FIGURE 13.33 Multizone air-handling unit. where qs  internal sensible load T  temperature difference between room and air leaving coil (usually If a fresh-air duct is not installed, and the outside air is allowed to infiltrate into the premises, we use Eq. (13.30) with the sensible part of the load of the infiltration air added to qs , because the outside air infiltrating becomes part of the internal load. Once the amount of air to be handled is determined, choose a coil of face area such that the coil-face velocity V would be not much more than 500 ft /min. Some coil manufacturers recommend cooling-coil-face velocities as high as 700 ft /min. But there is danger of moisture from the cooling coil being carried along the air stream at such high velocities. Q Ac where Q  air flow, ft3 /min Ac  coil face area, ft2 V  air velocity, ft /min The number of rows of coils can be determined by getting the manufacturers capacity ratings of the coils for three, four, five, six, etc., rows deep and choosing a coil that can handle not less than the sensible and latent load at the working suction temperature. Multizone air-handling units (Fig. 13.33) are used to control the temperature of more than one space without use of a separate air-handling unit for each zone. When a zone thermostat calls for cooling, the damper motor for that zone opens the cold deck dampers and closes or throttles the warm deck dampers. Thus, the same unit can provide cooling for one zone while it can provide heating for another zone at the same time. This style of unit is not typically used in new installations, but may be encountered in existing facilities. Various controllers are available to raise these units closer to efficiencies expected in current designs. In large and complex buildings, there will be many spaces that have different inside design conditions. The reason for this is that it is very difficult, in many cases, to provide an air-handling unit that discharges supply air at a given set of conditions to satisfy the various spaces with different design conditions. Common practice is to combine areas with similar design requirements into a zone. Each zone is then served by a separate air-conditioning unit independently of the other zones. In some cases, a zone may be satisfied by use of reheat coils to satisfy the zone requirements. However, reheating the low-temperature supply air consumes energy. Hence, this practice is in disfavor and in many cases prohibited by building codes and governmental policies. Other formerly well-established systems, such as multizone and dual-duct systems, are also considered energy consumers and are also in disfavor. Exceptions that permit these systems by codes and statutes are for specific types of manufacturing or processing systems, or for areas where the cooled space must be maintained at very specific temperatures and humidities, such as computer rooms, libraries, operating rooms, paper and printing operations, etc. The most common system in use for zone control with central air-handling equipment is the variable-air-volume system. (See Art. 13.31.) (H. E. Bovay, Jr., Handbook of Mechanical and Electrical Systems for Buildings, McGraw-Hill Publishing Company, New York.) To meet the great demand for cheaper air-conditioning installations, manufacturers produce packaged, or preassembled, units. These vary from 4000 to 30,000 Btu/ hr for window units and 9000 Btu/hr and up for commercial units. Less field labor is required to install them than for custom-designed installations. Packaged window units operate on the complete cycle shown in Fig. 13.30, but the equipment is very compactly arranged. The condenser is air cooled and projects outside the window. The cooling coil extends inside. Both the cooling-coil fan and condenser fan usually are run by the same motor. These packaged units require no piping, just an electric receptacle of adequate capacity. Moisture that condenses on the cooling coil runs via gutters to a small sump near the condenser fan. Many manufacturers incorporate a disk slinger to spray this water on the hot condenser coil, which vaporizes it and exhausts it to the outside. This arrangement serves a double purpose: 1. Gets rid of humidity from the room without piping. 2. Helps keep the head pressure down with some evaporative cooling. Floor-type and ceiling-type packaged units also contain the full air-conditioning cycle (Fig. 13.30). The air-cooled packaged units are usually placed near a window to reduce the duct runs required for the air-cooled condenser. Roof-type packaged units are available for a variety of applications. These units contain a complete cooling cycle (usually with an air-cooled condenser) and a furnace. All controls are factory prewired, and the refrigeration cycle is completely installed at the factory. Necessary ductwork, wiring, and gas piping are supplied in

