Handbook of air conditioning and refrigeration / Shan K

FIGURE 30.2 (Continued)
Effect of Filter Final-Initial Pressure Drop Difference on
System Performance
AC SYSTEMS: CENTRAL SYSTEMS AND CLEAN-ROOM SYSTEMS 30.23
During the calculation of the system pressure loss of an air system for the purpose of selecting fans
and AHUs, one should use the final pressure drop of the filter �pf.f, in in. WC (Pa), to provide the
required volume flow rate whenever filters are either clean or loaded with specified capacity. For
purposes of energy estimation, the average pressure drop of the filters during the working period
�pf,m, in in. WG, should be employed instead of final or initial pressure drops.
For most prefilter and ultrahigh-efficiency air filter assemblies used in AHUs, the difference between
the final pressure drop and the initial pressure drop of the filter assembly is 0.4 to 0.8 in. WC
(100 to 200 Pa). Let us consider an air system whose design volume flow rate V˙ is 30,000 cfm
(14,160 L/s), and whose system pressure loss when filters are loaded �psy is 2.2 in. WC (550 Pa).
At V˙ � 30,000 cfm (14,157 L/s), the difference between the final and initial pressure drops is 0.5
in. WC (125 Pa).
The effect of the difference in final and initial pressure drops of a filter on system performance
is illustrated in Fig. 30.2. In Fig. 30.2a, an airfoil-blade centrifugal fan is used. The fan and AHU
are selected according to these criteria: V˙ � 30,000 cfm (14,160 L/s) and �psy � 2.2 in. WC (550
Pa). If the filters are loaded, the operating point is P. If the filters are clean, the system pressure
drops to 1.7 in. WG (425 Pa). The operating point moves along the selected fan curve to point Q,
which has V˙ � 33,000 cfm (15,573 L/s) at �p � 1.7 in. WC (425 Pa).
In Fig. 30.2b, a forward-curved centrifugal fan is used. A forward-curved fan has a slightly flatter
fan curve at operating point Q, and the system volume flow rate increases to 33,500 cfm (15,810
L/s). For Fig. 30.2c, a vane-axial fan is used. A vane-axial fan has a very steep fan curve. Therefore,
at point Q the system volume flow rate is increased to only about 31,000 cfm (14,630 L/s).
The air system in a clean-room system is always a constant-volume system. The fan power inputs
at point Q for airfoil blade and vane-axial fans are approximately the same as at point P,
whereas for forward-curved fans, their fan power input is comparatively greater.