Total Heat Exchanger Area Research Articles

05/02702 An approach for the optimum design of heat exchangers

We improved our model for minimization of entropy production rate in diabatic tray distillation. The entropy production rate had contributions from heat and mass transfer on the trays and from heat exchangers connected to the trays. The area of the heat exchangers were not used as free variables, but calculated from four different area distribution rules. The total heat exchange area and its distribution over the column had a significant effect on the entropy production rate in the system. In the limit of infinitely large forces, the results of an adiabatic column were obtained, as expected. As the forces became smaller, the distribution of the area became more important. The area distribution rule with constant average force in each heat exchanger had the lowest entropy production rate of the rules used. Possible consequences for column design were discussed, and a sketch with the characteristics of an optimum distillation column was presented. Two minimization algorithms were used with similar outcome; the Matlab® function fmincon being the fastest.

A novel near-zero CO 2 emission thermal cycle with LNG cryogenic exergy utilization

A novel liquefied natural gas (LNG) fueled power plant is proposed, which has virtually zero CO 2 and other emissions and a high efficiency. Natural gas is fired in highly enriched oxygen and recycled CO 2 flue gas. The plant operates in a quasi-combined cycle mode with a supercritical CO 2 Rankine-like cycle and a CO 2 Brayton cycle, interconnected by the heat transfer process in the recuperation system. By coupling with the LNG evaporation system as the cycle cold sink, the cycle condensation process can be achieved at a temperature much lower than ambient and high-pressure liquid CO 2 ready for disposal can be withdrawn from the cycle without consuming additional power. The net energy and exergy efficiencies are found to be over 65 and 50%, respectively. In the case computed (but not optimized), the required total heat exchanger area is estimated to be about 460 m 2/MW electricity produced. Besides electricity and condensed CO 2, the byproducts of the plant are H 2O, liquid N 2 and A r.

Distribution of heat exchange in optimum diabatic distillation columns

We improved our model for minimization of entropy production rate in diabatic tray distillation. The entropy production rate had contributions from heat and mass transfer on the trays and from heat exchangers connected to the trays. The area of the heat exchangers were not used as free variables, but calculated from four different area distribution rules. The total heat exchange area and its distribution over the column had a significant effect on the entropy production rate in the system. In the limit of infinitely large forces, the results of an adiabatic column were obtained, as expected. As the forces became smaller, the distribution of the area became more important. The area distribution rule with constant average force in each heat exchanger had the lowest entropy production rate of the rules used. Possible consequences for column design were discussed, and a sketch with the characteristics of an optimum distillation column was presented. Two minimization algorithms were used with similar outcome; the Matlab ® function fmincon being the fastest.

Design Study of Part-Flow Evaporative Gas Turbine Cycles: Performance and Equipment Sizing—Part I: Aeroderivative Core

The evaporative gas turbine cycle is a new high-efficiency power cycle that has reached the pilot testing stage. This paper presents calculation results of a new humidification strategy based on part flow humidification. This strategy involves using only a fraction of the compressed air for humidification. Thermodynamically, it can be shown that not all the air needs to be passed through the humidification system to attain the intrinsic good flue gas heat recovery of an EvGT cycle. The system presented also includes live steam production and superheating by heat from the hottest flue gas region. The humidifier only uses the lower temperature levels flue gas heat, where it is best suited. The analyzed system is based on data for the aeroderivative Rolls Royce Trent as a gas turbine core. Part II of this two-part paper presents the results based on data for the industrial gas turbine ABB GTX100. Simulation results include electric efficiency and other process datas as functions of degree of part flow. A detailed model of the humidifier is also used and described, which produces sizing results both for column height and diameter. Full flow humidification generates an electric efficiency of 51.5% (simple cycle 41%). The efficiency increases when the humidification air flow is reduced, to reach a maximum of 52.9% when air flow to the humidification amounts to around 12% of the intake air to the compressor. At the same time, total heat exchanger area is reduced by 50% and humidifier volume by 36% compared to full flow humidification. This calls for a recommendation not to use all the compressed air for humidification.

Design Study of Part-Flow Evaporative Gas Turbine Cycles: Performance and Equipment Sizing—Part II: Industrial Core

