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INTRODUCTION
The evaporative cooling tower was originally developed as a water conservation device, designed to reduce dependence on “once through” cooling systems and successfully replaced many such systems. The development of the cooling tower also expanded the ability of designers to provide efficient cooling in areas without large water supplies. Cooling towers are now applied widely across a range of industries around the world.
Characteristics of closed circuit cooling tower
• A closed circuit cooling tower in which the process fluid does not contact the cooling air.
• Different types of fluids can be cooled including water, plating solutions, quenching oils, chemicalsolutions, gases, refrigerants and air.
• The fluid cannot contact the atmosphere.
• Only a small quantity of water is required in the open evaporating water circuit.
• Can be natural, forced or induced draft.
• Reduced water treatment and corrosion.
• Reduced pumping requirement.
• Can be heavier and larger than open circuit alternatives.
• Increased fan energy requirement.
• The tubes are sensitive to local dry areas caused by solid deposits.
• Common in industrial applications.
5.3. COMPONENTS USED IN COOLING TOWER
5.3.1. frame & casing
A cooling tower casing acts to contain water within the tower, provide an air plenum for the fan, and transmit wind loads to the tower framework. It must have diaphragm strength, be watertight and corrosion resistant, and have fire retardant qualities. It must also resist weathering, and should present a pleasing appearance.
Currently, wood or steel framed, field-erected towers are similarly cased with fire-retardant fiberreinforced polyester corrugated panels, overlapped and sealed to prevent leakage. Factory-assembled steel towers utilize galvanized steel panels, and concrete towers are cased with precast concrete panels.
Fiber
Fiberglass is a type of fiber reinforced plastic where the reinforcement fiber is specifically glass fiber. The glass fiber may be randomly arranged, flattened into a sheet, or woven into a fabric. The plastic matrix may be a thermosetting plastic – most often epoxy, polyester resin or vinylester, or athermoplastic.
The glass fibers are made of various types of glass depending upon the fiberglass use. These glasses all contain silica or silicate, with varying amounts of oxides of calcium, magnesium, and sometimes boron. To be used in fiberglass, glass fibers have to be made with very low levels of defects.
Fiberglass is a strong lightweight material and is used for many products. Although it is not as strong and stiff as composites based on carbon fiber, it is less brittle, and its raw materials are much cheaper. Its bulk strength and weight are also better than many metals, and it can be more readily molded into complex shapes. Applications of fiberglass include aircraft, boats, automobiles, bath tubs and enclosures, swimming pools, hot tubs, septic tanks, water tanks, roofing, pipes, cladding, casts, surfboards, and external door skins.
Other common names for fiberglass are glass-reinforced plastic , glass-fiber reinforced plastic. Because glass fiber itself is sometimes referred to as "fiberglass", the composite is also called "fiberglass reinforced plastic." This article will adopt the convention that "fiberglass" refers to the complete glass fiber reinforced composite material, rather than only to the glass fiber within it.
Specifications
Material : fiber
Dimensions : 2 feet height
1.5 feet breadth
1.5 feet depth
Fiber sheet : 6mm thickness
5.3.2. Evaporator
The evaporator is kind of heat transfer apparatuses where the heat transfer is done by forced convection or natural convection. And it’s an important component of refrigeration system and air conditioning system.
Evaporation process is rejection of water (or other liquids) by concentrating the solution. The required time for this process can by shortened by increasing the surface area, the solution is exposed to it, or by exposing the solution to heating to a higher temperature.
Specifications
Material : aluminum
Type : simple
draught fan
Cooling tower fans must move large volumes of air efficiently, and with minimum vibration. The materials of manufacture must not only be compatible with their design, but must also be capable of withstanding the corrosive effects of the environment in which the fans are required to operate. Their importance to the mechanical draft cooling tower’s ability to perform is reflected in the fact that fans of improved efficiency and reliability are the object of continuous development.
Propeller type fans predominate in the cooling tower industry because of their ability to move vast quantities of air at the relatively low static pressures encountered. They are comparatively inexpensive, may be used on any size tower, and can develop high overall efficiencies when “system designed” to complement a specific tower structure.
The rotational speed at which a propeller fan is applied typically varies in inverse proportion to its diameter. The smaller fans turn at relatively high speeds, whereas the larger ones turn somewhat slower. This speed-diameter relationship, however, is by no means a constant one. If it were, the blade tip speed of all cooling tower fans would be equal. The applied rotational speed of propeller fans usually depends upon best ultimate efficiency.
Specifications
Voltage : 220-240V AC
Frequency : 50Hz
Power : 35W
Current : 0.23Amps
Material : plastic
5.3.4. Electrical motor
Electric motors are used almost exclusively to drive the fans on mechanical draft cooling towers, and they must be capable of reliable operation under extremely adverse conditions. The high humidity produced within the tower, plus the natural elements of rain, snow, fog, dust, and chemical fumes present in many areas combine to produce a severe operating environment.
Specifications
Voltage : 220V AC
Frequency : 50Hz
Power : 35W
Material : plastic
Rotations : 1200rpm
Ability to lift : 1.7m
5.3.5. Water distribution system
Distribution systems are subjected to a combination of cold water and maximum oxygenation. Therefore, the materials utilized should be highly resistant to both corrosion and erosion. Historically proven materials are hot-dip galvanized steel, cast iron, and redwood stave pipe. Because of the relatively low pressures to which cooling tower piping is subjected, the use of various types of plastic pipe and nozzles has also become a mark of quality construction. Except for relatively small diameters, the plastic pipe utilized is usually fiber reinforced. Precast and prestressed concrete pipe and flumes are also utilized on concrete towers.
