Design of SHEVS

Study on Smoke and Heat Exhaust Ventilation System (SHEVS)

Available facilities for designing of SHEVS

Natural smoke exhaust system

• Low Cost • Wind effect affects to the system • Reliability is low since unavailability of a clear and easy path to move smoke naturally

from fire room to the natural smoke ventilators • Difficult to remove smoke which come from rooms situated at the center of the floor

Powered smoke exhaust system

• High cost • Wind effect does not affect to the system • Smoke outlet from the building can be located remote from the building • Smoke can be exhausted at the fire room itself • Reliability - High

Separate duct system

Can be operated as an isolated system

SHEVS

Natural Powered

Seperate System

Light well Seperate Duct System

MVAC System

Supply Air Duct Line with light well

Return Air Duct Line with Common Exhaust Duct

With a well-controlled damper system, smoke can be exhausted at the identified room itself

Available space for duct is very low

Duct cost - Expensive (If MVAC duct system can be used for smoke extraction with fire and smoke rated duct system, ducting cost can be reduced)

Light well

Low cost Easy installation This is an isolated system, can be operated separately System includes with automatically operated fire & smoke dampers, lightwell, powered

fan etc. Light well consists of non-fire resistant materials like glass (cannot be used without any

modifications) Smoke pathway goes through corridors (at evacuation stage in large fire) Distances from some rooms to light well are somewhat high Controlling system of replacement air inlets is complex

Use Fresh air supply duct of MVAC system and light well

This system includes with supply air diffusers (grilles), supply air duct, duct line that bypasses the AHU and fresh air duct line of MVAC system, light well, fire and smoke dampers, powered fan etc.

Return air duct of MVAC system

This system includes with return air grilles, return air duct, duct line, which bypasses the AHU, exhaust duct of MVAC system, fire and smoke dampers, powered fan etc.

Fire rated return duct for MVAC system saves money instead of using separate duct system Single return air duct path covers several numbers of rooms. Therefore, in a case of fire, this

system cannot exhaust required air volume from the fire room (if calculated volume is exhausted). However, this can be achieved with a high capacity fan.

Estimation of exhaust flow rate

Codes, standards, and guidelines

Code/ Standard Calculations based on 1. BS (British Standards

Institution) 2. NFPA (National Fire

Protection Association)

Fire size

3. Simplified method for designing a mechanical smoke exhaust system for high-rise Building - NRCC (National Research Council Canada)

Building size (ACH level)

1. Fire Size • Difficult to decide suitable smoke reservoir therefore each room should be individually

addressed (i.e. each room is considered as smoke reservoir). 1. Individual exhaust system for each room

a. Using individual duct system for each room b. Using well sized duct system

- Limited space and MVAC ducting system is already sized. 2. Duct with well controlled damper system

a. Cost is high b. Complex controlling system

3. High capacity exhaust system 4. MVAC duct system considering smoke reservoir as the area where AHU duct system

covers • Reliability - high

2. National Reserch Council Canada • Guidelines - Specified Method for Designing a Mechanical Smoke Exhaust System for High-Rize

Buildings - by G.T. Tamura and K. Tsuji . • A vertical shaft is used to transport Smoke and Heated air from the fire floor to outside.

• The flow rate need to be extracted from a fire floor is based on recommended ACH level and leackage data of the shaft.

• Difficult to make predictions on clear height at evacuation period and smoke layer propoties but facilitate to firefighting operations

Buildings with compartmented HVAC system 4 ACH Buildings with central HVAC system 6 ACH Buildings with central HVAC system and loose interior construction of floor enclosure 8 ACH

Estimation of Replacement air (Inlet air)

BS 7346 - part 4 4.3 Combined use of natural and powered ventilators Natural and powered ventilators should not be used together for exhaust in the same smoke reservoir or for inlet air supply in the same smoke reservoir. A smoke and heat exhaust system should consist of: a) a natural exhaust system with a natural replacement air supply system; or b) a natural exhaust system with a powered replacement air supply system; or c) a powered exhaust system with a natural replacement air supply system; or d) a smoke and heat exhaust system relying on a powered exhaust system and a powered replacement air supply system (push and pull system). Items b) and d) should not be designed unless a fully engineered and detailed description of the system is supplied showing how the system works under the design conditions. 6.8.2Recommendations 6.8.2.1Natural and powered ventilators should not both be used for inlet air in the same reservoir, neither should the inlet air arrangements make use of both powered and natural ventilators for the same system except in the Circumstances outlined in 6.8.2.2. 6.8.2.2Where for design reasons it is desirable to use both natural and powered inlet supplies a fully engineered and detailed description of the system should be supplied showing how the system works under the design conditions. Designing of SHEVS according to the guidelines of BSI

Assumptions 1. All rooms have been considered as offices 2. Fire occurs in a room of the building 3. Fire is assumed to be t-squared fire untill it becomes steady level & fast fire

