Ask the Expert: Ventilation and Fan Requirements

Ask the ExpertD. 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.

Most industrial and lab fans are specified by FTP (usually easier to measure because you can often avoid measuring VP) but you need to know how to estimate both and how the fan curves and table were generated. (Using FTP? Or FSP?)

Now, to estimate the new desired FSP or FTP you could use the fan laws. Here are some of the pertinent ones:

Q2/Q1 = (n2 / n1)

SP2/SP1 = (n2 / n1)^2

V2/V1 = (n2 / n1)

power2/power1 = (n2 / n1)^3

SP2/SP1 = (Q2 / Q1)^2

SP2/SP1 = (V2 / V1)^2

FSP2/FSP1 = (V2 / V1)^2

FTP2/FTP1 = (V2 / V1)^2

where

1 = initial conditions
2 = desired conditions

n = fan speed, rpm
Q = flowrate
V = face velocity (of the hood)
SP = any pressure
FSP = fan static pressure
FTP = fan total pressure
power = power at motor, hp

You must measure the existing face velocity, Q and FSP (or FTP). You know your desired face velocity, V, from standard 5154.1. Then estimate the new FSP (or FTP).

For example, if you measure existing V1 = 95 fpm, Q1 = 1000, and FTP1 = 3.5″ w.g. and your desired V2 = 125 fpm,

FTP2 = FTP1 x (V2/V1)^2 = 3.5″ x (125/95)^2 = 6.06″ w.g.

Q is proportional to V.

Q2/Q1 = (V2 / V1)

and

Q2 = Q1 x (V2 / V1) = 1000 x 125/95 = 1315 cfm

So now you have your SOP. Go to the fan manufacturers curves or tables and choose a new fan (in this case, based on Q and FTP.)

And you can use the fan laws to predict your new motor power requirements, too.

This previous discussion assume no dampers are used in the system to modify flow.

As for measuring FSP, take traverses in the duct, if possible when measuring VP; keep a good distance from inlet and outlet. FTP pressure measurement is nice because you often only need to measure SP in and out, which is quite stable and usually doesn’t require a traverse.

Most lab exhaust sytems have the fan mounted near a short stack and the static pressure out of the fan is typically small. In some cases you could assume SPout = 0 but take a close look at the system and its plans before making that assumption.

– Jeff

Courtesy of:  Jeff Burton (www.eburton.com) via Yahoo ListServ

Visit Jeff’s website at www.eburton.com for many great resources on industrial hygiene fundamentals, ventilation, ih calculations, ethics and much more!

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