AbsolutAire HMA Service Manual
INSTALLATION, SERVICE
AND ENGINEERING GUIDE
MAKE-UP AIR
BURNER
HMA Series with
Adapto Pilot
Patent No. 3,186,697
The MAKE-UP AIR GAS BURNER is well suited for allevi-
ating heating problems caused by excessive ventilation.
Where it is necessary or desirable to replace large
amounts of air, heaters of this type are used to temper the
incoming air and thus relieve the building heating plant of
an extra load. No build-up of contaminants is possible because the make-up air is never recirculated; once heated it
enters and blends with the air in the building and is discharged by the exhaust system.
INSTALLER: Inform and demonstrate to
the user the correct operation and
maintenance of this appliance. Inform the
user of the hazards of storing flammable
liquids and vapors in the vicinity of this
appliance and remove such hazards.
USER: Retain this manual for future
reference. If other than routine service
or maintenance as described in this
manual is required, contact a qualified
service agency. DO NOT ATTEMPT
REPAIRS. An inadvertent service error
could result in a dangerous condition.
CODE COMPLIANCE IS THE SOLE
RESPONSIBILITY OF THE INSTALLER.
WARNING: If the information in
these instructions is not followed
exactly, a fire or explosion may
result causing property damage,
personal injury or death.
Do not store or use gasoline or other
flammable vapors and liquids in the
vicinity of this or any other appliance.
WHAT TO DO IF YOU SMELL GAS
Do not try to light any appliance.
Do not touch any electrical switch;
do not use any phone in your
building.
Immediately call your gas supplier
from a neighbor's phone. Follow the
gas supplier's instructions.
If you cannot reach your gas
supplier, call the fire department.
Installation and service must be
performed by a qualified installer,
service agency or the gas supplier.
MIDCO International Inc.
4140 WEST VICTORIA STREET, CHICAGO, ILLINOIS 60646 / (312) 604-8700 FAX: (312) 604-4070
PRINTED IN U.S.A.
192
8470-40
I OPERATING PRINCIPLES
The MIDCO Make-up Air Burner is designed to operate
in a duct of flowing fresh air. Fuel gas is fed directly to
the burners; kinetic energy of the air stream furnishes
combustion air. It will function properly at the velocity
and pressures associated with the usual ventilating systems.
■ The burner must be installed to fire with, and parallel
to, the air flow. By virtue of velocity impact and suction
generated by the diverging shape of the combustion baffles, air is induced through the air ports into the combustion zone. The air supply is constant, though only that
which mixes with the gas flowing from the burner ports
takes part in combustion.
■ When a very small quantity of gas is admitted to the
burner, sufficient mixing takes place in the low fire slot
within the burner casting and combustion takes place in
this zone. Since the low fire zone is contained within the
burner casting it is effectively shielded from fire disrupting uncontrolled air entry.
■ As the gas supply is increased the flame progresses
into the intermediate fire zone where an additional supply of air is available. At higher or full capacity, mixing
occurs at the larger air ports of the high fire zone augmented by air spilling over the end of the baffles.
■ On a reduction of gas supply the reverse sequence
takes place, the flame receding to a location of lesser air
supply until the low fire zone is reached.
■ The system above is suitable for a turndown ratio of
approximately 30 to 1.
tor, usually 15% to 20%. The static pressure rating of
the blower should also be selected on the basis of the
o
standard 70
rating temperature. The entire system will,
of course, operate at approximately this temperature in
warm weather and the design pressure drop will apply
as calculated. In cold weather the portion of the heater
downstream of the burner will remain at 70
o
, though the
inlet side will run at outdoor temperatures, which will result in slightly reduced pressure losses.
■ The total pressure rating of the blower must include
allowance for the resistance of the heater and pressure
drop at the burner, together with pressure losses at the
inlet screen, inlet louvres filters, outlet louvres if used,
plus the external pressure rating of the heater, if any.
■ The MIDCO Make-Up Air Burner will also operate
satisfactorily when located downstream of the blower,
which is known as a push-thru system, though a mixing
plenum may be required at the heater discharge opening. Blower and motor selections must be made on the
basis of corrections for the coldest anticipated inlet temperature. In this system the heater outlet CFM will vary
due to the expansion of air on heating.
