In Soviet period designers were engaged in design of ventilation.
Even the young designer before works found the corresponding Construction Norms and Regulations in the small organization and performed work on design of ventilation according to its requirements. In standard design situations it guaranteed standard result.
At the largest organizations branch institutes worked. Their design groups could solve qualitatively any problem, make design of ventilation of any complexity
Now everything looks differently:
Under impression of ease of assembly of systems of ventilation from ready accessories absolutely casual people undertook design.
They consider that it is possible to make system of ventilation as if of cubes:
to estimate the necessary productivity of the fan;
to find according to the catalog suitable (usually axial!) fan;
to attach to it a network of air ducts and a lattice.
But such scheme can work only in very short symmetric network.
When lengthening a network the fan hoots, but air does not drive. If the network short, but asymmetrical, then air goes through one of branches. Such network can usually be adjusted.
Under impression of several failures such designers begin to look for computer programs for ventilation calculation. But only the one who can do also without them can use such programs.