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Flash evaporation in a superheated water liquid jet
Desalination 206 (2007) 311–321
Flash evaporation in a superheated water liquid jet Adel K. El-Fiqia*, N.H. Alia, H.T. El-Dessoukyb, H.S. Fathc, M.A. El-Hefnic a
Nuclear Research Center, Atomic Energy Authority, PO Box13759, Abo-Zabal, Inshas, Egypt email: [email protected] b NWFP — University of Engineering and Technology, Egypt c Alexandria University, Mechanical Engineering Department, Alexandria, Egypt
Received 29 December 2005; accepted 1 May 2006
Abstract The present paper gives an experimental coverage of the flashing process through a superheat liquid jet using tap water at low pressures. The spray nozzles of diameters <0.4 mm, and the working injection pressure is up to 6 bar, the experimental study was carried out with a degree of superheat ranging between 2 and 18 K, inlet feed temperatures from 40 to 70°C, and at different feed flow rates by measuring the inlet and outlet temperatures through the flash chamber, in vacuum. The amount of the flashed vapor can be determined by condensing it and comparing with the calculated one at different feed flow rates. The relation between the degree of superheat and the amount of the flashed vapor can be evaluated through this work. Also, through this experimental work, the flashing efficiency was measured and there was a good agreement with the one proposed by other authors. Keywords: Superheat; Flash vaporization; Flashing efficiency; Saturation temperature, Spray nozzles
1. Introduction Flashing phenomena take place when a liquid’s temperature exceeds a certain degree of superheat. In other words, flashing is the phenomenon observed when the surrounding liquid conditions suddenly change and become lower than its satu*Corresponding author.
rated conditions. At such a variation as a sudden pressure drop, the liquid, initially at equilibrium, becomes superheated, and the whole energy cannot be contained in the liquid as sensible heat, and the surplus heat is converted into latent heat of vaporization. The temperature of the liquid decreases quickly towards the equilibrium value, so the degree of superheat can be defined as ∆Tsup =
Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.05.017
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Tin – Tsat. The phenomena can be manifested in the chemical and process plants where liquid superheat is essential. Flashing is a process which gives rise to a vaporization flow rate more significant than that obtained during simple evaporation. It is very quick phenomena caused by abrupt pressure drop which transforms the initially subcooled liquid into superheated. The industrial applications of flashing are varied. One of the most important concerns the seawater desalination as in multistage flash evaporators. Other systems appear when they contain both energy stored and superheated liquid such as drying processes use this phenomenon, for example paper sheet drying and sterile loads drying in vapor sterilization cycles. Flash evaporation is also used in geothermal power plants to generate vapor which drives the turbines producing electricity. Also, in the ocean thermal energy conversion (OTEC), the power plants use the fact that water layers in oceans are at different temperatures to produce energy. Due to the sudden phase change, flash evaporation causes a sudden temperature drop of the liquid, it cools quickly cool and is used in cooling the hot parts of a shuttle by water spraying under low pressure conditions. Flashing phenomenon is used in some multi-component film deposition processes, where a liquid mixture is evaporated and then condenses again as a thin layer on the required surface [1]. Although the phenomena have positive applications in many engineering processes, they have equal disadvantages in some processes, such as nuclear reactor cooling system, where it is a potential source of nuclear accidents. The flash evaporation can happen at nuclear power plants breaking the core cooling system of a reactor. This is known as the loss of coolant accident (LOCA). In this case the high pressure in the cooling system drops suddenly to the surrounding atmospheric pressure. The superheated coolant explodes into vapor due to flash evaporation. A thorough understanding of the physics governing the phase change is useful in the design of boilers, heat exchangers, steam
generators, refrigeration, distillation desalination equipment, and the containment of liquefied gases. The phenomenon has received a considerable attention in the nuclear power industry where containment structures, and emergency cooling and safety systems and equipment must be designed to remedy reactor loss of coolant accidents (LOCA). Knowledge of the transient coolant heat and mass transfer rates could be used to facilitate the design as well as to verify the accuracy of various computational models currently used to predict the thermal and hydraulic response of a nuclear reactor to loss-of-coolant accidents. Flashing, on the other hand, is caused by large pressure drops, converting the initially sub-cooled liquid bulk into superheated liquid. The phenomenon is initially more violent at the surface and causes the liquid to acquire a very heterogeneous temperature composed of superheated, saturated, and sub-cooled liquid. The greater turbulence perpetuates the phenomenon, which results in higher rates of mass transfer, and eventually causes cooling throughout the liquid. Flashing may occur in the absence or presence of bulk nucleation. Bulk nucleation may also, originate from exceeding the spontaneous nucleation temperature which causes the bulk of the liquid to explode into the vapor state. The absence of nucleation sites and dissolved gases within the liquid causes flashing to occur only at the surface. After a period, the liquid bulk cools down and behaves in a similar manner as a liquid undergoing evaporation. The two processes may be represented by different boundary conditions imposed at the liquid– vapor interface. Evaporation is caused by local surface instability corresponding to what is believed to be a linear temperature change at the boundary. In contrast, flashing with a rapid change in the total pressure corresponds to a step change in surface temperature [2]. 2. Literature survey Miyatake et al. [3] discussed through their ex-
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perimental work on the spray flash evaporation the effects of the degree of superheating, the spray flow rate and nozzle diameter on the spray flash at 60°C. Through this study they concluded that the rate of flash evaporation of a liquid jet is extremely faster than that of flowing superheated liquid in conventional MSF evaporators and superheat pool water. As the degree of superheating ∆Tsup = Tin – Tsat increases, the behavior of θ (dimensionless temperature) shows a rapid decrease. It is found that when ∆Ts increases to some extent, the spray flash evaporation undergoes two exponential decaying processes after the elapse of time lag (to). An empirical equation suitable for predicting the variation of liquid is
(We / Re ) exp ( ∆T / 35) ≥ 24 1/ 8
s
and this equation is correlated well with the experimental results. The difference between the time at intersection of the two asymptotes (ti) and the time lag of initiation of flashing (to) is related only to the nozzle diameter (d). As Zo= U to, the value of (Zo/d) is related mainly to ∆Ts. Miyatake et al. [4] presented the effect of liquid temperature on the spray flash evaporation at temperatures of 40 and 80°C, From the experimental results, a more general empirical equation suitable for predicting the variation of liquid temperature in the center of jet with residence time was deduced. It was seen that, even at lower liquid temperatures, spray flash evaporation still had higher evaporation performance and faster evaporation rate than the flash evaporation occurring in other systems. Sebastain and Nadeau [5] studied the falling jet flash evaporator for vintage treatment theoretically and experimentally. A mono-stage falling jet flash evaporator was constructed and instrumented to follow the system parameters: pressures, temperatures, and flow rates. A steady-state model was built and validated by using the experimental results. The thermodynamic aspects of the prototype are discussed through the work. The
