Condensing Heat Exchanger Research Articles

Novel flue gas waste heat recovery system equipped with enthalpy wheel

Waste heat recovery from flue gas is an efficient way to increase the thermal efficiency of a gas-fired boiler. This paper proposes a novel flue gas waste heat recovery system, which comprises a condensing heat exchanger and an enthalpy wheel. The flue gas firstly flows through the condensing heat exchanger for heating boiler water, and then flows through the enthalpy wheel. The wheel, covered with desiccant material, acts as a medium for further heat and moisture transfer from the flue gas to the oxidizing air, resulting in an increase in the dew point temperature of the flue gas. Experimental results show that the average boiler efficiency reaches 106% and the average total recovery efficiency reaches 88%. The dew point temperature of the flue gas discharged from the boiler increases to around 60 °C, higher than that of conventional flue gas (around 55 °C). Consequently, more latent heat is recovered in the condensing heat exchanger by virtue of the proposed system. A mathematical model is established for further analysis based on the experimental results. The rotation speed of the enthalpy wheel and backwater temperature are key factors influencing the system performance.

A novel electrical charging condensing heat exchanger for efficient particle emission reduction in small wood boilers

Small-scale biomass combustion is an important source of fine particles in ambient air, causing adverse health and environmental effects. Thus, there is a clear need to develop efficient and feasible flue gas cleaning technologies for small-scale combustion appliances. In this study a novel electrical charging condensing heat exchanger (eCHX) for combined fine particle removal and efficient heat recovery from flue gases was demonstrated in a small biomass-fired boiler. The method is based on the combination of a shielded corona charger and a condensing heat exchanger, where fine particles are removed by the electrophoretic, thermophoretic and diffusiophoretic forces. The eCHX was found to decrease >80% of fine particle mass (PM1) emissions and >40% of particle number emissions with simultaneous high thermal efficiency in the heat exchanger. The usage of the condensing heat exchanger without electrical charging resulted in 40% decrease in PM1 emissions when compared to the usage of a traditional tube heat exchanger. The advantage of the eCHX system is that it replaces the conventional heat exchanger in boilers, making it a compact and inexpensive solution, when compared to additional flue gas cleaning devices installed after the boiler.

A theoretical fundamental investigation on boilers equipped with vapor-pump system for Flue-Gas Heat and Moisture Recovery

There is tremendous surplus heat in flue gas from gas boilers which cannot be efficiently recovered by conventional condensing heat exchangers. As one of heat recovery systems, boilers equipped with vapor-pump system (BEVP system) is complicated. To improve system performance, theoretical investigation and mathematical models are required. However, these are not proposed in previous work. In this study, thermodynamic work principles and moisture loop of BEVP system are analyzed. It utilizes the ‘constant vapor flux’ property of natural gas combustion and the humidity level of combustion generated vapor flux depends on the humidity level of combustion air. The system is divided into two subsystems for better understanding. Subsystem I is used for dehumidification, and subsystem II is for total heat recovery, serving as a ‘vapor pump’. The core optimization principle of BEVP system is to decrease the moisture transfer driven force of vapor pump, which is used for total heat recovery in Subsystem II. A mathematical model is established to quantitatively characterize the BEVP system. Analytical solutions are derived with clear physical significances and additivity property. The mathematical model is used to conduct performance analyses under various conditions. The systematic feasible domain is developed with iso-efficiency lines.

Investigation of crystallization risk in different types of absorption LiBr/H2O heat transformers

Absorption heat transformers are utilized to upgrade the energy of low-temperature heat sources. When the working fluid in such systems is LiBr/H2O solution, there is a possibility of crystallization phenomenon at their generator outlet. This phenomenon has to be avoided because otherwise it stops the system operation. In the present work, the possibility of crystallization, under different operating conditions such as different temperatures in the condenser, evaporator, absorber and absorbing evaporator temperatures as well as different effectiveness of applied heat exchangers, is investigated. Using engineering equation solver software, the investigation is carried out for three types of heat transformers including single-stage, double-effect and triple-effect for identical values of input heat. The results show that at higher absorber temperatures especially in the double and triple-effect absorption heat transformers, crystallization may occur at the outlet of expansion valves in addition to the generator outlet. Moreover, lower values of the condenser temperature and heat exchangers effectiveness increase the possibility of crystallization risk in all the three types investigated.

