# Problem #119 HVAC & Refrig NCEES Practice Exam



## breezy_moto (Mar 7, 2018)

Can anyone provide some guidance with this problem? Really struggling with where the 62F comes from and how the reheat process line is drawn. 

Thanks!


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## Slay the P.E. (Mar 7, 2018)

breezy_moto said:


> Can anyone provide some guidance with this problem? Really struggling with where the 62F comes from and how the reheat process line is drawn.
> 
> Thanks!


The air conditioning unit has a cooling coil which discharges the air at condition CC (CC in this case is 52db,51wb). Then, there is a reheat coil which performs simple heating (constant humidity ratio) until the desired room supply condition is achieved. Let's call this condition (S). We only know S will be to the right of CC, so locate CC on the psych chart and draw a horizontal line through CC. This horizontal line is the reheat process, and is shown in green in the chart below. S will be somewhere on this line to the right of CC.

You are also given the room (or return) air condition which we will call (R) and is 75db, 50%rh. We know the air process as it goes from the supply registers to the return grills is a straight line from (S) to (R) with a slope given by the SHR of 0.66. Therefore, locate (R) on the psych chart and draw through it a line with SHR=0.66, which is shown as a dark blue line in the chart (you need a psych chart with a protractor). We know (S) will be on this line to the left of (R).

Therefore, (S) will be at the intersection of these two lines. That's how you obtain the 62F. You can calculate the heat required for the CC-S process by reading the enthalpy change directly from the chart, or by using an equation like they did in their solution.


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## breezy_moto (Mar 7, 2018)

Slay the P.E. said:


> The air conditioning unit has a cooling coil which discharges the air at condition CC (CC in this case is 52db,51wb). Then, there is a reheat coil which performs simple heating (constant humidity ratio) until the desired room supply condition is achieved. Let's call this condition (S). We only know S will be to the right of CC, so locate CC on the psych chart and draw a horizontal line through CC. This horizontal line is the reheat process, and is shown in green in the chart below. S will be somewhere on this line to the right of CC.
> 
> You are also given the room (or return) air condition which we will call (R) and is 75db, 50%rh. We know the air process as it goes from the supply registers to the return grills is a straight line from (S) to (R) with a slope given by the SHR of 0.66. Therefore, locate (R) on the psych chart and draw through it a line with SHR=0.66, which is shown as a dark blue line in the chart (you need a psych chart with a protractor). We know (S) will be on this line to the left of (R).
> 
> ...


Ah! Thank you for the detailed explanation! I was missing the reheat concept of constant humidity ratio.


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## Slay the P.E. (Mar 7, 2018)

breezy_moto said:


> Ah! Thank you for the detailed explanation! I was missing the reheat concept of constant humidity ratio.


You're welcome. Have you checked out the free sample pages (20+ pages) from our Psychrometrics and HVAC System Calcs for the HVACR Exam Study Problems book? This topic (and many more) are thoroughly discussed there. You can download the free sample pages HERE


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## breezy_moto (Mar 8, 2018)

Slay the P.E. said:


> You're welcome. Have you checked out the free sample pages (20+ pages) from our Psychrometrics and HVAC System Calcs for the HVACR Exam Study Problems book? This topic (and many more) are thoroughly discussed there. You can download the free sample pages HERE


I'll check it out. Was actually thinking of purchasing your practice exam solutions and working through your exam once I get through the NCEES exam.


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## Slay the P.E. (Mar 8, 2018)

breezy_moto said:


> I'll check it out. Was actually thinking of purchasing your practice exam solutions and working through your exam once I get through the NCEES exam.


Cool. Keep in mind, though, that our exam is TFS; we don’t have an HVACR exam yet.


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## Mech Uruk Thraka (Jul 21, 2020)

How would we get from 38 F ADP to 51db/52wb supply air?

Would we have injected hot water vapor after the cooling coil?


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## usernamerequired (Jun 27, 2022)

gotta bump this, been looking all over the web for this, this is now #19 in the current ncees practice exam, i had no idea the reheat process line was horizontal, this was the only thing i was caught up on, absolutely no mention in the solution, so kudos to slay for this help, also ncees hvac practice exam btw


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