Systems design is the application of the scientific method to selection and assembly of components or subsystems to form the optimum system to attain specified goals and objectives while subject to given constraints and restrictions. The scientific method is defined in Art. 1.8. Goals, objectives, and constraints are discussed later. Systems design of buildings, in addition to correcting defects in traditional design, must provide answers to the following questions: 1. What does the client actually want the building to accomplish (goals, objectives, and associated criteria)? 2. What conditions exist, or will exist after construction, that are beyond the designers control? 3. What requirements for the building or conditions affecting system performance does design control (constraints and associated standards)? 4. What performance requirements and time and cost criteria can the client and designers use to appraise system performance? Collection of information necessary for design of the building starts at the inception of design and may continue through the contract documents phase. Data collection is an essential part of systems design but because it is continuous throughout design it is not listed as one of the basic steps. For illustrative purposes, the systems design procedure is shown resolved into nine basic steps in Fig. 1.10. Because value analysis is applied in step 5, steps 4 through 8 covering synthesis, analysis, and appraisal may be repeated several times. Each iteration should bring the design closer to the optimum. In preparation for step 1, the designers should secure a building program and information on existing conditions that will affect building design. In step 1, the designers use the available information to define goals to be met by the system. Goals. These state what the building is to accomplish, how it will affect the environment and other systems, and how other systems and the environment will affect the building. Goals should be generalized but brief statements, encompassing all the design objectives. They should be sufficiently specific, however, to guide generation of initial and alternative designs and control selection of the best alternative. A simple example of a goal is: Design a branch post-office building with 100 employees to be constructed on a site owned by the client. The building should harmonize with neighboring structures. Design must be completed within 90 days and construction within 1 year. Construction cost is not to exceed $500,000. When systems design is applied to a subsystem, goals serve the same purpose as for a system. They indicate the required function of the subsystem and how it affects and is affected by other systems. Objectives. With the goals known, the designers can advance to step 2 and define the system objectives. These are similar to goals but supply in detail the requirements that the system must satisfy to attain the goals. FIGURE 1.10 Basic steps in systems design in addition to collection of necessary information. In listing objectives, the designers may start with broad generalizations that they later develop at more detailed levels to guide design of the system. Some objectives, such as minimization of initial costs, life-cycle costs and construction time, should be listed. Other objectives that apply to the design of almost every building, such as the health, safety, and welfare objectives of the building, zoning, and Occupational Safety and Health Administration regulations, are too numerous to list and may be adopted by reference. Objectives should be sufficiently specific to guide the planning of building interior spaces and selection of specific characteristics for the building and its components: appearance, strength, durability, stiffness, operational efficiency, maintenance, and fire resistance. Also, objectives should specify the degree of control needed for operation of systems provided to meet the other

(percent passing) Atterberg limits Granual materials (35% or less passing No. 200 sieve) Group A-1 Group A-3 Group A-2 A-1-a A-1-b Stone or gravel fragments Gravel and sand mixtures Fine sand that is nonplastic Silty gravel and sand Silty gravel and sand Clayey gravel and sand Clayey gravel and sand Percent Passing: No. 10  50% No. 40  30% No. 200  15% No. 40  50% No. 200  25% No. 40  50% No. 200  10% Percent passing No. 200 sieve  35% Percent passing No. 200 sieve  35% Percent passing No. 200 sieve  35% Percent passing No. 200 sieve  35% PI  0 (nonplastic) Silt-clay materials (More than 35% passing No. 200 sieve) Group A-4 Group A-5 Group A-6 Group A-7 Silty soils Silty soils Clayey soils Clayey soils Clayey soils Percent passing No. 200 sieve  35% Percent passing No. 200 sieve  35% Percent passing No. 200 sieve  35% Percent passing No. 200 sieve  35% Percent passing No. 200 sieve  35% Highly organic Group A-8 A-8 Peat and other highly