Handout No. 4
Tips for Getting the Ventilation-Related
Questions Answered Correctly
by D. Jeff Burton (jeff@eburton.com)
What is this handout?
This 14-page handout will help you find the “best” answer for
ventilation related questions. It provides tips and suggestions on how
to better your chances when taking multiple-choice exams.
Why do we have exams?
But first, a little review. Tests and exams are curious, indeed.
They mostly check our knowledge, memory, and our ability to
memorize and to think under pressure.
Today’s well-written exams also monitor our judgement, intuition, and
decision-making skills.
Other factors?
Exams and tests don’t easily measure our effectiveness with people,
negotiating skills, sense of responsibility, compassion, honesty, social
consciousness, management skills, and other attributes of a
“qualified” professional.
But, unfortunately, there is no other independent, uniform and
unbiased way to determine if you’ve studied hard, passed a course, or
are prepared to take on the responsibilities of a professional.
Plus, without knowledge all those other factors are useless. So, exams
are worthwhile. At least it shows you have the foundations of a
professional.
Your responsibility
Your study and preparation will help you build that foundation. The rest
is up to you.
Methodical approach
But like anything, knowing how to take a test is important. A thoughtful,
methodical approach to taking exams may mean the difference
between passing and not-passing.
“Poor test taker?”
The following tips and suggestions will help those who are “poor test
takers” but who have prepared themselves with the knowledge and
experience necessary to pass the exam, given a fair opportunity.
(c) 2010
D. Jeff Burton
Handout No. 4
Page 1
Time Considerations
1. Time
Before the exam begins, estimate the average time available for each
question.
Example. If the exam consists of 250 questions and you have 7
hours to complete the exam, then
t = 420 minutes/250 questions
= 1 min 40 sec per question (on average)
2. Time duration
Recognize that some questions will take less time. Some answers
will take less than a minute each to identify; others, a longer time,
e.g., 5 minutes each. The “easy questions” typically come from
subjects you know well.
Some questions will be difficult, requiring you to perform multiple
calculations, for example, or will require you to think, remember and
evaluate.
Example. If 60% of the questions (150 of 250) take one minute per
question, that leaves an average of just less than three minutes per
question on the other 100 questions.
3. Watch your time
Don’t let your frustration or fear of failure lull you into taking an
inordinate amount of time on any one question. If you only have three
minutes for the “hard questions,” then taking 10 minutes on any one
question will reduce the time you have to answer other questions.
Finding the right answer
4. Skip and keep a list
For questions that are likely to be too time-consuming, skip them and
add them to a “skipped questions” list you keep. Or, mark, flag or
label the questions you want to return to on a computer-based test.
You can return to these “problematic” questions later.
5. Work the entire exam
As feasible, go through the entire exam (or that portion open to you
during the allotted time), and answer those you know and can
complete quickly. Keep a list of “skipped questions” and another list
of “need another review” questions. (However, see Paragraph 7 for a
caution about this second list of “unsure answers.”)
Example. Suppose you complete 200 questions in 4 hours. You’ve
kept a list of 50 questions to return to. That leaves you 3 hours to
complete those, at about 3.5 minutes each.)
6. Find the answer
the exam?
(c) 2010
D. Jeff Burton
Recognize that the answer for one question might be included in
another questions somewhere else in the exam!
See Sample Exam Question #8 at the end of this handout.
Handout No. 4
Page 2
7. First impression
But ------->
Keep in mind that your first inclination or first impression of the
“correct answer” is usually the right one.
Be extremely careful when reviewing your answers that you don’t
change your answers unless you have positive proof that you worked
the wrong problem or you misread the question.
Example. You thought you read “velocity” and calculated the answer
for “velocity” when, as it turns out during your review, the question
actually asked for “velocity pressure.”
8. Best answer
Recognize that you are looking for the “best answer” to the questions.
Multiple choice answers are never like this:
a.
b.
c.
d.
white
red
red
red
---->
a. absolutely right
b. absolutely wrong
c. absolutely wrong
d. absolutely wrong
They are more like this:
a.
b.
c.
d.
most white
gray
pink
more pink (or red)
a.
b.
c.
d.
the best answer
partly true but only if...
probably not correct or true
probably wrong or wrong
In almost every case, two or more answers which have elements of
“truth.” You are looking for the one with the “most truth,” the best
answer.”
