Air Wet-bulb Temperature Research Articles

Study on average Nusselt and Sherwood numbers in vertical plate channels with falling water film evaporation

In this paper, the effects of variable parameters on the average Nusselt and Sherwood numbers between moist air and liquid water film in vertical plate channels are investigated using an approach proposed by previous work. The validity of data is also investigated. Traditional correlations for the average Nusselt and Sherwood numbers are not comprehensive enough as they cannot indicate the effects of inlet parameters on heat and mass transfer process. In this research, correlations for the average Nusselt and Sherwood numbers are developed in terms of moist air inlet Reynolds number, ratio of water to moist air inlet mass flow rate, ratio of channel length to half channel width and moist air inlet dimensionless temperature. The definition of the moist air inlet dimensionless temperature takes into account the effects of moist air inlet dry-bulb and wet-bulb temperatures and water film inlet temperature. The correlations show that the moist air inlet dimensionless temperature has the greatest influence on the average Nusselt number compared with that of the other dimensionless parameters, but it has the lowest influence on the average Sherwood number. The results would be helpful to understand the heat and mass transfer behavior between moist air and liquid water film.

Experimental Investigation of Condensation Heat Transfer in Ammonia Evaporative Condensers

This paper reports experimental data obtained for ammonia condensation heat transfer in an evaporative condenser. Experiments were carried out on a specially designed set-up equipped with both measuring and control devices, and a data acquisition system. Experiments were conducted for ammonia saturation temperature of 22°C to 40°C and condenser heat rejection rate ranging from 2000W to 6000W, during steady state condensation of ammonia. The experimental investigation aimed to determine thermal performances of the ammonia evaporative condenser under different operating conditions, namely as a function of: temperature difference between ammonia condensation and inlet air wet bulb temperature, cooling water mass flow rate and ammonia mass flow rate. Water film temperature distribution along the condenser's height has also been studied. Experimental results have been further compared to data in the open literature by authors with similar experimental investigations. The paper concluded that experimental results are in fairly good agreement with other authors work and consequently reliable. The paper also addresses the current ecological aspect implied by refrigerants, by using ammonia. Apart from its attractive thermo-physical properties, as compared to HFCs, ammonia is also an environment friendly natural refrigerant, known for its zero ozone depleting potential and zero global warming effect.

MATLAB Simulation and Validation of Fluid Properties in the Cross Flow Wet Cooling Tower

The efficiency of steam-based thermal power plant includes the performance of various components such as condenser, boiler, turbine and cooling tower. The cooling tower is a device which is used to reject the waste heat through the cooling water to the ambient air. The performance of cooling tower is normally obtained by means of energy analysis using properties of the fluid at the inlet and outlet. The performance of cooling tower is gauged with its efficiency and effectiveness. The variation of fluid properties inside the cross flow wet cooling tower along with the direction of fluid flow has not been studied exhaustively. As the phenomenon of heat and mass transfer in cross flow wet cooling tower is bidirectional and complex. This paper consists of extraction of fluid properties based on heat and mass transfer principle across a cross flow wet cooling tower using MATLAB simulation. Hence with the view to carry out analysis of the cross flow wet cooling tower, wet-bulb temperature and humidity of air along with the horizontal plane and temperature of water along with the vertical plane are obtained using MATLAB simulation and the results of the simulation are validated through experimentation.

