HEAT  TRANSFER The transfer of heat is the next problem to consider after the heat has been produced. It must be moved to the space where it is to be used. Heat always flows from a warmer to a cooler substance; consequently, there must be   a   temperature   difference   before   heat   can   flow. Naturally,  the  greater  the  temperature  difference,  the faster  the  heat  flow.  Two  objects  that  have  different temperatures,  when  placed  together,  tend  to  equalize their temperature. Heat travels in heating systems from one  place  to  another  by  three  different  methods.  All three of these methods are used in most heating systems. They are discussed in the paragraphs that follow Conduction Conduction is the flow of heat from one part of a substance to another part of the same substance or from one   substance   to   another   when   they   are   in   direct contact. When one end of a stove poker is held in a flame, the  other  end  will  soon  be  too  hot  to  hold.  This indicates   that   the   heat   is   being   conducted,   or transferred, from one end of the poker to the other end. Such  a  transfer  of  heat  is  called  conduction. Conduction is used to transfer heat through the walls of a stove, furnace, or radiator so that the warmth can be used for heating. Some materials do not conduct heat as well as others. For example, if a piece of wood had been  used  instead  of  the  poker,  the  end  of  the  wood away from the fire would have remained cool. Those materials  that  offer  considerable  resistance  to  heat flow are referred to as insulators or poor conductors. Convection Convection  is  the  transfer  of  heat  by  means  of mediums,  such  as  water,  air,  and  steam.  When  air  is heated,  it  expands,  becomes  lighter  in  weight,  and rises. The cooler air, which is heavier, then flows in to replace the warm air. Thus a convection current is set up. Water, when heated, acts in the same way as air. The water next to the heating surface becomes warmer, lighter, and rises. This action allows the cooler water to flow in next to the heating surface and become heated. Convection  is  a  very  important  factor  in  a  heating system.  It  is  this  force,  developed  by  heating  the medium, which circulates that medium to the space to be heated. Radiation Radiation  is  the  transfer  of  heat  through  space. When a hand is held in front of a stove, it is quickly warmed  by  means  of  radiation.  In  this  same  manner, the earth receives its heat from the sun. Radiated heat is transferred by heat waves, similar to radio waves. Heat waves do not warm the air through which they pass, but they must be absorbed by some substance  to  produce  heat.  For  example,  when  you stand in the shade of a tree, you feel cool because the leaves and limbs are absorbing the heat waves before they reach you. When  heat  waves  strike  an  object,  some  are reflected,   some   may   pass   through,   and   the   object absorbs the rest. Polished metals are the best reflectors known; therefore, they are a poor absorber of heat. A poor  absorber  is  also  a  good  radiator.  Rough  metal absorbs heat more readily than a highly polished metal, and it also loses heat faster by radiation. The color of a substance also affects its absorbing power. A black surface absorbs heat faster than a white one.  That  is  why  light-colored  clothes  are  cooler  in summer than are dark-colored clothes. Q1.  Heat  can  be  produced  or  generated  by  what methods? Q2. What two types of measurements directly affect the safety, efficiency, and reliability of heat plant operations? Q3. Temperature is the measurement of what? Q4.   Convert   82   degrees   Fahrenheit   to   degrees Celsius. Q5.  Heat  travels  in  heating  systems  by  what  three methods? COMBUSTIBLE  FUELS Learning  Objective:  Understand  the  types  and characteristics of combustible gases and fuel oils used in  heating  systems. If  electricity  and  coal  are  disregarded,  the  fuels most   commonly   used   with   heating   equipment   are either gas or petroleum. Next, we will take a brief look at  the  types  and  characteristics  of  combustible  gases and fuel oils used for heating. TYPES OF GASES Gaseous  fuels  are  usually  classified  according  to their  source  that,  in  turn,  determines  their  chemical composition. The heat valve (Btu per cubic foot) varies with  the  types  of  gas  and  determines  the  quantity 4-3