The “distracter”
9. Don’t get
bogged down
There is often an answer, called the distracter, which could easily
appear to be correct for the less careful or the less knowledgeable.
The “distracter” is used to help assure that you actually know the
issues involved and have chosen the correct answer based on your
adequate knowledge and experience.
Again, recognize because two or more answers could be
the “answer,” you may find yourself thinking, “well if you assume X,
then “a” is correct; but if you assume Y, then “b” is correct; etc.
Try to avoid such complexities.
Choose the best, the most logical, the most common, the most
traditional, the most likely, or the most expected answer.
Many very fine minds have floundered on the “but what if you assume
X?” approach to taking exams. They waste time, get frustrated, and
make unnecessary mistakes.
(c) 2010
D. Jeff Burton
Handout No. 4
Page 3
10. Pay attention
Read each question very carefully. Reread it to be sure you know
exactly what it says, exactly what information is given, and what
exactly is being asked for.
Underline if necessary. Write each input and desired output on
scratch paper, if necessary. Again, many fine people have failed
exams because they “misread” or misunderstood questions.
11. Ignore extra info
By the same token, ignore extraneous information that is sometimes
provided which is not required to get the answer.
Some questions may give you “all the available information” (just like
the real world) and you will be required to sift through the input data to
determine which inputs are necessary to get the desired answer.
See Sample Question #4 at the end of this handout.
12. Psych out the writer
Try to put yourself in the shoes of the question writer. “If I were the
expert writing this exam question, what would I expect the exam taker
to do?”
13. Look for the wrong
answer too
Recognize that you should be looking for the “wrong” answers as
well as the “right answer.”
For example, in the extreme case, if you know that 3 of the 4 possible
answers are wrong, then the remaining answer has to be correct
(even if you don’t actually know it is correct.)
If you know that 2 of the 4 answers are wrong, then at the very least,
you have a 50% chance of guessing the correct one.
This is completely acceptable because it demonstrates that you have
at least a partial knowledge of the problem and its solution. In a
sense, it is the means to receiving “partial credit” for your incomplete
knowledge.
Think about it. If you have narrowed the acceptable answers to only
two possible choices on 100 questions, you will (by the law of
averages) get 50 of those “correct,” a 25% “partial credit!” (You could
get 25% correct by randomly picking answers.)
If questions have as an answer, “d. all of the above,” and one of the
other answers is wrong, then “d” cannot be correct.
Again, narrowing your choices to two possible answers.
(c) 2010
D. Jeff Burton
Handout No. 4
Page 4
14. Distracters, etc
(Also, “distractors.”) As mentioned earlier, many questions will have
answers that could be characterized as:
a.
b.
c.
d.
almost correct (the distracter)
most correct
probably not correct
not correct, even ridiculous
It is often not too difficult to spot a “ridiculous” answer because either
the question writer was trying to be funny, or has run out of semilogical, seemingly-correct answers.
15. Watch for lapses
Recognize that the question writer is human, not perfect, and may not
perfectly compose the question, or leave out information.
For example, if “STP” is not stated, assume STP.
Likewise, if temp and pressure are given, assume density correction
calculations are necessary.
Calculations and Estimations
Much of the exam may involve estimating or calculating answers (for
ventilation, noise, heat stress, statistics, radiation, air sampling, and
so forth). The following thoughts are offered for exam questions
requiring estimations, calculations, or the use of charts and graphs.
Most of the principles introduced here are demonstrated in the
“Sample Questions” provided at the end of the handout.
16. Provided equations
Because many basic equations and charts are now provided in exam
handouts, exam question writers are not likely to compose a
question that you can simply plug into one of the given equations.
Too easy.
They will expect you to make multiple calculations, or convert input
data to some other units, or solve first for some other unknown in
order to move on to the desired answer.
17. Watch for the ringer
Because it is important to know how and when to use an equation, a
question may include correct-looking answers but which will be
wrong because you didn’t take some critical step or make some
important conversion.
See Sample Question #3 at the end of the handout.
18. Use “unit analysis”
(c) 2010
D. Jeff Burton
Also, called “dimensional analysis,” this approach relies on looking
the input units (dimensions like ft/min) and the answer units.
Manipulating (multiplying, converting, dividing) the input units should
result in the desired output units. Again, see Sample Question #3.
Handout No. 4
Page 5
19. Know your calculator
Most exams allow only certain models and brands of calculator. (Preprogrammed calculators are not allowed at most exams.) You may
purchase a new calculator for the exam. Be sure you know how to
use all the new calculator’s functions, e;g, log keys, exponential keys,
statistical functions, if allowed.
20. Conversions
Before the exam, be sure you know how to make conversions, e.g.,
horsepower to KWh, mg to lbs, etc.
21. Carry units and signs
Be sure to write down and carry all your signs and units as you solve
calculation problems, step by step. This will help to avoid using the
wrong units, solving equations upside down, getting the wrong sign,
and so forth.
Suppose a question asks for the volume flowrate (Q, in standard
cubic feet per minute) and provides average velocity (3000 standard
feet per minute) and duct area (1.096 ft2 ). Carry all units along as you
work the problem.
Q = VA = 3000 ft/min x 1.096 ft2
= 3,207 ft3/min
22. Know your equations
Be sure you know what the equations (e.g., on the ABIH Candidate
Handbook) solve and what each of the terms means and its units.
Practice solving for each term in the equation. Know how to convert
units of given data into the required units for the equation. See
Handout No. 12.
Suppose question “No. 240” asks:
240. What is the reported air volume flowrate (Q, in standard cubic
feet per minute) and for an average duct velocity = 2200 standard
feet per minute in a round duct with a diameter = 12 inches.
a.
b.
c.
d.
1727.9 scfm
1821 scfm
2805 scfm
248,800 acfm
You would (find and) use “Q = VA,” the airflow equation.
Note that you would need to convert “inches” to “feet” in order to get
the correct answer.
Q = VA = (2200 ft/min) x π (12/12 feet)2 / 4
= 2200 ft /min x 0.7854 ft2 = 1727.9 ft3/min
(c) 2010
D. Jeff Burton
Handout No. 4
Page 6
23. Significant figures
Recognize that some (but not all) questions will adhere to recognized
rules for significant figures in the answers. When possible select the
answer that follows the rules of significant figures.
Suppose the potential answers for the last question (Paragraph 22,
Question “No. 240”) were given as follows:
a. 1727.88 scfm
b. 1700 scfm
c. 2800 scfm
d. 248,800 acfm
Note that “b” is the “Best answer” because it follows the rules for
significant figures. (The input “2200 scfm” has only two significant
figures.)
Answer “a” is the distracter. (“But it could be right if we ignore
significant figure rules.”)
Answer “c” is the “ringer.” It is actually the answer if you make a
mistake and use “Q = V/A = 2200/0.7854,” a common mistake if you
don’t carry along the signs and units in your calculation work.
Answer “d” is another “ringer.” This is the answer if you forget to convert
“12 inches” to “1 foot.”
Answer “d” is also “wrong” because it says, “acfm.” (Actual cfm.) Here
you could eliminate answer “d” because it is simply written wrong.
Again, you can avoid critical calculation mistakes if you will write down
and carry your units along as you solve the problem.
24. Standard conditions
If the question does not mention anything about “standard conditions,”
“STP,” or “NTP,” then assume the problem is at standard conditions (or
for routine IH calculations), NTP. If the problem or question provides
temperature and pressure conditions, you will most likely will have to
estimate the density correction factor (or in some way take into account
air density) in order to determine the correct answer.
Note:This may not always be case, e.g., where more information than
you actually need is provided or you have a distracter.
25. Likely questions
(c) 2010
D. Jeff Burton
Since you won’t have infinite study time, devote your study and practice
to those equations and charts most likely to be on the exam. Reason,
for example, suggests that because of its importance and universality,
ventilation is likely to have a stronger presence on an exam compared
to radiation (even though they have about the same number of
equations on the exam “handout”).
Handout No. 4
Page 7
Other Preparations before the Exam
26. Medications
Everyone responds differently to medications. In most cases it is
inadvisable to take medications (or drugs or alcohol) the night before
or on the day of the test that could dull your senses, make you sleepy,
or impair your thinking processes and memory.
27. Practice Exams
Before you take “the” exam, take practice exams. Although the same
questions will not be seen, similar questions are likely to be
encountered. Plus it will give you a good idea of how to proceed, what
approaches works best for you, how to relax, how to manage your time,
and it provides an opportunity to apply the thoughts and suggestions
found in this handout. See the “Learning through Exercises” in the IHW.
28. Relax.
During the exam, take a little time to relax. Every ten questions, for
example, you might close your eyes, take a couple of deep breathes,
and relax. Stand and stretch at least hourly.
