Page 74 - Industrial Plants

equipment so that any external means aimed at limiting comes from the axial fans, whose noise can be
noise propagation, such barriers or other obstacles, significantly higher than the aggregate noise from other
may be quite expensive. sources.
The growing attention paid by the market to noise Noise generated by fans depends heavily on their
emission of air coolers is highlighted by Fig. 1, where operating point, defined by the air flow rate, the static
the use of low noise fans and standard noise fans has pressure and the rotational speed. In addition to this,
been reported. As shown, the trend of the last three different blades profiles and sizes allow for different
years is to go for low noise equipment. noise results to occur.
The way the various operating parameters affect noise
emission of fans can be evaluated by means of
formulae, like the one that follows, referred to a low
noise fan:
L = K + 30 • log (v ) + 10 • log (Q • H / 1000)
w 10 tip 10
where:
• L [dB(A)] is the sound power level;
w
• K is a constant depending on the fan geometry;
• v [m/s] is the peripheral speed;
tip
• Q [m /s] and H [Pa] are the volumetric flow rate
3
and the static pressure, respectively.
The application of this equation to practical cases
indicates that noise generation is usually much more
Figure 1: Type of fans on executive Projects in the affected (up to 5 times) by the peripheral speed
latest three years (Boldrocchi database). rather than by the volumetric flow rate or the static
pressure. This explains why low noise air coolers are
usually equipped with slow rotating fans. The
What are the major sources concept is indeed well taken into account while

of noise in an Air Cooled Heat developing the thermodynamic and mechanical
design of air coolers, together with all other technical
Exchangers? constraints that affect feasibility and costs. For

instance, despite the formula above suggests that
An ACHE has several noise sources that can be easily fans working at very low rotational speed should be
ranked as below: very silent, it does not consider the risk of resonance
• axial fans; with the structure and related impact on noise, nor
• belt transmissions; the impact on the mechanical efficiency of the fan.
• electric motors; Another source of noise of air coolers is represented
• turbulence created by obstacles located upstream by belts transmissions, which are wrapped around
Figure 2: Ongoing and downstream the fans; the drive pulleys and used to couple the electric
assembly of the air • other vibrating parts (e.g., structural parts). motors to the fans. Related noise is affected by the
coolers at site rotational speed of the belts so that fast rotating
(Renco S.p.a.)
Among these sources, very often the major contribution belts are more noising than slow rotating belts.

IndustrIal Plants - May 2022
72