Designing low noise burners inside firetubes

ELSEVIER

Applied Acoustics, Vol. 51, No. 2, pp. 121b139, 1997 0 1997 Else&r Science Ltd. All rights reserved

Printed in Great Britain PII: SOOO3-682X(96)00054-0 0003-682X/97/317.00+ .OO

Designing Low Noise Burners Inside Firetubes

Gary SamP and Jim Jordan6

“Director, NATCO Research and Development, Tulsa, OK, USA bProject Engineer, ZEECO Inc., Tulsa, OK, USA

(Received 19 April 1996; revised version received 19 September 1996; accepted 23 October 1996)

ABSTRACT

Frequently, natural draft burner designs used in indirect heaters fail to meet the low noise standard of 85-88 dBA three feet from the flame arrestor. The noise encountered with indirect burner designs has been shown to be related to the gas velocities of the nozzle andfiretube. Testing showed that when the nozzle velocity was st@cienily greater than the firetube velocity, the low frequency rumble that accompanies current designs ceased. Data ObtainedfromJield testing was used to construct a relationship between the burner noise level and the gas volume expansion ratio, burner air to fuel ratio, mixture flow rate, orifice velocity, burner area, and the number of burners. Knowing the above variables, which are easily calculated, a prediction of the burner noise was made. 0 1997 Elsevier Science Ltd

Keywords: Burners, combustion, firetubes, indirect heaters, natural draft.

INTRODUCTION

Indirect heating systems are unique in that, unlike open flares and other combustion processes, the burner is enclosed within a firetube. These differ- ences can be seen in Fig. 1 which displays an assembled heating unit used in indirect heating systems. The enclosed design complicates matters as gas velocities within the firetube and access to secondary air become critical issues. The arrangement of the burner within the firetube and flame arrestor is seen in Fig. 2.

Several low noise burner designs have been found to exceed the low noise criteria of 85-88 decibels at a distance of three feet from the flame arrestor.

121

122 G. Sams. J. Jordan

These problems have been corrected by increasing the fuel pressure and removing a burner in multiple burner installations or decreasing the burner size in single burner applications. Therefore, a similar heat release is achieved but at a higher nozzle velocity.

It is speculated that this noise is a result of the velocity of the gases in the nozzle (1 in Fig. 2) failing to be significantly greater than the velocity of the combustion products in the firetube (2 in Fig. 2). This is expected as the primary air inspirated into the venturi mixer by the fuel is typically less than

Fig. 1. Firetube arrangement for heating process fluids.

Flame Arrest0

Fig. 2. Burner arrangement.

Designing low noise burners inside firetubes 123

that required for complete combustion. As a result, a secondary air flow is established within the firetube once the flammable mixture has emerged from the burner nozzle. As a result, the firetube velocities can frequently be greater than the nozzle velocities. The low nozzle velocity cannot overcome the higher firetube velocity; the result is a distinct low frequency pulsating rumble.

In order to ensure the nozzle velocity has sufficient energy to overcome the firetube velocity, a simple comparison can be made between the kinetic energies in each gas stream. At nozzle-to-firetube energy ratios lower than 5.35, the energy associated with the nozzle velocity is less than the energy associated with the flame in the firetube. Many low noise designs fail to meet this ratio limit. By increasing the fuel pressure, the energy ratio exceeds the minimum value and the low frequency rumble ceases.

An improved burner design criterion was established so that each burner had a theoretical energy ratio, or velocity head ratio, greater than 5.35. It should also be noted that velocity head ratios significantly larger than 5.35 (-80-100) would also create excessive noise. It is suspected that there is a window of relatively quiet burner operation between velocity head ratios of 7 and 50. An explanation of the

The velocity head ratio

A method of quantifying the

velocity head ratio is included below.

relationship between the firetube and nozzle velocities from known parameters can be found in the mixture to fuel relationship for natural draft burners displayed below.’

where R = ft3 of air-gas mixture per ft3 of gas (mixture to fuel ratio), K =Factor for throat size and venturi slope, F, =Factor for throat to port area ratio, Fb =Factor for port size and depth, P =Pressure of gas back of orifice, in of water, d =Specific gravity of gas (air = 1 .O), d, =Specific gravity of air-gas mixture (air = 1 .O), Qg =Gas input rate, ft3 per h, T =Absolute temperature of air-gas mixture in burner head, “F plus 460.

