Forced Draft Heat Exchanger
Air is delivered from the fan and passes over the heat exchanger tubes. This setup is particularly useful if you do not wish to draw corrosive or erosive process gases over the fan motor and parts.
Induced Draft Heat Exchanger
Air on the supply side is drawn across the heat exchanger and then the fan, prior to being discharged. This is not a good setup if the air on the suction side is not ‘clean’. Induced fans are not given preference for extracting corrosive process gases because the gas passing over the fan can damage the fan (this would not occur for a forced air fan which would have no process gas flowing over its parts).
This 3D model shows a full view of both a forced draft and induced draft cooling cell.
3D Model Annotations
Induced Draft Cooling Cell
Air passes across the heat exchanger, then through the fan. Because air passes through the heat exchanger before the fan, this cooling cell is classified as an induced draft cooling cell.
The fan ring surrounds the blades. The clearance between the fan blades and ring should be minimal in order to reduce air leakage past the blade tips; this helps the fan maintain high efficiency.
Fan Blades and Hub
The fan blades are attached to the fan hub, and the hub is connected via a key to the shaft. The geometry and number of fan blades defines the operating characteristics of the fan. Generally, axial fans are suitable for high flow, low resistance applications.
The plenum provides an enclosed space between the heat exchanger and the fan. As air passes through the plenum, its pressure decreases and velocity increases. This allows the fan to discharge the air further away from the cooling cells, which prevents the heated air being recirculated to the fan’s inlet side.
The heat exchanger tubes carry the flowing medium that will be cooled by the air. For most refrigeration and cooling systems, the medium being cooled is usually water, or a water glycol mixture. The tubes provide a large contact surface area between the air and cooled medium, this yields a high heat transfer rate and corresponding high cooling cell efficiency.
A three-phase motor provides the mechanical force to rotate the fan blades. Pulleys and a gearbox may be part of the drive train depending upon the design. It is possible to install variable frequency drive motors to control the fan speed, but other solutions (pulleys etc.) are cheaper and may be suitable if the fan is operated only in on/off mode (no speed control required).
Forced Draft Cooling Cell
Air passes through the fan, then across the heat exchanger. Because air passes through the fan before the heat exchanger, this cooling cell is classified as a forced draft cooling cell.