Cooling Tower Design Calculation Review

Summary

In this video lesson, Dr. Christie Patton Luke, a chemical engineering professor, provides an overview of the fundamental concepts in designing cooling towers. The focus is on cooling water by mixing it with air, optimizing mass transfer through turbulence and packing. Key topics include approach temperature, cooling range, and the use of psychrometric charts. The process involves energy and mass balances, heat transfer equations, and numerical integration techniques for calculating tower dimensions. The video also highlights the importance of determining air flow rates based on liquid flow requirements to ensure efficient cooling.

Takeaways

• 😀 Cooling towers aim to cool water by mixing it with air, where water falls through rising air, creating mass transfer.
• 😀 Maximizing turbulence within the tower is essential for efficient mass transfer, achieved using packing grades or troughs.
• 😀 The approach temperature is the difference between the temperature of the exiting water and the incoming air.
• 😀 The cooling range represents the difference between the inlet and outlet temperatures of the liquid water.
• 😀 The minimum temperature achievable by a cooling tower is three degrees Celsius above the wet bulb temperature, due to practical limitations.
• 😀 The process involves energy balances between the gas and liquid phases, where the gas enthalpy change equals the liquid enthalpy change.
• 😀 Heat transfer in the tower is primarily due to convection, and the heat transfer coefficient is critical for calculating heat loss due to vaporization.
• 😀 Fick's law helps estimate humidity change, and various equations can relate enthalpy changes between gas and liquid phases.
• 😀 Design of cooling towers includes plotting equilibrium data and determining the operating line, with gas and liquid inlet temperatures used to calculate required values.
• 😀 Determining the air flow rate requires figuring out the minimum gas flow rate and then adjusting to meet operational requirements, using techniques from absorption and stripping processes.

Q & A

What is the primary goal of a cooling tower in chemical engineering?

The primary goal of a cooling tower is to cool water by mixing it with air, where the water falls through rising air to maximize mass transfer by increasing turbulence.

What is the difference between 'approach temperature' and 'cooling range' in cooling tower design?

The 'approach temperature' refers to the temperature difference between the liquid leaving the tower and the air entering the tower, while the 'cooling range' is the difference between the inlet and outlet temperatures of the liquid water.

How does a cooling tower achieve efficient mass transfer?

Efficient mass transfer in a cooling tower is achieved by maximizing turbulence, which is often done through packing grades or troughs, rather than just increasing flow rates.

What factors limit how close the cooling tower can bring the water temperature to the wet bulb temperature?

The cooling tower cannot reach the wet bulb temperature due to practical constraints like evaporation, which leads to water loss. Typically, the minimum temperature is about 3°C above the wet bulb temperature.

What is the role of psychrometric charts in cooling tower design?

Psychrometric charts are used in cooling tower design to plot the relationship between the enthalpy of the gas (air) and the temperature of the liquid water, helping to visualize the equilibrium and operating lines during the design process.

What is the significance of the enthalpy change in cooling tower operations?

Enthalpy change is significant because it helps to quantify the heat and mass transfer occurring within the cooling tower, and is used in energy balance equations to determine the cooling efficiency and tower design parameters.

How do you calculate the air flow rate required for a cooling tower?

The air flow rate is determined by calculating the minimum required air flow based on the water cooling flow rate, using methods similar to those in absorption or stripping processes. The pinch point is used to calculate the minimum gas flow rate, which is then adjusted by a factor of 1.3 to 1.5.

What is the method to estimate the height of packing in a cooling tower?

The height of packing in a cooling tower is estimated through numerical integration of the transfer unit, which involves calculating the difference in enthalpy between the operating line and the equilibrium curve, and using this to determine the number of transfer units and total height of packing.

What are the key variables used in cooling tower design calculations?

The key variables include water flow rate (L), dry air flow rate (G), air humidity, liquid temperature, gas enthalpy, and the heat transfer coefficient. These variables are used to determine mass and heat transfer parameters.

How does the use of film coefficients affect cooling tower design calculations?

Using film coefficients in design calculations helps account for the heat and mass transfer occurring at the interface between the air and water. Without film coefficients, the overall mass transfer coefficient is used, which simplifies the calculations but may reduce accuracy.