Steam condensation in a 2D model of an annular gap steam
injector
Andreas Nilsson
Department of Chemical Engineering, Lund Institute of Technology, May 2007
Abstract
The Tetra VTIS steam injector is an annular gap steam injector used for sterilization purposes of liquid food
products such as milk. The sound levels are high and knowledge of the condensation process is lacking. An
experimental rig to study the condensation process is constructed. A test injector with sight glasses to make
visual observations and to take photographs is used. The injector geometry is a two dimensional model of a basic
annular gap steam injector. In the visualization injector steam and water is mixed with a two-phase jet as a result.
The two-phase jet condenses in the water under different regimes. One regime is comparable to what can be seen
in the real injector, oscillatory jet regime, with a jet oscillating back and forth in the condensation chamber. The
condensation length measured increases as the level of heating increase. The length decreases with higher
pressures due to density effects and higher subcooling. Higher product flows with increased mixing and
turbulence as a result also decrease the condensation length.
Introduction
Continuous direct steam injection is used to
quickly raise the temperature of a process media,
either for pure heating or for a sterilization process.
The benefit of the direct contact condensation
process is high heat transfer rates and low fouling
compared with other methods such as heat
exchangers. In the product portfolio of Tetra Pak the
Tetra Therm Aseptic VTIS steam injector is an
annular gap steam injector. It is used for sterilization
purposes of liquid food products such as milk. The
main function of the steam injector is to inject steam
into the process media under high pressures to reach
a desired sterilization temperature.
The knowledge of the condensation process is
lacking and sound levels in the steam injector are
high. Therefore an investigation of the condensation
process in a visualization injector is desired. The
work in this study is aimed at an understanding of
the steam injection process taking place in the Tetra
VTIS steam injector.
In the sterilization process the incoming product
to the injector is preheated to approximately 80°C
before entering the injector. In the injector the
temperature is raised to about 140°C and pressures
are held at >3.5 barg to prevent boiling. The product
is held at this high temperature in approximately
four seconds in the holding tube before it enters a
expansion chamber where it is flash cooled. The
flash cooling takes place in partial vacuum where
the pressure is controlled so that the amount of
vapour flashed off is equal to the amount of water
added in the steam injection step [1].
The injector is of annular gap type (also called
ring nozzle steam injector), Figure 1. The steam is
injected coaxial in an angle into the product flow.
The product flow is divided into two streams in the
injector, one to the product gap and the other into the
center and led out to the cone.
Tetra VTIS steam injector
The Tetra VTIS (Vacuum Thermal Instant
Sterilizer) is the process module where the
preheating, sterilization and cooling takes place. In a
direct UHT (Ultra high temperature) treatment the
milk is pumped through a closed system where it is
preheated, high temperature heated, cooled,
homogenised and packed aseptically [1]. The Tetra
VTIS steam injector is located after the preheating
and it is where the high temperature sterilization
occurs.
Figure 1. Tetra VTIS steam injector [2]
The product gap of the injector is adjusted with
washers of different size at the back of the injector,
to set the desired gap width. The adjusted gap width
lets the injector work in wider area of capacities of
product flow. There are four Tetra VTIS steam
injectors available with capacities stretching from
800 to 35 000 L/h.
experiments is similar to the ones used in the Tetra
VTIS injector.
Condensation regimes and jet length
Previous work has reported regime maps to
classify the how the condensation process takes
place [3], [4]. Three distinct regimes are found in the
literature; chugging, bubbling and jetting. The
chugging regime is a periodic build-up and collapse
of the steam cavity with large pressure fluctuations
as a result. Bubbling is a build-up of steam cavity
outside the nozzle that oscillates or even detaches
some part of the steam. Jetting is considered to be
the stable regime regarding hydrodynamic
steam/water flow regime and noise levels [5].
