COOLING TOWER DESIGN FOR CENTRAL GENERATORS OF CUET, BANGLADESH. Mohammad Sharif Khan, Golam Mainuddin, Abu Sadat Mohammad Sayem, Nadeem Nafis

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1 Proceedings of the 4 th BSME-ASME International Conference on Thermal Engineering 7-9 December, 008, Dhaka, Bangladesh COOLING TOWER DESIGN FOR CENTRAL GENERATORS OF CUET, BANGLADESH. Mohammad Sharif Khan, Golam Mainuddin, Abu Sadat Mohammad Sayem, Nadeem Nafis ABSTRACT: Department of Mechanical Engineering, Chittagong University of Engineering & Technology (CUET) Chittagong, Bangladesh. sharif_khan587@yahoo.com Cooling tower has a very crucial impact on any kind of power plant like steam power plant, nuclear power plant, gas turbine power plant etc. as the segment of circulating water system. The circulating water system supplies cooling water to the turbine condensers with the assistance of the cooling tower. The system also supplies lesser amounts of auxiliary cooling water for turbine and steam generator buildings, for the fire protection system. Cooling tower is not installed only for a power plant but also in industries of various kinds for cooling purposes like paper mills, garments, chemical industries etc. Our concentration regarding the project and thesis is to design a cooling tower for the central generators of CUET to remove the heat from the generators during operation. There are three gas generators in generator house of CUET each of 0KW rated output. Each generator set is fitted with a radiator engine cooling by cooling water. Radiator is of forced convective type installed with a fan of 5 KW capacities. Thus the three fans consuming about 8% of output power & in addition the fans produce noise. So we have decided to replace this radiator cooling system by an appropriate induced draft wet cooling tower.. KEYWORDS: Power Generator, Wet Cooling Tower, Counter Flow.. INTRODUCTION: Cooling towers are heat exchangers that are used to dissipate large heat loads to the atmosphere. When water is used as the heat transfer medium, wet, or evaporative, cooling towers may be used. Wet cooling towers rely on the latent heat of water evaporation to exchange heat between the process and the air passing through the cooling tower. Although cooling towers can be classified several ways, the primary classification is into dry towers or wet towers, and some hybrid wet-dry combinations exist. In wet cooling towers, heat transfer is measured by the decrease in the process temperature and a corresponding increase in both the moisture content and the wet bulb temperature of the air passing through the cooling tower. Wet cooling towers typically contain a wetted medium called "fill" to promote evaporation by providing a large surface area and/or by creating many water drops with a large cumulative surface area. Our goal is to design an induced draft counter flow type wet cooling tower for three generators of capacity 50KW each. The main advantages of mechanical draft cooling towers are the assurance of moving the required quantity of air at all loads and climate conditions, low initial capital and construction costs and a low physical profile. Counter flow type is more efficient then counter flow and wet cooling tower has

2 greater heat transfer rate than dry cooling tower. Circulating cooling system of water with the help of cooling tower is more efficient than radiator. The present trend in materials for wet cooling towers favors concrete structures with plastic fill, drift eliminators also with plastic, fan stacks fan blades with galvanized steel, valves and nozzles. The concrete-plastic combination results in longer life and less maintenance.. OBJECTIVES: Although there is a radiator with each generator unit, the cooling tower for these generators circulating water can be a right choice due to following reasons. To improve heat transfer from the generator engine. To save energy consumption by radiator fan. To reduce vibration and hence noise due to radiator fan. 3. METHODOLOGY: For designing a cooling tower following steps can be followed. To study about the various components and terminology of cooling tower theoretically. To collect related data or information about circulating water system of water jacket of the engine of gas generator. To study for designing suitable cooling tower. To select the appropriate type of cooling tower according to optimal sense. To design the selected cooling tower. 4. COOLING TOWER: Cooling towers may either use the evaporation of water to reject process heat and cool the working fluid to near the wet-bulb air temperature or rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, chemical plants, power plants and building cooling. The towers vary in size from small roof-top units to very large hyperboloid structures that can be up to 00 meters tall and 00 meters in diameter, or rectangular structures that can be over 40 meters tall and 80 meters long. Smaller towers are normally factory-built, while larger ones are constructed on site. 4. Working Principle of Wet Cooling Tower: Wet cooling towers have a hot-water distribution system that showers or sprays the water evenly over a lattice network of closely set horizontal slats or bars called fill, or packing. The fill thoroughly mixes the falling water with air moving through the fill as the water splashes down from one fill level to the next by gravity. Outside air enters the tower via louvers in the form of horizontal slats on the side of the tower. The slats usually slope downward to keep the water in the intimate mix between water and air enhances heat and mass transfer, which cools the water. Cold water is then collected in a concrete basin at the bottom of the tower where it is pumped back to the condenser or returned to the natural body of water. Now the hot, moist air leaves the tower at the top. 3

3 5. COOLING TOWER DESIGN & CALCULATION: 5. Design Layout: Fig : Design Layout Fig 3: Make up water system Wet cooling tower calculations involve energy and mass balances. The energy balances will be based on the first law steady state steady flow equation. Three fluids entering and living the system:. the cooling water. the dry air 3. the water vapor in the air 5. Energy Balance: Hot water in A Hot air out (humid) W A, h WA h a, ω, h v Ρ i Cold air in h a, ω, h v B W B, h WB Cold water out 3 ρ o 4

