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CHAPTER 4
EXPERIMENTS
In this chapter, the single-effect desalination system and the reverse osmosis system
used for experiments in the present study are described. Different components of the
system together with their designed specifications have been elaborated. The
methodologies adopted during experimental studies are also introduced here. The
operating conditions have been selected as close to real scale operation of a Multi-
effect Desalination (MED) and Reverse Osmosis (RO) unit. Efficient design of the
evaporator plays a key role in the thermal performance of a MED system. A single
tube vertical desalination evaporator has been used to study the characteristics in a
greater detail. In this experimental study, 12 different kinds of tube profiles have been
considered for the design of the evaporator. Copper-Nickel (90-10) and Aluminum
have been chosen as the materials for the design of evaporators.
4.1 The Desalination Unit
A Single-effect desalination system was designed and fabricated in the Thermal
Process Lab 1 at National University of Singapore. The system utilizes waste heat in
the form of hot water (45-70
0
C) as the heating source instead of steam. A schematic
diagram of the system is shown in Figure 4.1. A photograph of the system is shown in
Figure 4.2.The key components of the desalination system are the shell and tube type
two-phase heat exchanger/evaporator, feed water tank, hot water tank, vacuum pump,
blow down pump and chilled water tank. Saltwater with variable concentrations
(15,000-35,000 ppm) was used as feed in the feed water tank for experimental studies.


0
C.
In the desalination rig, hot water enters the shell-side at the top, left at the bottom and
returned to the hot water tank for recirculation after heating. Feed water flows through
tubes entering from the bottom of the evaporator. The flow arrangement is counter-
current inside the evaporator. The hot water flow rate can be controlled by means of a
ball valve. In the evaporator, an average vacuum pressure of 80 mbar is maintained
with the help of a liquid ring vacuum pump. The feed saltwater reaches the saturation
temperature corresponding to the evaporator pressure by absorbing heat from the hot
water.
The generated vapour from the evaporator is taken away by the vacuum pump. The
level of feed saltwater inside the evaporator can be monitored through the sight glass
attached to it. As it is difficult and not desirable for 100% recovery of freshwater from
seawater due to an increasing level of concentration and scaling problems, part of the
feed saltwater which is not evaporated is either returned to the feed tank or purged to
the drain by means of a blow down pump.
A continuous flow of chilled water is maintained inside the vacuum pump. The
vapour is condensed by directly mixing with the chilled water and returned to the
chilled water tank. The water is then recirculated to the vacuum pump at a

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100
temperature less than 15
0
C. The vapour production is measured from the difference of
feed flow rate and rejected brine flow rate using a flow totalizer. The level difference
in the feed water tank in a continuous steady state operation also indicates the vapour
production when rejected brine solution is returned to the feed tank.

a. Material Carbon Steel.
Thickness: 2 mm
b. Capacity 600 litre
c. Insulation Rockwell insulation with aluminum
jacket.
Thickness: 50 mm
4.Liquid ring
Vacuum Pump
a. Rotor Star type rotor made of bronze
b. Capacity

250 m
3
/hr at 80 mbar
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4.2 Design of the components
In designing the desalination system, the careful selection and sizing of the
components were made for smooth running of the system. A photograph of the system
is shown in Figure 4.2.

Figure 4.2 A Photograph of the desalination Rig
Details of different components of the system and their design considerations are discussed in
the following section.
4.2.1 Evaporator

