Air Precooling Research Articles

A study on multi-nozzle arrangement for spray cooling system in natural draft dry cooling tower

Natural draft dry cooling tower (NDDCT) technology is especially attractive to power plants built in arid regions with limited water resource. However, high ambient temperature in summer deteriorates the performance of built NDDCT. To address this problem, evaporative pre-cooling technology has been developed by using nozzles to disintegrate water into fine droplets to achieve quick evaporation. The pre-cooled air flowing through radiator, has an enhanced heat exchange with the hot working fluid in the tube side. This paper reports a spray cooling system for the experimental tower built in UQ by combining several nozzle LNN1.5 to cool the inlet air and consequently improve the cooling efficiency of the NDDCT. To minimize water usage, a careful arrangement of spray nozzles should be investigated to achieve the maximum cooling outcome. With five nozzles installment, the inlet air is cooled by 6.3°C, corresponding to 51.2% cooling efficiency. A dimensionless analysis is presented to correlate cooling efficiency with influencing factors. The advantage of this pre-cooling system lies in the efficient water usage: more than 96% of the injected water extracts substantial heat from hot air and evaporates into vapor, leading to a pre-cooled airflow.

Performance enhancement of a vapor compression system by condensate

Air-conditioning systems consume large amount of energy and generate considerable amount of condensate when operated in hot and humid climates. This study considers utilizing the generated condensate to pre-cool the air entering evaporators to improve systems performance. Experimental and analytical investigations are carried out by using an air-conditioning system of 4.75 kW (16207 BTU/h) cooling capacity incorporated with an air pre-cooler and a condensate tank that acts as a thermal storage. During the pre-cooling-on period, the air inlet temperature of the evaporator is decreased by 5.7°C (10.3°F) and the compressor power consumption is reduced by 5.1% because of the decrease in discharge pressure. The coefficient of performance increases by 30.7% and second-law efficiency by 24.2%. The air pre-cooling is practical for 4–5 hours daily during the daytime if a 450–1000 kg of condensate cold thermal storage is employed. The deviation is 6.7% between the analytical and experimental results.

Forced-air precooling treatment enhanced antioxidant capacities of apricots

Effects of forced-air precooling on fruit quality and antioxidant system of apricot were evaluated in this study. Apricots were forced-air precooled for 24 hr at 0°C, and then stored at 0°C for up to 20 days. Results showed that precooling treatment slowed down a decrease of titratable acidity (TA) and total soluble solids (TSS) during storage period, whereas it had no effect on pH and L* value. On the 15th day of storage, associated with higher flavonoids content, precooled fruit had 73.6% higher DPPH and 183% higher ABTS radical-scavenging activities than the control. In addition, the peak of ascorbate peroxidase (APX) was earlier, and H2O2 content was reduced by precooling treatment with respect to control. These results indicate that precooling treatment could maintain the quality of apricot by enhancing antioxidant activity. Practical applications Fruit with high antioxidants implies high nutritional value. In this study, we found forced air precooling induced an increase of antioxidant activity of apricots. Precooling treatment should be used to enhance the quality and nutritional value of apricots.

Correlation analysis of transient heat transfer characteristics for air precooling aggregate

In dam works, air precooling of aggregate is a common and effective method to avoid temperature cracks in concrete structure. In order to offer a reliable design theory for the air precooling process to avoid unreasonable energy consumption, the transient heat transfer characteristics of the aggregate are intensively analyzed. The combined structure of the aggregate and the interstitial space in the hopper is treated as a porous structure, and the space-average method is used to simulate the transient heat transfer process. Simulation results show that size of the hopper and the average air velocity in the cross section have great influence on the transient heat transfer process of the aggregate, while the porosity in the range of 0.4‒0.5 has little influence. Nomograms are abstracted from simulation results, and then correlations of the compared excess temperature are precisely fitted to predict the air precooling transient heat transfer process of the aggregate.

