Step 1: Calculate the Basic Laboratory Supply Airflow
When there is no special process exhaust, the basic supply airflow can first be estimated according to the air change rate:
Supply Airflow Qs = Room Volume V × Air Changes per Hour N
For example, a laboratory is 10 m long, 6 m wide, and 3 m high, giving a room volume of 180 m³. If the design air change rate is 8 times per hour:
Qs = 180 × 8 = 1,440 m³/h
The air change rate should only be used for preliminary estimation. The actual design should also consider outdoor air requirements for occupants, sensible heat load, pollutant concentration, and applicable laboratory design standards. The higher airflow required to satisfy safety conditions should be selected.
Step 2: Calculate the Exhaust Airflow
The total laboratory exhaust airflow normally consists of local exhaust and general room exhaust:
Qe = Fume Hood Exhaust + Extraction Arm Exhaust + Equipment Exhaust + General Room Exhaust
Fume hood exhaust airflow can be calculated according to the effective sash opening area and face velocity:
Qf = 3,600 × A × v
Where:
A is the sash opening area, m²;
v is the design face velocity, m/s.
If the opening area is 0.8 m² and the face velocity is 0.5 m/s, the required exhaust airflow is approximately:
Qf = 3,600 × 0.8 × 0.5 = 1,440 m³/h
For multiple fume hoods, their rated exhaust airflow should not simply be added together. When VAV control is used, system capacity should be determined according to the simultaneous-use factor, minimum exhaust airflow, and maximum operating condition.
Step 3: Establish the Supply–Exhaust Airflow Offset
A negative-pressure laboratory should satisfy:
Exhaust Airflow > Supply Airflow
A positive-pressure clean laboratory should satisfy:
Supply Airflow > Exhaust Airflow
The airflow offset can be expressed as:
ΔQ = Qe − Qs
For example, if the exhaust airflow is 3,000 m³/h and the supply airflow is 2,700 m³/h, the negative-pressure airflow offset is 300 m³/h.
This airflow must enter through door gaps or transfer-air openings, thereby establishing an airflow direction from the cleaner area toward the contaminated area.
Can the Airflow Offset Be Converted Directly into Room Pressure?
No. A fixed airflow offset does not necessarily produce a fixed room pressure differential. Room pressure depends on door gaps, wall leakage, transfer-air openings, and other leakage areas. It can be represented using a leakage model:
Q = C × ΔPⁿ
Where C represents the room leakage characteristics, while n depends on the airflow conditions through the leakage paths.
The airflow offset can therefore be used for preliminary design estimates, but the final result must be confirmed through on-site room-pressure testing and air-valve commissioning.
Which Parameters Should Be Considered During Engineering Selection?
When purchasing supply and exhaust airflow control equipment, confirm:
Design airflow range;
Target room pressure;
Duct static pressure;
Valve dimensions;
Control accuracy;
Response time;
Control and communication signals.
CAV valves can be used in conventional constant-airflow laboratories. VAV valves are more suitable when the number or operating status of fume hoods changes or when energy-saving control is required.
For laboratories requiring stable room pressure and rapid response, Venturi valves can be combined with differential-pressure sensors and room controllers.
Frequently Asked Questions
Why does room pressure remain unstable after the airflow has been balanced?
Possible causes include frequent door opening, poor room airtightness, fluctuating fan static pressure, or unsynchronized supply and exhaust valve responses.
Is a higher exhaust airflow always safer?
No. Excessive exhaust increases energy consumption and may cause difficulty opening doors, airflow turbulence, and reduced fume hood performance.
What information should be provided before procurement?
Provide the room dimensions, experiment type, equipment exhaust requirements, number of fume hoods, target pressure differential, minimum and maximum airflow, and BMS interface requirements.
The supplier can then configure suitable CAV, VAV, or Venturi valves to coordinate supply airflow, exhaust airflow, and room-pressure control.