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parameters that are concerned in the study are pressure drop inside the separation chamber, condenser and vacuum system characteristics and the change in the generated sprays inside the chamber. Through the study, water is used as the reference fluid for simulations as its thermodynamic properties are known. As the numerical simulation code was verified well, it allowed them to extend their investigation to a bigger industrial unit. A new pilot unit using a two-stage evaporation technique was built and studied. Muthunayagam et al. [6] reported in their study a vapor diffusion model and an energy balance equation at the droplet surface to show the feasibility of the desalination of seawater at typical ocean surface temperature between 26 and 32°C by using a low-pressure vaporization process. They prepared an experimental apparatus to check the model validity and they found a good agreement between both the theoretical and experimental work. The range of the vacuum inside the test rig varied between 10 and 18 mm mercury during this work. The yield of fresh water was about 3–4% at the lower range of vacuum and the upper range of temperatures. The results matched well with the model prediction. Also, small values of water injection pressures, of about one bar, were found to be adequate when a swirl nozzle was used during the experiment. Kitamura et al. [7] made an experimental attempt to study the critical superheat for flashing of superheated liquid jets. This experimental work was carried out by an injection of water and ethanol through a long nozzle to a vacuum chamber. Through the work, the superheated spray had two different patterns: complete flashing and twophase vapor–liquid effluent. The complete flashing occurred when the temperature of the liquid jet was high enough above the liquid bubble point that corresponded to the pressure in the flash chamber. For the two-phase effluent, it happened when the temperature was close to the bubble point. The critical superheat above which the complete flashing occurred was correlated by an empi-
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rical equation on the basis of the bubble growth rate in the superheated liquid. Gemci et al. [8] experimentally studied cavitation and flash boiling atomization of water–acetone binary mixtures by using nitrogen as the propellant gas. A sharp edge orifice was used during this study to create cavitation bubbles. This research was used to study the spray breakup region and the influence of the nozzle internal conditions (length and diameter) on liquid atomization involving the flash boiling and cavitation mechanisms. Mean droplet diameters were measured as a function of the injection temperature, pressure and the flow rate ratio of the thermodynamic conditions upstream of the orifice induce internal bubble formation. A further improvement in the spray quality was achieved by increasing the propellant gas amount. This study of the binary acetone–water solution provides a model system for improving the atomization of hydrocarbon liquid through addition of a low boiling component at a reduced gas to oil ratio. Also, spray images were captured at the nozzle exit to visualize spray formation and breakup length. Peter et al. [9] studied the phenomenon of flashing and shattering of superheat liquid jet in the field of low pressure. They clarified that there are four physical characteristics of the jet in the area of low pressure and these are: non shattering liquid jet, partially shattering liquid jet, completely shattering jet and flare flashing liquid jet. They studied both the temperature distribution in the axial direction along the liquid jet and the radial temperature distribution of the liquid jet. Also, they studied the distribution of the droplet size and the contribution of bubble nucleation and growth to the liquid jet shattering in the area of low pressure. Brown and York [10] studied the sprays formed by flashing liquid jets by using rough orifices and sharp edge orifices. Liquids which are forced from a high pressure area to another one of low pressure often cross the equilibrium pressure for the liquid temperature and disintegrated into spray by partial
evolution of vapor. Their study of the sprays from water and feron-11 jets analyzed drop sizes, drop velocities and spray patterns. The mechanism which controls the process was analyzed and the data about the process were collected. A critical superheat was found, where the jet of liquid was shattered by rapid bubble growth. Also the bubble growth rate was controlled with the Weber number, where its critical value for low viscosity liquid was 12.5, the mean drop size also correlated with the Weber number and the degree of superheat. They also compared spray from orifices with other techniques and it was found to be comparable in all aspects except temperature. Due to the limited number of works, the present study will concern the parameters that influence the flashing jet and the ways to enhance the flash efficiency. In this study, many parameters will be taken into consideration: • Increase of the range of the operating temperatures and increase of the number of measurement points through this range. • The number of spray nozzles used through this work is four. • Comparison between the operating medium of the fresh water with most of the documented studies. • Measuring flashing efficiency and comparing it with the empirical formula proposed by Miyatake et al. [11]. The main objective of the present study is to enhance the flashing efficiency through a flashing chamber under low pressure by using small-diameter spray nozzles working at low injection pressures. The study also concerns the main parameters that affect the flashing efficiency such as feed flow rates, feed temperatures, vacuum, etc. So, through the experimental work that will be carried out under this study, the following will be investigated: • the effect of feed water flow rates, feed water temperatures, and different vacuum on the produced vapor and on the flashing efficiency;
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• the effect of the water level inside the flash chamber on the produced vapor, on the flashing efficiency; • the effect of the feed flow rates, the feed temperatures, and the vacuum on the product yield. 3. Experimental apparatus The experimental set-up is schematically shown in Fig. 1a. This set-up includes two loops, the main loop is used for injecting hot water in the flash chamber and it consists of many parts, the secondary loop is used for cooling water to condense in the flashed vapor inside a condenser. The entire test facility was constructed of iron tubes (0.5 inch), stainless steel and glass components. The test loop is designed to work in a flow range of 0–30 l/h, and a temperature range of 40–80°C. These conditions are realized in a closed loop, the hot water supply to the test section consists of a recirculating pump, water heaters, supply tank, y-filter, controlling valves (ball and global) and measuring flow meter. The steam is removed to a separate condensing loop. This loop
Fig. 1a. The main test section, the inlet feed line, the vapor outlet line to the condenser.
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consists of a Pyrex condenser (tube and shell), a recirculating pump to condensate the liberated steam, a collecting flask for the condensing steam, a vacuum system is connected at the condenser outlet for removing the non-condensable gases, needle valve for controlling the vacuum system, a sensitive balance for measuring the condensate vapor, and data acquisition system connected to the temperature sensors. Some parts of the experimental facility are shown in Figs. 1b. A y-filter with high efficiency is used during the process and replaced from time to time for cleaning. This filter is placed immediately upstream of the test section. There are 4 sprayers inside the test section. These sprayers could be modified to provide a variety of profiles but only one set is used during all experiments, the diameter of the sprayer nozzle is about 0.4 mm. The liberated steam is condensed in the tube and shell heat exchanger, and the condensed steam is measured by using the sensitive
Fig. 1b. Some parts of the experimental set-up: collecting flask, sensitive balance, vacuum pump, data acquisition system, and control needle valve.
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balance. The main hot water in the primary circuit is measured by a rotameter, its range is from 0 to 30 l/h. Also, the cooling water is measured by an orifice meter with a diameter of 6 mm. Temperatures, mass flow rate, pressures of both loops are measured and recorded at different points in the circuits. 3.1. Instrumentation The temperature is measured through the entire points of the set-up using thermocouples of type k (Cromel-Alumil) with a diameter 0.2546 mm and a range of –100–1372°C. The flow rate of the main loop is measured by a rotameter with an accuracy of ±3% of full scale, and the flow rate of the secondary loop is measured by the orifice meter with a diameter of 6 mm. The vacuum through the system is measured using a vacuum gauge, the data acquisition system (ChartScan/ 1400) of resolution equal to 0.1°C is controlled using a PC, and a sensitive balance for measuring the collected vapor of specification of a capacity of 2000 g, readability of 0.01 g, standard deviation of 0.01 g and the tolerance of ±2.5 mg. So the % errors in the measuring instrumentation in vacuum, feed flow rate, temperature and condensate vapor are 2%, 5.2%, 2%, 0.05%, respectively.
