板式间接蒸发冷却器热工特性的实验研究

对自行研制设计的间接蒸发冷却换热试件开展了实验，研究了影响换热器换热性能的因素。结果表明：板式间接蒸发冷却器换热效率随二次空气入口的速度升高、一次空气入口的温度、二次空气入口的湿球温度升高而变大，随一次空气入口的速度变大而变小。实验结果对于深入认识间接蒸发冷却器的换热机理及开展换热器的优化设计有着很大的指导意义。     

Indirect evaporative cooling of air to a sub-wet bulb temperature

Indirect evaporative cooling is a sustainable method for cooling of air. The main constraint that limits the wide use of evaporative coolers is the ultimate temperature of the process, which is the wet bulb temperature of ambient air. In this paper, a method is presented to produce air at a sub-wet bulb temperature by indirect evaporative cooling, without using a vapour compression machine. The main idea consists of manipulating the air flow inside the cooler by branching the working air from the product air, which is indirectly pre-cooled, before it is finally cooled and delivered. A model for the heat and mass transfer process is developed. Four types of coolers are studied: three two-stage coolers (a counter flow, a parallel flow and a combined parallel-regenerative flow) and a single-stage counter flow regenerative cooler.It is concluded that the proposed method for indirect evaporative cooling is capable of cooling air to temperatures lower than the ambient wet bulb temperature. The ultimate temperature for such a process is the dew point temperature of the ambient air. The wet bulb cooling effectiveness (E_wb) for the examples studied is 1.26, 1.09 and 1.31 for the two-stage counter flow, parallel flow and combined parallel-regenerative cooler, respectively, and it is 1.16 for the single-stage counter flow regenerative cooler. Such a method extends the potential of useful utilisation of evaporative coolers for cooling of buildings as well as other industrial applications.     

Going below the wet-bulb temperature by indirect evaporative cooling: Analysis using a modified ε-NTU method

The objective of this paper is to study a method to achieve sub-wet bulb temperature by indirect evaporative cooling of air (without using a vapor compression machine). For this purpose, an analytical model is developed based on the effectiveness-NTU method (ε-NTU). The main idea for achieving a sub-wet bulb temperature by indirect evaporative cooling of air is by indirectly pre-cooling the working air before it enters the wet passage. It is shown that a modified analytical model for indirect evaporative coolers could be based on the ε-NTU method for sensible heat exchangers when proper adjustments are made by redefining the potential gradients, transfer coefficient, heat capacity rate parameters and assuming a linear saturation temperature-enthalpy relation of air. This modified model is used to find the performance of a regenerative indirect evaporative cooler. The model results show very good agreement with results from experimental measurements and a numerical model.     

Modelling of a direct evaporative air cooler using artificial neural network

This paper proposes the use of artificial neural networks (ANNs) to predict various performance parameters of a direct evaporative air cooler. For this aim, an experimental evaporative cooler was operated at steady‐state conditions, while varying the dry bulb temperature and relative humidity of the entering air along with the flow rates of air and water streams. Using some of the experimental data for training, a three‐layer feed‐forward ANN model based on back propagation algorithm was developed. This model was used for predicting various performance parameters of the cooler, namely the dry bulb temperature and relative humidity of the leaving air, mass flow rate of the water evaporated into the air stream, sensible cooling rate, and effectiveness of the cooler. Then, the performance of the ANN predictions was tested by applying a set of new experimental data. The predictions usually agreed well with the experimental values with correlation coefficients in the range of 0.969–0.993, mean relative errors in the range of 0.66–4.04%, and very low root mean square errors. This study reveals that, as an alternative to classical modelling techniques, the ANN approach can be used successfully for predicting the performance of direct evaporative air coolers. Copyright © 2007 John Wiley & Sons, Ltd.     

