Exergoeconomic analysis of a combined water and power plant 3

Experiments CHAPTER 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 Multieffect 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 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. ___________________________________________________________________________ 97 Experiments 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 ___________________________________________________________________________ 98 Experiments 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 45650C. 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 countercurrent 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 ___________________________________________________________________________ 99 Experiments 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 a. Shell and Tube 1. Evaporator b. Tube Material & Geometries. c. Shell Material d. Insulation a. Material 2. Heating Medium b. Capacity Tanks c. Insulation a. Material 3. Feed Tank b. Capacity c. Insulation 4.Liquid ring Vacuum Pump a. Rotor b. Capacity Shell and Tube type Evaporator Total number of tubes: 175 I. Single-fluted Aluminum. II. Smooth Copper-Nickel. III. Corrugated Copper-Nickel. IV. PTFE coated Aluminium tube Carbon Steel with 9.5 thickness. Rock wool insulation Thickness: 25 mm Carbon steel Thickness: 2.5 mm. 100 litre Rockwell insulation with aluminum jacket. Thickness: 50 mm Carbon Steel. Thickness: mm 600 litre Rockwell insulation with aluminum jacket. Thickness: 50 mm Star type rotor made of bronze 250 m3/hr at 80 mbar ___________________________________________________________________________ 100 Experiments 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 ___________________________________________________________________________ 101 Experiments 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 ___________________________________________________________________________ 102 Experiments 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 bar. Figure 4.5 shows the cross section of fluted tube profile. ___________________________________________________________________________ 103 Experiments 4.2.1.2 Smooth Copper-Nickel (90-10) Tube Profile Smooth Copper-Nickel (90-10) tube profile has been considered for the second tubebundle 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 tubeprofiles. 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 Helix angle Pitch Groove width 69.8 degree 7.25 mm 2.25 mm ___________________________________________________________________________ 104 Experiments Tube Thickness Inside diameter Outside diameter 1.65 mm 13 mm 16.2 mm 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 tape pulls = no effect Cure test 50 + Firm rubs with MEK soaked cloth = no effect 1800C (Continuous) Thermal resilience 2400C (Intermittent) Ref: Technical bulletin of Whitford Pte. Ltd. ___________________________________________________________________________ 105 Experiments 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 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 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. Vent with valve Cover socket Insulation 1.5 m Heater 0.7 m 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 ___________________________________________________________________________ 106 Experiments 25 25 0.3 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. ___________________________________________________________________________ 115 Experiments 4.4.1.3 Wire Coil Insertions The wire coil insertions used in this experiment for heat augmentation purposes are basically springs made from stainless steel grade 316. Unlike the copper zinc insertions used initially, these stainless steel insertions not have a central cylindrical rod. This drastically reduces the pressure drop experienced by feed water flowing through the evaporator tubes and improves energy efficiency. In addition, these insertions closely resemble those featured in earlier literatures. Figure 4.16 shows both insertions used in the experiment. Figure 4.16 A photograph of the insertions used in the experiment 4.4.1.4 Hot and Feed Water Baths Both the hot and feed water baths are of the same model and functions to maintain water temperatures at specific desired values. The feed water bath provides a constant supply of feed water into the evaporator tube for flashing while the hot water bath plays the role of a heater for the incoming feed water. The water baths have a capacity of 10 litres and can supply a maximum pumping power of 25KW. Variables such as the heat medium temperature and feed temperature are altered using the hot water bath and feed water bath, respectively. Thick nylon hoses serve as connection ___________________________________________________________________________ 116 Experiments between the inlets and outlets of the water baths and the copper pipes. Figure 4.17 below shows the water bath used for this experiment. Figure 4.17 Hot water bath 4.4.1.5 Tube Shell Setup Figure 4.18 3D view of shell tube heat exchanger . ___________________________________________________________________________ 117 Experiments 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. Hot water Feed water Figure 4.19 Flow pattern within the shell tube heat exchanger with insert (Longitudinal view). 