Application

Sequence of Operation

Ventilation – Constant Volume Mode

Siemens Industry, Inc.

Application Note, App 6751

140-1331

2015-07-07

Ventilation – Constant Volume Mode

During Constant Volume (CV) operation, the active supply airflow minimum is used to

ensure that the room always receives enough supply air for proper ventilation. If

necessary, the application raises the general exhaust flow to keep the supply flow from

dropping below the minimum. See

Calculating Exhaust Flow Setpoint

for more

information.

In CV mode the active supply minimum is the setpoint. The active supply airflow

minimum is used to ensure that the room always receives enough supply air for proper

ventilation.

During CV operation, the air volume out of the supply box will equal the active supply

airflow minimum as long as this is sufficient to maintain proper room pressurization.

NOTE:

Regardless of the operation—VAV or CV temperature control—airflow out of both the

supply and general exhaust box will vary as needed in order to maintain proper room

pressurization whenever HOOD VOL changes.

AVS Calibration

Calibration of the air velocity transducer(s) is periodically required to maintain accurate

air velocity readings. Depending on the value of CAL SETUP, calibration takes place

either at fixed time intervals or whenever the application goes into unoccupied mode.

When calibration is in progress, CAL AIR equals YES. After calibration, CAL AIR

returns to NO.

The application uses Autozero Modules connected to AUTOZERO DO8. This means

that the supply and general exhaust flow control devices do not close during calibration

of the transducers.

NOTE:

The LCM does not monitor Fume Hood flow changes for 3 seconds during AVS

calibration.

Airflow Control

Supply flow and general exhaust are controlled by feedback loops that operate control

dampers so that the measured flows maintain their setpoints. The feedback gains SUP

P GAIN and GEX P GAIN are adjustable.

The damper command points (SUP DMPR CMD and GEX DMPR CMD) indicate the

rate at which the dampers move. The points do not indicate damper position. A value

of:

 100% indicates that the damper is being opened as quickly as possible

 10% indicates that the damper is being opened slowly

 0% indicates that the damper does not move at all

 A negative number indicates that the damper is closing at a corresponding rate; for

example, a value of -100% means the damper is closing at full speed