Other regimes also found in literature are
oscillatory interface regime where the steam mass
flux is low; a clear interface between the steam and
water can be seen. The water hammer regime which
is when the steam flow rate is greater than the
condensation rate and saturation temperature is
reached with steam left in the condensation
chamber.
Most of the regime study and estimation of
condensation length in literature is done in quiescent
water under atmospheric pressures. In 1972 Kerney
et al. [6] used a semi-empirical approach to set up
relation for the steam penetration depth of a pure
steam cavity. Since then this theory has been
developed and used for estimation of cenodensation
length in quiescent water. The conditions in these
cases differ from the conditions in the Tetra VTIS
where the flow of water mixes the steam cavity with
improved heat transfer as a result.
In the Tetra VTIS the level of heating is also
higher where in the start of the jet the mixing is high
with improved heat transfer. As the jet stretches out
in the condensation chamber the velocities and
subcooling decreases with less heat transfer as a
result.
Figure 2. The visualization injector
In order to perform the experiments a
surrounding system had to be set up. The
experimental rig supplies the visualization injector
with water and steam at temperatures, pressures and
flowrates set by the experimenter.
Previous experimental work at Tetra Pak with
direct contact condensation has shown that there is a
need for removal of non-condensable gases [7]. The
solubility of nitrogen and oxygen is decreased as the
temperature is raised in the steam injector. These
gases are unwanted since they not only may be
confused with steam but it lowers the partial
pressure of the steam in the steam cavity. Therefore
a deaeration branch is installed.
The rig consists of injector, tank, vacuum vessel,
pumps, heat exchangers and measurement devices,
Figure 3. It is connected to a 5,5 bar g steam line and
cooling water.
Material & methods
To make visual observations of the condensation
process possible, a test injector had to be
constructed, further called visualization injector.
The injector, Figure 2, consists of a house, a
cover, two inlets and an outlet. The flow through the
injector is defined by two plastic inlays, formed to
the desired geometry, inserted into the injector and
kept in place by four bolts. The house and cover of
the injector is hollow and glass plates, acting as
channel walls, make observations of the process
possible. The geometry of the channels used in the
Figure 3. Process layout
Water is held in the tank at approximate 90°C. In
order to heat the water in the tank steam is injected
direct into the water well below surface. The water
is the pumped through the first heat exchanger to the
injector. Steam is injected and the steam amount is
controlled via a control valve. After the injector a
control valve is used to keep the desired
backpressure. Before the hot water enters the tank at
atmospheric pressure a heat exchanger is used to
prevent boiling. When the rig is in deaeration mode
the vacuum vessel is used and a part of the water,
and the air, is flashed off.
The operation conditions in which the
experiments are made is similar to the ones used in
the Tetra VTIS process. The subcooling of the water
in the experiments can be varied with the
backpressure and inlet temperature. Inlet
temperature in the experiments was set to 80°C,
similar to the real injector, reducing the subcooling
to be set by backpressure of the injector if saturation
of the steam is assumed. The subcooling varies with
the flow direction in the chamber where the
condensation occurs. The operating conditions for
the visualization injector are displayed in Table 1.
The camera was set in a fixed position so that the
image analysis and image processing is made much
easier. The injector and plastic inlays are in the same
position in each picture which allows one picture to
be used to relate a distance measured in pixels to a
distance in metric units, and to set reference
positions such as jet start.
Regimes
In the experiments three classes of regimes are
found: oscillatory interface regime, chugging regime
and jetting regime. The latter, jetting regime is
characterized by only one connected steam cavity
length witch includes a wide area of jets, it is
therefore divided into a number of sub regimes: Jet
formation, oscillatory jet, oscillatory jet with
pulsating upwards flow and jetting with jet length
stretching outside the injector. A summary of the
regimes found is seen in Figure 4.
