Exergoeconomic analysis of a combined water and power plant 3

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 wate

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 (45-700C) 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

The evaporator is provided with a recirculation system for returning the blow down

saltwater back to the feed tank

Figure 4.1 Schematic diagram of the desalination system

4.1.1 Operation of the desalination unit

The desalination unit is operated under different conditions Experiments were carried

out by using both real and simulated seawater (salt mixed with water at variable

concentrations) (15,000-35,000 ppm) was used as feed The results obtained did not

differ significantly The feed tank with a capacity of 600 litres contains the salt water

A water level indicator (refer to Figure 4.2) is attached to the feed tank The feed tank

is connected to the evaporator through copper pipes, valves, and fittings A flow meter

indicates the volume flow rate of feed water to the evaporator The feed tank has a

heating element of 15 kW to control the temperature of the feed water entering the

evaporator

The feed pump circulates the feed water to the evaporator Feed water flows through

the tubes from the bottom of the evaporator The heating medium tank contains hot

water as the heating fluid in the shell-side of the evaporator It has a capacity of 100

litres with a heating capacity of 24 kW The temperature range for the hot water is

45-650C

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

temperature less than 150C 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

Concentrations (ppm) of the feed brine solution, product fresh water and rejected

brine solutions are measured by a conductivity meter which indicates the salinity The

operation of the desalination plant is controlled by a control panel installed in the rig

All the pumps and motors are provided with on-off control The temperatures of the

feed tank and heating medium tank are controlled by temperature controllers

4.1.2 Specifications of the components

Table 4.1 Components Specifications

1 Evaporator

a Shell and Tube Shell and Tube type Evaporator

Total number of tubes: 175

b Tube Material &

Geometries

I Single-fluted Aluminum

II Smooth Copper-Nickel

III Corrugated Copper-Nickel

IV PTFE coated Aluminium tube

c Shell Material Carbon Steel with 9.5 thickness

d Insulation Rock wool insulation

Vacuum Pump

a Rotor Star type rotor made of bronze

b Capacity 250 m3/hr at 80 mbar

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

The design of the evaporator is crucial for any thermal desalination process Efficient

design of the evaporator can considerably enhance the thermal performance of a

desalination system The evaporator used in the single-effect desalination system for

experimental study is basically a shell and tube two-phase heat exchanger Hot water

is allowed to flow through the shell side of the evaporator from the top There is a

baffle in the middle of the evaporator to enhance the heat transfer performance in the

shell-side The evaporator mainly consists of three parts: the top cover, shell with the

tube-bundles and bottom cover There is a sight glass in the top cover of the

evaporator for visual observation of the water level in the evaporator The evaporator

is shown in Figure 4.3

Figure 4.3 A Photograph of the Evaporator

In total, there are 175 tubes inside the tube bundle Inside diameter of the shell is 400

mm with a thickness of 9.5 mm The tube layout in the bundle is shown in Figure 4.4

Figure 4.4 The layout of tubes inside the evaporator

The shell construction material is Carbon steel having a length of 500 mm The tube

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

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-bundle of the evaporator in this study The advantage of Copper-Nickel (90-10) lies in

its superior thermal conductivity and can be fabricated thin compared to other

tube-profiles To be used preferably in marine conditions, as it forms a protective film which is

multi-layered in flowing seawater It can resist marine bifouling The tube thickness

considered for this study is 1.65 mm The inside and outside diameters of the tube are

13 mm and 16.2 mm, respectively The tubes are connected to the shell by the

expansion and O-ring

4.2.1.3 Corrugated Copper-Nickel (90-10) Tube Profile

Corrugated copper-nickel (90-10) material was used for the third tube-bundle The

thermal performance of the evaporator with corrugated Cu-Ni tube profile has been

compared with that of smooth profile The corrugated tube-bundle is shown in Figure

4.5

Figure 4.5 Corrugated Cu-Ni (90-10) Tube-bundle

The details of the geometry of the corrugated tubes are shown in Table 4.2

Table 4.2 Specification of Corrugated Cu-Ni (90-10) tube profile

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

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

1800C (Continuous)

