Cleaver-Brooks CBH 40 - Page 55

Gas flow
The volume of !{as flow is measured in terms of
cubic feet and is determined by a meter reading.
The !{as flow rate required for maximum boiler
output depends on the heating value (BTU/CU.
FT.) of the gas supplied. The supplying utility can
provide this information.
To obtain the required number of cubic feet per
hour of gas, divide the heating value (BTU/CU.
FT.) into the required burner input (BTU/HR.).
See Table 3 for input requirements.
TABLE 3
Boiler
Maximum Input
Horsepower BTU/Hr.
25
1,046,000
30
1,255,000
40
1,674,000
50
2,092,000
60
2,511,000
70
2,929,000
80
3,34 7,000
100
4,184,000
Pressure Correction
The flow rate outlined in the previous section is
figured on a "base'' pressure which is usually at-
mospheric or 14.7 psi.
Meters generally measure gas in cubic feet at
"line" or supply pressure. The pressure at which
each cubic foot is measured and the correction fac-
tor for this pressure must be known in order to con-
vert the quantity indicated by the meter into the
quantity which would be measured at "base"
pressure.
To express the volume obtained from an actual
meter reading into cubic feet at base pressure it is
necessary to multiply the meter index reading by
the proper pressure factor obtained from Table 4.
Conversely: to determine what the meter index
reading should be in order to provide the volume
of gas required for input, divide the desired flow
rate by the proper pressure correction factor. This
answer indicates the number of cubic feet at line
pressure which must pass through the meter to
deliver the equivalent number of cubic feet at base
pressure.
Regulator
Inlet
Pressure
1
psig
2
psig
3
psig
4
psig
5
psig
6
psig
7 psig
8
psig
TABLE 4
Pressure Correction factors
Pressure
Factor
Regulator
Inlet
Pressure
1.05
9
psig
1.11
10
psig
1.18
11
psig
1.25
12
psig
1.32
13
psig
1.39
14
psig
1.45
15
psig
1.52
Pressure
Factor
1.59
1.66
1.72
1.81
1.86
1.93
2.00
Chapter 5-Adjustment and Maintenance
As an example: assume that a 70 horsepower
boiler is installed at 2,000 feet above sea level; is
equipped with a standard gas train; and that 1,000
BTU natural gas is available with an incoming gas
pressure of 3 psig. The pressure and flow require-
ments can be determined as follows:
Pressure
Correction for the 2,000 feet altitude must be made
since altitude has a bearing on the net regulated
gas pressure. The standard gas train requires 5.0"
WC gas pressure at sea level (Table 1 ). Table 2
indicates a correction factor of 1.13 for 2,000 feet.
Multiplying these results in a calculated net regu-
lated gas requirement of approximately 5.7" we.
This is the initial pressure to which the regulator
should be adjusted. Slight additional adjustment
can be made later, if necessary, to obtain the gas
input needed for burner rating.
flow
Since the gas flow rate is based on standard con-
ditions of flow, correction must be made for the
supply pressure through the meter of 3 psig. Deter-
mine the flow rate by dividing the BTU content of
the gas into the burner input (Table 3) and
"correct" this answer by applying the correction
factor for 3 psig (Table 4 ).
BTU/HR Input
BTU/CU. FT.
2,929,000
1,000
2,929
1.18
CFH (Cubic feet/Hour)
OR
2,929 CFH (At 14.7 #
atmospheric "base" pressure)
THEN
2482 CFH
This is the CFH (at line pressure) which must pass
through the meter so that the equivalent full input
requirement of 2,929 CFH (at base pressure) will
be delivered.
Checking Gas flow
Your gas supplier can generally furnish a chart
developed to determine the cubic feet/hour reading
from the meter based on the number of seconds per
revolution of the 10 cubic feet dial. This provides a
knowledge of the flow rate after a relatively short
observation period.
Lacking a chart of this nature it is possible to
"clock the gas meter" as follows:
(1) Turn off all other gas appliances that may
be served by the meter.
( 2) Set burner at high fire.
(3) Note meter reading and record consumption
for 3 minutes.
5-13