DuctSmokeDetector Application Guide HVAG53
DuctSmokeDetector Application Guide HVAG53
DuctSmokeDetector Application Guide HVAG53
G U I D E
Duct Smoke
Detectors
A P P L I C A T I O N S
G U I D E :
D U C T
S M O K E
D E T E C T O R S
Duct Smoke
Detectors
Contents
Section 1:
Introduction..........................................................................................................2
Section 2:
Characteristics of Smoke in HVAC Systems ................................................3
Section 3:
Duct Smoke Detection Equipment .................................................................4
Section 4:
Typical Air Handling Systems...........................................................................5
Section 5:
How Duct Detection Systems are Used to Control Smoke.........................6
Section 6:
Procedure for Duct Smoke Detector Application and Installation.............7
Section 7:
Maintenance and Service of Detectors for Use in Ducts..........................10
Section 8:
Glossary of Terms.............................................................................................12
Foreword
The purpose of this guide is to provide much needed information concerning the proper use of smoke detectors in duct applications. Duct mounted
smoke detectors are designed to provide a specific type of protection that cannot be duplicated by any other type of system. However, there has been
a tendency to misapply these devices in the past by attempting to use them as a substitute for an early warning smoke detection system. This fact, coupled with new methods of detecting smoke in ducts, has prompted the writing of this industry guide. Fire protection engineers, mechanical and electrical
engineers, fire alarm system designers and installers should find the contents both educational and informative.
This information is intended as a technical guide, as distinct from mandatory requirements.
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Section 1
ing a capacity greater than 15,000 cfm (7080 L/sec), and serving more
than one story.
Introduction
Purpose of Duct Smoke Detection
6.4.2.2
Return system smoke detectors are not required when the entire space
served by the air distribution system is protected by a system of area smoke
detectors. (NFPA 90A, 2012, 6.4.2.2)
National and local safety standards and codes recognize the ability of air
duct systems to transfer smoke, toxic gases, and flame from area to area.
Sometimes smoke can be of such quantity as to be a serious hazard to life
safety unless blowers are shut down and dampers are actuated. The primary purpose of duct smoke detection is to prevent injury, panic, and
property damage by reducing the spread (recirculation) of smoke. Duct
smoke detection also can serve to protect the air conditioning system itself
from fire and smoke damage, and can be used to assist in equipment protection applications, for example, in the ventilation/ exhaust duct work of
mainframe computers and tape drives.
6.4.2.3
Fan units whose sole function is to remove air from inside the building to
outside the building. (NFPA 90A, 2012, 6.4.2.3)
Application Documents
The International Mechanical Code (IMC) specifies that If the space is air
conditioned and the A/C unit is over 2,000 CFM, a duct smoke detector is
required to be installed in the return air duct. IMC 2012, 606.2.1
*Exception: Smoke detectors are not required in the return air system
where all portions of the building served by the air distribution system are
protected by area smoke detectors connected to a fire alalrm system in
accordance with the International Fire Code. The area smoke detector shall
comply with Section 606.4.
There are several important documents that provide guidance concerning the performance, application and installation of duct smoke detectors:
U.L. Standard 268A, Standard for Smoke Detectors for Duct Applications
NFPA Standard 90A, Installation of Air Conditioning and Ventilating
Systems
NFPA 92A, Standard for Smoke-Control Systems Utilizing Barriers and
Pressure Differences
NFPA 72, 2013 17.7.4.3 and section 907.3.1 of the 2015 edition of the
International Fire Code states, Detectors that are installed in the air duct
system shall not be used as a substitute for open air protection.
Area smoke detectors are the preferred means of controlling smoke spread:
NFPA Standard 72, National Fire Alarm Code
Duct smoke detectors can only detect smoke when smoke laden air is
circulating in the ductwork. Fans may not be running at all times, such
as during cyclical operation or during temporary power failure.
Duct smoke detectors sample great volumes of air from large areas of
coverage. They cannot be expected to match the detection ability of
area detectors.
Typical Scenarios
Dirt contaminated air filters can restrict air flow causing a reduction in
the operating effectiveness of the duct smoke detectors.