The energy balance considering all modes of heat transfer between the droplets and the surrounding environment is
+ h fg dM f / dt + Ad σ ε (Ta − Td )
(1)
where the terms in the right-hand side represent heat terms as a result of conduction/convection, phase change, and radiation respectively [3]. By neglecting the conduction/convection and radiation terms, the above equation will be M f Cp d ( Td ) / dt = h fg dM f / dt
M f CpTin − M f CpTout + M v CpTout −M v h = 0
(3)
By comparing both the feed flow rate and the flashed mass, the value of the flashed mass is too small compared with the feed flow rate. So Eq. (3) can be rewritten as M f cpTin − M f CpTout − M v h fg = 0 ∴ M v = M f Cp ∆Tsup / h fg
(4)
∴ M v = M f Cp ∆Tsup / h fg
(5)
4. Results and discussion
3.2. Determination of flash vapor
M f C p d (Td ) / dt = Ah (Td − Ts )
by using mass conservation, this equation leads to Eq. (4). In another way, for the steady-state condition, the evaporated mass flow rate is calculated from the energy balance on the injected water inside the flash chamber, by neglecting the thermal losses through the walls of the flash chamber. Thus the energy released by the sudden drop in the feed temperature is completely used in vaporizing a quantity of the injected feed water, by illustrating the energy balance on the chamber,
(2)
The parameters that affect on the spray flashing phenomena are summarized as follows. 4.1. Residence time The values of the pressure in the flash chamber allow the evaporation to take place till the temperature at the droplet surface reduces to the value of the saturation temperature corresponding to the flash chamber pressure. Vaporization will take place as long as the saturation vapor pressure at the droplet temperature exceeds the ambient vacuum pressure. The surface tension induced pressure at the droplet (∆p) is 2σ/r where σ is the surface tension coefficient and r is
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− uo ⎤ / g ⎥⎦
(6)
From this equation the residence time for an average initial velocity equal to 3 m/s is approximately 140 ms, also the drag forces on the droplet will further increase the residence time of the droplet.
20
60
16
inlet temp. sat. temp. M co nd.
58 56
8 54 4
52 50 0
360
720
1080
1440
1800
∆Tsup which is the driving force for the flash evaporation process is known as
Fig. 2. Feed temperature, saturation temperature, condensate vapor at Mf = 14.8 kg/h, Tin = 60°C, vacuum = 0.85 bar.
(7) 62
4.3. Flashing efficiency (η)
20
Temperature (°C)
60
The efficiency of the spray flash evaporation is known as the ratio between the steam generation rate to the maximum steam generation rate or it is defined as the ratio of the amount of actual evaporation to that of the theoretical evaporation. η = (Tin − Tout ) / ( Tin − Tsat )
0 2160
Time (s)
4.2. The superheat of the working liquid
∆Tsup = Tin − Tsat
12
Md (gm)
0.5
62
inlet temp. sat. temp. Mcond.
58
All the experiment measurements were taken at steady-state conditions, the data were recorded for each run after reaching the steady state for a period of time of 30 min, using a computer system connected to a data acquisition system. Figs. 2–4
12
56 8
54
4
52
(8)
16 Md (gm)
t = ⎡( uo2 + 2 gh ) ⎢⎣
indicate the effect of changing the depressurization inside the flash chamber from 0.85, 0.87, to 0.9 bar, at a feed flow rate of 14.8 kg/h, and inlet feed temperature of 60°C. It is clear that increasing the depressurization leads to an increase in the degree of superheat, which leads to enhancement of the shuttering of the injected flow through the jet of the nozzles. So there is an increase in the amount of flashed vapor inside the flash chamber, followed by an increase in the amount of the con-
Temperature (°C)
the droplet radius. So for a droplet of diameter ~0.5 mm this pressure is about 11 Pa which is much less than the lowest vacuum in our experiment, so the effect of the surface tension is neglected. It is necessary that the evaporation time is less than the residence time to ensure the complete evaporation. As the droplet is injected at an initial velocity Uo, so the actual residence time can be calculated using Newton’s equation for motion with the assumption that the droplet diameter is constant, and the temperature of the droplet just away from the nozzle exit is constant along the height h
317
50 0
360
720
1080 1440
1800
0 2160
Time (s)
Fig. 3. Feed temperature, saturation temperature, condensate vapor at Mf = 14.8 kg/h, Tin = 60°C, and vacuum = 0.87 bar.
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20
54
16
52
12
50
8
48
4
46 44
0 0
360
720
1080
1440
1800
Vapor product (g/30 min)
56
180
24
inlet temp. sat. temp. Mcond.
58
Md (gm)
Temperature (°C)
200
28
60
Tinlet=60 oC Tinlet=65 oC
160
Tinlet=69 oC 140 120 100 80 60 40
Time (s) 20
Fig. 4. Feed temperature, saturation temperature, condensate vapor at Mf = 14.8 kg/h, Tin = 60°C, vacuum = 0.90 bar.
50
100
150
200
250
300
Absolute vacuum (mbar)
Fig. 5. Relation between the absolute vacuum and the amount of flashed vapor at different temperatures.
80 Evaporated mass rate (g/30 min)
densate vapor. Remember that all measurements of the amount of the condensate vapor were taken under steady-state conditions, and each run was done in a period of time equal to 30 min. If we compare these measurements with the values of the flashed vapor gained from Eq. (4), we will find a good agreement. Fig. 5 shows the values of the flashed mass with the absolute vacuum inside the flash chamber and it is clear that the flashed mass is a decreasing function of the pressure. Fig. 6 shows the values of the flashed mass with the inlet feed temperature to the flash chamber, also the amount of the flashed mass is an increasing function of the inlet feed temperature. This will be summarized in the next figures as a relation between the flashed mass and the degree of superheat. Figs. 7 and 8 show that the flashed mass is proportional to the degree of superheat, although we used different feed flow rates and inlet temperatures. If this situation studied by doing the energy balance to the flash chamber, it was concluded that the amount of the flashed vapor is proportional to the degree of superheat as it can be seen from Eq. (4) which indicates that Mv α (∆Tsup.). The measurements of the evaporated mass obtained through this work have a standard deviation between 0.87 and 0.93. as the feed flow rate =
0
vac.=0.88
70
vac.=0.90
60
vac.=0.91
50 40 30 20 10 0 48
50
52 54 Temperature (°C)
56
58
Fig. 6. Feed temperature vs. the amount of the evaporated mass rate at different vacuum.