4.4.1.6 Measuring Instruments The remaining components of the experimental setup include • Resistance temperature detectors (RTDs) attached to a data logger shown in Figure 4.20, • Flow meters (refer to Figure 4.21), • Vacuum pump (refer to Figure no. 4.22), ___________________________________________________________________________ 118 Experiments The RTDs basically measure the temperatures at the inlet and outlet of the tube shell setup. The flow meters are used for the monitoring of the water flow rates from the hot and feed water baths. The function of the vacuum pump is to maintain the pressure at approximately 50°C saturation pressure (12kPa). The heating medium and feed flow rate will be regulated with the use of ball valves. Lastly, the copper pipes and nylon hoses links the components together. The following Figure 4.23 provides a graphical representation of the various components as well as the entire experimental setup. Figure 4.20 RTDs are connected to this data logger where temperatures are recorded. ___________________________________________________________________________ 119 Experiments Figure 4.21 Flow meters which are used to regulate the heating medium and feed flow rates. Figure 4.22 Vacuum pump. ___________________________________________________________________________ 120 Experiments 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 would have to be checked and adjusted to the desired conditions. In the following sections a brief overview of the entire experimental procedure is presented: [1] Both the hot and feed water bath would have to be filled to their maximum capacity. They are then switched on and calibrated to the desired values. The regulators on both water baths are in the ‘closed’ position. ___________________________________________________________________________ 121 Experiments [2] The data logger is switched on and the individual channels are checked for readings to ensure that the RTDs are correctly and properly connected. [3] Once the water baths have reached their designated values, the regulators are switched to the ‘open’ position. The heating medium and feed flow rate are then adjusted to the required values using the ball valves. [4] The vacuum pump is switched on and the data logger is set to record the temperatures from the RTDs. Simultaneously, the stopwatch is started to time the experimental run which last for approximately 60 minutes. The temperatures are recorded at four locations in the tube shell setup: hot water inlet, hot water outlet, feed water inlet and feed water outlet. The temperatures are recorded at minute intervals. [5] The heat medium and feed flow rates have to be monitored and adjusted using the ball valves as they tend to fluctuate throughout the course of the experimental run. [6] Once the experimental run has completed, both the water baths and the vacuum pumps are switched off. The condensate is collected from the vacuum pump and measured in a measuring cylinder. The experimental variables are shown in Table 4.6. Each tube configuration is tested against these variables. Table 4.7 Experimental variables. Variables Heat Medium Temperature Feed Temperature Heating Medium Flow Rate Feed Flow Rate Units Range °C °C Gallons per minute GPM Gallons per minute 65 - 85 (5°C interval) 25 - 65 (10°C interval) 0.3 - 0.7 (0.1GPM interval) 0.05 - 0.15 (0.025GPM GPM interval) ___________________________________________________________________________ 122 Experiments 4.4.1.8 Instrumentation The system is equipped with appropriate instruments to measure different operating variables. Short descriptions of the different instrumentations used in the experiment are given below. The calibration graphs of these equipments are enclosed in Appendix B. RTD Sensors The system is instrumented with RTD sensors for measuring temperatures at different location of the feed and hot water flow. Two RTDs are used in the two tanks as temperature sensors to control the temperature of the tank. Two RTDs are provided at the inlet and outlet of the vacuum pump to measure the chill water temperature. Flow meter The flow rates of the feed water and hot water are measured by the flow meters each with 3.5 m3/hr capacity. Flow totalizer A flow totalizer is used in the system for noting the difference between the feed flow rate and rejected brine flow rate which gives the indirect measurement of the vapour production for a given experimental run. It is a turbine type flow totalizer. Pressure gauge ___________________________________________________________________________ 123 Experiments To measure the inlet and outlet pressure of the evaporator two pressure gauges are used in this desalination system. The gauges are dial gauge type having a dial of about 100 mm and the pressure range is from vacuum to bar(g). Conductivity meter To measure the salinity of the feed water, rejected brine and product water, a conductivity meter is used. The conductivity measures the salinity in terms of conductivity of the solution in micro siemens which in turns after calibration indicates the salinity in ppm. Data logger A hydra data logger is used in the system to monitor the temperatures at different locations of the system. 