Material Properties Flashcards

How does adding propylene glycol to a hydronic water loop change the fluid's properties?

Answer: Specific heat goes down and viscosity goes up

Glycol mixtures carry less heat per pound and are thicker, so you need more flow and more pump head for the same capacity.

Low-e coatings on glazing mainly improve performance by changing which surface property?

Answer: They lower long-wave infrared emissivity

Low-e coatings cut radiant heat transfer across the glazing cavity, which lowers the U-factor.

Which material gives a building the most thermal mass per unit volume?

Answer: Normal-weight concrete

Normal-weight concrete is dense (~140-150 lb/ft³), so it stores far more heat per unit volume than wood or insulation.

Why is a dielectric union installed where copper pipe connects to steel pipe?

Answer: To prevent galvanic corrosion between dissimilar metals

Copper and steel are far apart on the galvanic series, so an electrical break is needed to keep the steel from corroding when the two are joined in water.

Fibrous and cellular insulations work mainly by taking advantage of which property?

Answer: The low thermal conductivity of trapped still air or gas

Still air conducts only about 0.025 W/m·K, and insulation keeps that air from moving, which stops convection.

What happens to the effective thermal conductivity of fiberglass insulation when it absorbs moisture?

Answer: It increases, which lowers the R-value

Water conducts heat about 20+ times better than air, so wet insulation loses a lot of its R-value.

Which combination of surface properties makes a 'cool roof'?

Answer: High solar reflectance and high thermal emittance

A cool roof reflects most sunlight and gives off the heat it does absorb, which keeps the roof surface cooler.