Cooling Tower

Safety Procedures

Please be aware of any potential hazards prior to operating any experiment. In the event of exposure to hot water, run cool water over the affected area, contact lab personal, and use the emergency protocols at the exits to contact Health and Safety if additional medical attention is required.

Other Safety Concerns:

• Wear a lab coat and safety glasses at all times.

• The hot water tank can be very hot (> 60°C).

• Be aware of leaking areas and dry using rags or paper towels. Contact technical staff is it is significant.

Programs, Equipment, and Instruments

• Anemometer

• Thermocouples

• Pump

• Fan with controller

Objectives

1. Use the Merkel Equation to measure how the mass transfer group KαV/L(=NTU) varies as a function of L/G.

2. To do so,

   1. Ensure the wet bulb thermocouples are wet, then turn the system (control box, pump, and fan ) on.

   2. Set the bath temperature set point to something reasonable. If at steady state the green light on the PID controller is either always on or always off then your set point is not reasonable (look up "pulse width modulation for heater" if you're curious about why the light blinks like it does).

   3. Allow the system to reach steady state, then record all temperatures and mass flow rates. Vary L and/or G and repeat.

Tips

1. Example 10 on pg. 12-18 in Perry's Handbook  (8th ed.) suggests using the Chebyshev method to approximate the KαV/L(=NTU) integral. Do not use this method! Instead, use a numerical integration function in MATLAB such as integral.

2. You can use the nomograph in Fig. 12.8b in Perry's Handbook (8th ed.) to check that your calculation of KαV/L(=NTU) is reasonable.

3. When plotting NTU vs. L/G, your data is NOT linear. And it is NOT exponential. See references 4 & 5 for suggested models to fit.

4. Under certain operating conditions, the air exit stream will be saturated with water and the temperature probes become unreliable. Think why this might be and what other measuring devices are available to get around this problem.

References

1. Genskow L., et al. In: Perry's Chemical Engineers Handbook, Green, D., Perry, R., Eds., 8th ed.; McGraw Hill: New York, 2008, pp 12-3 - 12-22.

2. The derivation for the Merkel Equation originally posted here follows that of P.C. Chau for the UCSD CENG 176 lab (removed as of January 2022) but can also be found in various handbooks or textbooks, including references 6 & 7.

3. Young et al. A Brief Introduction to Fluid Mechanics, 5th ed. Wiley, NJ (2011), pp. 285.

4. Shryock H.A., Baker, D.R. J. Heat Transfer, 1961, 83, 339-349.

5. Lemouari, et al. Applied Thermal Engineering, 2007, 27, 902-909.

6. Geankoplis C.J. Transport Processes and Separation Process Principles, 4th ed. Prentice Hall, NJ (2012), pp. 645-657.

7. McCabe W.L., et al. Unit Operations of Chemical Engineering, 7th ed. McGraw Hill, NY (2005), pp. 628-640.