In any air-conditioning installation involving a duct system, invariably there is an accession of heat by the moving air in the ducts between the coils and supply grilles when air is supplied below room temperature. If the ducts are located through much of their length in the conditioned space, then, of course, this heat absorption has no effect on the total load and frequently may be disregarded.
More frequently, however, the supply ducts must pass through spaces that are not air- conditioned. Under these conditions, the heat absorbed by the air in the ducts can be regarded as an additional load on the cooling equipment. The temperature rise in a duct system of a cooling installation depends on the following factors:
? Temperature of the space through which the duct passes
? Air velocity through the duct
? Type and thickness of insulation, if any
The first factor establishes the temperature differential between the air on either side of the duct walls. The dew point of the air surrounding the duct may also have some effect on the heat pickup, as condensation on the duct surface gives up the heat of vaporization to the air passing through the system. The highest air velocities consistent with the acoustic requirements of the installation should be used, not only for economy in the sheet metal material used, but also to reduce the heat pickup in the ducts.
The amount of heat absorbed by a unit area of sheet-metal duct conveying chilled air is almost directly proportional to the temperature difference between the atmosphere surrounding the duct and the chilled air, irrespective of the velocity of the latter. The heat pickup rate will be influenced somewhat by the outside finish of the duct and by the air motion, if any, in the space through which the duct passes.
Heat leakage in Btu per hour per square foot per degree difference in temperature for uncovered galvanized iron ductwork will be between 0.5 and 1.0, with an average value of 0.73. The rate of leakage, of course, will be greatest at the start of the duct run and will gradually diminish as the air temperature rises. Covering the duct with the equivalent of 0.5-in. rigid insulation board and sealing cracks with tape will reduce the average rate of heat pickup per square foot of surface per hour to 0.23 Btu per degree difference.
To summarize, the designer of a central unit system should observe the following:
? Locate the equipment as close to the conditioned space as possible.
? Use duct velocities as high as practical, considering the acoustic level of the space and operating characteristics of the fans.
? Insulate all supply ducts with covering equivalent to at least 0.5 in. of rigid insulation, and seal cracks with tape.
Air ducts for transmission of air in a forced-air heating, ventilation, or air-conditioner system must be carefully designed from the standpoint of economy, as well as for proper functioning. When designing air ducts, the following methods may be used:
? Compute the total amount of air to be handled per minute by the fan, as well as the fractional volumes composing the total, which are to be supplied to or withdrawn from different parts of the building.
? Locate the supply unit in the most convenient place and as close as possible to the center of distribution.
? Divide the building into zones, and proportion the air volumes per minute in accordance with the requirements of the different zones.
? Locate the air inlets or outlets for supply and recirculation, respectively. At the positions so located on the building plans, indicate the air volumes to be dealt with. The position of the outlets and inlets should be such as to produce a thorough diffusion of the conditioned air throughout the space supplied.
? Determine the size of each outlet or inlet based on passing the required amount of air per minute at a suitable velocity.
? Calculate the areas, and select suitable dimensions for all branch and main ducts. Do this based on creating equal frictional losses per foot of length. This involves reducing the velocities in smaller ducts.
? Ascertain the resistance of the ducts that sets up the greatest friction. In most cases, this will be the longest run, although not so invariably. This will be the resistance offered by the duct system as a whole to the flow of the required amount of air.
? Revise the dimensions and areas of the shorter runs so that the ducts themselves will create resistances equal to the longest run. This will cut down the cost of the sheet metal, and the result will be just the same as if dampers were used. Too high a velocity, however, must be avoided.
? To compensate for unforeseen contingencies, volume dampers should be provided for each branch.