2400C (Intermittent) Ref: Technical bulletin of Whitford Pte Ltd

4.2.2 Feed Water tank

The system contains a feed tank of 600 litres capacity to store the feed water The

feed tank is shown in Figure 4.6 The tank is cylindrical type with a height of 1.5 m

and inside diameter of 0.7 m The tank is made of stainless steel (SS 316) with a wall

thickness of 2 mm A level gauge is attached with the tank to observe the water level

in the tank The tank is insulated with rockwool covered by an aluminum jacket

Three heaters each with a capacity of 5 kW are included inside the tank for preheating

the feed water The temperature inside the tank is controlled by a temperature

controller with a solid-state relay A RTD sensor is used to measure the temperature

of the water

Figure 4.6: Schematic diagram of the feed water tank

4.2.3 Heating Medium Tank

Hot water is considered as the heating medium for the desalination system instead of

steam The idea is to implement the waste heat utilization concept which may be in

Vent with valve

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

45-650C 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 m3/hr at 80 mbar A star

type rotor rotates inside the pump and there is a continuous supply of chiller water for

efficient operation Figure 4.8 shows the photograph of the vacuum pump

Heate

0.455 m

Insulation

Vent with valve Socket point

660 mm

Cover Socket

point

Figure 4.8 A photograph of the Vacuum pump

4.2.5 Blowdown pump

A specially designed blowdown pump is used in the system for recirculation of the

rejected brine from the evaporator The blowdown pump is of low NPSH (net positive

suction head) as it has to work against negative pressure in its inlet The NPSH of this

centrifugal blowdown pump is 0.5 and its impeller is made of stainless steel, as it has

to deal with brine solution Figure 4.9 shows the blow down pump

Figure 4.9 A photograph of the Blowdown pump

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/m3) 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

continuous manner

• After starting the blowdown pump, a particular flow rate was fixed by

adjusting the valve at the by pass line

• Steady state condition was attempted for each experimental run by observing

the different operating variable of the system Once the steady state was

obtained, different measurement values were noted

• The pressure inside the evaporator was maintained fixed by controlling the

vacuum pump flow rate

• The data were taken for each 15 minutes of operation

• The concentrations of the rejected brine and product water were monitored by

the conductivity meter

The changes of the operating variables during the experimental investigations are

listed in Table 4.4

Table 4.4 Operating variables and their ranges during the experiment

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

setup provides a good platform to examine the heat transfer capabilities of single tube

evaporators with permutations (refer to Figure 4.24) in material, different

augmentation techniques as well as other experimental variables The detailed

technical specification and its characteristics are described in the following sections

4.4.1 Experimental Setup

A lab scale single tube vertical heat exchanger was designed and fabricated, as shown

in Figure 4.10 in the thermal process lab of National university of Singapore to

investigate the effect of pitch and depth on heat transfer characteristics The single

evaporator tube experiment was set up in the thermal process laboratory after coming

up with its modular design, fabrication and testing All of the components used, were

either fabricated locally or obtained directly from the laboratory, except for the

evaporator tubes

4.4.1.1 Evaporator Tubes

Four different types of materials were used for the evaporator tubes, namely Stainless

Steel, Aluminum-Brass, Copper-Nickel and Copper However, the copper evaporator

tube exhibited too much of corrosion effects despite running only a few sets of

experiments (total run time of about 3 hours) Even though copper is a good

conductor of heat with a thermal conductivity of 401W·m−1·K−1 (second only to

silver), it would not be economically feasible to be replacing the tube so often due to

corrosion Hence, in choosing the best material, the life cycle of the evaporator tube

also has to be given consideration and thus, despite copper having the best

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

Table 4.5 Common Materials used as MED evaporator tubes in Desalination industry

Copper and its constituent alloys Best performance index, but relatively

poor corrosion resistance

Brasses (Alloy of copper and zinc) Slightly better resistance to corrosion but

lower thermal conductivity

Stainless Steel

High corrosion resistance and industrial life (up to 40 years) but lower conductivity compared to other materials

Aluminum Bronzes Lower cost and better conductivity than

steel tubes

Nickel iron aluminum bronze Highly resistant to corrosion but costly

4.4.1.2 Evaporator Tubes profiles

In total, twelve evaporator tubes were fabricated locally for this project Among the

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