This is Part II of a two-part paper and presents calculation results of a part-flow EvGT cycle based on gas turbine data for the ABB GTX100 (modified for intercooling). The evaporative gas turbine cycle is a new high-efficiency cycle that has reached the pilot testing stage. This paper presents calculation results of a new humidification strategy based on part-flow humidification. This strategy involves using only a fraction of the compressed air for humidification. Thermodynamically, it can be shown that not all the air needs to be passed through the humidification system to attain the intrinsic good flue gas heat recovery of an EvGT cycle. The presented system also includes live steam production and superheating, by heat from the hottest flue gas region, for injection. The humidifier then only uses the lower temperature levels, where it is best suited. The analyzed system is based on data for the ABB GTX100.gas turbine in intercooled mode. Part I of this two-part paper presents the results based on data for the aeroderovative Rolls Royce Trent. Simulation results include electric efficiency and other process data as function of degree of part flow. A detailed model of the humidifier is used, which produces sizing results both for column height and diameter. Paper I includes detailed description of the modeling. For the GTX100 system, full-flow humidification generates an electric efficiency of 52.6% (simple cycle 36.2%). The efficiency is virtually unaffected if the air portion to humidification is cut to 60% of accessible compressor air (represents 48% of compressor intake). If 30% of air from the compressor after cooling bleed (24% of intake) is led to the humidifier, the efficiency is reduced to 52.2%. On the other hand is the total heat exchanger area reduced by 20% and column volume by 50%. This calls for a recommendation not to use all the compressed air for humidification. It is recommended to use 15–30% of compressor intake air. The exact economic optimum depends on local fuel prices, CO2 taxes, interest rates, etc.

Thermodynamic optimization of finite time vapor compression refrigeration systems

The optimization of refrigeration systems involves finding the optimum operating point at which the coefficient of performance of the system is maximum. For most vapor compression refrigeration systems, optimum performance is obtained by proper distribution of the total heat exchanger area between the evaporator and condenser of the system. In this paper, finite time thermodynamic models of a simple vapor compression refrigeration system, a two stage vapor compression refrigeration system (TSS), a dedicated mechanical subcooling vapor compression refrigeration system (DMSS) and an integrated mechanical subcooling vapor compression refrigeration system (IMSS) are presented to study the systems with respect to the heat exchanger parameters. In addition, the model of the TSS is used for investigating the optimum performance with respect to interstage temperature, while the models of the DMSS and IMSS are used for investigating the optimum performance with respect to the subcooler saturation temperature.

Thermophysical Properties of Ammonia–Water Mixtures for Prediction of Heat Transfer Areas in Power Cycles

In power cycles using ammonia–water mixtures as the working fluid, several heat exchangers are used. The influence of different correlations for predicting thermophysical properties on the calculations of the size of the heat exchangers is presented. Different correlations for predicting both the thermodynamic and the transport properties are included. The use of different correlations for the thermodynamic properties gives a difference in the total heat exchanger area of 7%, but for individual heat exchangers, the difference is up to 24%. Different correlations for the mixture transport properties give differences in the predicted heat exchanger areas that are, at most, about 10% for the individual heat exchangers. The influence on the total heat exchanger area is not larger than 3%. A difference in the total heat exchanger area of 7% would probably correspond to less than 2% of the total cost for the process equipment. Experimental data and correlations developed for the ammonia–water mixture transport properties are very scarce. The evaporation and condensation processes involving ammonia–water mixtures are also not fully understood.

Thermoeconomic Analysis on the Performance Characteristics of a Multi-Stage Irreversible Combined Heat Pump System

A cycle model of a multi-stage combined heat pump system, which includes the irreversibility of finite rate heat transfer across finite temperature differences and the irreversibilities inside the working fluid, is established and used to investigate the influence of these irreversibilities on the performance of the system. The profit of operating the heat pump system is taken as an objective function for optimization. The maximum profit is calculated for a given total heat transfer area or total thermal conductance of heat exchangers. The coefficient of performance, heating load, and power input at the maximum profit are determined. The distribution of the heat transfer areas or the thermal conductances of heat exchangers and the temperature ratios of the working fluids of two adjacent cycles in heat exchange processes are optimized. The results obtained here are generally significant. They are suitable for an arbitrary-stage irreversible and endo- reversible combined heat pump system. [S0195-0738(00)01104-3]

Design and rating of an integrated mechanical-subcooling vapor-compression refrigeration system

Subcooling of the refrigerant at the exit of the condenser in a simple vapor-compression refrigeration system allows the refrigerant to enter the main cycle evaporator with low quality. Thus, allowing the refrigerant to absorb more heat in the evaporator; thereby improving the coefficient of performance (COP) of the system. In an integrated mechanical-subcooling vapor-compression refrigeration system, the subcooling is performed by utilizing a small integrated vapor-compression refrigeration cycle, known as the subcooler cycle. This subcooler cycle is coupled to the main cycle at the exit of the condenser and it utilizes the main cycle condenser for rejecting the heat. In this paper, thermodynamic models of an integrated mechanical subcooling system are developed to simulate the actual performance of the subcooling system, particularly with respect to the subcooler saturation temperature in addition to heat exchanger areas. It is demonstrated that the performance of the overall cycle is improved over the corresponding simple cycle. This improvement is found to be related to the refrigerant saturation temperature of the subcooler. The model is also used for predicting an optimum distribution of the total heat exchanger area between the evaporator and condenser.

