Design Priniples. Honeywell AUTOMATIC CONTROL SI Edition

SMOKE MANAGEMENT FUNDAMENTALS

Smoke Management System, Active: A system that uses fans to produce airflows and pressure differences across smoke barriers to limit and direct smoke movement.

Smoke Management System, Passive: A system that shuts down fans and closes dampers to limit the spread of fire and smoke.

Smoke Control Zone: An indoor space enclosed by smoke barriers, including the top and bottom, that is part of a zoned smoke control system (NFPA 92A).

Smoke Damper: A device designed to resist the passage of air or smoke that meets the requirements of UL 555S,

Standard for Leakage Rated Dampers for Use In

Smoke Control Systems.

Stack Effect: A movement of air or other gas in a vertical enclosure induced by a difference in density between the air or other gas in the enclosure and the ambient atmosphere. The density difference is caused by temperature-pressure differences between the air inside a building and the air outside a building. The air inside the building moves upwards or downwards depending on whether the air is warmer or colder, respectively, than the air outside.

UL: Underwriter’s Laboratories Inc.

UPS: Uninterruptible Power Supply.

OBJECTIVES

Designing a smoke management system requires agreement on the system objectives. The following is a partial list of potential system objectives:

— Provide safety for the occupants

— Extend egress time

— Provide safe egress route

— Provide safe zones (tenable environment)

— Assist firefighters

— Limit property damage

— Limit spread of smoke away from fire area

— Clear smoke away for visibility

— Provide elevator usage during fires as an egress route for the handicapped

DESIGN CONSIDERATIONS

GENERAL

Four points must be stressed in developing a smoke management system:

1. The smoke management system can be properly designed only with agreement on the objectives of the system.

2. The smoke management system must be designed as a complete mechanical control system that is able to function satisfactorily in the smoke management mode.

The smoke management system should be designed independently of the HVAC system and then integrated, where feasible, without sacrificing functionality of the smoke control system.

3. The smoke management system must be designed to be reliable, simple, and maintainable.

4. The smoke management system must be designed to minimize the risks of failure and must be tested periodically.

Sensors providing status of operation and building automation controls providing system monitoring and printed records can assist in the testing process.

Present active smoke control systems use active methods and follow two basic design approaches to preventing the movement of smoke from the fire zone:

— Providing static pressure differences across penetrations in smoke barriers, such as cracks around doors.

— Providing adequate velocity of air through large openings in smoke barriers, such as doors in an open position.

Although these two methods are directly related, it is more practical to use one or the other to design with and measure the results.

Methods used to activate smoke control systems require careful consideration. For zoned smoke control, care must be taken in using smoke detectors to initiate a pressurization strategy. If a smoke detector that is not in the smoke zone goes into alarm, the wrong smoke control strategy will be employed.

If a pull station is activated from a nonsmoke zone, the wrong smoke control strategy could again be employed.

Any alarm activation of a smoke management system that is common to all strategies in the building, such as stairwell pressurization, atria, and exhaust, is acceptable.

For a smoke management system to function reliably, building leakage must be controlled during and after construction. Any penetrations of smoke barriers and walls used for pressurization must be carefully considered in order to maintain the intended smoke control.

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SMOKE MANAGEMENT FUNDAMENTALS

Smoke management typically includes control of fires by automatic sprinklers. Designing smoke management systems for sprinklered buildings is quite practical. However, designing smoke management systems for buildings that do not have sprinkler systems is extremely difficult. Complicating the design task are problems with estimating the fire size and dealing with higher static pressures (or airflows).

Smoke vents and smoke shafts are also commonly used as a part of the smoke management system to vent pressures and smoke from fire areas; however, their effectiveness depends on the nearness of the fire, the buoyancy of the smoke, and other forces driving the smoke.

LAYOUT OF SYSTEM

Smoke management equipment should be located in a building where it can best facilitate smoke control for various building layouts. The following guidelines apply:

— Follow the drawings and specifications for the job.

— Locate the smoke controls near the mechanical equipment used to control the smoke.

— Try to minimize the length of runs for sensors, actuators, power, and communications wiring in order to reduce the possibility of wiring being exposed to areas where there might be a fire.

Appendix A of NFPA 92A describes an example of a

Firefighters’ Smoke Control Station (FSCS). The FSCS allows firefighters to have control capability over the smoke control equipment within the building. The FSCS must be able to show clearly if the smoke control equipment is in the normal mode or the smoke control mode. The example in NFPA 92A includes location, access, physical arrangement, control capability, response time, and graphic depiction. This example is for information only and is not a requirement.

CODES AND STANDARDS

The integration of fire alarm and smoke control is covered in

UL 864, Standard for Control Units for Fire-Protective

Signaling Systems. Compliance with this UL standard for engineered smoke control systems requires the following:

— Compliance with NFPA 92A, Recommended Practice for Smoke Control Systems

— End-of-process verification of each control sequence

— Annunciation of any failure to confirm equipment operation

— Automatic testing of dedicated smoke control systems

Controls that meet UL Standard 864 are listed under UL

Category UUKL. Standby power and electrical supervision items listed in UL864 are optional for smoke control systems.

