Emerson E2 Evaporative Condensers, Fan Control, Condenser Split Mode, Fast Recovery

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 11-2and Figure 11-3. Figure 11-2shows the typi- cal layout of an air-cooled condenser. Figure 11-3shows the typical layout of an evaporative condenser.

Figure 11-2- Air Cooled Condenser Diagram

11-4 • E2 RX/BX/CX I&O Manual 026-1610 Rev 13 14-SEP-2011