8 kg/h, Cp = 4.18 kJ/kg.k, hfg at 56°C = kj/kg, this will lead to the constant of proportional =, so in this case the deviation between the measured and calculated constant (Mf Cp / hfg) is about 15%. Figs. 9 and 10 indicate the relation between the degree of superheat and the flash efficiency when using the fresh water. These figures show a
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319
13
8
12 11
Evaporated mass rate (g/3 min)
Evaporated mass rate (g/3 min)
7 6 5 4 3 2
10 9 8 7 6 5 4 3
1
2
0
0
1 0
2
4
6
8
10
12
0
2
4
Degree of superheat (°C)
good agreement between the measured flash efficiency and the proposed equation for the flash efficiency by Miyatake et al. [11]. We proposed another equation from our work governing the relation between the flash efficiency and both the degree of superheat and the inlet feed temperature, and it has a good agreement with Miyatake’s equation. Fig. 11 indicates a comparison between the measured amount of the flashed vapor and the calculated one with some errors due to measurements. 5. Conclusions An extensive experimental work on the flashing process using fresh liquid was carried out. The work concerned the spray flash evaporation occurring in a superheated water jet injected through circular nozzles of a diameter less than 0.4 mm into a low-pressure vapor zone. This study was carried out on liquid water having an initial temperature ranging between 40 and 70°C, vacu-
8
10
12
Fig. 8. Evaporated mass rate vs. the degree of superheat at feed flow rate = 11.3 kg/h, inlet feed temperature ~68°C.
1
Flashing efficiency
Fig. 7. Evapoarated mass rate vs. the degree of superheat at feed flow rate = 8 kg/h, inlet feed temperature ~56°C.
6
Degree of superheat (°C)
0.9 0.8
Miyatake11
0.7
Exp. 0.6 0.5 0
1
2
3
4
5
6
7
8
9
10
11
12
Degree of superheat (°C)
Fig. 9. Flashing efficiency at feed flow rate = 8 kg/h, inlet temperature ~50°C, level = 46.5 cm.
um ranging between 60 and 250 mbar, feed flow rate ranging between 4 and 15 kg/h. The detailed data on the effect of the degree of superheating, the inlet feed flow rates, the inlet feed temperatures, vacuum, and flashing efficiency on the behavior and performance are presented. The results show that:
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(∆Tsup), both the flashing efficiency and the flashed vapor increases. The relation between the flashed vapor and the degree of superheat is a proportional relation, the factor of proportionality between the flashed vapor and the degree of superheat is obtained from the energy balance through the flash chamber, Mv = (Mf Cp ∆Tsup.) / hfg, and there is an agreement between the calculated and the measured values.
1
Flashing efficiency
0.9 0.8
M iyatake 11 0.7
Exp.
0.6 0.5 0
2
4
6
8
10
12
14
16
18
Symbols
Degree o f superheat (°C)
Fig. 10. Flashing efficiency at feed flow rate = 14.8 kg/h, inlet feed temperature = 69°C.
cal., vac.=0.88
Measured
cal.,vac.=0.9
measured
cal.,vac.=0.91
measured
cal.,vac.,=0.92
measured
Evaporated mass rate (g/3 min)
8 7 6 5 4 3 2 1 0 1
2
3
4
5
6
7
8
9
10
Time (3 min)
Fig. 11. Comparison between measured and calculated flashed vapor at different vacuum at Mf = 8 kg/h, Tin = 53°C.
• Increasing the degree of superheat leads to an increase in the flashing vapor to a certain amount. • Decreasing the water level inside the flash chamber, increases slightly both the flashing efficiency and the amount of the flashed vapor. • With the increase of the degree of superheat
Ad C Cp do g h
— — — — — —
hfg M m′ p Q′ r t T Tz
— — — — — — — — —
U
—
V, U — W — Z —
Droplet area, m2 Salt concentration, ppm Specific heat of liquid at p = c, J/kg.K Orifice diameter, m Gravity acceleration, m/s2 Traveling distance of the droplet, or brine level, m Latent heat, kJ/kg Flow rate, kg/h Specific weir load, kg/s.m Pressure, pa Volume flow rate, m3/h Radius of droplet, m Time, s Temperature, °C Liquid temperature at the center line of the jet, °C Mean velocity of the liquid in the nozzle, m/s Flow velocity, m/s Evaporation rate (mass flux), kg/s.m2 Vertical distance from the nozzle exit, m
Greek µ ρ θ η ω
— Dynamic viscosity, N.s/m2 — Liquid density, kg/m3 — Dimensionless temperature = (T* – Tsat) / (Tin – Tsat) — Flash efficiency = (Tin – Tout) / (Tin – Tsat) — Salinity, g/l
A.K. El-Fiqi et al. / Desalination 206 (2007) 311–321
σ — Surface tension, N/m ∆′ — Non-equilibrium allowance, °C ∆T — Temperature difference, °C Subscripts and superscripts a — b — d — eq — f — in — out, o sat — sup — v,s —
Atmosphere Boiling point Droplet Equilibrium Feed flow rate Inlet — Outlet Saturation Superheat Vapor
References [1] D. Saury, S. Harmand and M. Siroux, Flash evaporation from a water pool: Influence of the liquid height and the depressurization rate, Int. J. Therm. Sci., 44 (2005) 953–965. [2] R.J. Peterson, S.S. Grewal and M.M. El-Wakil, Investigations of liquid flashing and evaporation due to sudden depressurization, Int. J. Heat Mass Transfer, 27(2) (1984) 301–308. [3] O. Miyatake, T. Tomimura, Y. Ide and T. Fujii, An experimental study of spray flash evaporation, De-
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salination, 36 (1981) 113–128. [4] O. Miyatake, T. Tomimura, Y. Ide, M. Yuda, and T. Fujii, Effect of liquid temperature on spray flash evaporation, Desalination, 37 (1981) 351–366. [5] P. Sebastian and J.P. Nadeau, Experimental and modeling of falling jet flash evaporators for vintage treatment, Int. J. Therm. Sci., 41 (2002) 269–280. [6] A.E. Muthunayagam, K. Ramamurthi and J.R. Paden, Modelling and experiments on vaporization of saline water at low temperatures and reduced pressures, Appl. Therm. Eng., 25 (2005) 941–952. [7] Y. Kitamura, H. Morimitsu and T. Takahashi, Critical superheat for flashing of superheated liquid jets, Ind. Eng. Chem., Fundamentals, 25(2) (1986) 206– 211. [8] T. Gemci, K. Yakut and N. Chigier, Cavitation and Flash Boiling Atomization of Water/Acetone Binary Mixtures, Spray Systems Technology Center, Carnegie Mellon University, Pittsburgh. [9] E. Mhina Peter, T. Akira and H. Yujiro, Flashing and shattering phenomena of superheated liquid jets, JSME Int. J., Series B: Fluids Therm. Eng., 37(2) (1994) 313–321. [10] R. Brown and J.L. York, Sprays formed by flashing liquid jets, AIChE J., 8(2) (1962) 149–153. [11] O. Miyatake, Y. Koito, K. Tagawa and Y. Maruta, Transient characteristics and performance of a novel desalination system based on heat storage and spray flashing, Desalination, 137 (2001) 157–166. [12] H.W.M. Witlox, Flashing Liquid Jets and Two-Phase Dispersion, Contract Research report 403/2002, HSE, Health and Safety Executive, 2002.
