Cooling Tower Types
Cooling towers fall into two main categories: Natural draft and Mechanical draft.
Natural draft towers use very large concrete chimneys to introduce air through the media. Due to the large size of these towers, they are generally used for water flow rates above 45,000 m3/hr. These types of towers are used only by utility power stations.
Mechanical draft towers utilize large fans to force or suck air through circulated water. The water falls downward 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 their fan diameter and speed of operation
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Mechanical draft towers are available in the following airflow arrangements:
1. Counter flows induced draft.
2. Counter flow forced draft.
In counter flow towers, air flows opposite to the water flow. Air enters the tower beneath the fill and is drawn up vertically into the tower. Above the fill, hot water is introduced through low pressure spray nozzles to divide the hot water over the surface of the fill in fine droplets. The cooling air draws heat from the water as it progresses to the bottom of the tower. The drift eliminator above the spray nozzle captures water droplets and returns the water to the circulating system. A cold water basin contains the water after its interaction with the air flow.
In cross flow towers, air flow is directed across the water flow. Air flow enters the vertical faces of the tower to meet the fill. Hot water is distributed to the fill, perpendicular to the air flow, by gravity through perforated basins. The air passes through the fill, past the water flow into an open area while gravity distributes the water through nozzles across the fill. The turbulent air will flow through the fill structure to maximize the contact with the water thus drawing heat out of the water. A cold water basin contains the water after its interaction with the air flow.
COOLING TOWER TERMS AND DEFINITIONS
Cooling Range
The difference in temperature between the hot water entering the tower and the cold water leaving the tower is the cooling range.
Approach
The difference between the temperature of the cold water leaving the tower and the wet bulb temperature of the air is known as the approach
Wet bulb temperature
Temperature as indicated by a thermometer the bulb of which is kept moist by wick over which air is circulated. This is theoretically, the lowest temperature to which water can be cooled and it depends on the dry bulb temperature and relative humidity of the ambient air.
Drift
Water droplets that are carried out of the cooling tower with the exhaust air. Drift droplets have the same concentration of impurities as the water entering the tower. The drift rate is typically reduced by employing baffle-like devices, called drift eliminators, through which the air must travel after leaving the fill and spray zones of the tower. Proper tower design can minimize drift loss.
Dry bulb temperature
External outdoor temperatures as indicated by a dry bulb thermometer in degree C.
Heat Load
The amount of heat to be removed from the circulating water within the tower. Heat load is also an important parameter in determining tower size and cost. It is of primary importance that an accurate heat load determination be made. If the heat load calculations are low the cooling tower will be undersized. If the calculations are high, oversized more expensive equipment will result.
Ton
An evaporative cooling ton is 3024 kcal/hr.
Effectiveness=Range/(Range+ Approach)
Makeup
The amount of water required to replace normal losses caused by blow down, drift, and evaporation.
Cooling tower Location selection
When selecting the location, sufficient clearance should be allowed for the free flow of air to the inlet of the tower and for its discharge from the tower. Obstructions will reduce airflow causing a reduction in capacity.
Cooling tower location should be such that the air discharge will not cause condensation on nearby surfaces or wetting because of drift.
Cooling towers should be located so that noise created by air or water is not a source of annoyance.
Cooling tower selection and performance is based on water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb temperature.
THE factors affecting Cooling Tower performance are:
1. Dry bulb and wet bulb temperature of the air
2. Capacity C
3. Range
4. Approach
5. Efficiency of contact between air and water
6. Heat load
Ambient wet bulb temperature is a condition measured by a device called a psychrometer. A psychrometer places a thin film of water on the bulb of a thermometer that is twirled in the air. After about a minute, the thermometer will show a reduced temperature. The low point when no additional twirling reduces the temperature is called the wet bulb temperature.
Cooling tower performance is related to ambient wet bulb conditions. Higher wet bulb temperatures occur in the summer when higher ambient and relative humidity occurs.
The measured wet bulb temperature is a function of relative humidity and ambient air temperature. Wet bulb temperature essentially measures how much water vapor the atmosphere can hold at current weather conditions. A lower wet bulb temperature means the air is drier and can hold more water vapor than it can at a higher wet bulb temperature.
Cold water temperature from cooling tower depends upon mainly wet bulb temperature and cooling range. Wet bulb is the lowest theoretical temperature, which the water can be, cooled. Practically the cold water temperature approaches but does not be equal to wet bulb temperatures, because some drive is required for evaporation of water. Temperature difference between cold water temperature and Wet bulb temperature of air is called approach. Minimum approach is 3 degree C.
Capacity of a cooling tower plays a vital part. For example, a cooling tower sized to cool 4000 m3 per hour through a 10.5 C range might be larger than a cooling tower to cool the same 4540 m3 per hour through a 15.50 range.
Range is determined not by cooling tower, but by the process it is serving.
Range in Degree C = Heat load in kcals per hour / Water Circulation Rate in LPH.
Heat load imposed on a cooling tower is determined by the process being served. The degree of cooling required is controlled by the desired operating temperature level of the process. The size and cost of the cooling tower is proportional to the heat load.