Watlow Optimizing Your Process System with the Series 988 Controller - Manual (Page 25)

General Software Features
3.5
Optimizing Your Process System with the WATLOW Series 988
Dead band
Overview
The dead band prompts, and ,
located in the PID menus, determine the
amount of interaction between heat (reverse
acting) and cool (direct acting) control outputs.
The dead band directly offsets the target set
point of the cool control output.
With a positive dead band, both control out-
puts will never be on at the same time. With
the process in a positive dead band, the output
value is determined by adding the percent heat
output to the percent cool output and only
applying the result to the correct output —
cooling action if the sum is negative and
heating action if it is positive.
If the dead band is set to a negative value, the
heat and cool outputs can both be ON at the
same time.
Requirements
The dead band feature is standard on any
Series 988 controller with two control outputs.
The dead band prompts will appear if the
control outputs are configured for heat/cool or
cool/heat.
Sample Application
An engineer for an environmental chamber manufacturer, who is design-
ing the heating and cooling system for a new chamber, wants to minimize
the energy costs of operating the chamber. She has chosen the 988 and
will configure the heat and cool outputs with a positive dead band.
When the chamber temperature is near ambient the cooling and heating
systems had a tendency to buck one another, resulting in inefficient use
of energy. The engineer started with a dead band of five degrees, but in
the process of tuning the system for optimal control, the setting was
reduced to two degrees. This made the system more energy efficient and
reduced wear on the refrigeration system.
TL
TL
W WA
PROCESS
L1 L2 L3 L4
DEV
% OUT DISPLAY
MODE
AUTO
MAN
SERIES 988
input 1
output 2
output 1
limit
limit
sensor
Environmental Chamber
heater
chiller
temperature sensor