Section 1: Theory of Heat

Section 1: Theory of Heat Unit 2: Matter and Energy

Objectives • After studying this chapter, you should be able to: • Define matter • List the three states in which matter is commonly found • Define density • Discuss Boyle’s Law • State Charles’ Law

Objectives (cont’d.) • Discuss Dalton’s Law as it relates to the pressure of different gases • Define specific gravity and specific volume • State two forms of energy important to the air -conditioning (heating and cooling) and refrigeration industry

Objectives (cont’d.) • Describe work and state the formula used to determine the amount of work in a given task define horsepower • Convert horsepower to watts • Convert watts to British thermal units

Matter • Described as any substance that occupies space and has mass • Is made up of atoms • Exists as a solid, liquid or a gas • The state of matter is determined by heat content of the matter

Solids Figure 2.1 Solids exert all their pressure downward. The molecules of solid water have a great attraction for each other and hold together

Liquids Figure 2.2 The water in the container exerts pressure outward and downward. The outward pressure is what makes water seek its own level. The water molecules still have a small amount of adhesion to each other. The pressure is proportional to the depth

Gases Figure 2.3 Gas molecules travel at random. When a container with a small amount of gas pressure is opened, the gas molecules seem to repel each other and fly out

Important Definitions • Mass: property of matter that responds to gravitational attraction • Weight: force that matter (solid, liquid, or gas) applies to a supporting surface when it is at rest • Density: mass to volume relationship • Water has a density of 62.4 lb/ft3

Key Definitions (cont’d.) • Specific gravity: density of a substance divided by the density of water • Specific volume: volume of one pound of a gas (measured in ft3/lb)

Specific Gravity • Example: • Density of water = 62.4 lb/ft3 • Density of aluminum = 171 lb/ft3 • Specific gravity of aluminum: • Density of aluminum / Density of water = (171 lb/ft3)/(62.4 lb/ft3) = 2.74 • Specific gravity is unitless

Typical Densities and Specific Gravities Figure 2–4 Table of density and specific gravity.

Gas Laws • Always use absolute pressures and temperatures when working with gas laws • Boyle’s Law: relates pressure and volume • Charles’ Law: relates volume and temperature • General Law of Perfect Gases: relates pressure, volume and temperature • Dalton’s Law: relates pressures of gases in a mixture

Boyle’s Law

Boyle’s Law (cont’d.) Figure 2–5 Absolute pressure in a cylinder doubles when the volume is reduced by half.

Charles’ Law

Charles’ Law (cont’d.) • If V1 = 2000 ft3, T1 = 535R and T2 = 590R: • V2 = (V1 x T2)/ T1 = (2000 ft3 x 590 R)/ 535 R • V2 = 1,180,000 / 535 = 2205.6 ft3 Figure 2–6 Air expands when heated.

General Law of Perfect Gas • Combination of Boyle’s and Charles’ Laws

Dalton’s Law Figure 2–7 Dalton’s Law of Partial Pressures.The total pressure is the sum of the individual pressures of each gas.

Energy • Electrical energy drives motors and pumps in HVAC/R systems • Energy cannot be created or destroyed, but can be converted from one type to another • Heat energy provides comfort heating and flows from a warmer substance to a cooler substance • Electrical energy purchased by the kWh, fuel oil by the gallon, natural gas by the cubic foot

Energy (cont’d.) Figure 2–11 (A) If two substances of different temperatures are moved close to each other, heat from the substance with the higher temperature will flow to the one with the lower temperature. (B) Heat energy is still available at these low temperatures and will transfer from the warmer substance to the colder substance.

Work • Work = Force x Distance • Force is given in pounds, distance in feet • The units of work are foot-pounds, ft-lbs • Example: How much work is done to move a 150-pound object 100 feet? • Work = Force x Distance = 150 pounds x 100 feet • Work = 150 x 100 = 15,000 ft-lbs

Power • The rate at which work is done • Work per unit time, ft-lbs/min • Rated in horsepower • 1 hp = 33,000 ft-lbs/min • Electrical power measured in watts • 1 hp = 746 watts • 1 watt = 3.413 Btu • 1 kW = 3,413 Btu

Power (cont’d.) Figure 2–13 When a horse can lift 660 lb to a height of 50 ft in 1 min, it has done the equivalent of 33,000 ft-lb of work in 1 min, or 1 hp.

Summary • Matter takes up space, has mass, and can be in the form of a solid, a liquid, or a gas • Key terms • Weight • Mass • Density • Specific gravity • Specific volume

Summary (cont’d.) • Key laws • Boyle’s Law • Charles’ Law • General Law of Perfect Gases • Dalton’s Law

Summary (cont’d.) • Electrical energy and heat energy are two forms of energy • Key measurements • Measurements of fossil fuels, natural gas, coal, and electricity • Work = Force x Distance • Horsepower • Watts • Btu

Section 1: Theory of Heat