burner turndown ratio

8/12/2019 Burner Turndown Ratio

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ratios as a very desirable characteristic in allboiler systems.

But is that really the case? Or is it a mar-keting myth, advanced by boiler andburner manufacturers?

If we are to test this theory then, logi-cally, we must first address the question of

what exactly is meant by burner and/orboiler turndown. If one accepts our simpledefinition of boiler turndown (ratio) asthe ratio of repeatable maximum to mini-mum boiler output, then it would appearthat high turndown should be more fuel-efficient than a lower turndown ratio, be-

A n Internet search for boilerturndown or burner turn-down using one of the searchengines with advanced search capabilities

will return a substantial number of docu-ments. Most of these documents are com-mercial, not academic or scientific, in na-ture, and all of them advance the thesis thatturndown is always good, and the moreturndown one has the better, regardless of the specific applicati on or the type of boilereven (or especially) the Scotch ma-rine firetube boiler. So the conventional

wisdom, clearly, promotes high turndown

BOILERTURNDOWN

How much is enough?

BSE June 2002 31

FEATURE ARTICLEby David Thornock and Larry Clark

Excessair

PercentO2

PercentCO2 300

9.5

12.1

15.0

21.1

24.5

28.1

31.9

35.9

40.3

2.0

2.5

3.0

4.0

4.5

5.0

5.5

6.0

6.5

44.9 7.0

10.7

10.4

10.1

9.6

9.3

9.0

8.7

8.4

8.2

7.9

83.1

83.0

82.8

82.5

82.3

82.1

81.9

81.7

81.5

81.2

82.6

82.5

82.3

82.0

81.1

81.6

81.4

81.1

80.9

80.6

82.2

82.0

81.8

81.5

81.3

81.1

80.8

80.6

80.3

80.0

81.7

81.5

81.4

81.0

80.8

80.5

80.3

80.0

79.7

79.4

81.2

81.1

80.9

80.5

80.2

80.0

79.7

79.5

79.2

78.8

80.8

80.6

80.4

79.9

79.7

79.5

79.2

78.9

78.6

78.2

80.3

80.1

79.9

79.4

79.2

78.9

78.6

78.3

78.0

77.6

79.8

79.6

79.4

78.9

78.7

78.4

78.1

77.8

77.4

77.0

79.4

79.1

78.9

78.4

78.1

77.8

77.5

77.2

76.8

76.4

78.9

78.7

78.4

77.9

77.6

77.3

77.0

76.6

76.2

75.8

78.4

78.2

77.9

77.4

77.1

76.7

76.4

76.0

75.6

75.2

320 340 360 380 400 420 440 460 480 500

Power burners Recommended ranges

Net stack temperature F

TABLE 2.Combustion efficiencies for burners firing natural gas.


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cause performance is being opti-mized to the actual load profile.Burner turndown has also been de-fined as the reciprocal of the heatrelease of the burner normalized by the full firing rate. 1 That definition

would, too, imply the desirability of high turndown. However, withoutan analysis of the boiler loads, oper-ating cycles, and overall boiler effi-ciencies, those assumptions shouldnot be made.

This article will examine the en-ergy loss and resulting impact onboiler efficiency when comparing aboiler operating with a modulatingburner having a 10:1 turndown tothe same boiler operating at a 4:1turndown. The energy balance ap-proach taken is both simple andstraightforward and presents a rea-sonable evaluation that should beunderstood by anyone familiar withboiler and burner theory.

Boiler SelectionThe boiler selected for this study

was a 750 Bhp Scotch marine hot-wa-

ter boiler firing only natural gas. Thistype of boiler was chosen because of

32 June 2002 www.ABMA.com

BOILER TURNDOWN

its relatively low stack temperaturescompared to, for instance, a high-pressure steam boiler. If our argumentis correctthat higher turndown may not always yield higher overall effi-cienciesthen the hot-water boilerrepresents a best-case scenario for

the heat loss analysis because at higherstack temperatures, the differences be-tween high- and low-turndown oper-ations should be even greater.

The boiler-water outlet tempera-ture was set at 220 F, resulting in acorresponding stack-gas outlet tem-perature of approximately 251 F athigh fire. Proprietary heat-transferprograms were used to calculate exit-gas temperatures for the 10:1 turn-down, 4:1 turndown, pre-purge, andpost-purge cases.

Key parameters and calculated re-sults for the case study are shown inTable 1.

