Page contents
How do I calculate the ventilation rate of a room and size its duct network?
The ventilation rate is first set by regulation: fresh air flow per occupant in workplaces (25 m³/h per person in an office, 30 in a meeting room, 45 to 60 in a workshop) or extraction flows per room in housing (French order of 24 March 1982, tables below); it is also checked as an air change rate, the number of times the room volume is renewed per hour (Q = n × V). This flow, divided by an air velocity chosen to limit noise (3 to 5 m/s in quiet rooms, 6 to 8 m/s in main ducts), gives the section and the diameter of the ducts; the pressure losses of the network at this flow set the fan pressure. Ventilating costs twice: the electricity of the fan and, above all, the heating of the fresh air, 0.34 Wh per m³/h and per degree-hour. Example: an office of 10 people needs 250 m³/h, i.e. a 160 mm duct at 4 m/s, and heating that fresh air costs 4,700 kWh a year for 2,321 degree-days.
Ventilation rate and sizing formulas
- Q: fresh air flow, in m³/h; N: number of occupants; qpers: regulatory flow per person (French Labour Code: 25 to 60 m³/h depending on the room)
- n: air change rate, in volumes per hour (0.5 to 1 ach in housing, 2 to 6 in workplaces); Vroom: volume of the room, in m³
- D: duct diameter, in m, for the flow Q in m³/s and the chosen air velocity v, in m/s
- Eheating: annual energy to heat the fresh air, in Wh; 0.34 Wh/(m³·K): heat capacity of air; DD: heating degree-days of the site (base 18 °C), in K·day/year
- Pfan: absorbed power, in W, for Q in m³/s, ΔP pressure loss of the network in Pa and η fan efficiency
Fresh air flow and duct diameter according to the room
| Room | Occupants | Flow per person | Fresh air flow Q | Air velocity | Calculated diameter | Duct chosen |
|---|---|---|---|---|---|---|
| Office | 10 | 25 m³/h | 250 m³/h | 4 m/s | 149 mm | 160 mm |
| Meeting room | 20 | 30 m³/h | 600 m³/h | 5 m/s | 206 mm | 250 mm |
| Light-work workshop | 15 | 45 m³/h | 675 m³/h | 6 m/s | 199 mm | 200 mm |
| Physical-work workshop | 20 | 60 m³/h | 1,200 m³/h | 7 m/s | 246 mm | 250 mm |
Annual energy cost of ventilation against flow
| Fresh air flow | Heating of the fresh air | Fan power | Fan electricity |
|---|---|---|---|
| 100 m³/h | 1,894 kWh/year | 17 W | 146 kWh/year |
| 250 m³/h | 4,735 kWh/year | 42 W | 365 kWh/year |
| 500 m³/h | 9,470 kWh/year | 83 W | 730 kWh/year |
| 1,000 m³/h | 18,939 kWh/year | 167 W | 1,460 kWh/year |
| 2,000 m³/h | 37,879 kWh/year | 333 W | 2,920 kWh/year |
Detailed explanations and calculation in Mecaflux
Ventilation renews and cleanses the indoor air and provides fresh air necessary for well being of the occupants.
- Natural ventilation by temperature difference, using convection of cold air taken from outside that rises when warmed, This ventilation can be calculated as for pulling up a flue with a heat input (power generator) corresponding to the heat in the rooms.
- Mechanical ventilation is to create a dynamic air movement with a fan, which compensates the aeraulic head loss related to flow in air ducts.
The nine principles of ventilation(INRS)
• Wrapping at maximum area pollutants
• Capter au plus près de la zone d'émission
• Collecting at closer to the emission zone
• Using the natural movements of pollutants
• Induce sufficient air velocity
• Uniformly distributing the air speed at the zone
collection
• Compensate for air outlets with corresponding air inflow
• Avoid the air currents
• Reject the polluted air outside of the areas of fresh air inlet
regulation based on two principles: :
• Aeration should be general and permanent ;
• It must scan the entire housing with installed air inlets in the main rooms and outlets placed in service rooms.
| extracts Flow rates expressed in m3/h | ||||||||||
| Number main rooms | kitchen | Bathroom | Other bathroom | single WC | WC multiple | |||||
| 1 | 75 | 15 | 15 | 15 | 15 | |||||
| 2 | 90 | 15 | 15 | 15 | 15 | |||||
| 3 | 105 | 30 | 15 | 15 | 15 | |||||
| 4 | 120 | 30 | 15 | 30 | 15 | |||||
| 5 et plus | 135 | 30 | 15 | 30 | 15 | |||||
When the ventilation is ensured by a mechanical device that automatically module housing air renewal, so that the rate of indoor air pollution do not constitute any health hazard, and the condensation can be avoided, except transiently, the rates defined in the table above may be reduced.