Flash evaporation in a superheated water liquid jet Adel K. El-Fiqia*, N.H. Alia, H.T. El-Dessoukyb, H.S. Fathc, M.A. El-Hefnic a
Nuclear Research Center, Atomic Energy Authority, PO Box13759, Abo-Zabal, Inshas, Egypt email: [email protected] b NWFP — University of Engineering and Technology, Egypt c Alexandria University, Mechanical Engineering Department, Alexandria, Egypt
Received 29 December 2005; accepted 1 May 2006
Abstract The present paper gives an experimental coverage of the flashing process through a superheat liquid jet using tap water at low pressures. The spray nozzles of diameters <0.4 mm, and the working injection pressure is up to 6 bar, the experimental study was carried out with a degree of superheat ranging between 2 and 18 K, inlet feed temperatures from 40 to 70°C, and at different feed flow rates by measuring the inlet and outlet temperatures through the flash chamber, in vacuum. The amount of the flashed vapor can be determined by condensing it and comparing with the calculated one at different feed flow rates. The relation between the degree of superheat and the amount of the flashed vapor can be evaluated through this work. Also, through this experimental work, the flashing efficiency was measured and there was a good agreement with the one proposed by other authors. Keywords: Superheat; Flash vaporization; Flashing efficiency; Saturation temperature, Spray nozzles
1. Introduction Flashing phenomena take place when a liquid’s temperature exceeds a certain degree of superheat. In other words, flashing is the phenomenon observed when the surrounding liquid conditions suddenly change and become lower than its satu*Corresponding author.
rated conditions. At such a variation as a sudden pressure drop, the liquid, initially at equilibrium, becomes superheated, and the whole energy cannot be contained in the liquid as sensible heat, and the surplus heat is converted into latent heat of vaporization. The temperature of the liquid decreases quickly towards the equilibrium value, so the degree of superheat can be defined as ∆Tsup =
Presented at EuroMed 2006 conference on Desalination Strategies in South Mediterranean Countries: Cooperation between Mediterranean Countries of Europe and the Southern Rim of the Mediterranean. Sponsored by the European Desalination Society and the University of Montpellier II, Montpellier, France, 21–25 May 2006. 0011-9164/07/$– See front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.desal.2006.05.017
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Tin – Tsat. The phenomena can be manifested in the chemical and process plants where liquid superheat is essential. Flashing is a process which gives rise to a vaporization flow rate more significant than that obtained during simple evaporation. It is very quick phenomena caused by abrupt pressure drop which transforms the initially subcooled liquid into superheated. The industrial applications of flashing are varied. One of the most important concerns the seawater desalination as in multistage flash evaporators. Other systems appear when they contain both energy stored and superheated liquid such as drying processes use this phenomenon, for example paper sheet drying and sterile loads drying in vapor sterilization cycles. Flash evaporation is also used in geothermal power plants to generate vapor which drives the turbines producing electricity. Also, in the ocean thermal energy conversion (OTEC), the power plants use the fact that water layers in oceans are at different temperatures to produce energy. Due to the sudden phase change, flash evaporation causes a sudden temperature drop of the liquid, it cools quickly cool and is used in cooling the hot parts of a shuttle by water spraying under low pressure conditions. Flashing phenomenon is used in some multi-component film deposition processes, where a liquid mixture is evaporated and then condenses again as a thin layer on the required surface [1]. Although the phenomena have positive applications in many engineering processes, they have equal disadvantages in some processes, such as nuclear reactor cooling system, where it is a potential source of nuclear accidents. The flash evaporation can happen at nuclear power plants breaking the core cooling system of a reactor. This is known as the loss of coolant accident (LOCA). In this case the high pressure in the cooling system drops suddenly to the surrounding atmospheric pressure. The superheated coolant explodes into vapor due to flash evaporation. A thorough understanding of the physics governing the phase change is useful in the design of boilers, heat exchangers, steam
generators, refrigeration, distillation desalination equipment, and the containment of liquefied gases. The phenomenon has received a considerable attention in the nuclear power industry where containment structures, and emergency cooling and safety systems and equipment must be designed to remedy reactor loss of coolant accidents (LOCA). Knowledge of the transient coolant heat and mass transfer rates could be used to facilitate the design as well as to verify the accuracy of various computational models currently used to predict the thermal and hydraulic response of a nuclear reactor to loss-of-coolant accidents. Flashing, on the other hand, is caused by large pressure drops, converting the initially sub-cooled liquid bulk into superheated liquid. The phenomenon is initially more violent at the surface and causes the liquid to acquire a very heterogeneous temperature composed of superheated, saturated, and sub-cooled liquid. The greater turbulence perpetuates the phenomenon, which results in higher rates of mass transfer, and eventually causes cooling throughout the liquid. Flashing may occur in the absence or presence of bulk nucleation. Bulk nucleation may also, originate from exceeding the spontaneous nucleation temperature which causes the bulk of the liquid to explode into the vapor state. The absence of nucleation sites and dissolved gases within the liquid causes flashing to occur only at the surface. After a period, the liquid bulk cools down and behaves in a similar manner as a liquid undergoing evaporation. The two processes may be represented by different boundary conditions imposed at the liquid– vapor interface. Evaporation is caused by local surface instability corresponding to what is believed to be a linear temperature change at the boundary. In contrast, flashing with a rapid change in the total pressure corresponds to a step change in surface temperature [2]. 2. Literature survey Miyatake et al. [3] discussed through their ex-
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perimental work on the spray flash evaporation the effects of the degree of superheating, the spray flow rate and nozzle diameter on the spray flash at 60°C. Through this study they concluded that the rate of flash evaporation of a liquid jet is extremely faster than that of flowing superheated liquid in conventional MSF evaporators and superheat pool water. As the degree of superheating ∆Tsup = Tin – Tsat increases, the behavior of θ (dimensionless temperature) shows a rapid decrease. It is found that when ∆Ts increases to some extent, the spray flash evaporation undergoes two exponential decaying processes after the elapse of time lag (to). An empirical equation suitable for predicting the variation of liquid is
(We / Re ) exp ( ∆T / 35) ≥ 24 1/ 8
s
and this equation is correlated well with the experimental results. The difference between the time at intersection of the two asymptotes (ti) and the time lag of initiation of flashing (to) is related only to the nozzle diameter (d). As Zo= U to, the value of (Zo/d) is related mainly to ∆Ts. Miyatake et al. [4] presented the effect of liquid temperature on the spray flash evaporation at temperatures of 40 and 80°C, From the experimental results, a more general empirical equation suitable for predicting the variation of liquid temperature in the center of jet with residence time was deduced. It was seen that, even at lower liquid temperatures, spray flash evaporation still had higher evaporation performance and faster evaporation rate than the flash evaporation occurring in other systems. Sebastain and Nadeau [5] studied the falling jet flash evaporator for vintage treatment theoretically and experimentally. A mono-stage falling jet flash evaporator was constructed and instrumented to follow the system parameters: pressures, temperatures, and flow rates. A steady-state model was built and validated by using the experimental results. The thermodynamic aspects of the prototype are discussed through the work. The