The mixture to fuel ratio, R, can be used to calculate the amount of primary air inspirated into the mixer according to eqn (2). The primary air is then used in eqn (3) to calculate the nozzle velocity. The velocity head ratio is defined as the ratio of the velocity heads (V2/2 g) multiplied by the densities of the gases. Equation 4 shows how the nozzle velocity is used to obtain the velocity head ratio. In eqn (5) the minimum velocity head ratio is found. At this point, the velocity of the gases in the firetube and nozzle are approximately equal; only the ratio of the densities of the gases in the firetube (at

124 G. Sams, J. Jordan

- 2400” F) and nozzle (ambient: N 75” F) are considered in the calculation of the minimum velocity head ratio which was determined to be 5.35.

(R - l>Qg = Qp (2)

(Qg + QP) = v

36ooAb ’

& v;pn VHR=F=-

$P/ yp1.

(3)

(4)

Minimum - VHR = - = q?f

= 5.35 (5)

where: Qp =Primary air rate (ft3/h), Ab =Open area of burner nozzle (ft2), VHR =Velocity head ratio. V, =Velocity of gases at the nozzle (ft/s), Vf =Velocity of gases in the firetube (ft/s), g =Gravitational constant (32.2 ft/s2), p,, =Density of gases at the nozzle (lb/ft3), py =Density of gases in the firetube (1 b/ft3)

MATERIALS AND METHODS

A series of burner tests were conducted using a water filled 72” ODx20 Lg. indirect bath heater equipped with a 24” firetube and 20’ Lg. stack. Flameco Inc., Tulsa, OK, USA provided a flame arrestor equipped with (2) 36” OD flame cells. The arrestor was designed to permit the burner assemblies to be changed easily. The configurations found in Table 1 were used to conduct the low noise burner tests. No sound absorbing material was used in the arrestor housing.

TABLE 1 Configuration for Low Noise Burner Testing

Burner configuration

Mixer Burner Orifice nozzle size

Design pressure

(psig)

1 6” Eclipse compound 24F- 1 Ferrofix l/2” & 39164” 18&7 2 Dual 5” Eclipse compounds 20F-1 Ferrofix 7/16” 7 3 5” Eclipse compound 20F-1 Ferrofix 7/16” & 37/64” 18 & 7 4* 4” Eclipse compound 16F- I Ferrofix l/2,, 20

* See note in Table 2.

Designing low noise burners insidejretubes 125

The gas flow rate was measured with a calibrated Rockwell gas meter. The stack gas temperature, combustion efficiency, excess air percentage, oxygen, carbon monoxide. carbon dioxide, and nitrogen oxide concentrations were measured and recorded with an Enerac 2000 stack gas monitor.

Gas flow to the burners was controlled by manually adjusting a ball valve. Pressure to the burner was measured with a O-60 psig gauge. The sound level in dBA was measured with a Radio Shack model 33-2035 sound meter. All tests were conducted at Natco’s manufacturing facility on May 16-17 1995 in Electra, TX. USA.

For each burner test, the measured gas flow rate (SCF/h), fuel pressure (psig), noise 3 from the flame arrestor (decibels), combustion efficiency, stack temperature (“F), carbon monoxide (ppm), carbon dioxide (“A), excess air (%), nitrogen oxides (ppm) and bath temperature (“F) were recorded.

