Heat Exchanger Surface Area Research Articles

2단 응축 히트펌프 온수시스템의 사이클 해석 및 성능분석

The heat pump water heater developed in this research consisted of one evaporator, one compressor, 1st condenser, 2nd condenser, one expansion valve, one water tank, one recirculation circuit and etc. The performance of heat pump water heater was tested and analyzed. The quantities of output water changed linearly from 2380 to <TEX>$660{\ell}/h$</TEX>, and the output water temperature changed curvedly from 29.9 to <TEX>$44.5^{\circ}C$</TEX> when the opening rate of recirculation valve changed from 0 to 100%. The COP of heat pump water heater increased from 3.0 to 3.8 when the quantities of output water changed from 660 to <TEX>$2380{\ell}/h$</TEX>. When the temperature distributions of water tank were measured during 50 minutes after turning on the heat pump, the temperature stratification by the level appeared apparently. When the inlet water temperature changed from 30 to<TEX>$50^{\circ}C$</TEX>, the output energy of heat pump hardly changed. The surface area of double pipe heat exchanger changed from 0.429 to <TEX>$6.254m^2$</TEX> when the compressor capacity increased from 1.0 to 50.0 PS.

Development of Thermodynamic, Geometric, and Economic Models for Use in the Optimal Synthesis/Design of a PEM Fuel Cell Cogeneration System for Multi-Unit Residential Applications

Thermodynamic, geometric, and economic models are developed for a proton exchange membrane (PEM) fuel cell system for use in cogeneration applications in multi-unit residential buildings. The models describe the operation and cost of the fuel processing sub-system and the fuel cell stack sub-system. The thermodynamic model reflects the operation of the chemical reactors, heat exchangers, mixers, compressors, expanders, and stack that comprise the PEMFC system. Geometric models describe the performance of a system component based on its size (e.g., heat exchanger surface area), and, thus, relate the performance at off-design conditions to the component sizes chosen at the design condition. Economic models are based on data from the literature and address the cost of system components including the fuel processor, the fuel cell materials, the stack assembly cost, the fuel cost, etc. As demonstrated in a forthcoming paper, these models can be used in conjunction with optimization techniques based on decomposition to determine the optimal synthesis and design of a fuel cell system. Results obtained using the models show that a PEMFC cogeneration system is most economical for a relatively large cluster of residences (i.e. 50) and for manufacturing volumes in excess of 1500 units per year. The analysis also determines the various system performance parameters including an electrical efficiency of 39% and a cogeneration efficiency of 72% at the synthesis/design point.

A novel method to improve the performance of heat exchanger—Temperature fields coordination of fluids

The methods to enhance the heat transfer in heat exchanger may be classified into two levels: one is to improve the heat transfer coefficient; the other is to upgrade the whole arrangement of the heat exchangers. For the second level, the performance of heat exchanger can be upgraded by increasing the coordination degree between the temperature fields of cold and hot fluids. When the temperature distributions are similar to each other, the temperature difference field (TDF) is more uniform, which means that the temperature fields are more coordinated with each other. For the cross-flow heat exchanger, rearrangement of the heat exchange surface area should improve the heat transfer effectiveness, which is even equal to that of the counter-flow heat exchanger when the surface area is reassigned optimally. For the multi-stream heat exchanger, the thermal performance is also dependent on the uniformity of the TDF, and the parallel-flow arrangement may achieve higher heat exchange effectiveness than the counter-flow arrangement, which indicates that the performance of heat exchanger depends on the coordination degree of temperature fields instead of the flow arrangement.

A novel method to improve the performance of heat exchanger??Temperature fields coordination of fluids

The methods to enhance the heat transfer in heat exchanger may be classified into two levels: one is to improve the heat transfer coefficient; the other is to upgrade the whole arrangement of the heat exchangers. For the second level, the performance of heat exchanger can be upgraded by increasing the coordination degree between the temperature fields of cold and hot fluids. When the temperature distributions are similar to each other, the temperature difference field (TDF) is more uniform, which means that the temperature fields are more coordinated with each other. For the cross-flow heat exchanger, rearrangement of the heat exchange surface area should improve the heat transfer effectiveness, which is even equal to that of the counter-flow heat exchanger when the surface area is reassigned optimally. For the multi-stream heat exchanger, the thermal performance is also dependent on the uniformity of the TDF, and the parallel-flow arrangement may achieve higher heat exchange effectiveness than the counter-flow arrangement, which indicates that the performance of heat exchanger depends on the coordination degree of temperature fields instead of the flow arrangement.

