The Curious Definition Of Fan Static Pressure PART-2

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1 The Curious Definition Of Fan Static Pressure PART-2 Mat Ansari PE Figure-3 shows AMCA test type designations. (Luckily someone at AMCA had binary code in mind. If 0 is no duct and 1 is ducted, the 00, 01, 10, 11, which is the binary 0,1,2,3 reminds us of A,B,C,D.) Since we are trying to make sense of the Fan Static Pressure definition, let us start by analyzing Type -C or fan as an exhauster configuration. The reason for starting with Type - C will become obvious a little later. (Most Articles dealing with fan pressu re profiles use graphs and charts showing various pressures and their relationship. I personally don t think that helps in understanding the concept and therefore I will not use them in this post. There is really no substitute for looking at each of the reference planes in the figures below and then using a pencil and paper work out the relationships for yourself.) NOTE: In all the figures below, the duct dimensions match the fan dimensions for simplicity.

2 Figure-4 shows an exhauster configuration. The pressure calculations are very straightforward: 0.5 Entrance Loss 2.0 Duct & Fittings Loss 1.0 Exit Loss (Wa sted kinetic 4000 FPM Velocity) 3.5 Fan Total Pressure Required By definition: FSP FTP FVP, and so, FSP = = 2.5 Everything looks nice and simple but we have a little glitch here. Note the Exit loss of 1.0. This is the Velocity Pressure lost at the Fan Outlet. (From Part-1 of this post, this is also the Fan Velocity Pressure, FVP). The problem is this: We don t

3 know what the fan outlet velocity is going to be till the fan is selected and therefore you cannot run the above Total Pressure calculation when you design the duct. And since the Total Pressure loss is required for selecting the fan we are in a classic catch-22 situation. Figure-5 shows this catch with the missing information designated by?. Here is how the situation is resolved: The manufacturer agrees to subtract the FVP from his FTP values and call this new term FSP. The pressure and flow data are then cataloged as Flow/FSP table or chart. In Part-1 we defined FSP FTP FVP. It looked odd then, but now you know why it has to be that way for Type-C configurations. For fans used as exhausters t he designer simply takes his Total Pre ssure losses and ignores the exit loss. In our example above this is ( ) = 2.5. Now you can look up the Flow/FSP table to select the fan. You need not worry about the fan energy lost at the fan discharge, it has already been subtracted out of the Total Pressure and designated as FSP.

4 Before we end let us write the FTP and FSP and rearrange them using simple algebra. This rearrangement will give us some insights which might not be obvious or intuitive. First the FTP: FTP = TP 2 TP 1 = (SP 2 + VP 2 ) TP 1 But (SP 2 = 0), for an exhauster, Therefore, FTP = VP 2 TP 1 Next the FSP: FSP FTP - FVP = TP 2 TP 1 FVP = SP 2 + VP 2 (SP 1 + VP 1 ) FVP = SP 2 + VP 2 - SP 1 VP 1 - FVP But (SP 2 = 0) for an exhauster, and (FVP = VP 2 ) Therefore, FSP = - SP 1 VP 1 = - (SP 1 + VP 1 ) = - TP 1 [Check: In Figure-2: FSP = - (-3.5) 1 = +2.5 ] So it is not very intuitive, but the equation is telling us that for an exhauster, we can find FSP by negating the inlet Total Pressure (note double negation above). N O T E: YOU HAVE T O B E E X T R E M E L Y C A R E F U L A B O U T T H E S I G N O F T E R M S IN T H E S E E Q U A T IONS. I S E E M ISTAK E S M A D E A L L T H E T I M E. R E C E N T L Y I WAS L O O K ING AT A FIGURE IN A N A R T ICLE A B O U T FAN PR E S S U R E S I N T H E ASHRAE J O U R N A L. N O T H ING W A S M A K I N G S E N S E T ILL I R E A L I Z E D T H A T T H E I R F IGURE H A D A N IN C O R R E C T S IGN IN T H E E X P R E S S I O N F O R FTP. (SIN C E B O T H T H E A U T H O R S H A D P HD A T T H E E N D O F T H E IR NAMES I AM ASSUMING IT W A S J U S T S L O P P Y E D IT ING ON T H E P A R T O F T H E JOURNAL STAF F.)

5 A S A N O T H E R E X A M P L E T A K E A L O O K A T T H E 2012 ASHRAE HANDBOOK, C H A P T E R 21 FANS, FIG. 6. H E R E T H E Y A R E S H O W ING P R E S S U R E G R A P H S F O R T H E E X H A U S T E R C O N F IG U R A T ION WE H A V E B E E N D IS C U S S ING. T H E Y S H O W F A N S T A T IC P R E S S U R E BELOW D A T U M O R N E G A T IV E. T H E M A G N I T U D E I S C O R R E C T, B U T H O W C A N FSP B E N E G A T IVE? C ONTINUED IN PART-3