RESULTS AND DISCUSSION

Effects of orifice size and heat release on noise level and stack oxygen

Figures 3-5 display the effects of increasing heat release on the noise level and stack oxygen concentration. At a given heat release, larger orifice sizes resulted in a reduction in combustion noise. The larger orifice tests also had stack oxygen levels lower than the smaller orifice sizes. This is expected as the higher velocities travelling through the smaller orifice would inspirate more

A/,’ ,I

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,’ ,’

/’

/ n”’

2.6 3.9 4.9 5 5.6 8.1 6.6 7.3 1.9 8.3 8.9

Hut Reluw (MM Btdhr)

-Noise-1R”Orifce ~N~oi~e-39M*Odfim . swkoxy!Jsn-1rorifkm + slack oxygen - 39m4” olifice _

Fig. 3. Heat release vs noise level and stack oxygen concentration using a 6” compound mixer with a 24 F-l FerroCx.


air, resulting in higher stack oxygen concentrations. Inspection of Figs 3 and 6 demonstrate this point. At a heat release of -8 MM Btu/hr, a 6” compound mixer fired with a l/2” orifice (point “A” in Figs 3 and 6) produced an inspirator credit 90% greater than that produced by a 39/64” orifice (point “B” in Figs 3 and 6). The inspirator credit is the energy contained in the air/ fuel mixture from the burner nozzle that will off-set some of the stack draft created by the difference in density between the cool ambient air and the hot

t ’ g 85

f 3 I P ‘5

80 z 1

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,/’ /

,/

U’

.

. 70,.,, I I : ) I I I :=I 0

2.6 3.0 4.9 5.0 5.6 6.1 8.8 7.3 7.9 a.3 0.9

Hul R.kala (MM EN/ho

t-Noise - 7/W Orifice l Stack Oxygen _ 7116” Orifice

Fig. 4. Heat release vs noise level and stack oxygen concentration using 2-5” compound mixer with 20 F-l Ferrofix.

-N&e - 7118” orifka 6 Noise - 37/84’ orifice + stack0xyQell-7/16’olifke * stack oxy!pl - 37m4’ 0liRc.e J

Fig, 5. Noise level and stack oxygen concentrationusing a 5” compound mixer with 20 F-l Ferrofix.

Designing low noise burners insiakjiretubes 127

stack gases. This draft, or small vacuum is approximately 0.01 inches of water column per foot of stack height.

Obviously, as the heat release for a given burner configuration increases, the combustion noise also increases. Also, as the heat release increases, the stack oxygen concentration decreases. This is expected as an increase in release requires more oxygen for complete combustion. The increase in inspirator credit caused by increasing the fuel gas flow rate through the same

85 -

93

91 f

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.03 0.09 0.1

MiXWIWphtW~lnchaOfWrrCoknrll

~6~compwndwl1~0llnm --tScmpoundr*~*Chlfim -~--2-5'Campoundw/7/10'O1ifim

0 5~CLlmpmdwi7/16olmm 6 5'CmQomd~37m4'olinca

Fig. 6. Mixer inspirator credit vs burner noise, mixer type, and orifice size.

Fig. 7. Heat release vs burner noise level, mixer type, and orifice size.

128 G. Sams. J. Jordan

size orifice does not entrain enough excess air to sustain the stack oxygen concentration.

Figure 3 and Fig. 4 show that at the same heat release rate, two 5” compound burners with a 7/ 16” orifice produced nearly the same sound level as a single 6” compound burner operating with a larger 39/64” orifice. It can be seen in Fig. 5 that at the same heat release rate, a single 5” burner with a larger orifice was quieter than the same burner with a smaller orifice.

