Variation of air condition parameters, in the conditions of the presence of carbon dioxide, in the duct of ventilation installations

. Industrial ventilation takes into account the quality of the air inside the IAQ (Indoor Air Quality) and the degree of exposure of workers. In industrial targets the emission regime of contaminants can be 10–100 times higher than in non-industrial targets, but for many pollutants the IAQ levels can be the same. (cid:3) The parameters of the state of the air which are of interest for the technique of ventilation and air conditioning are: air temperature and humidity, atmospheric pressure and air speed. For the study on the variation of the air state parameters inside the industrial ventilation installations, in the experiment laboratory, on the study of the industrial ventilation systems, the experiment was performed on the variation of the air state parameters in the conditions of circulation through the ventilation duct. an amount of carbon dioxide. Prior to the introduction of carbon dioxide into the duct, the ventilation system was started, at nominal parameters. During the experiment, the operating parameters of the drive motor were changed using a frequency converter on levels 50; 40; 30; 20; 10 and 5 Hz. The ventilation system used was structured by means of flow converters which were fixed in the open position. The ventilation system as well as the flow variators were operated by the SCADA type command and control system The paper will present the analysis of the variation of state parameters (temperature, humidity, absolute pressure and air speed) by introducing a constant amount of carbon dioxide in the ventilation duct. (cid:3) Keywords


Introduction
The state of a thermodynamic system is defined by all the physical, chemical, biological, etc. quantities that characterize it.Some of these quantities can be measured directly and are called state parameters, for example temperature (T), volume (V), pressure (p).State parameters change their value when external conditions change.
Depending on the spatial or temporal variation of the state parameters, a thermodynamic system can be in: -steady state -its state parameters remain constant in space and time; -steady state -different parameters reach constant values, but which are nevertheless different from those in other areas of the system; -stationary non-equilibrium state -parameters vary from one point to another, but remain constant over time; -non-stationary non-equilibrium state -parameters vary both in space and time.Thermodynamic equilibrium can only be achieved in closed systems.In open systems, specific to biological systems, energy processes do not reach the state of equilibrium, but only a stationary state (dynamic balance) of maintaining the organization of energy processes, which ensures development through the exchange of substance and energy.(Al. Cristea, 1968), (Bancea O., 2009).

Concepts of air state parameters
The main role of the ventilation installations, and especially of the air conditioning ones, is to create indoor conditions and air quality independent of external atmospheric influences.
The main quantities involved in defining ventilation and air conditioning installations are: air pressure, air velocity and temperature.
Temperature, humidity, absolute pressure and air speed are meteorological elements when their hourly or daily variation is followed, and they are climatic elements when their behavior is followed over long periods.
These parameters are the most important characteristics of the ambient air, having a direct influence on human health and on the effectiveness with which they perform their work.(Alexandru Cristea, et al., 1971), (Băltărețu R., Teodorescu C., 1970), (H.Goodfellow, E. Tahti, 2001).

Absolute pressure
Pressure is one of the most important state parameters characterizing the state of a fluid.It is defined as the ratio between the force with which a fluid acts on a surface and its area.
Within fluids each layer serves as a support for all the layers above it.Gases being compressible, the action between the layers that compose them makes the density higher the lower the layer is placed.Therefore, the static pressure of the gas will also increase in the same direction.(Al.Cristea, Terețean T.Ș., 1976), (I.Matei, R. Moraru, M. Samoilă, G. Băbuț, 200), (W.L. Le Roux, 1972).
Different types of pressures can exist in nature and in technical installations: a) atmospheric pressure pb.The pressure exerted by the gaseous envelope that surrounds the globe is called atmospheric pressure or barometric pressure.It varies with: the altitude (due to the weight of the air), with the state of the weather (the date of the movement of atmospheric air masses) and with the geographical position on the globe; b) absolute pressure pa.Absolute pressure is the pressure of a fluid considered relative to absolute zero pressure.It is the pressure that is used in all thermotechnical relations; c) overpressure ps.When in technical installations the absolute pressure is higher than the atmospheric pressure, the difference between them is called overpressure or manometric pressure; d) depression pv.When in technical installations the absolute pressure is lower than the atmospheric pressure, the difference between them is called depression, underpressure, vacuum or vacuumometric pressure.

