NCEES HVAC+R Practice Exam #138 Solution (error?)

[NHEngineer037]

Problem 130:

The pump has the following to overcome:

160' wc + 20' wc (friction) + 1 atmosphere.

(160+20)*.0438 = 77.9 psi

1 atm = 14.6 psi

77.9 + 14.6 = 92.5 psia

Regardless of the upstream pressure at the pump, the outlet still needs to be 92.5 psia.  That is the highest pressure in the system.

The solution ignores atmosphere and adds in the 20 psig seen upstream of the pump.  I don't get it.  This seems like another half baked solution out of this practice exam.  Maybe I don't know what I'm doing. 

Has anyone felt hung up on this?  The solutions are close enough to select the right answer either way.. but its the approach I want to be sure I have down. 

Thanks in advance,

Ryan

 

[Audi Driver P.E.]

If you have a gauge pressure measurement, atmospheric pressure is accounted for.

Also, is the question asking what additional pressure the pump needs to add?

 

[justin-hawaii]

I believe this is question #138 on the NCEES 2016 HVAC practice exam.  

The question asks for the minimum pressure rating (psi) of the piping system.  

I do believe the question is not worded very well.  I think they are assuming that the pump is sized to overcome the vertical lift of 160 ft and the 20 feet of friction, which is 77.9 PSI.  In addition, the pump has an inlet pressure of 20 psig.  Therefore the outlet of the pump will be 97.9 psi.  The pressure at the outlet will be 20 psig.  

Inlet of pump:  20 psig ->  Outlet of pump:  97.9 psig  -> Discharge after 160 ft height gain and 20 ft friction:  20 psig

The piping must meet the maximum pressure in the system which is 97.9 psig.  You do not need to add 14.7 psi to this value, because we are only concerned with the pressure rating of the piping.  Yes, you have 14.7 psi acting on the water but you also have 14.7 psi acting on the outside of the pipe, thus these two values "cancel out".  

 

[NHEngineer037]

Justin! That is what I was missing!

Well done.

Thank you,

Ryan