Chapter 7: Ventilation

Question 1

The goal of ventilation is to remove airborne contaminants from worker’s _______________.

Answer 1

Breathing Zone.

Question 2

What is Local Exhaust ventilation method?

Answer 2

Removing contaminated air from breathing zone (eg. fume hood)

Question 3

What is Dilution ventilation method?

Answer 3

Bringing in a bunch of clean air to reduce contaminant concentration.

Question 4

VENTILATION EQUATION IS….

Answer 4

Q = VA

V=air velocity in fpm (foot per minute)
A=cross sectional area (ft2)

Note A = pir^2 or pid^2/4
Note ducts are always described using diameter

the unit of Q = cfm (cubic foot per minute)

Question 5

What is the equation for the conservation of airflow where air moves from a large air duct to a smaller one?

Answer 5

V1A1=V2A2

Question 6

What is the equation for conservation of airflow where air from 2 different ducts combine and flow into one duct?

Answer 6

Q1+Q2=Q3

Question 7

Describe the basic local exhaust system.

Answer 7

Hood - entry point
Duct - conduit of travel
Air cleaner - device to remove contaminant
Fan - energy source
Exhaust - exit point

HOOD-duct-AIR CLEANER-duct-FAN-duct-EXHAUST

Question 8

The two types of energy associated with ventilation are:

Answer 8

Potential energy: stored

Kinetic energy: moving

Question 9

Energy in ventilation is really ___________.

Answer 9

PRESSURE

Question 10

In ventilation, kinetic energy is called _______ pressure

Answer 10

VELOCITY

Question 11

In ventilation, potential energy is called _______ pressure.

Answer 11

STATIC

Question 12

Describe velocity pressure

Answer 12

• aka kinetic/moving energy
• the force applied by moving air hitting a surface
• velocity pressure is always positive
• It exists only when air is in motion

Question 13

Describe static pressure

Answer 13

• is the potential/stored energy exerted in all directions by air molecules. This energy is always present and acts independently of velocity pressure. In a duct, static pressure tends to expand or collapse a pipe, depending on whether static pressure is positive or negative. Static pressure is measured normal to the direction of air flow.

Question 14

In ventilation, are we concerned more with static or with velocity pressure? Why?

Answer 14

We are concerned with static pressure.

We want to know what the static pressure is inside the duct relative to the atmosphere outside. That will help us determine if the contaminants will be pushed into the duct or not.

Question 15

Is static pressure positive or negative?

Answer 15

It depends.
In supply systems, static pressure will be positive on the discharge side of the fan.

In exhaust systems, a negative static pressure will exit on the inlet side of the fan.

Question 16

Air moves from ____ static pressure to ____ static pressure.

Answer 16

High to low

Question 17

Formula for total pressure is…

Answer 17

the sum of static pressure and velocity pressure.

TP=SP+VP
units are inches of H20

Question 18

Units of Pressure…the various conversions

Answer 18

1 atm =
101,325 pascals = 14.7psi = 760mmHg = 33.93 ft H2O

Note ventilation uses inches of water…you will need to convert ft H2O to inches H2O

Question 19

What happens to VP, SP, and TP upstream of a fan?

Answer 19

Upstream means before or “input”
VP = pos
SP = neg
TP = neg

Question 20

What happens to VP, SP, and TP downstream of a fan?

Answer 20

Downstream means after or “output”
VP = pos
SP = pos
TP = pos

Question 21

Velocity is _______ proportional to the square root of velocity pressure.

What is the formula?

Answer 21

DIRECTLY.

V = 4005 --| VP
V = 4005 square root VP

where,
V = velocity (fpm)
VP = velocity pressure (inches of H2O)

Question 22

In ventilation, energy is used to do what 2 things?

Answer 22

1. Accelerate air into the system
2. Energy is used to overcome resistance from:
   - – friction along the walls
   - – turbulence in the water

Question 23

What is a hood?

What are the 2 hood categories?

Answer 23

A hood is the airflow entrance to the local exhaust ventilation system.

The two hood categories:
1) external hoods (snorkel hood, kitchen hood); does not enclose the contaminant

2) enclosing hoods (fume hood)