Total Heat Exchanger Research Articles

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.

空調システムによる暖冷房負荷の外気への放熱特性

To investigate the characteristics of exhaust heat out of air conditioning equipment to outside air is very important. Using four representative types of heat source systems, an analysis was made with a model building by varying the quantity of internally generated heat, both with and without total enthalpy heat exchangers. The quantity and characterisitics of exhaust heat differ with heat source systems in summer. In winter the quantity of exhaust heat from the equipment is small overall. Next, the characteristics of artificial generated heat are changed to the characteristics of artificial exhaust heat that include all latent heat transfer. The chracteristics of artificial exhaust heat are different from characterisitics of artificial generated heat or characteristics of exhaust heat out of the equipment

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.

Optimal allocation of a heat-exchanger inventory in heat driven refrigerators

This paper reports the thermodynamic optimization (or entropy generation minimization) of a heat-driven refrigeration plant, that is, a refrigerator without work input, which is driven by a heat source. The treatment accounts for the heat transfer irreversibilities of the three heat exchangers, and for the finiteness of the total heat-exchanger inventory. The operating conditions for maximum refrigeration rate are determined. It is shown that the heat-exchanger inventory must be divided optimally between the three heat exchangers. For example, half of the inventory must be placed in the heat exchanger used to reject heat to the ambient. The maximum refrigeration rate per unit of total heat exchanger inventory is reported. These thermodynamic optimization principles are then applied to a refrigerator driven by heat transfer from a solar collector.

Flow length effect in heat exchangers with turbulent flow at low prandtl number

This work brings out the effect of entry length on the mean overall heat transfer coefficient of a heat exchanger for turbulent flow with low Prandtl Number. For this purpose simple heat transfer correlations are proposed in the thermal entry length region and with these correlations the correction factors are calculated for selected special cases. The effect of flow parameters and total heat exchanger length has also been investigated. Finally comments are made regarding thermal boundary conditions and in its light the commonly used design method is assessed.

Analytical and Experimental Study on Rotary Total Heat Exchangers for Summer Time Conditions

Analytical and Experimental Study on Rotary Total Heat Exchangers for Summer Time Conditions

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.

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.

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.

Power and Refrigeration Plants for Minimum Heat Exchanger Inventory

What has been accomplished to date on the question of how to minimize and allocate the heat exchanger inventory in power plants and refrigeration plants is summarized in a table. This table also highlights the three objectives of the present study. The first objective is to devise a much simpler model and analysis to reproduce in closed form Ibrahim et al.'s conclusions for fixed power and minimum UA (total heat exchanger inventory). The second objective is to consider the reverse of the refrigeration problem, and to minimize the total UA while holding the refrigeration load fixed. It will be shown analytically that (UA)[sub H] = (UA)[sub L] is once again a feature of the optimal design, the H and L referring to hot and cold ends of the heat exchanger. The third objective is to minimize the total heat exchanger inventory while holding the refrigerator power input fixed, and to show analytically that (UA)[sub H] = (UA)[sub L]. The overriding objective of this note is to demonstrate in the simplest analytical way possible that power plants and refrigeration plants share a common optimization principle. That principle is that the needed heat exchanger inventory is minimum when it is divided evenlymore » between the two ends of the cycle.« less

Biophysical Ecology and Heat Exchange in Insects

When used with observations of behavior and physiology of animals in known microclimates, a biophysical approach is a powerful tool for predicting body temperatures of insects. For ectothermic insects, solution of the energy budget equation and use of operative temperature models have been used to determine the range of temperatures which an insect can exhibit in a given environment. Knowledge of body temperature has allowed predictions of when important behaviors arepossible in the field, thereby directly relating biophysical models to fitness parameters of animals. A proper understanding of the physiological mechanism(s) controlling heat exchange is prerequisite to application and interpretation of information obtained using biophysical techniques. For endothermic insects, physiological regulation of heat exchange forces a more complicated analysis. Evaluation of thoracic heat exchange alone (aside from indicating whether insects are regulating Tth) is of little utility for either quantifying total heat exchange, or evaluating thermoregulatory mechanisms without further information. Further studies of biophysics and physiology of endothermic insects during flight are needed to correct these deficiencies. Application of biophysical techniques has allowed predictions of behavior of flying insects based onprinciples of heat exchange which cannot be examined directly. Analyses of endothermy of restinghoneybee swarms and hives indicate that these “superorganisms” regulate temperature rather precisely over a remarkable range of environmental temperature using mechanisms equivalent to thoseused by resting endothermic vertebrates.

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.

