Handbook of air conditioning and refrigeration / Shan K

17.54 CHAPTER SEVENTEEN
Static Regain Method
according to the requirement to float the particles, either by calculation or by experience. On the basis
of the determined air velocity, the cross-sectional area and, therefore, the dimension of the duct
can be estimated and then rounded to a standard size. The total pressure loss of the duct system �p t,
in in. WC (Pa), along the critical path can be calculated as
�pt � K�
2g �� c
f1L1 D1 � � � ��� f nL n
D n
� C 1� v 1 2 �� f 2L 2
D 2
� C n� v n 2�
� C 2� v 2 2
(17.76)
where v 1, v 2,...,v n � mean air velocity at duct sections 1, 2, ...,n, respectively, fpm (m/s)
C 1, C 2,...,C n � local loss coefficients at duct sections 1, 2, ...,n, respectively
K � 5.35 � 10 �5 for I-P unit (1 for SI unit)
This method sizes the air duct so that the increase of static pressure (static regain) due to the reduction
of air velocity in the supply main duct after each branch takeoff nearly offsets the pressure loss
of the succeeding duct section along the main duct. As a consequence, the static pressure at the
common end of the diverging tee or wye of the sized duct section remains approximately the same
as that of the preceding section.
A rectangular duct section 1-2 between the cross-sectional planes 1 and 2 is illustrated in
Fig. 17.22. The size of this duct section is to be determined. Let v 1 and v 2 be the mean velocities at
FIGURE 17.22 Pressure characteristics of a main duct section.