Controlling Loads; Cooling Output; Standard Regulation And Continuous Regulation; Standard Regulation: [Cre = N] - Emerson xm678d Operation And Installation Manual

19 Controlling Loads

19.1. Cooling Output

Regulation is performed according to the temperature
measured by the thermostat probe (physical probe or
virtual probe) obtained by a weighted average
between the two probes following the formula:
value_-
for_room_regu- =
lation
If the temperature increases and reaches the setpoint
plus differential, the solenoid valve opens and then
closes when the temperature reaches the setpoint
value again.
In case of error in the thermostat probe, the opening
and closing time of the solenoid valve is configured
by the Con and CoF parameters.
19.2. Standard Regulation
and Continuous
Regulation
The regulation can be performed in three ways:
• Standard regulation - the goal of the first regulation
(standard regulation) is to reach the best superheat
via a classic temperature regulation obtained using
hysteresis.
• Continuous regulation - the second regulation
allows the valve to realize a high performance
temperature regulation with a good factor of
superheat precision. This kind of regulation can be
used only in centralized plants and is available only
with the electronic expansion valve by selecting the
[CrE = Y] parameter.
• Use of evaporator valves - the third kind of
regulation is through the use of evaporator valves
[CrE = EUP]; in this configuration, the valve is
placed at the end of the evaporator. In any case, the
regulation is performed via the PI regulator that
determines the valve opening percentage.
Cooling Output
(rPA*rPE + rPb*[100-rPE])
100

19.2.1. Standard regulation: [CrE = n]

The HY parameter is the differential for the standard
ON/OFF regulation. In this case, the int parameter is
neglected.
19.2.2. Continuous regulation: 
[CrE = Y]
The HY parameter is the proportional band of the PI
that is in charge of the room temperature regulation. It
is recommended to use at least [HY = 12°F/6.0°C].
The int parameter is the integral time of the same PI
regulator. Increasing the int parameter causes the PI
regulator to react slowly and is true vice versa. To
disable the integral part of regulation, set [int = 0].

19.2.3. Evaporator valves: [CrE = EUP]

In this case, the system performs a regulation of the
temperature without considering the superheat (in
fact, the valve is at the end of the evaporator). The HY
parameter is the proportional band for the temperature
regulation and int is the integral time for the
regulation. In this case, there is no superheat
regulation.

19.3. Defrost

19.3.1. Defrost starting

In any case, the device checks the temperature that is
read by the configured defrost probe before starting
the defrost, after that:
• (If RTC is present) Two defrost modes are available
through the tdF parameter: defrost with electrical
heater and hot gas defrost. The defrost interval is
controlled by the parameter EdF: (EdF = rtC)
defrost is made in real time depending on the hours
set in the parameters Ld1 to Ld6 in workdays and in
Sd1 to Sd6 on holidays; (EdF = in) the defrost is
made every IdF time.
• Defrost cycle starting can be operated locally
(manual activation by means of the keyboard or
digital input or end of interval time) or the command
can come from the master defrost unit of the LAN.
In this case, the controller will operate the defrost
cycle following the parameters it has programmed.
At the end of the drip time, it will wait until all the
other controllers of the LAN finish their defrost
cycle before restarting the normal regulation of the
temperature according to dEM parameter.
• Each time any of the LAN controller begins a
defrost cycle, it issues the command into the
network making all the other controllers start their
own cycle. This allows a perfect synchronization of
Controlling Loads • 33