Recirculated and Replacement
Air
Recirculated And Replacement Air
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Introduction
Replacement air
Replacement air distribution
Replacement air flow rate
Room pressure
Environmental control
Environmental control air flow rate
Air changes
Air supply temperatures
Air supply vs. Plant heating costs
Replacement air heating equipment
Cost of heating replacement air
Air conservation
Evaluation of employee exposure levels
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Introduction
 Supply systems are used for two purposes:
- To create a comfortable environment and.
- To replace air exhausted from a building.
 The pressure in a building will be lower than atmospheric pressure
(case of ‘negative pressure’) if the amount of replacement air supplied
to the building is lower than the amount of air exhausted.
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Replacement Air
 The amount of air entering a building is equal to the the flow
rate of exhaust air.
 Replacement air is necessary to:
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Ensure that exhaust hoods operate properly.
Insufficient replacement air and a ‘negative pressure’ condition
cause an increase in the static pressure.
 Eliminate high-velocity cross-drafts through windows and doors.
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Cross-drafts can interfere with the function of exhaust hoods and
can also disperse contaminated air from one section of the building
to another, besides affecting other processes, unsettling dust and
like materials to cause recontamination.
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Replacement Air
 Ensure operation of natural draft stacks such as combustion flues.
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Negative pressures can cause backdrafting of flues, and hence health
hazards. For e.G. Combustible products release carbon monoxide.
Secondary effects include difficulty in maintaining flames in burners,
poor operation of air controls, corrosion damage in stacks and heat
exchangers due to condensation of water vapor in flue gases.
 Eliminate cold drafts on workers.
 Discomfort is not caused, overall ambient temperature and hence
working efficiency is not reduced.
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Replacement Air
 Eliminate differential pressures on doors.
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Differential pressures are a cause of difficulty in opening and
closing doors and uncontrolled movements.
 Conserve fuel.
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Lack of replacement air results in cold conditions and hence
installation of heating equipment is required. Sometimes
overheating occurs that leads to uncontrolled circulation of warm
air. To tackle this, exhaust fans will be provided that further
aggravates the problem.Heat is wasted without curing the problem.
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Replacement Air Distribution
 Replacement air distribution is critical especially in process industries
like pharmaceuticals, electronics, paints and facilities like indoor firing
ranges, particularly non-turbulent air-flow.
 When turbulence is caused due to mixing effect caused by high-throw
diffusers, local exhaust hoods are redesigned to draw in more air to
overcome it and hence prevent the upset of contaminant control. Thus
energy costs rise.
 To get rid of turbulence, air may be provided through a supply air
plenum, or a perforated duct with the face of the plenum covered with
perforations to diffuse air.
 Critical attention must be given to the feeding of air into the plenum
and avoid high-velocity.
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Replacement Air Flow Rate
 The replacement air flow rate should be approximately equal to
the total air flow rate removed from the building.
 Factors considered in determination of actual removed flow rate
are:
 Quantity of air removed.
 Determination of air exhaust locations.
 Testing.
 Necessity for a particular piece of equipment.
 Reasonable projections for future requirements, etc.
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Room Pressure
 ‘Negative pressure’ is desirable, when contaminants are to be
prevented from escaping into the surrounding areas.
 For a clean environment to exist, positive pressure conditions are
maintained.
 Either of the conditions are achieved by obtaining a ratio of the exhaust
- supply flow differential.
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Environmental Control
 It involves the control of following factors:
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Toxic contaminants.
Heat load in the work space.
High horsepower demand of machines.
High light levels in the plant and corresponding greater heat
release and
indoor temperature.
Depletion in efficiency levels and healthful working conditions.
And increase in tolerance limit of products.
 The control is achieved by various methods adhering to process
specifications, using technologies like automated building control and
direct digital control (DDC).
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Environmental Control Air Flow Rate
 The design supply air flow rate depends on several factors besides
health and comfort requirements. They can be enlisted as:
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Satisfactory dilution of contaminants assuming perfect distribution of air
and solvent vapors (the mixing factor ‘k’ is also included).
The location of air supply and exhaust outlets such that the air passes.
 Through the zone of contamination (so that spot ventilation can be
considered). ‘Negative pressure’ is desirable, when contaminants are
to be prevented from escaping into the surrounding areas.
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Environmental Control Air Flow Rate
 The location of air supply and exhaust outlets such that the air
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passes through the zone of contamination.
To attain optimum utilization of air supply it is distributed in the
‘living zone’ of the space i.E. Below 8-10 feet level (where
majority of people and processes are located).
Multiple point entry provides uniformity of air delivered and
minimizes.
The re-entry of contaminated air that occurs when large
volume of air is to be delivered.
During cold weather, large amount of air is removed by mixing
rapidly.
The cooler air with the warmer air in the space.