Table 1. Operating conditions for the visualization
injector
Product flow
Product velocity in gap
Temp. inlet
180-469
5-12
80
L/h
m/s
°C
Steam flow
Steam velocity in gap
Heating
5-36
10-100
10-60
kg/h
m/s
°C
Backpressure
Initial subcooling
2.7-4.5
61-76
barg
°C
Image capturing device
To capture images from the condensation
process Sony XCD-X710 is used. It is a computer
controlled industrial camera with the possibility to
take pictures in series. The condensation process is
fast so the camera is set to take pictures with a short
shutter time so that the images are sharp and details
can be seen. With a short shutter time the camera
does not let enough light in to illuminate the object
and the image becomes black.
To illuminate the steam cavity both backlight
and frontlight is used. The frontlight helps to bring
out the details in the picture and it is possible to see
the steam/water interfaces as a cloud like formation
with light from front. The backlight is used to bring
out the contours of steam cavity which is helpful
when determine the surface area of the jet or the jet
length.
A camera software was developed in Visual
basic, it is used for camera control, triggering the
camera, triggering the lights and to process and save
the image.
Figure 4. Summary of the regime classification
With low steam mass flux two regimes can be
observed, oscillatory interface and chugging. An
example of oscillatory interface regime can be seen
in Figure 5. This regime is characterized by pure
steam in the steam gap and an oscillatory
steam/water interface direct at the steam exit.
Figure 5. Oscillatory interface regime
The other regime observed with low steam mass
flux is chugging, Figure 6. In this regime the
steam/water interface position is very irregular. The
chugging regime can be described as a periodic
build-up and collapse of the steam cavity. The steam
cavity penetrates the water and condenses rapidly, as
the steam cavity collapse a negative pressure is
generated in the steam gap and water flows up in the
steam channel. The steam then builds up pressure
and pushes the water out of the nozzle to form a
cavity again. When chugging occurred in the
experiments large pressure fluctuations in the system
were observed, caused by the irregular pulses of
steam build-up and collapse.
Figure 6. Chugging regime
As the steam mass flux is increased the
penetration length of the steam condensation
increases. The condensation zone starts to form a jetlike cloud, Figure 7.
Figure 7. Jet formation regime
With furthermore increasing steam mass flux the
jet formation regime transits to the oscillatory jet
regime, Figure 8. This regime is characterized by a
two-phase jet with condensation lengths reaching
longer than the mixing zone and a larger extent of jet
length oscillation than the jet formation regime
discussed above. The two phase jet occur in the
lower part of the condensation zone of the injector.
Further increase of the steam mass flux causes the
jet to stretch further out in the condensation zone.
Since this is the same type of fluctuating jet as
discussed above this regime is classified as the same
type of oscillatory jetting regime. The difference is
the higher steam mass flux resulting in longer
condensation length.
Figure 8. Oscillatory jet regime 1 and 2
The first jetting regime, jet formation, can also
transit to another form of oscillatory jet regime,
when the steam flux is increased. This regime is the
oscillatory jet with pulsating upwards flow, Figure 9.
This regime is also characterized by the same twophase jet stretching farther in the injector than the jet
formation regime. There are also similarities with
the oscillations from left to right as the first type of
oscillatory jet regime, but in this regime there is a
also a constant steam-escape causing an oscillatory
up and down motion of the steam. The lower of the
two steam cavities is more cloudlike than the upper.
This is explained by the two phase jet formed when
the water and steam is mixed, it appears as the water
flow is not capable of mixing all the steam to a twophase jet, and some of the steam escapes. This
regime was observed with low flow velocities of the
water and at low pressures.
Figure 9. Oscillatory jet regime with upwards flow 1
and 2
As the steam flow is further increased the
condensation length stretches beyond the injector
exit leaving a two phase flow in the entire
condensation chamber, called outside jet regime.
The full regime classification can be seen in [8].
