Air conditioning is a field of engineering that deals with the design, construction, and operation of equipment used to establish and maintain desirable indoor air conditions. It is used to maintain the environment of an enclosure at any required temperature, humidity, and purity. Simply stated, air conditioning involves the cooling, heating, dehumidifying, ventilating, and purifying of air.

One of the chief purposes of air conditioning aboard ship is to keep the crew comfortable, alert, and physically fit. None of us can long maintain a high level of efficiency under adverse environmental conditions. We have to maintain a variety of compartments at a prescribed temperature with proper circulation. These compartments must have the proper moisture content, the correct proportion of oxygen, and an acceptable level of air contamination (dust, airborne dirt, etc.). We also have to provide mechanical cooling or ventilation in ammunition spaces to prevent deterioration of ammunition components. We have to provide them in gas storage spaces to prevent excessive pressure buildup in containers and contamination in the space caused by gas leaks. Finally, we must provide cooling and ventilation in electrical/electronic equipment spaces. his is done to maintain the ambient temperature and humidity, as specified for the equipment.

To properly air-condition a space, the humidity, heat of the air, temperature, body heat balance, the effect of air motion, and the sensation of comfort is considered.

HEAT OF AIR

The heat of air is considered from three standpointssensible, latent, and total heat.

SENSIBLE HEAT is the amount of heat, which, when added to or removed from air, changes the temperature of the air. Sensible heat changes can be measured by the common (dry-bulb) thermometer,

Air always contains some water vapor. Any water vapor in the air contains the LATENT HEAT OF VAPORIZATION. (The amount of latent heat present has no effect on temperature and it cannot be measured with a dry-bulb thermometer.)

Any mixture of dry air and water vapor contains both sensible and latent heat. The sum of the sensible heat and the latent heat in any sample of air is called the TOTAL HEAT of the air.

TEMPERATURES

To test the effectiveness of air-conditioning equipment and to check the humidity of a space, you must consider two different temperatures-the dry-bulb and wet-bulb temperature.

Measurement of Temperatures

The DRY-BULB TEMPERATURE is the temperature of sensible heat of the air, as measured by an ordinary thermometer. In air conditioning, such a thermometer is known as a dry-bulb thermometer because its sensing bulb is dry.

The WET-BULB TEMPERATURE is best explained by a description of a wet-bulb thermometer. It is an ordinary thermometer with a loosely woven cloth sleeve or wick placed around its bulb and which is then wet with distilled water. The water in the sleeve or wick is evaporated by a current of air at high velocity (see next paragraph). This evaporation withdraws heat from the thermometer bulb, lowering the temperature by several degrees. The difference between the dry-bulb and the wet-bulb temperatures is called the wet-bulb depression. when the wet-bulb temperature is the same as the dry-bulb, the air is said to be saturated; that is, evaporation cannot take place. The condition of saturation is unusual, however, and a wet-bulb depression is normally expected.

The wet-bulb and dry-bulb thermometers are usually mounted side by side on a frame that has a handle and a short chain attached. This allows the thermometers to be whirled in the air, providing a high-velocity air current to promote evaporation. Such a device is known as a SLING PSYCHROMETER (fig 10-13) When using the sling psychrometer, whirl it rapidly-at least four times per second. Observe the wet-bulb temperature at intervals. The Point at which there is no further drop in temperature is the wet-bulb temperature for that space.

MOTORIZED PSYCHROMETERS (fig 10-14) are provided with a small motor-driven fan and dry-cell batteries. Motorized psychrometer are generally preferred and are gradually replacing sling psychrometer.

Relationships Between Temperatures

You should clearly understand the definite relationships of the three temperatures-dry-bulb, wet-bulb, and dew-point.

When air contains some moisture but is not saturated, the dewpoint temperature is lower than the dry-bulb temperature; the wet-bulb temperature lies between them. As the amount of moisture in the air increases, the difference between the dry-bulb temperature and the wet-bulb temperature becomes less.

Figure 10-13.-A standard sling Psychrometer.

Figure 10-14.-Exposed view of motorized psychrometer.

When the air is saturated, all three temperatures are the same.

By using both the wet-bulb and the dry-bulb temperature readings, you can find the relative humidity and the dew-point temperature on a psychometric chart (fig 10-15)

DEW-POINT TEMPERATURE.- The wet-bulb temperature lines are angled across the chart (see fig 10-15). The dew-point temperature lines are straight across the chart (indicated by the arrows for wet bulb and dew point). Find where the wet-bulb and dry-bulb lines cross, interpolate the relative humidity from the nearest humidity lines to the temperature-line crossing point. Then, to find the dew point, follow the straight dew-point line closest to the intersection across to the right of the chart and read the dew-point temperature. For example, find the wet-bulb temperature of 70F. Next, trace the line angling down to the right to the dry-bulb temperature of 95F. Finally, to find the dew-point temperature, follow the dew-point temperature lines nearest the intersection straight across to the right of the chart. The dew-point line falls about one-third of the way between the 55F mark and the 60 mark. You can see that the dew-point temperature is about 57F.

RELATIVE HUMIDITY.- To find the relative humidity (see fig 10-15), first find the dry-bulb temperature. Read across the bottom, find 95F and follow straight up to the intersection of the wet- and dry-bulb readings. The relative humidity arc nearest the intersection is 30 percent. However, the intersecting line is below 30 percent and higher than 20 percent. You can see that the relative humidity is about 28 percent.