20
The FIRING RATE area values have been obtained considering a surround-
ing temperature of 68°F (20°C), and an atmospheric pressure of 398” W.C.
and with the combustion head adjusted as shown on page 7.
The burner may be required to operate with combustion air at a higher tem-
perature and/or at higher altitudes.
Heating of air and increase in altitude produce the same effect: the expan-
sion of the air volume, i.e. the reduction of air density.
The burner fan's delivery remains substantially the same, but the oxygen
content per cubic meter and the fan's head are reduced.
It is therefore important to know if the maximum output required of the burner
at a given combustion chamber pressure remains within the burner's firing rate range even at different temperature and altitude con-
ditions. Proceed as follows to check the above:
1 - Find the correction factor F in the Table (A) for the plant's air temperature and altitude.
2 - Divide the burner's delivery Q by F in order to obtain the equivalent delivery Qe:
3 - In the firing rate range of the burner, Fig. (B), indicate the work point defined by:
Qe = equivalent delivery
H1 = combustion chamber pressure
The resulting point A must remain within the firing rate range.
4 - Plot a vertical line from Point A as shown in Figure (B) and find the maximum pressure H2 of the firing rate.
5 - Multiply H2 by F to obtain the maximum reduced pressure H3 of the firing rate.
If H3 is greater than H1, as shown in Fig. (B), the burner delivers the output required.
If H3 is lower than H1, the burner's delivery must be reduced. A reduction in delivery is accompanied by a reduction of the pressure in
the combustion chamber:
Qr = reduced delivery
H1r = reduced pressure
Example, a 5% delivery reduction:
Qr = Q x 0.95
H1r = H1 x (0.95)
2
Steps 2 - 5 must now be repeated using the new Qr and H1r values.
Important: the combustion head must be adjusted in respect to the equivalent delivery Qe.
Qe = Q : F
(MBtu/hr)
H
3
= H
2
x F
(“ W.C.)
Qe
MBTU/h
A
H
2
H
1
H
3
“ W.C.
(B)
D2617
APPENDIX
- Burner firing rates according to air density
above sea level
average barom.
pressure
CORRECTION FACTOR
F
Air temperature
°F (°C)
ft
m
“ W.C.
mbar
0 (0°C)
41 (5°C)
50 (10°C)
59 (15°C)
68 (20°C)
77 (25°C)
86 (30°C)
104 (40°F)
0
329
658
987
1316
1645
1974
2303
2632
2961
3290
3947
4605
5263
5921
6579
0
100
200
300
400
500
600
700
800
900
1000
1200
1400
1600
1800
2000
399
394
389
385
380
376
372
367
363
358
354
346
337
329
321
313
1013
1000
989
978
966
955
944
932
921
910
898
878
856
836
815
794
1,087
1,073
1,061
1,050
1,037
1,025
1,013
1,000
0,988
0,977
0,964
0,942
0,919
0,897
0,875
0,852
1,068
1,054
1,042
1,031
1,018
1,007
0,995
0,982
0,971
0,959
0,946
0,925
0,902
0,881
0,859
0,837
1,049
1,035
1,024
1,013
1,000
0,989
0,977
0,965
0,954
0,942
0,930
0,909
0,886
0,866
0,844
0,822
1,031
1,017
1,006
0,995
0,983
0,972
0,960
0,948
0,937
0,926
0,914
0,893
0,871
0,851
0,829
0,808
1,013
1,000
0,989
0,978
0,966
0,955
0,944
0,932
0,921
0,910
0,898
0,878
0,856
0,836
0,815
0,794
0,996
0,983
0,972
0,962
0,950
0,939
0,928
0,916
0,906
0,895
0,883
0,863
0,842
0,822
0,801
0,781
0,980
0,967
0,956
0,946
0,934
0,923
0,913
0,901
0,891
0,880
0,868
0,849
0,828
0,808
0,788
0,768
0,948
0,936
0,926
0,916
0,904
0,894
0,884
0,872
0,862
0,852
0,841
0,822
0,801
0,783
0,763
0,743
(A)
H
1
r = H
1
x
(
)
Qr
Q
2