Figure 7 compares the heat release rate and the produced noise of all the mixers and orifice sizes considered in the study. It is again apparent that, for a given mixer and heat release, a larger orifice operates more quietly. Com- paring the mixers, it is evident that, for a given heat release and similar orifice size operating at high pressures, a 6” mixer is more quiet than the 5” mixer. The 5” compound mixer with a 7/16” orifice operating at high fuel pressures was considerably louder than the dual 5” mixers with the same orifice operating at lower fuel pressures. This tends to indicate that increased burner areas (reduced nozzle velocities) and/or reduced fuel pressures decrease the noise of combustion. This assumes that the minimum velocity head ratio, discussed earlier, has been achieved.

Effects of nozzle velocity, and mixture flux on noise level

Figure 8 displays a plot of the nozzle velocity vs the combustion noise level for each mixer and nozzle combination tested. At a fixed velocity, larger orifice sizes were more quiet despite having a higher heat release value. For example, at a nozzle velocity of -95 ft/sec, a 37/64” orifice at 5 psi (point

Nou* vek.city (fuS)

& 6’ comwund WI l/z' orific% -t 6” Compound w/391(14’ Orifice -0 2-S” Compound w/ 7/W Orifice

0 5’ compound w/ 7/w orifxe + 5” compwnd VI/ 37/64’ oriliee

Fig. 8. Nozzle velocity vs burner noise, mixer type, and orifice size.

Designing low noise burners insi&$retubes 129

“C” in Figs 7, 8 and 10) released over 7 MM Btu/h at a noise level of 88 decibels. A smaller sized 7/l 6” orifice in the same mixer/burner configuration released only 5.5 MM Btu/h at 10 psi (point “D” in Figs 7, 8 and 10) yielding a combustion noise of 91 decibels. Considering the orifice velocity in Fig. 10, the noisy-low heat release burner (“D”) theoretically had orifice velocities over 1400 ft/s. The value for the quiet-high heat release burner (“C”) was over 20% less at 1100 ft/s.

77i q i 7oIl 12UO 17ca 2200 2700 32Qll

Nozzb Mixhan Flow Rata (SCFmuam inch

-- 8” compound w/ l/2” olifka

c_

--t 6. compwnd w/ 3W’ OImm - * - 2-S” compound WI 7/w Oflflm 0 5” compoulld w/ 7/w OrlRcs * sccqoundwl37m4*o

Fig. 9. Gas and primary air mixture flow rate per square inch of burner nozzle area vs noise level, mixer type, and orifice size.

+ 800 800 1000 1200 1400 1600 1500

oriR=v.l=ew=G

-6’cMnpoundw(l~Oiifk4 -0’crmpoundwl3911)4’Orilka -d- 2-5’Cmpoundw/7/1COrMa, 0 5. compound WI 7116. OImce * 5’ canpwnd WI 37184’ Ormca

Fig. 10. Orifice velocity vs noise level, mixer type, and orifice size.


The fuel gas and primary air flux through the burner nozzle is considered in Fig. 9. This plot is essentially identical to Fig. 8 only the numerical values of the ordinates have changed. It can be seen that for a constant mixture flux of 1700 SCF/h/in2, two 5” compound mixers with 7/16” orifices (point “E” in Figs 7 and 9) were 3 decibels louder than a single 5” compound with the same size orifice (point “F” in Figs 7 and 9) while producing a heat release almost two times that of the single compound mixer. A 3 decibel increase in the sound level is expected as the two identical burners produced twice the acoustical energy of the single burner while doubling the heat release. The effect of doubling the sound power and the resulting 3 decibel increase can be seen more clearly in eqn (7).

Considering Figs 8 and 9 alone, one might conclude that smaller burners (5”) are more quiet than larger burners (6”). This is not necessarily the case; at a constant nozzle velocity or mixture flux, the heat release of the larger burners was significantly greater than that of the smaller burners.

