Future Design MCT-CM, User Manual

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MCT-CM
65
The
Defrost Type selection is used to choose the defrost mode. Available selections are Disabled, Timed
Defrost and Timed Process. When Timed Defrost is selected, defrost occurs on regular intervals based on the
interval time entered on the defrost screen in the MCT runtime application. When Timed Process is selected,
defrost also occurs on regular intervals based on the interval time entered on the defrost screen in the MCT
runtime application; however, the selected enable loop/monitor must also be below the enable setpoint.
The interval timer will begin counting down to the next defrost as long as the selected enable loop/monitor
remains below the enable setpoint. If at any time the selected loop/monitor rises above the setpoint, the defrost
interval timer will be reset. This provides a means for enabling defrost only when the control loop or monitor
point associated with the function is below a certain temperature in which defrost may be required on regular
intervals.
The
Enable Loop/Monitor selection is shown only when defrost is set to timed process. These fields are used
to select any of the available loop or monitor points configured in the system to be used as the enable point for
defrost.
When a loop is selected as the enable source:
The set point of the selected loop is used as the source value and is compared to the defrost enable set point.
When the loop set point is equal to or less than the defrost enable set point, the defrost logic will be activated.
This insures that defrost will operate as required, since temperatures can rise if the cooling coil begins to ice
over and cooling capacity is lost. If the air temperature was used, the coil could ice over causing air temperature
to rise, preventing defrost from activating when it is required.
When a monitor point is selected as the enable source:
The process value of the selected monitor point is used as the source value and is compared to the defrost
enable set point. When the monitor point is equal to or less than the defrost enable set point, the defrost logic
will be activated. This offers the ability to provide a type of “adaptive” defrost by placing the sensor on the
cooling coil or cold plate and monitoring surface temperature. It could then be used to activate defrost only
when the surface temperature is operating at or below freezing in which ice can accumulate.
The
Enable Setpoint field is shown only when defrost is set to timed process. It is used to enter the setpoint
for the selected enable loop/monitor below which the defrost interval timer will be started.
The
Terminate Setpoint field is shown only when defrost is set to timed process. It is used to enter the setpoint
for the selected enable loop/monitor above which the active defrost will be stopped and the precool period
started. When used in conjunction with a monitor input to measure coil temperature for example, it allows
defrost to be stopped once the coil temperature has risen to a point at which all ice would be melted off. This
allows regular conditioning to resume earlier than waiting the entire defrost duration.