On a certain method of power boiler blast air pre-heating

The method for preliminary heating of boiler blast air has been developed; as per this method, the air before being blown to boiler air heater is pre-heated with exhaust natural gas combus-tion products in surface heat exchanger with combustion products cooled below dew point temperature. It is suggested to use water vapor condensate produced by this process as heat system make-up water. Thermal calculation for condensing heat exchanger installed down-stream of BKZ-420-140 NGM power boiler is made, as a result of the calculation, the following parameters were obtained for different values of ambient air: heating power of heat exchanger; consumption of exhaust gases for pre-heating of boiler blast air provided that they are cooled below dew point temperature; proportion of exhaust gases transferred to heat ex-changer; consumption of water vapor condensate produced from gas fuel combustion prod-ucts; temperature, moisture content and dew point of exhaust gases on exit of boiler unit; in-crease in boiler fuel consumption coefficient due to operation of condensing heat exchanger; heat transfer coefficient. Cost effectiveness due to pre-heating of BKZ-420-140 NGM boiler blast air in condensing surface heat exchanger at Samara Power Station has been determined.

A novel design of rotary regenerative condensing heat exchanger for the dehydration from high humidity flue gas

A rotary regenerative condensing heat exchanger was proposed for dehydration of the flue gas outflowing from the desulfurization tower in coal-fired power plant. An analytical model of heat and mass transfer processes in the regenerative condensing heat exchanger was developed. A calculation program was built by using an iterative solution technique to compute the heat transferred from flue gas to cooling air and the condensation rate of water vapor in the flue gas. The distributions of flue gas temperature, matrix temperature, water vapor mass fraction and condensate water mass were analyzed. The effects of matrix materials, operational and structural parameters of the rotor on the condensation efficiency were examined. The condensation efficiency from high to low for different matrix materials are in the sequence: SiC ceramic > carbon steel > fluorine plastic > Borosilicate Glass. Once the rotation speed, the height and radius of the rotor are larger than specific values, their changes are less effective on the condensation efficiency. Compared with the fluorine plastic tube condensing heat exchanger, the rotary regenerative condensing heat exchanger was more effective and economical on the whole volume and the capital cost to meet the same requirement of condensation efficiency.

The prediction of heat efficiency and pollutant emission of non-rated loaded condensing heat exchangers

In order to study the working conditions of condensing heat exchangers under non-rated load conditions, the orthogonal test method was conducted on the condensing gas water heaters by experiment firstly in this paper. Eighty groups of the experimental data about the relationship between the design variables and the object functions were obtained, among them, the design variables include excess air coefficient, flue gas flow, water flow and water flow temperature, and the objective functions include heat exchanger efficiency, NOx and CO emission concentration. The above experimental data were divided into two groups for the training and prediction of neural networks. Here two neural networks of BP artificial neural network (BPNN) and generalized regression neural network (GRNN) will be used to predict the heat efficiency and pollutant emission of condensate heat exchanger at partial load rate. The results show that both BPNN and GRNN structures achieve the required prediction accuracy, and the prediction accuracy of GRNN is higher than that of BPNN. The condensing heat exchanger can achieve higher heat exchanger efficiency and lower concentration of NOx and CO emission at the 90–100% load rate. When the load rate is lower than 30% or higher than 100%, the heat efficiency of the heat exchanger is decreased, and the concentration of pollutant emission is increased rapidly.

Analysis of the Efficiency of Using the Heat of Natural Gas Combustion Products to Preheat Forced Air Supplied to the Boiler

Preheating of forced air in a surface-type heat exchanger prior to supplying it to the air heater of the boiler by using flue gases while cooling them below the dew point is proposed. Athermal calculation of the condensing heat exchanger installed downstream of the boiler (model BKZ-420-140 NGM) was performed to determine the thermal capacity of the heat exchanger, flow rate of flue gases required to preheat forced air supplied to the boiler while cooling them below the dew point, flow rate of the water vapor condensate obtained from the gaseous fuel combustion products, temperature of flue gases at the outlet of the boiler unit, and fuel utilization factor of the boiler resulting from the use of the condensing heat exchanger. The water vapor condensate obtained from flue gases in the surface heat exchanger can be used as make-up water for the heat network. It was established that depending on the ambient air temperature, the percentage of flue gases directed to the heat exchanger for preheating forced air of the BKZ-420-140 NGM boiler assuming their cooldown below the dew point constitutes 3.9 to 20.7 and 7.6 to 24.4% of their total amount, when heating air to 20 and 30°C, respectively. The economic effect in the monetary terms due to installation of the condensing heat exchanger downstream of the BKZ-420-140 NGM boiler amounts to 12, 691.7 and 17,242.2 thousand RUB/year, when heating forced air to 20 and 30°C, respectively. Considering five power-plant boilers installed at the Samara CHP operated by “Samarsky” branch of the PJSC “T Plus,” the amount of savings should reach 63.5 and 86.2 million RUB/year, respectively.