Effects of orifice velocity, gas flux, fuel pressure, and velocity head ratio on noise level

The effects of the orifice velocity on the noise level three feet from the flame arrestor are considered in Fig. 10; the fuel pressure recorded in psig for each test is displayed beside the data point. For the same pressure, mixer, and burner nozzle, it can be seen that the gas velocities across the orifice increased by a maximum of only 13% as the orifice size decreased from 39/64” to l/2” in the 6” burner and 37/64” to 7/ 16” in the 5” burner. However, as mentioned earlier with points “C” and “D”, the orifice velocity increased over 36% as the orifice size decreased from 37/64” (“C”) to 7/16” (“D”) at a constant nozzle velocity (Fig. 8). The heat release at point “C” was 30% greater than that at point “D”. The nozzle velocity remained constant at these two points while the pressure just upstream of the orifice increased from 5 to 10 psi. The orifice velocities of points “C” and “D” were found to approach the sonic velocity limit. The sonic limit is the speed of sound in a media at a specified temperature and pressure. The flow velocity of gases through an orifice cannot travel at rates higher than this theoretical value;2 the speed of sound is determined by eqn (6).

v, = J144kgpv (6)

where: VS =Speed of sound (ft/s), k =Ratio of specific heat at constant pressure to the specific heat at constant volume ( Cp/Cy), V =Specific volume of media (ft3/1 b), g =Gravitational constant, P =Pressure (psi).


As the orifice size was decreased, the combustion noise reduced as less fuel gas was available for combustion. Similar trends are seen in Fig. 11. As the fuel pressure increases, heat release increases. During testing, higher pressures (> 10 psi) were not obtainable with the larger orifice sizes due to a limited gas supply. For the same reason, actual design pressure for the high pressure burners was not obtainable (18 psig).

The gas flux through the orifice is displayed in Fig. 12. Once again, the trends are identical to those observed with the orifice velocity (Fig. 10) and similar to those observed with the fuel pressure (Fig. 11). For a constant orifice gas flux, the noise level and corresponding heat release rates of the smaller diameter orifice were lower than those for larger orifice diameters. For example, at an orifice flux of m 25 000 SCF/h/in2, a 6” compound mixer with a 39/64” orifice released over 7 MM Btu/h of heat (point “G” in Fig. 7 and 12) while maintaining a combustion noise level of 88 decibels. The same mixer with a smaller l/2” orifice released only 5 MM Btu/h of heat at a noise level of 87 decibels (point “H” in Figs 7 and 12).

Considering the effects of the size of the mixer, at a constant orifice flux of w 32 000 SCF/h/in2, a 6” compound mixer with a 39/64” orifice (point I in Figs 7 and 12) released over 9 MM Btu/h at a noise level of 93 decibels. A 5” compound mixer with a 37/64” orifice (point J in Figs 7 and 12) released about 8.5 MM Btu/h at 92 decibels.

These results indicate that a reduction of both the nozzle and the orifice velocities (both fuel pressure dependent) to levels that yield velocity head ratios just greater than the suggested minimum value of 5.35 will result in

Fig. 11. Fuel pressure vs burner noise, mixer type, and orifice size.


quieter combustion. This is accomplished by using larger orifice sizes, burner nozzles, and mixers with more cross-sectional surface area.

This idea is reinforced in Fig. 13. As the velocity head ratio increases, the noise level sharply increases, especially for the burners with larger cross-sectional areas (dual 5” and single 6”). Therefore, a minimum velocity head ratio above the lower limit of 5.35 is suggested. It is speculated that if tests were conducted under conditions that yield velocity head ratios below 5.35,

79 t 0

77 J I .,__ 4

15000 2OcPm 25000 3oooo 35000 4wOo 45ooa

Orifice Gas Flow Rate (SCF/hr)/squan inch

+B” Compound w/ l/2’ orifice --c 6’ Compound w/ 3W Orifice 4 2-S Compound w/ 7116’ Orifce

0 5’ COmwllnd w/ 7/w orifice * 5” Comoound w/ 37iS4’ Orifice /

Fig. 12. Gas flow rate per square inch of orifice area vs noise level, mixer type, and orifice size.

95 jp- ~---~ -.---

93 -- !’

