Rule number one is to avoid elbows and bends whenever possible. However, the fact is that many installations require at least one elbow. There are two precautions you can take when installing elbows to achieve optimum airflow.
First, allow a 2-3 foot straight run out of the fan before the first elbow. This allows airflow to be uniform before passing through the first elbow. An installation that has a 90-degree elbow immediately after the fan exhaust port will cause air to flow back into the fan. This will reduce fan performance and increase noise.
Second, use a long radius angle to help ensure optimum airflow and minimum airflow noise. The shortest smooth, inner surface duct with the least number of elbows will provide optimum fan performance.