■ Refer to MIDCO for further information regarding spe-
cial applications or push-thru system.
II AIR SUPPLY
The heater will generally contain the blower equipment
within the same casing in which the burner is located.
Any of the standard type blowers can be used whether
of the centrifugal or axial flow (van-axial, tube-axial or
propeller). In either case the air flow must be substantially straight and velocity must be within the proper
range to develop the desired turndown and capacity
within the space allowed.
■ The MIDCO Make-Up Air Burner is designed to oper-
ate in a make-up air heater and an air stream taken directly from outdoors. Recirculation is not permitted because of the possibility of build-up of contaminants, and
is prohibited by local health codes for this reason. However, it can be used with recirculation in certain process
heating applications, such as ovens.
■ To avoid stratification of the heated air, the burners
are usually located on the suction side of the blower to
take advantage of the mixing effect of the blower. This
is known as a pull-thru system. In selecting a blower for
this case, catalog CFM ratings can be used without correction since the heater outlet temperature of approximately 70
temperature. Motor horsepower rating will also apply directly for operating conditions, though an increase of up
to 20% will occur when the inlet air is unheated and enters the heater at below zero temperatures. This will
load the motor a corresponding 20% but ordinarily this
condition will exist only for a short period during start up
and is compensated for by the service factor of the mo-
o
will correspond to the standard blower rating
FIGURE 1 Combustion Pattern
-2-
III BURNER CAPACITY.
AIR FLOW AT BURNER
BASED ON 70o (STD)
BLOWER RATING
DESIGN
VELOCITY
FPM
3200
2850 (ideal)
2500
DESIGN
PRESSURE
DROP W.C.
.70"
.55"
.45"
Reduce capacity 4% for each 1,000 feet altitude over 2,000 feet.
* Flame lengths are given to the end of the main mass of flame, excluding any isolated
wisps or flashes, and for normal operation -- that is, with cold inlet air (design minimum).
BURNER CAPACITY (PER FOOT)
MAXIMUM
CAPACITY
BTU/Hr.
550,000
550,000
550,000
NAT & LP GAS
MINIMUM
CAPACITY*
BTU/Hr.
19,000
18,000
17,000
TURN
DOWN
29:1
30:1
32:1
APPROXIMATE LENGTH OF FLAME
PROJECTING BEYOND END OF
BURNER AND PROFILE PLATE*
450,000
BTU/Ft.
7 - 8"
9 - 10"
11 - 12"
500,000
BTU/Ft.
10 - 11"
12 - 13"
14 - 15"
550,000
BTU/Ft.
13 - 14"
15 - 16"
17 - 18"
TABLE 1 Design Data (Pull-Thru System)
AI R F LOW VARIATIONS DUE TO HEA TIN G IN A PULL-THRU SY STE M
■ Velocity and pressure drop figures shown in Table 1 apply to standard design conditions. However, these condi-
tions apply only when air enters the heater at 70
o
and requires little or no heat. In practice with the burner operating,
these figures 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 1 apply to conditions shown
in Table 2.
■ Request MIDCO Bulletin 8470-51 for procedure for testing a heater with warm inlet temperatures.
DESIGN
VELOCITY AT
BURNER
(from Table 1)
3200 fpm
2850 fpm
2500 fpm
EXAMPLE: A heater rated for 100o rise, 70o 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
DESIGN
PRESSURE DROP
AT BURNER
(from Table 1)
.70" W.C.
.55" W.C.
.45" W.C.
TEMP
TABLE 2 Air Flow Variations
IV HEAT REQUIREMENTS, BURNER
LENGTH AND PROFILE AREA
The fuel consumption is determined only by air delivery of
the heater and the desired temperature rise. The air delivery of the heater or group of heaters is usually equal to, or
slightly in excess of, the total exhaust potential of the
building ventilating system. The temperature rise is the
difference between the coldest anticipated outdoor temperature and desired outlet temperature, usually a few degrees above indoor temperature.
■ The burner capacity must be accurately correlated to
the heat demand for best results. If it were undersized, it
is obvious that in severe weather the outlet temperature
AIR
RISE
75
100
75
100
75
100
ACTUAL COLD
AIR VELOCITY
AT BURNER
o
o
o
o
o
o
o
.
