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Performance is analyzed for kinetic energy variation in high velocity stream heat exchangers that are subjected to external heat transfer. Analytical solutions are obtained using the method of inverse operators. They are verified against the reported expressions in the appropriate limits for constant kinetic energy system as well as for perfectly insulated conditions. Kinetic energy decay in hot stream enhances performance that exceeds the conventional heat exchanger effectiveness. For kinetic-to-thermal energy ratio and dimensionless characteristic length constant each equaling unity on the hot side, the terminal effectiveness under balanced operation is 136% for counter-flow arrangement and 82% for parallel-flow in the absence of external heat load. On the contrary, decay on the cold side lowers performance. For unbalanced flow, kinetic energy change in the higher heat capacity rate fluid has a lesser impact on the effectiveness. Thermal interaction with the ambient generally has a deleterious effect on the hot stream effectiveness of the cryogenic system. However, the performance improves under certain conditions such that the rise in fluid temperature caused by kinetic energy deterioration promotes heat loss to the surroundings.

This paper evaluates an optimal airside thermal-hydraulic performance of compact heat exchangers. Seventy-five airside surfaces have been chosen, i.e. standard reference of Kays and London, to represent louver-fin, strip-fin, wavy-fin, plain-fin, pin-fin, finned circular tubes and finned flat tubes. A robust evaluation method is implemented by an estimation of the heat transfer rate per unit pumping power with and without considering the heat exchanger compactness. Experimental data of Colburn j-factor and Fanning friction factor (f) are used to estimate both the heat transfer rates and the friction power, respectively. The results demonstrate that strip-fin surface 1/10-27.03 shows an optimal heat transfer values per unit friction power when the compactness is taken into consideration. Nonetheless, pin-fin surface PF-4(F) shows an optimal heat transfer rate per pumping power when neglecting the importance of the compactness. The geometries having an optimal thermal-hydraulic performance for each airside type (with or without considering the compactness) are recommended as benchmarks to assess the performance of similar airside surfaces.

This work focuses on proposing generalized air-side thermal-hydraulic correlations for wavy-fin compact heat exchangers. The new correlations are modeled from experimental and numerical f- and j-factor data reported in the literature and presented in this paper. We have simulated twenty-one new geometries, using experimentally validated three-dimensional model implemented in ANSYS Fluent program, to obtain heat-transfer and fluid-flow characteristics of unreported geometries. Data of 50 geometries are used to propose new f and j correlations with an rms error of 9.23% and 9.90%, respectively. Testing and comparing the present correlations with those reported in the literature demonstrate that the present air-side f- and j-factor correlations are more accurate and generalized for regular wavy-fin compact heat exchangers.

An optimal assessment of heat transfer and friction power performance is investigated to address a robust method for facilitating performance evaluation of different wavy-fin arrangements. Two benchmark geometries are presented to compare the performance of all the reported wavy-fin arrangements with and without considering heat exchanger compactness. New 21 wavy-fin geometries are numerically studied to represent the unreported arrangements in the literature. The performance results of 105 geometries showed that the reference standard geometries of Kays and London could be used as benchmarks to evaluate the airside arrangements, with and without considering the compactness effect, respectively. Heat transfers versus friction power plots are found to be a useful approach for performance evaluation purpose. The surface GK2 is found to have the optimal thermal-hydraulic performance when comparing the heat exchanger compactness, whereas GI1 and GK1 might represent the geometries having optimal performances when neglecting the compactness importance.

The analysis for fin thermal performance under dehumidifying conditions is conducted using a numerical approach. Finite-Element method is used to account for the non-linear effects due to temperature dependent parameters and humidity-temperature relationship. The temperature profiles, as well as the overall fin efficiency, are studied. The fin efficiency is found to be highly dependent on the operating conditions and the degree to which the heat transfer coefficient is a function of temperature. It is found that the effect of temperature-dependent thermal conductivity on fin performance is less as compared to heat transfer coefficient over the considered range of operating conditions.

Thermal performance characteristics of orthotropic material annular fins subjected to dehumidifying operating conditions are studied using two-dimensional finite element method. The method accounts for non-linear temperature–humidity ratio psychrometric relationship under both partially and fully wet operating conditions. Limits of one-dimensional solutions for orthotropic material fins subject to dehumidifying conditions are investigated in somewhat more detail by introducing dimensionless parameters. The dry fin parameter that fully governs fin efficiency under a set of operating conditions is also defined. The optimum fin dimensions are also estimated for a set of operating conditions and fixed fin volume.

