TFS practice problem of the week...

[Slay the P.E.]

A steam turbine with an isentropic efficiency of 76% admits steam at 100 psia, 750F. The turbine discharge is at 2 psia. The shaft power developed in kW per lbm/h of steam flowing through the turbine is most nearly:

(A) 0.08

(B) 0.10

(C) 0.27

(D) 262

 

[Vel2018]

Hi,

So this is what I got using Mollier 0.067kW time 3min 46 sec.

Using tables I got 0.077kW time 2min 24 seconds.

I think it made me faster on tables just because I already input the numbers, each step. OR maybe I am just slow in the chart.

 

[mongolianbbq]

a?

 

[mongolianbbq]

Hi,

So this is what I got using Mollier 0.067kW time 3min 46 sec.

Using tables I got 0.077kW time 2min 24 seconds.

I think it made me faster on tables just because I already input the numbers, each step. OR maybe I am just slow in the chart.

I used the chart and got it in about a minute. I think it helps because my super heated steam tables don't have 750 C so i would have to interpolate or estimate and estimating is easier for me with the chart.

 

[Vel2018]

I used the chart and got it in about a minute. I think it helps because my super heated steam tables don't have 750 C so i would have to interpolate or estimate and estimating is easier for me with the chart.

Yes, its just I am bad at the chart, I used the chart in the MERM appendix using only ruler, did not point my pencil or write a line since its not allowed during test, that's what I got from using chart, my estimate was h1=1405 and h2=1105 that't definitely not within 2% from the value calculated using tables. In this case, there are no other options so would choose A. 

 

[Vel2018]

Oh crap I screw it up, was supposed to be about 1060 h2. Probably when I blinked it went up one line lol it should be same value 0.077kW using the chart.

 

[mongolianbbq]

Oh crap I screw it up, was supposed to be about 1060 h2. Probably when I blinked it went up one line lol it should be same value 0.077kW using the chart.

The merm chart was too small for me. I ended up downloading one online and printing it out and putting it in my resource binder.

 

[Slay the P.E.]

The merm chart was too small for me. I ended up downloading one online and printing it out and putting it in my resource binder.

You downloaded this one, right? wink-wink-nod-nod. It's color coded!

https://www.slaythepe.com/free-resources.html

 

[Vel2018]

You downloaded this one, right? wink-wink-nod-nod. It's color coded!

https://www.slaythepe.com/free-resources.html

awesome! thanks! 

 

[MikeGlass1969]

I got (B).    h1=1403.4 and h2r=1039.8  h2i=925

Mine is wrong.  your chart reads h2i=1050

Yep.  Should be (A)...   I read the .2 psia curve initially, I think.

 

[Slay the P.E.]

Happy Friday. Here's a fresh practice problem for you.

A military jet flies at a speed of 495 miles/hour at an altitude of 49,000 feet. A commercial airplane is flying at the same Mach number as the military jet but at an altitude of 27,000 feet. A plot showing the variation of temperature with altitude in the atmosphere is provided for your potential use. The speed (ft/s) of the commercial jet is most nearly:

(A)  515

(B)  726

(C)  757

(D)  795

 

[MA_PE]

Mach is speed relative to the speed of sound.  If two planes are flying at the same Mach number then they are flying at the same speed (independent of altitude). 

495 mph converts to 726 ft/s.

Answer is (B)

 

[Vel2018]

Mach is speed relative to the speed of sound.  If two planes are flying at the same Mach number then they are flying at the same speed (independent of altitude). 

495 mph converts to 726 ft/s.

Answer is (B)

I think not, temperature are different just have to equate both v1/(kgcRT1)^1/2=(v2/kgcRT2)^1/2

You get  v2

 

[MikeGlass1969]

Speed of sound is a function of absolute temperature.  So I came up with 512 ft/sec.

Answer should be (A)

 

[Slay the P.E.]

Speed of sound is a function of absolute temperature.  So I came up with 512 ft/sec.

Answer should be (A)

Are you sure your answer is in ft/s?

 

[Vel2018]

Answer should be C

 

[MikeGlass1969]

Are you sure your answer is in ft/s?

Damn...  I screwed up again.  

 

[MA_PE]

I think not, temperature are different just have to equate both v1/(kgcRT1)^1/2=(v2/kgcRT2)^1/2

You get  v2

oops.  I forgot that the speed of sound varies with temp/changes in atmosphere.  I'm a structural guy.

 

[MikeGlass1969]

You downloaded this one, right? wink-wink-nod-nod. It's color coded!

https://www.slaythepe.com/free-resources.html

Sure!!!  Where was this 2 years ago when I was studying for my exam????   BUT NO.....   Mike had to use the Black and White one and had to blow it up to 11X17 so his eyes wouldn't cross...

 

[Slay the P.E.]

Sure!!!  Where was this 2 years ago when I was studying for my exam????   BUT NO.....   Mike had to use the Black and White one and had to blow it up to 11X17 so his eyes wouldn't cross...

LOL

well, you know what will really suck? When the exam transitions to closed-book CBT next year. I dunno if the on-screen reference book will have a Mollier diagram, but if it does, just imagine people putting a ruler against the computer monitor... yikes