Optimum allocation of heat transfer surface area for cooling load and COP optimization of a thermoelectric refrigerator

The theory of finite time thermodynamics is applied to analyze and optimize the performance of a thermoelectric refrigerator, which is composed of multi-elements. For the fixed total heat transfer surface area of two heat exchangers, the ratio of the heat transfer surface area of the high temperature side heat exchanger to the total heat transfer surface area of the heat exchangers is optimized for maximizing the cooling load and the coefficient of performance of the thermoelectric refrigerator. The effects of various parameters on the optimum performance are analyzed. The results may provide guides for the analysis and optimization of practical thermoelectric refrigerators.

Thermoeconomic optimization of combined cycle power plants

Recognition and acceptance of irreversibility in the design and operation of any power plant is essential due to the constraints in finite resources (such as finite time of operation, finite size of plant, finite capital investment, finite power production, etc.). Thermoeconomic optimization of a combined cycle power plant, comprised of an arbitrary number of internally irreversible Carnot-like heat engines, is studied in this paper considering finite resource constraints. The efficiency of a multistage endoreversible combined cycle power plant corresponding to maximum power production or minimum operating cost is observed to be identical to that of a single endoreversible heat engine under the same operating conditions. An increase in the number of stages reduces power production but increases total annualized cost of the plant. Inventory control of heat exchanger surface areas or their thermal conductances and the directions for heat transfer augmentation to get maximum benefits are also discussed in this work. Flexibility in selecting different working fluids at different operating pressures is identified for optimal design and operation of the combined cycle power plant.

99/00790 Effect of in-bed recirculation of particle on the design of heat transfer surface of dense phase zone in circulating fluidized bed

The circulating fluidized bed combustion technology has been extensively developed in recent years all over the world. The size of fed coal ranges from 0 to 10 mm and the mean diameter of coal ranges from 2 mm to 3.5 mm leading to a the fluidized bed which consists of a dense phase zone (large particle bed) and a dilute phase zone (small particle bed). This means that the coal combustion fraction in the dense phase zone is large, so one must pay more attention to the heat balance calculation and amount of heat exchange surface area in the dense phase zone. If the heat exchange surface area is not sized correctly, the circulating fluidized bed boiler can not be operated in a steady manner. This paper analyses the heat balance and heat exchange surface design in the dense phase zone for a circulating fluidized bed boiler of 75 t/h steam capacity which is widely used in China. The effect of in-bed particle recirculation on the amount of heat exchange surface area in the dense phase zone is also described.

Intelligent computer aided optimization on specific power of an OTEC Rankine power plant

Specific power, power per unit total heat exchanger surface area, of a heat engine is chosen to be the objective function in the design of an OTEC (Ocean Thermal Energy Conversion) Rankine power plant. An ICAI (Intelligent Computer Aided Instruction) computer software, CyclePad, with Refrigerant-12 as the working fluid operating between a warm ocean water heat source and a cold ocean water heat sink is used in the design process. Through manipulation of the boiler pressure and condenser pressure, the specific power of the OTEC plant is calculated and an upper bound is determined. This bound can direct the design of current and future OTEC heat engines to provide beneficial applications in industry.

Theoretical analysis of the performance of a regenerative closed Brayton cycle with internal irreversibilities

Using the technique of finite-time thermodynamic analysis, the performance of a closed Brayton cycle with regeneration is assessed. Specifically, analytical expressions for the power output and the thermal efficiency, as functions of the pressure ratio and the reservoir temperatures, are derived. The analysis also takes into account all the irreversibilities associated with finite-time heat transfer processes. The paper also shows that the power output can be maximized with judicious selection of parameters such as the heat exchanger surface areas and the heat conductances. Maximum power is obtained when the effectiveness of the hot- and cold-side heat exchangers are related in a specific manner, and this power output is a strong function of the regenerator effectiveness.

