Application

Sequence of Operation

Pressurization Control

Siemens Industry, Inc.

Application Note, App 6750

140-1328

2015-07-07

Sequence of Operation

The following paragraphs present the sequence of operation for BACnet LCM-OAVS

VAV Room Pressurization with HW Reheat and Two Venturi Air Valves.

Pressurization Control

The goal of pressurization control is to maintain a fixed difference between the

volumes of total supply air and total exhaust air (see the following figure). The

controller selects supply and exhaust setpoints to balance flows while meeting supply

air requirements, and the supply and exhaust flows are individually controlled by

respective feed-forward table statements and feedback loops in order to meet the

setpoints.

Room Airflow Balance

The difference between total supply flow and total exhaust flow is the room airflow

balance as shown in these calculations:

VOL DIFFRNC = TOTL EXHAUST – TOTL SUPPLY

-or-

VOL DIFFRNC = (HOOD VOL + GEX AIR VOL +

OTHER EXH) – (SUP AIR VOL + OTHER SUP)

The controller uses these calculations to maintain VOL DIFFRNC at the VOL DIF

STPT.

NOTE:

VOL DIFFRNC and VOL DIF STPT are positive numbers in a room that is negatively

pressurized and negative in a positively pressurized room.

Application 6750 has the ability to maintain a different volume differential setpoint

during occupied mode than during unoccupied mode. When OCC.UNOCC = OCC,

VOL DIF STPT = OCC DIF STPT. When OCC.UNOCC = UNOCC, VOL DIF STPT =

UOC DIF STPT.