Emerson E2 - Manual (Page 146)

11-4 • E2 RX/BX/CX I&O Manual 026-1610 Rev 13 14-SEP-2011
The refrigerant temperature is subtracted from the
value of an ambient air temperature sensor. The result is
the temperature differential. It is this differential value
that is compared to the PID setpoint for the purpose of
determining the amount of total fan capacity to activate.
11.2.2 Evaporative Condensers
In an evaporative condenser, water is sprayed across a
condenser coil, which cools the refrigerant as water is
evaporated. Control of the evaporative condenser is simi-
lar in ways to the air cooled strategy in that the Condenser
Control application uses PID control to activate or deacti-
vate fans (thus increasing or decreasing the amount of
evaporative cooling).
Like air-cooled condensers, evaporative condensers
may be controlled by discharge pressure or temperature.
They may also be controlled by water sump temperature.
In addition, each evaporative condenser may have up to 16
“override” sensors (either temperature or pressure) that
allow the condenser to be overridden to fast recovery
mode (See Section 11.2.5, Fast Recovery).
11.2.3 Fan Control
Condenser Control applications can control three dif-
ferent kinds of fans: single-speed fans (up to 12 stages),
two-speed fans, and variable-speed fans. All fan types are
controlled by PID control; in other words, the Condenser
Control application generates a percentage from 0% to
100% that corresponds to the amount of total fan power
that should be active.
Single-speed fans translate the PID percentage into a
percentage of total fan capacity. For example, if the PID
percentage is 75%, then 75% of all condenser fan stages
will be ON.
Two-speed fans use Cut In/Cut Out setpoints to trans-
late the percentage into an OFF, LO, or HI fan state.
Variable-speed fans simply use the percentage to
determine the fan speed. Thus, a 51% PID percentage will
result in the fan running at 51% maximum speed.
VS-SS combined fan setup is for the combination of a
variable-speed fan with single-speed fans. First, under PID
control, the variable-speed (VS) fan turns on. Then as the
PID percentage rises above VS fan’s maximum speed, the
E2 switches on additional single-speed fan stages.
11.2.4 Condenser Split Mode
The E2 is capable of operating condensers in a special
mode that reduces its condensing capacity. This special
mode is called split mode.
Split mode is most often used in cold climates during
periods of low outside air temperature. Split mode is also
sometimes used when heated refrigerant from the refriger-
ation system is being used as reclaim heat by an HVAC
unit.
The most common way E2 achieves split mode in an
air-cooled condenser with single-speed fans is to lock OFF
50% of the total number of fans. You may choose to lock
OFF all odd-numbered fans, even-numbered fans, the first
half of all fans, or the last half of all fans.
Split mode can also be achieved by activating a valve
that bypasses a portion of the tubing in the condenser man-
ifold. The resulting decrease in surface area results in
reduced cooling.
11.2.5 Fast Recovery
Under certain conditions the system pressure may
increase too quickly above the condenser setpoint to be
reduced effectively by normal condenser control. The E2
provides a user-definable fast recovery rate setpoint at
which all the condenser fans are turned ON to reduce sys-
tem pressure.
For air-cooled and temperature-differential condenser
strategies, discharge pressure is always used as the control
value that determines fast recovery. You may choose to
enable or disable fast recovery, and also to include a delay
when transitioning from one mode to the other.
For evaporative condensers, up to 16 “override” tem-
perature sensors may be combined to yield a single over-
ride value that is used for fast recovery. Fast Recovery is
always used in an evaporative condenser.
11.2.6 Hardware Overview
An overview of the input and output devices that make
up a typical Condenser Control application is shown by
Figure 11-2 and Figure 11-3. Figure 11-2 shows the typi-
cal layout of an air-cooled condenser. Figure 11-3 shows
the typical layout of an evaporative condenser.
Figure 11-2 - Air Cooled Condenser Diagram