Design and rating of dedicated mechanical-subcooling vapour compression refrigeration systems

Subcooling of the refrigerant at the exit of the condenser in a vapour compression refrigeration system allows the refrigerant to enter the main cycle evaporator with a lower quality and thus allows the refrigerant to absorb more heat in the evaporator, thereby improving the coefficient of performance (COP) of the system. In a dedicated mechanical-subcooling vapour compression refrigeration system, the subcooling is performed by utilizing a small dedicated vapour compression refrigeration cycle. This cycle is coupled to the main cycle at the exit of the condenser. In this paper, thermodynamic models of the dedicated mechanical-subcooling systems are developed to simulate the actual performance of the system, particularly with respect to the subcooler saturation temperature in addition to the heat exchanger areas. It is demonstrated that the performance of the overall cycle is improved over the corresponding simple cycle and this improvement is found to be related to the refrigerant saturation temperature of the subcooler. The models are also used to predict an optimum distribution of the total heat exchanger area between the evaporator and condenser.

The Influence of Several Major Irreversibilities on the Performance Characteristics of an n-Stage Combined Heat Pump System

A universal cycle model of an n-stage combined heat pump system, which includes the irreversibility of finite-rate heat transfer across finite temperature differences, the heat leak loss between the external heat reservoirs, and the irreversibilities inside the working fluid, is established and used to investigate the influence of these irreversibilities on the performance of the system. The coefficient of performance is taken as an objective function for optimization. A general optimum relation is derived for a given specific heating load and a given total heat-transfer area of heat exchangers. The general performance characteristic curves of the system are obtained. The maximum coefficient of performance with non-zero specific heating load is determined. Some key design variables, such as the specific heating load, the temperature ratios of the working fluid in the isothermal processes, the distribution of the heat-transfer areas of heat exchangers, and the total power input of the system, are optimized. The optimal performance of an arbitrary-stage irreversible, endoreversible, and reversible combined heat pump system can be directly derived from the results in this paper.

Design and performance evaluation of reciprocating refrigeration systems

The characteristic performance curves of vapor-compression refrigeration systems are defined as a plot between the inverse coefficient of performance (1/ COP) and inverse cooling capacity (1/ Q ̇ evap ) of the system. Using the actual data of a simple vapor-compression system, performance curves of the system are obtained. The curves were found to be linear and this linear relation between 1/ COP and 1/ Q ̇ evap is explained in the light of various losses of the system, resulting from the irreversibilities losses due to finite rate of heat transfer in the heat exchangers and non-isentropic compression and expansion in the compressor and expansion valve of the system, respectively. A finite-time thermodynamic model which simulates the working of an actual vapor-compression system is also developed. The model is used to study the performance of a variable-speed refrigeration system in which the evaporator capacity is varied by changing the mass-flow rate of the refrigerant, while keeping the inlet chilled-water temperature as constant. The model is also used for predicting an optimum distribution of heat-exchanger areas between the evaporator and condenser for a given total heat exchanger area. In addition, the effect of subcooling and superheating on the system performance is also investigated.

Design and rating of a two-stage vapor-compression refrigeration system

The non-isentropic compression processes in the compressor of a vapor-compression system consume more power with increasing thermal lift because of the increasing slope of the constant isentropic line on a log (p)-h diagram. However, in a two-stage system (TSS), compression is performed in stages, thereby allowing the compressors to operate efficiently close to the saturation curve. In this paper, thermodynamic models of the TSS are developed to simulate system performance with respect to the interstage temperature and heat-exchanger parameters. The TSS performance is better than for a simple cycle and depends on the interstage temperature of the flash intercooler. Models are used to derive the optimum total heat exchanger area between the evaporator and condenser.

The influence of multi-irreversibilities on the performance of a heat transformer

A general cycle model including several major irreversibilities existing usually in real heat transformer systems is established and used to investigate the optimal performance of a heat transformer affected by the irreversibilities of finite-rate heat transfer between the working fluid and the external heat reservoirs, heat leak losses of the heated space, and internal dissipation of the working fluid. The coefficient of performance and the specific heating load of the cycle system are used in turn as objective functions for optimization. The maximum values of these parameters are calculated for a given total heat-transfer area of heat exchangers. The dimensionless specific heating load versus coefficient of performance curves describing the general performance characteristics of heat transformers are presented. The practical operating region of the heat transformer is discussed.

