required  for  a  specific  heating  requirement.  The types principally in use are natural gas, manufactured gas, and liquid petroleum gas (table G, appendix II). Natural Gas Natural gas is a mixture of combustible gases and usually  small  amounts  of  inert  gases  obtained  from geologic formations. While the composition of natural gas  varies  with  the  source,  methane  (CH₄)  is  always the major constituent. Most natural gases also contain some  ethane  (C₂H₆)  along  with  small  amounts  of nitrogen  and  carbon  dioxide  (CO₂).  Natural  gas  is colorless and odorless in its natural form; however; a distinctive odor is usually added as a safety factor for detecting  leaks.  Natural  gas  mixes  readily  and completely  with  combustion  air  and  thus  is substantially free from ash and practically smokeless. T h e s e    c h a r a c t e r i s t i c s    c o n t r i b u t e    t o    g o o d environmental pollution control. From a standpoint of trouble-free   performance,   ease   of   handling,   and control, natural gas offers many advantages that make it the most desirable of all heating fuels. Manufactured Gas The  common  manufactured  gases  are  carbureted water gas, oil gas, and producer gas. These gases are roughly   one-half   hydrogen   and   one-third   methane, plus  small  amounts  of  carbon  dioxide,  nitrogen,  and oxygen. They   are   made   by   converting   low-grade liquid or solid fuels to the gaseous form by destructive distillation   (cracking)   of   oil   or   coal,   by   the steam-carbon  reaction,  or  by  a  combination  of  both processes.  These  gases  are  ordinarily  used  at  or  near the  production  point  because  of  high  manufacturing costs rule out the added expense of distribution. Liquefied Petroleum Gas Liquefied  petroleum gases are hydrocarbon gases normally obtained as a by-product of oil refineries or by  stripping  natural  gas.  These  compounds  are normally   gaseous   under   atmospheric   conditions; however, they can be liquefied by moderate pressure at normal  temperatures. The  principal  LPG  products  are  propane  (C₃H₈) and  butane  (C₄H₁₀).  Propane,  the  most  common,  is available by the bottle or cylinder and in bulk form. Its boiling point is -44°F (note that this is very close to that of refrigerant R-22). Butane is generally available in bulk form. It boils or vaporizes at 32°F. In other words, if the temperature of butane is 32°F or lower, at atmospheric pressure, it remains a liquid, and heat must be applied to bring it to the gaseous state. Note in table G, appendix II, the high heating. values of propane and butane. Fuel Oils Fuel oils are derived from crude oil, which consists primarily  of  compounds  of  hydrogen  and  carbon (hydrocarbons),   and   smaller   amounts   of   oxygen, nitrogen,  and  depending  on  the  source,  sulfur. Practically  all  fuel  oil  is  either  a  product  or  a by-product   of   refining   crude   oil   by   the   fractional distillation process or by cracking. The   Bureau   of   Standards,   United   States Department of Commerce, standardizes commercially used fuel oils. The oils are numbered in grades 1, 2, 4, 5,  and  6  and  are  titled  commercial  standard  grades (CSG).  These  grades  are  identified  in  the  Navy  by military  specifications  and  are  intended  for  use  in oil-burning  equipment  for  the  generation  of  heat  in furnaces  for  heating  buildings,  for  the  generation  of steam,  and  for  other  purposes.  A  more  in-depth discussion  of  fuels  and  their  characteristics  is contained   in   Fundamentals   of   Petroleum, NAVEDTRA  10883.  A  comparison  of  fuel  oils  by grade is given in table H, appendix II. Q6. What  are  the  principal  types  of  gases  used  in heating? Q7. Fuel oils consist primarily of what compounds? WARM-AIR  HEATING  EQUIPMENT Learning  Objective:  Identify  the  different  types  of warm-air  heating  units  and  equipment  and  basic installation and maintenance guidelines. Advances  in  the  field  of  warm-air  heating  have made it one of the most popular and widespread forms of  heating  in  use  today.  It  has  the  advantage  of adaptability  with  various  fuels  and  can  be  used  in  a variety  of  buildings,  including  barracks,  hangars, personnel  housing,  schools,  and  theaters.  It  is  likely, therefore,  that  at  one  time  or  another  you  will  be responsible for performing technical maintenance and repair and installation of warm-air heating equipment and systems. The different types of heating equipment that will be discussed include unit heaters, electric and gas- and 4-4