ENVH 557: Ventilation Class, Lab Exercise Outline
CLASS WILL MEET AT THE FIELD GROUP LAB IN THE ROOSEVELT BUILDING
All numbered sections have questions for which you should give a written answer in your lab report.
Your lab report due Friday 2/24.
Here is the system:
Instrument Review
- Pitot tube: measures Pv -- Use this for measurements inside the duct
- Hot wire anemometer: measures air velocity directly – use this for measurements at the hood face
- Static Pressure measurements : Used to measure Ps at the duct walls (U-tube manometer,
magnehelic gauge, digital micromanometer)
- Tachometer: measures the fan speed
Calculations Review
Q = V*A
TP = SP + VP
Lab Exercises, Section 3: Hood measurements
3.1- Before you do the exercise below, measure the face velocity for the enclosed hood and the capture hood.
Note: To measure the face velocity of a hood, you need to measure the average over the entire opening
cross section at the entry plane. The easiest way to do this is to divide the opening into a number of equal
smaller areas and measure the velocity at the center-point in each of the small areas. For a square opening
this is usually done by creating a grid of measurement points; for a circular entry, this is done by taking
measurements along a series of circular rings, each having equal area. (See the diagram in section B below.
3.2 Sketch a diagram and show your measurements for each hood below:
3.3
Face velocities
3.3.1 - What is the average face velocity for the enclosure hood? ____________
3.3.2 - What is the average face velocity for the capture hood?
____________
3.3.3 - What is the Q for the enclosure hood?
____________
3.3.4 - What is the Q for the capture hood?
____________
3.3.5 - Would the average velocity be the same as averaging the Pv measurements from a pitot tube?
4.1
Enclosure Hood measurements
4.1.1. - What is the hood static pressure for the enclosures hood? __________
4.1.2. – Compute is the coefficient of entry for the enclosure hood?
4.1.3. - Is this enclosure really a compound hood? Why?
5.1. Capture hood
5.1.1. – What is the capture velocity of the capture hood opening at distances of x=d/4 x=d/2 and x=d
x=d/4 __________ x=d/2 __________ and x=d __________
How do these measurements compare to the average face velocity?
5.1.2. - Measure the capture hood static pressure? __________
5.1.3. - What is the coefficient of entry for the capture hood? __________
5.1.4. - How does the Ce you determined compare to a value from a text book?
5.1.5. - Can you measure the vena contracta region? Sketch it
6.0 System modeling & measurement
6.1.1 - Measure the velocity pressure, static pressure, and total pressure, in each branch of the system by a
pitot traverse. Use the diagram below as your guide. Show your work on the diagram.
Unequ al increm ents, equ al a reas
6.1.2 - Measure the velocity static and total pressures in the main duct after the junction by a pitot traverse.
Repeat these measurements at the point just before the fan. Show your work on the diagram.
Unequ al increm ents, equ al a reas
6.1.3 – Compute the average velocity in each duct section of the system.
6.1.4 – Compute Q from your measurements in each branch and in main section of the duct.
Does the total Q from the branches add up in the main section? Does the TP add up?
6.1.5 – Make a table to summarize the VP, SP and TP at different points in the system.
Graph the system by plotting these pressure readings at different points in the system.
7.0 System modifications: What happens if you partially close off the blast gate?
7.1.1 - How is the air flow redistributed in the branches as you close the blast gate?
7.2.2 - Measure the static and total pressure at 3 points in the main duct with the gate partly closed.
- Estimate the loss coefficient for the blast gate: Is you measurement reliable?
Extras:
- What happens when you remove the grill in the enclosure hood?
- How is this data interpreted?
What would results be compared to? (Hint think Ce=1)
- What simple baffle modification at the capture hood would allow for greater capture velocity?
Try your idea and measure the result.