# MERM Chapter 32 Questions



## JoeysVee (Jul 25, 2009)

I noticed problem 32.1 and 32.2 both used F for equation 29.99 but it seems to me like you would use R. How do you know when to use F and when to use R? Both chapter 29 and 32 say T is in units of R on the 1st page of that respective chapter.

So should it be R or F in equation 29.99?


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## MikeR (Jul 26, 2009)

JoeysVee said:


> I noticed problem 32.1 and 32.2 both used F for equation 29.99 but it seems to me like you would use R. How do you know when to use F and when to use R? Both chapter 29 and 32 say T is in units of R on the 1st page of that respective chapter.
> So should it be R or F in equation 29.99?


You are it should be technically R but in this situation if you use R or F you get the same answer. The +460 cancels. Try it with both units.


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## JoeysVee (Jul 26, 2009)

MikeR said:


> You are it should be technically R but in this situation if you use R or F you get the same answer. The +460 cancels. Try it with both units.


It does not give you the same answer if you use R or F.


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## buick455 (Jul 27, 2009)

JoeysVee said:


> I noticed problem 32.1 and 32.2 both used F for equation 29.99 but it seems to me like you would use R. How do you know when to use F and when to use R? Both chapter 29 and 32 say T is in units of R on the 1st page of that respective chapter.
> So should it be R or F in equation 29.99?


What revision of MERM are you working as the units in the twelfth use R in 32.2?


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## JoeysVee (Jul 28, 2009)

Thanks for the response. I'm using the 12th ed. If you notice they change the temp back to F when they use the formula 29.99.


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## TouchDown (Jul 28, 2009)

Just looking in my MERM... I kind of agree that whether it's R or F in the formula, they should cancel.

In looking at the solutions, the equation in question looks like the one they use to feed into 29.99, which is 29.18.

In 29.18, since they use a multiplier T2 = T1(p2/p1)^((k-1)/k)) that the units need to be in R.

Once you have solved for T2 from that, then you can plug into 29.99, and in that formula since it's add/subtract, then it can be either R or F since they would cancel out and you are looking at the delta between the two:

Ns,comp = (T2-T1)/(T'2-T1) it's just a ration of the deltas, so as long as the T's are all in the same units, whether F or R, they should work. Still, goes back to it must be a requirement of equation 29.18 which needs to have absolute temp units?


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## JoeysVee (Jul 28, 2009)

I stand corrected! When I used all degree R, I left the neg sign in but the T1 term isn't neg once you add 460 to it......duh!!!!! :brickwall:

I also only meant 32.1 not 32.2.

Thanks for thew help guys!


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