D. Jeff Burton, PE, CIH, noted author and OH&S contributor, lends a hand on this “Ask the Expert” question.
Q: How do I go about taking measurements for an old, underperforming laboratory fan for specification of a new fan?
A: When specifying a fan, the two basic numbers you need are Q and SP — called the “System Operating Point,” SOP.
There are two ways of determining “SP.” According to AMCA , one is the Fan Total Pressure (FTP) and the other is Fan Static Pressure (FSP).
FTP represents all energy requirements for moving air through the ventilation system. FTP is calculated by adding the absolute values of the average total pressures found at the fan. If the normal sign convention is followed, then a formula for FTP is:
FTP = TPoutlet – TPinlet
substituting for TP gives
FTP = SPout + VPout – SPin – VPin
If VPout equals VPin, i.e., if the average inlet and outlet velocities are equal, then the VP terms in the above equation cancel, leaving:
FTP = SPoutlet – SPinlet
The fan static pressure (FSP) is defined as the fan total pressure minus the average velocity pressure out of the fan.
FSP = Fan TP – VPout
(The fan static pressure is not defined as the static pressure out minus the static pressure in.)
Substituting the value of FTP into the FSP equation:
FSP = SPout + VPout – SPin – VPin – VPout
The VP (out) terms cancel, leaving
FSP = SPout – SPin – VPin
FSP represents the system losses, i.e., the amount of static pressure converted to useless heat or noise. (more…)