10.4 PAPER MACHINE WET END

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In a paper machine, the fluid pulp is introduced at one end (the ‘‘wet end’’)

of the machine, where the fluid flows across the couch roll to begin the

drying process, as shown in Fig. 10-5. The wet paper moves over a screen

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FIGURE 10-3 Schematic of the metal cut-off saw and enclosure.

Copyright © 2003 Marcel Dekker, Inc.

to the suction rolls, and the drying and sizing process is continued down the

machine (Salmon et al., 1975).

The major source of noise around the wet end of the paper machine is

the couch roll suction air movement, the pumps, and the whipper roll. The

whipper roll provides a mechanical beating action on the felt of the paper

machine to keep the web felt clean. The air around the wet end of the paper

machine has a very high humidity, so it is not practical to provide an

acoustic enclosure for the machine itself.

10.4.1 Analysis

The noise level at the wet end of the operator aisle in front of the machine

was in the range from 92 dBA and 94 dBA. The sound pressure level spec-

Case Studies in Noise Control 483

FIGURE 10-4 Sound pressure level spectrum for the metal cut-off saw noise at the

operator’s location (1) before installation of the enclosure, LA ј 97 dBA, and (2)

after installation of the enclosure, LA ј 84 dBA.

Copyright © 2003 Marcel Dekker, Inc.

trum around the wet end is shown in Fig. 10-6. Higher levels of approximately

100 dBA were measured around the couch roll.

The operator usually spent about 1 hour making adjustments around

the couch roll area рLA ј 100 dBA) and about 2 hours making general

observations in other areas around the machine, where the sound level

was 92 dBA. The remainder of the 8-hour day was spent at the control

station, where the sound level was about 94dBA. The allowable exposure

time, according to the OSHA standards discussed in Chapter 6, may be

calculated from Eq. (6-2):

LA1 ј 100 dBA; T1 ј 2 hours

LA2 ј 92dBA; T2 ј 6:063 hours

LA3 ј 94dBA; T3 ј 4:595 hours

The corresponding noise exposure dose (NED) for the situation if no noise

control measures were implemented may be determined from Eq. (6-3):

NED ј

1

2 ю

2

6:063 ю

5

4:595 ј 0:500ю0:330ю1:088 ј 1:918 > 1

This noise exposure dose is not in OSHA compliance.

If it is impractical to enclose the noise source (the paper machine wet

end), then an alternative approach would be to enclose the operator. A

personnel booth may be used to house the operator and the operating controls.

The operator would need to go outside the enclosure to make adjustments

around the couch roll and to make general observations along the

machine. If the operator can spend 5 hours each day in the booth, where the

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FIGURE 10-5 Layout of the area around the wet end of the paper machine.

Copyright © 2003 Marcel Dekker, Inc.

noise level is less than 85 dBA, the corresponding noise exposure would be in

OSHA compliance:

NED ј 0:500 ю 0:330 ю 0 ј 0:830 < 1

If a personnel acoustic enclosure were selected to reduce the operator’s

noise exposure, the required transmission loss for the wall facing the

machine may be estimated as follows. The sound pressure level outside

the enclosure may be approximated by Eq. (7-73) with the direct sound

field term, Q=4_r21

, neglected:

Lp1 _ LW _ 10 log10рR1=4Ю ю 0:1 ј LW _ 10 log10 R1 ю 6:1 dB (10-6)

Case Studies in Noise Control 485

FIGURE 10-6 Sound pressure level spectrum (1) around the wet end of the paper

machine, LA ј 94 dBA, Lp ј 99 dB, and (2) within the personnel enclosure,

LA ј 75 dBA, Lp ј 85 dB.

Copyright © 2003 Marcel Dekker, Inc.

The term R1 is the room constant for the space outside the enclosure and LW

is the power level of the source of noise. The sound pressure level within the

enclosure may be estimated from Eq. (7-71):

Lp2 ј ЅLW _ 10 log10 R1_ ю 10 log10Ѕр4Sw=R2Ю ю 1_ _ TL ю 0:1 (10-7)

The quantity Sw is the surface area of the enclosure wall facing the noise

source. Note that, for a wall having dimensions of about 4.27m

(14 ft) _ 2.45m (8 ft), the parameter рSw=2_Ю1=2 ј 1:287m (4.22 ft). It is

likely that the operator would be located within 1.3m of the wall; therefore,

the condition of Eq. (7-71) would apply.

