Issue:
Head Pressure Control
Resolution:
The head pressure control algorithm for a particular refrigeration circuit is initiated when a compressor output for that circuit is energized. The condenser fans will remain off until the saturated condensing temperature becomes greater than the "lower band limit". Condenser fans are staged on and off based upon the saturated condensing temperature. Fans are staged on if the saturated condensing temperature rises above the "upper band limit" and staged off if the saturated condensing temperature falls below the "lower band limit". Fan staging (on or off) will continue as long as the saturated condensing temperature is outside the upper and lower limits.
If fan control remains at a given stage, other than the highest stage, for longer than 30 minutes then the unit will do an efficiency check to see if a fan stage can be added. This efficiency check will automatically be initiated if the saturated condensing temperature does not fall below the "efficiency check point". If the saturated condensing temperature does fall below the "efficiency check point" then fan control will remain at its present stage.
If four fan stage changes occur within a 10 minute time period, then the lower limit of the condenser fan control band is changed from the "lower band limit" to a temperature equal to the "lower band limit" minus the "temporary low limit suppression" (adjustable through Human Interface from 0 to 20 degrees). This new temperature will remain the lower band limit temperature will for a period of one hour. This prevents unit from operating at conditions that promote short cycling of condenser fans.
A Low Ambient Head Pressure Control algorithm is active on all units, regardless of whether the unit is configured as having the low ambient option. In a low ambient condition, head pressure may be maintained on each cooling circuit by small changes in fan capacity, as opposed to a full fan stage (which would provide excess capacity). To provide head pressure control in low ambient conditions, the UCM has a Low Ambient Fan Control Actuator output signal for each cooling circuit. When the head pressure control function has reduced capacity to one fan stage on a circuit, the Saturated Condensing Temperature is controlled by modulating the capacity of that condenser fan stage between 0 and 100% via the Low Ambient Actuator output. This feature allows for low ambient compressor operation down to 0 degrees.
NOTE: Minimum condenser fan stage on and off time is 5.2 seconds.
Facts:
- Intellipak Air-Cooled Chiller 20-60
Document Identifier: DOC-77057
WARNING:
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