Performance Analyses of Counter-Flow Closed Wet Cooling Towers Based on a Simplified Calculation Method

As one of the most widely used units in water cooling systems, the closed wet cooling towers (CWCTs) have two typical counter-flow constructions, in which the spray water flows from the top to the bottom, and the moist air and cooling water flow in the opposite direction vertically (parallel) or horizontally (cross), respectively. This study aims to present a simplified calculation method for conveniently and accurately analyzing the thermal performance of the two types of counter-flow CWCTs, viz. the parallel counter-flow CWCT (PCFCWCT) and the cross counter-flow CWCT (CCFCWCT). A simplified cooling capacity model that just includes two characteristic parameters is developed. The Levenberg–Marquardt method is employed to determine the model parameters by curve fitting of experimental data. Based on the proposed model, the predicted outlet temperatures of the process water are compared with the measurements of a PCFCWCT and a CCFCWCT, respectively, reported in the literature. The results indicate that the predicted values agree well with the experimental data in previous studies. The maximum absolute errors in predicting the process water outlet temperatures are 0.20 and 0.24 °C for the PCFCWCT and CCFCWCT, respectively. These results indicate that the simplified method is reliable for performance prediction of counter-flow CWCTs. Although the flow patterns of the two towers are different, the variation trends of thermal performance are similar to each other under various operating conditions. The inlet air wet-bulb temperature, inlet cooling water temperature, air flow rate, and cooling water flow rate are crucial for determining the cooling capacity of a counter-flow CWCT, while the cooling tower effectiveness is mainly determined by the flow rates of air and cooling water. Compared with the CCFCWCT, the PCFCWCT is much more applicable in a large-scale cooling water system, and the superiority would be amplified when the scale of water distribution system increases. Without multiple iterative calculations and extensive experimental data, the simplified method could be used to effectively analyze the thermal performance of counter-flow CWCTs in operation. It is useful for optimization operation of counter-flow CWCTs such that to improve the energy efficiency of the overall cooling water system.

Irrigation Time Detection System (ITDS): Irrigation Scheduling Using Explicit Neural Network Modeling of Plant and Environmental Variables

This study was conducted in four phases to construct models through them the plant water status can be estimated. These relations and models were implemented as a software and embed into a hardware device. The data were collected in the greenhouse and open field conditions during 2010-2015 and modeled by a multilayer feed forward neural network using MATLAB software. In the first phase, 7 variables including vapor pressure deficit, photosynthetically active radiation, wet bulb temperature, leaf temperature (Tl), air temperature (Ta), Ta and Tl difference (Ta-Tl) and relative humidity (RH) were measured for tomato and cucumber. High coefficient of determination (R) values (more than 0.7) was obtained in all fitted models when all variables are incorporated. However, starting from a complete set of variables, the result of stepwise backward elimination of variables showed that removing VPD, PAR, WbT and Tl has a trivial effect on R values. Therefore in the second phase, only 3 variables (Ta, Ta-Tl and RH) were measured to fit the model for 4 greenhouse and 10 open field crops. For each crop, the best model with the highest R value was obtained. These models were used to design an Irrigation Time Detection System (ITDS) in the third phase. The main parts of the system consist of an artificial neural network (ANN) module as the intelligent part of this system implemented on a cell phone coupled with a microcontroller based hardware device. Finally, in the fourth phase, ITDS was tested successfully on tomato and cucumber in the greenhouse.

Application of Universal Thermal Climate Index (UTCI) for assessment of occupational heat stress in open-pit mines.

The purpose of this article is to examine the applicability of Universal Thermal Climate Index (UTCI) index as an innovative index for evaluating of occupational heat stress in outdoor environments. 175 workers of 12 open-pit mines in Tehran, Iran were selected for this research study. First, the environmental variables such as air temperature, wet-bulb temperature, globe temperature, relative humidity and air flow rate were measured; then UTCI, wet-bulb globe temperature (WBGT) and heat stress index (HSI) indices were calculated. Simultaneously, physiological parameters including heart rate, oral temperature, tympanic temperature and skin temperature of workers were measured. UTCI and WBGT are positively significantly correlated with all environmental parameters (p<0.03), except for air velocity (r<−0.39; p>0.05). Moreover, a strong significant relationship was found between UTCI and WBGT (r=0.95; p<0.001). The significant positive correlations exist between physiological parameters including oral temperature, tympanic and skin temperatures and heart rate and both the UTCI and WBGT indices (p<0.029). The highest correlation coefficient has been found between the UTCI and physiological parameters. Due to the low humidity and air velocity (~<1 m/s) in understudied mines, UTCI index appears to be appropriate to assess the occupational heat stress in these outdoor workplaces.