Heat transfer effect on the specific heating load of heat pumps

Abstract The maximum possible specific heating load that can be obtained from two-heat-reservoir heat pumps with a set of high-temperature heat sinks and low-temperature heat sources is analyzed. The heat pumps considered in this paper include (1) externally and internally reversible, (2) externally irreversible and internally reversible, (3) externally reversible and internally irreversible and (4) externally and internally irreversible heat pumps. The irreversibilities are assumed to be caused by heat transfer only. The specific heating load, defined as the heating load per unit total heat exchanger surface area, is adopted as the objective function for the heat pump performance analysis in this paper.

Heat transfer effect on the specific cooling load of refrigerators

Abstract The maximum possible specific cooling load that can be obtained from two-heart-reservoir refrigerators with a set of high-temperature heat sinks and low-temperature heat sources is analyzed. The refrigerators considered in this paper include (1) externally and internally reversible, (2) externally irreversible and internally reversible, (3) externally reversible and internally irreversible and (4) externally and internally irreversible refrigerators. The irreversibilities are assumed to be caused by heat transfer only. The specific cooling load, defined as the cooling load per unit total heat-exchanger surface area, is adopted as the objective function for the refrigerator performance analysis in this paper.

Maximum obtainable specific power of high-temperature waste heat engines

An endoreversible Carnot cycle is presented in this paper for a heat engine using higt-temperature waste heat. The endoreversible Carnot cycle is a modified Carnot cycle, where the heat-transferred between the heat engine and its surroundings is the only irreversible process. Since the energy input (waste heat) to the heat engine is free, the cost of the output power of the heat engine depends mainly on the size of the heat exchangers. A specific power, power per unit area of heat exchanger surface area, is adopted as the objective function for the performance analysis of the heat engine. The relation between the maximum obtainable specific power and the temperature range in which the high-temperature waste heat engine operates is found.

Specific heating load of thermoelectric heat pumps

A mathematical model for calculating the performance of a thermoelectric heat pump to achieve a specific heating load is presented. The effects of Joulean heating and thermal conduction are considered. An elaborate heat transfer model is employed to relate the thermal resistances between the thermoelectric system and its surroundings. A specific heating load, heating load per unit of total heat exchanger surface area, is adopted as the objective function for the thermoelectric heat pump performance analysis in this paper.

Heat transfer effect on the specific power availability of heat engines

The maximum possible specific power (specific power availability) that can be obtained from heat engines with a set of high temperature heat source and low temperature sink is analyzed. The heat engines considered in this paper include (1) externally and internally reversible, (2) externally irreversible and internally reversible, (3) externally reversible and internally irreversible and (4) externally and internally irreversible engines. The irreversibilities are assumed caused by heat transfer only. The specific power, defined as the power output per unit total heat exchanger surface area, is adopted as the objective function in determining power economics in this paper.

Decay Heat Removal by Natural Circulation in a 550-kW(electric) SP-100 Power System for a Lunar Outpost

In this paper, the natural-circulation decay heat removal capability of a 550-kW(electric) SP-100 reactor power system for a lunar outpost is investigated. A transient thermal-hydraulic model of the decay heat removal loop (DHRL) is developed to investigate the effects of the radiator surface area, the dimensions and elevation of the decay heat exchanger (DHE), and the diameter of the rise and down pipes on the passive decay heat removal of the system. The effect of gravity is also investigated in order to examine the applicability of earth-based test results to the actual system on the lunar surface. Results show that natural circulation of lithium coolant in the DHRL would keep the SP-100 reactor safely coolable after shutdown. However, the lithium coolant in the adiabatic rise pipe, directly downstream from the reactor core, could overheat by as much as 175 K above its nominal operation value of 1355 K at [approx] 200 s after shutdown. This coolant temperature increase can be reduced by as much as 50 K by increasing the height of the DHE duct to 15 cm; a further increase in the duct height would have little effect n the decay heat removal. Increasing the elevation of the DHEmore » slightly improves the decay heat removal.« less