Duct smoke detection may be useful in preventing injury and property damage in instances such as the following:
Applications
NFPA 90A2, Standard for the Installation of Air Conditioning and Ventilating
Systems, specifies that Smoke detectors listed for use in air distribution
systems shall be located as follows:
6.4.2.1
(1) Downstream of the air filters and ahead of any branch connections in air
supply systems having a capacity greater than 2,000 cfm (944L/sec).
Since the primary purpose for detecting smoke in the HVAC system is to
automatically initiate action to minimize the spread of smoke through the
air handling system, it follows that the nature of the smoke to be expected
in various parts of the system should be understood. The following discussion is a theoretical description of smoke characteristics as they pertain to
this application, as yet, not supported by fire test data.
6.4.2.1
(2) At each story prior to the connection to a common return and prior to
any recirculation or fresh air inlet connection in air return systems hav1
A P P L I C A T I O N S
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Section 2
S M O K E
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of smoke per given time period), increases proportionately. Stated differently, in a one-foot square duct an air velocity of 100 feet per minute would
transport 100 cubic feet of air each minute. Increasing the velocity to 400
feet per minute would increase the volume of air transported to 400 cubic
feet per minute, thereby reducing the concentration of smoke particles by
a factor of four, assuming that the rate of smoke generation was the same
in both cases.
10
5
2
1
0.5
0.2
B Extinction principle
(according to Hosemann)
Relative Sensitivity
0.1
0.05
0.02
0.01
0.1
0.2
Particle Diameter
0.3
0.4
0.5
2
m
From the above discussion, one can begin to get an idea of what smoke
is in the various parts of an air handling system. Specifically, the farther
away the point of observation from the fire source, the cooler the smoke will
be and the more visible because of the growth of sub-micron particles by
agglomeration and recombination. There probably is some loss in quantity,
i.e., the mass of the smoke particles at the point of entry to a duct system
is probably greater than at some finite point downstream. However, at concentrations of concern, losses by precipitation are probably negligible.
Section 3
Change in Concentration
A much more important consideration is the change in concentration
because of dilution by clean air from other return air ports in the duct
system. A simple mathematical consideration will show that the original
concentration is given by Q/V where Q is the quantity of smoke particles
and V the volume in which they are dispersed. If a return duct has four
branches, each with equal air flow (cfm), then after they join, the concentration will be reduced to Q/4V.
Air velocity has no direct effect upon the characteristics of smoke. It does
affect the concentration because as velocity is increased, the total volume
of smoke being produced and dispersed at some rate, Q/dt (the quantity
4
S Y S T E M
S E N S O R
EXHAUST TUBE
4-WIRE
POWER BOARD
POWER BOARD MODULE COVER
SENSOR MODULE
COVER
METAL
SAMPLING TUBE
(sold seperately)
SENSOR HEAD
MAGNET TEST LOCATION
Exhausted Air
Detector
Sampling Point
Duct width
Power Supply
Controller
Exhaust tube
Section 4
Air Duct
Conduit
Electrical Box
Detector
Conditioning Section
Another duct detection device may be the light beam type detector. This
unit performs best when utilizing long beam paths, e.g., in the large plenums rather than small ducts.
Aspiration detection units are extremely sensitive and can detect minute
changes in air particle concentration (Figure 5). An air sampling detection tube or head is inserted in the air stream of the duct. Air is drawn out
and through the detection device. The detection unit typically is set at an
operating level of normal background particle concentration. An alarm will
sound when the concentration exceeds a preset alarm threshold value.
The cooling coil lowers the temperature of the air passing through either by
using chilled water or direct expansion of a refrigerant gas supplied from a
remote refrigeration compressor.
The heating coil raises the temperature of the air passing through it by
means of steam, hot water, or electric heaters.
A P P L I C A T I O N S
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Humidifiers add moisture in the form of dry steam directly into the air stream.
A humidifier is shut off whenever the system supply fan is not running to
avoid moisture damage to the duct system.
Fan Section
This section consists of one or more fans powered by a single electric
motor. The fan section may be placed before or after the conditioning coils.
The fan section may include a device to regulate the static pressure
developed by the fan. This device might be an inlet vane (vortex) damper,
discharge damper, suction damper, or a device to regulate the fan speed.
