Thermal Process Control Lap 4 Thermal Energy Notes: 1) Temperature Measurement a) Define temperature i) A measure of the amount of heat contained in a solid, liquid, or gas ii) Result of molecular motion in a substance (1) Molecular motion stops at absolute zero b) Describe two types of relative temperature measurement and give an application i) Fahrenheit scale (1) Water freezes at 32 F and boils at 212 F ii) Celsius scale (1) Water freezes at 0 C and boils at 100 C c) Describe how to convert between relative temperature scales i) TF = (9/5 x /TC) + 32 ii) TC = 5/9 x (TF 32) (1) Skill One Convert between Fahrenheit and Celsius temperature scales (a) Work sample problem(s) d) Describe three common methods of temperature measurement and give an application of each i) Measure a change in physical dimensions (1) Liquid in glass/glass bulb thermometer (a) Liquid in the thermometer expands due to an increase in temperature (i) Temperature is read from a scale on the thermometer (2) Bi-metallic thermometer
(a) Two different metals are joined together and wound into a helix (i) An indicating needle is attached to the helix (ii) Dissimilar metals expand at differing rates as the helix is exposed to a temperature change and causes the helix to rotate (iii) The needle deflects and the temperature is read from a scale ii) Measure a change in electrical properties (1) Thermoresistive devices (a) RTDs (i) Nickel, silver, and platinum are commonly used (ii) As temperature increases, the resistance also increases 1. Positive temperature coefficient (iii) Changes in electrical resistance are scaled to temperature units (iv) Typical ranges are -300 F to 600 F or -184 C to 316 C (2) Thermocouples (a) Based on the natural voltage difference existing between two dissimilar metals that are joined together (b) Provide for a broad range of temperature measurement (i) Typical ranges are -420 F to 4400 F or -251 C to 2427 C iii) Measure thermal radiation (1) Pyrometers detect thermal radiation
(a) Provide for non-contact temperature measurement (b) Useful for measuring very high furnace temperatures and other inaccessible objects (c) Typical ranges are 1400 F to 6300 F or 760 C to 3482 C e) Describe how to read a thermometer i) Use glass tube and bimetallic thermometers to measure temperature 2) Thermal Energy a) Define internal energy and give its units of measurement i) Heat energy contained in a substance that causes temperature ii) When heat is added to a substance, molecular motion increases resulting in an increase of temperature iii) The symbol used to represent this heat energy is U iv) The units of internal energy or heat energy (1) British Thermal Unit/BTU English system (a) 1 BTU = heat needed to raise the temperature of 1 lb of water 1 degree F (2) Joule SI system (a) 4.184 joules needed to raise the temperature of 1 gram of water 1 degree C (b) 4.184 joules = 1 thermochemical calorie (or small calorie) (c) 1000 small calories = 1 large calorie (dietary unit of measure b) Describe how to convert between BTU and joule energy units
i) Work sample problems c) Define specific heat capacity and give its units of measurement i) All substances have the ability to store internal energy (1) A pail of hot water will stay warm for some time (2) Some substances have higher heat capacity than others (a) Referenced to water, copper sheds heat about ten times faster and aluminum sheds heat about five times faster (i) Would these materials make good heat sinks? d) Describe how to use a table to find the specific heat capacity of a substance e) Describe how to calculate the amount of energy needed to raise the temperature of a substance f) Calculate the amount of energy needed to raise the temperature of a substance 3) Heat Transfer a) Define heat transfer and give its units of measurement i) Describes the movement of heat energy between two points ii) Always from hot to cold iii) Units of heat transfer (1) BTU/hr English system (2) Joules/s or Watts SI system iv) Thermal systems are designed to control or use heat transfer to accomplish a goal (1) Speedup or slowdown heat transfer b) Three forms of heat transfer
i) Conduction ii) Convection iii) Radiation c) Define conduction heat transfer and give an application i) Movement of heat through a solid material (1) Soldering d) Define two types of convection heat transfer and give an application i) Two typed of convection heat transfer (1) Free convection (a) When the moving fluid is a result of a natural unforced flow (i) Heating soup on a stove burner (2) Forced convection (a) When the moving fluid is caused by a pump or a fan (i) Heat exchangers (ii) Refrigeration systems (b) Activity 1 Demonstration of heat transfer e) Define radiation heat transfer and give an application i) Heat transmitted via electromagnetic waves (1) Solar (2) Infrared ii) Requires an emitter and an absorber (1) Darker objects absorb more heat 4) Thermal System Power a) Define thermal power and give its units of measurement i) Power is the amount of work performed in a given time (1) Units (a) English 1 hp = 550 ft/lb/sec
(i) In thermal systems usually used to describe the work done by the compressor or the electric motor powering the compressor (b) SI system Watt(1 joule/sec) or kw(1000 joules/sec) ii) Thermal power refers to the rate at which heat is transferred between materials (1) Units (a) English system Btu/hr (i) 1 Btu = 778.169 ft-lbs of mechanical work 1. The energy required to lift one pound a distance of 778.169 ft (b) SI system Watts or kw b) Describe how to convert between common power units i) Convert between common power units c) Describe how to calculate the heat transfer rate required to raise the temperature of a flowing fluid Calculate the heat transfer rate required to raise the temperature of a flowing fluid