Air temperature
The air temperature can be of two types: a. Outside air temperature, te.An important factor in the dimensioning of ventilation and air conditioning installations, it is determined near the earth's crust, on the one hand, by solar radiation and the absorption or average heat loss of the soil and, on the other hand, by the wind.
b. Indoor air temperature, ti.In the work area, this is a relatively good basis for characterizing a microclimate.Relatively small variations in the temperature of the indoor air are noticed immediately by the human body, which must quickly cope with the new changes, in order to maintain a constant heat exchange between the person and the surrounding environment.(Cioclea D., 2020), (I.Matei, R. Moraru, M. Samoilă, G. Băbuț, 2000), (. H. J. Burrows, R. Hemp, F. H. Lancaster, J. H. Quilliam, 1974).

Air humidity
In ventilation and air conditioning technology, the water vapor contained in the atmospheric air is of particular interest, because its quantity greatly influences the physical properties of the gas mixture.At a given temperature and barometric pressure, the water vapor content in one kilogram of air cannot exceed a certain limit, which is the amount of water vapor that saturates the air.

Air speed
The speed of indoor air movement is another parameter of thermal comfort.The feeling of discomfort is felt all the more when the temperature of the air in motion is lower than the temperature of the environment, and this, all the more when it falls from a certain part of the body (neck, ear).
To work well, a suction device must create at the point of release a strong enough airflow velocity to entrain and direct all the harmful particles towards the suction nozzle.The condition that is imposed is that the air speed at the place of generation is higher than the own speed of movement of the particles.For a particle floating in the air to be carried by an air current, it must have a velocity greater than the particle's suspension velocity.By the suspension velocity of a particle floating in the air is meant the maximum velocity of a current that does not yet succeed in displacing the particle from its equilibrium position.(Al. Cristea, 1968), (D.Cioclea, C. Lupu, I. Gherghe, 2013), (I.Gherghe, D. Cioclea, 2018).

Variable structure experiment system for the study of complex industrial ventilation networks
Fans are mechanical devices used in ventilation installations to circulate air.Depending on the construction characteristics, they ensure the required air flow in an installation and the coverage of pressure losses.Centrifugal fans are used to cover relatively high pressure losses, and axial fans for relatively low pressures.
The experimental system consists of a centrifugal fan-motor aggregate and a complex structure of rectangular tubing with dimensions of 300/400 mm.The ductwork is located on the southern, western and northern wall, the fresh air inlet, respectively the air outlet is made on the eastern wall, figures 1 ÷ 3.
Fig. 1 The structure of the experiment system Fig. 2 The structure of the experiment system on the northern wall on the west wall Fig. 3 The structure of the experiment system on the southeast wall The automation system of the flow variators is made with the help of servomotors and the command is made centrally with the help of a control desk, respectively with the help of a SCADA type program, figures 4 ÷ 8.

