Air handling and air conditioning systems are used not only to maintain indoor temperature or temperature and humidity level but to provide required air quality that is determined by a number of factors as carbonic acid concentration (CO2), various smells, organic substances, tobacco smoke, etc.

Air comfort in enclosed spaces depends on qualitative characteristics of indoor air, quantity and purity of supply air.

The recommended supply air volume calculation is based on carbon dioxide volume released by a human being while breathing within a certain time period.

Actually, carbon dioxide concentration in permanently occupied premises where the gas composition fluctuates with respect to vital functions of human beings is the major criteria of the sanitary air condition.
Excess as well as deficiency of carbon dioxide in the air we breathe is harmful for the human health. If we analyze the reasons of stale air inside the premise we also have to find out s solution that provides us with balanced air exchange and the best indoor air composition.

The amount of fresh air supply is specified by acting hygienic regulations, i.e. 20-60 m3/h per each person for living areas and 80 m3/h for each sportsman in a sport hall and 20 m3/h per each spectator. Calculation is done on assumption of full audience or all inhabitants inside. However the number of people inside the premise can vary within a day and even within a week, especially in sport halls, cinemas, trade houses, audiences and other public premises.

That would be not reasonable to provide maximum air supply to half or one-third or even one-quarter full public premises, especially with low physical activity of people inside.

What is the way out? That would be more reasonable to control indoor air exchange rate with respect to specific condition and time to ensure comfort conditions for people inside.
How to do that?

The most efficient solution is to install one or two carbon dioxide sensors and volatile organic compound sensors in addition to temperature and humidity sensors. A carbon dioxide sensor is a perfect indicator of human presence and their physical activity. In time periods when the premises are fully occupied maximum air capacity must be provided.

As number of people inside the premise decreases the carbon dioxide concentration in the air also decreases and the need of intensive air exchange rate also decreases. In such a case the carbon dioxide sensor sends the signal to the ventilation system to decrease the air capacity or even shutdown the system if other air parameters are within permissible limits. Air exchange is controlled in two ways:

• In series switching on/off of two or more fans or use of double-speed fans;
• Smooth air speed (air capacity) control with a frequency converter which is a more preferable way.

Air speed control aims at permanent air parameters maintaining and wise utilization of electric energy for this purpose. This helps to reduce electric energy demand consumption required for air handling and distribution. However following the energy saving policy does not mean saving by all means because the health of people in the premise and their job performance is a priority.

Except for the indoor gas composition, another air quality factors include air temperature and humidity. Whereas the speed controller regulates fan speed (air capacity) through the carbon dioxide and volatile organic compound sensors, the temperature and humidity sensors regulate temperature and humidity level in the room. These sensors are used for continuous operation and permanent maintaining of temperature and humidity characteristics.

In case of temperature or humidity increase inside the premise, the controller sends the signal to increase the air exchange rate accordingly.
Smooth air flow control with respect to internal parameters is very popular in Europe.
So, the Zürich University has a very good practical experience in automatic control of air handling units that are used for servicing 76 premises (lecture halls, audiences, laboratories) with a total area 15 000 m2 and air exchange 385 000 m3/h.

The Swiss hygienic standards allow indoor carbon dioxide concentration
0,1–0,15%. The ventilation systems in the Zürich University ensure maintaining of this concentration with 1000-1500 ml/m3 (1000–1500 ppm). That differs from external CO2 concentration by 0,06–0,07% only. To reach such effect the supply air speed must be within 12-30 m3/h per each person inside. For control of CO2 concentration the carbon dioxide sensors with measuring range up to 2000 ppm are used.

As a result of controllable air exchange technology implementation the operating time of ventilation and air handling units decreased by more than 40% as compared to standard operation mode. Consecutively the energy demand and maintenance costs reduced accordingly.

Carbon dioxide sensors have indoor, duct, display modifications. The measuring method is based on infra-red absorption. One carbon dioxide sensor may be mounted into a fan casing together with a volatile organic compound sensor or a humidity sensor.

In case of switching of two or more fans in series or in case of double-speed fan applications the ventilation system switches on only after the prior “approval command” of the sensors that determine the air quality. Air handling units turn on as the indoor air quality becomes too low and turns off when the air quality becomes consistent with set parameters. The set parameter range determines the frequency of the system turning on and off.

For smooth motor speed control and avoiding too high starting current we recommend using frequency converters with the output frequency up to 150 Hz.

Vents company is the world ventilation leader and the AirVents air handling unit is one of the best corporate ventilation products.
AirVents air handling units are equipped with new control and automation system that ensures professional but easy and user-friendly control of ventilation and air conditioning systems. This automation provides comfortable microclimate in any premises with minimum expenses.

The core of this system is a software programmable controller that operates jointly with a user’s remote control panel. The automation and control systems have easy and simple interface for correct regulation of air parameters in ventilation and air conditioning systems.

The air handling units AirVents are equipped with individual automation system that can be integrated into a building management system that includes specially designed controllers for ventilation system.

This solution ensures maintaining of set air parameters in any building, flexible control and settings.

Automation is designed to reduce operating costs and energy demand as well as increase of the equipment reliability.

National standardization associations in 27 counties approved a relevant European Standard EN 13779 “Ventilation performance requirements for ventilation and room-conditioning systems” that was prior approved by European Committee for Standardization on 16th of January 2004.

The expert experience of the balanced air exchange technology for periodically visited premises in EU proves that this technology reduces energy demand by 20-70% as well as reduces operating costs and ensures high air quality.

Author: Tarasenko Y.