Emerson E2 Boiler, Bypass Valve Control, Digital Scroll Compressor, Unequal Capacity Compressors

11.23.4.1 Digital Scroll Compressor

MCC can also utilize a single Digital Scroll Compres- sor. The assumption is that one of the Compressor Control Modules in the system may have a digital scroll instead of a standard scroll compressor installed. The Digital Scroll Compressor may not be the same size as the fixed scroll compressors. When a Digital Scroll Compressor is defined in the application, MCC will utilize the compressor con- trol modules with the digital scroll at all times, modulating the capacity of the compressor to maintain the Chilled Supply Temperature setpoint. All other compressor con- trol modules will be turned ON or OFF based on the rota- tion sequence defined above. Based on this strategy, the digital scroll compressor will have the fewest cycle counts and the most runtime of all the compressors in the system.

11.23.4.2Variable Frequency Drive Com-

pressor

MCC can utilize one compressor with a variable fre- quency drive. The method of control will be similar to the digital compressor strategy. The application will continue to allow only one variable capacity device to be defined in the system, making the Variable Frequency Drive and Dig- ital compressor options mutually exclusive.

If a Variable Frequency Drive compressor is to be used and it is a Control Techniques drive using the Modbus communication interface, the Control Techniques Drive association menu will support associating the drive to an MCC.

11.23.4.3 Unequal Capacity Compressors

Optionally, the MCC can support different capacity compressors. When this option is selected, the capacity of each defined compressor will be user specified. The con- troller will evaluate the capacities of all the compressors to determine the appropriate compressor to switch on or off in order to maintain the Supply Temperature at setpoint.

When unequal capacity compressors are defined, the controller will not use a simple round robin approach to switching compressors on or off (as with equal capacity compressors) and therefore compressor runtimes may vary significantly over time. However, for those compressors defined with identical capacity, the algorithm will attempt to keep both runtimes and cycle counts as even as possi- ble.

11.23.5 Bypass Valve Control

MCC can utilize the Chilled and Condenser Bypass Valves. These valves will be modulated to maintain user defined differential setpoints. For each valve, as the differ- ential pressure rises above the setpoint, the valve will modulate open. When the differential pressure falls below setpoint, the valve will modulate closed.

11.24 Boiler

The BX-300 and BX-400 will default to a maximum of two Boiler applications. Other models will have to license the Boiler Application. The Boiler will be available only for E2 versions 3.0 and higher.

11.24.1 Overview

The Boiler can be controlled two ways, via the boiler temperature or the boiler pressure. The E2 uses the current outside air temperature to calculate a new boiler control setpoint. If the boiler temperature (or boiler pressure) is less than the setpoint value then the boiler is turned ON. Otherwise it is turned OFF. The algorithm will work in an inverted fashion if the Minimum Outside Air Temperature is greater than the Maximum Outside Air Temperature.

Figure 11-1- Temperature Ranges

An Outside Air Temperature Boiler On and an Outside Air Temperature Boiler Off can be defined. If the outside air temperature is lower than the Outside Air Temperature Boiler On the boiler is turned ON. If the outside air tem- perature is higher than the Outside Air Temperature Boiler Off the boiler is turned OFF.

If defined, the Outside Air Temperature Boiler Off must be greater than the Outside Air Temperature Boiler On.

11.24.2 Alarms

There are four advisory generated by the Boiler Appli- cation:

•High Temperature

•Low Temperature

•High Pressure

•Low Pressure

Each advisory will have an associated type, priority, and delay.

Boiler Software Overview • 11-55