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parameters that are concerned in the study are pressure drop inside the separation chamber, condenser and vacuum system characteristics and the change in the generated sprays inside the chamber. Through the study, water is used as the reference fluid for simulations as its thermodynamic properties are known. As the numerical simulation code was verified well, it allowed them to extend their investigation to a bigger industrial unit. A new pilot unit using a two-stage evaporation technique was built and studied. Muthunayagam et al. [6] reported in their study a vapor diffusion model and an energy balance equation at the droplet surface to show the feasibility of the desalination of seawater at typical ocean surface temperature between 26 and 32°C by using a low-pressure vaporization process. They prepared an experimental apparatus to check the model validity and they found a good agreement between both the theoretical and experimental work. The range of the vacuum inside the test rig varied between 10 and 18 mm mercury during this work. The yield of fresh water was about 3–4% at the lower range of vacuum and the upper range of temperatures. The results matched well with the model prediction. Also, small values of water injection pressures, of about one bar, were found to be adequate when a swirl nozzle was used during the experiment. Kitamura et al. [7] made an experimental attempt to study the critical superheat for flashing of superheated liquid jets. This experimental work was carried out by an injection of water and ethanol through a long nozzle to a vacuum chamber. Through the work, the superheated spray had two different patterns: complete flashing and twophase vapor–liquid effluent. The complete flashing occurred when the temperature of the liquid jet was high enough above the liquid bubble point that corresponded to the pressure in the flash chamber. For the two-phase effluent, it happened when the temperature was close to the bubble point. The critical superheat above which the complete flashing occurred was correlated by an empi-
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rical equation on the basis of the bubble growth rate in the superheated liquid. Gemci et al. [8] experimentally studied cavitation and flash boiling atomization of water–acetone binary mixtures by using nitrogen as the propellant gas. A sharp edge orifice was used during this study to create cavitation bubbles. This research was used to study the spray breakup region and the influence of the nozzle internal conditions (length and diameter) on liquid atomization involving the flash boiling and cavitation mechanisms. Mean droplet diameters were measured as a function of the injection temperature, pressure and the flow rate ratio of the thermodynamic conditions upstream of the orifice induce internal bubble formation. A further improvement in the spray quality was achieved by increasing the propellant gas amount. This study of the binary acetone–water solution provides a model system for improving the atomization of hydrocarbon liquid through addition of a low boiling component at a reduced gas to oil ratio. Also, spray images were captured at the nozzle exit to visualize spray formation and breakup length. Peter et al. [9] studied the phenomenon of flashing and shattering of superheat liquid jet in the field of low pressure. They clarified that there are four physical characteristics of the jet in the area of low pressure and these are: non shattering liquid jet, partially shattering liquid jet, completely shattering jet and flare flashing liquid jet. They studied both the temperature distribution in the axial direction along the liquid jet and the radial temperature distribution of the liquid jet. Also, they studied the distribution of the droplet size and the contribution of bubble nucleation and growth to the liquid jet shattering in the area of low pressure. Brown and York [10] studied the sprays formed by flashing liquid jets by using rough orifices and sharp edge orifices. Liquids which are forced from a high pressure area to another one of low pressure often cross the equilibrium pressure for the liquid temperature and disintegrated into spray by partial
evolution of vapor. Their study of the sprays from water and feron-11 jets analyzed drop sizes, drop velocities and spray patterns. The mechanism which controls the process was analyzed and the data about the process were collected. A critical superheat was found, where the jet of liquid was shattered by rapid bubble growth. Also the bubble growth rate was controlled with the Weber number, where its critical value for low viscosity liquid was 12.5, the mean drop size also correlated with the Weber number and the degree of superheat. They also compared spray from orifices with other techniques and it was found to be comparable in all aspects except temperature. Due to the limited number of works, the present study will concern the parameters that influence the flashing jet and the ways to enhance the flash efficiency. In this study, many parameters will be taken into consideration: • Increase of the range of the operating temperatures and increase of the number of measurement points through this range. • The number of spray nozzles used through this work is four. • Comparison between the operating medium of the fresh water with most of the documented studies. • Measuring flashing efficiency and comparing it with the empirical formula proposed by Miyatake et al. [11]. The main objective of the present study is to enhance the flashing efficiency through a flashing chamber under low pressure by using small-diameter spray nozzles working at low injection pressures. The study also concerns the main parameters that affect the flashing efficiency such as feed flow rates, feed temperatures, vacuum, etc. So, through the experimental work that will be carried out under this study, the following will be investigated: • the effect of feed water flow rates, feed water temperatures, and different vacuum on the produced vapor and on the flashing efficiency;
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• the effect of the water level inside the flash chamber on the produced vapor, on the flashing efficiency; • the effect of the feed flow rates, the feed temperatures, and the vacuum on the product yield. 3. Experimental apparatus The experimental set-up is schematically shown in Fig. 1a. This set-up includes two loops, the main loop is used for injecting hot water in the flash chamber and it consists of many parts, the secondary loop is used for cooling water to condense in the flashed vapor inside a condenser. The entire test facility was constructed of iron tubes (0.5 inch), stainless steel and glass components. The test loop is designed to work in a flow range of 0–30 l/h, and a temperature range of 40–80°C. These conditions are realized in a closed loop, the hot water supply to the test section consists of a recirculating pump, water heaters, supply tank, y-filter, controlling valves (ball and global) and measuring flow meter. The steam is removed to a separate condensing loop. This loop
Fig. 1a. The main test section, the inlet feed line, the vapor outlet line to the condenser.
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consists of a Pyrex condenser (tube and shell), a recirculating pump to condensate the liberated steam, a collecting flask for the condensing steam, a vacuum system is connected at the condenser outlet for removing the non-condensable gases, needle valve for controlling the vacuum system, a sensitive balance for measuring the condensate vapor, and data acquisition system connected to the temperature sensors. Some parts of the experimental facility are shown in Figs. 1b. A y-filter with high efficiency is used during the process and replaced from time to time for cleaning. This filter is placed immediately upstream of the test section. There are 4 sprayers inside the test section. These sprayers could be modified to provide a variety of profiles but only one set is used during all experiments, the diameter of the sprayer nozzle is about 0.4 mm. The liberated steam is condensed in the tube and shell heat exchanger, and the condensed steam is measured by using the sensitive
Fig. 1b. Some parts of the experimental set-up: collecting flask, sensitive balance, vacuum pump, data acquisition system, and control needle valve.