S M O K E
D E T E C T O R S
System Type
Supply Side
Return Side
High Pressure
8 8
(2.78 m 2.78 m)
Low Pressure
6 (2.09 m) diameter
The air pressure inside a duct with respect to the air pressure outside the
duct is positive on the supply side of the fan and is negative on the return
side.
Terminal Section
System
Supply Side
Max In
Supply Side
Typical
Return Side
Max In
Return
Side Type
Typical
High Pressure
+5 (1.24)
+4.0 (.99)
2 (.50)
1 (.25)
Key Parameters
Low Pressure
+2 (.50)
+1.5 (.37)
2 (.50)
1 (.25)
The capacity of air handling systems are typically rated in cfm (cubic feet
per minute). This is determined by multiplying the cross sectional area of
the duct in square feet times the velocity of the air in feet per minute. (Note
for a metric system the capacity is rated in cubic meters per second.)
Air velocities used in duct systems are typically 500 ft/m (2.54 m/s) which
is the rating for most of the heating and cooling coils used in the systems.
Higher velocities may be encountered in applications where air must be
moved through longer ducts. Maximum velocities encountered there can
be as much as 4500 ft/min (22.9 m/s). The air velocity in the supply (fan
discharge) side will be larger than in the return side. Velocities in the return
side may be as low as 200-300 ft/m (1.02 1.52 m/s).
The temperature of the air in the mixed air and outside air sections are
affected by the outdoor temperature. In very cold climates, the temperature in these sections can be as low as 40F (40C).
The range of sizes in duct work varies greatly. The smaller ducts are similar in size to the air ducts used in a residential dwelling and may be less
than 1 1 (.3 m .3 m). The maximum duct sizes used in large air handling systems are the following:
Exhaust Air
Return Air
Cooling Coil
Heating Coil
Supply Fan
Discharge Air to
Terminal Section
Filter
Outside Air
Mixed Air Section
Conditioning Section
Fan Section
Terminal Section
S Y S T E M
S E N S O R
Section 5
In the typical fan system shown in Figure 7, when any duct smoke detector senses smoke, the fan system will be turned off and all the dampers
shown will go to their closed positions. These actions are intended to prevent smoke from being distributed via the air handling system to unaffected
areas of a building.
An alternate method that may be used is to only stop the supply fan and
keep the return fan running. In this method the outdoor air damper and the
mixed air damper go closed and the (exhaust) air damper is opened. This
technique will help exhaust smoke-laden air from the building. In addition
to the requirements of NFPA 90A, duct smoke detectors may also be used
within duct systems as detection devices in engineered smoke control systems. These systems are engineered for the specific building. They use
the building construction and air handling systems to contain and reduce
the spread of smoke.
NFPA 90A1 requires that when there is an approved fire alarm system
installed in a building, required duct smoke detectors must be connected to
the fire alarm system in accordance with the requirements of NFPA 72. NFPA
72 21.7.4 Smoke detectors mounted in the air ducts of HVAC systems shall
initiate a supervisory signal. 21.7.4.1 Smoke detectors mounted in the air
ducts of HVAC systems in a fire alarm system without a constantly attended
location or supervising station shall be permitted to initiate an alarm signal.
The International Mechanical Code has a similar requirement stating, if the
adopted building code does require a fire alarm system, the duct smoke
detectors must be connected to a fire alarm system so that activation of
any duct smoke detector will cause a visible and audible supervisory signal to be indicated at a constantly attended location, or cause an alarm
signal. IMC 2012 606.4.1
Exhaust Air
Mixed Air
Damper
Duct Smoke
Detector
Optional Return
Air Fan
Return Air
Typical Floor
Return
Heating Coil
Dampers
Outside Air
Cooling Coil
Supply Fan
Filter
Humidifier
Figure 7: Typical Single Zone System
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L1
Line Side
Load Side
L2
L3
Contactor
H
On
Off
Coil
Overload Safety
Controls
NC
Contactor
Auxiliary
Contact
Contactor
Section 6
Upon selection of the location for duct smoke detector installation, drill a
hole into the duct and measure duct air velocity, humidity, and temperature.
Refer to duct instrumentation for instrument use in this manual (pages 8-9).
Application
Measure room temperature and humidity in the area of intended duct smoke
detector installation.