Performance Enhancement of an Air Cooled Air Conditioner with Evaporative Water Mist Pre-cooling

The present paper deals with experimental investigation of the performance of air cooled split air conditioner, with evaporative water mist pre cooling to increase the cooling capacity and reduce the consumption power under hot and dry climate. This investigation considers how the performance can be enhanced by using water mist to pre-cool ambient air entering the condensers by adiabatic cooling process which depends on the ambient air wet bulb temperature; as well the condensing temperature and condensing pressure will be decreased accordingly. So the cooling capacity would be increased and consumption power would be decreased, consequently the energy ratio, EER would be improved. The performance of air cooled air conditioner with water mist pre cooling;ECAC was compared to the performance of air cooled air conditioner, ACAC which tested under the same ambient condition that is ranged from 25 oC to 52 oC. Test results show that ECAC operating at EER of 10.5 BTV/W. The ECAC had an EER of 47 % higher than that of ACAC under the same and most serve hot and dry condition of 52 oC and 10% relative humidity.

Effect of operation conditions on the second law analysis of a desiccant cooling system

In order to evaluate the potential of a desiccant based evaporative air conditioning system, a second law analysis of the systems was investigated. In this work, a wide range of working parameters that are regeneration temperature from 90°C to 110°C, volume flow rate from 2000m3 to 4000m3 and ambient air conditions which are relative humidity, ambient air temperature and wet-bulb temperature is carried out for performance calculation of the system. It is observed from results that the Carnot COP and the thermal COP of the system varies vice versa during the day. The highest value of Carnot COP and the thermal COP is about 25 and 0.62, respectively. Also, the cooling capacity has significant effect on the second law efficiency. The importance of this study shows that the second law analysis can provide useful information with respect to the theoretical upper limit of the system performance, which cannot be obtained from the first law of thermodynamic analysis alone.

Effects of Environmental Temperature Change on the Efficiency of Coal- and Natural Gas-Fired Power Plants.

Modeling studies predict that droughts and hotter water and air temperatures caused by climate warming will reduce the efficiency (η) of thermoelectric plants by 0.12-0.45% for each 1 °C of warming. We evaluate these predictions using historical performance data for 39 open- and closed-loop coal and natural gas plants from across the U.S., which operated under daily and seasonal temperature fluctuations multiples greater than future average warming projections. Seven to 14 years of hourly water (Tw), dry-bulb air (Ta), and wet-bulb air (Twb) temperature recordings collected near each plant are regressed against efficiency to attain estimates of Δη per 1 °C increase. We find reductions in η with increased Tw (for open-loop plants) up to 1 order of magnitude less than previous estimates. We also find that changes in η associated with changes in Ta (open-loop plants) or Twb (closed-loop plants) are not only smaller than previous estimates but also variable; i.e., η rises with Ta or Twb for some plants and falls for others. Our findings suggest that thermoelectric plants, particularly closed-loop plants, should be more resilient to climate warming than previously expected.

Investigation of the effect of packing location on performance of closed wet cooling tower based on exergy analysis

In this paper, the effect of packing location on thermal performance of Closed Wet Cooling Tower (CWCT) based on exergy analysis has been studied. The experimental study incorporates design, manufacture and testing of a modified counter flow forced draft CWCT prototype. The modification based on addition packing to the conventional CWCT. The variation of spray water temperature, air dry bulb temperature, air wet bulb temperature, enthalpy and relative humidity of air for different position along the tower are measured experimentally. Applying the exergy destruction method for the cooling tower; exergy destruction, exergy efficiency, exergy of water and air were calculated for two cases: CWCT with packing below the heat exchanger and CWCT with packing above the heat exchanger. It is highly important to analyze the exergy along the cooling tower height. Therefore, the exergy analysis of different elements along the height of the tower is carried out. Results show that the total exergy destruction of modified CWCT is higher when the heat exchanger is located above the packing at the highest point of the tower.

