Grain Systems PNEG-501 - Manual (Page 8)

8
Fan/Heater Application
FAN & ROOF VENT SELECTION
STEP 1. Determine the average grain depth
for a desired bin size:
LEVEL FILL -AVE GRAIN DEPTH (ft) =
EAVE HEIGHT (ft) - FLOOR HEIGHT ft)
PEAKED FILL- AVE GRAIN DEPTH (ft) =
EAVE HEIGHT (ft) + BIN DIAMETER
(ft) ÷12, Minus FLOOR HEIGHT (ft)
STEP 2. Select airflow rate in CFM/BU:
Use SUGGESTED AIRFLOW RATES (page 13-30) for
desired drying or aeration conditions.
STEP 3. Using the STATIC PRESSURE
TABLES:
For a selected grain, find the static pressure for the
corresponding AVE GRAIN DEPTH and AIRFLOW
RATE. Use the nearest height or approximate where
necessary. Where stirring machines are to be used, use
charts for stirring.
STEP 4. Determine bin capacity:
BUSHELS = AVE GRAIN DEPTH (ft) x 0.6594 x
DIAMETER (ft) x DIAMETER (ft)
This capacity includes 5% compaction.
Since the calculation uses an average grain depth, the
capacity may vary from published sales capacities.
STEP 5. Find the required TOTAL AIR
FLOW in CFM:
TOTAL AIRFLOW (cfm) = AIRFLOW RATE x
BUSHELS
IMPORTANT!
If more than one fan is desired for a system, divide the
TOTAL AIRFLOW by the number of fans to get the
CFM REQUIRED PER FAN for fan Selection.
STEP 6. Using the FAN PERFORMANCE
TABLE for vane axial or centrifugal fans, the TOTAL
STATIC PRESSURE from Step 3, and the CFM
PER FAN from Step 5, select the fan best
meeting the requirements. For best fan perfor-
mance, a fan should run in the mid-range of is static
pressure ratings, if possible. Should excessive
static pressures be encountered, as with small grain
or very high airflows, consult GSI engineering for
the best fan choice.
NOTE: Consult GSI engineering where hopper
tank aeration is in question.
Roof Vent Selection
STEP 7. Determine MINIMUM number of
roof vents:
NUMBER ROOF VENTS = TOTAL AIRFLOW
(cfm) ÷ 1800 (cfm per vent)
If less than a full bin is intended an increased
number of roof vents will be necessary. The same
fan will produce a greater airflow when the bin is
less than full, due to less resistance from the grain.
An inadequate number of vents may have two
undesired effects
1) adding to the static pressure causing poor
fan performance and less air through the grain than
intended, and
2) possible structural damage to the roof
due to this increased pressure. Ideally, there should
be near zero static pressure in the space between the
grain and the roof.
A good estimate of vent numbers particu-
larly in drying situations, would be using the CFM
from the FAN PERFORMANCE TABLE at one-
half the static pressure anticipated. This new value
should be divided by 1800 CFM/VENT.