FloAire Canopy Hood - Manual (Page 34)

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Using a Shortridge Instrument
The Shortridge is a sophisticated instrument that, with its built-in features, is basically a self contained test
and balance kit. It has a “velocity grid” for filter face readings, a “velocity probe” for ductwork readings, a
“differential pressure” function to check room pressure and static pressure, as well as a “temperature
probe” so it can calculate accurate values based on varying temperature (most equipment assumes
standard temperature and pressure), and many other useful features.
Hood Information
To calculate the CFM, the following information must first be acquired:
1. Hood size and length
2. Filter size and length
3. PSP width and length
Measuring Hood Static and Room Pressure
Static Pressure:
1. Measure hood static pressure at exhaust collar using the Shortridge instrument.
Static Pressure = _______________
Room Pressure:
For an exhaust hood to work properly, the kitchen should be at a slight negative pressure to the dining
area (caused by its air removal) and the building overall slightly positive. The dining room should be a
slight positive to the outdoors and the kitchen +0.02” w.c. or about 300 CFM positive. This will keep dust
and bugs outside and doors will be easy to open. The kitchen should be a slight negative to the dining
room to keep the odors in the kitchen. The kitchen should be balanced to slightly negative, 0” to -0.02”
w.c., to the dining room. Generally, if there is not enough negative, or if there is positive pressure there
will be smoke roll out from the hood that occurs due to the wind currents from people moving around in
the area, while if there is too much negative, there becomes a pressure problem on the building(opening
doors, drafts, hot water heaters, etc.)
1. Measure room static pressure using the Shortridge instrument. Adjust the supply fan to set room
to 0.02” negative to dining area.
Room Pressure = _______________
Calculating Make-up Air CFM with a Shortridge Instrument and Spreadsheet
1. Use the Velgrid mode of the Shortridge instrument to measure supply air velocity. Make sure to
take measurements every twelve (12) inches along the length of the PSP.
2. Locate the appropriate Excel spreadsheet for the PSP/Supply velocity and record all necessary
data. This spreadsheet will calculate the total supply CFM for the hood.
3. The spreadsheet is located on the manufacturer’s website.
Calculating Make-up Air CFM without a Shortridge Instrument
1. Compute the open area of the supply plenum of the hood. This area must be calculated at the
same plane that velocity readings are taken. Area can be calculated using the following formula:
Area (ft²) = Length (ft) x Width (ft)