Detectors should be listed by a third party testing laboratory for the environment measured in the duct and room at the installation site. Relocate
the detector if measurements fall outside of rated limits.
Locate supply or exhaust input/output ports, filters, diluters, dampers, chillers, heaters, humidifiers, dehumidifiers, air cleaners, control
devices, deflectors, bends and restrictors in engineering drawings and
specifications.
Installation
Supply duct smoke detector installation should be downstream of fans, filters, chillers, heaters, and humidifiers.
Installation of duct smoke detectors may be within the duct, protruding into
the duct, mounted in an enclosure with sampling tubes protruding into or
traversing the duct.
Duct smoke detectors in the return air stream should be located at every
return air opening within the smoke compartment, or where the air exits
each smoke compartment, or in the duct system before air enters the return
air system common to more than one compartment.
Drill only required holes for duct installation. A template specifying hole
diameters or entry dimensions should be provided to locate mounting holes
for detector enclosure, sampling tubes, access panels, or doors.
Detector installation within the duct can be of the pendant or beam type.
The pendant duct smoke detector should be mounted to an appropriate
electrical box. The box pendant extension arrangement should be mounted
from either the top or side walls extending to the center of the air duct
stream.
An access panel or door is incorporated in the duct side walls and is used
during installation and for maintenance and test.
S Y S T E M
S E N S O R
Air Duct
Conduit
Electrical Box
Select the proper sampling tube length for duct enclosure installation. If
duct is more than three feet wide, drill an appropriate diameter hole directly
opposite but two to three inches lower to support the sampling tube of
lengths longer than three feet.
Detector
Exhaust tube length is not usually a critical dimension. It may vary from a
stub to the full width of the duct. Be sure to follow the recommendations of
the manufacturer regarding the exhaust tube.
Install the sampling and exhaust tubes to the mounting holes on the duct
enclosure.
Tubes may be marked or, as indicated in manufacturers installation instructions, provided with inhibitors to ensure that the tubes are installed in their
proper duct input and output ports.
Dilution: If outside air mixes with circulated air, it can dilute combustion particle concentration and prevent a detector from sensing a fire. Remember
that detectors alarm only when combustion particles constitute a specified
percentage of air being sampled. To avoid dilution, detectors should be
located before fresh air intakes and before the exhaust air output.
Position holes or openings located along the length of the sampling tube
into the air flow. Secure tube position with locking means provided. A tube
visual indicator should be provided to indicate the tube hole location without dismantling the duct installation.
Prior to completing the installation, clean the duct enclosure, and check
indicators for hole orientation.
Air Filters: Air filters within ducts tend to collect paper, lint and trash
all flammable materials. For this reason, duct smoke detectors should be
located on the downstream side of filters.
Air Velocity: Duct smoke detectors are usually designed to be used in air
handling systems having a certain range of air velocities. Be sure to check
engineering specifications to make sure duct air velocity falls within these
parameters.
See NFPA 90A for details of where duct smoke detectors can be eliminated in systems with air flow greater than 2,000 cfm capacity or in buildings equipped with a
system of smoke detectors providing total area coverage.
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Duct Instrumentation
Pressure Reading Instruments
Used for reading:
Tube support hole only for ducts more than three feet wide
Expected air flow direction
Duct width
Exhaust tube
Manometer gauges are used in conjunction with pitot tubes, straight metal
tubes and various static pressure sensors.
Pressure drops are best read with a magnehelic gauge. For lower velocity
readings in the range between 400 and 2000 fpm a 14-inch manometer is
the most accurate instrument to use along with the pitot tube. For higher
velocity readings in ducts, the 10-inch vertical or the one inch inclined
manometer is required. See Figure 12 for example of manometer and gauge
use. See Figure 13 for manometer type, and Figure 14 for magnehelic
gauge.
Damper
Filter
Cooling
Coil
Total Discharge
Pressure
Heating
Coil
Louver
Pitot Tube
Traverse
Static Pressure Zero
RA
Total Suction
Pressure
Static Pressure
Readinds Across
Filters and Coils
Figure 12: After electrical and fpm readings are taken, the total volume of air,
fan pressure and pressure drops are read.