4 Where, h a = Enthalpy of dry air (J/Kg) ω = absolute humidity, mass of water vapor per unit mass of dry air. h v = Enthalpy of water vapor (J/Kg) W = Mass of circulating water per unit mass of dry air h ω = Enthalpy of circulating water(j/kg) The subscripts & refer to air inlet and exit and the subscripts A & B refer to circulating water inlet & exit respectively. The air leaving the system at is often saturated. h v From steam table, saturated vapor at given temperature h ω From steam table, saturated liquid at given temperature air = dry air + water vapor Neglecting the energy of make up water Energy in = Energy out Energy in by dry air + Energy in by water vapor + Energy in by circulating water = Energy out by dry air + Energy out by water vapor + Energy out by circulating water. h a + ω h v + W A h WA = h a + ω h v + W B h WB () Because of the low pressures and temperatures commonly encountered in towers, the above equation can be simplified with little error by the following approximations. For constant specific heats, h a h a = c pa (t a t a ) () 5.. Mass Balance: Loss of circulating water = Increase of water vapor in the air = make up water W A - W B = ω ω (3) 5.. Height of the Cooling Tower: P d = (ρ o - ρ i) H (4) Where, ρ o = outside air density ( at inlet air temperature) ρ i = inside air density at the exit of the fill. H = Height of the cooling tower. P d = driving pressure which should equal to the air pressure losses in the tower. 6. CONSIDERATION FOR DESIGN: Considering the design for different climate condition (winter, summer and rainy season). Neglecting forced convection heat transfer through the pipe. Tower may be erected on the roof of the generator room for availability of good flow of air. 6. Heat Removal by the Radiator: Cooling water heat release = 93 KW, Cooling water flow rate = 3.7 Kg/s, Cooling water heat release, Q = MC (T wi T wo ) Where, So, Q = MC (T wi T wo ) So water temperature drop by the radiator = 6 C 4 5

5 ω 6. Winter Season Let, the range = 80 C - 70 C = 0 C & the approach = 70 C - 5 C = 55 C 6.. Energy Balance: From equation () h a + ω h v + W A h WA = h a + ω h v + W B h WB W A h WA = C pa (t a t a ) +ω h v +W B h WB -ω h v 3 h WA = 340 KJ/Kg of water (at 50 C) from steam table, v WA =.00 m / Kg of water P sat = bar (at 8 C) from steam table, h v = 55.7 KJ/Kg vapor (at 8 C) from steam table W A h WA = C pa (t a t a ) + ω h v + W B h WB - ω h v, W A =.07Kgwater / Kgdryair 6.. Dry Air Required: = Circulating water inlet flow rate (m 3 /sec) / W A = 3.45 Kg da/sec = 07.47Kg da/min 6..3 Make Up Water: = ω ω Kg water/ Kg da = Total Outside Air Required: 0 3 m 3 /min = Dry air required + water vapor = Dry air + dry air ω = = Kg air/ min = 7.09 m 3 /min 6..5 Air Densities: P Pv Pv ρ o = + =.9 Kg/m 3 P Pv Pv ; ρ i = + =. 56 Kg/m 3 R T R T R T R T a 6..6 Height of the Tower: v From equation (4) H = 4.85; assumption, = 3 N/m 6..7 Power Requirement of the Pump: Pd a 5 Power, P = ρ gh Q, h = H + H, Where, H = height of the generator room, H = height of the tower H = 3.04 m, H = 4.85 m, h = H + H = = 7.89, hence P = 0.4 hp 6..8 Cooling Tower Efficiency: The efficiency of the cooling tower can be illustrated as % Where, t i = water inlet temp r. t o = water outlet temp.t wb = wet bulb temp of inlet air, Similarly the values of different properties can be calculated for summer & winter season. From which values can be tabulated, given below. v 6

6 Climatic condition Table.: Values of different parameters in various climatic conditions. Dry air require d (Kg/mi n) Make up water (m 3 /min) Outside air required (m 3 /min) Height of the tower (m) Power reqd. of the pump (hp) Cooling tower efficiency µ (%) Winter Summer Rainy season The highest values from the table from each column can be taken for consideration of factor of safety in design. 7. CONCLUSION: The proper design of cooling tower will ensure the longevity of the generator. The design of a cooling tower is really cumbersome task. The basis of my design was to determine basically the height of the tower. The radiator of each generator can lessen maximum of 6 C temperature of circulating water. What should be the maximum height of the tower for reducing temperature of 0-5 C was my goal to find out. Some heat also must be transferred by forced convection, radiation and conduction through the pipe that I neglected from calculation. I have tried my level best to design the cooling tower in optimal sense. Different companies I have visited that I mentioned earlier adopted a heat exchanger for transferring heat from generators circulating hot water to cooling tower cold water. They have adopted the heat exchanger for avoiding natural dust, debris, turbidity etc. and installed for high capacity generator unit (MW, MW) etc. By considering the cost effectiveness I didn t use the heat exchanger in my design because the heat exchanger is costly (a shell & tube type heat exchanger is cost about Tk ). In place of heat exchanger I have used a filter to remove natural fouling. I have also proposed in my design to make the tower wooden structured in place of steel by considering optimal cost sense. Above all whether I have designed properly or not but I have accomplished a deep knowledge about cooling tower design as well as construction that will make me pioneer for engineering. 8. REFERENCES: [] M.M. EI Wakil, st printed 985, Power Plant Technology. [] Gustaf A.Gaffert, fourth edition, Steam Power Stations. [3] John Maulbetsch, Maulbetsch Consulting, May 003, Cooling System Retrofit Costs EPA Workshop on Cooling Water Intake Technologies. [4] Department of Energy, Office of Fossil Energy's Power Plant Water Management R&D Program. 6 7