4.5 Uncertainty analysis Uncertainty analysis for the system has been made in Appendix C of the system. The instruments have the following uncertainties. Table 4.8 Uncertainty of measurement Instruments RTD Pressure gauge Flow meter Level gauge Uncertainty ±0.10C ±0.025 bar ±0.025 m3/hr ±0.05 m3/hr ___________________________________________________________________________ 124 Experiments Elements of Experiment Different Profile for Evaporator Tube Smooth Tubes Smooth Tubes with Insertion Corrugated Tubes Copper Nickel Aluminum Brass Stainless Steel Copper Copper Nickel Aluminum Brass Stainless Steel Copper All Aluminum Brass with variable corrugation pitch and depth (Refer to Table 4.1) Heat Medium Temperature Pressure Drop Heat Flow Rate Overall Heat Transfer Coefficient Heat Flow Rate Specific Heat Transfer Area Water Production Rate Figure 4.24 Diagram showing different variable of the experimental set up ___________________________________________________________________________ 125 Feed Flow Rate Performance Ratio Experiments 4.6 Experiments on Reverse Osmosis system Since RO involve membranes that are highly susceptible to fouling and damage, certain procedures and precautions must be adhered to when running the systems. Lack of understanding of the systems can lead to a rapid and irreversible damage to the membranes. It is thus necessary to become familiar with the basic components and operations of the RO systems. 4.6.1 Operations of the RO system The laboratory RO set-up was designed for a product capacity of 5m3/day. A schematic diagram of the RO system is shown in Figure 4.25. A 1m3 tank contains the feed water to be passed through the RO system. To ensure that steady state conditions are achieved, a copper coil is attached to a constant temperature bath and immersed in the feed water tank. This serves to keep the feed water temperature constant at all times. A submersible pump is also lowered into the feed water tank to ensure proper mixing of the feed water before it enters the RO system. The feed water is pumped through two built-in cartridge filters (a 10 µm one, followed by a 1µm one) via a centrifugal booster pump. The cartridge filters remove any impurities or suspended particles within the feed water. The booster pump delivers the feed water at a pressure of about 5.14 bar in order to pump the water through the cartridge filters and provide sufficient head for the high pressure pump. The high pressure pump provides the high pressure needed (55.16 bar to 62.16 bar) to carry out the RO process. A pressure regulating valve is used to control the pressure into the RO membranes. This globe valve controls the amount of reject water. When closed, less reject is allowed to pass through, resulting in higher pressure into the membranes. At higher pressure, there is a higher volume flow rate of product water and lower volume flow rate of reject water which is indicated by the reject and product flow meters. ___________________________________________________________________________ 126 Experiments Flash tank Spiral wound membrane Feed water tank Booster pump Cartridge filters High Pressure pump PRV Figure 4.25 Schematic of the RO experitmental set up Legend Valve Temperature sensor Pressure gauge Solenoid valve Pressure relieve valve Pressure regulator ___________________________________________________________________________ 127 Flow meter Experiments Figure 4.26 Photograph of the RO experitmental set up The feed water is channeled to one of two membranes installed in the RO system. Two spiral wound membranes by Hydranautics are installed in a parallel arrangement and only one membrane can be used at a time. Detailed specification of the membrane is given in Appendix D. Usage of the membranes is controlled by a series of valves which must be opened and closed manually. The feed water is then separated into a product and reject stream by the membranes. Both streams are channeled back to the feed tank where they are mixed before being re-circulated back to the RO system. 4.6.2 Calibration of measuring instruments Temperature and flow rates are measured by respective gauges on the RO system. Together with the conductivity meter, which measures the salinity of the water, the gauges have to be calibrated before the initial rounds of experiments begin so as to allow for actual readings to be obtained. Calibration charts are shown in Appendix B. ______________________________________________________________________________ 128 Experiments 4.6.3 Experimental program Saline water is used to simulate actual seawater. While the concentration of seawater varies at different parts of the world, seawater around Singapore generally has a concentration of around 33,000ppm. During salt water tests, operating conditions and feed water concentration are varied to determine how the RO system reacts to these changes. The range of experimental parameters used is shown in Table 4.9. Table 4.9: Range of experimental parameters 4.6.4 Operating variable Range Interval Feed concentration 25,000ppm – 33,000ppm 5,000ppm Pressure 55.16 bar - 62.16 bar Feed temperature 26oC – 32oC oC Experimental procedure Feed water is prepared by dissolving the appropriate amount of salt (NaCl) in about 500 litres of tap water. It is then dechlorinated by adding 100g of sodium meta-bi-sulphite (Chemkon) to every 500 litres of tap water. This step is crucial because the RO membranes cannot tolerate chlorine. The temperature of the constant temperature bath is set to 