Optimal Thermodynamic Approximation to Reversible Distillation by Means of Interheaters and Intercoolers

The purpose of this paper is to deal with the problem of heat and power integration on one side and the problem of minimizing heat exchange areas on the other side, in both conventional and nonconventional distillation columns. We consider the limiting case of columns operating at minimum reflux. The appropriate objective functions that one must consider are the entropy production rate and the total heat exchange area, respectively. This is done by means of optimal placement of a given number of interheaters (IHs) and intercoolers (ICs) in stripping and rectifying sections, respectively. To solve these problems, an appropriate thermodynamic model for both conventional and nonconventional distillative columns is formally presented. This model allows us to formulate an optimization problem involving thermodynamically reversible profiles in stripping and rectifying sections of the columns. Our approach differs from others previously reported in that multiple reversible profiles were identified for each section of the column which give rise to lower and upper bounds for the objective function of the minimization problem. In other words, we obtain two solutions for each column section: the first is a nonoptimal feasible one, and the second is an optimal but not necessarily feasible one. Finally, the comparison of our approach with a method based on pseudobinary reversible profiles is carried out. Optimizing with this curve, solutions will be generated with objective function values between our lower and upper bounds. Therefore, care would be taken in using a pseudobinary pinch point curve for the placement of intermediate heat-exchanger units especially when the difference between our upper and lower bounds for the objective function values is relatively great.

Performance Analysis of an OTEC Plant and a Desalination Plant Using an Integrated Hybrid Cycle

A performance analysis of an OTEC plant using an integrated hybrid cycle (I–H OTEC Cycle) has been conducted. The I–H OTEC cycle is a combination of a closed-cycle OTEC plant and a spray flash desalination plant. In an I–H OTEC cycle, warm sea water evaporates the liquid ammonia in the OTEC evaporator, then enters the flash chamber and evaporates itself. The evaporated steam enters the desalination condenser and is condensed by the cold sea water passed through the OTEC condenser. The optimization of the I–H OTEC cycle is analyzed by the method of steepest descent. The total heat transfer area of heat exchangers per net power is used as an objective function. Numerical results are reported for a 10 MW I–H OTEC cycle with plate-type heat exchangers and ammonia as working fluid. The results are compared with those of a joint hybrid OTEC cycle (J–H OTEC Cycle).

Optimization of Design Conditions for OTEC. In the Case of Annual Operation Performance.

In order to build a high-performance OTEC (ocean thermal-energy conversion) plant and operate it efficiently, it is necessary to optimize design conditions, such as velocities of cold sea water and warm sea water passing through heat exchangers, working fluid temperatures for evaporation and condensation, and warm and cold sea water temperatures. In this paper, the total heat exchanger area per annual net power is used as an objective function. Calculated results are shown for a 1000 kW OTEC plant. Ammonia is used as the working fluid. Okinawa is selected as the construction site.

Economic power generation from low-temperature geothermal resources using organic rankine cycle combined with vapour absorption chiller

Power generation with low temperature geothermal resources is not economically viable due to the poor thermal efficiency of organic Rankine cycles (ORC). A novel idea is proposed where a vapour absorption chiller (VAC) can be employed to lower the ORC condensing temperature, thus increasing its power output. This paper presents detailed analysis of a VAC that operates with water-lithium bromide to extract heat from the geothermal brine leaving the ORC vaporizer. The evaporator of the VAC serves as the ORC condenser. A computer program is coded to simulate the combined ORC-VAC performance and compare it with that of the conventional ORC. The results indicate that the ORC-VAC option would render power generation more economical. The organic fluid flow rate in the ORC per unit power output is cut by at least 50% in this case, thus reducing the ORC size and turbine cost. Moreover, even though the ORC-VAC includes more components, the total heat exchange area requirement per unit power output remains practically unchanged.

Optimal synthesis of heat-and-power systems with multiple steam levels

In this paper, the extension of the operating line method (Irazoqui, 1986, Chem. Engng Sci. 41, 5) for the optimal synthesis of heat-and-power systems with dominant power load to the more general case of multiple steam levels is made. The thermodynamic meaning of the optimality region, obtained as a necessary condition for optimality, is also uncovered. An interesting feature of the optimal solution for the integration problem with multiple steam levels is that integration gaps arise. The optimal solution is compared to other solutions which appear as natural alternatives to it on the basis of the same value of the total area of heat exchangers and the same total power output. They all show lower values of their efficiencies to generate shaft power.

Specific power optimization of closed-cycle OTEC plants

The specific power of a simple irreversible closed-cycle OTEC power plant is analyzed and optimized. The plant uses a working fluid such as ammonia to produce power. Specific power is the net output per unit total heat exchanger surface area. Net output generated by the working fluid is obtained in terms of the rate of heat added to the working fluid from the warm surface ocean water, less the rate of heat removed from the working fluid to the cold deep ocean water. A time factor is added to iimulate the heat exchanges between the OTEC plant and its surroundings. A mathematical expression is derived for the specific power output of the irreversible OTEC heat engine. It is found that there is a bound on the specific power output. This bound provides the basis for a practical engineering effort towards maximizing the per unit time and per unit total heat exchanger area production of work in power plants whose heat transfer area is constrained by economic consideration.