cooling towers
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  • 1. Khalid nawaz cell no: (0300-7024407) institute of chemical engineering and technology Lahore Pakistan
  • 2. The cooling tower is one of the most important device in chemical industries for example when the hot water come from heat exchanger we use the cooling tower to cool it. The purpose of cooling tower is to cool relatively warm water by contacting with unsaturated air. The evaporation of water mainly provides cooling. In a typical water cooling water tower, warm water flows countercurrent to an air stream. Typically, the warm water enters the top of packed tower and cascades down through the packing, leaving at the bottom.
  • 3. Air enters at the bottom of the tower and flows upward through the descending water. The tower packing often consists of slats of plastic or of packed bed. The water is distributed by troughs and overflows to cascade over slat gratings or packing that provides large interfacial areas of contact between the water and air in the form of droplets and films of water. The flow of air upward through the tower can be induced by the buoyancy of the warm air in the tower (natural draft) or by the action of a fan. The water cannot be cooled below the wet bulb temperature. The driving force for the evaporation of the water is approximately the vapor pressure of the water less the vapor pressure it would have at the wet bulb temperature.
  • 4. What Does A Cooling Tower Do?  Cooling Towers are used to transfer heat from cooling water to the atmosphere. – Promotes efficient water usage – Prevents environmental damage
  • 5. Where cooling tower used Used in power stations, oil refineries, petrochemical plants and natural gas plants. Cooling water is continuously circulated through heat exchangers to absorb heat from process material and machinery. Because it's cost efficient to reuse water and plants can't dump excessive amounts of hot water into rivers and lakes, cooling towers are used to remove the heat from the water, so it can be recirculated.
  • 6. How Does It Work?
  • 7. Parts Of Cooling Tower  Frame and casing. Most towers have structural frames that support the exterior enclosures (casings), motors, fans, and other components. With some smaller designs, such as some glass fiber units, the casing may essentially be the frame.  Fill. Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. There are two types of fill:  Splash fill: water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fills promote better heat transfer than wood splash fills.  Film fill: consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.  Cold-water basin. The cold-water basin is located at or near the bottom of the tower, and it receives the cooled water that flows down through the tower and fill. The basin usually has a sump or low point for the cold-water discharge connection. In many tower designs, the cold-water basin is beneath the entire fill. In some forced draft counter flow design, however, the water at the bottom of the fill is channeled to a perimeter trough that functions as the cold-water basin. Propeller fans are mounted beneath the fill to blow the air up through the tower. With this design, the tower is mounted on legs, providing easy access to the fans and their motors.
  • 8. Fill: Most towers employ fills (made of plastic or wood) to facilitate heat transfer by maximizing water and air contact. Fill can either be splash or film type. With splash fill, water falls over successive layers of horizontal splash bars, continuously breaking into smaller droplets, while also wetting the fill surface. Plastic splash fill promotes better heat transfer than the wood splash fill. Film fill consists of thin, closely spaced plastic surfaces over which the water spreads, forming a thin film in contact with the air. These surfaces may be flat, corrugated, honeycombed, or other patterns. The film type of fill is the more efficient and provides same heat transfer in a smaller volume than the splash fill.
  • 9.  Drift eliminators. These capture water droplets entrapped in the air stream that otherwise would be lost to the atmosphere.  Air inlet. This is the point of entry for the air entering a tower. The inlet may take up an entire side of a tower (cross-flow design) or be located low on the side or the bottom of the tower (counter-flow design).  Louvers. Generally, cross-flow towers have inlet louvers. The purpose of louvers is to equalize air flow into the fill and retain the water within the tower. Many counter flow tower designs do not require louvers.  Nozzles. These spray water to wet the fill. Uniform water distribution at the top of the fill is essential to achieve proper wetting of the entire fill surface. Nozzles can either be fixed and spray in a round or square patterns, or they can be part of a rotating assembly as found in some circular cross-section towers.  Fans. Both axial (propeller type) and centrifugal fans are used in towers. Generally, propeller fans are used in induced draft towers and both propeller and centrifugal fans are found in forced draft towers. Depending upon their size, the type of propeller fans used is either fixed or variable pitch. A fan with non-automatic adjustable pitch blades can be used over a wide kW range because the fan can be adjusted to deliver the desired air flow at the lowest power consumption. Automatic variable pitch blades can vary air flow in response to changing load conditions.
