What is Make-Up Air?
REPLACEMENT AIR = MAKE-UP AIR
Air will enter a building in an amount to equal the flow rate of
exhaust air whether or not provision is made for this replacement.
However, the actual exhaust & supplyflow rate will be less than the design
requirements of the plant when it is under negative air pressure. If the building
perimeter is fairly tightly constructed, thus blocking effective infiltration of
outside air. A severe decrease of the exhaust flow rate will result and many other problems may occur.
When a building is in a negative air condition, the resultant
in-plant environmental condition is undesirable since the influx
of cold outside air chills the perimeter of the building. Workers are
subjected to drafts, workspace temperatures are not uniform, and the
building heating system is usually overtaxed. Although the air may
eventually be tempered to acceptable condition by mixing as it moves
toward the building interior, this is an ineffective way of
transferring heat to the air and usually results in greater fuel usage. Using EnergyMaster patented distribution system ducts will enable the make-up air system to utilize the otherwise wasted heated heat that is stratified at the ceiling.
Experience has shown replacement air is necessary for the following
The proper operation of exhaust hoods: The lack of sufficient make
up air and a negative air pressure causes an increase of the static
pressure in which the exhaust fans must overcome. This causes a
reduction in the “cubic feet per minute” (CFM) of air from all exhaust
fans. THE RESULT: NEGATIVE AIR PRESSURE.
To eliminate high velocity cross-drafts through windows and doors:
Depending on the negative pressure created, cross-drafts may be
substantial. Cross-drafts not only interfere with the
proper operation of exhaust hoods, but also may disperse contaminated
air from one section of the building to another and can interfere with
the proper operation of process equipment such as open top solvent
degreasers. In the case of dusty operation, settled material may be
dislodged from surfaces and result in recontamination of the work area.
To ensure operation of natural draft stacks such as atmospheric
burner and other combustion flues: Moderate negative pressures can
result in back drafting of flues which may cause a dangerous health
hazard from the release of combustion products, principally carbon
monoxide, into the work area. Back drafting may occur in natural draft
stacks at negative pressures as low as 0.02” Water Gauge (See Table
7-2). Secondary problems include difficulty in maintaining pilot
lights in burners, poor operation of temperature controls, corrosion
damage in stacks and heat exhausters due to condensation of water vapor
in the flue gases.
NEGATIVE PRESSURES WHICH MAY CAUSE
UNSATISFACTORY CONDITIONS WITHIN BUILDINGS (Table 7-2)
Negative Pressure Inches of Water
Adverse Conditions Which May Result
0.01 to 0.02
Worker Companies-High velocity drafts through doors, window
and crack openings.
0.01 to 0.05
Natural Draft Inoperable-Ventilation through exhaust
ventilators, flow through stacks with natural draft greatly
0.02 to 0.05
Carbon Monoxide Exposure-Black drafting atmospheric burners
will take place in water heaters, unit heaters, furnaces and
other combustion equipment not provided with induced draft.
0.03 to 0.10
Mechanical Ventilation Greatly Reduced-Airflow amounts
reduced in propeller fans and exhaust system.
0.04 to 0.10
Doors Difficult to Open - Employee injury may result from
non-checked slamming doors.
0.10 to 0.25
Local Exhaust Ventilation Reduced- Centrifugal fan fume
exhaust flow reduced. Exhaust hood source capture zones
To eliminate cold drafts on workers: Drafts
not only cause discomfort and reduce working efficiency but also may
result in lower overall floor temperatures.
To eliminate differential pressure on doors: High differential
pressures make doors difficult to open or shut and, in some
instances, can cause personnel safety hazards. (SEE
To conserve fuel: Without adequate replacement air, uncomfortable
cold conditions near the building perimeter frequently lead to the
installation of more heating equipment in those areas in an attempt
to correct the problem. These heaters take excessive time to warm
the air and the over-heated air moving toward the building interior
and up to the ceiling makes those areas uncomfortably warm. These, in
turn, usually lead to the installation of more exhaust fans to remove the
excess heat, further aggravating the problem by increasing the hot
air at the ceiling. Heat is wasted without curing the problem.
REPLACEMENT AIR FLOW RATE
In most cases, replacement airflow rate should approximate the total
airflow rate of air removed from the building by exhaust ventilation
systems, process systems and combustion process. Determination of
the actual flow rate of air removed usually requires an inventory of
air exhausters. When conducting the
exhaust inventory is necessary not only to determine the quantity of
air removed, but also the need for a particular piece of equipment.
At the same time, reasonable projections should be made of the total
plant exhaust requirements for the next one to two years,
particularly if process changes or plant expansions are
contemplated. In such cases, it can be practical to purchase slight more
replacement air than what is immediately necessary with
the knowledge that the increased capacity will be required within a
short time. The additional cost of a larger unit is relatively small.
Having established the minimum air supply
quantity necessary for replacement air purposes, many plants have
found that it is wise to provide an additional supply air flow rate to establish a positive air pressure.
This will overcome natural ventilation leakage and further eliminate drafts at
the perimeter of the building.
Source : Industrial Ventilation – A
manual of recommended practice 21st Edition, American Conference of
Government Industrial Hygienists