Ducts for Air Conditioning

In designing a duct system for air conditioning, we must first determine air-outlet locations. If wall grilles are used, they should be spaced about 10 ft apart to avoid dead spots. Round ceiling outlets should be placed in the center of a zone. Rectangular ceiling outlets are available that blow in either one, two, three, or four directions.
Manufacturers catalog ratings should be checked for sizing grilles and outlets.
These catalogs give the recommended maximum amount of air to be handled by an outlet for the various ceiling heights. They also give grille sizes for various lengths of blows. It is obvious that the farther the blow, the higher must be the velocity of the air leaving the grille. Also the higher the velocity, the higher must be the pressure behind the grille.
When grilles are placed back to back in a duct as in Fig. 13.34, be sure that grille A and grille B have the same throw; for if the pressure in the duct is large enough for the longer blow, the short-blow grille will bounce the air off the opposite wall, causing serious drafts. But if the pressure in the duct is just enough for the short blow, the long-blow grille will never reach the opposite wall. Figure 13.35 is recommended for unequal blows because it allows adjustment of air and buildup of a higher static pressure for the longer blow.
In some modern buildings perforated ceiling panels are used to supply conditioned air to the premises. Supply ductwork is provided in the plenum above the suspended ceiling as with standard ceiling outlets. However, with perforated panels, less acoustical fill is used to match the remainder of the hung ceiling.

After all discharge grilles and the air-handling unit are located, it is advisable to make a single-line drawing of the duct run. The air quantities each line and branch must carry should be noted. Of the few methods of duct-system design in use, the equal-friction method is most practical. For most comfort cooling work, it is considered good practice not to exceed 0.15-in friction per 100 ft of ductwork.
It is also well to keep the air below 1500 ft /min starting velocity.
If a fresh-air duct is installed the return-air duct should be sized for a quantity of air equal to the supply air minus the fresh air.

It is advisable, where physically possible, to size the fresh-air duct for the full capacity of the air-handling unit.
For example, a 10-ton system handling 4000 ft3 /min of supply air 1000 ft3 / min fresh air and 3000 ft3 /min return air should have the fresh-air duct sized
for 4000 ft3 /min of air. A damper in the fresh-air duct will throttle the air to 1000 ft3 /min during the cooling season;
however, during an intermediate season, when the outside air is mild enough, cooling may be obtained by operating only the supply-air fan and opening the damper, thus saving the operation of the 10-ton compressor-motor unit.
As an example of the method for sizing an air-conditioning duct system, let us determine the ductwork for the first floor of the building in Fig. 13.36. Although a load analysis shows that the air requirement is 2979 ft3 /min, we must design the ducts to handle the full capacity of air of the packaged unit we supply. Handling less air will unbalance the unit, causing a drop in suction temperature, and may cause freezing up of the coil. If a 71⁄2-ton packaged unit is used, for example, the ducts should have a capacity at least equal to the 3000 ft3 / min at which this unit is rated.

Table 13.17 shows the steps in sizing the ducts. The 3000 ft3 /min is apportioned to the various zones in the store in proportion to the load from each, and the flow for each segment of duct is indicated in the second column of the table. Next, the size of an equivalent round duct to handle each airflow is determined from Table 13.8 with friction equal to 0.15 in per 100 ft. The size of rectangular duct to be used is obtained from Table 13.6.
The preceding example of ductwork design falls into the category of lowpressure duct systems. This type of design is used for most air-distribution systems that are not too extensive, such as one- or two-floor systems, offices, and residences.
In general, the starting air velocity is below 2000 ft /min, and the fan static pressure is below 3 in of water.
For large multistory buildings, high-velocity air-distribution duct systems often are used. These systems operate at duct velocities well above 3000 ft /min and above 3-in static pressure. Obvious advantages include smaller ducts and lower building cost, since smaller plenums are needed above hung ceilings. Disadvantages are high power consumption for fans and need for an air-pressure-reducing valve and sound attenuation box for each air outlet, resulting in higher power consumption for operation of the system.

Some of the more elaborate heating and air-conditioning installations consist of a high-pressure warm-air duct system and a high-pressure cold-air duct system.
Each air outlet is mounted in a sound attenuation box with pressure-reducing valves and branches from the warm- and cold-air systems (Fig. 13.37). Room temperature is controlled by a thermostat actuating two motorized volume dampers. When cooling is required, the thermostat activates the motor to shift the warm-air damper to the closed or throttled position and the cold-air damper to the open position.

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