板式间接蒸发冷却换热器的层流特性研究

间接蒸发冷却换热器与传统空调相比具有环保与节能的优点。由于蒸发冷却过程的传热传质机理复杂,本文针对板式间接蒸发冷却换热器,建立了三维稳态传热传质数学模型,并确定了合适的边界条件。通过数值模拟得到了压力场、温度场和浓度场的分布,并讨论了通道间距、速度、温度以及相对湿度等因素对换热效果的影响,为间接蒸发冷却换热器的设计提供了理论指导。     

Performance investigation of plain and finned tube evaporatively cooled heat exchangers

The performance of two evaporatively cooled heat exchangers is investigated under similar operating conditions of air flow rates and inlet hot water temperatures. The heat exchangers are plain and plate-finned circular tube types which occupy the same volume. Spray water, which is circulated in a closed circuit, is injected onto the exposed surfaces of the tubes and fins. The contact between air and spray water results in evaporative heat transfer. The tubes are copper, 10 mm o.d. The finned configuration is constructed by introducing 0.5 mm thick copper plates between the tubes, with a total area ratio of four. A substantial increase in heat transfer takes place for the plate-finned tubes. The increase is 92–140% for air velocities from 1.66 to 3.57 m s⁻¹. A model is used to calculate the thermal performance of the plain and finned tubes assuming a constant spray water temperature in the heat exchanger. The wet-finned surfaces show low fin efficiency compared with dry surfaces. An energy index defined as the ratio of volumetric thermal conductance to air pressure drop per unit length is found to be close for the two heat exchangers. This reveals higher thermal utilisation of the occupied volume by the finned tubes with the same energy index.     

Experimental Analysis of the Effects of Particulate Fouling on Heat Exchanger Heat Transfer and Air-Side Pressure Drop for a Hybrid Dry Cooler

It is well known that significant fouling by particulate matter can have a deleterious effect on the performance of enhanced surface heat exchangers, and the same is true for hybrid heat exchangers. Hybrid heat exchangers are heat exchangers that are typically run in dry mode to reject heat. When the ambient conditions require more heat rejection than can be provided by sensible heat transfer, a water pump is turned on and water flows over the fins, and the evaporation of water provides a further cooling effect. Fouling in dry-mode operation is physically similar to that of air-cooled heat exchangers, but in evaporative mode the flow of the water over the coil eliminates the impact of fouling. A hybrid dry cooler heat exchanger of 60 cm × 60 cm frontal area has been installed in a well-instrumented wind tunnel to measure the heat exchanger's performance. Hot water flows through the coil to provide the load, and air flows over the coil to provide cooling. During evaporative mode operation another stream of water flows over the outside face of the coil, adhering mainly to the louvered fins. The louvered fins are specially designed for optimized water flow during wetting mode. The fins are made of aluminum, the tubes are copper, and protection against corrosion is realized by a special E-coating. This coil has been tested clean and fouled with ASHRAE standard dust, for both dry and wet operation. Results are presented for the air-side pressure drop and overall heat transfer conductance of the coil under all conditions for which 50% increases in air-side pressure drop are found under heavy fouling. The influence of fouling on heat transfer is small. Also, using the wetting water to wash the fouling off the coil is investigated and is found to be of some limited utility.     

Experimental Evaluation of One,Two, and Three Stage Evaporative Cooling Systems

The thermal performance of four different arrangements of evaporative air coolers have been evaluated experimentally during the hot summer of Kuwait. The systems include one-stage direct evaporative cooling (DEC), one-stage indirect evaporative cooler (IEC) linked to an external cooling tower, two-stage indirect/direct evaporative coolers (IEC/DEC), and three-stage system of evaporative cooling and mechanical vapor compression (IEC/DEC-MVC). Two variables are used to evaluate the system thermal performance, the thermal effectiveness and the energy efficiency ratio (EER). The data show that the IEC/DEC has the highest EER, followed by the DEC, IEC/DEC-MVC, and IEC. The DEC has the lowest effectiveness, succeeded by the DEC/IEC, IEC, and IEC/DEC-MVC. Coupling MVC with IEC/DEC extends the cooling range and can cool the ambient air dry bulb temperature over a range of 40°C to 15°C at any value of ambient air wet bulb temperature. Two experimental correlations have been developed for each one of the tested systems. The first one relates the effectiveness to water to air mass flow ratio (L/G) or Reynolds number (Re), while the second correlation relates the EER with effectiveness and L/G and/or Re. These relationships are very important in designing and optimizing the studied evaporative cooling units.     

