Application

Sequence of Operation
Operating Without a Supply or Exhaust
25
Siemens Industry, Inc.
Application Note, App 2926
140-1308
2015-07-07
8
340
3.2
9
390
3.5
10
521
3.8
11
531
4.1
12
598
4.9
13
691
5.8
14
798
7.0
15
0
0
16
1200
10
Example 5 - Multi-point Table
The point values above are for the exhaust table. Open loop supply operation is
similar, but the points used are 1 through 16; not 30 through 46.
Since the actual airflow being measured is 0, the heating safety requirement for
minimum airflow is not met and heating will not occur. To re-enable heating, the FLO
value must be set to 0, but only after a thorough safety review that the electric heating
mechanism has sufficient internal safeguards (that is, resettable shutoffs) that would
operate in the event that the system at some time actually did have insufficient flow.
NOTE:
Since
PID Only Mode uses PID without a Venturi table and Open Loop Mode uses
Venturi table without PID, these two modes cannot be used at the same time.
Operating Without a Supply or Exhaust
It is possible to run this application without the supply or exhaust if the corresponding
flow coefficient is set to zero. When the flow coefficient is zero and the offboard air
module is not connected the air velocity sensor will not display a FAIL status and the
flow loop will be allowed to run with a flow value of zero. See the
Application Notes
section for more information.
Airflow Control
Both supply flow and general exhaust flow are controlled as follows:
An embedded table statement works together with a feedback loop to operate the
Venturi Air Valve (either supply or general exhaust) so that the measured air velocity is
maintained at setpoint.
In these applications, the supply and general exhaust Venturi Air Valves can operate in
one of three modes:
 Mode 1 – Operates with both a PID loop and a Venturi table.
This mode provides the best control and would be the most commonly used mode
for these applications. In this mode, the embedded Venturi table statements works
together with a PID feedback loop to operate the Venturi air valve so that the
measured air velocity is maintained at setpoint. The following several sections
describe this mode.
 Mode 2 - Operates with a PID loop, but no Venturi table.
In this mode, the controller operates with PID control based on a flow sensor input,