26oC to keep the feed water at that temperature. The submersible pump and RO system are put into operation. A few minutes are allowed for the water in the pipes, modules and feed tank to be mixed. Pressure into the membranes is set to 700psig by adjusting the pressure regulating valve. ______________________________________________________________________________ 129 Experiments The conductivities of the reject and product water are measured repeatedly until consistent readings are obtained. Reject and product flow rates are also noted and recorded. Steps and are repeated for subsequent pressures of 800psig-1,000psig. Steps 2-6 are then repeated for feed water temperatures of 28oC-32oC. During experiments, it is important to ensure that steady state conditions prevail. This is achieved by ensuring that the reject and flow rates are constant and that the concentration of feed, reject and product lines are constant as well. This requires the material balance across the RO system to be held at all times. 4.6.5 Water analysis To better understand the effectiveness of the RO system, water analysis is carried out to ascertain any improvement in water quality after RO. Samples of saline feed water at 33,000ppm, RO product water (obtained at membrane inlet pressure of 800psig and 28oC) were collected and sent for water testing. The following analyses were conducted: Table 4.10: Water analysis Test Equipment used Total dissolved solids Conductivity Meter LF538 (TDS) Turbidity HACH 2100N Turbidimeter ______________________________________________________________________________ 130 [...]... material balance across the RO system to be held at all times 4.6.5 Water analysis To better understand the effectiveness of the RO system, water analysis is carried out to ascertain any improvement in water quality after RO Samples of saline feed water at 33 ,000ppm, RO product water (obtained at membrane inlet pressure of 800psig and 28oC) were collected and sent for water testing The following analyses... obtained Calibration charts are shown in Appendix B 128 Experiments 4.6 .3 Experimental program Saline water is used to simulate actual seawater While the concentration of seawater varies at different parts of the world, seawater around Singapore generally has a concentration of around 33 ,000ppm During salt water tests, operating conditions and feed water concentration... spiral wound membranes by Hydranautics are installed in a parallel arrangement and only one membrane can be used at a time Detailed specification of the membrane is given in Appendix D Usage of the membranes is controlled by a series of valves which must be opened and closed manually The feed water is then separated into a product and reject stream by the membranes Both streams are channeled back to... evaporator tube for flashing while the hot water bath plays the role of a heater for the incoming feed water The water baths have a capacity of 10 litres and can supply a maximum pumping power of 25KW Variables such as the heat medium temperature and feed temperature are altered using the hot water bath and feed water bath, respectively Thick nylon hoses serve as connection ... operations of the RO systems 4.6.1 Operations of the RO system The laboratory RO set-up was designed for a product capacity of 5m3/day A schematic diagram of the RO system is shown in Figure 4.25 A 1m3 tank contains the feed water to be passed through the RO system To ensure that steady state conditions are achieved, a copper coil is attached to a constant temperature bath and immersed in the feed water. .. 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... 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... in earlier literatures Figure 4.16 shows both insertions used in the experiment Figure 4.16 A photograph of the insertions used in the experiment 4.4.1.4 Hot and Feed Water Baths Both the hot and feed water baths are of the same model and functions to maintain water temperatures at specific desired values The feed water bath provides a constant supply of feed water into the evaporator tube for flashing... fluctuate throughout the course of the experimental run [6] Once the experimental run has completed, both the water baths and the vacuum pumps are switched off The condensate is collected from the vacuum pump and measured in a measuring cylinder The experimental variables are shown in Table 4.6 Each tube configuration is tested against these variables Table 4.7 Experimental variables Variables Heat Medium... Nickel Aluminum Brass Stainless Steel Copper Copper Nickel Aluminum Brass Stainless Steel Copper All Aluminum Brass with variable corrugation pitch and depth (Refer to Table 4.1) Heat Medium Temperature Pressure Drop Heat Flow Rate Overall Heat Transfer Coefficient Heat Flow Rate Specific Heat Transfer Area Water Production Rate Figure 4.24 Diagram showing different variable of the experimental set . Hot and Feed Water Baths Both the hot and feed water baths are of the same model and functions to maintain water temperatures at specific desired values. The feed water bath provides a constant. supply of feed water into the evaporator tube for flashing while the hot water bath plays the role of a heater for the incoming feed water. The water baths have a capacity of 10 litres and can. 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