КОМП’ЮТЕРНЕ МОДЕЛЮВАННЯ ТЕПЛОВИКОРИСТОВУЮЧОЇ СИСТЕМИ ОХОЛОДЖЕННЯ ПОВІТРЯ НА ВХОДІ ГАЗОТУРБІННОЇ УСТАНОВКИ З ОТРИМАННЯМ КОНДЕНСАТУ

The results of the computer simulation of waste heat recovery gas turbine unit intake air cooling system by transformation of the exhaust gas heat with producing the condensate as a subproduct have been analized. With this a two-stage air cooler of combined type with water stage of air precooling and refrigerant stage of air deep cooling were used. The ways and system schemes of separate – according to temperature – withdrawing of condensate during air cooling process, its accumulation and using as a coolant for the stage of air precooling.

Experimental studies on improvement of coefficient of performance of window air conditioning unit

This paper presents the performance analysis of a window air conditioner unit incorporated with wick less loop heat pipes (WLHP). The WLHP are located on the evaporator side of the air conditioning unit. The working medium for the WLHP is R134a refrigerant gas, an alternate refrigerant. The supply and return humidity of room air, the heat removal rat, and the coefficient of performance of the unit are analyzed for various ambient and room temperatures before and after incorporation of WLHP. The performance curves are drawn by comparing the power consumption and humidity collection rates for various room and ambient temperatures. The results show that coefficient of performance of the unit is improved by 18% to 20% after incorporation of WLHP due to pre-cooling of return air by WLHP, which reduces the thermal load on compressor. Similarly, the energy consumption is reduced by 20% to 25% due to higher thermostat setting and the humidity collection is improved by 35% due to pre-cooling effect of WLHP. The results are tabulated and conclusion drawn is presented based on the performance.

Energy Impact of Air Pre-cooling on Screw Air Compressor

Air compressors are widely used in a variety of industries. The energy consumed by air compressors comprises a significant portion of the overall energy consumption in industrial plants. Both theoretical and experimental studies were conducted to investigate the impacts of air pre-cooling on the energy consumption of air compressor. Experiments were performed on a 11 kW screw air compressor. The paper presents the details of the modelling as well as the measured results. The results indicate that the energy consumption decreases by 0.65% for every 1℃ drop in suction temperature. The energy consumption decreases 0.243% for every 1% drop in suction relative humidity. Significant compressor energy consumption can be reduced by pre-treating or pre-cooling suction air.

Selection of wetted media for pre-cooling of air entering natural draft dry cooling towers

The performance of natural draft dry cooling towers (NDDCTs) can be improved by wetted-medium evaporative pre-cooling. However, the pre-cooling benefit is season dependent and is significantly affected by wetted media. To further investigate the effect of wetted medium type on pre-cooling performance, the current study simulates a pre-cooled NDDCT using two typical types of wetted media, i.e., film (Cellulose7060 and PVC1200) and trickle (Trickle125 and Trickle100). A MATLAB program was developed and was validated against published data. The model was then used to carry out case study of a 120m-high NDDCT pre-cooled using the selected wetted media with fixed thickness of 300mm. Both the medium performance and pre-cooled NDDCT performance were compared to recommend the characteristics of wetted media promising for evaporative pre-cooling. Simulation finds that the media with high or low cooling efficiencies and pressure drops are not promising for the studied pre-cooling case while those media with middle cooling efficiencies and pressure drops intend to produce much performance enhancement and Cellulose7060 is a representative of such kind of medium. For the 120m-high NDDCT, the wetted-medium pre-cooling can raise the heat rejection rate of the tower without pre-cooling from 45MW to 92MW, 76MW, 68MW and 65MW by Cellulose7060, PVC1200, Trickle125 and Trickle100, respectively at ambient temperature of 50°C and humidity of 20%.

Economic analysis of air-cooled chiller with advanced heat rejection

This study analyses economic benefits of an air-cooled chiller retrofitted with advanced heat rejection features. The rated capacity was 282 kW and operating variables were monitored comprehensively to develop random forest models. The top five variables to ensure model accuracy were identified. Electricity savings were predicted by the models with 3 advanced operating modes: variable speed control for condenser fans with an adjustable condensing temperature (the VSD mode), mist precooling of condenser air (the Mist mode), and their composites (the VSD + Mist mode). The integrated part load value (IPLV) was 2.90, 3.31, 2.93 and 3.34 for the normal mode, VSD mode, Mist mode, and the VSD + Mist mode, respectively. For a chiller system serving an office building, the VSD mode gave the highest economic benefit with a simple payback of 10.83 yrs and an internal rate of return of 4.38% over a 15-yr life cycle.