Proposed

I ; : Trend Lines h 0

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Area ~ Head Ratio v a

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0 5 10 15 20 25 30 35 40 45

operating V.locity Head Ratio

-c 8’ Compound VI/ l/2” orifice --c 8’ Compound WI 39&4’ Odtica + - 2-5” Compound w, 7/l S’ OdRce

0 5” Compound w/ 7/w Orinca * 5” Compound w/ 37m4’ orifice

Fig. 13. Operating velocity head ratio vs noise level, mixer type, and orifice size.

Designing low noise burners insidejiretubes 133

the noise levels would increase sharply due to the low frequency pulsating rumble. Several current low noise designs, which frequently fail to meet the low noise criteria, fall within this range of excessively low velocity head ratios. This area of excessive noise below velocity head ratios of 5.35 is approximated by the dotted trend lines in Fig. 13.

It is also apparent that with larger area burners, the effect of the orifice size on the velocity head ratio is minimal when compared to the difference between orifices of smaller area burners. Ratios differ by a maximum of about 5 with changes in mixer or orifice size on the larger area burners (dual 5”, single 6”). The effect of orifice size on smaller area burners (single 5”) is quite dramatic. Velocity head ratios differ by as much as 14 when the orifice of a 5” compound mixer is changed from 7/16” to 37/64”.

To ensure noise levels below 88 decibels, a rule of thumb would be to design 6” and dual 5” compound burners to operate at velocity head ratios below 10-14. A single 5” compound burner should operate below 25 for a 37/64” sized orifice and below 35 for a 7/16” orifice. All of these ratios, of course, must be larger than the minimum value of 5.35. It is speculated that smaller area burners (l-4”) may successfully operate at velocity head ratios much higher than 5 and 6” burners; possibly exceeding 100-200 in some cases.

It should also be noted that all of the tests, which were designed to operate above the minimum velocity head ratio, operated stably at all test conditions. It was possible to reduce, or turn down, the test pressure from the design pressure (or maximum attainable pressure) to 1 psig and less.

To this point, optimizing the velocity head ratio has been achieved by changing burner nozzle and orifice sizes, thereby setting the nozzle velocity to a value slightly greater than the firetube velocity. Another option in optimizing the velocity head ratio not investigated in this report involves adjustment of the firetube velocity. This would be accomplished during the initial design of the heater by increasing the diameter of the firetube. Lower firetube velocities would increase the number of burner and orifice combinations that would successfully operate in the heater by reducing the nozzle velocity required to meet the minimum velocity head ratio of 5.35 (see eqn (5)).

Effects of heat release density on firetube efficiency and percentage of excess air

The firetube heat release density is defined as the heat release rate divided by the cross-sectional area of the firetube (in*). For this test, the 24” firetube had an area of 415.5 in*. The firetube efficiencies calculated by the stack gas analyzer are plotted against the corresponding heat release densities in Fig. 14. It is recommended that heat release densities not exceed 21000 Btu/h in*.


Figure 14 shows that as the heat release density increases and the stack oxygen decreases (Fig. 15), the firetube efficiency increases. At 21000 Btu/h in*, near 65% firetube efficiency is seen. This is a typical efficiency for most firetubes. Above this point, the combustion changes from deflagration to detonation.

The effect of the heat release density on the percentage of excess air is seen in Fig. 15. As the heat release density increased, the amount of excess air decreased. At 21000 Btu/h/in*, no excess air was available. At this point, incomplete combustion occurs which is commonly indicated by high carbon monoxide and low stack oxygen concentrations.

Prediction of burner noise: low noise burner design

Based on data obtained from this study, it was determined that the noise produced by a burner is a function of several design parameters. These parameters include the heater duty (MM Btu/h), firetube inside diameter, firetube length (feet), number of firetubes, firetube efficiency at maximum capacity (typically 65%), fuel gas higher heating value (HHV) and gravity, burner nozzle/mixer size and type, and the operating pressure and excess air percentage (typically 15%) at the design conditions.