A counter-flow heat exchanger with longitudinal conduction in separating wall is analyzed along with external heat flux. The inhomogeneous system of model equations is solved with the method of variation of parameters. Closed-form expressions are obtained for balanced flow with three types of wall end-conditions and the zero heat in-leak limit verified against reported expressions. In addition, analytical solution is derived for the general unbalanced flow model with non-adiabatic wall ends that is validated against the limiting case of no axial conduction. It enables examining property variations and arbitrary external load by dividing the system into smaller segments and applying the derived expressions to each element. Heat in-leak has higher impact on deterioration in the low NTU region studied when directed toward the hot fluid. As the external parasitic enhances with NTU, a critical NTU is noted in the range 2.8–5.1 for the various cases analyzed. At this optimum condition, the net energy reaching colder stages is between 27% and 55%.

The thermal performance of hyperbolic profile annular fins subjected to dehumidifying operating conditions is studied. An analytical solution for completely wet fin is derived using an approximate linear temperature–humidity relationship. A numerical solution using actual psychrometric relationship for completely and partially wet operating conditions is then obtained to account for the actual temperature–humidity ratio psychrometric relationship under both partially and fully wet operating conditions. An excellent agreement is observed between analytical and numerical solutions for completely wet fin. The fin optimization is presented based on the analytical solution of completely wet fin. Finally, a finite element formulation is used for studying the two-dimensional effects of orthotropic thermal conductivity on the thermal performance of fin under partially and fully wet operating conditions.

Compressed natural gas is transmitted through a network of pipelines after its production from oil-gas wells. This gas is then cooled below the dew-point to remove the condensate that may accumulate in the transmission line, causing erosion and deposits. In this paper, the dew-point refrigeration system used with a gas compression system is investigated. The design conditions are obtained from the manufacturer data sheets, which are validated with a computer program to study both the design and rating of such systems. The coefficient of performance (COP) of the system at the design condition is 2.81, while the effectiveness of condenser is 0.84 and that of evaporator is 0.91. The system COP sensitivity with regard to inlet single-phase temperatures, superheat temperature and conductance of heat exchangers (UA), is examined. It is found that the system is more sensitive under design conditions as compared to performance operations. The effect of fouling that degrades the UA value of the condenser and evaporator is also studied. It is found that condenser fouling has a significant impact on the performance of the system. The impact of alternate refrigerants at optimized intermediate pressure, is also investigated.

In this paper, thermoeconomic considerations are given to heat exchanger inventory allocation in irreversible refrigeration cycles and heat pumps with finite thermal capacitances. Investigation is made with respect to specified rate of cooling and heating for the refrigeration cycle and heat pump, respectively. Exact expressions are obtained without the use of an internal irreversibility parameter. The optimum hot-to cold-end unit cost ratio resulted in unequal division of heat exchanger conductances for the heat pump in contrast to the endoreversible case where they were the same. At a constant evaporator to condenser fluid temperature ratio, the COP of both refrigeration and heat pump systems was found to vary in an almost asymptotic manner in contrast to the endoreversible case where it was constant. In conclusion, it is best to measure the effect of internal dissipation by deriving exact mathematical expressions as the effect on some performance parameters could be lost due to the averaging effect of the internal irreversibility parameter.

In this paper, a model is presented to predict properties and performance parameters of refrigeration systems under fouled conditions. This was based on the fact that the majority of the heat transfer in these systems occurs in the phase-change region. By applying the Buckingham Pi theorem, it was established through the use of the reversed Curzon–Ahlborn cycle that the effect of fouling on thermodynamic properties and performance parameters can be predicted but that this is possible through combining data under specific conditions. The fact that real refrigeration systems are based upon these cycles pointed to the conclusion that this can be used on actual industrial and residential systems. Therefore, this was demonstrated through application upon the simulated model of a simple vapor compression cycle. The advantage provided by the fouling prediction model is the saving of time and money as it will reduce the amount of simulations and/or experimental work needed.