Experimental investigation of a vapor compression system with condenser air pre-cooling by condensate

Air conditioning systems contribute to the largest share of energy consumption in building sector. On the other hand, the systems produce reasonable amount of condensate, especially when operating in humid climates. The aim of this study is to minimize the energy consumption and improve the performance of air conditioning systems utilizing condensate. Experimental investigation has been carried out to improve the performance of an air-cooled vapor compression system by pre-cooling air entering the condenser using condensate. A pre-cooler is incorporated on a 1.5ton-cooling capacity split-type air conditioning system to lower the air temperature entering the condenser sensibly. Performances of the air conditioning system with and without air pre-cooling are compared and reported in this paper. The results show that pre-cooling the air by about 4°C before entering the condenser lowers the compressor discharge pressure. The decrease in the discharge pressure resulted in the decrease in compressor power consumption by 6.1% and the cooling effect of the system is enhanced. The combined effect of the increase in the cooling effect and decrease in compressor power resulted in an increase in the coefficient of performance (COP) and second law efficiency of the system by about 21.4 and 20.5%, respectively.

Performance assessment and transient optimization of air precooling in multi-stage solid desiccant air conditioning systems

Renewable energy is one of the most promising solutions to both energy and global warming crisis. Energy consumption can be minimized considerably by utilizing solar energy in air conditioning systems operation. One of the popular solar air conditioning technologies is desiccant air conditioning. Nonetheless, conventional desiccant air conditioning systems have a relatively low coefficient of performance (COP). In consequence, two-stage desiccant air-conditioning systems are proposed to improve desiccant air conditioning systems’ COP. Moreover, a recently commercialized cooling method named Maisotsenko cooling cycle which is capable of cooling air near to its dew point temperature is considered to be integrated within the proposed multi-stage desiccant cooling systems.In this paper, three new two-stage desiccant air conditioning systems incorporating Maisotsenko cooling cycle are proposed and investigated in details for hot and humid climates such as UAE. Furthermore, air precooling is considered to improve two stage desiccant air conditioning systems’ COP. Moreover, full transient analysis and optimization are carried out in UAE within June–October. The proposed system can minimize the required solar heating during noon time as the ambient air dry bulb temperature rises. Average COP of the system during electricity load peak hours (10:00–14:00) for all five considered and combined months is 1.77. Average rate of heat input required to operate the system and average building cooling load are determined to be 100.3kW and 46.2kW, respectively. Therefore, system average COP is computed to be 0.46.

Parametric study on spray cooling system for optimising nozzle design with pre-cooling application in natural draft dry cooling towers

Pre-cooling of inlet air with water spray is proposed for performance enhancement of natural draft dry cooling towers (NDDCTs) during high ambient temperature periods. Previous experiments showed promising results on cooling enhancement using spray cooling. Spray nozzles characteristics are an essential element for designing spray cooling systems. A parametric analysis is conducted on the effect of different spray characteristics parameters and various hollow-cone droplet size distribution patterns on a spray cooling system performance for nozzle design optimisation. A validated Eulerian–Lagrangian 3-D model with a novel nozzle representation method for hollow-cone nozzles was used to perform the analysis. The results can help prepare design principles of effective spray nozzles for spray cooling systems. The results showed a clear trend of increasing spray cooling efficiency as droplet velocity increases due to better spray dispersion. 15% improvement in cooling performance was achieved by increasing droplet velocity from 20 to 80 m/s. A spray nozzle with high injection velocity, small droplet size distribution, large cone angle and a droplet size pattern of mean droplet size increasing towards the spray periphery was found to be the best for sprays with Dv90 ≤ 50 μm. Complete evaporation was successively achieved within 5 m distance using a spray with droplet size distribution of Dv90 = 20 μm and droplet velocity of 120 m/s at an air velocity of 1 m/s.

