Forced ventilation systems usually operate based on so-called time control, which means that the volume of exchanged air depends on time intervals. In this case, it may sometimes happen that the air is exchanged more intensively than necessary for the given situation. This is a downside of controlled ventilation – it leads to over-drying of the air and also to energy waste. However, even this single downside can be solved in a more economical way of ventilation – ventilation controlled by actual demand.
You might be wondering what to imagine under the term “actual demand”? Simply put, it refers to the necessary amount of air that needs to be brought into the interior based on real requirements. Systems controlled by actual demand are basically two – VaV and DCV.
DCV System
The DCV system (from English Demand Controlled Ventilation) is a continuous control ventilation system using air quality sensors. Its immediate performance is therefore dependent on the current air quality. The DCV system has dampers on the supply duct that are continuously adjustable. Each damper has its own air quality sensor, which continuously controls the necessary amount of supply air with its analog signal. The advantage is a more constant quality of indoor air, while the disadvantage is higher investment costs for the servo drives of the dampers with the possibility of continuous control.
DCV System Diagram
VAV System
VAV stands for Variable Air Volume, meaning ventilation with variable air volume. This ventilation system is used for the ventilation of office spaces directly in our company. Ventilation is ensured by a recuperative unit with a plate counterflow heat exchanger and control electronics from our own development. This unit operates in a mode that maintains constant pressure in the outlet/supply duct to individual offices.
Each office is equipped with one carbon dioxide sensor and a damper on the duct. The supply damper in each office is controlled by a control servo drive, which is regulated by the CO2 sensor. If all offices are empty and there is unexhaled air in them, the recuperative unit operates at minimal performance because all dampers are closed, and the desired pressure in the duct is easily achieved. When people enter the office, the concentration of CO2 gradually starts to rise, and upon exceeding the preset concentration level (in our case, 950 ppm), the sensor sends a command to open the supply damper. As a result, the pressure in the supply duct drops, and the recuperative unit automatically increases its performance (it tries to maintain the preset pressure in the supply duct) and thus begins to ventilate the office that has exhaled air. Once the concentration of CO2 drops below the lower limit (in our case, 800 ppm), the supply and exhaust damper closes again, and the given office stops being ventilated. This is how individual offices are ventilated, and the performance of the recuperative unit or the demand for the total necessary amount of air is transmitted to the recuperative unit via the current pressure in the supply duct.
The dampers in individual offices are equipped with a type of servo drive that ensures only the control of the dampers in an open/closed system, thus not allowing continuous adjustment of the closing damper positions.
The dampers in individual offices are equipped with a type of servo drive that ensures only the control of the dampers in an open/closed system, thus not allowing continuous adjustment of the closing damper positions.
VAV System Diagram
Initial System Regulation
During the first startup of the system, it was necessary to adjust the system so that each office was ventilated with a sufficient amount of air according to the number of people working in it.
The regulation of the ventilation system was carried out by first fully opening all dampers in all offices, and the amount of incoming air was measured at the furthest one. Then the pressure in the supply duct behind the ventilation unit was gradually increased to achieve the necessary air supply to this furthest office according to the number of people working in it.
Then, the process moved towards the ventilation unit through each office, setting the necessary amount of supply air using the maximum open position of the supply and exhaust damper. The maximum opening angle of the supply and exhaust dampers was thus individually set for each office using an adjustable end stop. The ventilation unit was then switched to normal operating mode, where the required pressure in the supply duct was set to the value that was determined during the initial regulation.
CO2 Concentration Record in the Meeting Room and Office Ventilated by the Above-Described VAV System