Note that the amount of excess airrequired at the higher turndown ra-tio is significantly greater than thatrequired for the lower turndowncondition. This is due to the need tomaintain good mixing at the burnerfront and cool the firing head. The

flue-gas O 2 concentrations used inthis analysis were 9 percent and 5

percent, respectively, for the 10:1and 4:1 turndown cases. As a practi-cal matter, burners operating atlower turndown ratios can typically be set up with lower O 2 , which

would result in addit ional energy savings.

Energy ProductionOur energy analysis focused on the

boilers energy input and losses. A control volume was used that in-cluded the burner and boiler lossesthrough the stack. The analysis didnot include the electrical and othernon-thermodynamic system losses,concentrating instead only on theboiler heat transfer and stack gaslosses.

The time frame used for the study was one to eight hours for the case of the boiler load conforming to a 10:1turndown. In other words, the en-ergy output required by the system is1 10 of the normal full-load energy de-mand. Therefore:

This required energy outputformed the basis of our analysis. For

example, if the boiler for our analysishad been a 1,000-Bhp boiler, then

Purge times

Exit gas temperatures

Exit gas temperatureduring purge

Exit gas temperature at 4:1 turndown

Exit gas temperature at 10:1 turndown

At high fire

At 4:1 turndown

At 10:1 turndown

1 min

15 sec

220 F

230 F

224 F

15%

28%

70%

Pre-Purge:

Post-Purge:

Percent of excess air

TABLE 1.Key parameters and calculated results for the case study.

Boiler load Boiler full rate10

=

Stack gas energy loss

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

M B t u

1 hour

10:1 turndown at 70 percent excess air 9.0 percent O 2

4:1 turndown at 28 percent excess air 5.0 percent O 2

0.1470.134

FIGURE 1. One-hour stack-gas energy loss.


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BSE June 2002 33

we could at a 10 :1 tu rndown as -sume a minimum energy-output re-quirement equivalent to a 100-Bhpboiler (as one might see duringnighttime operation). In that sce-nario, with the burner operating ata 10:1 turndown, it could run allnight without being shut down. Itthen follows that a boiler operatingat a 4:1 turndown would run for asignificantly shorter amount of timeto produce the same amount of en-ergy in either steam or hot water tothe system. The actual operatingtime, at the 4:1 condition, is easily calculated as:

Therefore:

This tells us that, in order to havethe same energy output as the boileroperating at a 10:1 turndown forone hour, the 4:1 turndown boilermust operate for 24 minutes out of

that hour.For this analysis, the effects of

convection and radiation have beenneglected because they are constantfor a given boiler at a specified outlettemperature, regardless of turn-down. Because the shell temperatureof a boiler is unrelated to the burnerturndown, the so-called shelllosses can be ignored.

Stack Gas AnalysisFor both the 10:1 and 4:1 turn-

down conditions, the energy lostduring boiler operation was evalu-ated using a stack-gas analysis. Themethodology for obtaining the fuelhigher heating value (HHV), prod-ucts of combustion (POC) at variousO 2 levels, and the enthalpy (h f ) of the mixtures is found in ASMEPTC-4. 2 Enthalpies were calculatedusing the JANAF tables. 3 Combus-

tion efficiencies associated with vari-ous stack temperatures for boilers fir-

ing natural gas are shown in Table 2.By evaluating the enthalpy remain-ing in the flue gas when it exits theboiler, the energy lost to the stack can be calculated using:

Stack losses = m(h out - h in )Prior to lighting the burner, the

prepurge cycle ensures that a mini-mum of four air changes throughthe boiler furnace are made throughthe combustion-air blower. Duringthis time, there is no heat beingadded to the system. However, asthe cooler air travels through thepressure vessel, it removes heatand, therefore, energyfrom theboiler control volume. With the useof the JANAF tables and the aboveequation, that energy loss can alsobe calculated.

Similarly, the post-purge cyclealso requires relatively cooler com-bustion air to be forced through thepressure vessel, resulting in anotherenergy loss that is calculated in thesame manner.

It should benoted that in or-

der to eliminateenergy lossesunrelated to ac-tual operationalconsiderationsand provide fora heat loss out of the stack that

was consis tentfor one hour,the 4:1 boiler

was eq uipped with a sta ck damper that re-mained closed

when the boiler was off (36 min.out of thehour). Note thatthis can also beaccomplished by closing theburner fresh-air

dampers whenthe burner is off.

ResultsThe results of operating the boiler

at the two turndown ratios for onehour are shown in Figure 1.

In the instant case, then, the 10:1turndown offers no improvement inefficiency over the lower-turndown-ratio burner; in fact, it is slightly lessefficient. This is due to the longer op-erating time (60 min vs. 24 min) athigh excess-air levels with hot gasescontinually exiting the boiler. Even

with the energy wasted during startupand shutdown (pre- and post-purgecycle losses), the 36-min. idle time of the 4:1 boiler conserves more energy than does the 10:1 case.