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balance. The main hot water in the primary circuit is measured by a rotameter, its range is from 0 to 30 l/h. Also, the cooling water is measured by an orifice meter with a diameter of 6 mm. Temperatures, mass flow rate, pressures of both loops are measured and recorded at different points in the circuits. 3.1. Instrumentation The temperature is measured through the entire points of the set-up using thermocouples of type k (Cromel-Alumil) with a diameter 0.2546 mm and a range of –100–1372°C. The flow rate of the main loop is measured by a rotameter with an accuracy of ±3% of full scale, and the flow rate of the secondary loop is measured by the orifice meter with a diameter of 6 mm. The vacuum through the system is measured using a vacuum gauge, the data acquisition system (ChartScan/ 1400) of resolution equal to 0.1°C is controlled using a PC, and a sensitive balance for measuring the collected vapor of specification of a capacity of 2000 g, readability of 0.01 g, standard deviation of 0.01 g and the tolerance of ±2.5 mg. So the % errors in the measuring instrumentation in vacuum, feed flow rate, temperature and condensate vapor are 2%, 5.2%, 2%, 0.05%, respectively.
The energy balance considering all modes of heat transfer between the droplets and the surrounding environment is
+ h fg dM f / dt + Ad σ ε (Ta − Td )
(1)
where the terms in the right-hand side represent heat terms as a result of conduction/convection, phase change, and radiation respectively [3]. By neglecting the conduction/convection and radiation terms, the above equation will be M f Cp d ( Td ) / dt = h fg dM f / dt
M f CpTin − M f CpTout + M v CpTout −M v h = 0
(3)
By comparing both the feed flow rate and the flashed mass, the value of the flashed mass is too small compared with the feed flow rate. So Eq. (3) can be rewritten as M f cpTin − M f CpTout − M v h fg = 0 ∴ M v = M f Cp ∆Tsup / h fg
(4)
∴ M v = M f Cp ∆Tsup / h fg
(5)
4. Results and discussion
3.2. Determination of flash vapor
M f C p d (Td ) / dt = Ah (Td − Ts )
by using mass conservation, this equation leads to Eq. (4). In another way, for the steady-state condition, the evaporated mass flow rate is calculated from the energy balance on the injected water inside the flash chamber, by neglecting the thermal losses through the walls of the flash chamber. Thus the energy released by the sudden drop in the feed temperature is completely used in vaporizing a quantity of the injected feed water, by illustrating the energy balance on the chamber,
(2)
The parameters that affect on the spray flashing phenomena are summarized as follows. 4.1. Residence time The values of the pressure in the flash chamber allow the evaporation to take place till the temperature at the droplet surface reduces to the value of the saturation temperature corresponding to the flash chamber pressure. Vaporization will take place as long as the saturation vapor pressure at the droplet temperature exceeds the ambient vacuum pressure. The surface tension induced pressure at the droplet (∆p) is 2σ/r where σ is the surface tension coefficient and r is
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− uo ⎤ / g ⎥⎦
(6)
From this equation the residence time for an average initial velocity equal to 3 m/s is approximately 140 ms, also the drag forces on the droplet will further increase the residence time of the droplet.
20
60
16
inlet temp. sat. temp. M co nd.
58 56
8 54 4
52 50 0
360
720
1080
1440
1800
∆Tsup which is the driving force for the flash evaporation process is known as
Fig. 2. Feed temperature, saturation temperature, condensate vapor at Mf = 14.8 kg/h, Tin = 60°C, vacuum = 0.85 bar.
(7) 62
4.3. Flashing efficiency (η)
20
Temperature (°C)
60
The efficiency of the spray flash evaporation is known as the ratio between the steam generation rate to the maximum steam generation rate or it is defined as the ratio of the amount of actual evaporation to that of the theoretical evaporation. η = (Tin − Tout ) / ( Tin − Tsat )
0 2160
Time (s)
4.2. The superheat of the working liquid
∆Tsup = Tin − Tsat
12
Md (gm)
0.5
62
inlet temp. sat. temp. Mcond.
58
All the experiment measurements were taken at steady-state conditions, the data were recorded for each run after reaching the steady state for a period of time of 30 min, using a computer system connected to a data acquisition system. Figs. 2–4
12
56 8
54
4
52
(8)
16 Md (gm)
t = ⎡( uo2 + 2 gh ) ⎢⎣
indicate the effect of changing the depressurization inside the flash chamber from 0.85, 0.87, to 0.9 bar, at a feed flow rate of 14.8 kg/h, and inlet feed temperature of 60°C. It is clear that increasing the depressurization leads to an increase in the degree of superheat, which leads to enhancement of the shuttering of the injected flow through the jet of the nozzles. So there is an increase in the amount of flashed vapor inside the flash chamber, followed by an increase in the amount of the con-
Temperature (°C)
the droplet radius. So for a droplet of diameter ~0.5 mm this pressure is about 11 Pa which is much less than the lowest vacuum in our experiment, so the effect of the surface tension is neglected. It is necessary that the evaporation time is less than the residence time to ensure the complete evaporation. As the droplet is injected at an initial velocity Uo, so the actual residence time can be calculated using Newton’s equation for motion with the assumption that the droplet diameter is constant, and the temperature of the droplet just away from the nozzle exit is constant along the height h
317
50 0
360
720
1080 1440
1800
0 2160
Time (s)
Fig. 3. Feed temperature, saturation temperature, condensate vapor at Mf = 14.8 kg/h, Tin = 60°C, and vacuum = 0.87 bar.
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A.K. El-Fiqi et al. / Desalination 206 (2007) 311–321 62
20
54
16
52
12
50
8
48
4
46 44
0 0
360
720
1080
1440
1800
Vapor product (g/30 min)
56
180
24
inlet temp. sat. temp. Mcond.
58
Md (gm)
Temperature (°C)
200
28
60
Tinlet=60 oC Tinlet=65 oC
160
Tinlet=69 oC 140 120 100 80 60 40
Time (s) 20
Fig. 4. Feed temperature, saturation temperature, condensate vapor at Mf = 14.8 kg/h, Tin = 60°C, vacuum = 0.90 bar.
50
100
150
200
250
300
Absolute vacuum (mbar)
Fig. 5. Relation between the absolute vacuum and the amount of flashed vapor at different temperatures.
80 Evaporated mass rate (g/30 min)
densate vapor. Remember that all measurements of the amount of the condensate vapor were taken under steady-state conditions, and each run was done in a period of time equal to 30 min. If we compare these measurements with the values of the flashed vapor gained from Eq. (4), we will find a good agreement. Fig. 5 shows the values of the flashed mass with the absolute vacuum inside the flash chamber and it is clear that the flashed mass is a decreasing function of the pressure. Fig. 6 shows the values of the flashed mass with the inlet feed temperature to the flash chamber, also the amount of the flashed mass is an increasing function of the inlet feed temperature. This will be summarized in the next figures as a relation between the flashed mass and the degree of superheat. Figs. 7 and 8 show that the flashed mass is proportional to the degree of superheat, although we used different feed flow rates and inlet temperatures. If this situation studied by doing the energy balance to the flash chamber, it was concluded that the amount of the flashed vapor is proportional to the degree of superheat as it can be seen from Eq. (4) which indicates that Mv α (∆Tsup.). The measurements of the evaporated mass obtained through this work have a standard deviation between 0.87 and 0.93. as the feed flow rate =
0
vac.=0.88
70
vac.=0.90
60
vac.=0.91
50 40 30 20 10 0 48
50
52 54 Temperature (°C)
56
58
Fig. 6. Feed temperature vs. the amount of the evaporated mass rate at different vacuum.
