Go to...
Boiler Compressor Cooling Tower Electric Motor Fan Simple Pump Simple Gas Turbine Steam Turbine

[Back to top]

For small compressors:

	31.5	63	125	250	500	1k	2k	4k	8k
Up to 1.5kW	90	89	89	88	91	94	94	92	89
2kW to 6 kW	95	92	92	91	94	97	97	95	92
7kW to 75kW	100	95	95	94	97	100	100	98	95

[Back to top]

Cooling towers are pieces of mechanical plant used in many air conditioning systems to cool the water used in the compressor coils. This is achieved by blowing air through a stream of water, whereby the evaporation of some of this water will lead to cooling of the remaining water.

There are four main types of cooling tower configuration which must be considered:

• Centrifugal fan
  • Blow-through type
  • Axial flow
• Propeller-type
  • Induced draft
  • Forced draft, ‘underflow’

The noise levels can be estimated by knowing the operating power of the cooling tower, as well as whether it is a centrifugal or propeller type. Note that the subtypes above are for the consideration of blow through effects only.

Type	Equation
Propeller <75kW	100+8log(kW)
Propeller >75kW	96+10log(kW)
Centrifugal <60kW	85+11log(kW)
Centrifugal >60kW	93+7log(kW)

In order to convert this overall SPL to a set of octave band SPLs, the following conversion factors should be applied:

Cooling tower type	31.5	63	125	250	500	1k	2k	4k	8k
Propeller Type	-8	-5	-5	-8	-11	-15	-18	-21	-29
Centrifugal Type	-6	-6	-8	-10	-11	-13	-12	-18	-25

Additionally, there are directional effects to be considered for the cooling towers when we measure the sound power level on a specific side. These effects should only be considered where the distance of measurement is >6m and are given in B&H Table 11.8 as:

Type of tower and location of measurement	31.5	63	125	250	500	1k	2k	4k	8k
Centrifugal fan blow through
Front	3	3	2	3	4	3	3	4	4
Side	0	0	0	-2	-3	-4	-5	-5	-5
Rear	0	0	-1	-2	-3	-4	-5	-6	-6
Top	-3	-3	-2	0	1	2	3	4	5
Axial flow, blow through
Front	2	2	4	6	6	5	5	5	5
Side	1	1	1	-2	-5	-5	-5	-5	-4
Rear	-3	-3	-4	-7	-7	-7	-8	-11	-3
Top	-5	-5	-5	-5	-2	0	0	2	4
Induced draft, propeller type
Front	0	0	0	1	2	2	2	3	3
Side	-2	-2	-2	-3	-4	-4	-5	-6	-6
Top	3	3	3	3	2	2	2	1	1
Underflow forced drafter propeller type
Any side	-1	-1	-1	-2	-2	-3	-3	-4	-4
Top	2	2	2	3	3	4	4	5	5

In cases where the measurement is to be taken close to the intake or discharge (< 6m), data is available but it appears to be independent of the power of the plant and so has not been considered here.

The Cooling Tower estimator tool looks like this:

And implements all the corrections defined above.

[Back to top]

For small motors (<300kW), the equations are as follows:

	Equation (SPL @ 1m)
Under 40kW	Lp=17+17log(kW)+15log(RPM)
Over 40kW	Lp=28+10log(kW)+15log(RPM)

Corrections can be applied to Totally Enclosed Fan Cooled (TEFC) types and Drip Proof Motor (DRPR) types:

	31.5	63	125	250	500	1k	2k	4k	8k
TEFC	14	14	11	9	6	6	7	12	20
DRPR	9	9	7	7	6	9	12	18	27

[Back to top]

Function LwFanSimple(freq As String, v As Double, P As Double, FanType As String)

From Biess and Hansen, the overall Sound Power of a fan can be approximated by:

Lw=10log(v)+20log(P)+40

Where v is the volumetric flowrate of the fan

and P is the static pressure of the fan

The following spectral corrections apply to different fan types:

