The basic element in a roof is a thin, waterproof covering, called roofing (Sec. 12). Because it is thin, it is usually supported on sheathing, a thin layer, or roof deck, a thick layer, which in turn, is carried on structural members, such as beams or trusses. The roof or space below should contain thermal insulation (Fig. 1.6c and d). Exterior walls enclose a building below the roof. The basis element in the walls is a strong, durable, water-resistant facing. For added strength or lateral stability, this facing may be supplemented on the inner side by a backing or sheathing (Fig. 1.5b). For esthetic purposes, an interior facing usually is placed on the inner side of the backing. A layer of insulation should be incorporated in walls to resist passage of heat. Generally, walls may be built of unit masonry, panels, framing, or a combination of these materials. Unit masonry consists of small units, such as clay brick, concrete block, glass block, or clay tile, held together by a cement such as mortar. Figure 1.5a shows a wall built of concrete blocks. Panel walls consist of units much larger than unit masonry. Made of metal, concrete, glass, plastics, or preassembled bricks, a panel may extend from foun- dation to roof in single-story buildings, or from floor to floor or from window header in one story to window sill of floor above in multistory buildings. Large panels may incorporate one or more windows. Figure 1.5c shows a concrete panel

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

Placing a brick on the mortar roll Blade The working part of the trowel that allows manipulation of the mortar A bricklayer rolling mortar STEP 1 Place the brick in the wall. STEP 2 Remove the excess mortar from the joint. STEP 3 Apply mortar to the next brick. STEP 4 Insert and remove mortar from the header face. Remember, this is a real skill that requires much practice and repetition. Surplus mortar squeezed from the bed joint must be removed with care. The final appearance of the work can be spoiled if mortar is smudged over the face of the wall and not cut off carefully. Forming a perp or cross joint is also something that requires continuous practice. With the brick held almost vertically in one hand and the trowel in the other, a suitable amount of mortar is picked up and spread on the header face of the brick. There are a number of techniques to achieve this, but it should always be remembered that the practice of wiping a small amount of mortar on the front and back edges of the header face will not produce a weatherproof joint. Good standards of workmanship demand that perp joints and bed joints should be full.