BLOCK 4: CHEMICAL CONTROLS - Unit 4: Intro to Ventilation (complete) Flashcards
A principle that states that mass cannot be created or destroyed. For a system operating at a steady state, this means that the mass entering must equal the mass exiting the system.
CONSERVATION OF MASS
For a ventilation system, this means that the amount of air coming in equals the amount of air going out. This can be stated by the equation (Q1 = Q2) where each Q is the volumetric flow rate at a point in the system.
A principle that states that energy cannot be created or destroyed; it can only change form.
CONSERVATION OF ENERGY
The area of a cross-section of duct. For a round duct, the cross section is a
circle. For a rectangular duct, the cross-section is a rectangle.
DUCT AREA
- For a round duct: 𝐴 = 𝜋𝑟2
- For a rectangular duct: 𝐴 = 𝐿 ∗ W
This is a measurement of air flowing through a point in the system.
Measure in units of volume per time such as cubic feet per minute (CFM)
VOLUMETRIC FLOW RATE (Q)
Quantity moving through a space at a certain time.
*Speed
*Amount
*Area
Q=VA
Can be thought of as the potential energy of the ventilation system; is exerted in all directions and is the result of a volume of air occupying the space in the duct. It is the pressure that tends to either collapse (negative) or expand (positive) the ductwork with the greatest amount of pressure near the fan.
STATIC PRESSURE (SP)
Static pressure is negative upstream of the fan (where air is “pulled” toward the fan) and positive downstream of the fan (where air is “pushed” away from the fan).
Can be thought of as the kinetic energy in a ventilation system; it is the pressure required to accelerate air (kinetic energy) to its current velocity. NOTE: The impact of moving air is always measured in the direction of flow, which will always make this positive.
VELOCITY PRESSURE (VP)
The sum of VP and SP (TP = SP + VP). It carries the sign of SP (either positive
or negative) and measures the energy content of the air stream.
TOTAL PRESSURE (TP)
The air velocity required to capture and transport a contaminant into a ventilation hood.
CAPTURE VELOCITY
The volumetric flow rate (Q) of a ventilation system must be high enough to achieve capture velocity where the contaminant is generated.
The measurement of airflow across the opening of a local exhaust capture hood; measured in fpm (feet per minute). It is one method of determining the performance of a hood and one of the ways used to determine if the ventilation system meets design criteria and baseline parameters.
FACE VELOCITY
The amount of airflow required inside a duct to keep a contaminant entrained throughout the system. It is expressed in feet per minute (fpm).
TRANSPORT VELOCITY
COMPONENTS OF A VENTILATION SYSTEM
Where the air is drawn into the hood, which captures, contains, or controls the emission source.
INLET
“What if I draw my.. finger here? Cause this is where all of it comes in.”
- Sgt. Loveless
COMPONENTS OF A VENTILATION SYSTEM
Carries the contaminant to the air cleaner and then to the outside area.
DUCT
“I love vents..”
COMPONENTS OF A VENTILATION SYSTEM
Scrubs, separates, removes, or filters the contaminant from the air.
AIR CLEANER
“I talked about it the least - but it’s actually the most important”
COMPONENTS OF A VENTILATION SYSTEM
Generates static pressure and moves the air.
FAN
“Can you tell what that weird thingy I drew here is?”
COMPONENTS OF A VENTILATION SYSTEM
Disperses the cleaned air into the outside environment.
OUTLET
“Air is let out of the… outlet - that’s right!”
Separated by the fan, the two sides are called _______________ (suction side), and
_______________ (blowing side).
UPSTREAM; DOWNSTREAM
If measurements are taken upstream from the fan, static pressure is _______________. If taken downstream from the fan, static pressure is _______________.
NEGATIVE (-); POSITIVE (+)
_______________ is the pressure exerted by air in motion; the pressure required to
accelerate air from zero to the current velocity.
VELOCITY PRESSURE (VP)
It is always measured in the direction of airflow which will always make VP positive.
The sum of the static and velocity pressures at a given point in the duct with due regard for sign.
TOTAL PRESSURE (TP)
Note that the sign for TP will always be the same as the sign for SP!
The point where the airstream diameter is the smallest is called the _______________
VENA CONTRACTA
To reduce contractions, hoods should be flanged to create a smoother transition of airflow into the duct.
