KITCHEN VENTILATION (EXTRACT) SYSTEM. DESIGN CALCULATIONS (1) EXTRACT FAN FLOW RATE (A). OPTION I
Based on the recommended extract rates for kitchen equipments given in th e ‘CIBSE guide B Table 2.11
Type of kitchen equipment on cooking island.
Recommended extract rate (L/s)
Number of kitchen equipment on cooking island
Boiling pans
Total Recommended extract rate (L/s)
6
1800
Cooking range
300
2
600
Baine Marie
200
2
400
Deep fat fryer
450
2
900
Coffee/tea urn
150
3
450
Stockpot stove
300
2
600
TOTAL EXTRACT RATE
Total Extract Rate
=
4750 L/S
=
4.75 m3 /s
4,750 L/s
Assuming that not all equipments are in use at any particular moment, a diversity of 80% 80 % will be applied. Therefore, the design flow rate
=
90 % x 4.75 m3
Hence, extract fan rate / duty
=
4.3 m3 /s x 3600
/s
= =
4.33 /sm 15480 3 /hr m
(2) GREASE FILTERS
Design flow rate
4.3m3
Suitable filter: -
Type DS
(154803 /hr) m
/s
20 / 6
Double sided vee-bank Capacity = 12,240 m3 /hr Comprising of 6 No. Stainless steel panel filters. Approximate size: 5500 x 700 x 600 mm high
(3) EXTRACT DUCT SIZING
Design flow rate
2.31 m3
(8316 3 /hr) m
/s
From IHVE Guide B, the recommended pressure loss for low velocity systems is: Up to 10 m/s
=
1 pa / m run
Round duct
=
Ø 600mm
Equivalent rectangular duct =
@ 8 m/s
550 x 550mm @ 7.64 m/s
Rectangular ducts are easier to fabricate, install and maintenance and offer better stiffness.
(4) FAN STATIC PRESSURE The cause of pressure drop considered were loss due to duct fittings (such as grease filters, bends, entries, contractions etc.), length of duct loss and finally the velocity pressure required for air flow. Velocity pressure, ∆Pf = 0.5 k ℓ v2 Where
∆Pf
pressure loss (N/m2)
ℓ
Air density (1.2 Kg/m3)
v
Air velocity (m/s)
k
Velocity pressure loss factor
(a) Entry loss at hood face. 0.5 k ℓ v2
Velocity pressure, ∆Pf
=
Hood face
=
6 m x 3.4 m
Design flow rate
=
8316 m3
Design velocity
=
2.31 / 20.4
= = =
0.5 k ℓ v2 0.5 x 0.5 x 1.2 x 0.1142 N/m2
Therefore, ∆Pf
20.4 m2
=
/hr
2.313 /s m
= =
0.114 m/s
2
0.004 N/m
(b) Loss across grease filters From “Vokes” catalogue on grease filters, the pressure loss ∆Pf is obtained as follows. Percentage flow rate =
= =
Design flow rate (m3 /hr) x 100 % Filters rated capacity (m 3 /hr) 8316 / 12240 68 %
From the catalogue performance graph, ∆Pf
= = =
6.5 mm of water 6.5 x 10-3 x1000 x 9.81 2
63.765 N/m
(c) Loss due to contraction at plenum to duct Velocity pressure, ∆Pf =
0.5 k ℓ v2
Aspect ratio, k
=
Area of Duct Area of plenum
=
0.34
= = =
0.5 k ℓ v2 0.5 x 0.34 x 1.2 x 7.642 N/m2
k Therefore, ∆Pf
2
11.904 N/m
=
0.55 m x 0.55 m 1.5 m x 0.7 m
(d) Loss due to duct bend Velocity pressure, ∆Pf
=
0.5 k ℓ v2
Angle of bend
=
200
Duct aspect ratio
=
1.0
Radius of curvature to duct width ratio
=
0.7
For a duct of aspect ratio 1, the velocity pressure loss factor k = 0.23 Thus, 200 bend velocity pressure loss factor, k 1 = 0.23 x 0.3 = 0.069 Therefore, ∆Pf
0.5 k ℓ v2 0.5 x 0.069 x 1.2 x 7.642 N/m2
= = =
2.417 N/m
=
∆Patm
2
(e) Loss along duct length Velocity pressure, ∆Pf
x3m
2
=
3.0 N/m
=
0.5 x 1.2 x 7.942
(f) Velocity pressure for air to flow.(k=1) Velocity pressure, ∆Pf
Total velocity pressure ∆PT
∆PT
=
0.5 k ℓ v2
=
35.022 N/m
=
0.004 +63.765 +11.904 +2.417 +3 +35.022
=
116.112 pa
=
117 N/m
2
2
Allowing for 25 % for other losses and dirty grease filters, ∆PT
=
117 x 1.25
≈
150 pa
=
(5) FAN SELECTION
As “WOODS” Model 12K fan capable of discharging/ delivering Capacity
0.5 m3 /s
Static pressure
150 pa
Speed
2800 rpm
Power
1-Ø, 240 V, 50 Hz
146.25 pa
