THERMODYNAMICS PREPARED BY DW & JD

Transcription

1 THERMODYNAMICS PREPARED BY DW & JD 1

2 HEAT FLOWS QUESTION: heat flows in 3 ways what are they? 2

3 HEAT FLOWS ANSWER: 1. CONDUCTON 2. CONVECTION 3. RADIATION 3

4 HEAT FLOWS CONDUCTION: the transfer of heat from molecule to molecule through a substance by chain collision. Eg. Heating a piece of metal on one end and the heat moves down to the opposite end. 4

5 HEAT FLOWS CONVECTION: heat transfer by the movement of molecules from one place to another. Eg : the sun heating up a black parking lot and creating upwards air currents. 5

6 HEAT FLOWS RADIATION: transfer of heat by passing from a source to an absorbent surface without heating the space in between. Eg: when the sun shines down on a car in the winter-time the car can absorb heat and become warmer inside than the outside air.( no the car is not running) 6

7 HEAT TRANSFER Heat transfer: heat always travels from an area of high temperature to an area of lower temperature. 7

8 BRITISH THERMAL UNIT BTU: is the unit for measuring the quantity of heat. It is defined as the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit. 8

9 FORMS OF HEAT There are 2 form of heat: SENSIBLE HEAT LATENT HEAT 9

10 SENSIBLE HEAT Is the energy of molecular motion and can be measured with a thermometer. 10

11 SENSIBLE HEAT Liquid water at a temperature of 200 degrees F is 12 degrees below it s boiling temperature and is said to be 12 degrees subcooled 11

12 LATENT HEAT Is the energy of molecular separation and it can t be measured with a thermometer. This type of heat causes a change of states. 12

13 LATENT HEAT QUESTION: 1 lb of water is heated from 32 degrees F to 212 degrees F. How many BTU s would be required to accomplish this? 13

14 LATENT HEAT ANSWER: it takes 180 BTU s to increase the water s temperature 180 degrees F 14

15 LATENT HEAT QUESTION: would this be adding sensible or latent heat to the water? 15

16 LATENT HEAT ANSWER: this would be sensible heat because we can measure this heat with a thermometer. 16

17 LATENT HEAT QUESTION: once the one pound of water reaches 212 degrees F how much more heat is required to boil all of the water into steam? 17

18 LATENT HEAT ANSWER: it would take another 970 BTU s to boil off the one pound of water into steam. 18

19 LATENT HEAT During the time that the one pound of water was changing to steam, it never increased the water temperature above 212 degrees F.. To get the water to change to steam, we added latent heat energy. The water changed state from liquid to vapor but did not change temperature. 19

20 LATENT HEAT To get the water to change to steam, we added latent heat energy. The water changed state from liquid to vapor but did not change temperature. 20

21 LATENT HEAT Latent heat changes the state of a substance, not the temperature. 21

22 SUPERHEATED After all the water as changed into steam if we continue to add heat the vapor will become superheated. In this case we added sensible heat and could measure the temperature change with a thermometer. 22

23 SUPERHEATED In this case we added sensible heat and could measure the temperature change with a thermometer. 23

24 SUPERHEATED Water vapor or steam at a temperature of 220 degrees F is 8 degrees above it s boiling temperature and is said to be 8 degrees superheated. 24

25 EFFECTS OF PRESSURE ON BOILING POINT As the pressure on an enclosed system goes up so does its boiling point. 25

26 KEY POINT WHEN PRESSURE GOES UP BOILING POINT GOES UP TEMPERATURE GOES UP 26

27 QUESTION What is the boiling point of water in degrees Fahrenheit? 27

28 ANSWER 212 DEGREES FAHRENHEIT 28

29 FACT The boiling point of water will rise 3 degrees Fahrenheit for every pound of pressure applied to it.(at sea level) 29

30 QUESTION A vehicle using an 8 lb radiator cap has pure water in it s cooling system. If the cooling system is performing at its rated 8 lb capacity, then what is the boiling point of the water? 30

31 ANSWER An 8 lb radiator cap will increase the boiling point of the water 3x8=24degrees Boiling point of water = 212 Boiling point with an 8 lb radiator cap at system pressure is =236 degrees Fahrenheit 31

32 QUESTION WHAT IS AMBIENT TEMPERATURE? 32

33 ANSWER Ambient temperature: refers to the temperature of the surrounding air. That which is outside the controlled space. 33

34 Pressure Pressure is an external force acting on another body that is in direct contact with it. 34

35 QUESTION What is atmospheric pressure and what is it s pressure at sea level? 35

36 ANSWER Atmospheric pressure is the weight of the air in the atmosphere acting on the surface of the earth. Atmospheric pressure is 14.7 PSI at sea level 36

37 VACUUM Any pressure less than atmospheric is what is referred to as a vacuum or partial vacuum. To read pressure less than atmospheric a vacuum gauge is needed. Vacuum gauges are calibrated in inches of mercury (hg) 37

38 HUMIDITY Is simply the water vapor or moisture in the air. Eg On a hot humid day the humidity in the air will condense in small droplets on the outside of a nice cold beer bottle. 38

39 EFFECTS OF HUMIDITY ON AC Before a refrigeration system can be fully effective in transferring heat from the inside of the vehicle,moisture in the air must be reduced. It is for this reason the system must work much harder on a humid day. 39