Hood Efficiency & Losses (Module 5)

Question 1

What is vena contracta?

Answer 1

-Where the creates loss normally occurs
-at the entrance to the duct

Question 2

How do you get rid of vena contracta?

Answer 2

Add a tapered entrance=way less energy loss

Question 3

What is a tapered entry?

Answer 3

• creates a gradual air flow into the hood
• all flow comes from front the same as for flanged hood

Question 4

What is hood entry loss?

Answer 4

• Major energy losses as the air moves into the hood and duct
• Represents the losses attributable to turbulence in the hood and transition

Question 5

What is the equation for hood entry loss? (he)

Answer 5

he =Fe VP

fe=hood entry loss factor
vp= average duct velocity pressure

Question 6

What is hood entry loss proportional to?

Answer 6

Velocity pressure

Question 7

What is the equation for hood losses?

Answer 7

SPh = -(VP + he)

he: hood entry lopss
vp: velocity pressure

Question 8

How is SPh measured?

Answer 8

-SPh is typically measured 4-6 diameters downstream of the hood
-This is typically done because hood turbulence may make closer reading invalid

Question 9

What is hood efficiency?

Answer 9

• the ratio of actual to ideal flow
• Coefficient of entry/hood flow coefficient, Ce

Question 10

When is Ce-1? (100% efficiency)

Answer 10

If the entry of air into the hood is smooth (ideal geometry), then
energy is not wasted on the entry

Question 11

What are circular lines in an air duct?

Answer 11

Lines of constant velocity

Question 12

What does the flange do>

Answer 12

Reduces vena contracta

Question 13

What are important factors in hood design that minimize hood entry loss?

Answer 13

• Hood geometry
• Number of enclosing sides
• Flow rate
• Use of slots, etc.