# Energy/Mass Balances HVAC EXAM



## Rachel F (Feb 11, 2020)

I'm using the PPI online HUB to help me study for the HVAC PE Exam. For the Energy/Mass Balances section of the test it's saying to read Ch. 20-21 which are inorganic chemistry and fuels and combustion... Is this a mistake? If so, which chapters in MERM should I use to study for this section?


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## OldSquaw (Feb 11, 2020)

Rachel F said:


> I'm using the PPI online HUB to help me study for the HVAC PE Exam. For the Energy/Mass Balances section of the test it's saying to read Ch. 20-21 which are inorganic chemistry and fuels and combustion... Is this a mistake? If so, which chapters in MERM should I use to study for this section? View attachment 16412


According to the NCEES syllabus, fuels and combustion are fair game on the HVAC exam (see applications/equipment and components/boilers and furnaces/fuel types and combustion). Inorganic chemistry...I'm not studying that, but maybe it is helpful for combustion problems.

I'm taking the HVAC exam as well. My plan for studying fuels and combustion was to just familiarize myself with the reference manual. I'm not sure if that's a good strategy or not, but that's what I'm doing. I could be wrong, but the combustion probably makes up a tiny portion (if any) of the HVAC exam. If they do ask you a question or two you can probably find the answer right in the reference manual. I guess I will find out.


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## Slay the P.E. (Feb 11, 2020)

OldSquaw said:


> I'm taking the HVAC exam as well. My plan for studying fuels and combustion was to just familiarize myself with the reference manual. I'm not sure if that's a good strategy or not, but that's what I'm doing. I could be wrong, but the combustion probably makes up a tiny portion (if any) of the HVAC exam. If they do ask you a question or two you can probably find the answer right in the reference manual. I guess I will find out.


@MikeGlass1969 can tell you Combustion was actually an important part when he took the HVAC exam. He doesn’t seem to be active on the boards anymore, but search his old posts.

At a minimum, do some problems where you have to figure out air-fuel ratios, (or alternatively, fuel-air ratios). Also problems calculating the percent of excess air (or alternatively percent theoretical air).


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## Slay the P.E. (Feb 12, 2020)

Also recommend “reverse” combustion analysis, in which the composition of combustion products is provided (given the composition of combustion products of a known hydrocarbon fuel, do you know how to calculate, say, the air-fuel ratio?)

Also, something like the attached (we consider this “advanced”)


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## MikeGlass1969 (Feb 12, 2020)

"I ain't dead yet, Slay"!!!! 

Slay speaks truth,  combustion problems had a significant presence on my exam.    I would study these hard.

For example be able to calculate the partial pressure of water in the flue gas.   or something along those lines


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## OldSquaw (Feb 12, 2020)

MikeGlass1969 said:


> "I ain't dead yet, Slay"!!!!
> 
> Slay speaks truth,  combustion problems had a significant presence on my exam.    I would study these hard.
> 
> For example be able to calculate the partial pressure of water in the flue gas.   or something along those lines


Thanks for the feedback! I guess this makes sense. I never really think about combustion in regards to HVAC.


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## OldSquaw (Feb 15, 2020)

Slay the P.E. said:


> Also recommend “reverse” combustion analysis, in which the composition of combustion products is provided (given the composition of combustion products of a known hydrocarbon fuel, do you know how to calculate, say, the air-fuel ratio?)
> 
> Also, something like the attached (we consider this “advanced”)
> 
> View attachment 16415


Can I PM you to check my answer or should I post here?


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## Slay the P.E. (Feb 15, 2020)

OldSquaw said:


> Can I PM you to check my answer or should I post here?


This problem is discussed in pages 2 and 3 of this thread (click the link below). However, we can talk about it here, absolutely. Can you show your work?


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## OldSquaw (Feb 15, 2020)

Slay the P.E. said:


> This problem is discussed in pages 2 and 3 of this thread (click the link below). However, we can talk about it here, absolutely. Can you show your work?


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## OldSquaw (Feb 16, 2020)

Here’s what doesn’t make sense to me about this problem (unless I made a mistake somewhere). 
 

Based on the information given, the water vapor in the exhaust gas would condense at 134F. Yet the saturation temperature of the water that is evaporating is 281F. So it’s impossible to get the water vapor in the exhaust gases to condense at 134F if the water is evaporating at 281F.


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## Slay the P.E. (Feb 16, 2020)

OldSquaw said:


> Here’s what doesn’t make sense to me about this problem (unless I made a mistake somewhere).
> 
> 
> Based on the information given, the water vapor in the exhaust gas would condense at 134F. Yet the saturation temperature of the water that is evaporating is 281F. So it’s impossible to get the water vapor in the exhaust gases to condense at 134F if the water is evaporating at 281F.


The water enters as a compressed liquid at 70F, so that is the inlet temperature of the water. So, for the water, the enthalpy at the inlet is calculated with the saturated liquid approximation for compressed liquids:

h(70F, 50psi) = h_f(70F) and NOT h_f(50psi) so you have to change that in your solution.
 

Water enters at 70F (is heated to 281F within the heat exchanger) and leaves at 281F. Gas enters at 750F and leaves at 134F. You may be able to have that in a counterflow arrangement.


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## OldSquaw (Feb 16, 2020)

Yes that makes sense. For some reason I had in my mind that it was a saturated liquid at  281F entering. Thanks! This problem definitely took me longer than 6 minutes, but that ok, it’s a great problem.

For the enthalpy I took hf at 50psia and subtracted the sensible heat from 281 to 70. But your way is much easier!


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## Audi Driver P.E. (Feb 21, 2020)

I don't know that the original question of "is this a mistake" has been addressed fully, by any of the above. Certainly the inorganic Chemistry section incorporates mass balance and has some energy balance concepts as well, likewise with fuels and combustion.  But, like you, on first blush I would consider the bulk of the conservation of energy principles to lie in a different section like perhaps chapter 22, based solely on the chapter title.

In really these concepts are sprinkled all through the Fluids and Thermodynamics sections of the text (Chapters 14-25).  So, no, it's not a mistake for PPI to point to 20 and 21 and say "study energy and mass balance in these chapters". It would simply appear they want you to focus on the mass and energy balance aspects of the materials presented in the chemistry and combustion sections for this area of test prep (those are key concepts in those chapters I can assure you). As has been noted above, there are likely to be problems on your exam that stem from these sections, because historically there have been, as was the case in my own exam, as well as in the exams NCEES has published for use in self study, and the point PPI is driving home here: the questions from these chapters will deal with energy and mass balance concepts.

As for your second question: which chapters should I study for energy and mass balance... well, I sorta already touched on that. The fluids and thermal sections of the text all incorporate these concepts quite heavily, however Chapter 22 has probably some of the more intensive focus on energy balance.  Just keep in mind that any time you are dealing with a system that is dynamic in any way both mass and energy balance is going to be key to proper understanding and breakdown of the problem into its basic elements.  When you have a static system, the key concept is going to be balance of forces.  Hope this helps.


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