29. Don’t cheat
It may be tempting to prepare and carry crib sheets into the exam, write
something on your hand, or program some information into your
calculator. Avoid such temptations. If you get caught your reputation
and career will be severely curtailed. Not to mention that you will never
know if you could have passed the exam without cheating.
Sample Questions that Illustrate the Principles of this Handout
Note: √√
indicates the “correct” answer for each question.
1. The primary purpose of a local exhaust ventilation system is to:
a.
b.
c.
d.
prevent reentrainment of air contaminants at a stack
contain, control, capture, and/or remove contaminants at their source √√
provide dilution ventilation of volatile substances
provide protective ventilation of micro-organisms
Evaluation: All answers are “correct” in some circumstances, but the key word is “primary.”
Answer “b” is the primary purpose, and thus the best answer.
2. At a point upstream from the fan in an exhaust system, the velocity pressure is VP = 2.00” w.g.
and the static pressure is SP = -3.50” w.g. What is the total pressure, TP?
a. 1.50 inches of water
b. -1.50 inches of water √√
c. 5.50 inches of water
d. -5.50 inches of water
Evaluation: You can look for the appropriate equation in the ABIH Handbook, under
“Ventilation.”
(c) 2010
D. Jeff Burton
Handout No. 4
Page 8
You should look for an equation that has TP, SP and VP.
On the Handout sheet you will find: “TP = SP + VP”
In order to be sure to obtain the correct answer, you should carry all signs and units along in the
calculation.
Total pressure = velocity pressure + static pressure
–1.50”wg
= + 2.00”wg
+ (–3.50”wg)
Note that any of the multiple choice answers might be obtained if you don’t carry the correct sign
in your calculation work.
3. At an open doorway the air flows out of a room at a rate of Q = 2300 cfm.
The open area is 3' x 7'. What is the average velocity of the air through the doorway?
a.
b.
c.
d.
48,300 fpm
2300 fpm
110 fpm √√
913 cfm
Evaluation: Note that the equation provides Q (flow rate) and dimensions for A (area) plus it
asks for V (velocity.)
Using the equation. You can look for the appropriate equation in the ABIH Handbook, under
“Ventilation.” You should look for an equation that has Q and V (velocity) and A (area).
On the exam Handout you will find: “Q = V x A,” which has all of the terms necessary to solve the
problem.
In order to obtain the correct answer quickly, you should carry all signs and units along in the
calculation.
Write your work on a sheet of scratch paper.
You will have to first estimate A = b x h = 3 ft x 7 ft = 21 ft2 (A not uncommon practice in these
types of questions is to require multiple calculations.)
Then, solving for V in the equation:
V=
Q
/
= 2300 ft3/min /
= 109.5238 ft/min
A
21 ft2
Note that the final units are in “ft/min,” the correct units for velocity.
Look for your answer among the multiple choices
Note that answer “c” has your answer and follows the rules of significant figures.
(c) 2010
D. Jeff Burton
Handout No. 4
Page 9
3. Continued. Using Unit analysis. Suppose you can’t find or think of the proper equation. You
should look at the input units and desired units.
The input units are ft2 and ft3/min. The desired units are ft/min (velocity). What can you do to the
input units to obtain ft/min?
The only way to obtain the desired output units are to divide ft3/min by ft2.
2300 ft3/min / 21 ft2 = 109.5 ft/min
A few extra thoughts:
If you hadn’t used the units you might have incorrectly estimated Q x A = 48,300 cfm (one of the
answers).
Finally, Note that since air density is not mentioned, you should assume STP.
(Although not needed in this question.)
4. Room air at STP enters 10 oven doors (2’ x 2’) at a rate of 100 cfm per oven. The air temperature
then rises to 600˚F before leaving each oven and recombining in a 16” exhaust duct. The
exhaust duct is insulated with 3” of fiber insulation and runs 75’ to the fan housing inlet, 14
inches in diameter. What actual volume flowrate of air must the fan be capable of handling?
a.
b.
c.
d.
4000 acfm at standard conditions
2000 acfm at 600˚F √√
1000 scfm
1000 acfm at 600˚F
Evaluation: Because the question mentions a temperature increase from STP, it likely will
require an adjustment for air density.
Note also that more information is given than is necessary (door size, 3” of insulation, 75’ of 16”
duct, fan 14” inlet) to find the desired answer (the actual volume flowrate flowing through the fan).
