Twin City Airflow Measuring System Using Piezometer Ring - ES-105 - Manual (Page 4)

TWIN CITY FAN & BLOWER | WWW.TCF.COM
5959 Trenton Lane N | Minneapolis, MN 55442 | Phone: 763-551-7600 | Fax: 763-551-7601
Selecting a pressure transducer with the appropriate
range is critical in order to get accurate measurements
using the piezometer ring. Since most transducers list
accuracy as a percent of full scale, if the range selected
is too high, this can have a significant impact on the
accuracy of the flow measurement. If the range is too
low, there is risk of damaging the instrument and/or get-
ting inaccurate readings or no reading at all.
The following steps are for sizing the pressure trans-
ducer for use with the piezometer ring flow measurement
system:
1. Determine the maximum flow rate in CFM that the
fan is expected to produce. This maximum should be
the greater of normal, maximum, and/or emergency
conditions.
2. Find the formula for calculating the actual flow rate
from page 1 of this document for the corresponding
size and type of fan being used.
3. Calculate the pressure drop corresponding to the
maximum flow rate determined in Step 1.
4. Select the pressure transducer with the smallest
range that includes the pressure drop calculated in
Step 3.
5. Now take the maximum range from the pressure
transducer selected in Step 4 and use that to calcu-
late the maximum flow rate that could be measured
with this transducer.
6. Determine an acceptable safety factor for sizing the
transducer.
7. Multiply the maximum flow rate from Step 1 by the
safety factor. If the maximum flow rate from Step 5
is less than the result, bump up the transducer to
the next largest size. Otherwise, the transducer from
Step 4 should be used.
Example:
Company XYZ has a size 270 BC SWSI fan to be
installed with design conditions of 12,000 CFM at 5
inches w.g. and standard density. What size transducer
should be used?
1. After speaking to the design engineer, it was deter-
mined that 12,000 CFM is the actual maximum and
most of the time the fan will be running closer to
10,000 CFM. Therefore, 12,000 CFM will be used for
the calculations.
2. The calculation for this fan type and size is:
ACFM = 4247.77 * ( ∆P) for standard density
Note that if the density was other than standard air,
the formula would be different.
3. By rearranging the formula in Step 2, the following
formula is obtained:
∆P = (ACFM/4247.77)^2
so, ∆P = (12000/4247.77)^2 = 7.98 inches w.g.
4. For the pressure transducer models being considered,
the ranges are 0-3, 0-6, 0-10, and 0-20. Therefore,
for this flow rate the transducer model is the 0-10
inches w.g. model.
5. The maximum for this transducer is 10 inches, which
corresponds to the following flow rate:
ACFM = 4247.77 * (10) = 13433 CFM
6. Since 12,000 CFM is the maximum and normal oper-
ating conditions are 10,000 CFM, a 10% safety factor
should be plenty for this application
7. From step 1, 12000 CFM * 1.1 = 13200 CFM. This
is less than 13433 CFM, so the 0-10 inch pressure
transducer is acceptable.
Transducer Sizing for Piezometer Ring