Leveling Bubble
Leveling Bolt
Scale Plate
Adjustment
for Zeroing
FPM
Direct
Feet per
Minute
Reading
Scale
Inches of
Water Gauge
Basic
1/100ths
Increments
Leg for
Table Setting
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Section 7
Figure 14: Magnehelic gauge for reading static pressures in air systems
CAUTION
11
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Notify the proper authorities that the smoke detector system is being tested.
All persons who would automatically receive a real fire alarm signal should
be notified to prevent an unnecessary response.
If a detector functions properly and its sensitivity is within specifications,
nothing further need be done to the detector. However the routine maintenance procedure described under Typical Maintenance Practices above
is recommended. If a detectors sensitivity is not within specifications, it
should be replaced, or refer to manufacturers recommended procedure.
It is not recommended that duct fires be used to test duct smoke detectors. This procedure does not provide a consistent, measurable method of
determining if the detectors are performing properly. The test procedures
and test equipment recommended by the detector manufacturer are
the best way to test these detectors.
The smoke generated is a chemical reaction and does not represent true
smoke. They produce cold smoke particles, which are larger and are not
easily detected. These particles are also dependent on relative humidity,
distance traveled from the source and time of activation. This phenomenon is caused by the smoke being a mist rather than suspended solids in
warm gases.
There is no industry standard smoke bombs. They may be made of different chemical substances and may not allow the detectors to respond
properly within a specified time limit or provide the required obscuration.
It may also be possible to pass a smoke bomb test and be out of the
required manometer range for sampling; giving the installer a false sense
of proper operation. The manometer test must be performed.
12
S Y S T E M
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Date Tested
Test Mode
Test Results
Example:
passed
none
13
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Section 8
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Glossary of Terms
Magnehelic Gauge
Gauge for reading static pressures in air systems.
Photoelectric Detector
A smoke detector using the principle of optical detection of visible particles of combustion.
Pitot Tube
A device used to measure the total pressure of a fluid stream. It is essentially a tube attached to a manometer at one end and pointed upstream
at the other.
Coil
A cooling or heating element made of pipe or tubing.
Coil, direct expansion
Coil using the direct refrigeration method.
Pressure
The normal force exerted by a homogenous liquid or gas per unit of area
on the wall of a container.
CFM
Unit volume of air flow Cubic Feet per Minute.
Pressure Differential
A pressure difference (usually specified in inches of water) between two
points in a duct.
Damper
A valve or plate regulating the flow of air or other fluid.
Return Exhaust
The duct which is used to return the air to the HVAC processing center for
conditioning.
Sampling Tube
Usually a round tube with holes that collects air from the duct and brings
it to the detection device.
Duct
A passageway made of sheet metal or other suitable material not necessarily leak-tight, used for conveying air or other gas at low pressures.
Smoke Detector
A device used to automatically sense the presence of particles of
combustion.
Stratification
A phenomenon where smoke or other gases travel in layers at different levels within the duct, rather than being evenly distributed throughout the duct.
Exhaust Tube
Usually a round tube that provides a path for sampled air to return from the
detection device to the duct.
Supply Duct
The duct which distributes conditioned air, i.e., cooled, heated, cleaned,
humidified, etc.
Fan
An air moving device comprising a wheel or blade and a housing or
orifice plate.
Thermo-anemometer
A device used to measure air velocity.
FPM
Unit velocity of air flow Feet Per Minute.
Venturi Principle
A tube with a narrow throat (a constriction) that increases the velocity and
lowers the pressure of the liquid or gas passing through it, creating a partial vacuum immediately after the constriction in the tube. The vacuum
created has a sucking effect (eduction), and a Venturi is commonly used
to introduce a liquid (such as a regenerant) or gas (such as air) into a flowing water stream.
Humidifier
A device to add moisture to the air.
Inch of Water
A unit of pressure equal to the pressure exerted by a column of water at a
temperature of 4.0C (39.2F).
Inlet Sampling Tube
Usually a round tube with holes that collects air from the duct and brings
it to the detection device.
Ionization Detector
A smoke detector using the principle of ion flow within a chamber to detect
visible and invisible particles of combustion (within a size range normally
encountered as a result of fire).
Light Beam Detector
14
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15
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2015 System Sensor. The company reserves the right to change specifications at any
16 time.
S M O K E
D E T E C T O R S
HVAG5301 2/15