Performance of Evaporative Condensers

Experimental investigation is conducted to study the performance of evaporative condensers/coolers. The analysis includes development of correlations for the external heat transfer coefficient and the system efficiency. The evaporative condenser includes two finned-tube heat exchangers. The system is designed to allow for operation of a single condenser, two condensers in parallel, and two condensers in series. The analysis is performed as a function of the water-to-air mass flow rate ratio (L/G) and the steam temperature. Also, comparison is made between the performance of the evaporative condenser and same device as an air-cooled condenser. Analysis of the collected data shows that the system efficiency increases at lower L/G ratios and higher steam temperatures. The system efficiency for various configurations for the evaporative condenser varies between 97% and 99%. Lower efficiencies are obtained for the air-cooled condenser, with values between 88% and 92%. The highest efficiency is found for the two condensers in series, followed by two condensers in parallel and then the single condenser. The parallel condenser configuration can handle a larger amount of inlet steam and can provide the required system efficiency and degree of subcooling. The correlation for the system efficiency gives a simple tool for preliminary system design. The correlation developed for the external heat transfer coefficient is found to be consistent with the available literature data.     

单元式露点蒸发冷却装置的实验研究

本研究设计了一台由干、湿通道相结合的单元式露点蒸发冷却装置,通过实验研究了蒸发冷却装置在空气经过一级冷却的模式1和经过二级冷却的模式2两种运行模式下,不同空气入口参数时的换热效果。实验结果表明,空气的入口温度越高,换热效果越好;低湿度时空气的进出口温差比高湿度时大,但其湿球效率和露点效率反而较低,这说明2种效率并不适用于不同湿度间的冷却效果对比;模式2运行时的换热效果比模式1好。与已有研究成果对比表明,该单元式露点蒸发冷却装置的湿球效率和露点效率分别可以达到120%和88%,为露点蒸发冷却装置的优化设计提供理论依据和优化方向。     

Simplified model for the determination of the steady state response of cooling systems

This paper looks at the propagation of temperature disturbances through above ambient cooling systems and its determination at steady state. A cooling system consists of a cooling tower, a pumping system and a network of heat exchangers or coolers arranged in parallel, in series or in a combination of both. Temperature disturbances are introduced into a cooling system in various ways, namely: changes in the wet bulb temperature, changes in the heat load and by the deterioration of the coolers performance due to fouling. The response of a cooling systems to temperature disturbances is shown to be a function of the overall system thermal effectiveness which in turn is a function of the cooling tower thermal effectiveness and of the network overall thermal effectiveness. The way the latter parameter is determined depends on the network structure.     

An Investigation on Cooling Performance of Air-Cooled Heat Exchangers Used in Coal Seam Gas Production

ABSTRACT

This paper reports an investigation into a practical cooling issue on a type of fan-forced finned-tube heat exchangers used in Queensland's coal seam gas (CSG) industry. CSG compression facilities in some production sites suffered underproduction in recent summers because of frequent automatic engine shutdowns. The problem is not expected by the manufacturer's design. However, it is suspected of being related to the control systems on the compression facilities triggering the overheating-protection shutdowns due to possible deficiencies in one or some water/gas cooling loops in the facilities’ air-cooled heat exchangers. Therefore, to understand which heat exchangers and what exact reasons cause the unexpected cooling issue, an investigation has been carried out on the cooler units of the gas compression facilities. A field instrumentation measurement on one operating cooler unit has been done, followed by an analysis using a one-dimensional analytical model and a three-dimensional computational fluid dynamics model. The experimental results are used to validate both the models. Then the cooling performance of the cooler unit under the summer peak condition is predicted by the verified models. The prediction suggests that the water inlet temperature in one particular cooler section is higher than its upper limit defined by the manufacturer, due to poor cooling at high ambient temperatures. The lower cooling performance is caused by large reductions in the cooler air speed and total heat transfer coefficient, which are related to less efficiency of the cooler fans, more airflow resistance, and fouling on both sides of the finned tubes.     

Numerical investigation on the heat and mass transfer in a direct evaporative cooler

A simplified mathematical model is developed to describe the heat and moisture transfer between water and air in a direct evaporative cooler. The mass of evaporated water is treated as a mass source of air flow, and the related latent heat of water evaporation is taken as a heat source in the energy equation. The momentum caused by water evaporation is taken into account in the momentum equations. The effective air viscosity and diffusion coefficient are decided experimentally. The models and methods are validated by comparing the numerical results with those of experiment for the same evaporative cooler. The influences of the inlet frontal air velocity, pad thickness, inlet air dry-bulb and wet-bulb temperatures on the cooling efficiency of the evaporative cooler are calculated and analyzed.The cooling effects of the direct evaporative cooler are predicted for use in four different regions in northwest China using the present numerical method and local weather data for air conditioning design. The predicted results show the direct evaporative cooler with high performance pad material may be well applied for air conditioning with reasonable choices for the inlet frontal velocity and pad thickness.     