In this paper, an experimental study on the nature of the effects of condenser fouling on some performance characteristics and properties of a simple vapor compression system is presented. This is performed by gradually blocking the condenser air passage and measuring the change in condenser pressure, superheat temperature at the compressor exit, compressor power consumption and COP. The room temperature is kept at approximately 21 °C while the ambient temperature is at 31.6 °C with R22 as the cycle refrigerant. The variation in the above-mentioned quantities is found to be logarithmic in nature and this is connected to heat exchanger theory for phase change processes. Comparison with other works is also done and a consistent behavior is concluded for both properties as well as performance parameters.

Using mechanical sub-cooling systems to increase COP of vapor compression cycles is a known method in literature to save energy and increase efficiency. Recently, much progress has been made with respect to investigation into its different aspects that can help to put it into practice. Numerical and experimental works are considered for the purpose of highlighting this progress. These can be categorized as: a) simulation of performance characteristics resulting from different refrigerant combinations in dedicated mechanical sub-cooling systems, b) variation in performance characteristics for a vapor compression cycle using integrated mechanical sub-cooling because of fouling, c) experimental study about consequences of employing a dedicated mechanical subcooling cycle with a simple vapor compression system, and d) experimental investigation about consequences of employing a subcooler in a two-stage refrigeration cycle. Some important results are discussed. Finally, some suggestions are made to provide direction into future research in this area to help put it into practice.

The effectiveness-NTU relations are valuable information for heat exchangers with fluids having significant variation in velocities in the presence of heat leak. In this paper, the closed-form relations for parallel-flow heat exchangers, when the heat leak is either on the cold or hot side of the heat exchanger in the presence of kinetic energy variation, are presented. The effectiveness was found to depend on NTU and fluid capacity ratio along with six other quantities that reflect the magnitude and axial distribution of the kinetic energy and heat leak on the hot and cold sides of the heat exchanger. The results are presented in a graphical form demonstrating the variation of effectiveness of the heat exchanger with the relevant parameters. It was demonstrated that in the limiting case, the solution reduces to the classical effectiveness-NTU relations for parallel-flow heat exchangers.

In this paper, thermoeconomic considerations are given to heat exchanger inventory allocation in vapor compression cycles with mechanical subcooling. Investigation is made with respect to constant work rate, heat rejection and cooling rates as well as heat transfer in the subcooler. It was found that no minima exist for any of the cost functions with respect to the absolute temperature ratio ( ξ ) and the average subcooling absolute temperature ratio ( θ 3 ). The derivatives for the integrated subcooling cycle can be generated from the derivatives of the dedicated subcooling cycle. It was concluded that the cost optimization of the integrated mechanical subcooling system is qualitatively the same as the dedicated subcooling system.

The optimal dimensions of annular fins of constant and variable cross sectional area when subjected to both heat and mass transfer were investigated by a numerical scheme. A non-linear model representing both heat and mass transfer mechanisms was solved using finite difference over-relaxation scheme. Numerical solutions are obtained for the dimensionless heat transfer rate for completely wet conditions as a function of important dimensionless parameters ( u , v , w ) for annular fins. The results are presented in a graphical form as well as regression equations for rectangular, triangular, convex and concave parabolic annular fins. It is shown that these dimensionless parameters represent modified version of fin parameter ( m o L ) , non-dimensional fin material volume and fin base ratio (outer radius of tube to thickness of fin at its conjunction with tube). Keeping the two dimensionless parameters constant ( u , v ) and considering w as the only independent variable, the maximum heat dissipated from the fin is obtained.

A numerical analysis is carried out to study efficiency and temperature distribution of annular fins of different fin profiles (constant and variable cross-sectional area) when subjected to simultaneous heat and mass transfer mechanisms. The temperature and humidity ratio differences are driving forces for heat and mass transfer, respectively. Actual psychrometric relations are used in the present work instead of a linear model between humidity ratio and temperature that has been used in the literature. A non-linear model representing heat and mass transfer mechanisms was solved using a finite difference successive over-relaxation method. Solutions are obtained for temperature distribution over the fin surface in addition to fin efficiency for both fully wet and partially wet fin surfaces. The numerical results are compared with those of previous studies. It was found that one of the linear models for the relation between the humidity ratio and temperature is a reasonable approximation.

