Cooling Tower
- Cooling water is used to condense the steam in condenser. temperature of Cooling water rises and it needs to be cool down again in order to use it (closed loop).
- Reduce temperature of water by extracting heat from water and emitting it to atmosphere. by means of evaporation of portion of water which carries extra heat of water thereby cooling it.
Atmospheric air is having certain capacity to absorb the water vapors at a given temperature. Water vapors are created due to evaporation of water droplets.
For evaporation of water, heat is required. This heat is obtained from the remaining water. So, the remaining water is cooled, as the heat is removed from it for evaporation. Rate of evaporation and hence, drop in cooling water temperature depends upon the following factors:
- Water surface area exposed to atmospheric air
- Time of expose
- Velocity of air flow
- Relative humidity of atmospheric air or difference between dry and wet bulb temperature
- Direction of air flow with respect to water flow
- Inlet water temperature
Finding Relative Humidity (how much water vapours the atmospheric air contains) : link and Use Psychrometic Chart
Types of Cooling Tower
- Natural draft cooling tower
- Hyperbolic cooling tower (Cross & Counter flow)
- Large heat duties
- Hot air moves upwards, Fresh air from bottom (delta T)
- Hyperbolic cooling tower (Cross & Counter flow)
- Mechanical Draft
- Induced draft cross flow cooling tower.
- Induced draft counterflow cooling tower.
Hyperbolic cooling tower is a natural draft cooling tower. Air velocity is created due to chimney effect.
Induced Draft Counter Flow Cooling Tower
Induced Draft Cross Flow Cooling Tower
Components
- louvers: air enters through
- spray nozzles
- fills
- distributors
- Drift Eliminators
- Fan: Create drift
- Basin
Working
- Fans create drift which pulls air, water falls downwards over fill surfaces which increase contact area of water and air.
- Cooling rates is determined by fan diameter, speed, fills system resistance, etc..
Terms
- Cooling range: The difference between the temperatures of hot water entering the tower and cold water leaving the tower is called cooling range.
- Approach: The difference between the temperature of cold water leaving the tower and the wet bulb temperature of atmospheric air is known as approach.
- Wet bulb temperature: Wet bulb temperature can be measured by using a thermometer with the bulb covered in wet muslin. This is the minimum temperature which can be achieved by purely evaporative cooling. Wet bulb temperature is always lower than the dry bulb temperature.
- Dry bulb temperature: Dry bulb temperature refers to the ambient air temperature basically. It is called dry bulb because the air temperature is indicated by a normal thermometer, not affected by the moisture of air.
- Drift: Water droplets that are carried out of the cooling tower with the exhaust air are called drift. Drift droplets have the same concentration of impurities as the circulating water of the tower.
- Drift eliminator: Drift loss rate is normally reduced by putting a baffle-like device, called drift eliminator, through which air has to pass after leaving the fill and spray nozzle zone of the tower.
- Fill: fills are arranged to increase the contact surface as well as the contact time between air and water. It provides better heat transfer and the efficiency of tower increases. Film type and splash type fills are used.
- Capacity: The amount of water (m3 /hr) that a cooling tower can cool through a specified range at a specified approach and wet bulb temperature is called capacity of that cooling tower
- Make-up: The amount of water required to make up the normal losses caused by blowdown, drift and evaporation is known as make-up.
- Heat load: The amount of heat to be removed from the circulating water per hour is called heat load of cooling tower. Heat load is equal to the rate of water flow per hour multiplied by range.
- Cycle of concentration (COC): When pure water is evaporated, minerals are left behind in the circulation water. As evaporation continues, the water becomes more concentrated than the original make-up water. The ratio of level of solids of the circulating cooling water to the level of solids of the fresh raw make-up water is called cycle of concentration (COC).
Losses
- Evaporation
- E = Circulation Rate x Delta_T x (Cp/Hv).
- Windage or drift loss
- W = 0.005% with drift eliminators
- Blowdown
- B + W = E/COC1.
Calculations
Amount of Cooling Water Required
Enthalpy of steam at exhaust at Pressure: 0.08 kg/cm2A, Temp: 45 C = __
Enthalpy of Condensate at 0.08 ATA, 41 C =
Treatment
- Anti-scale and corrosion treatment
- Biocide treatment
- Acid treatment
Questions
- Why Blowdown is required?
- Due to COC, concentration of solids in water becomes more than the original make-up water.
- minimize excessive concentration of these impurities, some quantity of circulating water is removed from the system.