Midco HMA-2 & HMA-2A Series - Manual (Page 7)

Midco International Inc.
7
8471 34
Installation
Net Profi le Area = Rated Fan (SCFM) ÷ Profi le Velocity (SFPM)
The Profi 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 profi 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 profi le opening should be
sized for a pressure drop of 0.4 ″ W.C. across the burner. The corresponding profi le velocity across
the burner should be determined from Chart 4 or use the following equation.
For the 675,000 BTU/hr/ft
Net Profi le Area = 5000 (SCFM) ÷ 3086 (SFPM)=1.62ft 2
For the 450,000 BTU/hr/ft
Net Profi le Area = 5000 (SCFM) ÷ 2182 (SFPM)=2.29ft 2
To calculate the profi le area needed for both cases:
Profi le Area = Net Profi le Area + Total Burner Area
For the 675,000 BTU/hr/ft
Profi le Area = 1.62 + 0.650 = 2.27 ft 2
For the 450,000 BTU/hr/ft
Profi le Area = 2.29 + 0.975 = 3.265 ft 2
To calculate the length of the profi le opening add burner length to the desired clearance:
For the 675,000 BTU/hr/ft case
12 inch + 4 inch (2 inch on each side) = 16 inch (1.3ft)
For the 450,000 BTU/hr/ft case
18 inch + 4 inch (2 inch on each side) = 22 inch (1.83ft)
To calculate the height of the profi le opening divide the profi le area by the profi 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 = 1.78ft (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 difference -- 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
___________________________________
Profi 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)