedure for thermal energy storages is demonstrated using a case study adapted from literature.

Increasing Energy Efficiency in Pulp and Paper Production by Employing a New Type of Latent Heat Storage

Abstract In pulp and paper production black liquor is burnt to recover base chemicals and to generate electricity and steam which is e.g. used in papermaking machines. In the event of paper tearing excess steam has to be condensed usually. We propose a latent thermal energy system to recover that energy and re-use it in the papermaking machines. A storage model was developed and experimentally verified by a lab-scale prototype. For the specific process, we designed a tube-bundle storage using thermally conductive HDPE as PCM and show that up to 30 MWh can be recovered per day. Finally, we describe the potential of storages to improve energy generation and efficiency in plants.

Mixed Integer Linear Programming Formulation for Sensible Thermal Energy Storages

Abstract In order to model the charging and discharging power of sensible thermal energy storages for the unit commitment problem with mixed integer linear programming more precisely, two extended formulations are proposed in this work. To this end, simulation data of a detailed simulation model, derived from measurement data of a fixed bed regenerator test rig, were investigated in order to determine the actual charging and discharging power of the storage in highly dynamic operation. The two formulations were derived from an original MILP storage model with fixed maximum charging and discharging power. The first proposed formulation adds linear constraints for charging and discharging power depending on the fill level. The second proposed formulation introduces auxiliary variables, taking into account the fill level at the end of the previous charging or discharging phase. The predicted operation for a test case of the original and the two proposed mixed integer linear formulations are compared with the achieved operation of the detailed simulation model. While the original formulation allows infeasible predictions for the storage operation, the two proposed formulations are able to describe the real operating behavior more detailed, therefore decreasing the infeasible predictions for operation as well as increasing the overall system performance.

Innovative Temperature Swing Adsorption Simulation Model for Biogas Upgrading

Abstract A multi-stage fluidized bed system for temperature swing adsorption (TSA), which uses solid amine sorbents for CO2 capture and a heat pump for further optimization of the energy consumption, is studied in detail. The present work introduces detailed models for the most important aspects of the process, in order to study interactions in a simulation model. The overall efficiency depends on three different aspects: adsorption efficiency, heat transfer and heat recovery with a heat displacement system. These three modeling aspects have been combined to gain information about different limiting factors of the proposed TSA process for biogas upgrading. With the new TSA process model, a tool has been created which enables the design and dimensioning of a biogas upgrading plant, as well as the comparison to other biogas upgrading processes.

A Novel Approach for Linearization of a MINLP Stage-Wise Superstructure Formulation

Abstract Mathematical programming using superstructure formulations has been used for cost efficient heat exchanger network synthesis (HENS) for about three decades now and significant improvements have been achieved since then. One major problem is the combinatorial nature of the underlying superstructure formulations which means that the mathematical complexity of the HENS problem scales exponentially with problem size. In this paper a novel approach using convex linear approximations is presented for simultaneous HENS. The linearization is carried out prior to optimization and the original Mixed Integer Non-Linear Programming (MINLP) problem is reformulated into a Mixed Integer Linear Programming (MILP) problem. For the linearized problem a global optimum can be obtained much faster compared to the original MINLP formulation. For all presented case-studies feasible solutions could be obtained, which compare well with results from other authors.

How to tighten a commonly used MINLP superstructure formulation for simultaneous heat exchanger network synthesis

Abstract MINLP superstructures for heat exchanger network synthesis allow the simultaneous optimization of utility heat loads, the number of heat exchanger units and their area requirements. The algorithms used to solve these nonlinear non-convex optimization problems solve MILP and NLP sub-problems iteratively to find optimal solutions. If these sub-problems are tightened, which means that the solution space is reduced but still includes all feasible integer solutions, the algorithms can potentially go through the solution space faster as branches can be excluded earlier. In this work, tightening measures for a commonly used MINLP stage-wise superstructure formulation are proposed and the impact of tighter variable bounds and additional inequality constraints is investigated using various case-studies taken from literature. It is shown that tighter formulations help the solver to find global optimal solutions and that the duality gap can be reduced significantly if the test cases could not be solved to global optimality.