arrangement in the tube-bundle is of triangular pitch having a pitch of 25.5 mm in
equilateral triangle. The clearance between the tubes is 6.5 mm.
Four different kinds of tube profiles have been considered for the evaporator design in
this research work. These are:
a. Aluminum Brass Tube.
b. Smooth Cu-Ni (90-10) Tube.
c. Corrugated Cu-Ni (90-10) Tube.
d. PTFE Coated smooth Aluminium Tube.
Each of the tube profiles has been discussed here.
4.2.1.1 Single-fluted Aluminum Tube
As the fluted surface exhibits higher heat transfer performance when used in the
evaporator of a thermal desalination system, single-fluted Aluminum tube profile has
been considered for this study. As there were several past investigations on the
double-fluted tube documented in the available literature, the aim here is to
investigate the thermal performance of the evaporator using tube profile with fluted
outside surface.
Aluminum is considered here as tube material for its superior thermal conductivity
and popularity in desalination industries from an economic point of view. The inside
and outside diameter of the tube are 13 mm and 19 mm, respectively. The thickness of
the tube is 3.25 mm and the length is 500 mm. The tubes are joined with shell and
using grommet joint at the bottom and top cover of the shell. The maximum
permissible pressure for this tube bundle is 4 bar. Figure 4.5 shows the cross section
of fluted tube profile.

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4.2.1.2 Smooth Copper-Nickel (90-10) Tube Profile
Smooth Copper-Nickel (90-10) tube profile has been considered for the second tube-

105
4.2.1.4 PTFE-Coated Smooth Aluminum Tube Profile
Scaling is considered to be the most serious problem in the operation of a desalination
unit. Several research investigations have been made to minimize scaling (Aly et al.,
2003, El-Dessouky and Ettouney, 2002,

Kalender and Griffiths, 2001). Poly Teflon
coated Aluminum tube was used to find performance in reducing scaling. The aim is
to investigate the thermal performance of the coated tube-bundle and scaling potential
on the inside surface of the tube. As coating inside tube surface is very difficult, the
thickness of the coating is maintained thin (75 micron) for better adhesiveness and
bonding strength. The specifications of the PTFE coating are outlined in Table 4.3.
Table 4.3 Technical Specification of PTFE Coating
Chemical Compound XYLAN 1400 RC/437 Green
Thickness ~75 micron
Dry film thickness 0.7-0.9 mm
Pencil hardness 4-6 H
Adhesion
1.0 mm cross hatch and place in boiling water 15 minutes;
after 5 tape pulls = no effect
Cure test 50 + Firm rubs with MEK soaked cloth = no effect
Thermal resilience
180
0
C (Continuous)
240
0

Heater

0.7 m
Insulation
1.5 m
Cover
socket

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the form of low pressure steam taken either from the waste heat recovery boiler or
bled steam. The tank capacity is 100 litres with a height of 660 mm. Construction
material of the tank is cast iron with a thickness of 2mm. The tank is insulated by 50
mm Rockwell insulation with aluminum jacket to prevent heat loss. The tank includes
2 heaters of 24 kW capacities to heat the recirculating water in a temperature range of
45-65
0
C. Temperature of tank is controlled by a temperature controller with a solid-
state relay. The function of controller is to maintain the temperature of the tank at the
desired temperature. A RTD sensor is used to measure the temperature of the tank.
The schematic diagram of the tank is shown in Figure 4.7.

Figure 4.7 Schematic diagram of the heating medium tank
4.2.4 Vacuum pump
The evaporator pressure is maintained in a range of 80-100 mbar by a liquid ring
vacuum pump. The capacity of the vacuum pump is 250 m
3
/hr at 80 mbar. A star

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4.3 Test procedure
A series of experiments were conducted in order to investigate the thermal
performance of the system under different operating conditions. During each
experiment the following procedure was carried out.
• The feed tank was first filled with supply water and the level was checked by
level gauge. As there was difficulty in accessing the actual pretreated
seawater, the feed water was mixed with desired amount of salts for a fixed
concentration in order to simulate the seawater. The mixing was done by the
feed pump in recirculation/by pass flow to the tank. The concentration of the
feed brine solution was checked by a conductivity meter.
• A definite amount of Ameroyal (Anti-scaling agent, 25 ml/m
3
) was mixed
with the feed water to prevent excessive foam formation inside the evaporator.
• The feed temperature was checked and the feed water was heated by the heater
on the set temperature of a given operation in the system.
• Hot water tank was checked and filled up with water.
• The heater of the hot water tank was switched on to set the temperature at
desired condition.
• The vacuum pump was started and continued to run until the pressure was set
to the minimum value. The chill water pump was started to circulate chilled
water from the chiller water tank to the vacuum pump.
• Feed water and hot water pump were started at desired flow rates in