Thermodynamic Optimization of Irreversible Power Cycles With Constant External Reservoir Temperatures

An extended version of the Bejan model of irreversible power plants is proposed using a log-mean temperature difference (LMTD) representation for both the high and low-temperature heat exchangers. The analysis focuses on minimizing the irreversibilities associated with the hot and cold heat exchangers. The results indicate that the maximum power output, external conductance allocation ratio, and second law efficiency are functions of the number total heat exchanger transfer units (N), and are asymptotic to Bejan’s original results as N → O. This asymptote represents a global power output maximum and occurs for either extremely high cycle flow rates or cycle phase change processes in both heat exchangers. The LMTD representation also shows that under optimal conditions, more conductance should be allocated to the low-temperature heat exchanger as N increases.

Finite-element solution of thermal conductivity of muscle during cold water immersion

The in vivo or effective thermal conductivity (keff) of muscle tissue of the human forearm was determined through a finite-element (FE) model solution of the bioheat equation. Data were obtained from steady-state temperatures measured in the forearm after 3 h of immersion in water at temperatures (Tw) of 15 (n = 6), 20 (n = 5), and 30 degrees C (n = 5). Temperatures were measured every 0.5 cm from the longitudinal axis of the forearm to the skin approximately 9 cm distal from the elbow. Heat flux was measured at two sites on the skin adjacent to the temperature probe. The FE model is comprised of concentric annular compartments with boundaries defined by the location of temperature measurements. Through this approach, it was possible to include both the metabolic heat production and the convective heat transfer between blood and tissue at two levels of blood flow, one perfusing the compartment and the other passing through the compartment. Without heat exchange at the passing blood flow level, the arterial blood temperature would be assumed to have a constant value everywhere in the forearm muscles, leading to a solution of the bioheat equation that greatly underpredicts keff. The extent of convective heat exchange at the passing blood flow level is estimated to be approximately 60% of the total heat exchange between blood and tissue. Concurrent with this heat exchange is a decrease in the temperature of the arterial blood as it flows radially from the axis to the skin of the forearm, and this decrease is enhanced with a lowered Tw.(ABSTRACT TRUNCATED AT 250 WORDS)

On ENSO Physics

Two extended integrations of general circulation models (GCMs) are examined to determine the physical processes operating during an ENSO cycle. The first integration is from the Hamburg version of the ECMWF T21 atmospheric model forced with observed global sea surface temperatures (SST) over the period 1970–85. The second integration is from a Max Planck Institut model of the tropical Pacific forced by observed wind stress for the same period. Both integrations produce key observed features of the tropical ocean-atmosphere system during the 1970–85 period. The atmospheric model results show an eastward propagation of information from the western to eastern Pacific along the equator, although this signal is somewhat weaker than observed. The Laplacian of SST largely drives the surface wind field convergence and hence determines the position of large scale precipitation-condensation heating. This statement is valid only in the near-equatorial zone. Air-sea heat exchange is important in the planetary boundary layer in forcing the wind field convergence but not so important to the main troposphere, which is heated largely by condensation heating. The monopole response seen in the atmosphere above about 500 mb is due to a combination of factors, the most important being adiabatic heating associated with subsidence and tropic-wide variations in precipitation. The models show the role of air-sea heat exchange in the ocean heat balance in the wave guide is one of dissipation/damping. Total air-sea heat exchange is well represented by a simple Newtonian cooling parameterization in the near-equatorial region. In the wave guide, advection dominates the oceanic heat balance with meridional advection being numerically the most important in all regions except right on the equator. The meridional term is largely explained by local Ekman dynamics that generally overwhelm other processes in the regions of significant wind stress. The principal element in this advection term is the anomalous meridional current acting on the climatological mean meridional SST gradient. The eastward motion of the anomalies seen in both models is driven primarily by the ocean. The wind stress associated with the SST anomalies forces an equatorial convergence of heat and mass in the ocean. Outside the region of significant wind forcing, the mass source leads to a convergent geostrophic flow, which drives the meridional heat flux and causes warming to the east of the main wind anomaly. West of the main anomaly the wind and geostrophic divergence cause advective cooling. The result is that the main SST anomaly appears to move eastward. Since the direct SST forcing drives the anomalous wind, surface wind convergence, and associated precipitation, these fields are seen also to move eastward.

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.

Heat transfer solutions in laminar co-current flow of immiscible liquids

Thermal entry region solutions are analytically determined for horizontal, co-current laminar flow of immiscible liquids in direct contact, inside circular tubes and parallel plate channels. The related eigenvalue problem for such a composite media is readily solved by extending the ideas in the recently advanced sign-count method. It is assumed a core-annular flow configuration for circular tubes and sheat-core flow for the parallel plates channel, without consideration of interface instabilities and stratified flow. First, the velocity problem is solved for fully developed flow and pumping power expressions established for different operating conditions. Then, the temperature problem is analytically handled to yield expressions for quantities of practical interest such as total heat exchange rates, along the duct length and, again, for different flow rates and pressure drop requirements. The analysis is illustrated through consideration of an application dealing with pumping of a very viscous oil with the addition of an external thin layer of a less viscous fluid (water). Pumping power and total heat exchange are then evaluated for both geometries and critically compared to the single fluid flow problem.

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.

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.