Midco International Inc.
7
8471 34
Installation
Net Pro
fi
le Area = Rated Fan (SCFM) ÷ Pro
fi
le Velocity (SFPM)
The Pro
fi
le Velocity should be determined based on the burner
fi
ring rates. If we choose to
fi
re the
burner at 675,000 BTU/hr/ft then the pro
fi
le opening should be sized for a pressure drop of 0.8
″
W.C. across the burner. If the
fi
ring rate is 450,000 BTU/hr/ft then the pro
fi
le opening should be
sized for a pressure drop of 0.4
″
W.C. across the burner. The corresponding pro
fi
le velocity across
the burner should be determined from Chart 4 or use the following equation.
For the 675,000 BTU/hr/ft
Net
Pro
fi
le Area = 5000 (SCFM) ÷ 3086 (SFPM)=1.62 ft
2
For the 450,000 BTU/hr/ft
Net
Pro
fi
le Area = 5000 (SCFM) ÷ 2182 (SFPM)=2.29 ft
2
To calculate the pro
fi
le area needed for both cases:
Pro
fi
le Area = Net Pro
fi
le Area + Total Burner Area
For the 675,000 BTU/hr/ft
Pro
fi
le Area = 1.62 + 0.650 = 2.27 ft
2
For the 450,000 BTU/hr/ft
Pro
fi
le Area = 2.29 + 0.975 = 3.265 ft
2
To calculate the length of the pro
fi
le opening add burner length to the desired clearance:
For the 675,000 BTU/hr/ft case
12 inch + 4 inch (2 inches on each side) = 16 inch (1.3 ft)
For the 450,000 BTU/hr/ft case
18 inch + 4 inch (2 inches on each side) = 22 inch (1.83 ft)
To calculate the height of the pro
fi
le opening divide the pro
fi
le area by the pro
fi
le length:
For the 675,000 BTU/hr/ft case
2.27
ft
2
÷ 1.3 ft = 1.75 ft (21 inch)
For the 450,000 BTU/hr/ft case
3.265
ft
2
÷ 1.83 ft = 1.78 ft (21.5 inch)
1. Conversion of SCFM to Actual CFM of air
SCFM = CFM x
ρ
_____
0.075
2. Air density as a function of Temperature --
ρ
= 1.35 x Barometric Pressure (in Hg)
T
(out)
+ 460
3. Change in Standard Barometric Pressure as a function of Altitude
Barometric Pressure (in.Hg) = 29.921x (1-6.8753 x 0.000001x altitude (ft))^5.2559
4. Temperature
di
ff
erence -- Temperature Rise = T
(out)
- T
(in)
5. Energy equation - - BTU/hr = SCFM x Temperature Rise x 1.08 / 0.92
Where: 1.08 is a sensible heat equation constant
1.08
=
0.2397
(
BTU
)
x 60
(
min
)
x 0.075
(
lb
)
____ ___ ___
lb
H
Ft
3
___________________________________
Pro
fi
le Setup Example
Continued
Profile Velocity
075
0
945
P
Δ
=
.
Profile Velocity
075
0
945
=
.
0
.
8
=
3086(SFPM)
Profile Velocity
075
0
945
=
.
0
.
4
=
2182(SFPM)