Experimental and theoretical investigation of a novel full-open absorption heat pump applied to district heating by recovering waste heat of flue gas

The industrial exhaust flue gas contains tremendous waste heat that cannot be efficiently recovered by conventional condensing heat exchanger. This paper proposes a novel full-open absorption heat pump for the total heat recovery of flue gas. The recovered heat is used for district heating. By applying direct-contact heat and mass transfer without solid transfer interfaces, the novel system can save considerable metallic tube or plate materials and therefore the initial investment can be reduced substantially. A prototype full-open absorption heat pump is developed and tested, achieving the COP (coefficient of performance) of 1.621 and the exit flue gas dew point of 36.2 °C at most. A detailed theoretical model in Eulerian–Lagrangian formulation is established, involving in conjugate heat and mass transfer and particle dynamics. The numerical simulation agrees well with the experiment results. On the basis of the validated model, three critical parameters, including excess air coefficient of the burner, moisture content of the objective flue gas and return water temperature of the district heating network, are selected to study their effects on system performances. As a controlled variable, especially, the excess air coefficient plays an important role by influencing the regeneration of the liquid absorbent in the generator. The full-open absorption heat pump outperforms the condensing heat exchanger by 19.7–178.1% in heat recovery capacity as the return water temperature increases from 40 to 55 °C.

Modeling and simulation of cross-flow transport membrane condenser heat exchangers

Transport Membrane Condenser (TMC) is a promising technology that works based on the capillary condensation property of the nano-scale ceramic porous membrane materials and can recover both waste heat and water in various industrial applications. For applications with high volume flow rate such as in power plants, a large number of TMC heat exchangers are required, which highlights the importance of the optimum design for the TMC exchangers in order to obtain the maximum heat transfer and minimum cost of the TMC heat exchanger unit. In this paper, the effects of different tube spacing and the inlet water vapor mass fraction on the overall performance of a membrane-based heat exchanger have been studied numerically using a combined condensation model based on the capillary condensation and condensation on a solid wall. The results were obtained in terms of the Euler number, dimensionless volumetric heat transfer density, and contours of the water mass fraction and temperature distribution inside the TMC heat exchanger.

Numerical and experimental investigation of flue gases heat recovery via condensing heat exchanger

The ever increasing environmental awareness introduces the trend of increasing the efficiency of power plants via implementing waste heat recovery. This study reports two numerical models suitable for analyses of a condensing heat exchanger. The numerically predicted results are compared with the results acquired experimentally with the use of a coal-fired water condensing heat exchanger installed in the local laboratory of thermal engineering. The first applied mathematical model is a stationary 1D model based on our own modification of the Colburn-Hougen model. The second one is a proposed 3D multiphase flow model based on the Euler model of mixture expanded by the model of condensation of water vapour from the flue gases. A thorough comparative analysis of the results of both the numerical models and the experimental ones showed very good correlation. The differences between all the acquired results for the temperatures of flue gases and heated water at the outlet of the heat exchanger did not exceed 4 °C. In cases of heat recovery with the occurrence of condensation, the latent heat has a significant influence on the flue gases and water temperatures at heat exchanger outlet.

Replacement of Present Conventional Condenser of Household Refrigerator by Louver Fin Micro-Channel Condenser

In this paper, numerical simulation of wire on tube condenser and louver fin micro-channel heat exchanger is discussed. Simulation data of both heat exchangers were compared with the experimental data, and they show good agreement. In case of louver fin micro-channel heat exchanger, prediction error of heat capacity was within ± 5%. In addition, from the result of simulation, it was found that, for same thermal performance, micro-channel heat exchanger occupies less space as well as requires less refrigerant charge than wire on tube condenser. Therefore, in this paper, two louver fin micro-channel condensers are proposed to replace wire on tube condenser of household refrigerator. Response surface methodology was used to find out the optimal condition of the louver fin micro-channel condenser.

Conjugate heat and mass transfer study of a new open-cycle absorption heat pump applied to total heat recovery of flue gas

Industrial exhaust flue gas contains massive heat and water. Effective recovery will make positive sense for energy saving and conservation. Traditional condensing heat exchanger is restricted by coolant water temperature and corrosion problem. This paper proposes a new open absorption heat pump (OAHP). The system works on the basis of directly contacting spray heat and mass transfer occurring in three main parts, including generator, condenser and absorber. Driven by high-temperature flue gas directly from a burner, the system generates strongly concentrated LiBr solution prepared for the absorption of moisture and heat from the objective flue gas. A detailed theoretical model is established in Eulerian-Lagrangian formulation to simulate the particle dynamics and the heat and mass transfer characteristics. Extraordinary temperature profiles different from those in common heat transfer are found in the generator and are explained by the conjugate heat and mass transfer. The effects of the driving flue gas temperature and the return water temperature on system performances are studied. The OAHP system achieves excellent heat recovery capacity even at high return water temperature, indicating superiority to the condensing heat exchanger.

