increases in temperature. The layer of air also absorbs some of the water vapor given off by the body, thus increasing its relative humidity. This means the body is surrounded by an envelope of moist air that is at a higher temperature and relative humidity than the ambient air. Therefore, the amount of heat that the body can lose to this envelope is less than the amount it can lose to the ambient air. When the air is set in motion past the body, the  envelope  is  continuously  being  removed  and replaced by the ambient air. This movement increases the rate of heat loss from the body. When the increased heat loss improves the heat balance, the sensation of a breeze is felt; when the increase is excessive, the rate of heat loss makes the body feel cool and the sensation of a draft is felt. SENSATION OF COMFORT From what you have just learned, you know that three factors are closely interrelated in their effects upon the comfort and health of personnel aboard ship. These factors  are  temperature,  humidity,  and  air  motion.  In fact,  a  given  combination  of  temperature,  humidity,  and air motion produces the same feeling of warmth or coolness as a higher or lower temperature along with a compensating humidity and air motion. The term given to the net effect of these three factors is known as the EFFECTIVE  TEMPERATURE.  Effective  temperature cannot be measured by an instrument, but can be found on  a  special  psychometric  chart  when  the  dry-bulb temperatures  and  air  velocity  are  known. The  combinations  of  temperature,  relative humidity, and air motion of a particularly effective temperature may produce the same feeling of warmth or coolness.  However,  they  are  NOT  all  equally comfortable.   Relative   humidity   below   15   percent produces  a  parched  condition  of  the  mucous  membranes of   the   mouth,   nose,   and   lungs,   and   increases susceptibility  to  disease  germs.  Relative  humidity  above 70  percent  causes  an  accumulation  of  moisture  in clothing.  For  best  health  conditions,  you  need  a  relative humidity ranging from 40 percent to 50 percent for cold weather and from 50 percent to 60 percent for warm weather. An overall range from 30 percent to 70 percent is acceptable. VENTILATION EQUIPMENT Proper circulation is the basis for all ventilating and air-conditioning  systems  and  related  processes. Therefore, we must first consider methods used aboard ship to circulate air. In the following sections, you will find  information  on  shipboard  equipment  used  to supply, circulate, and distribute fresh air and to remove used,  polluted,  and  overheated  air. In  Navy  ships,  the  fans  used  with  supply  and exhaust   systems   are   divided   into   two   general classes-axial  flow  and  centrifugal. Most fans induct systems are of the axial-flow type because  they  generally  require  less  space  for installation. Centrifugal fans are generally preferred for exhaust systems that handle explosive or hot gases. Because the motors of these fans are outside the air stream, they cannot ignite the explosive gases. The drive motors for centrifugal fans are less subject to overheating to a lesser degree than are motors of vane-axial fans. VANE-AXIAL FANS Vane-axial fans (fig. 10-16) are high-pressure fans, generally installed in duct systems. They have vanes at the discharge end to straighten out rotational air motion caused  by  the  impeller.  The  motors  for  these  fans  are cooled by the flow of air in the duct from the fan blades across the motor. The motor will overheat if it is allowed to operate while the supply air to the fan is shut off. TUBE-AXIAL FANS Tube-axial  fans  are  low-pressure  fans,  usually installed without duct work. However, they do have sufficient pressure for a short length of duct. CENTRIFUGAL FANS Centrifugal  fans  (fig.  10-17,  view  A)  are  used primarily  to  exhaust  explosive  or  hot  gases.  However, they may be used in lieu of axial-flow fans if they work better with the arrangement or if their pressure-volume characteristics  suit  the  installation  better  than  an axial-flow  fan.  Centrifugal  fans  are  also  used  in  some fan-coil assemblies, which are discussed later in this chapter. PORTABLE  FANS Portable  axial  fans  (fig.  10-17,  view  B)  with  flexible air hoses are used aboard ship for ventilating holds and cofferdams.  They  are  also  used  in  unventilated  spaces to clear out stale air or gases before personnel enter and for  emergency  cooling  of  machinery. 10-17