Emerson E2 Temperature Differential TD Strategy, TD Control Fail-Safes, Configuration

must be used.

A Multiple Input cell is a simple application that reads data values from its inputs, combines them using a user- defined combination strategy, and sends the combined value to the desired application input.

A common application for this is in HVAC control, where a single heating control value might come from an average of a number of temperature sensors throughout the building.

Two different types of Multiple Input cells may be used, depending upon whether the inputs to be combined are analog or digital sources.

11.19 TD Control

11.19.1 Overview

The TD Control application controls fans sequentially based on the temperature differential (TD) of the con- denser. When an increase is called for, the next fan will turn on when the time since the last fan state change is greater than the Fan On Delay Time setpoint (or will turn on immediately if the last change was more than the Fan On Delay). Additional fans will be staged on at Fan On Delay intervals while an increase in capacity is called for. When a decrease is called for, the next fan will stage off when the time since the last fan state change is greater than the Fan Off Delay Time setpoint (or will turn off immediately if the last change was more than Fan Off Delay). Additional fans will be staged off at Fan Off Delay intervals while a decrease in capacity is called for.

11.19.2Temperature Differential (TD) Strategy

TD strategy attempts to maintain a constant difference (i.e., TD setpoint) between the temperature of the refriger- ant and the ambient temperature. The TD setpoint allows you to set the constant that is to be maintained.

TD strategy uses drop leg temperature for the refriger- ant temperature and plenum temperature for the ambient temperature. The formula for determining the TD control value is:

TD = (dropleg temp) + (dropleg offset*) - plenum temp.

* DropLeg Offset is a user-configured parameter to allow fine-tun- ing of the drop leg temperature measurement.

In TD condenser control operation, when the TD rises above TD setpoint + (TD deadband / 2), an increase in fan capacity is called for. Similarly, when the TD drops below TD setpoint - (TD deadband / 2), a decrease in fan capac- ity is called for. TD can control up to four condenser fans.

11.19.3 TD Control Fail-Safes

TD Control operates as long as plenum and drop leg temperature inputs are available and the drop leg tempera- ture remains within a minimum/maximum range config- ured by the user. Otherwise, TD control uses various fail- safe modes to control the condenser fans.

In the event drop leg temperature is not available (due to probe failure), all fans will be staged on.

Drop Leg Temperature Above Maximum Drop Leg Setpoint

If the Drop Leg temperature (Drop Leg Temp) rises above the maximum drop leg temperature setpoint (Drop Leg Max), an increase will be called for until Drop Leg Temp drops below Drop Leg Max. This protects against the possibility of the Drop Leg Temp rising too high as a result of high plenum temperature.

Drop Leg Temperature Below Minimum Drop Leg Setpoint

If the Drop Leg temperature drops below the mini- mum drop leg temperature setpoint (Drop Leg Min) a decrease will be called for until Drop Leg Temp rises above Drop Leg Min. This protects against the possibility of the Drop Leg Temp falling too low as a result of low plenum temperature.

Plenum Temperature Not Available

If plenum temperature is not available (due to probe or communication failure), control will be based on the Drop Leg Min and Max setpoints.

11.19.4 Configuration

TD Control allows the user to operate a condenser at its designed temperature differential and saves energy by maintaining optimal condenser TD with the least amount of fans on as possible.

Standard discharge pressure control may attempt to operate the condenser below its designed TD and turn on more fans than necessary.

Set the number of condenser fans to be controlled in the Num Cond Fans field under Setup (4 is the default value).

11.19.5 Setpoints

TD setpoints are configured to allow control of the condenser at its designed TD during swings in ambient temperature.

For the Drop Leg Minimum setpoint, if the drop leg temperature falls below this setpoint, condenser fans will be cycled OFF regardless of the value of the TD setpoint.

For the Drop Leg Maximum setpoint, it is the maxi- mum allowable value of the drop leg temperature. If the

TD Control Software Overview • 11-49