8 kg/h, Cp = 4.18 kJ/kg.k, hfg at 56°C = kj/kg, this will lead to the constant of proportional =, so in this case the deviation between the measured and calculated constant (Mf Cp / hfg) is about 15%. Figs. 9 and 10 indicate the relation between the degree of superheat and the flash efficiency when using the fresh water. These figures show a
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319
13
8
12 11
Evaporated mass rate (g/3 min)
Evaporated mass rate (g/3 min)
7 6 5 4 3 2
10 9 8 7 6 5 4 3
1
2
0
0
1 0
2
4
6
8
10
12
0
2
4
Degree of superheat (°C)
good agreement between the measured flash efficiency and the proposed equation for the flash efficiency by Miyatake et al. [11]. We proposed another equation from our work governing the relation between the flash efficiency and both the degree of superheat and the inlet feed temperature, and it has a good agreement with Miyatake’s equation. Fig. 11 indicates a comparison between the measured amount of the flashed vapor and the calculated one with some errors due to measurements. 5. Conclusions An extensive experimental work on the flashing process using fresh liquid was carried out. The work concerned the spray flash evaporation occurring in a superheated water jet injected through circular nozzles of a diameter less than 0.4 mm into a low-pressure vapor zone. This study was carried out on liquid water having an initial temperature ranging between 40 and 70°C, vacu-
8
10
12
Fig. 8. Evaporated mass rate vs. the degree of superheat at feed flow rate = 11.3 kg/h, inlet feed temperature ~68°C.
1
Flashing efficiency
Fig. 7. Evapoarated mass rate vs. the degree of superheat at feed flow rate = 8 kg/h, inlet feed temperature ~56°C.
6
Degree of superheat (°C)
0.9 0.8
Miyatake11
0.7
Exp. 0.6 0.5 0
1
2
3
4
5
6
7
8
9
10
11
12
Degree of superheat (°C)
Fig. 9. Flashing efficiency at feed flow rate = 8 kg/h, inlet temperature ~50°C, level = 46.5 cm.
um ranging between 60 and 250 mbar, feed flow rate ranging between 4 and 15 kg/h. The detailed data on the effect of the degree of superheating, the inlet feed flow rates, the inlet feed temperatures, vacuum, and flashing efficiency on the behavior and performance are presented. The results show that:
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(∆Tsup), both the flashing efficiency and the flashed vapor increases. The relation between the flashed vapor and the degree of superheat is a proportional relation, the factor of proportionality between the flashed vapor and the degree of superheat is obtained from the energy balance through the flash chamber, Mv = (Mf Cp ∆Tsup.) / hfg, and there is an agreement between the calculated and the measured values.
1
Flashing efficiency
0.9 0.8
M iyatake 11 0.7
Exp.
0.6 0.5 0
2
4
6
8
10
12
14
16
18
Symbols
Degree o f superheat (°C)
Fig. 10. Flashing efficiency at feed flow rate = 14.8 kg/h, inlet feed temperature = 69°C.
cal., vac.=0.88
Measured
cal.,vac.=0.9
measured
cal.,vac.=0.91
measured
cal.,vac.,=0.92
measured
Evaporated mass rate (g/3 min)
8 7 6 5 4 3 2 1 0 1
2
3
4
5
6
7
8
9
10
Time (3 min)
Fig. 11. Comparison between measured and calculated flashed vapor at different vacuum at Mf = 8 kg/h, Tin = 53°C.
• Increasing the degree of superheat leads to an increase in the flashing vapor to a certain amount. • Decreasing the water level inside the flash chamber, increases slightly both the flashing efficiency and the amount of the flashed vapor. • With the increase of the degree of superheat
Ad C Cp do g h
— — — — — —
hfg M m′ p Q′ r t T Tz
— — — — — — — — —
U
—
V, U — W — Z —
Droplet area, m2 Salt concentration, ppm Specific heat of liquid at p = c, J/kg.K Orifice diameter, m Gravity acceleration, m/s2 Traveling distance of the droplet, or brine level, m Latent heat, kJ/kg Flow rate, kg/h Specific weir load, kg/s.m Pressure, pa Volume flow rate, m3/h Radius of droplet, m Time, s Temperature, °C Liquid temperature at the center line of the jet, °C Mean velocity of the liquid in the nozzle, m/s Flow velocity, m/s Evaporation rate (mass flux), kg/s.m2 Vertical distance from the nozzle exit, m
Greek µ ρ θ η ω
— Dynamic viscosity, N.s/m2 — Liquid density, kg/m3 — Dimensionless temperature = (T* – Tsat) / (Tin – Tsat) — Flash efficiency = (Tin – Tout) / (Tin – Tsat) — Salinity, g/l
A.K. El-Fiqi et al. / Desalination 206 (2007) 311–321
σ — Surface tension, N/m ∆′ — Non-equilibrium allowance, °C ∆T — Temperature difference, °C Subscripts and superscripts a — b — d — eq — f — in — out, o sat — sup — v,s —
Atmosphere Boiling point Droplet Equilibrium Feed flow rate Inlet — Outlet Saturation Superheat Vapor
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salination, 36 (1981) 113–128. [4] O. Miyatake, T. Tomimura, Y. Ide, M. Yuda, and T. Fujii, Effect of liquid temperature on spray flash evaporation, Desalination, 37 (1981) 351–366. [5] P. Sebastian and J.P. Nadeau, Experimental and modeling of falling jet flash evaporators for vintage treatment, Int. J. Therm. Sci., 41 (2002) 269–280. [6] A.E. Muthunayagam, K. Ramamurthi and J.R. Paden, Modelling and experiments on vaporization of saline water at low temperatures and reduced pressures, Appl. Therm. Eng., 25 (2005) 941–952. [7] Y. Kitamura, H. Morimitsu and T. Takahashi, Critical superheat for flashing of superheated liquid jets, Ind. Eng. Chem., Fundamentals, 25(2) (1986) 206– 211. [8] T. Gemci, K. Yakut and N. Chigier, Cavitation and Flash Boiling Atomization of Water/Acetone Binary Mixtures, Spray Systems Technology Center, Carnegie Mellon University, Pittsburgh. [9] E. Mhina Peter, T. Akira and H. Yujiro, Flashing and shattering phenomena of superheated liquid jets, JSME Int. J., Series B: Fluids Therm. Eng., 37(2) (1994) 313–321. [10] R. Brown and J.L. York, Sprays formed by flashing liquid jets, AIChE J., 8(2) (1962) 149–153. [11] O. Miyatake, Y. Koito, K. Tagawa and Y. Maruta, Transient characteristics and performance of a novel desalination system based on heat storage and spray flashing, Desalination, 137 (2001) 157–166. [12] H.W.M. Witlox, Flashing Liquid Jets and Two-Phase Dispersion, Contract Research report 403/2002, HSE, Health and Safety Executive, 2002.