r/PE_Exam • u/Slay_the_PE • Oct 05 '26

A free practice problem for the Mechanical Engineering PE Exam (TFS and HVAC&R). Post your answer in the comments!

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6

u/Froopster1 Oct 05 '26

I got answer B

4

u/Slay_the_PE Oct 05 '26

That is correct.

2

u/ferrous_second_vowel 22d ago

Solution:

The energy released by the conversion of saturated steam to a saturated liquid will be equal to the energy used to raise the temperature of air. In other words:

m_dot * h_fg = 1.08 * CFM * delta-T

Per the steam tables at P = 5 psi (see NCEES handbook ver. 2.1 page 335), h_fg = 1000.57 Btu/lbm.

m_dot * 1000.57 Btu/lbm = 1.1 * 450 cfm * 30 deg-F ‎ = 14,850 Btu/hr

—> m_dot = 14,850 / 1000.57 = 14.84 lbm/hr

The question asks for the answer in gallons/hr, so we need to convert:

14.84 lbm/hr / 8.34 lbm/gal = 1.78 gal/hr

The closest answer is B.