# Rules of thumb



## ikesdsu (Jun 6, 2012)

I was wondering how many people used rules of thumb to answer questions? If I wanted to double check an answer or didn't know where else to start I used rules of thumb like:

BTU=1.085 CFM delta T or BTU= 500 GPM delta T. In the 10 years I have been doing engineeering they have never lead me wrong so I used them on the test. I did pass the PE this time, so they can't be two bad.

Any thoughts?


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## MetsFan (Jun 8, 2012)

I definitely did. I figured it would save me time to get to the harder questions and hoped that the people who wrote the exam weren't too cruel.


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## r_mojo1 (Jun 8, 2012)

I used the hvac rules of thumb and worked great for me. I was able to pass the test first time out, and 7 years out of school and never have done any engineering work. I do management.


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## ksprayberry (Jun 14, 2012)

I fretted over using the "rules" of thumb on the exam. It turned out to be mainly my eyesight causing me problems when I would plot things on the Psychrometric chart more than anything. I love my Psycal program. A pair of reading glasses took care of that. I was concerned when I noticed some of the exams using the air density instead of 1.08 or 4.5. I worked them both ways during the exam just to be certain.

In industry, if you think about it, it's like an old truck, there's a half a round of play in the steering. For most work, the guy running the load calculation adds 10% safety, yet, they chose the outside air conditions from the .4% column, the boss didn't like the cfm per square foot and asks to bump up the airflow a little. If I'm lucky (I'm the manufacturer) they were kind to me and rounded up your airlow to the nearest 100 cfm..really, the balancing guy isn't going to balance the unit to 6,152 cfm..I can promise. He's going to get it to maybe +/-5 -10%. and call it done. If he doesn't do the "desktop" balance on you and never look at the equipment. Then, they picked the fan at standard air conditions even though the coil in a draw through position and the leaving air off the coil is 52 degrees. 1.08, 4.5, etc is just fine to me unless your at altitude, even then, it's fairly easy to correct.

There's a lot of variables. I've always understood the need to do it right and make sure you've accoounted for everything, but it's not a "Moon Shot".

Just my opinion.

Thanks

kelly

I didn't think anything about using the 500gpm delta t, it worked fine for me.


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