Experiments on the variation of state parameters when circulating through the ventilation network of carbon dioxide, CO2
The experimental system for the study of the variation of air condition parameters inside the ventilation installation is composed of a data acquisition system consisting of a humidity sensor, a temperature sensor, respectively a pressure A system for measuring the dynamic pressure in order to determine the circulation speed was also used, two MSA ALTAIR 5X and DRAGER X-am 8000 type multigas detectors, which can detect concentrations of O2, CO2, CO and CH4.
At the same time, a KIMO AMI 310 kit was used to measure air condition parameters, respectively a FLUKE 345 PQ electrical parameters measuring device.
The cross-section of the pipe is rectangular with sides of 300/400 mm, having a section of 0.12 m 2 ."Experimental system with variable structure for the study of complex industrial ventilation networks" was used to analyze the dynamics of toxic atmosphere formation.
The initial experimental conditions were as follows: The system consists of a cylinder of compressed carbon dioxide at a pressure of 65 bar, at a concentration of 100 % Vol. and a pressure reducer.The gas was introduced into the ventilation duct by means of a hose with an internal diameter of 8 mm.The discharge of carbon dioxide inside the ventilation pipe was achieved through the hose fixed on the wall of the pipe.
Prior to the introduction of carbon dioxide into the pipe, the ventilation system was started, at the nominal parameters.During the experiment, the operating parameters related to the drive motor were changed using a frequency converter in steps of 50; 40; 30; 20; 10 and 5 Hz.The ventilation system used was structured with the help of flow variators that were fixed in the open position.
The ventilation system as well as the flow variators were operated by the SCADA command and control system shown in figure 4 The variation of the monitored/measured parameters in relation to the frequency, under the conditions of the introduction of carbon dioxide into the ventilation pipe, is presented in table no. 2 The variation of the carbon dioxide concentration, the electrical parameters and the state of the air measured in the ventilation duct, in relation to the frequency variation, is shown in figures no. 28-37.

CONCLUSIONS
This work allowed highlighting the following findings: ➢ The parameters analyzed during the experiment regarding the variation of air condition parameters inside a ventilation installation by introducing a constant amount of carbon dioxide, were: condition parameters, aerodynamic parameters and electrical parameters; ➢ Variation of state parameters were analyzed in relation to frequency variation.In this sense, the operating regimes of the drive motor were used for the frequencies of 50, 40, 30, 20, 10 and 5 Hz; ➢ The air condition parameters, determined with the data acquisition system, showed a variable evolution in relation to the respective time with the frequency variation: • The air temperature had a variable evolution between 15.3 and 15.4 °C at 50 Hz, between 14.1 and 15.3 °C at 40 Hz, between 13.5 and 13.8 °C at 30 Hz, 13.2 to 13.6°C at 20 Hz, 13.1 to 13.2°C at 10 Hz, and 13.1 to 13.3°C at 5 Hz;

Cos ϕ
• Air humidity showed a variable evolution between 32.5 and 33.3 % RH, at 50 Hz, between 32.9 and 36.8 % RH at 40 Hz, between 36.1 and 38.1 % RH at 30 Hz, between 37.0 and 38.9 % RH at 20 Hz, between 37.1 and 38.9 % RH at 10 Hz and between 37.4 and 38.8 % RH at 5 Hz; • The differential pressure showed a variable evolution between 0.0 and 182.0 Pa in relation to the frequency used; • The absolute pressure showed a variable evolution between 949.1 hPa and 949.3 hPa in relation to the frequency used; ➢ The evolution of carbon dioxide concentration in relation to frequency showed a variation between 0.75 and 1.84% Vol.; ➢ The electrical parameters: the motor supply voltage, the absorbed electric current, the absorbed power and the power factor, presented a variable evolution in relation to the frequency; ➢ The air condition parameters determined manually inside the piping had a variable evolution in relation to the frequency as follows: air temperature with values between 13.1 and 15.4 °C, air humidity with values between 33.5 and 38, 6% RH, barometric pressure between 949.3 and 949.8 hPa, static pressure between 1 Pa and 164 Pa and air velocity inside the tubing between 0.8 and 9.12 m/s ; ➢ The circulation process of air enriched with suffocating/asphyxiating gases such as carbon dioxide presents an increased risk in terms of changing the functional parameters of the fan even in the conditions of a constant flow of carbon dioxide circulated; ➢ Under the conditions of an absolute, constant flow of carbon dioxide circulated by the ventilation installation, if the functional parameters of the fan change negatively, it may end up in a situation where the level of carbon dioxide concentration increases rapidly, exceeding the maximum limit allowed by the legislation in effect, for example twice in the case of experimentation;

Table nr .
2 Variation of air condition parameters, electrical parameters and carbon monoxide concentration in relation to electrical frequency