An adequate supply of fresh air is a necessary condition for life. If a person does not have a supply of air at all, after a few minutes, then loses consciousness and dies. Everyone is aware of this fact, but does everybody realize the consequences of long-term breathing a poor quality air? Stuffy air causes fatigue and drowsiness, which naturally reduces the efficiency and ability to concentrate. Nevertheless, many people spend most of the day in an environment that without ventilation. Children in schools where the levels of carbon dioxide move frequently in risk levels, adults at work, where they sit in closed offices with other colleagues. Even at homes the situation is not better, especially overnight, when people close their windows to save energy of heating.
A solution would be regular ventilation – once per hour open window. During the day it is possible but during the night it is difficult. It is advisable to use a much easier solution – a system of forced ventilation.
Forced ventilation systems usually operate on the basis of the time-controlled regulation, which means that the volume of the exchanged air is depending on time intervals. In this case, it may sometimes occur that the air is exchanged more intensely than it is necessary in the situation. It is the minus of the controlled ventilation – the air is redryied and there is wasting of energy. But even this only minus is possible to solve by a more economical way ventilation – ventilation controlled by a real need. You say what is meant by the “real need”? It is just the necessary amount of air which is needed to bring to the interior, depending on the actual requirements. There are basically two systems controlled by a real need – VaV and DCV.
DCV system (an acronym derived from the English designation Demand Controlled Ventilation) is a system of a control of ventilation by air quality sensors. DCV system is a system where the valves on the supply line are continuously adjustable. Each valve has again its air quality sensor, which, however, by means of its analog signal continuously controls the necessary amount of supply air. The advantage is the more constant indoor air quality; the disadvantage is the higher investment costs of servo-drives of the valves with the possibility of a continuous control.
Each office is equipped with a sensor of carbon dioxide – CO2 and a valve on the piping. The inlet valve in each office is controlled by the servo-drive, which is controlled by the CO2 sensor. If all offices are empty and there isn’t a stuffy, the air recovery unit operates at minimum power, because all valves are closed and it is easy to achieve the desired pressure in the pipeline. When people come into the office, the concentration of CO2 starts gradually to rise and in excess of a predetermined level of concentration (in our case 950 ppm) the valve gives the command to open the inlet valve. As a consequence, the pressure in the supply line and the recovery unit will automatically increase their power (they try to maintain the predetermined pressure in the supply line) and will therefore ventilate the office which has the stuffy air. Once the concentration of CO2 falls below the lower level (in our case it is 800 ppm) the inlet and exhaust valve again close a the given office stops to be ventilated. Thus, offices are selectively ventilated and the performance of the recovery units or the requirement on the total required amount of air is passed to the recovery unit through the current pressure in the supply line.
Valves in the individual offices are equipped with such type of servo-drive, which provides only the control of valves by means of the open / closed system, it does not enable the continuous positioning of the closing valves.
When you first start the system it is necessary to set the system so that each office was ventilated with sufficient amount of air according to the number of people working there.
Regulation of the ventilation system was designed so that first all valves were fully open in all offices and the outermost one the amount of incoming air was measured. Then there was a gradual increase of pressure in the supply piping behind the ventilation unit so as to achieve the required air supply to the outermost office according to the number of people working in it. Then we progressed toward the ventilation unit through each office so that it was always set the necessary amount of supply air by setting the maximum open position of the inlet and outlet valve. The maximum opening angle of the inlet and outlet valves was therefore for each office individually adjusted using the adjustable end stop. Ventilation unit was then switched to normal operation, where the required pressure in the supply line was set precisely for the value that was detected during the initial regulation. This ensures the necessary air exchange in every office in m3 / h corresponding to the number of workers in the office and even in the case that all offices are fully occupied and ventilated.