According to NFPA 92A, control sequences should allow smoke control modes to have the highest priority; however, some control functions should not be overridden. Examples of these functions are duct-static high pressure limit control (use a modulating limit control, if a concern) and shutdown of the supply fan on detection of smoke in a supply air duct.

Manual override of automatic smoke control systems should be permitted. In the event of multiple alarm signals, the system should respond to the first set of alarm conditions unless manually overridden.

All related energy management functions should be overridden when any smoke control mode is activated by an actual alarm or during the testing process.

During the planning stage of a project, design criteria should include a procedure for acceptance testing. NFPA 92A states that, “Contract documents should include operational and acceptance testing procedures so that all parties—designer, installers, owner, and authority having jurisdiction—have a clear understanding of the system objectives and the testing procedure.” 2 ASHRAE 5-1994 Commissioning Smoke

Management Systems is intended to ensure proper operation.

Legal authority for approval of smoke control systems is from the Authority Having Jurisdiction (AHJ). The AHJ uses local building codes as its primary standard. Local building codes are established using several reference standards or codes including the following:

— Model Building Codes:

– Building Officials and Code Administrators

International (BOCA), Inc.

– International Conference of Building Officials

(ICBO)

– Southern Building Code Congress, Inc. (SBCCI)

– Western Fire Chiefs Association (WFCA)

– National Mechanical Code (NMC)

– American with Disabilities Act (ADA)

— National Fire Protection Association (NFPA)

Standards:

– NFPA 92A, Recommended Practice for Smoke

Control Systems

– NFPA 92B, Guide for Smoke Management

Systems in Malls, Atria, and Large Areas

– NFPA 90A, Installation of Air

Conditioning Systems

— Underwriters Laboratories (UL) Standards:

– UL 555, Standard for Fire Dampers and

Ceiling Dampers

– UL 555S, Standard for Leakage Rated Dampers for Use In Smoke Control Systems

– UL 864, Standard for Control Units for Fire–

Protective Signaling Systems (UL Category UUKL)

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SMOKE MANAGEMENT FUNDAMENTALS

DESIGN PRINCIPLES

CAUSES OF SMOKE MOVEMENT

The movement or flow of smoke in a building is caused by a combination of stack effect, buoyancy, expansion, wind velocity, and the HVAC system. See Figure 1. These items basically cause pressure differences resulting in movement of the air and smoke in a building.

plane, approximately midheight, and exits above the neutral plane. See Figure 2. Air neither enters nor exits at the neutral plane, a level where the pressures are equal inside and outside the building.

NORMAL STACK EFFECT REVERSE STACK EFFECT

REGISTER

NEUTRAL

PLANE

CORRIDOR STAIRCASE BUILDING

SPACE

WIND

MECHANICAL

HVAC

SYSTEM

BUOYANT SMOKE

GAS

EXPANDS

BUILDING

STACK

M13022

Fig. 1. Factors Affecting the Movement of Smoke.

Before controls can be applied, it is necessary to first understand the overall movement of smoke.

NOTE: ARROWS INDICATE DIRECTION OF AIR MOVEMENT

∆

P = Pressure difference, Pa

Ks = Coefficient, 3460

To = Absolute temperature of outdoor air,

Kelvin (K)

Ti = Absolute temperature of air inside the shaft,

Kelvin (K) h = Distance from the neutral plane, m

C5153

Fig. 2. Smoke Movement Caused by

Normal or Reverse Stack Effect.

When it is colder inside than outside, there is a movement of air downward within the building. This is called reverse stack effect. With reverse stack effect, air enters the building above the neutral plane and exits below the neutral plane.

The pressure difference across the building’s exterior wall caused by temperature differences (normal or reverse stack effect) according to Design of Smoke Management Systems for Buildings published by ASHRAE is expressed as: 3

∆

P = Ks x

( 1

To

–

1

Ti

)

x h

Where:

STACK EFFECT

Stack effect is caused by the indoor and outdoor air temperature differences. The temperature difference causes a difference in the density of the air inside and outside of the building. This creates a pressure difference which can cause a vertical movement of the air within the building. This phenomenon is called stack effect. The air can move through elevator shafts, stairwells, mechanical shafts, and other vertical openings. The temperature-pressure difference is greater for fire-heated air which may containing smoke than it is for normal conditioned air. For further information on stack effect refer to the Building Airflow System Control Applications section.

When it is colder outside than inside, there is a movement of air upward within the building. This is called normal stack effect.

Stack effect is greater for a tall building than for a low building; however, stack effect can exist in a one-story building. With normal stack effect, air enters the building below the neutral

BUOYANCY

Buoyancy is the tendency of warm air or smoke to rise when located in cool surrounding air. Buoyancy occurs because the warmer air is less dense than the cooler air, resulting in pressure differences. Large pressure differences are possible in tall fire compartments.

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