An indirect evaporative chiller

A novel indirect evaporative chiller driven by outdoor dry air to produce cold water as the cooling source for air conditioning systems is introduced, and the principle and the structure of the chiller is presented. The cold water can be produced almost reversibly under ideal working conditions, with its temperature infinitely close to the dew point temperature of the inlet air. The key components of the chiller are an air cooler and a padding tower. To improve the heat transfer performance inside the chiller, a quasi-countercurrent air cooler was designed; a subsection linear method was used for the mathematical model of the padding tower. The first indirect evaporative chiller, designed and developed in 2005, has been in use in Kairui Building, a big hotel in Shihezi, Xinjiang Autonomous Region. The tested temperature of the water produced is below the wet bulb temperature of outdoor air and reached the average value of the dew point temperature and the wet bulb temperature of outdoor air. As the running components are only pumps and fans, the COP (cooling energy for room divided by power cost) of this chiller is high, and the drier the outdoor air, the higher COP the chiller obtained. Since no CFCs are used in this chiller, it would not cause any pollution to the aerosphere. Finally, the application prospect of the indirect evaporative chiller in the world is presented.     

Theoretical and testing performance of an innovative indirect evaporative chiller

An indirect evaporative chiller is a device used to produce chilled water at a temperature between the wet bulb temperature and dew point of the outdoor air, which can be used in building HVAC systems. This article presents a theoretical analysis and practical performance of an innovative indirect evaporative chiller. First, the process of the indirect evaporative chiller is introduced; then, the matching characteristics of the process are presented and analyzed. It can be shown that the process that produces cold water by using dry air is a nearly-reversible process, so the ideal produced chilled water temperature of the indirect evaporative chiller can be set close to the dew point temperature of the chiller’s inlet air. After the indirect evaporative chiller was designed, simulations were done to analyze the output water temperature, the cooling efficiency relative to the inlet dew point temperature, and the COP that the chiller can performance. The first installation of the indirect evaporative chiller of this kind has been run for 5 years in a building in the city of Shihezi. The tested output water temperature of the chiller is around 14–20 °C, which is just in between of the outdoor wet bulb temperature and dew point. The tested COP_r,s of the developed indirect evaporative chiller reaches 9.1. Compared with ordinary air conditioning systems, the indirect evaporative chiller can save more than 40% in energy consumption due to the fact that the only energy consumed is from pumps and fans. An added bonus is that the indirect evaporative chiller uses no CFCs that pollute to the aerosphere. The tested internal parameters, such as the water–air flow rate ratio and heat transfer area for each heat transfer process inside the chiller, were analyzed and compared with designed values. The tested indoor air conditions, with a room temperature of 23–27 °C and relative humidity of 50–70%, proved that the developed practical indirect evaporative chiller successfully satisfy the indoor air conditioning load for the demo building. The indirect evaporative chiller has a potentially wide application in dry regions, especially for large scale commercial buildings. Finally, this paper presented the geographic regions suitable for the technology worldwide.     

A new duplex Stirling engine concept for solar-powered cooling

The global cooling demand is one of the fastest growing energy demands and is putting a strain on the electricity infrastructure. Solar-powered cooling could provide most of the cooling demand due to the coincidence of the cooling demand and the solar irradiance. In particular, the solar-powered Stirling-cycle cooler has low maintenance requirement, high theoretical efficiency, and use of environmentally friendly gases. However, Stirling-cycle coolers are expensive due to high driving temperatures, complex heat exchangers, and expensive solar tracking so that they have so far only been successful at high-temperature difference applications. This study introduces a novel directly coupled solar Stirling cooler for which the hot engine cylinders are deployed inside evacuated tube collectors. The machine uses air as working fluid, and its driving mechanism is based on the free-piston, balanced compound technology that was patented by Finkelstein. A second-order mathematical model is used to investigate the performance of the machine for different cylinder arrangements, gas leakage rates, chilling temperatures, and solar irradiance. In addition, the regenerators are optimised to maximise the cold production. It is shown that mechanical frictions can be reduced to 20% by selecting an appropriate cylinder arrangement. The solar cooler achieves a maximum cold production rate of 367.5 W/m² without using external heat exchangers at load temperature of 7°C, which is comparable with photovoltaic powered coolers. In addition, the machine is relatively simple, has safe and quiet operation, uses ambient air as working gas, and is able to produce a wide range of chilling including sub-zero temperatures without changing the working gas. The direct thermal coupling of the Stirling cooler to evacuated tube collectors significantly reduces the complexity of the machine and removes intermediate heat transfer steps which reduce the performance. Thus, the suggested cooling technology has great potential for solar refrigeration, especially for low power and near ambient cooling.     

