Variable Air Volume System with VSD Fan Control

Application Principle

Variable Air Volume or VAV systems have some kind of sysetm to control the air volume.  The simplest control is On-Off control of fans but the damper control is more common and variable speed control of fans is the most economic method to control the air volume.  The system is usually designed to maintain constant static pressure in the supply duct.

The positive building static pressure is kept by regulating the airflow of both the supply and return fans.  Terminal devices or VAV boxes supply the conditioned space with a variable flow of constant temperature airflow.  Plant maintenance is centralised. The system may be designed so that cooling by outside air can be obtained without operating the refrigeration plant.  Room temperature can be achieved by:

• Air volume control
• Bypass damper control
• Control of refrigeration capacity

Detailed Description

This variable air volume system brings outside air and return air to the Air Handling Unit (AHU) (3). where the incoming air (8.) temperature and humidity can be controlled.  The main AHU components are supply fan (2.) heating coil (5.) cooling coil (6.), filter (7.), and humidity control equipment (not in above).  The three way valves for heating and cooling are remotely operated.  The temperature in the individual rooms is measure by thermostats (11.), which directly control the dampers in each room.  The supply air fan is speed controlled by an AC drive (4.) and it delivers the air to individual rooms throughout the building by supply air ducts.  The AC drive controls the air volume by keeping the air pressure constant.  The pressure is measured by the sensor (13.). The return air fan (1.) delivers the exhaust air (9.) out of the building or part of the air (10.) is returned back to the AHU.  The fan is controlled by an AC drive (14.) which keeps the room pressure measured by sensor (12.) constant.

Performance in Creating comfort zone

Constant volume - variable temperature system have been dominating the HVAC applications in the past for obvious reasons.  the implementation of variable volume control has been done with dampers or other mechanical control methods.

The advantages of variable air volume system implemented with variable speed control are:

• Fast control to keep the comfort zone limits
• Low consumption of electrical energy
• Lower consumption of heating and cooling energy than in constant air volume systems
• Simpler to keep noise level down