Numerical simulation of water spray in natural draft dry cooling towers with a new nozzle representation approach

Pre-cooling of inlet air with water spray is proposed for performance enhancement of natural draft dry cooling towers (NDDCTs) during high ambient temperature periods. Previous experiments showed promising results on cooling enhancement using spray cooling. In this study, a numerical Eulerian–Lagrangian 3-D model is used to simulate evaporating water sprays produced by real nozzles. A new adaptable method of hollow-cone spray representation in Eulerian–Lagrangian numerical models was developed to reproduce the real nozzle behaviour using experimentally measured initial spray characteristics and taking into account radial evolution of droplet size distribution and air/droplets momentum exchange. Experimental measurements from a wind tunnel test rig simulating NDDCTs inlet flow conditions have been performed for validation. Overall, a good agreement was obtained between numerical predictions and experimental measurements for the streamwise development of droplet size and velocity, and outlet air dry bulb temperature. An average deviation below 5.3% was achieved for all compared parameters. Moreover, the validated CFD model has provided insight into the experimental observations of local droplet velocity increase and higher air cooling in the lower region.

Water spray for pre-cooling of inlet air for Natural Draft Dry Cooling Towers – Experimental study

This paper deals with an experimental investigation of inlet air pre-cooling with water sprays aimed to enhance the performance of Natural Draft Dry Cooling Towers during high ambient temperature periods. An open-circuit wind tunnel with a test section of 1 × 1 m2 cross section and length of 5.2 m was employed to represent an inlet flow area section in a Natural Draft Cooling Tower. Experimental measurements of droplet evaporation and air cooling are presented. Nine high pressure, hollow cone nozzles were tested at various droplet sizes, air velocities (1, 2, 3 m/s), and injection rates under different ambient conditions. The water spray was characterized using a Phase Doppler Particle Analyser (PDPA). The effects of drop size distribution and air velocity on droplet evaporation, cooling effectiveness, and coverage area were investigated. The data shows clear trends of cooling enhancement with low air velocity or small droplet size distribution. It was found that the spray cooling efficiency, to a large extent, is dependent on spray coverage area. The experimental findings will benefit optimizing spray cooling performance in Natural Draft Dry Cooling Towers and nozzle arrangement.

Comparative study on the energy performance of chiller system in an institutional building with stochastic frontier analysis

This study demonstrates how to compare the energy performance of an existing air-cooled chiller with and without mist precooling of condenser air, considering different operating conditions between the two situations. A comprehensive set of operating data was gathered for each situation for over 4 months. Using stochastic frontier analysis, technical efficiency (TE) for the coefficient of performance (COP)—output—was evaluated for each set of operating conditions—inputs—involving weather and controlled operating variables. TE had an average of 0.64 and a standard deviation of 0.14 under mist precooling, while the corresponding values were 0.56 and 0.18 in the absence of mist precooling. A positive correlation was identified between the COP and TE. Based on the frequency distribution of TE, mist precooling enabled the chiller to attain higher performance more frequently under standardized operating conditions. Pearson correlations among the variables ascertain that TE has the highest correlation with condensing temperature, followed by the temperature of supply chilled water, so their precise control would give performance enhancement. TE can provide a basis of benchmarking chiller performance at different operating conditions and help explain the extent of improvement brought from the weather responsive control.

The Optimum Heat Transfer Area of the Air Precooler in a Two-Stage Evaporative Cooler

The main objective of this study is to experimentally evaluate the effect of the air precooler heat transfer area on the effectiveness of a two-stage evaporative air cooler. To the best of our knowledge, this effect has not been studied before. In the experimental unit used, there were three identical heat exchangers to precool the ambient air prior to its flow into the direct evaporative cooling unit. These heat exchangers can work individually or be combined to control the total heat transfer area of the air precooler. In addition, the effectiveness of the two-stage system was measured as a function of the ratio between the mass flow rate of water flowing over the packing and the mass flow rate of the dry air passing through the packing; the effectiveness also depends on the mass flow rate of water flowing into the precoolers. The data obtained showed that there is an optimum value for the air precooler heat transfer area at which the thermal performance of the two-stage evaporative cooler has the maximum value. Moreover, the system effectiveness improved with the rate of flow of water into the first and the second precoolers. Note that the optimum value of the precooler heat transfer area should be determined accurately for each set of design and operating conditions.

