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Case Control Circuits

Software Overview

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5.27.4.1 Defrost States

The defrost cycle for a Case Circuit application con-

sists of three steps. Of these three, steps #1 and #3 apply
only to cases with heated defrosts:

Pump Down -

The defrost cycle begins with this

step immediately after the refrigeration solenoid
is turned OFF. During the Pump Down phase, the
application waits for a user-specific amount of
time to elapse before turning on the defrost heat.
This allows refrigerant in the evaporator to be
evacuated before defrost heat is activated. The
compressor(s) remain ON during Pump Down.

Pump Down times may only be used for hot gas

and electric type defrosts.

Defrost -

During the defrost phase, refrigeration

is disabled. If using electric defrost heaters will
be ON. If using hot gas, heated refrigerant will be
pumped through the coil. This phase will con-
tinue until the defrost is terminated (see

Section

11.3.2.3

for information on how defrost is termi-

nated).

Run-Off -

After defrost heat is deactivated, the

application waits for the Run-Off time to pass
before re-entering refrigeration mode. This
allows melted frost on the evaporator to drain
from the coil so that it will not re-freeze when
refrigeration begins again. After the user-speci-
fied Run-Off time has passed, the defrost cycle
has ended.
Run-Off times may only be programmed for hot
gas and electric type defrosts.

5.27.4.2 Defrost Types

There are many different ways used to defrost a refrig-

erated case. A case controller is capable of using three dif-
ferent types of defrost.

Off Cycle (Timed)

Off Cycle defrost (also known as Timed defrost) is

simply a period of time during which refrigeration is sus-
pended. No heat is applied to the evaporator. The applica-
tion simply turns refrigerant flow OFF for the duration of
the defrost cycle.

When these defrost types are used, Pump Down and

Run-Off times are not necessary; therefore, they will not
be part of the defrost cycle.

Hot Gas and Reversed Cycle Hot Gas

Hot Gas and Reversed Cycle Hot Gas require the use

of hot gas from the refrigeration lines. During these types
of defrost, the application will open the valve and pump
heated refrigerant through the evaporator coil. Any user-

defined Pump Down and Run-Off times will be observed
as normal.

Electric Defrost

Electric defrost uses electric heaters to defrost the

evaporator coil. During electric defrost, the application
will turn the Defrost output ON, which will likewise acti-
vate the heaters connected to the power module’s defrost
relay.

Any user-defined Pump Down and Run-Off times will

be observed as normal.

5.27.4.3 Defrost Termination

Both the start time and the end time of a defrost cycle

are determined by the user. The Pump Down, Defrost, and
Run-Off stages all have fixed durations, and when the last
stage of the cycle is complete, defrost is terminated.

However, a case controller may be programmed to ter-

minate the Defrost stage of the defrost cycle early if the
temperature inside the case rises above a fail-safe tempera-
ture setpoint. For certain defrost types, defrost heat may
also be “pulsed” in order to keep the temperature below
the setpoint without terminating defrost.

Temperature Termination

One or more temperature sensors in the case circuit

may be designated as termination temperature sensors.
The values of these sensors are combined into a single
control value, and this value is compared to the setpoint. If
the termination control value is greater than the user-
defined setpoint, defrost will end, and the defrost cycle
will begin the Run-Off period (if applicable).

Termination sensors may be either analog temperature

sensors or digital closures (Klixons). Also, Case Circuit
applications may use the value of the analog case tempera-
ture sensors for use in temperature termination.

Pulsed Defrost

Pulsed Defrost is only available if the circuit is using

Electric or Hot Gas defrost.

Pulsed Defrost is similar to Temperature Termination,

except when the termination temperature rises above the
setpoint, defrost does not terminate. Instead, the output
that applies defrost heat is turned OFF. The output will
remain OFF until the temperature falls below the setpoint,
at which time the output will come back ON.

The Case Circuit application will continue to pulse

defrost in this manner until the defrost time has passed.
The defrost cycle will then begin the Run-Off period.

5.27.4.4 Demand Defrost

If a case is configured with a demand defrost sensor, a

case controller may use their inputs as a means of keeping
scheduled defrost cycles from occurring when frost levels