In perfectly still air, the layer of air around a body absorbs the sensible heat given off' by the body and 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.