Performance characteristics due to use of different refrigerant combinations in vapor compression cycles with dedicated mechanical sub-cooling are investigated. For scratch designs, R134a used in both cycles produced the best results in terms of COP, COP gain and relative compressor sizing. In retrofit cases, considering the high sensitivity of COP to the relative size of heat exchangers in the sub-cooler cycle and the low gain in COP obtained due to installation of a dedicated sub-cooling cycle when R717 is the main cycle refrigerant, it seems that dedicated mechanical sub-cooling may be more suited to cycles using R134a as the main cycle refrigerant rather than R717. With R134a as the main cycle refrigerant, no major difference was noted, by changing the sub-cooler cycle refrigerant, in the degradation of the performance parameters such as COP and cooling capacity, due to equal fouling of the heat exchangers.

In this paper, the rules of thumb used to predict evaporation losses in different evaporative heat exchangers are unified. The accurate prediction of all aspects of evaporative heat exchanger behavior is very important. Accurately predicting evaporation losses is significant since water, in this class of heat exchangers, is cooled primarily by evaporation of a portion of the circulating water that causes the concentration of impurities to increase. Furthermore, the design of hybrid cooling towers requires that the amount of evaporation be known. A single correlation is, therefore, developed for this class of heat exchangers that is simple and accurate with a wide range of applicability. The predicted values are in good agreement with experimental values as well as predictions made by reliable mathematical models.

Performance degradation due to fouling in a vapor compression cycle is investigated for various applications. Considering the first set of refrigerants i.e. R134a, R410A and R407C, from a first law standpoint, the COP indicates that R134a always performs better unless only the evaporator is being fouled. In contrast to this, from a second-law standpoint, the second-law efficiency indicates that R134a performs the best in all cases. Considering the second set of refrigerants i.e. R717, R404A and R290, from a first law standpoint, the COP indicates that R717 always performs better unless only the evaporator is being fouled. In contrast to this, from a second-law standpoint, the second-law efficiency indicates that R717 performs the best in all cases. Volumetric efficiency of R410A and R717 remained the highest under the respective conditions studied. Furthermore, performance degradation of the evaporator often has a larger effect on compressor power requirement while that of the condenser has an overall larger effect on the COP. A new performance degradation law is presented in light of the data generated, which can reduce the amount of experimentation and help predict relevant quantities of the refrigeration system.

An analysis was carried out to study the efficiency of annular fin when subjected to simultaneous heat and mass transfer mechanisms. The temperature and humidity ratio differences are the driving forces for the heat and mass transfer, respectively. Analytical solutions are obtained for the temperature distribution over the fin surface when the fin is fully wet. The effect of the atmospheric pressure on the fin efficiency was also studied, in addition to fin optimum dimensions. It is demonstrated that the closed-form solutions for a dry-fin case presented in many text books are special cases for the solutions presented in this paper.

Fouling of cooling tower fills is one of the most important factors affecting its thermal performance, which reduces cooling tower effectiveness and capability with time. In this paper, the fouling model presented in an earlier paper using the experimental data on fill fouling, is used to investigate the risk based thermal performance of the cooling tower. It is demonstrated that effectiveness of the cooling tower degrades significantly with time indicating that for a low risk level ( p =0.01), there is about 6.0% decrease in effectiveness for the given fouling model. The sensitivity analysis of the cooling tower is investigated for both rating and design calculation for different values of mass flow rate ratios. The effect of atmospheric pressure on the thermal performance of the cooling tower is also demonstrated.

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.

Thermodynamic analysis of HFC-134a vapor-compression refrigeration cycles is investigated by both the first and second laws of thermodynamics. Second-law analysis is carried out for both two-stage and mechanical-subcooling refrigeration cycles. The analysis is performed on each of the system components to determine their individual contribution to the overall system irreversible losses. It is found that most of the losses are due to a low compressor efficiency. Irreversibilities of expansion valves and condenser are also significant. In addition, it is shown that the optimum inter-stage pressure for two-stage and mechanicalsubcooling refrigeration systems is very close to the saturation pressure corresponding to the arithmetic mean of the refrigerant condensation and evaporation temperatures. These results are compared with the existing practice in the industry. Furthermore, theoretical results of a two-stage refrigeration system performance are also compared with experimental values for a CFC-22 system. Copyright © 1996 Elsevier Science Ltd and IIR