The author, during his perhaps too long a career, has come across many instances where the ductwork manufacturer has provided an inlet box to the fan, to give side entry. Reference to Chapter 5, Section 5.6 shows that this leads to a “system effect” such that the fan no longer gives its rated catalogue performance. The fan requires a fully developed symmetrical air velocity profile free from swirl at its inlet.
It is the system designer’s responsibility to provide this. Where an inlet box entry cannot be avoided, it should preferably be or-
20/25% greater than discharge
Note: Sheet metal duct design is always an approximation. In the smaller sizes the dimensions have therefore been rounded to the nearest 5 mm. In the larger sizes some rounding has also been made.
The basis of Table 3.6 is:
A Minimum loss: 0.5 x py in inlet diameter
B Maximum loss: 1.0 x pv In inlet diameter
Avoid this at it may cause slow surge
C & D 0.75 x py in inlet diameter
Round duct diameter = 1.265 x? p—Equ 3.40
Spiral inlet box:
Volume is seriously reduced Avoid this type
Figure 3.67 Fan inlet box losses
D + w
It is usual to design the system in the first instance on the basis of circular ducting and then to convert it into the equivalent rectangular cross-section. In many cases the depth may be kept constant for constructional reasons e. g. where the ducting is in a void above a false ceiling.
Posted in Fans Ventilation A Practical Guide