ASHRAE Journal - January 2010 - 38

ASHRAE59–2009 YE A R S JOURNAL conclusion designing duct systems must start with an understanding of how pressure losses are calculated and combined. then a design method should be selected based on what is available to the designer. if a computer and the t-method duct design program are available, and the designer has the information that the t-method needs to optimize systems (such as cost of energy, unit cost of ductwork, etc.), then the t-method will probably give the best solution. if the t-method is not available, the static regain method should be used on most medium to large systems to achieve balancing and the smallest sizes for given operating pressures. for small systems such as in residential buildings or single run systems, the equal friction method should be adequate. in all cases, total pressure should be used to analyze systems and smaller sizes or less efficient fittings should be used in noncritical paths to reduce costs and improve balancing. After the initial design of a system, it still needs to be analyzed for acoustics, heat transfer, duct construction, how the system should be installed, and how much leakage should be allowed. After installation, the system should be checked to see that leakage is within the specification and the system is balanced. With today’s computer programs, much better designs can be established that meet the owners’ requirements. However, a fundamental understanding of the design parameters is still important to the best design. which would have reduced the pressure required to operate the path containing Sections A and C to 0.66 in. wg. Because I still need 0.74 in. wg to operate the path for Sections A and B, I must add 0.08 in. wg to the path containing Sections A and C to balance the system. (With the original metric design, I must add 23 Pa to the path containing Sections A and B.) I cannot make the duct smaller because it would create a new critical path and increase the operating pressure. However, if I can use less efficient fittings, I still may be able to achieve a better balance and save additional money. By using pleated elbows in Section C for the I-P design, I will increase the total pressure drop from 0.30 to 0.32 in. wg. I have then reduced the excess pressure requirement to 0.06 in. wg (12 Pa). This will mean that I have used less expensive fittings and created a better balanced system. I should still change the initial parameters and calculate total cost for each to determine which design is optimum. The T-method of duct design was developed by ASHRAE to optimize systems considering equipment and operating costs.10 The theory is too complex to develop in a fundamentals article and it has already been discussed thoroughly in previous articles.10 But the basis is minimization of an objective function that represents the present-worth owning and operating costs of the duct system. In the T-method, the duct system is considered a tree structure. The branches and roots of the tree are systematically condensed, resulting in a single imaginary duct section or resistance with identical hydraulic characteristics. This equation is then minimized to cost and an optimized fan is selected. The system is then expanded to its original sections, distributing the available (optimized) total pressure as it expands. The complexity of the T-method requires designs to be done on a computer. 5. Idelchik, I., et al. 1986. Handbook of Hydraulic Resistance, 2nd ed. New York, New York: Hemisphere Publishing Corp., Harper & Row. 6. ASHRAE. Duct-Fitting Design Database. Atlanta, Georgia: ASHRAE. 7. McQuiston, F., Delahoussaye, R. 1994. “Duct fitting enhancements and data base development.” ASHRAE Transactions. Atlanta, Georgia: ASHRAE. Vol. 10, Pt. 1. 8. Carrier Air Conditioning Co. 1965. Handbook of Air Conditioning System Design. New York, New York: McGraw-Hill Book Co. 9. United McGill Corp. 1990. Computer Aided Duct Design: Comparing the Methods. Engineering report no. 144. Groveport, Ohio: United McGill Corp. 10. Tsal, R., et al. 1988. “T-method duct design, part 1: Optimization theory, part II: Calculation procedure and economic analysis.” ASHRAE Transactions. Atlanta, Georgia: ASHRAE. Vol. 94, Pt. 2, pp. 76 – 89. a s h ra e. o rg January 2010 References 1. ASHRAE. 1993. “Duct design.” ASHRAE Handbook–Fundamentals. Atlanta, Georgia: ASHRAE. Chapter 32. 2. ISO. 1983. ISO Standard 7807, Air Distribution-Straight Circular Sheet Metal Ducts with a Lock Type Spiral Seam and Straight Rectangular Sheet Metal Ducts-Dimensions. Geneva, Switzerland: International Organization for Standards. 3. Huebscher, R. 1948. “Friction equivalents for round, square and rectangular ducts.” ASHVE Transactions. Vol. 54, pp. 101 – 144. 4. Heyt, J., Diaz, M. 1975. “Pressure drop in flat-oval spiral air duct.” ASHRAE Transactions. Atlanta, Georgia: ASHRAE. Vol. 81, Pt. 2, pp. 221 – 232. 38 ASHRAE Journal

ASHRAE Journal - January 2010

Table of Contents for the Digital Edition of ASHRAE Journal - January 2010