Q$4.($a$)$ The owner intends to change the use of his building and the new building information is shown in the following:

Building Type: Shopping mall

Gross floor area: $2,400m^2$

Room height: $5$ m

Compartment: One per floor

Openable window area: $130m^2$ per floor

Design fire load $=1200M\frac{J}{m^2}$

$($i$)$ Justify with reasons whether a smoke extraction system is needed for the shopping mall. $(6$ marks$)$

$($ii$)$ The shopping mall is designed with a size of $8m4m$ confined fire area having a convective heat output of $800k\frac{W}{m^2}.$ The ambient air temperature is $17C.$ Using the following Morgan formula, determine the temperature rise of smoke layer, density of smoke and volumetric flow rate of smoke production if the bottom smoke layer to floor distance is kept at $3$ m $.$

$(5$ marks$)$

Given:

$M=0\mathrm{.}188P{y}^{\frac{3}{2}},=\frac{Q}{MC},V=M\frac{T_o+}{{}_o}{T}_o$

${}_s={}_o\frac{T_o}{T_o+}$

where

$M=$ mass flow rate $of$ smoke production $(k\frac{g}{s})$

$P,=$ perimeter $of$ fire $(m)$

$y,=$ distance between the floor and the bottom $of$ smoke layer $(m)$

$Q,=$ convective heat output $of$ fire $(kW)$

$,=$ temperature rise $of$ the smoke layer above ambient temperature $(K)$

$C_p,=$ specific heat $of$ smoke $(k\frac{J}{k}\frac{g}{K})=1\mathrm{.}0k\frac{J}{k}\frac{g}{K}$

$V,=$ volume flow rate $of$ smoke $(\frac{m^3}{s})$

$T_o,=absolute$ temperature $of$ ambient air $(K)$

${}_o$ density $of$ ambient air $(k\frac{g}{m^3})=1\mathrm{.}22k\frac{g}{m^3}at290K$

${}_s$ density $of$ smoke $(k\frac{g}{m^3})$

$0C,=273K$

To be continued...

PTHDOA$-$Piped Services and Fire Safety Installations B $($MBS$4243)$

Page $8$ of $14$

January $2021$