Losses in LEV system elements (module 5)

Question 1

What is total energy?

Answer 1

• potential (PE) plus kinetic (KE) – contained in the material of a system
• thermal energy

Question 2

What is heat?

Answer 2

The amount of the thermal energy transferred from one system to another

Question 3

What is friction loss?

Answer 3

Loss of energy from a moving gas which goes into heating up the containing walls, and which cannot be transferred back into the flow

Question 4

T or F: once converted to heat, energy is gone?

Answer 4

True

Question 5

What are the goals of an LEV designer?

Answer 5

• Prevent energy losses
• Convert as much SP as possible into VP

Question 6

What are the LEV components?

Answer 6

Hood, duct, filter, fan, stack

Question 7

What happens when a fan is turned on in a duct?

Answer 7

• an creates a vacuum in the hood and ductwork upstream
• negative SP upstream

Question 8

Why does air rush into the hood when a fan is turned on?

Answer 8

• air is driven by pressure gradients and wants to move from high to low pressure
• SP (potential) converted to VP (kinetic)

Question 9

Where does friction in a duct come from?

Answer 9

• Friction associated with turbulence and shear stresses in duct
• Shock from sudden velocity changes or flow separation (elbows, tapers, hood entry losses, etc)

Question 10

What are examples of resistance?

Answer 10

• Elbows
• Tapers
• Friction in duct
• Hood entry losses
• Fan losses
• Losses at valves, blast gates, air cleaners, etc.

Question 11

What happens when air encounters resistance?

Answer 11

Will cause SP and TP to decrease

Question 12

What is the equation for friction loss?

Answer 12

• SPloss = KLVP*d

K = loss factor per length of duct
L = length of duct
VP = velocity pressure
d = density correction factor

Question 13

Where do shock losses occur?

Answer 13

• expansions
• contractions
• elbows
• branch entries