Once the above design parameters have been selected, several other process parameters are fixed. These parameters include the orifice size, heat release density and flux (heat transfer across the firetube wall), primary air, secondary air, and fuel rates, mixture/fuel ratio, nozzle and firetube velocities, hot

.

Fig. 14. Heat release density vs firetube efficiency.


mixture densities and temperature in the firetube, inspirator credit, and the velocity head ratio. This data can be used to make a prediction of the noise level for a given burner design.

The sound power level-(PWL) is a dimensionless value (dB) quantifying the total acoustical energy radiated by a noise source (eqn (7)). The human audible frequency range is restricted to - 20-20 000 Hz; frequencies higher and lower than this range are not heard by human observers. Therefore, the PWL, which measures the total sound energy emitted, may not represent the amount of actual sound that is audible to human observers.

PWL = lOlog $ ( > 0

where PWL =Sound Power Level (dB), W =Acoustical Energy (watts), W, =Reference Acoustical Energy (lo-l2 watts). The dBA scale (or A weighted scale) has been established to approximate noise levels within the 20-16 000 Hz range. This range is frequently reported in 9 octave bands; these bands are denoted by the center frequency in each band ranging from 3 l-8000 Hz.

Using the correlation established by Narasimhan,3 the difference between the sound power level (dB) and the dBA scaled value did not exceed 4-5% for these tests. Knowing this, the measured dBA values were substituted into

Fig. 15. Heat release density vs excess air percentage.

136 G. Sam, J. Jordan

eqn (7) for the PWL and an acoustical energy (W) could be calculated for each test case.

A proprietary parameter termed the acoustical energy of combustion has been developed by Natco which establishes the dependence of combustion noise on the gas volume expansion ratio, air to fuel ratio, air plus fuel flow rate, orifice velocity, burner area, and the number of burners. The acoustical energy of combustion parameter allows one to predict the audible acoustical energy in watts based solely on values easily calculated from the design conditions. Figure 16 shows the relationship between the acoustical energy of combustion parameter and the acoustical power produced by each test. A linear and non-linear fit to the data is also displayed. For acoustical energy of combustion parameters greater than 2.75xE20, the linear relationship is used. For acoustical energy of combustion parameters lower than 2.75xE20, the non-linear relationship is used.

Once the acoustical energy is known, an approximation of the audible sound in dBA can be made using eqn (7). Examples of the noise levels and stack temperatures predicted vs the measured values are reported below in Table 2. The stack oxygen levels were not predicted but used as input data which assisted in determining the stack temperature.

The burner noise predictions come within + /-8% of the measured values. If the propane and high oxygen content cases are ignored, the estimations

.

Fig. 16. Burner acoustical power parameter vs acoustical power.

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138 G. Sam. J. Jordan

come within +/-3% of the measured values. Noise predictions for cases where the stack oxygen content is greater than 15% or where methane is not the source of combustion may not be adequately represented by the model. Stack temperatures, being a function of the stack oxygen concentration, were within + i-15% of the measured values.

CONCLUSIONS

Based on the field tests, it is evident that a minimum velocity head ratio of 5.35 at all operating pressures is needed for the least amount of combustion noise during operation. Ratios much greater than this minimum value will also produce excessive noise. A window of low noise operation (< 88 decibels) is found as the velocity head ratio falls between 5.35 and 10 for larger area burners (6” and dual 5” compound mixers), 25 for a single 5” compound mixer with a 37/64” orifice and 35 for a 7/16” orifice. The maximum heat release rates for the burners involved in the field test ranged from - 5- 7.5 MM Btu/h. Low noise burners should be designed to exceed the minimum velocity head ratio at 1 psig as the ratio tends to increase with increasing pressure. This will prevent excessive noise and rumble as the burner starts up and shuts down.

