Eqn not working

[Viper5]

I am trying to solve some HVAC problems which sometimes involves acquiring enthalpies. The MERM estimates these values using Eqn 38.18 and 38.19. In some instances, I get very different answers when trying to solve myself where the solution for these types of problems sometimes uses the low pressure air tables and other tines these equations. I am having difficulties in identifying what assumption the equation is making and knkwing when it is appropriate to use. 

For instance, 

@800 F,

App 23F gives air as h=306.65, but equation gives (.240)*(800F) = h = 193. 

 

[JHW 3d]

Just a shot in the dark, but is it possible you need to convert the temperature to an absolute scale before multiplying by 0.24? For example, to the Rankine scale. I don't have MERM in front of me right now, so apologies in advance if this is bad advice.

 

[Audi Driver P.E.]

@JHW 3d has it.  The equation 38.18 is not correct.  The Cp value they use is in degrees Rankine, and the corresponding temperature in that equation must be Rankine too.

Coincidentally, if you ignore that the table data corresponds to degrees R, and say look up 800F, the h value there is 191.81 (which is close to what you get multiplying .240*800), so this could be causing a lot of confusion, too.

 

[Viper5]

I see what you mean now. It looks like eqn 38.18 should be in absolute, but eqn 38.19 is correct for C and F.

Any other significant errors that you could make me aware of? I've noticed a few of my own, mostly related to the practice problems companion.  

 

[JHW 3d]

I see what you mean now. It looks like eqn 38.18 should be in absolute, but eqn 38.19 is correct for C and F.
Any other significant errors that you could make me aware of? I've noticed a few of my own, mostly related to the practice problems companion.  

It would depend on your edition and printing.

http://ppi2pass.com/submit-and-view-errata

 

[Viper5]

Thanks. I was able to see the errata for my version (13th, 3rd print) and there is no mention of this mistake. I guess it just hasn't been reported yet unless you were able to find it somewhere else. 

 

[P-E]

Enthalpy is overrated. 

 

[Audi Driver P.E.]

Thanks. I was able to see the errata for my version (13th, 3rd print) and there is no mention of this mistake. I guess it just hasn't been reported yet unless you were able to find it somewhere else. 

I have the 4th printing and noticed it wasn't there.  So, I reported it.  Will see if they decide to correct it or not.  They certainly need to.

 

[Audi Driver P.E.]

Thanks. I was able to see the errata for my version (13th, 3rd print) and there is no mention of this mistake. I guess it just hasn't been reported yet unless you were able to find it somewhere else. 

I have the 2nd printing and noticed it wasn't there.  So, I reported it.  Will see if they decide to correct it or not.  They certainly need to.

 

[JHW 3d]

On 10/15/2016 at 3:07 PM, Viper5 said: Thanks. I was able to see the errata for my version (13th, 3rd print) and there is no mention of this mistake. I guess it just hasn't been reported yet unless you were able to find it somewhere else. 

I have the 4th printing and noticed it wasn't there.  So, I reported it.  Will see if they decide to correct it or not.  They certainly need to.

Why on earth do you have the 4th printing? Someone at your office?

 

[Audi Driver P.E.]

Why on earth do you have the 4th printing? Someone at your office?

Uh woops, I have the 2nd printing and it's not in the errata, so I reported it.  :huh: