ASHRAE Material Properties 3 — Questions and Answers
Question 1: What is the approximate specific heat of dry air at constant pressure in I-P units?
- 1.0 Btu/lb·°F
- 0.17 Btu/lb·°F
- 0.24 Btu/lb·°F (Correct answer)
- 0.45 Btu/lb·°F
Correct answer: 0.24 Btu/lb·°F
For dry air, cp ≈ 0.24 Btu/lb·°F, which gives the 1.08 constant in the sensible heat equation (0.075 × 60 × 0.24).
Question 2: What density does ASHRAE use for standard air at sea level?
- 0.075 lb/ft³ (Correct answer)
- 0.062 lb/ft³
- 0.0807 lb/ft³
- 1.20 lb/ft³
Correct answer: 0.075 lb/ft³
Standard air is taken as 0.075 lb/ft³, which is about 1.2 kg/m³.
Question 3: A 2-inch layer of insulation has k = 0.25 Btu·in/h·ft²·°F. What is its R-value?
- R-4
- R-12.5
- R-8 (Correct answer)
- R-0.5
Correct answer: R-8
R = thickness / k = 2 / 0.25 = 8 h·ft²·°F/Btu.
Question 4: Which piping material has the highest coefficient of linear thermal expansion?
- Carbon steel
- Copper
- Ductile iron
- CPVC (Correct answer)
Correct answer: CPVC
Plastics like CPVC expand about 3-4 times as much as copper and about 5 times as much as steel, so they need more room for expansion.
Question 5: Compared with open-cell spray foam, closed-cell spray polyurethane foam has which characteristics?
- The same R per inch but lower density
- Higher R per inch and higher vapor permeance
- Lower R per inch and higher vapor permeance
- Higher R per inch and lower vapor permeance (Correct answer)
Correct answer: Higher R per inch and lower vapor permeance
Closed-cell SPF is rated at about R-6 to R-7 per inch and can act as a vapor retarder, while open-cell is about R-3.5 to R-3.8 per inch and lets vapor through.
Question 6: What is the approximate latent heat of vaporization of water at 212°F and atmospheric pressure?
- 540 Btu/lb
- 144 Btu/lb
- 970 Btu/lb (Correct answer)
- 1,060 Btu/lb
Correct answer: 970 Btu/lb
At 212°F, water takes about 970 Btu/lb to boil, which is why steam carries so much heat.
Question 7: About how much energy does it take to melt one pound of ice at 32°F?
- 288 Btu
- 12,000 Btu
- 970 Btu
- 144 Btu (Correct answer)
Correct answer: 144 Btu
The latent heat of fusion of ice is about 144 Btu/lb, and 2,000 lb × 144 Btu/lb gives the 288,000 Btu/day definition of a ton of refrigeration.
ASHRAE Material Properties 3
What is the approximate specific heat of dry air at constant pressure in I-P units?