1.5~3.5

Feed water temperature (
0
C)

30~50

Heating medium temperature (
0
C) 47-65
Chamber pressure (mbar) 60~130
Concentrations (ppm) 15,000; 25,000; 30,000 and 35,000

4.4 Vertical Single tube heat exchanger
Multi effect desalination consists of a number of evaporators/effects. These effects
individually make up the performance of the whole MED system. So, a single tube
heat exchanger experimental set up was used to study different tube profile (with or
without inserts) to find the heat transfer enhancement characteristics in detail and
improve the performance of the MED system. The objective of using this set up was
to get deeper understanding of the heat transfer mechanism inside a tube under the
same conditions as it would undergo in a desalination effect. It should be noted that,
this fabricated system is able to take into account all the possible complexities
encountered in a fully functional multi-effect desalination unit. This experimental

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112
conductivity, the results obtained for copper is not used for analysis here. All the
evaporator tubes come with the exact dimensions given below.

Figure 4.10 Schematic of the single tube experimental setup.

The Figure below shows four different heat exchanger tube materials to be tested:

Figure 4.11: Picture of the four evaporator tubes used
In the desalination industry today, different materials are used for evaporators. The
common materials used and their respective pros and cons are given in Table 4.4.
Feed water
bath Valve Temperature
sensor

Flow meter


twelve tubes are four smooth tubes of four different materials namely: Copper Nickel,
Aluminum Brass, Stainless Steel and Copper. The remaining eight tubes are
corrugated profile Aluminum Brass tubes with varying corrugation pitch and depth.
Figure 4.12 illustrates the dimensions for the corrugated tubes. Table 4.5 summarises
the specifications of the twelve evaporator tubes. Figures 4.13, 4.14 and 4.15 depict
both the smooth and corrugated tubes. Experiments
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114

Figure 4.12 Corrugated tube dimensions: Corrugation pitch p and corrugation depth h. Figure 4.13 Four smooth evaporator tubes of different materials: Copper Nickel,
Aluminum Brass Stainless Steel and Copper.
Table 4.6 Specifications of the twelve evaporator tubes
Material Type
Length
(mm)
Inner
Diameter
(mm)
Corrugation
Pitch p (mm)
Corrugation
Depth e (mm)
Copper


0.3

15 0.5
20 0.3
20 0.5

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25 0.3
25 0.5 Figure 4.14 Two evaporator tubes of different corrugation depths: (Top) Corrugation
depth 0.2mm, (Bottom) corrugation depth 0.6mm.
Figure 4.15 Three evaporator tubes of different corrugation pitch: (Top) Corrugation
pitch 10mm, (Centre) corrugation pitch 15mm, (Bottom) corrugation pitch 25mm. Experiments
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4.4.1.5 Tube Shell Setup Figure 4.18 3D view of shell tube heat exchanger .

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The most critical component in the entire experiment would be the tube shell setup as
it is where heat exchange occurs between the hot water and feed water. The heat
energy imparted from the hot water allows for flashing of the feed water to manifest.
Figure 4.18 shows a 3D graphical representation of the tube shell setup.
As observed from Figure 4.19, the hot water from the hot water bath enters from the
top, down the annulus and exits the shell side at the bottom. Feed water is introduced
from the bottom and it flows through the evaporator tube mounted in the centre,
making this a counter-flow configuration.

Figure 4.19 Flow pattern within the shell tube heat exchanger with insert
(Longitudinal view).
4.4.1.6 Measuring Instruments

components as well as the entire experimental setup. Figure 4.20 RTDs are connected to this data logger where temperatures are recorded. Experiments
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120

Figure 4.21 Flow meters which are used to regulate the heating medium and feed flow
rates.

Figure 4.22 Vacuum pump.

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Figure 4.23 Photograph of the experimental set up
4.4.1.7 Experimental Procedure
Before commencement of the experimental run, the various apparatus and equipment