deck_3354128 (1) Flashcards
Velocity from velocity pressure
4005(VP).5
V in FPM
4005(VP).5
Velocity in FPM for non- normal conditions
4005[VP/df].5
- DF from table 5.10 air density correction factor
What is velocity if VP is 2.7iw, temp is 100degrees F and BP is 31”hg
Table 5-10 air density correction factor @ 31mmhg, 100F the Df=.98
V=4005[2.7/.98].5
6648 fpm
Reynolds for Turbulent vs laminar flow
129.2 Q/d
d= in ft
Q=cfm
>4000= Turbulent
With a duct that is 1’ in diameter what is reynolds number for flow of:
1) 10 CFM
2) 100 CFM
1) 129.2 Q/D @10cfm
129. 2*10/1ft = 1292 -
2) 129.2*100/1 = 12,920 - >4000 so turbulent
Measurements to account for duct turbulance
Vavg
VPavg
Vavg= .9 Vcl
VPavg= .81 VPcl
Flanges and baffles
- Confine and restrict flow contour lines at a hood face
- Flanges reduce required Q by 25
- Flange width >= Squareroot of surface area of the hood
Design Requirements- Flow dist in hoods
Rectangular and round hoods
1) Make hood plenum as long as possible
2) ensure take off angle is
3) use internal baffles as needed
4) use multiple take offs for wide hoods
5) use a slotted face
Slot Hood Design Guidelines
Vslot= 2000 fpm
Vplenum=
- Use multiple take offs for slot width >10ft
- Use internal baffles/ slitter vanes if neede
Hood Entry Loss
- some loss due to vena contracts as air enters the face
- Tapered hoods, measure TP(&SPh ) at 1 Duct diameter past take off
- Plain and flanged hooods- measure 3D from hood face
- Two ways to determine He
Hood Entry Coefficent Ce
Hood Entry loss Coefficient Fd
Determine VP
(V/4005)2
he Hood Entry Loss
he= |SP|- VPavg
VPavg= .81(VPcl)
Hood Entry Coefficient
Ce Qactual/Qideal
Ce = (VP/SPh).5
Ce= (VP/ |sp| ) .5
Hood Entry Loss Coefficient, Fd
- Ce=
- Fd= 1-Ce2 / Ce2
- he= FdVP
SPh= (Fd*VP)+VP
Hood Entry Coefficient (Ce)
Ce= (VP/|sp|).5
Where VP= .81(VPcl)
Hood Entry Loss Coefficient (Fd)
Fd= 1-Ce2/Ce2
- Find VP, find |sp|, then computer Ce
Determining Sph from Fd
Sph= (Fd* VP)+VP
Fd from IVM fig 5-15
VP=.81(VPcl)
Hood Entry Loss from Fd
He= Fd* VP
Fd = figure 5-15 IVM
A hood Sp is -2iw. Centerline duct velocity pressure is 1.2iw
What is
- he
- Ce
- Fd
VP= .81(Vpcl) .81*1.2=.97iw
- he= |sp|- .97; 2.0-.97= 1.03 iw
- Ce= (VP/|sp|).5 = .7
- Fd= 1-Ce2/Ce2= 1-.72/.72 = 1.0
A round flanged faced hood is attached toa 10” diamter duct through a 90 degree takeoff at a designed flow rare of 800 cfm. What is expected Sph?
- Find V - V=Q/A= 1467 cfm
- Look up Fd from fig 5-15. Fd=.15
- SPh= (Fd+1)(V/4005)2
= .15 iw
Using Ce or Fd with measurement of Sph to mesure Q
Q=4005* Aduct* Ce* (Sqrt SPh)
Q=4005* Aduct * (sqrt SPh/Fd+1)
A tapered conical hood enters a 5” duct via a 100º take off. The hood Sp is 1.9 iw
What is flow rate through hood
- Table 5-15 of IVM for Entry Loss Fd= .18
- Q=4005* Aduct* (sqrt SPh/Fd+1)
692.94 CFM
Losses in compound hood
For a compound hood w/ Vs= 2000 fpm
and Vd= 3500 fpm. Whats the total Sph for a rectangular to round 90º take off
FsVPs+FdVPd+VPmax
( VPmax the larger of VPs or VPd)
Fs= 1.78 , Fd.25 (from 5-15 table)
SPh= (1.78)(2000/4005)2+ (.25)(3500/4005)2+ (3500/4005)2