  • 10. Cooling tower material  Wood--- frame, casing, louvers, fill, and cold water basin (or concrete)  Galvanised steel, various grades of stainless steel, glass fibre and concrete, aluminium and various types of plastics for some components  Large towers are made of CONCRETE  Plastics are widely used for FILL, including PVC, polypropylene and other polymers  Plastics also find wide use in nozzle materials
  • 11. Cooling Tower Types  Cooling towers fall into two main categories: Natural draft and Mechanical draft. Mechanical draft towers are available in the following airflow arrangements:  1. 2. 3. Counter flows induced draft. Counter flow forced draft. Cross flow induced draft.
  • 12. Cross flow Cross flow is a design in which the air flow is directed perpendicular to the water flow (see diagram below). Air flow enters one or more vertical faces of the cooling tower to meet the fill material. Water flows (perpendicular to the air) through the fill by gravity.
  • 13. Counter flow In a counter flow design the air flow is directly opposite of the water flow (see diagram below). Air flow first enters an open area beneath the fill media and is then drawn up vertically. The water is sprayed through pressurized nozzles and flows downward through the fill, opposite to the air flow.
  • 14. Natural draft, which utilizes buoyancy via a tall chimney. Warm, moist air naturally rises due to the density differential to the dry, cooler outside air. Warm moist air is less dense than drier air at the same temperature and pressure. This moist air buoyancy produces a current of air through the tower. This photo shows a single natural draft cooling tower as used at a European plant. Natural draft towers are typically about 400 ft (120 m) high, depending on the differential pressure between the cold outside air and the hot humid air on the inside of the tower as the driving force. No fans are used.
  • 15. • The natural draft or hyperbolic cooling tower makes use of the difference in temperature between the ambient air and the hotter air inside the tower. It works as follows: • Hot air moves upwards through the tower (because hot air rises) • Fresh cool air is drawn into the tower through an air inlet at the bottom. • Due to the layout of the tower, no fan is required and there is almost no circulation of hot air that could affect the performance. Concrete is used for the tower shell with a height of up to 200 m. These cooling towers are mostly only for large heat duties because large concrete structures are expensive.
  • 16. • Air drawn up of Cooling Towers through falling water • Fill located Natural Draft Cooling Towers tower inside • Air drawn across Types falling water • Fill located outside tower Cross flow (Gulf Coast Chemical Commercial Inc.) Counter flow 20 © UNEP 2006
  • 17. Types of natural draft cooling tower  There are two main types of natural draft towers: • Cross flow tower (left figure): air is drawn across the falling water and the fill is located outside the tower • Counter flow tower (right figure): air is drawn up through the falling water and the fill is therefore located inside the tower, although design depends on specific site conditions
  • 18. Types of Cooling Towers Mechanical Draft Cooling Towers • Large fans to force air through circulated water • Water falls over fill surfaces: maximum heat transfer • Cooling rates depend on many parameters • Large range of capacities • Can be grouped, e.g. 8-cell tower 22 © UNEP 2006
  • 19. • Mechanical draft towers have large fans to force or draw air through circulated water. The water falls downwards over fill surfaces, which help increase the contact time between the water and the air - this helps maximize heat transfer between the two. • Cooling rates of mechanical draft towers depend upon various parameters such as fan diameter and speed of operation, fills for system resistance etc. • Mechanical draft towers are available in a large range of capacities. Towers can be either factory built or field erected – for example concrete towers are only field erected. • Many towers are constructed so that they can be grouped together to achieve the desired capacity. Thus, many cooling towers are assemblies of two or more individual cooling towers or “cells.” The number of cells they have, e.g., a eightcell tower, often refers to such towers.
  • 20. Types of Cooling Towers Forced Draft Cooling Towers • • Advantages: suited for high air resistance & fans are relatively quiet • (GEO4VA) Air blown through tower by centrifugal fan at air inlet Disadvantages: recirculation due to high air-entry and low air-exit velocities 24 © UNEP 2006
  • 21. A mechanical draft tower with a blower type fan at the intake. The fan forces air into the tower, creating high entering and low exiting air velocities. The low exiting velocity is much more susceptible to recirculation. With the fan on the air intake, the fan is more susceptible to complications due to freezing conditions. Another disadvantage is that a forced draft design typically requires more motor horsepower than an equivalent induced draft design. The forced draft benefit is its ability to work with high static pressure. They can be installed in more confined spaces and even in some indoor situations. This fan/fill geometry is also known as blow-through.