Condensation length
The jet lengths for the experiments were
measured in Matlab. Figure 10 displays the
comparison of the jet length with a constant flow
velocity in the product gap of 8,5 m/s at different
pressures. From the figure it can be seen that
increased pressure leads to a shorter jet. This effect
is due to the density effects (lower specific volume)
and increasing subcooling. As the subcooling is
increased, the water that is mixed and exist in the
two phase jet can be heated to a higher temperature
before it is mixed with surrounding water. This leads
to that more steam can condensate per mixed water
volume which enhances the heat transfer.
140
2.7 barg
120
3.1 barg
4.0 barg
Jet length (mm)
100
4.5 barg
80
60
40
20
0
0
10
20
30
Heating, ΔT (°C)
40
50
60
Figure 10. Jet length comparison with constant
product flow velocity (8,5 m/s) and increasing pressure
The same comparison but with a constant
pressure of 3,5 bar g and increasing flow velocity of
the product is displayed in Figure 11. It appears as the
jets are shorter at higher product flow velocities. The
higher velocities cause a better mixing and
turbulence levels. This leads to increased heat
transfer and therefore a shorter jet. The lowest flow
velocity does not follow this trend, since it is in the
upwards flow regime where the steam escapes
upwards instead of stretches in the flow direction
and therefore it is a shorter jet.
140
6.8 m/s
10.3 m/s
Jet length (mm)
100
Acknowledgement
This work was carried out at as a Master work at
Tetra Pak with PhD Fredrik Innings as supervisor,
who is gratefully acknowledged along with Prof.
Stig Stenström, Dpt of Chemical Engineering LTH,
who was examiner.
References
[1] Tetra Pak Processing Systems AB, Dairy
processing handbook (1995).
[2] Tetra Pak Media box at
http://neworbis.tetrapak.com/irj/portal.
[3] Liang, K.-S.; Griffith, P. (1994) Experimental
and analytical study of direct contact
condensation of steam in water, Nuclear
Engineering and Design, Vol. 147, 425-435.
5.0 m/s
120
comparable to what can be seen in the real injector.
Increased level of heating cause the condensation to
go from oscillatory interface, through jet formation,
oscillatory jet regime and eventually the
condensation is outside the injector.
The condensation length measured increases as
the level of heating is raised. There is a clear trend
of higher pressures decreases the condensation
length due to lower specific volume of the steam and
subcooling. Higher product flows with increased
mixing and turbulence as a result also decrease the
condensation length.
12.0 m/s
80
[4] Gulawani, S.S.; Joshi, J.B.; Shah, M.S.;
RamaPrasad, C.S.; Shukla, D.S. (2006), CFD
analysis of flow pattern and heat transfer in
direct contact steam condensation, Chemical
Engineering Science, vol. 61, 5204-5220.
60
40
20
0
0
10
20
30
40
50
60
Heating, ΔT (°C)
Figure 11. Jet length comparison with constant
pressure (3,5 bar g) and increasing flow velocity
The heat transfer coefficient is estimated for the
experiments with a measurable jet length. In the
calculation of the coefficient the area of the jet is
defined as the jet lengths multiplied with the depth
of the channel due to the difficulties of determine jet
area. In the experiments the estimated heat transfer
coefficient varies from 0.2 to 2 MW/(m2 K), which
is in the same order as in [4] and [5].
Conclusions
The different regimes observed in the
visualization injector can be divided into three
branches, oscillatory interface regime, chugging
regime and jetting regime which includes a wide
area of jets. One of these, oscillatory jet regime 1 is
[5] Patel, R.; Varley, J.; Couch, M. (1996), Design
of Stable Steam Injectors for Continuous
Heating, Journal of Chemical Technology &
Biotechnology, Vol. 66, 327-339.
[6] Kerney, P.J.; Faeth, G.M.; Olson, D.R. (1972),
Penetration characteristics of a submerged
steam jet, AIChE Journal, vol. 18, 548-553.
[7] Von Schenk, M. (1972), Studium av
kondensation vid direktinsprutning av ånga i
strömmande medier
[8] Nilsson, A. (2007), Steam condensation in a 2D
model of an annular gap steam injector, LTH,
Lund