Adsorption-type dehumidifiers operate on the use of sorbent materials for adsorption of moisture from the air. Sorbents are substances that contain a vast amount of microscopic pores. These pores afford a great internal surface to which water adheres or is adsorbed. A typical dehumidifier based on the honeycomb desiccant wheel principle is shown schematically in Fig. 3-21. The wheel is formed from thin corrugated and laminated asbestos sheets rolled to form wheels of various desired diameters and thickness. The wheels are impregnated with a desiccant cured and reinforced with a heat-resistant binder. The corrugations in the honeycomb wheel form narrow flutes perpendicular to the wheel diameter. Approximately 75 percent of the wheel face area is available for the adsorption or dehumidifying flow circuit, and 25 percent is available for the reactivation circuit. In the smaller units, the reactivated air is heated electrically; in the larger units, it is heated by electric, steam, or gas heaters.
Figure 3-22 shows another industrial adsorbent dehumidifier of the stationary bed type. It has two sets of stationary adsorbing beds arranged so that one set is dehumidifying the air while the other set is drying. With the dampers in the position shown, air to be dried flows through one set of beds and is dehumidified while the drying air is heated and circulated through the other set. After completion of drying, the beds are cooled by shutting off the drying air heaters and allowing unheated air to circulate through them. An automatic timer controller is provided to allow the dampers to rotate to the opposite side when the beds have adsorbed moisture to a degree that begins to impair performance.
As mentioned previously, the dehumidifier (see Fig. 3-19) operates on the principles of the conventional household refrigerator. It contains a motor-operated compressor, a condenser, and a receiver. In a dehumidifier, the cooling coil takes the place of the evaporator, or chilling unit in a refrigerator. The refrigerant is circulated through the dehumidifier in the same manner as in a refrigerator. The refrigerant flow is controlled by a capillary tube. The moisture-laden air is drawn over the refrigerated coil by means of a motor-operated fan or blower.
The dehumidifier operates by means of a humidistat (see Fig. 3-20), which starts and stops the unit to maintain a selected humidity level. In a typical dehumidifier, the control settings range from dry to extra dry to continuous to off. For best operation, the humidistat control knob is normally set at extra dry for initial operation over a period of 3 to 4 weeks. After this period, careful consideration should be given to the dampness in the area being dried. If sweating on cold surfaces has discontinued and the damp odors are gone, the humidistat control should be reset to dry. At this setting, more economical operation is obtained, but the relative humidity probably will be higher than at the extra dry setting.
After 3 or 4 weeks of operation at the dry setting, if the moisture condition in the area being dried is still satisfactory, the operation of the dehumidifier should be continued with the control set at this position. However, if at the setting the dampness condition is not completely corrected, the control should be returned to the extra dry setting. Minor adjustments will usually be required from time to time. Remember that the control must be set near extra dry to correct the dampness conditions but as close to dry as possible to obtain the most economical operation.
An electric dehumidifier operates on the refrigeration principle. It removes moisture from the air by passing the air over a cooling coil. The moisture in the air condenses to form water, which then runs off the coil into a collecting tray or bucket. The amount of water removed from the air varies, depending on the relative humidity and volume of the area to be dehumidified. In locations with high temperature and humidity conditions, 3 to 4 gal of water per day can usually be extracted from the air in an average-size home.
When the dehumidifier is first put into operation, it will remove relatively large amounts of moisture until the relative humidity in the area to be dried is reduced to the value where moisture damage will not occur. After this point has been reached, the amount of moisture removed from the air will be considerably less. This reduction in moisture removal indicates that the dehumidifier is operating normally and that it has reduced the relative humidity in the room or area to a safe value.
The performance of the dehumidifier should be judged by the elimination of dampness and accompanying odors rather than by the amount of moisture that is removed and deposited in the bucket. A dehumidifier cannot act as an air conditioner to cool the room or area to be dehumidified. In operation, the air that is dried when passed over the coil is slightly compressed, raising the temperature of the surrounding air, which further reduces the relative humidity of the air.
