Method: Vary heat flux. Varies cooling medium rate.
Process: Net vapor rate zero.
Disadvantages: High maintenance for louvers and variable-pitch configurations. Variable speed rarely used due to cost.
Application: Most common method of control with air-fins.
Variants: Vary fan pitch, vary louver position, vary motor speed.
Configuration notes: Variable position louvers tend to be troublesome. Louvers must be designed for automatic control. Do not refit an automatic positioner to a set designed for manual operation. Variable fan pitch is also subject to maintenance problems and fans have to be shut down before the pitch mechanism can be worked on. Variable speed using a variable frequency motor is the most reliable, but most expensive and is rarely used.
Operation: Variable flow rate varies air outlet temperature. Variable LMTD controls heat removal.
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