Part Two
52
Temperature control
The Model 8682 receives a temperature input from a temperature sensor (1000
Ω
Platinum RTD).
The Model 8682 controller maintains temperature control by:
(1) Controlling supply and general exhaust for ventilation and cooling
(2) Controlling the reheat coil for heating
The Model 8682 has three supply flow minimum set points. The ventilation set point (
VENT MIN
SET
) is the minimum flow volume required to meet ventilation needs of the laboratory (ACPH).
The temperature supply set point (
COOLING FLOW
) is the theoretical minimum flow required
to meet cooling flow needs of the laboratory. The unoccupied set point (
UNOCC SETP
) is the
minimum flow required when the lab is not occupied. All of these set points are configurable. If
the Model 8682 is in the Unoccupied Mode, the controller will control the supply air flow to the
UNOCCUPY SET
ventilation rate, the supply flow will not be modulated for space cooling;
space temperature control will be maintained by modulating the reheat coil.
The Model 8682 continuously compares the temperature set point to the actual space temperature.
If set point is being maintained, no changes are made. If set point is not being maintained, and the
space temperature is rising, the controller will first modulate the reheat valve closed. Once the
reheat valve is fully closed the controller will begin a 3-minute time period. If, after the 3-minute
time period the reheat valve is still fully closed, the Model 8682 will then gradually begin
increasing the supply volume by 1 CFM/second up to the
COOLING FLOW
set point.
The controller, when controlling supply flow for cooling, will not increase the supply flow above
the
COOLING FLOW
ventilation rate. If the space temperature decreases below the set point, the
controller will first reduce the supply volume. Once the supply volume reaches its minimum
(
VENT MIN SET
), the controller will then start a 3-minute time period. If, after 3 minutes the
supply flow is still at the
VENT MIN SET
flow rate, the controller will begin modulating the
reheat coil open to meet the heating demand.
If the general exhaust is in the closed position and fume hood loads require additional replacement
air, the Model 8682 will override ventilation or temperature set points to modulate supply for
pressurization control. Temperature will then be controlled by the reheat valve in this sequence.
The control output items in the
DIAGNOSTICS
menu will show a percentage value. If control
direction for a given output is set to
DIRECT
, the diagnostic value will be percent
OPEN
. If
control direction for a given output is set to
REVERSE
, the diagnostic value will be percent
CLOSED
.
NOTE
: The greatest flow requirement dominates the supply flow. If hood replacement air
exceeds the ventilation or temperature flow minimums, the replacement air requirement
is maintained (minimums are ignored).