Figure 2 presents a graphical rep-resentation of the cumulative energy loss over an eight-hour operating pe-riod. Although the lower turndownboiler initially loses more energy, by the end of the first hour, it has recov-ered its advantage over the higherturndown boiler. As the operating

1 hr Full load Btuh10

t Full load Btuh4

4 1 =

:

t 4 1 hr10

0 4 hr or 24 min4 1:

.=

=

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Circle 23


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34 June 2002 www.ABMA.com

time continues, the advantage be-comes more significant.

So, strictly from a thermodynamicperspective, the conclusion can bedrawn thatcontrary to conventional

wisdomthere is not necessarily anefficiency advantage to operating withhigh turndown.

And there are other factors to beconsidered when specifying turn-down. Proponents of high-turndownburners will argue that it reduces ther-mal stresses on the boiler. And theremay be some validity to that argu-ment. However, once a Scotch marineboiler of quality construction reachestemperatures greater than 180 F, it iscapable of high-cycle operation with-out damage.

And, conversely, it is widely ac-cepted that high turndown burners

which by their nature require closercontrol of both fuel and air deliveryare more difficult to tune to and main-tain at optimum performance.

Also, higher efficiency in a non-condensing boiler is not always desir-able because stack temperature is in-

versely proportional to combustionefficiency. Obviously, as stack temper-atures decrease, approaching the dew

point of the flue gases, condensationin the stack can occur. This condition

is highly undesirable because it can bedamaging to the boiler and steam-sys-tem components. In a four-pass, wet-back boiler, such as the one selectedfor the instant case largely because of its higher efficiency and resultantlower stack temperature, measuredstack temperatures are typically in the

range of 30 to 50 F above the boilersaturation temperature at high fire.Condensation in this boiler will thenbe a function of the temperature and

water content of the combustion air(and, obviously, the resultant POCs),and we have found that the dew pointunder these operating conditions willbe in the range of 140 to 180 F. Theuse of a double-wall-insulated stack can help lessen these condensationproblems.

Burner turndown is not a panaceafor correcting low efficiency or com-pensating for poor equipment selec-tion. There are certainly applications

where higher turndown is desirable.In those processes that demand very tight maintenance of temperature orpressure setpoints, for example, thereis an obvious benefit from high turn-down. However, if a system has ex-tremely significant load swings in very

short cycle times, as some of thoseprocesses can, then a three- or four-

pass Scotch marine boilerregardlessof burner turndownmay not be theproper solution. There are boilersspecifically designed for these types of applications, such as specially designedtwo-pass firetube boilers that havelarger steam release and steam storage

volumes. Where thermal shock is acritical consideration, there are coil-type forced circulation watertubesteam generators that can be continu-ously cycled and provide output (not

just burner) turndown ratios as high as13:1. 4 Those steam generators, how-ever, are limited in size (generally lessthan 600 Bhp) and are significantly more expensive than firetube boilersin the same application.

Boiler and burner selection andturndown requirements are just someof the determinations that should bemade on a case-by-case basis by tech-nically qualified individuals for any contemplated boiler application.

References

1) Colannino, J. (1997, April). NOxand CO: Semi-empirical models forboilers. Presented at American PowerConference, 59th Annual Meeting.

2) American Society of MechanicalEngineers. 1964 (Rev. 1991) Perfor- mance Test Code - Steam Generating Units . ANSI/ASME PTC 4.1.

3) Chase, M.W. (1998). NIST-JANAF Thermochemical Tables.Fourth edition. J. Phys. Chem. Ref.Data, Monograph 9.

4) Clark, L.(1999, September).Coil-Type Steam Generators for Heat-ing Plant Applications. HPAC Engi- neering.

1.400

1.200

1.000

0.800

0.600

0.400

0.200

0.000

T o t a

l l o s s

( M B t u )

1 61 121 181 241 301 361 421

4:1 running for 24 minwith 1 minpurge per hour

10:1 running60 minper hour

4:1 must run only 24 minin order to produce theequivalent Btus of a 10:1

4:1 10:1

Time (min)

FIGURE 2. Cumulative stack losses over eight hours of operation.

About the author:

David Thornock is director ofengineering at Johnston Burner Co.He can be reached at davidthornock @johnsonboiler.com .

Larry Clark is regional sales man-

ager for MEPCO. He can be reached at [email protected].

BOILER TURNDOWN

burner turndown ratio

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