Air Conditioning

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.).

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 standpoints-sensible, latent, and total heat.

SENSIBLE HEAT Heat which causes a change in temperature of a substance. 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 Latent (hidden) heat when added or removed changes the state of a substance with no change in temperature. Example, the heat added to water to boil it into a gas (steam) The temperature remains at 212°F throughout the process. 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. 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 (not shown). 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 (Shown Right) are provided with a small motor-driven fan and dry-cell batteries.

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. 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.

DEW-POINT TEMPERATURE.- The wet-bulb temperature lines are angled across the chart. 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 70°F. Next, trace the line angling down to the right to the dry-bulb temperature of 95°F. 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 55°F mark and the 60° mark. You can see that the dew-point temperature is about 57°F.

RELATIVE HUMIDITY.- To find the relative humidity, first find the dry-bulb temperature. Read across the bottom, find 95°F 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.

BODY HEAT BALANCE

Ordinarily, the body remains at a fairly constant temperature of 98.6°F. It is important to maintain this body temperature. Since there is a continuous heat gain from internal body processes, there must be a continuous loss to maintain body heat balance. Excess heat must be absorbed by the surrounding air or lost by radiation. As the temperature and humidity of the environment vary, the body automatically regulates the amount of heat that it gives off. However, the body's ability to adjust to varying environmental conditions is limited. Furthermore, although the body may adjust to a certain (limited) range of atmospheric conditions, it does so with a distinct feeling of discomfort. The discussion that follows will help you understand how atmospheric conditions affect the body's ability to maintain a heat balance.

Body Heat Gains

The body gains heat by radiation, by convection, by conduction, and as a by-product of physiological processes that take place within the body.

The heat gain by radiation comes from our surroundings. However, heat always travels from areas of higher temperature to areas of lower temperature. Therefore, the body receives heat from those surroundings that have a temperature higher than body surface temperature. The greatest source of heat radiation is the sun. Some sources of indoor heat radiation are heating devices, operating machinery, and hot steam piping.

The heat gain by convection comes only from currents of heated air. Such currents of air may come from a galley stove or an engine.

The heat gain by conduction comes from objects with which the body comes in contact. Most body heat comes from within the body itself. Heat is produced continuously inside the body by the oxidation The chemical reaction by which oxygen combines chemically with the elements of the burning substance. of foodstuffs and other chemical processes, friction and tension within the muscle tissues, and other causes.

Body Heat Losses

There are two types of body heat losses-loss of sensible heat and loss of latent heat. Sensible heat is given off by radiation, convection, and conduction. Latent heat is given off in the breath and by evaporation of perspiration.

EFFECT OF AIR MOTION

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.

Aboard ships, fans used with supply and exhaust systems are divided into two general classes-axial flow and centrifugal. Most fans in duct 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.

Vapor Compression Units

The vapor compression chilled water circulating system differs from a refrigerant circulating (direct expansion) air-conditioning system. In vapor compression chilled water circulating systems, the air is conditioned by using a secondary refrigerant (chilled water) that is circulated to the various cooling coils. Heat from the air-conditioned space is absorbed by the circulating chilled water. Heat is then removed from the water by the primary refrigerant system in the water chiller. In large ton vapor compression systems, the compressor is a centrifugal type that uses R-11 or R-114 as the primary refrigerant.

Low Pressure System Operation

The operating cycle of the centrifugal refrigeration plant is basically the same as other refrigeration plants except for the method of compression.

Water flowing through the evaporator tubes is warmer than the refrigerant surrounding them. Heat is transferred to the refrigerant and the water leaves the evaporator at a lower temperature. Liquid refrigerant in the lower part of evaporator shell is boiled off to a vapor in the process. The compressor draws refrigerant vapor from the top of the evaporator producing a very low pressure (vacuum) in the shell which is needed to obtain the maximum cooling effect of the refrigerant. A system using R-11 will have a operational vacuum of 16 to 18 inches of mercury in evaporator (cooler) shell. Since parts of the system are under vacuum, any leaks on the low pressure side will allow air to enter. This air collects in the upper part of the condenser. Since this air is non-condensable it reduces the area of the condenser, which increases the condenser pressure. Most low pressure refrigerant systems have a built in purge system to remove non-condensables and water vapor. A purge unit uses a cooling coil to separate the refrigerant and water from the non-condensable gases and purge the gases by using a vacuum pump.