Experimental study of the application of two trickle media for inlet air pre-cooling of natural draft dry cooling towers

This paper is part two of a broader investigation into pre-cooling the air that enters natural draft dry cooling towers. Evaporative cooling of air is to some extent different from evaporative cooling of water. Two trickle media (Trickle125 and Trickle100) originally designed for evaporative cooling of water were studied in an open-circuit wind tunnel for evaporative cooling of air. Three medium thicknesses (200, 300 and 450mm) and two water flow rates (10 and 5l/minperm2 horizontally exposed surface area) were used in the tests. The air velocities ranged from 0.5 to 3.0m/s. The cooling efficiency and the pressure drop of the two media were curve fitted to yield a set of correlations. The pressure drop ranges for Trickle125 and Trickle100 were 0.7–50Pa and 0.6–41.6Pa, respectively. The cooling efficiencies of Trickle125 and Trickle100 fell within 15.7–55.1% and 11–44.4%, respectively. Generally, media with large effective surfaces provide high cooling efficiencies and high pressure drops; there is a trade-off between cooling efficiency and pressure drop when selecting a particular medium for a specific application. The water entrainment off the media was detected with water-sensitive papers, and both media had severe water entrainment at large air velocities.

Experimental study of film media used for evaporative pre-cooling of air

An open-circuit low-speed wind tunnel was used to study the performance of evaporative cooling with cellulose and Polyvinyl Chloride (PVC) corrugated media. These two film media were selected as part of a broader investigation on pre-cooling the entering air of natural draft dry cooling towers. The heat and mass transfer and pressure drop across the two media with three thicknesses (i.e., 100, 200 and 300mm) were experimentally studied in the wind tunnel. The test data were non-dimensionalized and curve fitted to yield a set of correlations. It was found that the pressure drop range of the cellulose media is 1.5–101.7Pa while the pressure drops of the PVC media are much lower with the range of 0.9–49.2Pa, depending on the medium thickness, air velocity and water flow rate. The cooling efficiencies of the cellulose media vary from 43% to 90% while the cooling efficiencies of the PVC media are 8% to 65%, depending on the medium thickness and air velocity. The water entrainment off the media was detected by water sensitive papers, and found that the cellulose media have negligible water entrainment under the studied conditions while care must be taken in the use of PVC media as water entrainment happens even at relatively low air velocities.

Fresh Air Pre-cooling and Energy Recovery by Using Indirect Evaporative Cooling in Hot and Humid Region – A Case Study in Hong Kong

Evaporative cooling (EC) is an energy efficient and environmental friendly technology which uses water evaporation to absorb the heat from surroundings and produce cooling air. EC has limited application cases in hot and humid regions because of the relatively low cooling capacity. In recent years, the study on hybrid EC and traditional mechanical cooling becomes the trend to break the region limitation of the technology. This paper carries out a case study for applying the Regenerative Indirect Evaporative Cooling System (RIECS) to an all-fresh-air hybrid air-conditioning system of a wet market in Hong Kong. The condense water from an air-handling unit (AHU) is collected and used as cooling medium sprayed onto the heat exchanger of an IEC equipment. At the same time, the contaminative and exhausted air from the wet market returns to the IEC equipment to cool the intake fresh air, so that the fresh air pre-cooling before it enters to the AHUs is realized. The energy saving and economic efficiency were analyzed and compared among RIECS, rotating heat recovery wheel system and the A/C system without heat recovery. The results show that RIEC system has the largest annual energy saving potential with a payback period of 2.9 years, which shows a great potential of its application in Hong Kong.

Use of Indirect Evaporative Cooling as Pre-cooling Unit in Humid Tropical Climate: An Energy Saving Technique

Evaporative cooling system is an environmentally-friendly and energy saving technology for air conditioning. However, in hot and humid climates, it is rather inefficient due to the high wet bulb temperature of the ambient air. In order to take advantage of indirect evaporative cooling (IEC) system in humid tropical climates, we present a hybrid system that combines IEC system and vapor compression system. In the IEC unit, the exhaust air from the conditioned room is used as the working air, and outdoor fresh air is used as the product air so that the IEC unit produces pre-cooled air for vapor compression system. In this study, two types of IEC units, namely, a conventional counter flow IEC unit and a novel counter flow IEC unit based on M-cycle, have been numerically analyzed. Results have indicated that the humid outdoor fresh air can be pre-cooled to a temperature below its dew point temperature when the wet bulb temperature of the exhaust air is lower than the dew point temperature of the outdoor air. In addition, the use of IEC reduces the cooling load of the vapor compression system, as a result, the hybrid system has a large potential to reduce electricity demand and energy consumption.