You might look for the appropriate equation in the ABIH Handbook, under “Ventilation.”
Unfortunately, the Handbook entry for “ventilation” does not include an equation for density
correction! You’re on your own.
If you don’t remember the density correction equation, or how to manipulate the Ideal Gas Law
(which could take many minutes), you should try to reason yourself to an answer.
You will note that some answers have “acfm” and some “scfm.” The question asks for the
“actual volume flowrate” of air that will be hot. This means that answers “a” and “c” cannot be
correct because they are in “scfm”, air flow at standard conditions. That narrows your choices to
“b” and “d.”
Now you know that 1000 cfm enters the ovens and must expand to some higher volume
flowrate because it is heated.
(c) 2010
D. Jeff Burton
Handout No. 4
Page 10
The means that “d” cannot be correct because no expansion is taken into account.
The only answer left, “b” must be the correct answer!
The actual math is this: The absolute temperature ratio (in degrees R) = d = 1060/530 = 2.
Therefore, Qstp x 2 = 2000 cfm at 600F.
5. The hood static pressure downstream from a grinder hood ( measured using a pitot tube) =
SPh = -1.69" wg. The connecting duct diameter is 6" and the hood shape is that of a standard
grinder hood having a coefficient of entry of Ce = 0.78. What air flowrate would this hood static
pressure suggest? (Assume non-STP; df = 0.95)
a. 410 scfm
b. 820 acfm
√
c. 1200 acfm
d. 1600 scfm
Evaluation: You can look for the appropriate equation in the ABIH Handbook, under
“Ventilation.” You should look for an equation that has “flowrate” (Q), SPh, Ce and A (Area, for a
6” duct).
On the Handout sheet you will find: “Q = 4005 • Ce • A (SPh)1/2 ”
Unfortunately, this is the equation for standard air, or STP.
You will have to remember where the density correction factor, df, is involved.
The proper place for “df” is shown below:
Also, remember to use the absolute value of SPh in this equation.
Q = 4005 • Ce • A (SPh/df)1/2
Q = 4005 • 0.78 • 0.1964 (1.69/0.95)1/2 = 818.314 acfm (or rounded, Q = 820 scfm)
Reasoning your way to the answer: Note that answers “a” and “d” cannot be correct because the
units are “scfm.” That leaves 2 possible answers. If you can’t remember where “df” goes in the
equation, perform the calculation without df using the Handout equation:
Q = 4005 • Ce • A (SPh)1/2
Q = 4005 • 0.78 • 0.1964 (1.69) 1/2 = 797.6 scfm or rounded, Q = 800 scfm)
Now, look at the density correction factor, df, provided in the question.
It suggests about a 5% difference from standard conditions. Which of the two remaining
answers, b or c, is more probable? Answer “c” is about 33% different and Answer “b” is about
3% different.
The best choice would be Answer “b.”
(c) 2010
D. Jeff Burton
Handout No. 4
Page 11
6. Round ducts on the suction side of a fan are considered better than square ducts because:
a.
b.
c.
d.
they resist collapsing better
the use less material for the same cross-sectional area
they provide better conditions for consistent particle transport
all of the above √√
Evaluation: From geometric principles alone, we know that answers “a” and “b” are true.
Even if we don’t know the status of answer “c,” and because at least two of the answers are
correct, “d” must be the best answer.
7. Someone adjusts the slot width on a side-draft hood from 2 inches to 1 inch (without any other
modifications to the ventilation system). This action would have the following effect:
a.
b.
c.
d.
increase capture velocities in front of the hood
reduce air flow through the hood √√
reduce the hood entry loss
no effect
Evaluation: Suppose you don’t know the answer to this question.
Flag it and come back later.
A little later you run across Question 10. (See Question 10 now.) Note that the action is similar
(narrowing the hood opening). It states that the effect will be to reduce flowrate. Yahoo! There’s
your answer.
8. A fan is rated in a manufacturer's catalog as delivering 10,500 scfm when FTP = 3.0 inch w.g.,
and the fan speed is n = 400 rpm. The drive motor requires 6.2 shaft horsepower at these
conditions. If the fan speed is increased to 500 rpm, determine the new Q, FTP, and shp.
a.
b.
c.
d.