Minimizing the velocity head ratio is essentially accomplished by reducing the firetube velocity or increasing the nozzle velocity. The latter is accomplished by decreasing mixer, orifice, and burner nozzle sizes. Ideally, just enough area should be provided by the orifice and burner nozzle for the gas and primary air to pass that the resulting velocity head ratio just exceeds the 5.35 limitation. Low noise operation may be best achieved at low fuel pressures and stack oxygen concentrations above 2%. A disadvantage to many low noise, low pressure designs is a significant reduction in turndown cap- ability as low noise combustion ( < 90 decibels) is generally not obtainable at elevated fuel pressures.

From this study, two primary conclusions can be made. First, it was determined that nozzle velocities slightly greater than the firetube velocity are needed for relatively quite operation (< 88 dBA). Second, the development of the acoustical energy of combustion parameter appeared to adequately predict the noise produced by burners inside of firetubes within + /-3%. It is emphasized that the results from these tests are based on only five commonly used burner and orifice size configurations; this is a small fraction of the burner/orifice combinations available. The accuracy of the acoustical energy of combustion parameter and the reliability of the minimum velocity head ratio theory could be improved by examining other burner configurations, possibly with fuels other than natural gas.

Designing low noise burners insidejretubes 139

REFERENCES

1. Shnidman, L., Gaseous Fuels - Properties, Behavior, and Utilization, American Gas Association, New York, NY, 1954.

2. Reed, R. D., Furnace Operations, Gulf Publishing Company, Houston. TX, 1981.

3. Narasimhan, N. D., Predict Flare Noise. Hydrocarbon Processing, 1986, April, 133-136.

4. Beranek, L. L., Acoustics, McGraw Hill Book Company, New York, NY. 1954. 5. Bragg, S. L., Combustion Noise. Journal of the Institute of Fuel, 1963, January,

12-16. 6. Crane Co., (Engineering Division), Flow of Fluids Through Valves, Fittings, and

Pipe, Crane Co., New York, NY. 7. De Biase J. L., Criteria and Design Specifications for Noise Control of Indus-

trial Plants. Noise and Vibration Control Engineering-Proceedings from the Purdue Noise Control Conference. 14-16 July, Lafayette, IN, pp. 142-149.

8. Grumer, J. and Harris, M. E., Flame-Stability Limits of Methane. Hydrogen, and Carbon Monoxide Mixtures. Industrial and Engineering Chemistry, 1952, (July), 1547-l 553.

9. Grumer, J., Predicting Burner Performance with Interchanged Fuel Gases. Industrial and Engineering Chemistry, 1949, (December), 27562761.

10. Gruner W. J., A Unitary Method for Prediction of Rocket Engine Noise. Noise and Vibration Control Engineering-Proceedings from the Purdue Noise Control Conference, 1416 July, Lafayette 1971, IN, 503-509.

11. Seebold J. G., Noise Control in Outdoor Process Plants. Noise and Vibration Control Engineering-Proceedings from the Purdue Noise Control Conference. 14 16 July, 1971, Lafayette, IN, 163-168.

12. Smith, T. J. B. and Kilham, J. K., Noise Generation by Open Turbulent Flames. The Journal of the Acoustical Society of America, 1963, 35, 715-724.

13. Walker, P. L. and Wright, C. C., Hydrocarbon Burning Velocities Predicted by Thermal Versus Diffusional Mechanisms. Journal of the American Chemical Society, 1952, 74, 37693773.

14. Walker, P. L. and Wright, C. C., Stability of Burner Flames for Binary and Tertiary Mixtures of Methane, Carbon Monoxide and Water Vapour. Fuel, 1952,31, 374.

15. Wilson, C. W., Lifting and Blowoff of Flames from Short Cylindrical Burner Ports. Industrial and Engineering Chemistry, 1952, 44, 2937-2942.

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Designing low noise burners inside firetubes