The pressure level difference between inside and outside of the enclosure

may be found by combining Eqs (10-6) and (10-7):

_Lp ј Lp1 _ Lp2 ј _10 log10Ѕр4Sw=R2Ю ю 1_ ю TL ю 6:0 dB (10-8)

By using acoustic treatment inside the enclosure, an average surface

absorption coefficient of __ _ 0:30 could be achieved. For preliminary

design, let us try a ratio of wall area (wall facing the paper machine) Sw

to total wall surface area So (i.e., Sw=SoЮ of 0.15:

4Sw

R2 ј

4рSw=SoЮр1 _ __Ю

__ ј р4Юр0:15Юр1 _ 0:30Ю

р0:30Ю ј 1:40

Let us consider a sound pressure level reduction of _Lp ј 15 dB. Making

these substitutions into Eq. (10-8), we obtain the following estimate of the

required transmission loss for the wall facing the paper machine:

TL ј 15 _ 6 ю 10 log10р1:40 ю 1Ю ј 13 dB

Let us suppose that the wall facing the paper machine is a 2 _ 4 frame

construction with 1

2-inch plywood sheets attached to both sides of the frame.

In the 1000 Hz octave band, the transmission loss is approximately 43 dB

рat ј 50:1 _ 10_6Ю for the wall (Reynolds, 1981). Let us suppose the wall has

two windows, each 0.914m (3 ft) _ 1.524m (5 ft). The windows are aluminum

frame windows, with double glazing and a 12.7mm (1

2 in) thick air space

between the panes. The transmission loss in the 1000 Hz octave band for this

window is approximately 40 dB рat ј 100 _ 10_6Ю (Reynolds, 1981).

The overall transmission loss for the partition may be found from Eq.

(4-173). The total surface area of the windows is as follows:

S1 ј р2Юр0:914Юр1:524Ю ј 2:786m2 р30 f t2Ю

Suppose the partition dimensions are 4.267m (14 ft) _ 2.438m (8 ft) high.

The wall portion has a surface area as follows:

S2 ј р4:267Юр2:438Ю _ 2:786 ј 10:403 _ 2:786 ј 7:617m2 р82 f t2Ю

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Copyright © 2003 Marcel Dekker, Inc.

The overall transmission loss for the wall facing the paper machine may be

calculated:

_at ј

_Sjat;j

Sw ј Ѕр2:786Юр100Ююр7:617Юр50:1Ю_р10_6Ю

р10:403Ю ј 63:46_10_6

The wall transmission loss is as follows:

TL ј 10log10р1=_atЮ ј 10 log10р63:46_10_6Ю ј 42 dB > 13dB

Therefore, the proposed enclosure wall construction should yield satisfactory

acoustical results.

10.4.2 Control Approach Chosen

To protect the paper machine operator from excessive noise exposure at the

wet end, an operator enclosure was provided. The paper machine operating

controls and main instruments were placed within the booth, and doubleglazed

viewing windows were provided on the wall of the booth facing the

machine to allow the operator to observe the machine operation continuously.

A solid wood core door with gaskets and drop closure seals was

provided in one of the side walls for entry into the enclosure.

The operator’s booth was constructed with 2_4-inch framing with 1

2-

inch (12.7mm) thick plywood sheathing on the inside and outside. The

dimensions of the room were 4.27m (14 ft) _ 3.05m (10 ft) _ 2.44m (8 ft)

high. Two double-glazed windows, 3 ft_5ft (0:914m_1:524mЮ, were provided

for operator observation of the machine operation. The ceiling and

upper one-half of the walls were covered with acoustic tile to reduce internal

noise levels. The roomwas provided with lighting, heating and air conditioning

for operator comfort.

The sound pressure level spectrum inside the booth is shown in Fig.

10-6. The A-weighted sound level was 75dBA, and the overall sound pressure

level was 85 dB, with the main contribution at the lower frequencies

(below 250 Hz). This sound level was well below the 85 dBA required to

meet OSHA criteria.

10.4.3 Cost

The personnel enclosure was constructed in-plant for a cost of approximately

$7500, including materials and in-plant labor. Although greater

noise attenuation could be achieved by purchasing commercial acoustic

enclosures (NIOSH, 1975), the higher attenuation was not required in this

application. A typical commercially available personnel enclosure of the size

needed for this application would cost approximately $15,000.

Case Studies in Noise Control 487

Copyright © 2003 Marcel Dekker, Inc.

10.4.4 Pitfalls

Most of the problems with the use of the personnel enclosure tend to be

non-acoustical. For example, it is essential that the operator has a clear and

unobstructed view of the paper machine for monitoring purposes. This

consideration places a restriction on the location of the booth and on the

size and location of the windows in the booth.

It is important that the enclosure door be sealed effectively to prevent

noise ‘‘leakage’’ around the door. The windows should be double-glazed to

provide the largest attenuation of sound passing through the windows.

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