Constant volume - Variable temperature systems have been dominating the HVAC applications in the past as they are simple to design, install and operate, and have a relatively low initial cost. However, they react slowly, have high energy consumption and can be noisy.
Variable Air Volume or VAV systems are usually designed to maintain constant static pressure in the supply duct. This can be achieved with a damper, on/off control or a variable speed drive. A variable air volume system with variable speed control gives fast reaction to temperature changes, low consumption of energy and low noise levels.
This is a simplified solution of the variable air volume system with VSD fan control. The return air fan it is tracking the positive pressure by following the supply fan speed, keeping return airflow equal to the supply airflow. A system like this can be easier to keep stable than one with static pressure controls.
A variable speed chilled water plant reduces installation and operation cost. It enables the use of smaller pumps and piping and also allows the chilled water temperature rise to remain constant during partial load conditions and chilled water flow to vary in proportion with the load. Using variable speed AC drives improves the controllability of the whole chilled water system, giving accurate temperature control.