Deposition and corrosion characteristics of liquid-solid droplets on tubular corrosion probes in desulfurized flue gas

Abstract In coal-fired boilers, low-temperature corrosion occurring in the back-end is always threatening. Particularly, the desulfurized flue gas from outlet of the flue gas desulfurization (FGD) unit is quite aggressive. The flue gas reheater placed after FGD is usually operating under a much more severe condition than the condensing heat exchangers before FGD. In this paper, field corrosion tests were conducted in a coal-fired heating plant and tubular corrosion probes made of different grades of steels were tested. The total testing period was 6 weeks. The corroded probes were then analyzed by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), and the deposition and corrosion mechanisms were studied. The droplets entrained by the flue gas resulted in deposition and corrosion on the probe, especially on the windward side. During the tests, 2507 and 2205 showed good corrosion resistance, while ND, 304L, 316L suffered considerable corrosion.

F-Gas regulation-Possible solutions for the retrofit dead end

The EU F-gas regulation of 2006 and the recast of 2014 are forcing the market to reduce the use of refrigerants with high global warming potential (GWP). As a result, the production of hydrofluorocarbon (HFC) refrigerants with higher GWP decreased, making the prices of all HFC gases to increase. Any maintenance problem in a refrigeration system asks to evaluate the retrofitting of the gas making necessary to know the expected behaviour of the system. This paper aims to discuss the gases that can substitute the now-a-days HFCs, and the impact the retrofit will cause in a real air-conditioning systems. Many studies on retrofit address the behaviour of the refrigeration cycle, but usually, do not take into account the behaviour of the system as a whole. This paper models a water-to-water air-conditioning system taking into consideration the evaporator and condenser heat exchangers, the refrigeration cycle, the air-conditioning loop and the heat exchanger to the acclimatized area. Moreover, the paper studies the performance of the system when subjected to high condensing temperatures. The paper concludes that all possible retrofit solutions need to use flammable gases that make the refrigeration power of the equipment to reduce.

Thermodynamic analysis of a combined power/refrigeration cycle: Combination of Kalina cycle and ejector refrigeration cycle

In the present study, a new power and refrigeration cycle is investigated which is a combination of a Kalina cycle and an ejector refrigeration cycle (ERC). In the proposed configuration of the combined cycle, an ejector refrigeration cycle is inserted into the Kalina cycle to recover heat from ammonia poor solution which leaves the separator at high temperature/pressure and does not contribute to power generation in Kalina cycle Working fluid of the Kalina cycle and ERC are ammonia-water solution and R134a, respectively. The combined cycle is simulated by EES software and details of the applied mathematical model and developed simulation program are extensively reported. The effect of five key operational parameters of the combined cycle (i.e. turbine inlet pressure, turbine inlet temperature, concentration of ammonia-water basic solution, condenser outlet temperature and pressure of refrigerant in heat exchanger) on the combined cycle performance parameters (refrigeration capacity, power production, thermal efficiency, exergy of produced power, exergy of refrigeration and exergy efficiency) is analyzed and physical mechanisms behind the determined results are reported. Additionally, variation of performance parameters with heat exchanger pressure is examined with different refrigerants (R134a, R152a and R290) to determine the effect of refrigerants on system performance. The results show that thermal efficiency of the combined cycle increases with increasing turbine inlet temperature and concentration of ammonia-water solution but decreases with rising condenser outlet temperature and heat exchanger pressure. A maximum thermal efficiency point is determined in the analyzed range of the turbine inlet pressure. Exergy efficiency increases with rising turbine inlet pressure, turbine inlet temperature and concentration of ammonia-water solution but decreases with increasing condenser outlet temperature and heat exchanger pressure. Refrigeration capacity and thermal efficiency results of the combined cycle are the highest for the operation of ERC with R290 and the lowest with R134a. Exergy efficiency is the lowest for ERC operation with R290 and the highest with R134a.