Annual Variations of Environmental Conditions in Naturally Ventilated Classrooms in a Tropical Environment: Comparison with Proposed Guidelines

Internal classroom conditions are important aspects of the learning environment of students. A suite of relevant guidelines for acoustic, visual and thermal comfort parameters in naturally and/or mechanically ventilated classrooms in tropical environments is proposed. Data measured during winter and rainy seasons in a typical provincial school in central Thailand were then compared. Only thermal parameters, viz. wet bulb globe temperature, air temperature and relative humidity met the proposed guidelines values at all times during the school year. Mean sound levels and signal-to-noise ratios did not meet the requirements at some time during this period. Mean illuminance in classrooms only reached proposed guideline values when all internal lights were operating which occurred mainly during the rainy season. Illuminance in front of blackboards and whiteboards never reached proposed thresholds at any time.

Cellulose-pad water cooling system with cold storage

The present study develops a cooling system using water as the working medium which is cooled at night by cellulose-pad cooling tower (CWCT) and stored for cooling application at daytime. That is, it utilizes the natural energy drawn from diurnal ambient air temperature difference. A cooling system was built and tested. It is found that the coefficient of performance of CWCT for heat dissipation of water at night, COPnt, is between 3.8 and 11 and varies linearly with the evaporation temperature glide DG (difference between cold water temperature in the storage tank and wet-bulb temperature of ambient air). The COP for room cooling at daytime run with air cooler in a room, COPday, is between 8.8 and 12.6. For day cycle operation, the measured overall cooling COPo is 5.1. COPo is expected to reach 9.4 at room temperature 45 °C.

Feasibility study on two schemes for alleviating the underground heat accumulation of the ground source heat pump

The long time running of the ground source heat pump (GSHP) system in cooling dominated areas will cause heat accumulation and then the temperature increase under the ground, resulting in the cooling performance degradation of the GSHP system, as well as other problems, such as the change of thermal and microbial environment, and even the organic carbon reserves in the soil. In order to attemper the heat accumulation, a scheme that the cooling tower be operated in transitional (mid) seasons is proposed for the hybrid GSHP system assisted with cooling tower. The temperature difference (Δt) between the air wet-bulb temperature and the ground temperature is defined as the threshold for the cooling tower to start up in transitional seasons. It has been found that the scheme can effectively alleviate the increase of the ground temperature by simulating the system performance with TRNSYS software. However, the system’s total power consumption in 30 years increases due to the extra running time of the cooling tower. 8∼12°C is found to be the optimum range of Δt for the studied hybrid GSHP systems by comparing the system performances under different set values of Δt. In order to find further efficient scheme, a hybrid GSHP system combined with a chiller cooling system is also proposed and studied. However, the simulation result shows that the system power consumption has not got down effectively, for the reason that the energy efficiency ratio (EER) of the chiller used in the TRNSYS model is lower than that of the heat pump.

Evaporation characteristics analysis of water film on a spherical solid particle

A theoretical study has been conducted to analyze the transient evaporation characteristics of water film on a spherical solid particle, with the water free surface exposed to air fluid. An analytical solution for transient heat transfer of a spherical solid/water bilayer structure with a combined sensible and latent heat transfer boundary condition at the water/air interface that is movable due to the water evaporation is derived based on the method of variable separation. The water film evaporation characteristics are analyzed for different air temperatures and solid particle materials. The results show that when water evaporation begins, the water surface temperature initially changes sharply, then more gently, and eventually stabilizes at the air wet-bulb temperature. The latent heat needed for the water evaporation may come from both the water film by heat conduction and the bulk air by heat convection, or from the latter only, depending on the magnitude of the air wet-bulb temperature relative to the initial water temperature. The water film transient evaporation characteristics are affected more by the heat capacity than by the thermal conductivity of the solid particle.

