Chapter 7: Ventilation Flashcards
The goal of ventilation is to remove airborne contaminants from worker’s _______________.
Breathing Zone.
What is Local Exhaust ventilation method?
Removing contaminated air from breathing zone (eg. fume hood)
What is Dilution ventilation method?
Bringing in a bunch of clean air to reduce contaminant concentration.
VENTILATION EQUATION IS….
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)
What is the equation for the conservation of airflow where air moves from a large air duct to a smaller one?
V1A1=V2A2
What is the equation for conservation of airflow where air from 2 different ducts combine and flow into one duct?
Q1+Q2=Q3
Describe the basic local exhaust system.
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
The two types of energy associated with ventilation are:
Potential energy: stored
Kinetic energy: moving
Energy in ventilation is really ___________.
PRESSURE
In ventilation, kinetic energy is called _______ pressure
VELOCITY
In ventilation, potential energy is called _______ pressure.
STATIC
Describe velocity pressure
- 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
Describe static pressure
- 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.
In ventilation, are we concerned more with static or with velocity pressure? Why?
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.
Is static pressure positive or negative?
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.
Air moves from ____ static pressure to ____ static pressure.
High to low
Formula for total pressure is…
the sum of static pressure and velocity pressure.
TP=SP+VP
units are inches of H20
Units of Pressure…the various conversions
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
What happens to VP, SP, and TP upstream of a fan?
Upstream means before or “input”
VP = pos
SP = neg
TP = neg
What happens to VP, SP, and TP downstream of a fan?
Downstream means after or “output”
VP = pos
SP = pos
TP = pos
Velocity is _______ proportional to the square root of velocity pressure.
What is the formula?
DIRECTLY.
V = 4005 --| VP V = 4005 square root VP
where,
V = velocity (fpm)
VP = velocity pressure (inches of H2O)
In ventilation, energy is used to do what 2 things?
- Accelerate air into the system
- Energy is used to overcome resistance from:
- – friction along the walls
- – turbulence in the water
What is a hood?
What are the 2 hood categories?
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)
