Methods of Sound Measurement for Water Cooling Towers
To understand how to measure sound, it’s important to first understand the nuances of sound and how it is formed. In essence, sound can be categorized as a form of energy that transmits through vibrations. These vibrations create small, repeating pressure disturbances that disseminate through the air along a path until they reach a receiver — our ears. These pressure disturbances are then measured in frequency (Hz), which is the number of oscillations (cycles) per second emitted from a vibrating object. In terms of our ability as humans to pick up these frequencies, we can generally hear a range of about 20 Hz to 20,000 Hz.
Why is Sound Important for Water Cooling Towers?
Sound is a critical component for understanding more about the performance of your cooling system. It’s formed from a combination of different functions, such as fan turbulence and motor mechanics. Sound from water cooling towers provides owners and manufacturers with a specification of the overall integrity of the system. This is important for two reasons:
To determine compliance with occupational safety regulations. This helps to determine how the overall sound level is impacting plant personnel as they conduct their daily responsibilities.
To predict the contribution of cooling tower sound to the overall sound level from a facility. This helps to determine how the overall sound level is impacting distant locations such as residential areas and locations near the plant facility boundaries.
How to Measure Vibrations in Water Cooling Towers
1. Select the right instrument.
As proper usage of all testing equipment is essential to obtain a valid measurement, it is important to adhere to the procedures and precautions outlined by each instrument manufacturer. A standard industrial accelerometer should be used for both the velocity and displacement measurements. Be sure to check the low-frequency response of the accelerometer and confirm that it has a flat response at the running speed of the fan (this is important to get an accurate displacement reading). Generally, vibration analysis systems use accelerometers to capture the time waveform signal of acceleration versus time, then convert it to amplitudes of velocity or displacement.
Note: The low-frequency response for most industrial accelerometers provided by the vendor is the -3 dB point, which means the accelerometer sensitivity is down by about 30% from the nominal value at that frequency. For greater accuracy, select an accelerometer that has a response that is suitable to measure vibration at fan speed.
2. Understand the requirements.
Testing of cooling towers for vibration level performance against contract requirements must employ an FFT analyzer equipped with limit filters set to exclude extremely low and high frequencies. Ideally, the full range of vibration frequencies should be specified by the purchaser and agreed to by the manufacturer prior to any valid measurement. If no frequency range has been established, we suggest using the range listed below:
Belt Drive: Upper-frequency limit should be no greater than 6 times the motor speed
Gear Drive: Upper-frequency limit should be no greater than 3.25 times the gear mesh frequency
3. Choose the best mounting method.
Mounting methods and placement of sensors are critical to the success of any vibration acceptance test. Use the following mounting methods to achieve the best results:
Stud: Stud mounting a sensor is by far the best method to use for mounting accelerometers, as it ensures you receive the most accurate response.
Magnet: Magnets are widely used to easily and temporarily mount accelerometers to a machine. However, over time, they can lose strength and become less effective.
Probe or Stinger: Probes or stingers are mounted to the accelerometer and held against the surface by hand. This method typically has a high resonance and should be avoided, if possible.
4. Record vibration levels.
You will want to record measurements for the 1) rotating machinery; 2) motor, gear reducer, and driveshaft; 3) belt drive fans; and 4) fan stack, if applicable. Measurements should be taken with all fans operating at full speed with the motor power at or near design, and with the water flow distributed to all cells as recommended by the manufacturer. Heat load is not mandatory during mechanical draft cooling tower sound measurements, and water flow rates may be measured with a pitot tube equipped with an air-over-water manometer.
For mechanical draft towers, the following deviations from design conditions shall not be exceeded:
Circulating water flow: ± 15%
Fan driver power: ± 10%
Fan speed: ± 5%
For natural draft towers, the following deviations from design conditions shall not be exceeded:
Circulating water flow: ± 10%
Cooling range: ± 20%
Wet bulb temperature: ± 8.5 °C (15 °F)
Dry bulb temperature: ± 14 ° C (25 °F)
Note: The absence of heat load on natural draft towers will influence water pullback in the air inlet which may affect measurement results. Additional exceptions apply, and weather conditions will be noted. Testing will not be conducted during precipitation, snow cover, or climatic conditions. For more information, purchase our Test Code for Measurement of Sound from Water Cooling Towers PDF.
Additional Sound Testing Information
Of course, the details within this article are a general outline to stress the importance of sound measurement for water cooling towers. Our recommendation is that you contact a professional agency to conduct any necessary testing. For additional information on the specifics of codes and standards relative to this topic, please consider the following resources:
We Provide Impartial Testing Services
Whether it’s an HVAC system or a natural draft cooling tower at a power plant, sound testing is an important factor in understanding how these systems impact the surrounding environment. We provide independent third-party compliance tests to help with methods of sound measurement for all types of water cooling tower systems. This means we can contractually secure an independent testing agency to evaluate the performance of your cooling towers — avoiding any agency which may have an interest in the outcome of the assessment or affiliation with the manufacturer, purchaser, or our organization. The sound test is executed by a mutually-acceptable CTI Licensed Test Agency or independent contractor. Contact us today to find out how we can help ensure your cooling towers conform to local building codes and safety standards.