It is easiest and cheapest to direct couple a fan to the motor shaft so the fan must be designed to work at about 1700 rpm. Forward inclined impellers will produce a much greater velocity flow than backwards. Higher velocity means more pressure potential, higher power, and more noise. Backward inclined are quieter, use less power, and will have to be larger to get the same flow. There is no difference in "push" vs. "pull" between the two types. A fan can only push. You cannot pull a gas, as it has no tensile strength. Gas entering a fan is pushed into the fan by the higher pressure gas outside the fan, and thus approaches from all directions equally. Gas leaving a fan has momentum and will keep moving in the same direction until it mixes and diffuses.
