Friday, August 7, 2009

Studying for the P.E. Exam

Last year, I talked about being halfway to the P.E. Exam. It's now a year later and I'm about one year away from being eligible to take the exam.

The 2010 tests in April and October 2010. I think I graduated 2 weeks too late to take the April exam, plus it would be right about the time of a certain special event, so it will probably be October.

I've started to gather materials and start studying. I've got a couple of sample exams and a P.E. prep manual. Lindeburg is what most of my coworkers used during their exam, so that's what I'll start with.

I've started with an older edition of the sample exam and reference manual. They are a lot cheaper and I can get a sense of whether I not I like the book before I buy the latest version next year. I did get the latest sample exam, because the exam has a new format.

The exam takes all day and is split into 2 parts. It's open book. No graphing calculators. The new format has 40 multiple choice questions in the morning on general mechanical engineering topics. The afternoon allows you to choose what subject you want to concentrate on ("HVAC and Refrigeration, Mechanical Systems and Materials, or Thermal and Fluids Systems"), once again 40 multiple choice questions. The new format also mixes SI and Imperial units (the P.E. exam has traditionally been strictly Imperial).

I haven't been studying for that long, but here are a few of the useful things I've learned so far:
* UNITS- Checking the units of the problem are critical to getting the right answer they're looking for. Unit Analysis [DOC] a great check to make sure you're on the right track, too.
* Having handy conversion factors is a must. Kurt Geick's book has been great in that department.
* No computer! Not having Google to do unit conversions and quickly look up definitions threw me at first. I've got a decent supply of reference materials to help me out, but I need to get faster at using them.
* One thing that's great about graphing calculators is you can see what you entered and see if you typed something in wrong. I discovered a newer scientific calculator that comes pretty close in idiot-proofing calculations that's proven quite handy. All it really does that a normal scientific doesn't is makes it more legible and easier to track calculations, but that can be a big help in solving problems quickly and accurately.

As far as sample questions, here's one from the obsolete format of the exam to give you an idea. The suggested allotted time for this problem is about 24 minutes.


An electrical generator and an air conditioning unit on a passenger aircraft use 68 lbm/min of compressed air at 100 psia and 640 degrees F bled off from one of the jet engine compressor sections. The air conditioner runs on an open cycle and maintains the passenger cabin at 80 degrees F and 12 psia. The electrical generator is driven by an air turbine whose expansion is a polytropic process with a polytropic exponent of 1.2. The mechanical efficiency of the turbine is 85%. Prior to entering the air turbine, compressed air from the engine is cooled to 250 degrees F in a crossflow heat exchanger using ram air from outside the aircraft. The pressure drop across the heat exchanger is negligible.

Here's my sketch of the problem:

(a) Assuming air is a real gas, what power (in kW) is developed by the turbine?

(b) Assuming that air is a real gas, what is the total cooling load [NOTE: in Tons] from the passenger cabin?

(c) Repeat (a) assuming air is an ideal gas, the turbine expansion is isentropic, and the efficiency remains the same.

(d) Repeat (b) assuming air is an ideal gas, the turbine expansion is isentropic, and the efficiency remains the same.


Answers in the comments.


Clay said...

(a) W = 50.7 kW
(b) Q = 3.4 Tons
(c) W = 78.65 kW
(d) Q = 12.4 Tons

Tim said...

I have no doubt you will ace it. You're already bright enough, and I know with the pre-test prep you're planning you will breeze right through it.

And do you get a bonus or a raise with that PE?



Clay said...

From talking with the older engineers, you get a small bump in pay. From what they say, not as much as you'd expect for a consulting firm who's livelyhood IS stamped drawings. Also, almost all officer positions in the company are reserved for P.E.'s.

Does the Corps bump you up a GS grade or something when you get a P.E. while working for them?

Also, just out of curiosity, when did you take the P.E.? What was it like? Did they have a similar pass/fail rate then (it's a little better than 50/50 nowadays)?

Leah B said...

Its good that you're starting to study now, but I have been advised to not be overly prepared. I test this October for my PE (Civil). The Lindeberg is the best... Good luck with it!