  • 22.  Forced draft cooling tower • How it works: air is blown through the tower by a fan located in the air inlet • Advantages: • Suited for high air resistance due to centrifugal blower fans • Fans are relatively quiet • Disadvantages: Recirculation due to high air-entry and low air-exit velocities, which can be solved by locating towers in plant rooms combined with discharge ducts
  • 23. Types of Cooling Towers Induced Draft Cooling Towers • Two types • Cross flow • Counter flow • Advantage: less recirculation than forced draft towers • Disadvantage: fans and motor drive mechanism require weather-proofinh 28 © UNEP 2006
  • 24. • Two types of induced draft cooling towers: cross flow and counter flow • Advantage: Less recirculation than forced draft towers because the speed of exit air is 3-4 times higher than entering air • Disadvantage: Fans and the motor drive mechanism require weather-proofing against moisture and corrosion because they are in the path of humid exit air
  • 25. Types of Cooling Towers Induced Draft Counter Flow CT • Hot water enters at the top • Air enters at bottom and exits at top • Uses forced and induced draft fans (GEO4VA) 30 © UNEP 2006
  • 26. Types of Cooling Towers Induced Draft Cross Flow CT • Water enters top and passes over fill • Air enters on one side or opposite sides • Induced draft fan draws air across fill (GEO4VA) 31 © UNEP 2006
  • 27. Air/water systems Dry-bulb and wet-bulb temperature wet cloth/wick Air flow Dry bulb temperature: the ordinary temperature you measure with e thermometer Evaporation requires energy. The wick and therefore the thermometer bulb decreases in temperature below the dry-bulb temperature until the rate of heat transfer from the warmer air to the wick is just equal to the rate of heat transfer needed to provide for the evaporation of water from the wick into the air stream. The temperature reached is called the wet-bulb temperature
  • 28. Cooling tower Performance
  • 29. Performance  These measured parameters and then used to determine the cooling tower performance in several ways. These are: 1. Range 2. Approach 3. Effectiveness 4. Cooling capacity 5. Evaporation loss 6. Cycles of concentration 7. Blow down losses 8. Liquid / Gas ratio  We will now go through each of these parameters on the next slides
  • 30. Assessment of Cooling Towers 1. Range Hot Water Temperature (In) Range Difference between cooling water inlet and outlet temperature: Range (°C) = CW inlet temp – CW outlet temp (In) to the Tower (Out) from the Tower High range = good performance Approach Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 35 © UNEP 2006
  • 31. • The range is the difference between the cooling tower water inlet and outlet temperature. • The formula for cooling tower range in degrees Celcius is cooling water inlet temperature minus cooling water 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.
  • 32. Assessment of Cooling Towers 2. Approach Approach (°C) = CW outlet temp – Wet bulb temp Low approach = good performance Range Hot Water Temperature (In) Approach Difference between cooling tower outlet cold water temperature and ambient wet bulb temperature: (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 37 © UNEP 2006
  • 33. • Approach is the difference between the cooling tower outlet cold-water temperature and ambient wet bulb temperature. • The formula for approach is CT Approach in degrees Celcius is cold water outlet temperature minus the 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.
  • 34. Assessment of Cooling Towers 3. Effectiveness Hot Water Temperature (In) = Range / (Range + Approach) Range Effectiveness in % High effectiveness = good performance Approach = 100 x (CW temp – CW out temp) / (CW in temp – Wet bulb temp) (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 39 © UNEP 2006
  • 35. • Cooling tower effectiveness 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 • Effectiveness = Range / (Range + Approach) • The formula for cooling tower effectiveness is: CT Effectiveness (%) = 100 x (CW temp – CW out temp) / (CW in temp – WB temp) • The higher this ratio, the higher the cooling tower effectiveness.
  • 36. Assessment of Cooling Towers 4. Cooling Capacity = mass flow rate of water X specific heat X temperature difference High cooling capacity = good performance Approach Heat rejected in kCal/hr or tons of refrigeration (TR) Range Hot Water Temperature (In) (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) 41 © UNEP 2006
  • 37. Assessment of Cooling Towers 5. Evaporation Loss Hot Water Temperature (In) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2) Approach = theoretically, 1.8 m3 for every 10,000,000 kCal heat rejected Range Water quantity (m3/hr) evaporated for cooling duty (In) to the Tower (Out) from the Tower Cold Water Temperature (Out) Wet Bulb Temperature (Ambient) T1-T2 = Temp. difference between inlet and outlet water 42 © UNEP 2006
  • 38.  The main areas for improving the energy efficiency of cooling towers are: 1. Selecting the right cooling tower (because the structural aspects of the cooling tower cannot be changed after it is installed) 2. Fills 3. Pumps and water distribution system 4.Fans and motors  We will go through these one by one on the next slides.
