Specific Heat

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characteristic is used in refrigeration. The heat of the air, of the lining of the refrigerator, and of the food to be  preserved  is  transferred  to  a  colder  substance,  called the  refrigerant. Three methods by which heat may be transferred from  a  warmer  substance  to  a  colder  substance  are conduction,   convection,   and   radiation.   These principles are explained in chapter 4 of this TRAMAN. SPECIFIC  HEAT SPECIFIC HEAT is the ratio between the quantity of heat required to change the temperature of 1 pound of any substance 1°F, as compared to the quantity of heat required to change 1 pound of water 1°F. Specific heat is equal to the number of Btu required to raise the temperature  of  1  pound  of  a  substance  1oF.  For example, the specific heat of milk is .92, which means that 92 Btu will be needed to raise 100 pounds of milk 1oF. The specific heat of water is 1, by adoption as a standard, and specific heat of another substance (solid, liquid,   or   gas)   is   determined   experimentally   by comparing it to water. Specific heat also expresses the heat-holding capacity of a substance compared to that of water. A key RULE to remember is that .5 Btu of heat is required  to  raise  1  pound  of  ice  1oF  when  the temperature  is  below  32°F;  and  .5  Btu  of  heat  is required  to  raise  1  pound  of  steam  1°F  above  the temperature of 212°F. SENSIBLE  HEAT Heat  that  is  added  to,  or  subtracted  from,  a substance   that   changes   its   temperature   but   not   its physical  state  is  called  SENSIBLE  HEAT.  It  is  the heat that can be indicated on a thermometer. This is the heat  human  senses  also  can  react  to,  at  least  within certain ranges. For example, if a person put their finger into a cup of water, the senses readily tell that person whether it is cold, cool, tepid, hot, or very hot. Sensible heat  is  applied  to  a  solid,  a  liquid,  or  a  gas/vapor  as indicated  on  a  thermometer.  The  term  sensible  heat does not apply to the process of conversion from one physical state to another. LATENT  HEAT LATENT HEAT, or hidden heat, is the term used for the heat absorbed or given off by a substance while it is changing its physical state. When this occurs, the heat  given  off  or  absorbed  does  NOT  cause  a temperature  change  in  the  substance.  In  other  words, sensible  heat  is  the  term  for  heat  that  affects  the temperature of things; latent heat is the term for heat that affects the physical state of things. To understand the concept of latent heat, you must realize  that  many  substances  may  exist  as  solids,  as liquids,  or  as  gases,  depending  primarily  upon  the temperatures and pressure to which they are subjected. To change a solid to a liquid or a liquid to a gas, ADD HEAT; to change a gas to a liquid or a liquid to a solid, REMOVE HEAT. Suppose you take an uncovered pan of cold water and put it over a burner. The sensible heat of the water increases and so does the temperature. As you continue adding heat to the water in the pan, the temperature  of  the  water  continues  to  rise  until  it reaches 212°F. What is happening? The water is now absorbing its latent heat and is changing from a liquid to a vapor. The heat required to change a liquid to a gas (or,  the  heat  that  must  be  removed  from  a  gas  to condense  it  to  a  liquid)  without  any  change  in temperature   is   known   as   the   LATENT   HEAT   OF VAPORIZATION. Now suppose you take another pan of cold water and  put  it  in  a  place  where  the  temperature  is  below 32°F.  The  water  gradually  loses  heat  to  its surroundings, and the temperature of the water drops to 32°F until all the water has changed to ice. While the water is changing to ice, however, it is still losing heat to  its  surroundings.    The  heat  that  must  be  removed from a substance to change it from a liquid to a solid (or, the heat which must be added to a solid to change it to a liquid) without change in temperature is called the LATENT  HEAT  OF  FUSION.  Note  the  amount  of heat required to cause a change of state (or the amount of heat given off when a substance changes its state) varies   according   to   the   pressure   under   which   the process takes place. Figure 6-2 shows the relationship between sensible heat and latent heat for one substance –  water  at  atmospheric  pressure. To  raise  the temperature of 1 pound of ice from 0°F to 32°F, you must add 16 Btu. To change the pound of ice at 32°F to a pound of water at 32°F, you add 144 Btu (latent heat of fusion). There is no change in temperature while the ice is melting.   After  the  ice  is  melted,  however,  the temperature  of  the  water  is  raised  when  more  heat  is applied. When 180 Btu are added, the water boils. To change a pound of water at 212°F to a pound of steam at 212°F,  you  must  add  970  Btu  (latent  heat  of vaporization). After the water is converted to steam at 212°F, the application of additional heat causes a rise in the temperature of the steam. When you add 44 Btu 6-3

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