Midco HMA-2 & HMA-2A Series - Manual (Page 5)

Midco International Inc.
5
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- Velocity and pressure drop fi gures shown in Table 2 apply to standard design conditions.
However, these conditions apply only when air enters the heater at 70° and requires little or no heat.
In practice with the burner operating, these fi gures will vary, especially with inlet air at the minimum
design temperature. This is because the blower handles a constant volume of air regardless
of its temperature. When the air is heated expansion takes place ahead of the blower and just
downstream of the burner. It follows therefore that when the air is being heated a lesser volume
enters the heater, causing a reduction, both in velocity and pressure drop at the burner. Table
2 shows the variation for minimum temperature air entry and with full input to the burner. Flame
lengths given in Table 2 and Chart 5 apply to conditions shown in Table 2.
Air fl ow at burner Approximate length of fl ame
based on 70° (std) Burner Capacity (per foot) projecting beyond end of
blower rating Natural & Propane Gas burner and profi le plate *
Design
Design Pressure Max. Min.
velocity drop capacity capacity Turn 550,000 650,000 750,000
FPM ″ W.C Btu/Hr Btu/Hr Down per ft per ft per ft
3200 .7″ 550,000 19,000 29:1 13″ 16″ 20″
2850
.55″ 550,000 18,000 30:1 15″ 19″ 22″
(ideal)
2500 .45″ 550,000 17,000 32:1 17″ 20″ 24″
Reduce capacity 4% per each 1000 ft altitude over 2000 ft.
* Flame lengths are given to the end of the main mass of fl ame, excluding any isolated
wisps or fl ashes, and for normal operations ie. with cold inlet air (design minimum)
Velocity Pressure Air Actual cold Actual cold
at burner drop at temp. air velocity air pressure
(from Table 1) burner rise at burner drop at burner
3200 fpm .7″ W.C.
75° 2750 fpm .52″ W.C.
100° 2600 fpm .46″ W.C.
2850 fpm .55″ W.C.
75° 2450 fpm .41″ W.C.
100° 2300 fpm .36″ W.C.
2500 fpm .45″ W.C.
75° 2150 fpm .33″ W.C.
100° 2000 fpm .29″ W.C.
EXAMPLE: A heater rated for 100° rise, 70° outlet temperature at a design velocity of 2850 fpm
and a design pressure drop of .55″ W.C., will in actual operation pass 2300 fpm over
the burner with a .36″ W.C. drop when air enters at -30°.
HMA 2 & HMA 2A fl ame length as a function of pressure drop and fi ring rate applies to push and pull
through systems using 100% fresh standard air.
Air Flow Variations
Due to Heating in a
Pull-Thru System
Installation
Table 2 - Design Data for Pull-Thru System
Table 3 - Air Flow Variations for Pull-Thru System
35
0
10
15
20
25
30
5
1.2 0.2 0.4 0.6 0.8 1 0
HMA 2 & HMA 2A Flame Length
750,000 Btu/ft
650,000 Btu/ft
550,000 Btu/ft
Pressure Drop ( " W.C.)
Flame Length ( inches )
Chart 5 - HMA 2 & 2A Flame Length