ASHRAE Journal - January 2010 - 33

ASHRAE59–2009 YE A R S JOURNAL This friction chart can be used for most applications with minimal error. Corrections for surface roughness or variations due to small temperature or pressure variations from standard are generally not necessary (see chapter 32 in the ASHRAE Handbook–Fundamentals1 for a more detailed discussion). As easy as friction charts are to use, they are only for round ductwork. Friction loss for rectangular,3 flat oval,4 and other shapes of duct are determined by using the equivalent round diameter. The general equation for equivalent round is given by: De = 1.55 A p 0.250 0.625 Dpt = fitting total pressure loss, in. wg (Pa) pv = velocity pressure, in. wg (Pa ) The 1993 ASHRAE Handbook–Fundamentals1 has a limited number of tables of loss coefficients for use with the example problems. In June 1994, ASHRAE published a computerized Duct Fitting Database that contains the loss coefficients for over 220 fittings.6,7 Use of this database is menu-driven and straightforward and it can be linked to existing duct design programs. Calculating the pressure loss of a fitting manually is also straightforward by rearranging Equation 8: Dpt = C pv (9) (7) where De = equivalent round diameter for equal air flow, length and fluid resistance, in. (mm) A = cross-sectional area of the duct, in.2 (mm2) p = perimeter of the duct, in. (mm) When using the equivalent round method of calculating the friction losses with the friction chart, use the same air quantity as in the non-round shape and the equivalent round diameter, De, to determine the friction rate. Dynamic losses are caused from fittings that change the air flow path’s direction and/or area and result from flow disturbances. Entries, exits, transitions, junctions, elbows and obstructions are all examples of where dynamic losses occur in systems. It is difficult to predict from theoretical calculations what these losses will be so they are almost always determined from experimental data. Idelchick has a thorough discussion on all the parameters that effect these losses.5 Rather than print volumes of data on pressure losses under various operating conditions for all the fittings, the data is condensed into a non-dimensional loss coefficient, C. The loss coefficient is determined experimentally by measuring the change in total pressure between two stations that contain the fitting, subtracting the friction loss between the two stations, then dividing by the velocity pressure. This is represented in equation form by: Dp C= p t v where C = local loss coefficient, dimensionless January 2010 (8) However, you must be aware of what velocity pressure the loss coefficient data is referenced to. If the fitting is a single-flow fitting with equal inlet and outlet areas, then there is only one possibility. But if the areas are unequal, you must determine if the reference velocity pressure is for the inlet or the outlet. Also, if you are calculating the pressure loss of a junction, you must determine which leg to use to calculate the velocity pressure for use in Equation 9. For example, prior to the 1993 data, the loss coefficients in the Handbook–Fundamentals were referenced to the common section of junctions, which in a supply system, is the section upstream of the section that you are calculating the losses. This did not allow you to sum loss coefficients for a section because the junction loss would be referenced to the upstream section’s velocity pressure, while the rest of the fittings were referenced to the velocity pressure in the section. The 1993 Handbook–Fundamentals now presents the loss coefficients referenced to the section that the pressure losses are being calculated. The ASHRAE Database program shows both methods (which must produce the same pressure loss). To calculate the total pressure losses in a section, the friction loss between two stations must be added to the sum of the fitting dynamic losses. Figure 3 is a diagram of an example that will be used to calculate a section’s losses. To calculate the total pressure losses between stations 1 and 2 for Section B, we must determine the friction loss, Dpf,1-2, the sum of the loss coefficients, SC, and its velocity pressure, pv: ASHRAE Journal 33

ASHRAE Journal - January 2010

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