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Abstract
Water scarcity is a serious problem addressing a major challenge that faces the entire Middle East Region. So, water desalination presents a potential solution, even it only addresses a portion of the water needed to resolve the problem of water scarcity.
One of the most important industrial application of the flashing process is the sea water desalination for fresh water production. Flashing phenomena can happen with a superheat liquid pool or in a superheated liquid jet. This phenomenon is very important in all thermal desalination plants and in particular the Multi-Stage flash (MSF) plants.
In this study, an experimental work has been carried out to investigate the phenomenon of the spray flash evaporation that happen in a low-pressure flash chamber. The study uses swirl nozzles of diameter around 0.5 mm, short length, and a ”down-ward” flow jet using a low injected pressure in the range of 1-6 bar. The nozzle type that used through the work gives a hollow cone spray with very fine water droplets.
The present experimental work analyzed the flashing jet process, the flashing efficiency and the amount of the flashed vapor. The influence of the following factors on these process parameters has been investigated:
The degree of superheating (ATsup.).
The inlet feed temperature (Tin).
The inlet feed flow rate (Mf).
The effect of the distance that the droplet travelling through it in the flash chamber.
The present work has been carried out using both the fresh water and saline water. In addition, the work was carried out in a range of inlet feed temperatures between ”40•C - 70•C” in order to match the trend in the new desalination plant design to work at low temperatures, with different feed flow rates, the vacuum ranges from 0.25to 0.06 abs. bar, and a degree of superheat ranging from 2 to IS•C.
The results of the experimental study have shown the following:
1. The degree of superheating is the main parameter affects both the flashing efficiency and the amount of flashed vapor. Although the increase in the degree of super heat is followed by an increase in the flash efficiency, through this study itis found that, increasing the degree of superheat from 3 to 7°C lead to an increase in the flashing efficiency (11) 10%, while from 7 to 11°C lead to an increase in the flash efficiency by 5%, from 11 to 16°C, lead to an increase in the flash efficiency by 2%, and from 17 to 20”C shows a neglegible influence in the flashing efficiency. Increasing the degree of superheat increases also the amount of the flashed vapor as /). T sup’ =Tin - T sat.’ So to increase the degree of superheat, we
should decrease the saturation temperature.
2. Increasing the inlet feed temperature increases the flashing efficiency and the rate of the flashed vapor. Increasing the inlet feed temperature from 60 to 65°C, leads to an increase in the flash efficiency by about 7% while a slight increase in the flashing efficiency is shown if the inlet feed temperature increases from 65 to 69°C. Low temperature operation between 40 - 70°C, has the advantages of i-avoiding using chemicals and, ii- reduced the heating energy level.
3. The variation of the water level inside the flash chamber and the inlet feed flow rate show a negligible effect on both the flashing efficiency and the amount of the flashed vapor.
Based on the experimental results a correlation has been developed to calculate the flashing efficiency as a function of the inlet feed temperature and the degree of superheat. In order to verify the adequacy of the developed correlation, it is compared with that developed by Miyatake et al.[9], and show close agreement.