2750 fpm
2600 fpm
2450 fpm
2300 fpm
2150 fpm
2000 fpm
ACTUAL COLD
AIR PRESSURE
DROP AT
BURNER
.52" W.C.
.46" W.C.
.41" W.C.
.36" W.C.
.33" W.C.
.29" W.C.
would be too low. On the other hand, oversizing will displace the burner turndown range, wasting some of the
maximum capacity potential and causing too great a temperature rise at low fire. However, the wide turndown
range allows some latitude in sizing since in actual practice the extreme range will rarely be required.
■ THE CALCULATIONS AND THE EXAMPLE FOLLOW-
ING APPLY TO A PULL-THRU SYSTEM. Calculations
are simple because, as noted above, the usual outlet temperature will approximately match standard conditions
o
(70
). Push-thru systems require corrections for varying
air temperature at the blower, as do high outlet temperature systems, which are not covered by these instructions.
-3-
HEA T REQUIREMENTS
INSTRUCTION
1. Calculate temperature rise. (Outlet
temperature usually designed for 75
o
85
.)
2. Determine permissible rise in mild
weather (usually 4
o
to 6o).
3. Calculate turn down ratio
4. *Calculate maximum hourly BTU
5. Calculate minimum hourly BTU
* Formula above includes 1:1 constant for heat content of air and .92 factor, which is an average ratio of net and gross
heating value of common fuel gases.
** Assume a heater with an air delivery of 27,500 cfm. Minimum outdoor temperature -20
o
85
.
o
to
FORMULA
Outlet temperature
less inlet temperature
Maximum rise
Minimum rise
Cfm x maximum rise x 1.1
.92
Maximum input
Turn down ratio
EXAMPLE**
o
105
o
5
105
5
27,500 x 105 x 1.1
.92
3,300,000
21
o
. Desired outlet temperature
o
= 21 to 1
o
= 157,000
= 3,300,000
BUR NER LENG TH
The ideal burner length is such as to sufficiently spread the heat in front of the blower to yield uniform outlet temperatures. Other factors to consider are: air pressure drop at the burner, flame length, turn-down ratio, burner to hater casing clearance, and the maximum per foot capacity rating of the burner.
INSTRUCTION
1. Divide maximum hourly BTU by 550,000
to determine minimum feet of burner
2. Check your heater cross section for
physical fit of burner
3. Recalculate per foot capacity for actual
burner length
4. Determine air velocity at burner
See Section IV and Figure 6
FORMULA
Total maximum capacity
Per foot maximum capacity
Total maximum capacity
Actual burner length
See Section II and Table 1
EXAMPLE
3,300,000
550,000
Assume 6
1/2 ft. straight burner desirable
3,300,000
6
Assume 2850 fpm (.55" pressure drop)
= 6 ft.
= 510,000 per ft.
1/2
5. Calculate minimum BTU requirements
per foot
6. Determine burner minimum capacity
Total minimum capacity
Burner length
See Table 1
157,000
6
Minimum = 18,000 BTU/ft. therefore 24,000 acceptable
= 24,000 BTU per ft.
1/2
rating
7. Check operating characteristics
See Tables 1 and 2
Flame length = approx. 13"
Actual velocity at -20
o
Actual pressure drop at -20
= 2,300 ft.
o
= .36" W.lC.
PROFILE ARE A
In calculating velocity consider only the open area (profile area), less the area occupied by the burner. The burner
width at the flame baffle is 7
for areas of other sections.
1. Calculate total burner area
2. Calculate net profile area
3. Calculate gross profile area
4. Calculate length of profile opening
5. Calculate height of profile opening
7/8"; thus a one-foot section will cover 941/2 square inches or .65 square feet. See page 10
INSTRUCTION
Feet of Burner x area per foot
Burner & net area combined
Burner length plus 4" each end
FORMULA
Total cfm
o
70
velocity
Gross profile area
Length
EXAMPLE
61/2 x .65 = 4.2 sq. ft.
27,500
= 9.6 sq. ft.
2850
4.2 + 9.6 = 13.8 sq. ft.
78" + 4 + 4 = 86" (7.15 ft.)
13.8
= 1.93 ft. = 23"
7.15
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