Pumps 75 Generators 75 Compressors 75 Impact Pile drivers 95 Jackhammers 75 Rock drills 80 Pneumatic tools 80 Other Saws 75 Vibrators 75 or three-shift construction work and delaying work that requires overtime. Also, some Federal agencies have promulgated regulations requiring noise readings on projects (Table 17.2). In accordance with such readings, local officials may place a construction project in one of the following categories: Unacceptable: Noise levels exceed 80 dB for 1 h or more per 24 h, or 75 dB for 8 h per 24 h. Normally unacceptable (discretionary): Noise exceeds 65 dB for 8 h per 24 h or loud repetitive noises on site. Normally acceptable (discretionary): Noise does not exceed 65 dB for more than 30 min per 24 h. Acceptable: Noise does not exceed 45 dB for more than 30 min per 24 h. Energy conservation in construction projects is part of the overall problem of the conservation of energy resources of the nation as a whole. Contractors should be alert to and aware of any ways to bring this about. They can help by using recycled building materials, recycling materials used during demolition, and demonstrating sensitivity to depletion of endangered natural resources. The term systems building is used to define a method of construction in which use is made of integrated structural, mechanical, electrical, and architectural systems. The ultimate goal is integration of planning, designing, programming, manufacturing, site operation, scheduling, financing, and management into a disciplined method of mechanized production of buildings. Application of these systems should be controlled by an engineer-construction management firm rather than by use of prevailing contract building-management procedures. Housing Production. The greatest concentration of effort has occurred in the realm of structural framing, leading to development of mass-production methods. These have, in general. been of the following types: Panel type, consisting of floors and walls that are precast on site or at a factory and stacked in a house-of-cards fashion to form a building. Volumetric type, consisting of boxes of precast concrete or preassembled steel, aluminum, plastic, or wood frames, or combinations of these, which are erected on the site after being produced in a factory. Component type, consisting of individual members of precast-concrete beams and columns or prefabricated floor elements, which are brought to the site in volume, or mass produced. These systems have displayed inherent disadvantages that came about through lack of opportunity to use the entire systems-building process, and because the systems often were not able to attain sufficient volume production to pay for the many fixed costs and start-up costs for the component factories that were built. Experience has shown, however, that housing can be built efficiently and economically from standardized, modular designs. Some examples are as follows: Mobile homes manufactured, complete with floor, roof, walls, electric wiring, plumbing, and cabinetry, by techniques similar to those used by automobile manufacturers. Built in factories, mobile homes are trucked to the sites and then installed and finished by on-site builders. Packaged homes, also known as prefabricated or panelized. They are factory subassembled and, with an assortment of other building components, are delivered to the site for final assembly. Modular homes, also known as sectional homes, manufactured in off-site factories, usually on an assembly line similar to that used by mobile-home manufacturers. Completely furnished, three-dimensional sections, including mechanical systems, of one or more rooms are factory assembled and then delivered to the sites by truck, railroad, or barge. (A. D. Bernhardt. Building Tomorrow: The Mobile/Manufactured Housing Industry, The MIT Press, Cambridge, Mass.) The lessons learned from past experiences offer the following guidelines for construction managers: Early commitment must be made to use of a specific system in the design stage. The systems builder must control the design. Shop drawings should be started early and refined during the design. Preparations of schedules, cost estimate, and bids must involve all the members of the team, that is, the designers, owner, and contractor, because there will be many last-minute proposed changes and details that will have to be challenged

FIGURE 13.34 Grilles placed back to back on an air-conditioning duct with no provision for adjustment of discharge. FIGURE 13.35 Vanes placed in a duct to adjust air flow to back-to-back grilles, useful when air throws are unequal. When low-cost steam is available, low-pressure, single-stage absorption systems may be more economical to operate than systems with compressors. In general, steam consumption is about 20 lb /hr per ton of refrigeration. Tower cooling water required is about 3.7 gal /min per ton with 85
F water entering the absorption unit and 102
F water leaving it. As an alternative, high-pressure steam, two-stage absorption units are available that operate much more efficiently than the low-pressure units. The high-pressure units use about 12 lb of steam per ton of refrigeration at full load. This is a 40% savings in energy. 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. FIGURE 13.36 Ductwork for cooling a store. 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 air1000 ft3 / min fresh air and 3000 ft3 /min return airshould 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 TABLE 13.17 Duct Calculation for 71/2-ton Packaged Unit for Store

Suction temperaturewhether at 35
F, 40
F, 45
F, etc. Head pressurewhether serviced by a water-cooled or air-cooled condenser. Manufacturers of compressor-motor units rate their equipment for Btu per hour output and kilowatt and amperage draw at various conditions of suction and discharge pressure. These data are available from manufacturers, and should be checked before a compressor-motor unit is ordered. Although capacity of a compressor decreases with the suction pressure (or back pressure), it is usual practice to rate compressors in tons or Btu per hour at various suction temperatures. For installations in which the latent load is high, such as restaurants, bars, and dance halls, where a large number of persons congregate, the coil temperature will have to be brought low enough to condense out large amounts of moisture. Thus, we find that a suction temperature of about 40
F will be required. In offices, homes, etc., where the latent load is low, 45
F suction will be satisfactory. Therefore, choose a compressor with capacity not less than the total cooling load and rated at 104
F head temperature, and a suction temperature between 40 and 45
F, depending on the nature of the load. Obtain the brake horsepower of the compressor at these conditions, and make sure that a motor is provided with horsepower not less than that required by the