• If it is a CMV (controlled mechanical ventilation) Single flow the minimum flows to respect are :
| Nb main rooms | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Total minimum flow in m3/h | 35 | 60 | 75 | 90 | 105 | 120 | 135 |
| Minimum flow in kitchen m3/h | 20 | 30 | 45 | 45 | 45 | 45 | 45 |
•And if it is a humidity controlled ventilation, minimum ventilation rates are further reduced :
| Nb main rooms | 1 | 2 | 3 | 4 | 5 | 6 | 7 |
| Total minimum flow in m3/h | 10 | 10 | 15 | 20 | 25 | 30 | 35 |
Office premises without physical work |
25 m3 per hour |
Local restoration, sale, assembly |
30 m3 per hour |
Workshops with local light physical work |
45 m3 per hour |
Other workshops and local |
60 m3 per hour |
- Local non-specific pollution: Pollution mere presence of man
- To specific local pollution: pollution from the emission of harmful or embarrassing products
- Local ventilation: capture pollutants as close as possible to their source
- General Ventilation: dilution of the pollutant with a new air into the workroom
- Natural ventilation: ventilation only made by external opening
- Mechanical Ventilation: ventilation performed by mechanical devices, provides fresh air flow.
- Mechanical local non-specific pollution ventilation : Minimum flow of fresh air per person depending on the local
The hourly rate of brewing - or HRB - is the number of times the air is renewed in a room for a period of one hour.. Unlike the hourly turnover rates, the air flow in the room is not new and can come from an air intake followed by filtration.
For example, with a system of supply and return air with a flow rate of 30 m³ per hour, the HRB will be 3 for a room of 10 m³..
hourly turnover rates – HTR – is the number of times the air is changed in a room for a period of one hour. Unlike the hourly rate of brewing, the air blown into the room is new.
For example, with a system of supply and extract with a flow of 30 cubic meters per hour, HTR will be 3 for a room of 10 m³.
Mechanical ventilation is the cause of two types of energy expenditure:
- Energy expenditure of the fan
Expenses related to the fan is calculated by multiplying the engine power (in kW) by the annual operating time (8760 hours if the operation is continuous). This corresponds to the major losses in the air ducts and minor head loss of the system and fan efficiency
- The cost of heat (in winter) corresponding to the need to heat the air renewed
Expenses related to heating (E Wh / year) can be calculated as follows:
- 0.34 = constant wastage for ventilation W/K/ (m³/h) (W: Watt - K: Kelvin)
- Flow = airflow m³/h.
The flow is generally not known,it is calculated from the ratio of the air volume of the building, which is renewed in one hour. The basis of calculation may be 1 vol/h, which means that all air from the building is renewed in an hour.. - DDU (Degree Days unified) K.day/year = measure calculated for each department, equal to the difference between the mean temperature of the day and 18 ° C, every day.
- 24 hours / day to reduce the ratio in hour.
E = 0.34 * Flow * DDU * 24
Exemple : Building with a living area, heated of 2 380 m² (ceiling height 2.50m). Consumption of heating : 563 000 kWh. The CMV (controlled mechanical ventilation) is a power of 1150 W and renews the air up to 1 vol/h.
E = 0.34 * 5 950 * 2 321 * 24 = 112 689 190 Wh =>112 689 kWh
This consumption represents about 20% of annual heating consumption of condominiums.
Frequently asked questions
What is the difference between air change rate and recirculation rate?
Both are computed the same way, flow divided by room volume, but the air change rate counts only the fresh air taken from outside, whereas the recirculation (or air turnover) rate counts all the air supplied, including air returned and recycled after filtration. A 10 m³ room supplied with 30 m³/h has a turnover of 3 ach; its air change rate is 3 only if those 30 m³/h are fresh air.
What air velocity should I choose in the ducts?
Noise and pressure losses rise quickly with velocity: take 3 to 5 m/s in ducts serving bedrooms, offices and classrooms, 5 to 8 m/s in the main ducts of offices and shops, up to 10 to 12 m/s in industry. At the grilles, go down to 2 to 3 m/s. A lower velocity needs larger ducts but reduces the fan power, which varies as the cube of the velocity at equal flow.
How can the heating cost of ventilation be reduced?
By supplying only the flow needed when it is needed (humidity-controlled ventilation or CO₂-sensor modulation) and by recovering heat from the extracted air: a heat-recovery unit returns 70 to 90 % of the energy of the exhaust air, which brings the 18,900 kWh/year of 1,000 m³/h in the example down to 2,000 to 6,000 kWh. The extra fan uses a few hundred kWh a year, far less than the heat recovered.
How do I size the ventilation network of my own building?
Mecaflux Standard sizes each duct for the chosen flow and velocity among commercial diameters, adds up the pressure losses of ducts, bends, grilles and filters in the inventory, then finds the operating point of the fan whose curve you enter. For a network with many grilles, Mecaflux Pro3D, standalone-licensed network software, draws the ducts in 3D and balances the flow of each branch itself. For a building or a workshop to ventilate, Mecaflux carries out the study of your project and delivers the calculation report, or offers training on the software applied to your own installation.