what is the basis for fan selection

7/27/2019 What is the Basis for Fan Selection

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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 # 1The 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 to tal pressure forboth 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 t he air. This energy is represented in the increase in total pressureand can be converted to static or velocity p ressure. These two quantities are

interdependent; the fan performance cannot be evaluated by consideration of one or

other alone."

Key Point # 3

Most impor tantly, 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 fl ow is noted by Total Pressure . To

obtain t he fan static pressure requirement for f an selection where the fan total pressure is kno wn, 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 th e 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 resu lt of t his mix-up : " Most indus tr ial ex haus ter r atin g tab les are based on Fan Stat ic Pressure wh ich cannot be read

directly from t he calculation sheet" (Indust rial Ventilation, SMACNA).

Responding to th is 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 asthe 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

illust rative example, where two dif ferent sizes of fans are selected fo r the same 3000 cfm @ Fs

= 1wg (tables 1 and 2).

In the firs t case a designer selects Fan size of 20 PLR; The New York Blower Co. (NYB) Bulletin

051 (Table 1).


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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 p ressure between the two options (Table 1 an 2).

Figure 1 is provided to illust rate the results.

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

a. Parabola 0-1-3 is the characteristic of thedesigned or existing Vent System with3000cfm @ Fs1 = line 1-1c = 1wg.b. Parabola 0-1c is the characteristic ofthe Fan Outlet Velocity Pressure Fvpo1 = line1b-1c = 0.1wg.- Fan 12 PLR, 3000cfm@Fs=1 wg; 3200rpm a. Parabola 0-2 is the characteristic ofimaginary Vent System with 3000cfm @ Fs2 =line 2-2b = 1wg.b. Parabola 0-2b-3c is the characteristicof Fvpo2 = line 2b-1b = 0.8 wg.- Fan 12 PLR, 3400cfm@Fs=1 wg; 3200rpma. Parabola 0-1-3 is the characteristic ofthe designed Vent System with 3400cfm @Fs3= line 3-3c = 0.7 wg.b. Parabola 0-2b-3c is the characteristicof Fvpo3 = line 3c-3b = 1 wgKey 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 = 1wg have different Fan Total Pressures

Ft.

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

The real capacity o f 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


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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.

what is the basis for fan selection

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