26
Short Circuit Hoods
A. Supply
All cooking equipment should be off. The hood
exhaust should also be off.
1. Measuring Velocities
• Velocity measurements should be made
with a digital 2.75 in. (70 mm) rotating vane
anemometer or its equivalent.
• One velocity measurement should be taken
for every 8 in. (200 mm) of short circuit
opening length, starting tight against one
edge of the opening, and finishing tight
against the other edge. The anemometer
should be placed at the bottom edge of the
opening, flush with the bottom lip as shown
in Fig. 45. Both squareness and placement
are important for accuracy.
2. Calculate the average slot velocity.
3. Calculate the CFM per linear foot by dividing
the average velocity by a conversion factor of
5.52 per ft. or 1.68 per m.
4. Calculate the hoods exhaust volume by
multiplying the CFM per linear foot by the total
hood length.
Fig. 45
Testing Hood Air Volume
Example: 4 ft. (1.22 m) short circuit hood (36 in. (.914 m) short circuit opening):
Number of readings = 36 in. / 8 in. => 6 readings (.914 m / .2 m => 6 readings)
Average slot velocity =
Sum of Velocity Readings
Number of Readings
(Imperial)
=
786 + 900 + 1126 + 1048 + 1111 + 1115
6
=
6086
6
= 1014.3 fpm
(Metric)
=
1335 + 1529 + 1913 + 1780 + 1888 + 1894
6
=
10339
6
= 1723 m/hr
CFM per linear foot =
Average Slot Velocity
Conversion Factor
=
1014.3 fpm
5.52
= 183.8 cfm/linear foot
=
1723 m/hr
1.68
= 1026 m
3
/hr
Hood supply volume = CFM/linear foot (or m
3
/hr) x Total Hood Length
= 183.8 x 4 ft. = 735.2 cfm
= 1026 x 1.22 m = 1252 m
3
/hr
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