Corrosion‐resistant coating development with potential application in equipment of low‐temperature waste heat recovery

AbstractIn order to improve corrosion resistance of a condensing heat exchanger, a fluoropolymer, perfluoroalkoxy alkane (PFA), fine powder in 30 wt% aqueous slurry was coated on metal coupons and a coating of high quality finish was obtained. The coating was characterized by microscopy and thickness measurements. The thickness of the coating can be controlled by the number of layers used for the application of wet spray. Hot acid bath corrosion tests showed that the coated coupon possesses the highest adhesion strength and excellent corrosion resistance (1.8–6.7 × 10−3 mm/year), comparable to super alloys such as Inconel (1.1–26.2 × 10−3 mm/year) and Hastelloy (0.3–19.8 × 10−3 mm/year). A heat transfer coefficient analysis showed that across a heat transfer metal tube coated with PFA, the heat transfer resistance of the gas side film is one order of magnitude larger than the resistance from the coated layer. The developed coating could provide an alternative material solution for condensing heat exchangers used in low‐grade waste heat recovery.

Investigation of the effect of pressure increasing in condensing heat-exchanger

The effect of pressure increase was observed in steam condensation in the intermediate coolers of multistage steam ejector. Steam pressure increase for ejector cooler amounts up to 1.5 kPa in the first ejector stage, 5 kPa in the second and 7 kPa in the third one. Pressure ratios are equal to 2.0, 1.3 and 1.1 respectively. As a rule steam velocities at the cooler inlets do not exceed 40…100 m/s and are subsonic in all regimes.The report presents a computational model that describes the effect of pressure increase in the cooler. The steam entering the heat exchanger tears the drops from the condensate film flowing down vertical tubes. At the inlet of heat exchanger the steam flow capturing condensate droplets forms a steam-water mixture in which the sound velocity is significantly reduced. If the flow rate of steam-water mixture in heat exchanger is greater than the sound velocity, there occurs a pressure shock in the wet steam.On the basis of the equations of mass, momentum and energy conservation the authors derived the expressions for calculation of steam flow dryness degree before and after the shock. The model assumes that droplet velocity is close to the velocity of the steam phase (slipping is absent); drops do not come into thermal interaction with the steam phase; liquid phase specific volume compared to the volume of steam is neglected; pressure shock is calculated taking into account the gas-dynamic flow resistance of the tube bundle. It is also assumed that the temperature of steam after the shock is equal to the saturation temperature.The calculations have shown that the rise of steam pressure and temperature in the shock results in dryness degree increase. For calculated flow parameters the velocity value before the shock is greater than the sound velocity. Thus, on the basis of generally accepted physics knowledge the computational model has been formulated for the effect of steam pressure rise in the condensing heat exchanger.

Simulation of parameters of the vapor-liquid compression cooling system of power machines

The paper presents a mathematical model of the cooling system with vapor-liquid compression installation modified with an additional allowance for the effect of mass flow rate of ambient air on a heat transfer in the system’s heat exchanger-condenser. Also the correction factors considering the difference between the mean log and the mean arithmetic temperature pressure are introduced. The results of numerical simulation showed that the efficiency of the proposed cooling system is 149% and 200% higher than the classical at the ambient temperature plus 10°C and 50°C respectively.

Design and Simulation of Condensing Heat Exchanger

The conventional heat recovery techniques recover only sensible component of the flue gas heat and a significant portion of the flue gas heat is lost to the atmosphere. Condensing heat exchangers are designed to retrieve both sensible and latent heat components of the heat present in flue gas by promoting condensation of water vapour present in the flue gas. It has several challenges due to non-availability of the low-temperature heat sink. The paper explains numerical simulations based on analytical modeling of the heat and mass transfer processes involved in condensation of moisture in the flue gas in a cross-counter flow tubular heat exchanger- of size 1.752 m × 1.274 m × 2.230 m which comprises of 78 tube passes and 6 tubes with cooling water flowing inside and flue gas over the tubes. The performance of the system is exhibited as a function of different parameters. A mathematical model using MS-Excel and VB Macros as a tool is developed to study the effect of operating parameters on the performance. The governing equations and correlations based on mass and energy balances for the system are used to compute variables such as flue gas exit temperature, cooling water outlet temperature, mole fraction and condensation rates of moisture. The equations are solved using an iterative solution technique with calculations of heat and mass transfer coefficients and physical properties. The paper focuses on the maximization of heat recovery and the various parameters affecting the heat and moisture recovery and condensation. The effect of operating parameters like inlet temperatures of both the fluid streams and flow ratio is studied in order to achieve the maximum heat and moisture recovery. The condensation and heat transfer rates increase significantly by increasing the flow ratio and decreasing the inlet cooling water temperature.