  • 39. • Once a cooling tower is in place it is very difficult to significantly improve its energy performance. A number of factors are of influence on the cooling tower’s performance and should be considered when choosing a cooling tower: capacity, range, approach, heat load, wet bulb temperature, and the relationship between these factors. We will start with capacity. • Capacity, in terms of heat dissipation (in kCal/hour) and circulated flow rate (m3/hr), are an indication of the capacity of cooling towers. However, these design parameters are not sufficient to understand the cooling tower performance. • For example, a cooling tower sized to cool 4540 m3/hr through a 13.9 0C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5 0C range. Therefore other design parameters are also needed.
  • 40. Factors affecting Cooling tower performance Wet Bulb Temperature  WBT of air entering the cooling tower determines operating temperature levels throughout the plant, process or system.  Recirculation raises the effective WBT of the air entering the tower with corresponding increase in cold water temperature.
  • 41.   • • • • • Wet bulb temperature is an important factor in performance of evaporative water cooling equipment, because it is the lowest temperature to which water can be cooled. For this reason, the wet bulb temperature of the air entering the cooling tower determines the minimum operating temperature level throughout the plant, process, or system. The following should be considered when pre-selecting a cooling tower based on the wet bulb temperature: Theoretically, a cooling tower will cool water to the entering wet bulb temperature. In practice, however, water is cooled to a temperature higher than the wet bulb temperature because heat needs to be rejected from the cooling tower. A pre-selection of towers based on the design wet bulb temperature must consider conditions at the tower site. The design wet bulb temperature also should not be exceeded for more than 5 percent of the time. In general, the design temperature selected is close to the average maximum wet bulb temperature in summer. Confirm whether the wet bulb temperature is specified as ambient (the temperature in the cooling tower area) or inlet (the temperature of the air entering the tower, which is often affected by discharge vapors recirculated into the tower). As the impact of recirculation cannot be known in advance, the ambient wet bulb temperature is preferred. Confirm with the supplier if the cooling tower is able to deal with the effects of increased wet bulb temperatures. The cold-water temperature must be low enough to exchange heat or to condense vapors at the optimum temperature level. The quantity and temperature of heat exchanged can be considered when choosing the right size cooling tower and heat exchangers at the lowest costs. .
  • 42. Factors affecting Cooling tower performance Fill Media Effects  Function: Heat exchange between air and water is influenced by surface area of heat exchange, time of heat exchange and turbulence in water effecting thoroughness of intermixing.  Due to fewer requirements of air and pumping head, there is a tremendous saving in power with the intervention of film fill.  Recently, low clog film fills with higher flute sizes have been developed to handle high turbid waters. (sea water)
  • 43. Facts  The flow rate through a cooling tower in a typical 700 MW power plant is about 315,000 gal/min.  Cooling towers have 95% efficiency, losing only 5% of circulating water to evaporation.  Power consumption is minimal due to few moving parts.  Cooling towers save plants thousands to millions of dollars per year in water consumption and recycling costs.
  • 44. Energy Saving Opportunities in Cooling Towers Follow manufacturer’s recommended clearances around cooling towers and relocate or modify structures that interfere with the air intake or exhaust. Optimise cooling tower fan blade angle on a seasonal and/or load basis. Correct excessive and/or uneven fan blade tip clearance and poor fan balance. On old counter-flow cooling towers, replace old spray type nozzles with new square spray ABS practically non-clogging nozzles. Replace splash bars with self-extinguishing PVC cellular film fill. Install new nozzles to obtain a more uniform water pattern Periodically clean plugged cooling tower distribution nozzles. Balance flow to cooling tower hot water basins. Cover hot water basins to minimise algae growth that contributes to fouling. Optimise blow down flow rate, as per COC limit. Replace slat type drift eliminators with low pressure drop, self extinguishing, PVC cellular units. Restrict flows through large loads to design values.