Design inputs

1. Ambient Air Temperature (To) 31°C 304K 2. Ambient Air Density (ρamb) 1.22kg/m3 3. Table 1 — Default values of design fires (BS 7346 – Part 4)

Occupancy Fire area (Af) m2

Fire perimeter (P) m

Heat release rate per unit area (qf) kW·m–2

Offices Standard response sprinklers 16 14 225

4. Table 2 — Minimum clear height above escape routes(BS 7346 – Part 4) Type of building Minimum height (Y)

Minimum height (Y)

Non-public building, e.g. offices, apartments, open hall type prisons

2.5 m

5. Table 3 — Convective heat flux at the room opening (BS 7346 – Part 4) Type of room Convective heat flux Qw

(MW) Sprinklered offices 1

6. Page 50BS 7346 – Part 4 Rooms becomes fully involved in fire if Ts>= 550 Flashover does not take place if Ts< 550

7. Smoke in to adjacent space (BS 7346-Annex C)

Dd 0.9

Dw Dd

h W

W 1.4 m Ce 0.19

Cd 0.65 P 14 m

h 2.1 m 8. Replacement air inlet velocity 5m/s

Design informations

• Latest architectural drawings • MVAC duct & grilles layouts • MVAC duct sizes • Codes, standards & guides

o Table 1 — Default values of design fires (BS 7346 – Part 4) o Table 2 — Minimum clear height above escape routes(BS 7346 – Part 4) o Table 3 — Convective heat flux at the room opening (BS 7346 – Part 4) o Properties of smoke flows into adjacent space (BS 7346- Part 4-Annex C,) o Assesment for flashover (BS 7346 – Part 4 - Page 50) o Replacementair (Inlet air) - (BS 7346 - part 4 - section 4.3, 4.5.2.2, 6.8.2.)

Design outputs

• Working points of fans (Volumetric air flow rate of exhaust air & replacement air & Static head) • Minimum nos of exhaust air inlets • Quantity and sizes of replacement air inlets

• Installation plan for duct, fans, exhaust air inlets, dampers& inlets of replacement air • Typical drawings for damper arrangement, fan installation, inlets of replacement air etc • Detailed drawings for above • Specifications for duct, fans, exhaust air inlets, inlets of replacement air, dampers, hangings &

mountings etc • Schedule for accessories (grilles, dampers, duct quantity, etc) • Operating sequance

Calculations

Mass flow rate of the smoke (Mf=0.19PY1.5) 10.51 kg/s

BS7346 pt4 Eq: B.2

Convective Heat (Qc=0.8*Afqf) 2880 kW

Section 6.2.2. Recommendation (d)

Ts-To(=Qc/M/Cp) 273.91 °C= 546.91 K

BS7346 pt4 Eq: B.3

Smoke layer temperature (Ts) 304.91 °C = 577.91 K

Volumetric flow rate (V= MTs/(ρambTamb) 16.38 m3/s (34715.56 CFM =16383.82 l/s)

BS7346 pt4 Eq: F.5

Calculation for minimum No of exhaust points

Critical Exhaust Rate (Mcrit= 1.3(gdn5Tamb(Ts-To)/Ts

2)1/2) 0.51 kg/s

BS7346 pt4 Eq: F.9

Min No of Exhaust points 20.70 Nos

Assessment of flashover

According to Annex C of BS 7346 Page 50 Smoky gases passing through a vertical opening w=(CePWh3/2)/(W3/2+1/Cd(CeP/2)2/3) 3/2 2.06 kg/s

BS7346 pt4 Eq: C.1

Dw = 1/Cd(Mw/(2W))2/3 1.89 m

BS7346 pt4 Eq: C.2

Layer temperature of smoky gases out of the fire room into adjucent space

Tw= Qw/(CMw) 31+0.25x225x16/(1x2.18)

Layer temperature (Tw) 467.89 °C

BS7346 pt4 Eq: C.3 & C.4

Tw< 550 °C - Flashover does not take place

Replacement air inlets

Effective area for replacement air @ 5m/s inlet velocity (=V/5) (16.38m3/s) / (5m/s)

3.276 m2

Computer Based Analysis For Smoke Behavoir

A sample CFD analysis was done for 5th floor of Block 6 to analize smoke behavoir using a Fire Dynamic

Simulator (FDS)

Software Detail

Figure 1: FDS Software detail

Inputs to the Simulator

Fire detail

Fire is considered to be t-squared (unsteady) fire untill it becomes steady & within this period it behaves

equation 19 of ASHRAE page 52.13 & then it becomes steady state.

Steady Fire

Steady state fire size = qfAf (225kW/m2 x 16m2) = 3600kW

BS-7346 part 4

Burner Detail

Heat Release rate per unt area: 225 kW/m2

Ramp-up time: 1 s

Heat release rate values was tabulated using above equation against time and generated charts.

Results of FDS

5th floor without SHEVS

SHEVS - for fireroom