Application

Sequence of Operation
Pressurization Control
8
Siemens Industry, Inc.
Application Note, App 2922
140-1301
2015-07-07
Sequence of Operation
The following paragraphs present the sequence of operation for LCM-OAVS
Application 2622: VAV Room Pressurization with HW Reheat and Slow Venturi Air
Valves — One Exhaust, One Supply.
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.
When operating the supply or general exhaust without a flow sensor, the respective
flow is controlled in open loop mode directly from the Venturi tables.
Optional Rate Limiting of Actuators
Under specific circumstances, such as when operating open loop with setpoint tracking
and differently sized supply and exhaust, it may be necessary to adjust the actuator
travel rates to improve pressurization control.
The example below illustrates when a rate limit might be needed, such as when equal
travel times for the supply and exhaust actuators yield large differences in their
respective flow rates.