Experimental investigation into the thermal-flow performance characteristics of an evaporative cooler

This study investigates the thermal-flow performance characteristics of an evaporative cooler. The derivation of the Poppe [1] and Merkel [2] analysis for evaporative coolers are presented and discussed. Performance tests were conducted on an evaporative cooler consisting of 15 tube rows with 38.1 mm outer diameter galvanized steel tubes arranged in a 76.2 mm triangular pattern. From the experimental results, correlations for the water film heat transfer coefficient, air–water mass transfer coefficient and air-side pressure drop are developed. The experimental tests show that the water film heat transfer coefficient is a function of the air mass velocity, deluge water mass velocity as well as the deluge water temperature, while the air–water mass transfer coefficient is a function of the air mass velocity and the deluge water mass velocity. It was found that the correlations obtained for the water film heat transfer coefficient and the air–water mass transfer coefficient compare well with the correlations given by Mizushina et al. [3]. Relatively little published information is available for predicting the air-side pressure drop across deluged tube bundles. The present study shows that the pressure drop across the bundle is a function of the air mass velocity and the deluge water mass velocity.     

Effect of Nonuniformities in Vapor Saturation Pressure and Coolant Velocity on Noncondensable Contaminant Accumulation in Single-Pass Air-Cooled Condensers

In the present work, noncondensable contaminant accumulation in tubes of single-pass, multiple-row, cross-flow condensers is investigated. A mathematical model of condensation in such heat exchangers is proposed, which considers changes along the tubes in the saturation temperature, coolant velocity, and heat transfer effectiveness. The model equations are solved with an original algorithm. The vapor distribution, the global effectiveness of the condenser, and the pressure drop between the inlet and outlet plena are determined under different working conditions.     

Application of Transient Analysis Methodology to Quantify Thermal Performance of Heat Exchangers

A transient testing technique is developed to evaluate the thermal performance of industrial-scale heat exchangers. A Galerkin-based numerical method with a choice of spectral basis elements to account for spatial temperature variations in heat exchangers is developed to solve the transient heat exchanger model equations.

Testing a heat exchanger in the transient state may be the only viable alternative where conventional steady-state testing procedures are impossible or infeasible. For example, this methodology is particularly suited to the determination of apparent fouling levels in component cooling water system heat exchangers in nuclear power plants. The head load on these so-called component coolers under steady-state conditions is too small to permit meaningful testing. An adequate heat load develops immediately after a reactor shutdown when the exchanger inlet temperatures are highly time-dependent. The application of the analysis methodology is illustrated herein with reference to an in-situ transient testing carried out at a nuclear power plant. The method, however, is applicable to any transient testing application.     

Supervision of the thermal performance of heat exchanger trains

In oil refining, heat exchanger networks are employed to recover heat and therefore save energy of the plant. However, many heat exchangers in crude oil pre-heat trains are under high risk of fouling. Under fouling conditions, the thermal performance of heat exchangers is continuously reduced and its supervision becomes an important task. The large number of heat exchangers in pre-heat trains and the change of operation conditions and feedstock charges make the daily supervision a difficult task. This work applies an approach to follow the performance of heat exchangers [M.A.S. Jerónimo, L.F. Melo, A.S. Braga, P.J.B.F. Ferreira, C. Martins, Monitoring the thermal efficiency of fouled heat exchangers – A simplified method, Experimental Thermal and Fluid Science 14 (1997) 455–463] and extends it to monitor the whole train. The approach is based on the comparison of measured and predicted heat exchanger effectiveness. The measured value is computed from the four inlet and outlet temperatures of a heat exchanger unit. The predicted clean and dirty values of effectiveness are calculated from classical literature relations as a function of NTU and of heat capacity ratio (R). NTU and R are continuously adjusted according to mass flow rate changes. An index of fouling is defined for the whole network and the results show the performance degradation of the network with time. The work also suggests that Jerónimo’s index of fouling can be used to estimate the fouling thermal resistance of heat exchangers.