Material Properties Flashcards

What is the approximate specific heat of dry air at constant pressure in I-P units?

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).

What density does ASHRAE use for standard air at sea level?

Answer: 0.075 lb/ft³

Standard air is taken as 0.075 lb/ft³, which is about 1.2 kg/m³.

A 2-inch layer of insulation has k = 0.25 Btu·in/h·ft²·°F. What is its R-value?

Answer: R-8

R = thickness / k = 2 / 0.25 = 8 h·ft²·°F/Btu.

Which piping material has the highest coefficient of linear thermal expansion?

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.

Compared with open-cell spray foam, closed-cell spray polyurethane foam has which characteristics?

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.

What is the approximate latent heat of vaporization of water at 212°F and atmospheric pressure?

Answer: 970 Btu/lb

At 212°F, water takes about 970 Btu/lb to boil, which is why steam carries so much heat.

About how much energy does it take to melt one pound of ice at 32°F?

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.