Air-operated humidifying units operate in the same manner as electrical units, except that they utilize a pneumatic hygrostat as a humidity controller and an air operator to open or close the steam valve (see Fig. 3-17). Adecrease in relative humidity increases the air pressure under a springloaded diaphragm to open the steam valve wider. An increase in relative humidity reduces the pressure under the diaphragm and allows the valve to restrict the steam flow. In a humidifier operation of this type, the steam supply is taken off the top of the header (see Fig. 3-18). Any condensate formed in the supply line is knocked down to the humidifier drain by a baffle inside the inlet of the humidifier-separating chamber.
Any droplets of condensation picked up by the stream as it flows through the humidifier cap when the steam valve opens will be thrown to the bottom of the reevaporating chamber. Pressure in this chamber is essentially atmospheric. Since it is surrounded by steam at supply pressure and temperature, any water is reevaporated to provide dry steam at the outlet. The humidifier outlet is also surrounded by steam at supply pressure to ensure that there will be no condensation or drip at this point. Aclamp-on temperature switch is attached to the condensate drain line to prevent the electric or pneumatic operator from opening the steam valve until the humidifier is up to steam temperature.
Dry-steam electrically operated humidifiers operate by means of a solenoid valve, which is energized by a humidistat. When the relative humidity drops slightly below the desired level set by the humidistat (see Fig. 3-16), a solenoid valve actuated by the humidistat admits steam from the separating chamber to the reevaporating chamber. Steam passes from this chamber through the muffler directly to the atmosphere. The fan (which is energized when the solenoid valve opens) assists in dispersing the steam into the area to be humidified. When the relative humidity reaches the desired level, the humidistat closes the solenoid valve and stops the fan.
Figure 3-15 shows the essential parts of the pan-type humidifier. The main part is a tank of water heated by low-pressure steam or forced hot water where a water temperature of 200°F (93°C) or higher is maintained. The evaporative-type humidifier is fully automatic, the water level being controlled by means of a float control. In operation, when the relative humidity drops below the humidity-control setting, the humidifier fan blows air over the surface of the heated water in the tank. The air picks up moisture. The air is blown to the space to be humidified. When the humidity control is satisfied, the humidifier fan stops.
An air washer essentially consists of a row of spray nozzles inside a chamber or casing. Atank at the bottom of the chamber provides for collection of water as it falls through the air and comes into intimate contact with the wet surface of the chamber baffles. The water is generally circulated by means of a pump, the warm water being passed over refrigerating coils or blocks of ice to cool it before being passed to the spray chamber. The water lost in evaporation is usually replaced automatically by the use of a float arrangement, which admits water from the main tank as required. In many locations, the water is sufficiently cool to use as it is drawn from the source. In other places, the water is not cool enough and must be cooled by means of ice or with a refrigerating machine.
The principal functions of the air washer are to cool the air passed through the spray chamber and to control humidity. In many cases, the cooling coils are located in the bottom of the spray chamber so that as the warm spray descends, it is cooled and ready to be again sprayed by the pump. In some cases, the water is passed through a double-pipe arrangement and is cooled on the counter-current principle.
Figure 3-14 shows a sketch of an air washer. In this case, the spray pipes are mounted vertically. In some instances, the spray pipes are horizontal so that the sprays are directed downward. As some of the finer water particles tend to be carried along with the air current, a series of curved plates or baffles is generally used, which forces the cooled and humidified air to change the direction of flow, throwing out or eliminating the water particles in the process.
Humidifiers, by definition, are devices for adding moisture to the air. Thus, to humidify is to increase the density of water vapor within a given space or room. Air humidification is affected by vaporization of water and always requires heat for its proper functioning. Thus, devices that function to add moisture to the air are termed humidifiers, whereas devices that function to remove moisture from the air are termed dehumidifiers.
As previously noted, air humidification consists of adding moisture. Following are the types of humidifiers used in air-conditioning systems:
? Spray-type air washers
? Pan evaporative humidifiers
? Electrically operated humidifiers
? Air-operated humidifiers