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Myth About The Fan Static Pressure

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 What is the basis for fan selection: FSP or FTP?

The ASHRAE Handbooks’ statements regarding the fundamentals for fan selection are controversial, misleading, and erroneous.

Key Point # 1

The ASHRAE Equipment Handbook says: Fan total pressure is a true indication of the energy imparted to the air stream by the fan . . . An energy loss in a duct system can be defined only as a total pressure loss . . . By using total pressure for both fan selection and air distribution system design, the design engineer is assured that he is using the correct fundamentals. These fundamentals will apply equally well to both high and low velocity systems”.

Key Point # 2

The ASHRAE Equipment Handbook adds: " As stated before, a fan impeller imparts static and kinetic energy to the air. This energy is represented in the increase in total pressure and can be converted to static or velocity pressure. These two quantities are interdependent; the fan performance cannot be evaluated by consideration of one or other alone."

Key Point # 3

“Most importantly, remember that FSP is a defined term.” ASHRAE Handbook, Fundamentals

Key Point # 4

The ASHRAE Fundamentals 2001, page 34.6 indicates: “System resistance to air flow is noted by Total Pressure”. “To obtain the fan static pressure requirement for fan selection where the fan total pressure is known, use Ps = Pt - Pv,o      (18), where: 

Ps - fan static pressure

Pt - fan total pressure

Pv,o - fan outlet velocity pressure.”

 

Key Points # 1 an #2 state that the Fan Total Pressure, which equals to the system resistance, is the basis for fan selection.

Contradicting to the above-mentioned statement, Key Points #3 and # 4 apply the Fan Static Pressure (with lower value than the system resistance) as the “requirement for fan selection”.

 

As the result of this mix-up: "Most industrial exhauster rating tables are based on Fan Static Pressure which cannot be read directly from the calculation sheet" (Industrial Ventilation, SMACNA).

 

Responding to this mix-up the article “The Myth about the Fan Static Pressure” in the “Engineered Systems” of August 2006 demystifies the myth by using graphical illustrations as the clearest demonstration of the illogical mix-up and the proper basis for the fan selection - the Fan Total Pressure.  

This fan selection process evaluation can be better demonstrated through the use of an illustrative example, where two different sizes of fans are selected for the same 3000 cfm @ Fs = 1”wg (tables 1 and 2).

In the first case a designer selects Fan size of 20 PLR; The New York Blower Co. (NYB) Bulletin 051 (Table 1).

twofantable.jpg

In the second case a contractor selects the least expensive Fan size 12 PLR. (Table 2)

In both instances it appears that the both fans have the same CFM and Fs. However, there is a noticeable difference in Ft pressure between the two options (Table 1 an 2).

Figure 1 is provided to illustrate the results.

fans12-20curves.jpg

- Fan 20 PLR, 3000cfm@Fs =1”wg; 1000rpm

a.  Parabola 0-1-3 is the characteristic of the designed or existing Vent System with 3000cfm @ Fs1 = line 1-1c = 1”wg.

b.   Parabola 0-1c is the characteristic of the Fan Outlet Velocity Pressure Fvpo1 = line 1b-1c = 0.1”wg.

-  Fan 12 PLR, 3000cfm@Fs=1”wg; 3200rpm

a.    Parabola 0-2 is the characteristic of imaginary Vent System with 3000cfm @ Fs2 = line 2-2b = 1”wg.

b.   Parabola 0-2b-3c is the characteristic of Fvpo2 = line 2b-1b = 0.8”wg.

-  Fan 12 PLR, 3400cfm@Fs=1”wg; 3200rpm

a.    Parabola 0-1-3 is the characteristic of the designed Vent System with 3400cfm @ Fs3= line 3-3c = 0.7”wg.

b.   Parabola 0-2b-3c is the characteristic of Fvpo3 = line 3c-3b = 1”wg

Key Point # 5. The Table 1, Table 2, and Fig.1 expose an error, which occurs when the Fan Static Pressure Fs is applied:

1.   The fans 20 PLR and 12 PLR with the same Fs = 1”wg have different Fan Total Pressures Ft.

2.       The fan 20 PLR has Ft = 1.1”wg and Fan-12 PLR has Ft = 1.8”wg.

The real capacity of the fan 12 PLR in the Vent System comes to 3400 cfm @ Ft = 1.7”wg and Fs = 0.7”wg.

conclusion.

1.   The Fan Static Pressure (FsP) is unacceptable basis for fan selection. “Most importantly, remember that the "FsP" is a defined term.” (ASHRAE Handbook, Fundamentals). And “It cannot be read directly from the calculation sheet." (Industrial Ventilation, SMACNA).

2.   Fans do not impart in a Vent System only the sP as a single value. “As stated before, a fan impeller imparts static and kinetic energy to the air. This energy is represented in the increase in total pressure and can be converted to static or velocity pressure. These two quantities are interdependent; the fan performance cannot be evaluated by consideration of one or other alone.(ASHRAE Handbook, Equipment )

3.   The attention-grabbing fact is that a designer must recognize that no matter what methods have been applied to calculate the vent systems (Equal Friction, Velocity Reduction, Static Regain, or Constant Velocity, etc.,) the results eventuate in value of the Total Pressure, not in value of the Static Pressure.

4.By using total pressure for both fan selection and air distribution system design, the design engineer is assured that he is using the correct fundamentals. These fundamentals will apply equally well to both high and low velocity systems(ASHRAE, Equipment Handbook)

5.   Thus, Fan Total Pressure, evidently and logically, is the only proper basis for the fan selection.

6.   As the essential energy saving measure the fan manufactures must provide graphs, illustrating System / Fan trade-offs: fan curves, air capacity and total pressure, fan and motor mechanical efficiency, energy consumption, and noise levels.

 

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