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Air Changes
 ‘Number of air changes per minute or per hour’ is the ratio of
ventilation rate (per minute or per hour) to the room volume.
 The required ventilation depends upon the intensity of a problem and
not the size of a room and hence no. Of air changes per unit time is a
poor criteria for control and has limited applicability.
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Air Supply Vs. Plant Heating Costs
 Even if supply air were drawn into the building, there is an additional
loading on heating systems and the fuel costs will rise.
 For efficient heat transfer and independence of operation, the same
flow rate of outdoor air can be introduced through separate
replacement air heaters, without exceeding the overall fuel costs.
 In order for an equilibrium to be established, the heat outflow from the
building must balance the heat inflow.
 Design the supply system to provide sufficient heating to counter air
entering the building through infiltration, to prevent freezing so as to
obtain additional energy saving during downtime.
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Replacement Air Heating Equipment
 The basic requirements of air heater are:
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Continuous operation capability
Constant pre-selected discharge temperature under varying
conditions of temperature
 Air heaters are classified by property of source of heat as:
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Steam heaters (single coil, multiple coil and bypass)
Hot water units
Indirect fired gas and oil units
Direct fired natural gas and LPG units
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COST OF HEATING REPLACEMENT AIR
 Equations for estimating replacement air heating costs on hourly and
yearly basis:
QN
C
q
0.154 Q dg T c 
Yearly cost (C2) 
q
Hourly cost (C1)  0.001
where:
Q = air flow rate in cfm
N = required heat in BTU/hr/1000cfm
T = operating time in hours or week
q = available heat per unit of fuel
dg = annual degree days
c = cost of fuel in $ / unit.
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Air Conservation
 Four methods to reduce heating and cooling costs:
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Reduction in the total flow of air handled.
Delivery of untempered outdoor air to the space.
Recovery of energy from the exhaust air and.
Recovery of warm, uncontaminated air from processes.
 Reduced flow rate.
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It is achieved by taking stock of all exhaust and supply systems in the plant
and marking them as necessary, replaceable and obsolete, and handling the
airflow rate by implementing the changes.
 Untempered air supply.
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Cold outdoor air can be supplied untempered or moderately tempered to
dissipate sensible heat loads on the workers and provide effective
temperature relief from radiant heat loads.
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Air Conservation
 Energy recovery
 Involves use of heat exchange equipment to extract heat from the
air stream before exhaust by using any of the following equipment :
 Heat exchangers, heat wheel, fixed plate exchanger, heat pipe, run
around coil exchangers, etc
 Factors of consideration for selection of a heat exchanger are:
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Nature of exhaust system
Isolation of contaminated exhaust stream
Temperature of exhaust system
Space requirements
Nature of air stream
Need for a bypass
 Involves recirculation (return) of air from industrial exhaust
systems
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Air Conservation
Selection of monitors
Four basic components of a complete monitoring system:
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Signal transfer
Detector/ transducer
Signal conditioner
Information processor
The choice of detection method depends on whether the contaminants are
particulates or non- particulates
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Evaluation Of Employee Exposure Levels
CR 
1   CE  KRCM 
1  KR 1   
Where:
CR= air cleaner discharge concentration after recirculation in mg / m3
Η = fractional air cleaner efficiency
CE = local exhaust duct concentration before recirculation in mg / m3
KR = fraction of recirculated exhaust stream that is composed of the
recirculation return air (0 to 1.0)
CM = replacement air concentration in mg / m3
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Evaluation Of Employee Exposure Levels
CB 
QB
CG  CM 1  f   Co  CM  f  KBCR  1  KB CM 
QA
Where:
CB = 8 - hr TWA worker breathing zone concentration
After recirculation, mg/m3
QB = total ventilation air flow before recirculation
QA = total ventilation air flow after recirculation
CG = general room concentration before recirculation, mg/m3
F = fraction of time workers spend at work-station
Co = 8 - hr TWA breathing zone concentration at work station before
Recirculation
KB = fraction of worker’s breathing zone air that is composed of
recirculation
( 0 to 1.0)
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Design of Recirculated Air Systems
 Systems should be designed to bypass to the outdoors and not
to recirculate.
 Humid air from wet collectors can cause high humidity and
hence auxiliary ventilation may be required.
 Since exit concentration of collectors varies with time, design of
data and testing programs should consider all operational time
periods.
 Layout and design of recirculation duct should provide adequate
mixing with other supply air and avoid uncomfortable drafts on
workers.
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Design Of Recirculated Air Systems
 In lieu of a monitoring system, a secondary air cleaning
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system is more reliable and is easy to maintain.
Odor and nuisance value of contaminants should be compared
to official TLV values.
Records should be maintained for routine tests, maintenance
procedures and recirculating systems.
Periodic testing of workroom should be done.
Appropriate signs should be displayed in prominent places.
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