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Handout No. 4 Tips for Getting the Ventilation-Related Questions Answered Correctly by D. Jeff Burton ([email protected]) 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