Development and implementation of a virtual outside air wet-bulb temperature sensor for improving water-cooled chiller plant energy efficiency

Over the past decades, virtual meters or sensors have been rapidly developed and widely adopted within a number of different fields especially in the building section, and have enabled many intelligent features that would otherwise not be possible and economical. Ambient wet-bulb temperature is normally measured either by a psychrometer, or a combination of dry-bulb temperature sensor and relative humidity sensor. These physical sensors are notoriously costly and problematic considering their accuracy and reliability. Furthermore, ambient wet-bulb temperature is critical in a water-cooled chiller plant to enhance its holistic energy efficiency through specific control strategies, such as cooling tower temperature relief, condenser water supply temperature reset, and so on. This paper introduces a virtual ambient wet-bulb sensor through a black-box method using one low-cost dry-bulb temperature sensor and local weather data, either typical meteorological year (TMY) or real-time weather data. A control algorithm for the cooling tower fan operation was also developed to calibrate this virtual wet-bulb sensor, thus to minimize its measuring deviation. Two case studies in Beijing, China and Texas, USA were adopted to demonstrate the feasibility of this strategy under different climate scenarios. It shows that the virtual sensor combined with the control algorithm has high potential to improve the accuracy of the ambient wet-bulb temperature, and guarantee the chiller plant performance by the proposed control strategies.

Evaluation of the effectiveness-NTU method for countercurrent humidifier

An analytical method is advantageous in designing and optimizing the humidifier of the humid air turbine cycle. In this paper, the energy effectiveness-NTU model for the countercurrent humidifier is built and verified by the 141 groups of test data obtained from an experimental facility of pressurized packing humidifier. The effect of pressure and inlet conditions including water/air mass flow rate ratio, inlet water temperature and inlet air wet bulb temperature on the heat capacity rate ratio and the energy effectiveness is analyzed. A sensitivity analysis is performed in order to indicate the key inlet parameter to improve the energy effectiveness of countercurrent humidifier. It is shown that the maximum deviation of the energy effectiveness calculated by the analytical method is 22.7% comparing to the experimental data. However, the maximum deviation is not more than 16% when considering the actual operating conditions for the humidifier of heat capacity rate ratio being greater than1.5. Heat capacity rate ratio changes monotonically with pressure and inlet parameters, and the energy effectiveness has an inflection point of minimum value when heat capacity rate ratio is equal to 1. Sensitivity analysis shows that water/air mass flow rate ratio is the most critical factor.

Applicability of Universal Thermal Climate Index (UTCI) in occupational heat stress assessment: a case study in brick industries.

The present study aimed to investigate the applicability of Universal Thermal Climate Index (UTCI) as an innovative and science-based index in public health researches, in occupational heat stress assessment. All indoor and outdoor workers (200 people) of Brick industries of Shahroud, Iran participated in the research. First, the environmental variables such as air temperature, wet-bulb temperature, globe temperature, air velocity and relative humidity were measured; then UTCI and WBGT (wet-bulb globe temperature) indices were calculated. Simultaneously, physiological parameters including systolic and diastolic blood pressure, oral temperature, skin temperature, tympanic temperature and heart rate of workers were measured. UTCI and WBGT indices were 34.2 ± 2°C, 21.8 ± 1.8°C in the outdoor environments and 38.1 ± 4.4°C, 24.7 ± 3.3°C at the indoor environments, respectively. There were the weak inverse relationships between UTCI and WBGT indices at the outdoor environments and physiological responses such as systolic blood pressure, and diastolic blood pressure. However, there were no similar results for indoor environments. The significant relationships were found between UTCI and WBGT at both indoor and outdoor environments. Both UTCI and WBGT indices are suitable for assessing the occupational heat stress. Although, UTCI index seems more appropriate for heat stress assessment in the environments with low humidity and air velocity.