-	63	125	250	500	1k	2k	4k
Forward curved centrifugal	-5	-10	-15	-20	-25	-28	-31
Backward curved centrifugal	-10	-11	-10	-15	-20	-25	-30
Radial or paddle blade	3	-3	-10	-11	-15	-19	-23
Axial	-8	-8	-6	-7	-8	-12	-16
Bifurcated	-3	-3	-4	-5	-7	-8	-11
Propeller fan (approx)	-3	-4	-1	-8	-12	-13	-20
Variable inlet vanes - 100%	0	0	0	0	0	0	0
Variable inlet vanes - 80%	8	5	4	4	4	4	4
Variable inlet vanes - 60%	8	7	6	5	5	5	5
Variable inlet vanes - 40%	3	2	1	0	0	0	1

[Back to top]

Pumps are pieces of mechanical plant which move a fluid through a system. The motor on the pump is usually the main source of noise.

To estimate the sound levels from pumps, the estimator tool uses the method from Engineering Noise Control (Bies and Hansen) Table 11.10:

Speed range(RPM)	Equation
	Under 75kW	Above 75kW
3000-3600	72+10log(kW)	86+3log(kW)
1600-1800	75+10log(kW)	89+3log(kW)
1000-1500	70+10log(kW)	84+3log(kW)
450-900	68+10log(kW)	82+3log(kW)

The Pump estimator tool looks like this:

The corrections to derive sound pressure at 1m are as follows:

Octave Band	31.5	63	125	250	500	1k	2k	4k	8k
Correction	-13	-12	-11	-9	-9	-6	-9	-13	-19

[Back to top]

Gas turbines have 3 primary sources of noise, the casing, inlet, and exhaust noise. Changes in SWL from these sources vary with the power of the turbine in MW. From Biess and Hansen , overall SWLs can be estimated by the following equations:

• Casing: Lw = 120 + 5log(MW)
• Inlet: Lw = 127 + 15log(MW)
• Outlet: Lw = 133 + 10log(MW)

From the single number SWL we can then determine octave bands by subtracting the following values from the equations above.

-	31.5	63	125	250	500	1000	2000	4000
Casing	10	7	5	4	4	4	4	4
Inlet	19	18	17	17	14	8	3	3
Outlet	12	8	6	6	7	9	11	15

The dialogue box which accepts the inputs of turbine power, enclosure type, and which sound source (casing, inlet or exhaust) is shown below. The tool also incorporates reductions for enclosures.

The tool allows a user to make adjustments to the desired turbine inputs and displays a live sound power spectrum which will be inserted once ok is clicked.

[Back to top]

Steam turbine noise emissions may be estimated in direct relation to their power, however, these turbines are typically much smaller than gas turbines and their power measured in kW rather than MW. From Biess and Hansen , an estimation of the SWL radiated is:

• Lw = 93 + 4log(kW)

From the single number SWL we can then determine octave bands by subtracting the following values from the above.

	31.5	63	125	250	500	1000	2000	4000
Casing	11	7	6	9	10	10	12	13

The tool allows a user to make adjustments to the desired turbine inputs and displays a live sound power spectrum which will be inserted once ok is clicked.The tool also incorporates reductions for enclosures.

[Back to top]

It is often the case that these turbines will be externally wrapped with insulation, in such cases, additional noise reductions should be taken in accordance with the table below.

	Type 1a	Type 2b	Type 3c	Type 4d	Type 5e
31.5	2	4	1	3	6
63	2	5	1	4	7
125	2	5	1	4	8
250	3	6	2	5	9
500	3	6	2	6	10
1000	3	7	2	7	11
2000	4	8	2	8	12
4000	5	9	3	8	13
8000	6	10	3	8	14

Type a) Glass fibre or mineral wool thermal insulation with lightweight foil over the insulation

Type b) Glass fibre or mineral wool thermal insulation with 20 or 24 gauge aluminium

Type c) Enclosing metal cabinet for the entire packaged assembly, with open ventilation holes and with no acoustic absorptive lining inside the cabinet

Type d) Enclosing metal cabinet for the entire packaged assembly, with open ventilation holes and with internal acoustic absorptive lining inside the cabinet

Type e) Enclosing metal cabinet for the entire packaged assembly with all ventilation holes into the cabinet muffled and with acoustic absorptive lining inside the cabinet

[Back to top]