CO2 as an energy-saving control
• An optical filter that passes only the required wavelength.
• A detector to measure the amount of infrared energy. The Command family of wall modules from CentraLine includes models for CO2 sensing and control. The greater the level of CO2 in the chamber, the less infra-red energy reaches the detector. This has been found the most accurate measuring method, the CO2 being selectively filtered among all other influences. There is no need for calibration. Sensors may also contain a proportional controller or/and a simple limiting switch; small applications can then be handled directly by the sensor/controller. The sensors provide a linear output signal (0 to 10 V or 4 to 20 mA) representing the CO2 concentration in parts per million (ppm). The signal can be scaled for different measuring ranges, depending on the sensor accuracy and application needs. In general, sensors should have a range of 0 to 2000 ppm CO2. In large, open buildings such as cinemas and theatres, the sensor should be in the exhaust-air duct. In other buildings it is better to have a wall-mounted unit for individual control in every room. Applying control The design of the CO2 control system depends on the HVAC system. In new systems, the controls are designed to provide a fresh-air supply at the minimum fan speed. If the minimum speed is insufficient for air quality, heating or cooling, the fan speed increases. Where existing systems are to be adapted for CO2 control, the best of several solutions should be selected. • In small installations it may be acceptable to switch the fan off and on according to the sensor information. There must be a limiting switch in the sensor.
• Systems with mixing chambers can be extended with CO2 proportional control and a means to select the maximum signal from the existing temperature control and the new air-quality control. This extension kit is therefore independent of the existing building automation system.
•Fresh-air systems can only be extended with air-quality control output to the fan frequency converter. In some cases the fan motor must be replaced to apply a frequency converter (as the insulation class will be too low). The controls will comprise a CO2 sensor, a proportional controller and a maximising device.
• In the case of systems with a mixing chamber and a frequency converter, only the building-automation application can provide the necessary functionality, so it may be necessary to upgrade the complete air-conditioning control. Air-quality sensors based on oxidixable gases, such as odours and carbon monoxide, can be used where CO2 is not the primary control variable — such as restaurants and changing rooms in sports facilities. The percentage energy savings achieved by applying air-quality control in a HVAC control system are typically in the double-digit range. The controls also increase HVAC plant life and reduce equipment noise. Building managers should now seriously consider CO2 control of HVAC systems now that it has become a highly cost-effective means to save energy and prolong the life of HVAC plant. Harry Swinbourne is UK business manager for CentraLine by Honeywell.