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SUPPLY DUCT SYSTEMS (Additional information. Read sections 18.4, 18.5, & 18.6)

SUPPLY DUCT SYSTEMS (Additional information. Read sections 18.4, 18.5, & 18.6). General

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SUPPLY DUCT SYSTEMS (Additional information. Read sections 18.4, 18.5, & 18.6)

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  1. SUPPLY DUCT SYSTEMS (Additional information. Read sections 18.4, 18.5, & 18.6) • General • The supply duct system conveys conditioned air from an AHU to the spaces of the building to be conditioned. The power that causes the air to move through the supply ducts is supplied by the fan in the AHU unit and the motor that drives the fan. • Ductwork categories • Supply duct systems are designed and constructed in many different ways. They may be categorized as follows:

  2. SUPPLY DUCT SYSTEMS • Round vs. rectangular: • Both types have advantages and disadvantages, and both find applications where one is definitely superior to the other. • A round duct, however, is more efficient than a rectangular duct in performing the same task; it is also smaller in cross-sectional area and has less duct wall exposed to moving air. • An 18 in dia round duct, for example, has the same air-carrying capacity as a 26 in wide and 11 in high rectangular duct. • The round duct has a cross-sectional area 254.5 sqin and a perimeter of 4.7 ft, while the rectangular duct has 286 sqin area and a perimeter of 6.2 ft. The rectangular duct has 32% more metal in it, and should cost proportionately more. • One big disadvantage of round duct is its height. If the net clear height of a furred space above suspended ceiling is, for example, 14 in, an 18 in diameter duct cannot be installed therein; however, its equivalent 26 in wide by 11 in high rectangular duct will fit the space easily.

  3. SUPPLY DUCT SYSTEMS • Metallic vs. nonmetallic • Supply ducts may be further categorized according to the materials of which the ducts are made. • A great majority of metallic ducts are made of galvanized steel. • Next in popularity in metal ducts is aluminum. Aluminum ducts are light in weight, but basic cost per pound is higher than galvanized steel. • Other metals used under special circumstances are copper and stainless steel: appearance, resistance to some types of chemical attack. • Use of fiberglass, compressed paper, plastic, cement-asbestos, vitrified clay, and concrete as non-metallic ducts.

  4. SUPPLY DUCT SYSTEMS • Low velocity vs. high velocity • A duct system may also be categorized by air velocities. • Low-velocity ducts systems: • Characterized by air velocities in the range of 400 to 2500 fpm, but not usually in excess of 2000 fpm. • The vast majority of supply ducts, over 90%, are designed for low-velocity performance. • LV systems are lower in initial cost, have a lower energy consumption, and, consequently, lower in operating costs. • The design of LV duct systems is done on the basis of 0.1 in WG static pressure loss per 100 ft of equivalent length. • If any deviation from this value is made, it should be in the direction of less static pressure loss per unit length. • High-velocity duct systems: • Characterized by air velocities in the range of 2500 to 7000 fpm, although velocities normally do not exceed 6000 fpm. • HV ducts are smaller for a given air quantity and require less space for their installation, but they have a higher initial cost and even greater operating cost.

  5. SUPPLY DUCT SYSTEMS • Duct frictional resistance • Any type of duct system offers frictional resistance to the movement of supply air. • The frictional resistance of a supply duct varies in proportion to the square of the ratio of the velocity at two different velocities, and the fan power varies as the cube of this ratio. • If a supply duct, for example, is carrying 5000 cfm of air at 1000 fpm, and a second supply duct is carrying the same quantity of air at 2000 fpm, the frictional resistance of the second duct per foot of duct length will be four times higher than that of the first duct: (2000/1000)²; and the power required to overcome this frictional resistance will be eight times as much: (2000/1000)³.

  6. SUPPLY DUCT SYSTEMS • The AHU fan must develop a pressure equivalent to the frictional resistance of the ductwork in order to supply air in the spaces to be conditioned. • This pressure is measured in terms of inches of water. • The total pressure in the duct system is the sum of static and velocity pressures. • Static pressure is the outward push of air against duct surfaces as a result of the compressive force applied by the fan. • Velocity pressure is the directional thrust of supply air due to its velocity. • A decrease in duct size increases velocity pressure and decreases static pressure; an increase in duct size decreases velocity pressure, increases static pressure.

  7. SUPPLY DUCT SYSTEMS • Duct equivalent length • The fittings (elbows, tees, branch connections, etc.) and accessories (dampers, extractors, etc.) in the ductwork offer additional frictional resistance. An additional length has to be added to the actual measured length of the ductwork to take care of the resistance offered by these items to supply air. This additional length is called equivalent length, and the sum of actual measured length and equivalent length is known as total equivalent length (TEL). • TEL = L + (C*L) • where L = actual measured length • C = a coefficient of duct system complexity (0.4 for simple duct systems, 1 for very complex duct systems).

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