Evaporative Condensers
The Design of Countercurrent Evaporative Condensers with the Hybrid Method
Abstract
The heat and mass transfer in evaporative condensers are complex to model analytically and numerical simulations, when applied to multi-phase fluid dynamics in complex paths, often involve too high computational costs. Experimental campaigns at full scale of different heat transfer geometries and tube arrangements involve long lead times and high costs as well. The aim of the present work is to overcome the present limitations and to apply a new method to evaluate the overall performance of the countercurrent evaporative condensers, starting from the experimental, numerical or analytical data with a small scale approach. A test bench has been purposely designed and built up in order to reach and keep constant all the parameters determining the evaporative condenser heat transfer performance. In previous experimental contributions available in the literature, the air conditions were not controlled: here, an air handling unit placed before the evaporative condenser inlet allows to set up temperature and relative humidity of air in large ranges. An extended experimental campaign has been carried out to get affordable data to be used to find a relationship correlating the dry bulb temperature and relative humidity of air after its interaction with water and the condenser tubes surfaces, while all the parameters were set up and controlled. The regression function fits well the experimental data as the predicted values of temperature and relative humidity are characterized by a maximum percent deviation lower than 2.5% and 4% respectively. An iterative procedure was then implemented to determine the conditions of air going through the evaporative condenser in order to extend small scale results to full scale performance according to real geometries. The effect of the water flow rate on the cooling capacity was investigated and the results show that an increase of 50% of the sprayed water leads to an increase of 14% of the performance.
( Applied Thermal Engineering - Elsevier UK 2017 )
http://10.1016/j.applthermaleng.2017.11.076
Abstract
The heat and mass transfer in evaporative condensers are complex to model analytically and numerical simulations, when applied to multi-phase fluid dynamics in complex paths, often involve too high computational costs. Experimental campaigns at full scale of different heat transfer geometries and tube arrangements involve long lead times and high costs as well. The aim of the present work is to overcome the present limitations and to apply a new method to evaluate the overall performance of the countercurrent evaporative condensers, starting from the experimental, numerical or analytical data with a small scale approach. A test bench has been purposely designed and built up in order to reach and keep constant all the parameters determining the evaporative condenser heat transfer performance. In previous experimental contributions available in the literature, the air conditions were not controlled: here, an air handling unit placed before the evaporative condenser inlet allows to set up temperature and relative humidity of air in large ranges. An extended experimental campaign has been carried out to get affordable data to be used to find a relationship correlating the dry bulb temperature and relative humidity of air after its interaction with water and the condenser tubes surfaces, while all the parameters were set up and controlled. The regression function fits well the experimental data as the predicted values of temperature and relative humidity are characterized by a maximum percent deviation lower than 2.5% and 4% respectively. An iterative procedure was then implemented to determine the conditions of air going through the evaporative condenser in order to extend small scale results to full scale performance according to real geometries. The effect of the water flow rate on the cooling capacity was investigated and the results show that an increase of 50% of the sprayed water leads to an increase of 14% of the performance.
Numerical Investigations on two-phase flow modes in evaporative condensers
Abstract
Falling film evaporation over horizontal tubes consists of simultaneous heat and mass transfer processes: in an evaporative condenser it improves the heat rejection from the condensing refrigerant to the air. The liquid flow is generally influenced by viscous, gravity, tension effects, liquid mass flow rate, tube diameter and spacing and distance from the feeding system. In this work, a two-dimensional numerical model of the falling film evaporation on horizontal tubes is presented. The temporal change characteristics of the film flow process were studied and different types of flow (stable film and drops modes) were investigated, by varying the ratio between the water-to-air mass flow ratio. The effect of the tubes arrangement on the flow mode was analyzed too: an increase of 73 % of the longitudinal pitch corresponds to an increase of 66.7 % of the minimum water mass flow rate that prevents the film break-up. The trade-off curve for a given geometry was obtained: at a specific air mass flow rate, a transition zone between the stable film to the drops mode conditions occurs was individuated, with an uncertainty of 10% referring to a water mass flow rate variation of 10%.
( Applied Thermal Engineering - Elsevier UK 2015 )
http://dx.doi.org/10.1016/j.applthermaleng.2015.10.099,
Abstract
Falling film evaporation over horizontal tubes consists of simultaneous heat and mass transfer processes: in an evaporative condenser it improves the heat rejection from the condensing refrigerant to the air. The liquid flow is generally influenced by viscous, gravity, tension effects, liquid mass flow rate, tube diameter and spacing and distance from the feeding system. In this work, a two-dimensional numerical model of the falling film evaporation on horizontal tubes is presented. The temporal change characteristics of the film flow process were studied and different types of flow (stable film and drops modes) were investigated, by varying the ratio between the water-to-air mass flow ratio. The effect of the tubes arrangement on the flow mode was analyzed too: an increase of 73 % of the longitudinal pitch corresponds to an increase of 66.7 % of the minimum water mass flow rate that prevents the film break-up. The trade-off curve for a given geometry was obtained: at a specific air mass flow rate, a transition zone between the stable film to the drops mode conditions occurs was individuated, with an uncertainty of 10% referring to a water mass flow rate variation of 10%.
Numerical and Experimental Performance Analysis of Evaporative Condensers
( 71st Conference of the Italian Thermal Machines Engineering Association, ATI2016, 14-16 September 2016, Turin, Italy )
( Energy Procedia Elsevier UK 2016 )
http://dx.doi.org/10.1016/j.egypro.2016.11.004
Maria Fiorentino, Giuseppe Starace
Abstract
The heat rejection of industrial plants is often made with evaporative condensers as their choice meets energy efficiency requirements. In this work some numerical simulations of the evaporative condenser heat and mass transfer processes were carried out at the tube scale: 2D and 3D approaches were performed using the Ansys Fluent R.16.2 ( VOF model). The time resolved characteristics of the film flow process were studied and two different types of flow (stable film and drops mode ) were investigated, by varying the water-to-air mass flow ratio.
The decrease of the water-to-air mass flow ratio was found that led to the film break-up into droplets.
An experimental test rig was designed and built up for future validation works and to give designers new relations to quantify heat transfer performance depending on real working conditions..