Optimal Design of Crossflow Heat Exchangers

The design of plate-fin and tube-fin crossflow heat exchangers is discussed. The transfer surface area of crossflow heat exchangers is used ineffectively because of the nonuniform distribution of the heat transfer across the volume of the exchanger. The optimal distribution of the transfer surface area for maximum heat exchanger effectiveness and constant total surface area is determined. It is found that a Dirac delta distribution of the transfer surface aligned along the diagonal of the crossflow exchanger gives the best performance; equal to that of a counterflow device. Design guidelines for optimal area allocation within crossflow heat exchangers are established. Compared to conventional designs, designs following these guidelines may lead to either a higher exchanger effectiveness for equal pressure drops and surface area, reduced pressure drops for equal exchanger effectiveness, or reduced weight and a near cubic form of the exchanger core for equal pressure drops and effectiveness.

Radon modelling and the heat transfer surface area of the British hot dry rock geothermal reservoir

A simple thermal model is developed to evaluate the heat exchange surface area of a hot dry rock geothermal reservoir. This model, in conjunction with the Rn model of Andrews and coworkers, is applied to RH12/RH15 system of the British HDR reservoir. Results suggest that although the estimated Rn transfer surface area represents the actual swept surface area, it is between 25 and 45 times larger than the heat transfer surface area. The difference is explained as due to clustering of several fractures within a range of thermal interaction, over the duration of the circulation. It is also shown that the decline in the measured temperatures of circulation fluids since 1985 to the present is consistent with the heat exchange surface area going through a maximum in 1987.

77 – 4.2 K downlead concept based on thin-film BiCaSrCu oxide

Owing to an excellent match of lattice constants, a downlead concept consisting of thin films of BiCaSrCu oxide (BCSCO) grown expitaxially on the channel walls of a Bi 4Ti 3O 12 (BT) ‘honeycomb’ substrate is put forward. The highly symmetric honeycomb geometry leads to an interesting cancellation of the self-induced magnetic fields in the BCSCO films, suggesting that flux melting may not be problematic. This cancellation condition occurs for a specific, realistic ratio of wall thickness to channel width ( √ 5 4 ) in the BT honeycomb. Relationships between the honeycomb-geometry parameters and the superconducting-film parameters are derived, and for the case of the downlead conducting 75 kA it is found that all substrate and film parameters are within achievable limits. Estimates of the expected consumption of liquid helium due to the 77 − 4.2 K gradient across the downlead suggest that 70 cm 3 h −1 could be achieved provided the downlead has a very efficient heat-transfer coefficient with the evaporated helium gas. The honeycomb substrate should act as an efficient heat-exchanger (surface area ≈ 10 4 cm 2).

Specific power bound of real heat engines

Industrial heat engine optimization usually takes the form of determining minimum heat exchanger area per unit net power or minimum cost per unit net power rather than the form of maximum efficiency. A specific power, power per unit total heat exchanger surface area, is adopted to be the objective function for engine optimization in this paper. Specific power output of an irreversible heat engine coupled with its heat source and sink is analyzed. When time is explicitly considered in the energy exchanges between the engine and its surroundings, it is found that there is an upper bound on the specific power output of the engine. This bound can guide the evaluation of existing real heat engines or influence the design of future heat engines.

Specific power bound of a finite-time closed Brayton cycle

SYNOPSIS The purpose of this paper is to define a finite-time closed Brayton cycle and apply it to an indirect gas-cooled nuclear reactor plant for its optimization. Practical engineering heat engine power optimization usually takes the form of determining minimum heat exchanger area per unit net power output or minimum cost per unit power output, rather than determining the maximum cycle efficiency. The objective function of the optimization described in this paper is specific power, power output per total heat exchanger surface area. The specific power output of a real indirect gas-cooled nuclear reactor plant coupled with its heat source and sink is analyzed. It is found that there is an upper bound on the specific power output of the nuclear power plant. This bound can guide the evaluation of existing real plants or influence design of future power plants.