Q = 14,250 scfm; FTP = 6.2 in. of water; shp = 13.6
Q = 13,125 scfm; FTP = 4.7 in. of water; shp = 12.1 √√
Q = 12,253 scfm; FTP = 4.3 in. of water; shp = 11.6
Q = 11,158 scfm; FTP = 3.7 in. of water; shp = 10.4
Evaluation: Note that this question requires you to make multiple calculations—maybe up to
12 calculations! Talk about time consuming!
(The fan law equations are found at the bottom of the Ventilation sheet in the ABIH Handbook.)
However, note that if any one result in answers a-d is wrong, then that entire answer is wrong.
Also, if any one result is correct, the entire answer is correct. Therefore, you need only make
one calculation: new Q = 13,125 scfm, which is answer “b.”
Note also that “scfm” indicates STP conditions. Note also that in this question, the rules of
significant figures were not followed. A better answer would have been Q = 13,000 scfm.
(c) 2010
D. Jeff Burton
Handout No. 4
Page 12
9. For all practical purposes, use of a pitot tube is limited to velocities greater than:
a.
b.
c.
d.
200-300 fpm
400-600 fpm √√
800-1000 fpm
>1600 fpm
Evaluation: The velocity pressure, VP, at 400 fpm = 0.01” w.g. Anything less than that is virtually
impossible to read using typical testing and monitoring equipment.
You could use the V vs. VP equation but this is a question that actually relies more on your
experience and reason than on your ability to perform calculations.
10. The sash of a non-bypass lab fume hood is lowered, reducing the air flowrate through the
hood. After lowering the sash, the absolute value of the hood static pressure, SPh, will:
a. increase √√
b. decrease
c. stay the same
d. there is no way to know.
Evaluation: If the sash were closed entirely, SPh would be at a maximum. Again, watch the
sign of SP.
Absolute means “the positive value” of what is a negative number according the ventilation sign
convention. Note that the question provides the answer for Question No. 8.
11. The velocity of air in a duct can be estimated by measuring the velocity pressure. What
instrument is best-suited for obtaining this pressure measurement?
a. a vaned anemometer
b. a pitot tube √√
c. a thermal anemometer
d. a balometer
Evaluation: The question relies solely on your knowledge and experience.
But you could reason it out. “Anemometers” measure velocity and are secondary instruments.
The velocity pressure could then be estimated from the measured velocity (using Bernoulli’s
Equation) but that is a lot of work and probably doesn’t fit the definition of “best-suited.” (Be
sure read the question carefully!)
So answers “a” and “c” are suspect.
A balometer measures flowrate, with the same problems of anemometers.
The mostly likely, “best-suited” answer is “c” because the pitot tube is a primary instrument and
measures VP directly.
(c) 2010
D. Jeff Burton
Handout No. 4
Page 13
12. Toluene is evaporated in a glove box at the rate of 0.50 lb/hr. Assuming the air density
correction factor, df = 0.88, how much air flow is needed to reduce the concentration to 10% of the
LEL? (LEL = 1.27%, MW = m.w. = 92.1, s.g. = 0.866, assume perfect mixing.)
a. 31 acfm
b. 45 acfm
c. 58 acfm
d. 91.6 scfm
Evaluation: You can find a formula on the ABIH Handout, but it requires input data in “pints/min.”
where ER = evaporation rate in pints/min, C = acceptable concentration, K = mixing factor
In order to use the ABIH Handout equation, you will have to convert “0.5 lb/hr” into “pints/min.”
Water weighs 8.34 lb/gallon x 1 gallon/8 pints = 1.043 lb/pint = 0.959 pints/lb (water)
Toluene weighs 8.34 lb/gallon x 0.866 = 7.22 lb/gallon = 0.903 lb/pint = 1.108 pints/lb (toluene)
Therefore:
pints/min toluene = 0.5 lb/hr x 1.108 pints/lb
= 0.0.554 pints/hr x 1 hour/60 minutes
= 0.0092 pints/min
Now use the ABIH Handbook equation:
Note that “perfect mixing” means K = 1.
Note also that the ABIH equation assumes STP.
-- You will have to divide by df = 0.88, in order to get the correct answer in acfm.
= 27.5 scfm/0.88 = 31.2 acfm
These types of questions seem “tricky” but they really help determine if the Candidate really knows
how to deal with such problems. They require the Candidate to do more than simply “plug into an
equation.” They, in reality, help determine whether a Candidate is experienced and knowledgeable
enough to be a “CIH.”
So best of success! -- jeff
(c) 2010
D. Jeff Burton
Handout No. 4
Page 14