Calculation of ventilation with mechanical induction
(calculation of forced ventilation)
Forced ventilation, it is the mechanical impulse ventilation foreseeing the existence of certain devices for the transport of air flow in its system. These devices are called fans, the selection of which is the ultimate goal, which involves calculation of forced ventilation, that is, the calculation of ventilation with mechanical induction.
Basic stages of calculation
Calculation of ventilation with mechanical induction (forced ventilation) can be divided into several stages. The first of these, as foreseen by natural ventilation calculation, is to determine the required indoor airflow. There are several methods for calculating the amount of tidal air. According to one of them, the inflow of air, L, m3/h, for the ventilation and air conditioning system should be determined by calculation and accept biggest of the expenses necessary for the provision of sanitary-hygienic norms and (or) standards of explosion safety.
Necessary flow of inflow air is determined separately for the warm, transition and cold period of the year under the following conditions:
- for surpluses of apparent heat:
L = LWZ + [3,6Q - cLWZ (tWZ - tin )] / [c(ti - tin)] , where
LWZ - air flow that is removed from the service area or working area of the room by systems of local suction and for technological needs, m3/h;
Q - excessive apparent heat flow in the room, W;
tWZ - the temperature of the air removed from the service area or the working area of the room by systems of local suction and for technological needs, оС;
tin - air temperature in the room, оС;
ti - the temperature of the air removed from the room outside the service area or the working area, оС;
c = 1,2 kJ/m3 оС - heat capacity of air.
- by weight of harmful and explosive substances released:
L = LWZ + [mРО - LWZ (qWZ - qin)] / (qi - qin), where
mРО - the mass of each of the harmful or explosive absorbent substances coming from the air space, mg/h;
qWZ qi - concentration of harmful or explosive substance in the air, which is removed in accordance with the area served by either the working area of the room and outside, mg/m3;
qin - concentration of harmful or explosive substance in the air supplied to the premises, mg/m3;
At simultaneous allocation in the premises of several harmful substances having the effect of the total action, the air exchange should be determined, summing up the air flow, calculated for each of these substances.
- for excess moisture (water vapor):
L = L + [W - 1,2(dWZ - din)] / [1,2(di - din)], where
W - excess moisture in the room, g/h;
dWZ - moisture content of air that is removed from
the working area, or the zone, which is serviced by systems of internal nozzles and for technological needs, g/kg;
di - moisture content of air removed from the premises outside the working area or service area, g/kg;
din - moisture content of air supplied to the room, g/kg;
For rooms with excessive moisture, it is necessary to check the adequacy of air exchange to prevent the formation of condensate on the inner surface of the external enclosure structures under the design parameters outside air during the cold period of the year.
- for excess heat:
L = LWZ + [3,6QHF - 1,2LWZ (IWZ - Iin)] / [1,2(Ii - Iin)], where
QHF - excess full heat flow in the room W;
IWZ - specific air enthalpy that is removed from the working area or area serviced by the system by local suction systems and for technological needs, kJ/kg;
Iin - enthalpy of air supplied to the room, kJ/kg;
Ii - specific air enthalpy that is removed from the outside of the service area or from the working area, kJ/kg;
- at normalized multiplicity of air exchange:
L = VР n, where
VР - room volume, m3; for premises of 6 m or more height should be taken VР = 6 A, where A - area of the room m2;
n - normalized multiplicity of air exchange, h -1.
- on the standardized specific flow of supply air:
L = Ak;
L = Nm, where
k - normalized flow of supply air to 1 m2 floor area of the room, m3 /(h m2);
m - normalized flow of supply air for 1 person, 1 workplace, 1 visitor or unit of equipment, m3/h.
Having determined the type of building in which the premises are located, guided by the relevant regulatory document for the given building and the technical requirements for the design of the ventilation system, make appropriate calculations. From the calculated values of air exchange for the further calculations the maximum is taken. If, often in practice, engineering design questions do not contain complete information, or do not exist at all, the recommended multiplicity of air exchange for the respective types of premises is guided. This is a initial stage for compulsory ventilation calculation.
At the next stage, based on the technical assignments and building drawings of the building, begin to construct axonometric schemes of ventsystems. The diagrams indicate the distribution of air flow in rooms, the direction and amount of air in each of them.
Further, having established the amount of air through the highways and branching systems, the number of air separation devices, make the calculation of the cross sections of vent channels and the selection of lattices, anesthetics or diffusers, respectively, their live cross section. In doing so, the recommended speeds of air in the channels and at the exit of the grids for different types of buildings are taken into account. For example, for industrial premises, the speed in steel ducts should not exceed 8 m/s, at the exit from the grids - 3 m/s, for public buildings, these values are respectively 5 and 1.5 m/s, residential - 4 and 1 m/s . Conduct calculation and selection of the relevant components - heaters, filters, various valves, etc. This can be counted as a third step for calculating mechanical ventilation.
At the fourth stage, compulsory ventilation is performed on the aerodynamic calculation of the vent system. It takes the longest and most loaded branch of the air network. Taking into account the material of air ducts, their length, number and types of local resistances (taps, configurations and diffusers, tees, etc.), taking into account the pressure drop on the grids, the aerodynamic resistance of the ventilation network is determined in the components of the system.
And finally, the final phase for calculating ventilation with mechanical induction is actually the fan's choice. The values of the required cost, the resistance of the network, as well as some of the design features of the room being served are decisive factors when choosing a fan or venting system for a given vent system.