Section 1: Theory of Heat

Section 1: Theory of Heat. Unit 1: Heat, Temperature, and Pressure.

Heat, Temperature, and Pressure. Heat energy moves from one substance to another as well as between the molecules of a single substance When we need to be more specific than “hot” or “cold,” we refer to temperature “Heat” and “temperature” are not the same System pressures are obtained by using a refrigeration gauge manifold.

Temperature. The level of heat or heat intensity Measured with thermometers English system: Fahrenheit (F) Metric system: Celsius (C) Fahrenheit absolute scale: Rankine (R) Celsius absolute scale: Kelvin (K) Absolute zero: Temperature at which all molecular movement stops (−460°F).

Thermometers. An illustration of 2 thermometers. (A) shows Fahrenheit and Rankine thermometer and (B) shows a Celsius and Kelvin thermometer. The molecular motion stops at 460 degrees Fahrenheit, 0 degrees Rankine, negative 273 degrees Celsius, and 0 degrees Kelvin. Water freezes at 32 degrees Fahrenheit, 492 degrees Rankine, 0 degrees Celsius, and 273 degrees Kelvin. Water boils at 212 degrees Fahrenheit, 672 degrees Rankine, 100 degrees Celsius, and 373 degrees Kelvin. 0 degrees Fahrenheit is 460 degrees Rankine..

Converting Celsius to Fahrenheit. To convert a Celsius temperature to Fahrenheit °F = (1.8 ×°C) + 32° Example: Convert Celsius temperature of 20°C °F = (1.8 ×°C) + 32° °F = (1.8 × 20°C) + 32° °F = 36° + 32° °F = 68° So, 20°C = 68°F.

Converting Fahrenheit to Celsius. To convert a Fahrenheit temperature to a Celsius temperature °C = (°F − 32°) ÷ 1.8 Example, Convert Fahrenheit temperature of 50°F °C = (°F − 32°) ÷ 1.8 °C = (50°F − 32°) ÷ 1.8 °C = 18° ÷ 1.8 °C = 10° So, 50°F = 10°C.

Introduction to Heat. Heat cannot be created or destroyed Heat can be measured and accounted for Heat can be transferred from one substance to another Heat travels from a warmer substance to a cooler substance.

British Thermal Units. Figure 1.6 One British thermal unit (Btu) of heat energy is required to raise the temperature of 1 lb. (pound) of water from 68°F to 69°F..

Conduction. Heat energy travels from one molecule to molecule within a substance Heat energy travels from one substance to another Heat does not conduct at the same rate in all materials.

Example of Conduction. Heat will travel through a copper rod when placed near fire.

Convection. Heat transfers through a fluid from one substance to another Natural convection utilizes natural fluid flow, such as the rising of warm air and the falling of cooler air Forced convection uses fans or pumps to move fluids from one point to another.

Example of Convection. Baseboard heating. [image] An illustration shows how the heater functions The heater has a decorative metal protective cover and a hot water baseboard The heater takes the cool air from floor in a room and gives out the warm expanded air which rises because it is now lighter The rising effect draws cooler air into the bottom The warm air is naturally converted to cool air and it goes down to the bottom of the room and is taken by the heater The temperature outside the room is 10 degrees Fahrenheit.

Radiation. Radiant heat passes through air, heating the first solid object with which the heat comes in contact These heated objects, in turn, heat the surrounding area Radiant heat can travel through a vacuum.

Example of Radiation. Radiant heat can travel through space without heating it Example of radiation: An electric heater that glows red.

Sensible Heat. Heat transfer that results in a change in temperature of a substance Sensible heat transfers can be measured with a thermometer Example of a sensible heat transfer: Changing the temperature of a sample of water from 70°F to 71°F.

Latent Heat. Latent heat transfers result in a change of state of a substance with no change in temperature Also referred to as hidden heat Latent heat transfers cannot be measured with a thermometer Example of a latent heat transfer: Adding heat to boiling water makes it boil faster, but does not raise the water temperature.

Three Terms for Latent Heat. Three terms that are important to understand when referring to latent heat transfers: Latent heat of vaporization Latent heat of condensation Latent heat of fusion.

How Water Responds to Heat. [image] The horizontal axis is labeled heat content in British thermal unit and values ranges from 0 to 1400 in increments of 100 The vertical axis is labeled degrees and consists of both Celsius and Fahrenheit respectively and the values are negative 17 8 0 0 0 32 10 0 50 37 8 100 65 5 150 93 3 200 100 0 212 and 121 1 250 A circle labeled 2 is pointed towards the point 0 0 A circle labeled 1 is pointed towards the point 0 50 A circle labeled 3 is pointed towards the point 0 0 A circle labeled 1 is pointed towards the point 144 50 A circle labeled 4 is pointed towards the point 360 200 A circle labeled 5 is pointed towards the point 1320 200 The point 75 38 is labeled as 144 British thermal unit latent heat The point 21 26 is labeled as 16 British thermal unit sensible heat The point 263 128 is labeled as 180 British thermal unit sensible heat The point 963 216 is labeled as 970 British thermal unit latent heat The point 1333 260 is labeled as sensible heat.

Specific Heat. Defined as the number of Btus required to raise the temperature of 1 pound of a substance 1 degree Fahrenheit Specific heat of water is 1.00 Specific heat of ice is approximately 0.50 Specific heat of steam is approximately 0.50 Specific heat of air is approximately 0.24.

Sizing Heating Equipment: Formula. Q = W × Specific Heat × ΔT Where: Q = Quantity of heat needed W = Weight of the substance ΔT = Change in temperature.

Sizing Heating Equipment: Example. Example: 1000 pounds of steel must be heated from 0°F to 70°F How much heat is required to accomplish this? The specific heat of steel is 0.116 Btu/lb/°F Substituting in the above formula gives us Q = 1000 lb/h × 0.116 Btu/lb/°F × (70ºF − 0°F) Q = 1000 × 0.116 × 70 = 8120 Btu/h.

Pressure. Defined as the force per unit of area Often expressed in pounds per square inch (psi) Example: If a 100-pound weight rests on a surface of 1 square inch, the pressure is 100 psi Example: If a 100-pound weight rests on a surface of 100 square inches, the pressure is only 1 psi.

Illustration of Pressure. [image] 2 weights are shown The first weight is a cube resting on a 1 square inch surface area It exerts a pressure of 1 Pound The second weight is a combination of an inverted square frustum resting on a 1 square inch surface area and a cube is placed over the top side of the frustum It exerts a pressure of 100 Pounds.

Atmospheric Pressure. The atmosphere we live in has weight The atmosphere exerts a pressure of 14.696 psi at sea level (often rounded off to 15 psi) 14.696 psi at sea level is known as a standard condition Measured with a barometer.

A Mercury Barometer. [image] An image of Mercury Barometer is shown It shows a mercury puddle at the bottom in a container with an inverted glass tube placed over it The height of the glass tube is marked as 29 92 inches The top of the image reads Mercury drops causing a vacuum The center portion of the image reads Mercury atmosphere pushes down.

Pressure Gauges. Bourden tube: measures pressure in a closed system Compound gauge and high-pressure gauge used to take readings in the field or shop Gauges read 0 psi when opened to the atmosphere Gauge pressures are measured in pounds per square inch gauge, psig.

Bourdon Tube. It is made of a thin substance such as brass, with one end fastened to the pressure. The tube straightens when pressure increases. When attached to a needle linkage, pressure changes are indicated..

A Picture of a Bourdon Tube. [image] A photo of Bourdon Tube.