Calculate Dry, Wet Bulb Temperature from Psychrometric Chart in Cooling Tower
Ambient air is used to remove the heat from heated water in a cooling tower. Humidity of air flow is increased during the process due to evaporation of water. When dry bulb temperature and wet bulb temperature are read, absolute humidity and relative humidity values can be calculated or figured out using psychrometric chart.
Air inflow and outflow streams
The purpose of cooling tower is removing heat from hot water stream and as a result, a mass transfer and energy transfer is happened.
- During cooling tower process, a part of water is evaporated from the hot water stream to the ambient air flow. When this evaporation is happened, considerable heat is removed from the hot water stream to the ambient air flow. That means, temperature of the hot water stream is decreased.
- Due to the evaporation of water to the ambient air flow, now air flow has more water vapor in its outflow than inflow. So, temperature and humidity of air flow is increased during this mass and energy transfer process.
Example to calculate humidity from dry and wet bulb temperatures using psychrometric chart
A hot water flow is cooled from 550C to 450C in a cooling tower. Ambient air is flown through the falling water and following details of air flow are found.
Air inflow
- Dry bulb temperature: 250C
- Wet bulb temperature: 120C
Air outflow
- Dry bulb temperature: 350C
- Wet bulb temperature: 300C
Because, two air parameters are known, we can easily locate absolute humidity and relative humidity. Let's take a psychrometric chart and locate points as mentioned below.
Read absolute and relative humidity of inlet air
Identify the point 1 (black circle) (air inflow) from dry bulb temperature and wet bulb temperature of inlet air flow.
Absolute humidity: Draw a horizontal line (green colour) through point 1 and locate where dry bulb line is intersected. Approximately, value of absolute humidity is 3.5 g moisture / 1 kg of dry air.
Relative humidity:Point 1 is located between curves of 10% and 20% relative humidity curves. Approximately, point 1 is more close to the 20% relative humidity curve and we can assume relative humidity of point 1 as 16% or 17%.
Read absolute and relative humidity of outflow air
Identify the point 2 (black circle) (air outflow) from dry bulb temperature and wet bulb temperature of output air flow.
Absolute humidity: Draw a horizontal line (green colour) through point 1 and locate where dry bulb line is intersected. Approximately, value of absolute humidity is 25 g moisture / 1 kg of dry air.
Relative humidity:Point 2 is located between curves of 60% and 70% relative humidity curves. Approximately, point 2 is more close to the 70% relative humidity curve and we can assume relative humidity of point 1 as 68%.
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