24-Bld-A4 Building Environmental Control Systems · May 2017
Question 13 of 14: Psychrometric Relationships at Equal Dry-Bulb Temperature
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Notes on this paper
National Exams May 2017 — 07-Bld-A4, Building Environmental Control Systems (Building Engineering). 3-hour closed-book exam: Section 1 (four 20-mark essay/calculation questions) + Section 2 (ten 2-mark multiple-choice questions).
Reference texts: ASHRAE Handbook — Fundamentals (2021 ed.); ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality (2022 ed.); McQuiston, Parker & Spitler, Heating, Ventilating, and Air Conditioning: Analysis and Design (6th ed.); National Building Code of Canada 2020 (NBCC); National Energy Code of Canada for Buildings 2020 (NECB).
Question 2.9: Psychrometric Relationships at Equal Dry-Bulb Temperature (2 marks)
Given/Find. Both air samples share the SAME dry-bulb temperature T (the problem's stated premise). Test each option against psychrometric relations that hold at fixed T.
Options (a), (b), (c), (d) are all consistent monotonic relations at fixed T. At a common dry-bulb temperature, RH, humidity ratio W, dew point, enthalpy h and wet-bulb temperature T′ all increase together — so higher RH gives higher dew point (a) and higher W (d); higher enthalpy implies higher W and hence higher RH (b) and a higher wet-bulb temperature, hence a higher saturation humidity ratio at that wet-bulb Ws (c). All four hold and so are each capable of being true.
Option (e) contradicts the problem's own premise. The problem states both samples have the SAME dry-bulb temperature, i.e. $T_1=T_2$ always — so the condition "$T_1'>T_2'$" referring to a difference in dry-bulb temperature between the two samples can never actually occur under the stated scenario. It is therefore the one statement that "can't be true" given the problem as posed.
