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Outside Air Control

General

Outside Air Control (OAC) is a subsystem of the automatic control system of HVAC systems and usually includes control of the temperature, humidity, pressure, and flow rate of the ventilation air and its mixture (if any) with return air from the conditioned space. These automatic control sequences damper and equipment operation to meet ventilation requirements and provides safe operation of the equipment, using pneumatic, mechanical, electrical, and electronic control devices.

OAC is typically integrated with the air handling units using a damper section. In some cases, a separate OA unit will be used to condition only the intake air and then subsequently and separately mix it with return air or distribute the conditioned OA directly to the space in 100% fresh air applications.

Outdoor Air Control of Draw-through Air Handling Unit

The OAC should also consider controlling the flow of exhaust air in order to maintain the desired air pressure in the building. In most buildings a slight positive pressure is desired to minimize any air infiltration from undesired sources. Fixed minimum outdoor air control provides ventilation air, space pressurization (exfiltration), and makeup for exhaust fans.

Dampers

Automatic dampers are used in the ventilation component of air-conditioning to control outside airflow. They may be used (1) for modulating control to maintain a controlled variable such as mixed air temperature or supply air duct static pressure; or (2) for two-position control to initiate operation such as opening minimum outside air dampers when a fan is started.

While parallel-blade dampers are adequate for two-position control, opposed-blade dampers are preferable, because they normally provide better control.

Control of Outdoor Air Quantity

The following is an extract from the ASHRAE HVAC Applications Handbook chapter titled Automatic Control.

For systems without return fans, the outdoor air damper is interlocked to remain open only when the supply fan operates. The outdoor air damper should open quickly when the fan turns on to prevent excessive negative duct pressurization. In some systems, the fan's on-off switch opens the outdoor air damper before the fan is started. The rate of outdoor airflow is determined by the opening of the damper and by the pressure difference between the mixed air plenum and the outdoor air plenums.

For systems with return fans, there are two variations of fixed minimum outdoor air control. Minimum outdoor airflow is determined by the pressure (airflow) difference between the supply and return fans. If the outdoor air supplied is greater than the difference between the supply and return fan airflows, a variation of economizer cycle control is used.

In systems using 100% outdoor air, all air goes to the fan and no air is returned. The outdoor air damper is interlocked and usually opens before the fan starts.

Economizer cycle control reduces cooling costs when outside conditions are suitable, that is, when the outdoor air is cool enough to be used as a cooling medium. If the outdoor air is below a high-temperature limit, typically 65°F, the return, exhaust, and outdoor air dampers modulate to maintain a ventilation cooling set point, typically 55 to 60°F. The relief dampers are interlocked to close, and the return air dampers to open, when the supply fan is not operating. When the outdoor air temperature exceeds the high-temperature limit set point, the outdoor air damper is closed to a fixed minimum and the exhaust and return air dampers close and open, respectively.

Enthalpy economizer control

considers enthalpy rather than temperature. Here the high-temperature limit interlock system of the economizer cycle is replaced in order to further reduce energy costs when latent loads are significant. The interlock function can be based instead on (1) a fixed enthalpy upper limit, (2) a comparison with return air so as not to exceed return air enthalpy, or (3) a combination of enthalpy and high-temperature limits.

VAV warm-up control

during unoccupied periods requires no outdoor air; typically, outdoor and exhaust dampers remain closed. However, in systems with a return fan, the outdoor air damper should be positioned at its minimum position, and supply airflow cfm should be limited to return air airflow cfm to minimize positive or negative duct pressurization.

Night cool-down control

(night purge) provides 100% outdoor air for cooling during unoccupied periods. The space is cooled to the space set point, typically 9°F above outdoor air temperature. Limit controls prevent operation if outdoor air is above space dry-bulb temperature, if outdoor air dew-point temperature is excessive, or if outdoor air dry-bulb temperature is too cold, typically 50°F or below. The night cool-down cycle is initiated before sunrise, when overnight outside temperatures are usually the coolest. When outside air conditions are acceptable and the space requires cooling, the cool-down cycle is the first phase of the optimum start sequence.

Links to Related Topics

Ventilation
Indoor Air Quality
Filtration
Controls
Central Plant System
Central (or Built-Up) System
All-Air Central Systems
All-Air Central Reheat Systems
All-Air Central Dual Duct
All-Air Central Multizone
All-Air Rooftop
Air-Water Central Systems
All Water Central Systems
Two Pipe System
Three Pipe System
Four Pipe System