Refrigerant vapor drawn from the evaporator enters the compressor through inlet vanes. The inlet vans consist of a number of wedge shape blades connected to mechanical linkage, and are usually controlled by a thermostat. This arrangement serves as a capacity control for the compressor. When a reduced load results from a fall in chill water temperature at the thermostat, the vanes tend to close reducing the flow of vapor to the compressor. When the compressor is cycled off the vanes close, allowing the compressor to start unloaded, reducing the power required to start the compressor.

The compressor discharges the refrigerant vapor, heated by compression to the condenser. In the condenser the hot refrigerant vapor is cooled by sea water or a closed cooling system (keel cooled). The heat from the vapor is transferred to the cooling water, causing the refrigerant vapor to condense to a liquid. The pressure in the condenser corresponds to the saturated refrigerant temperature normally 105°F, for R-11 this pressure corresponds to 11 Psig.

Liquid refrigerant drains from the condenser to a float chamber or flash cooler. In a flash cooler (shown) a portion of liquid refrigerant is allowed to flash to a vapor, returning to the compressor inlet. The latent heat of vaporization absorbed when the liquid refrigerant flashes to a vapor is drawn from the remaining liquid, which is cooled to the saturated temperature corresponding to the lower pressure. In a float chamber, a valve in the bottom of the chamber maintains a liquid level serving as a seal between the high pressure in the condenser and low pressure in the evaporator (chiller), as the level of refrigerant rises, the valve opens allowing refrigerant to enter the evaporator. The flow of refrigerant is restricted by a port opening in the float valve, which servers as an expansion device. The pressure drop through the port in the float valve causes a decrease in pressure, causing a portion of the refrigerant flashes to vapor. The latent heat of vaporization absorbed when the liquid refrigerant flashes to a vapor is drawn from the remaining liquid, cooling it to the saturated temperature proportional to the lower pressure. The refrigerant enters the evaporator (chiller) and the cycle repeats.

Centrifugal Compressors

Centrifugal compressors work the same way as the centrifugal pump. The centrifugal compressor consists of an impeller rotating within a casing. Suction vapor enters at the center of the impeller. The impeller blades spinning centrifugal force causes the vapor to forced outward where it leaves the impeller with velocity due to its motion. The refrigerant vapor then enters the spiral shaped casing (volute) where the reduction in velocity results in pressure. Thus the pressure at the compressor discharge is greater than the inlet. The compressor produces the pressure differential required within the system.

There are two types of centrifugal compressors, the opened type, and closed, or hermetic type. The opened centrifugal compressor is driven by an external power source, such as an electric motor, steam turbine, or internal combustion engine. The disadvantage of an open type compressor is a shaft seal is required. Due to the high speed at which these compressors run (3600 RPM or higher) a shaft seal is possible point of leakage.

In a closed , or hermitic compressor the motor and compressor are sealed in one casing and the need for a shaft seal is eliminated. A means of cooling the motor windings is provided by refrigerant from the cooler, or from the chill water system. This cooling allows for physically smaller motors of the same horsepower as the open type of compressors.

Compressors can be single stage or two stage. A two stage compressor has two impellers and volutes. Vapor from the first stage is piped to the suction of the second stage. A two stage compressor can develop sufficient refrigerant discharge pressure at 3600 RPM, allowing them to be mounted directly on the shaft of an electric motor.  Single stage compressors require gear drives to turn the impeller at a higher RPM to develop sufficient refrigerant discharge pressure.