Extremum seeking control for efficient operation of hybrid ground source heat pump system

The Hybrid Ground Source Heat Pump (GSHP) systems combine the renewable geothermal energy and cooling tower for rejecting the cooling load, which is often adopted for high cooling demand. Model based control can be limited due to variations in ambient conditions, ground-loop heat exchanger (GHE) and equipment characteristics, cost and reliability of sensors. A self-optimizing control scheme is proposed for efficient operation of the hybrid GSHP based on Extremum Seeking Control (ESC), with feedback of the total power consumption and the control inputs of the relative flow rate of cooling tower and the water pump speed. The cooling capacity of the heat pump regulates the evaporator leaving water at 7 °C. A Modelica based dynamic simulation model is developed for a Hybrid GSHP system, with the vertical GHE model adopted from Modelica Buildings Library. The transient heat transfer is implemented with a finite volume method inside and outside the borehole. The proposed ESC scheme is evaluated under the scenarios of fixed cooling load, ramp change in the evaporator inlet water temperature, diurnal sinusoidal cycle of air wet-bulb temperature, and realistic ambient and cooling load condition. Simulation results show the proposed ESC strategy effectively achieves nearly optimal efficiency without the need for plant model.

외기온도 변화특성을 고려한 중앙냉방시스템의 에너지 절감 최적제어에 관한 연구

Abstract In this study, the optimal control method for minimizing of energy consumption for central cooling systemwith proper occupant comfort level is researched by simulation. The optimal control method is that the optimal settemperatures such as the condenser water temperature, supply air temperature, and chilled water temperature with environment variable change such as outdoor air dry-bulb and wet-bulb temperatures are obtained by suggested optimal control algorithm with maximum and part building load. The TRNSYS program is used for system modelingand the control performances with the suggested optimal control method are compared with the existing control method of fixed set points. The suggested optimal control method shows better responses in energy consumption in comparison with existing control ones. Key Words : Central Cooling System, Control variables, Environment variables, Optimal Control, Optimal Set Temperatures * Corresponding Author : Byung-Cheon Ahn(Gachon Univ.) Tel: +82-31-750-5309 email: bcahn@gachon.ac.kr Received April 29, 2015 Accepted June 11, 2015Revised (1st May 20, 2015, 2nd June 2, 2015)Published June 30, 2015

Performance analysis of proposed hybrid air conditioning and humidification–dehumidification systems for energy saving and water production in hot and dry climatic regions

Performance of integrative air-conditioning (A/C) and humidification–dehumidification desalination systems proposed for hot and dry climatic regions is theoretically investigated. The proposed systems aim to energy saving and systems utilization in fresh water production. Four systems with evaporative cooler and heat recovery units located at different locations are proposed, analyzed and evaluated at different operating parameters (fresh air ratio, supply air temperature and outside air wet bulb temperature). Other two basic systems are used as reference systems in proposed systems assessment. Fresh water production rate, A/C cooling capacity, A/C electrical power consumption, saving in power consumptions and total cost saving (TCS) parameters are used for systems evaluations and comparisons. The results show that (i) the fresh water production rates of the proposed systems increase with increasing fresh air ratio, supply air temperature and outdoor wet bulb temperature, (ii) powers saving of the proposed systems increase with increasing fresh air ratio and supply air temperature and decreasing of the outdoor air wet bulb temperature, (iii) locating the evaporative cooling after the fresh air mixing remarkably increases water production rate, and (vi) incorporating heat recovery in the air conditioning systems with evaporative cooling may adversely affect both of the water production rate and the total cost saving of the system. Comparison study has been presented to identify systems configurations that have the highest fresh water production rate, highest power saving and highest total cost saving. Numerical correlations for fresh water production rate and total system energy consumption are developed and presented in terms of the controlling parameters.