Specifications
Material : pvc pipe
Size : 0.5 inch diameter
5.3.6. Valves
Valves are used to control and regulate flow through the water lines serving the tower. Valves utilized for cooling tower application.
Nozzles
Plastics are also widely used for nozzles. Many nozzles are made of PVC, ABS, polypropylene, and glass-filled nylon.
. Working principle of cooling tower
Cooling tower is essentially a heat and mass transfer device. It removes heat from the water and loses a fraction of water during heat transfer to the air. When air is blown from downwards it came in contact with upcoming water the enthalpy of which is higher and at the time of contact this air causes evaporation of water droplet. It is known that in order to get evaporated water requires to gain certain amount of latent heat. It does so from nearby droplet and evaporates into vapour form. Due to removal of sensible heat, remaining water droplets loses temperature and cools down. Due to higher surface area in packing heat transfer rate increases and water is further cooled to the require temperature. Air moving upward takes away certain amount of water content in (vapour form) with it which is not desirable. In order to recover lost water drift eliminators are provided. Upcoming air loses their velocity and causes certain amount of vapour to be converted into water. The water falls downward and collected in the basin
Performance
These measured parameters and then used to determine the cooling tower performance in several ways.
5.5.1. Range.
This is the difference between the cooling tower water inlet and outlet temperature. A high CT Range means that the cooling tower has been able to reduce the water temperature effectively, and is thus performing well. The formula is:
Equation 1 CT Range
? (°?) = ? (°?)−? (°?)
5.5.2. Approach.
This is the difference between the cooling tower outlet coldwater temperature and ambient wet bulb temperature. The lower the approach the better the cooling tower performance; although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance.
Effectiveness.
This is the ratio between the range and the ideal range (in percentage), i.e. difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach). The higher this ratio, the higher the cooling tower effectiveness.
Equation 3 CT Effectiveness
? (%) =(? – ?) (? – ?) × ?
5.5.4. Cooling capacity.
This is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.
5.5.5. Evaporation loss.
This is the water quantity evaporated for cooling duty. Theoretically the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected. The following formula can be used (Perry):
Equation 4 Evaporation Loss
? (? ) = ?.? × ?.? (? ) × (? −?)
T1 - T2 = temperature difference between inlet and outlet water
5.5.6. Cycles of concentration (C.O.C).
This is the ratio of dissolved solids in circulating water to the dissolved solids in makeup water.
5.5.7. Blow down losses depend upon cycles of concentration and the evaporation losses and is given by formula:
Equation 5 Blow down
? = ?.?.?.− ?
5.5.8.Liquid/Gas (L/G) ratio.
The L/G ratio of a cooling tower is the ratio between the water and the air mass flow rates. Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynamic rules also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used:
? (? −?) = ? (? −?)
Equation 6 Liquid/Gas ratio
?=(? −?) (? −?)
Where:
L/G = liquid to gas mass flow ratio (kg/kg) T1 = hot water temperature (°C) T2 = cold-water temperature (°C)
h2 = enthalpy of air-water vapour mixture at exhaust wet-bulb temperature
h1 = enthalpy of air-water vapour mixture at inlet wet-bulb temperature
5.6. ADVANTAGES OF COOLING TOWER
• Stable water temperature independent of ambient.
• Little water treatment.
• No water loss
• Possible heat reclaim.
• Low power consumption.
• Moderate capital cost.
6. EXPERIMENTAL RESULTS AND DISCUSSION
The following parameters are considered in the experiments conducted on cooling tower.
i. L/G ratio is maintained between 0.5-5.5.
ii. The temperature changes of hot water at inlet are neglected.
iii. The inlet temperature of cooling air is assumed to be constant.
iv. Fluid friction inside the tower during experimentation is neglected.
v. The pressure loss in pipes is neglected.
By considering above parameters the following readings are obtained.
T1-Inlet air temperature
T2-Outlet air temperature
T3-Inlet water temperature
T4-Outlet water temperature
T5-Temperature of water at tank
T6-Wet bulb temperature
The results are presented in table mentioned below
With bed materials (forced draft configuration)
Ambient wet bulb temp-= 27°C
Specific heat of water Cpw= 4.186 kJ/kg-k
CONCLUSION
1. The range of cooling tower increases with increase in flow rate irrespective of configuration of cooling tower (fluidized system with forced draft or induced & forced draft system).
2. The use of fluidized bed cooling tower increases the cooling rate by 50 % compared to normal cooling tower.
3. The increase in inlet temperature of water decreases the effectiveness as same quantity of air is available for cooling for all operating temperatures of cooling tower.
4. As L/G ratio decreases, the cooling rate increases.
5. The effectiveness of cooling tower is increased by 30 to 40% for combination of forced and induced draft system compare to that of forced draft.
6. The cooling capacity is increased by 30 to 40% for combination of forced and induced draft system compare to that of forced draft. Compared to non fluidized system, the cooling capacity is increased by 90 to 110%.
7. For optimum utilization of fluidized bed the flow rate of cooling air should be increased.
8. In forced draft and induced draft combine system, the water carried away by cooling water is almost doubled compare to that of forced draft system.