Injector air handling unit – supply and extract on one motor

An air handling unit with supply and extract on a single motor? With an injector, yes.

An injector (ejector, jet pump) is a well-known device used in gas burners, spray guns and many other machines. According to Bernoulli’s principle, when a duct narrows the velocity increases and the static pressure drops (the Venturi effect). The low pressure in the throat can draw in another stream of air. The diagram below shows how an injector air handling unit works.

Filters, heaters, coolers and humidifiers are deliberately left out – they are not essential to the idea.

How an injector air handling unit works – step by step

A single fan does two jobs at once: it extracts air from the room and – instead of pushing it straight back – accelerates it in the injector nozzle. The fast jet becomes a “pump” for the fresh air.

1. Extract. The fan draws air from the occupied space (the primary, motive stream).
2. Exhaust branch. Part of the stream leaves through a control damper to a cross-flow or rotary heat exchanger and out through the exhaust. Whatever we throw out, the same amount of fresh air must come in – a simple mass balance.
3. Recovery no. 1 – the exchanger. Fresh air from the intake passes through the same exchanger and is warmed (in winter) or cooled (in summer) by the exhaust air.
4. Injector. The rest of the fan flow enters the nozzle, speeds up and, in the low-pressure zone, entrains the already preheated fresh air (the secondary, induced stream).
5. Recovery no. 2 – mixing. In the mixing tube both streams equalise their temperature and velocity, and the diffuser recovers part of the static pressure needed to overcome the supply ductwork.
6. Supply. A mix of fresh and extract air is delivered to the room. A bypass lets the air skip the injector – e.g. for closed-loop operation or intensive purging.

The key parameter of an injector is the entrainment ratio – the ratio of the induced (fresh) air flow to the motive flow. In simple low-pressure air-to-air injectors it is usually between a few tenths and about one, depending on the nozzle velocity, the geometry of the mixing tube and the resistance of the supply and intake ductwork.

Where it makes sense: spaces where recirculation is allowed

• retail halls, industrial halls, uniform public spaces, etc.,
• spaces with uniform heat gains and uniform air demand.

Advantages

• A high degree of double heat recovery (mixing of the air streams plus a cross-flow or rotary exchanger).
• One motor.
• With proper CO₂-based control we supply only as much outdoor air as we actually need.
• Depending on demand, the unit can work like a fan coil – heating and cooling in a closed loop or with only a small share of fresh air.
• Such a unit removes the need for separate heating and cooling terminal units. It can even run fully recirculated – like a fan coil (fresh air damper closed, injector bypass open).

Disadvantages

• Less effective purging (e.g. at night in summer), because drawing in fresh air through the injector requires “recycling” air from the extract. This can be solved with several dampers in the unit (supply-only mode) and transfer dampers between the room or the extract duct and the outside, so that overpressure pushes the air out. To save energy the unit can also be coupled with an extract fan and a bypass around the injector – the unit would then run at higher capacity and use favourable outdoor conditions, e.g. cooling a room that heated up during the day with outdoor air.
• Noise – the jet in the nozzle is fast, and jet noise rises very steeply with velocity (roughly with the eighth power). Silencers after the injector and a moderate nozzle velocity are needed.
• A new technology to develop – there are no off-the-shelf series units, selection software or manufacturer’s curves; every unit needs calculations and measurements.

Further limitations to keep in mind

• Lower aerodynamic efficiency. Mixing streams of different velocities causes losses. A gas injector itself typically reaches 20–35% efficiency, while an EC fan reaches 60–75%. One motor does not automatically mean lower electricity use – the fan has to produce a higher pressure to accelerate the air in the nozzle. The gain lies mainly in heat (recirculation + exchanger) and in a simpler, cheaper design.
• Sensitivity to system resistance. When the fresh air filter gets dirty, a diffuser is throttled or duct resistance rises, the amount of entrained fresh air changes. Without measurement (CO₂, airflow) the hall can easily end up under-ventilated.
• Supply and extract are not independent. Both streams are coupled through one impeller. Room over- or under-pressure cannot be set freely without additional dampers.
• Recirculation carries pollutants. Dust, odours and aerosols return to the room. The recirculated stream needs proper filtration (e.g. ePM1 or ePM10 depending on the application).
• Frost and condensation. In freezing weather the exchanger needs frost protection (exchanger bypass, damper, preheating), and mixing cold air with humid room air can produce fog in the mixing section.

Control: CO₂, fan speed and dampers

An injector unit has practically three “knobs”:

• fan speed (EC motor or inverter) – sets the total supply flow and the nozzle velocity, i.e. the strength of entrainment,
• the damper in the exhaust branch – sets how much air we exhaust, and therefore how much fresh air comes in,
• the injector bypass – switches between ventilation and closed-loop operation (fan coil mode).

The simplest sensible logic: a CO₂ sensor in the occupied zone with a setpoint of 800–1000 ppm opens the exhaust damper and raises the fan speed when the concentration rises. When CO₂ drops, the unit returns to a high recirculation share and heats or cools the room like a fan coil. Add a minimum damper position so that the required outdoor air flow is always guaranteed, and a temperature sensor after the mixing section that controls the heater and the frost protection.

Worked example: what double recovery gives

A hall with a 3000 m³/h unit, of which 30% (900 m³/h) is fresh air. Outdoors –10°C, indoors 20°C, exchanger temperature efficiency 60%.

• Fresh air after the exchanger: –10 + 0.6 × (20 – (–10)) = 8°C.
• After mixing in the injector: 0.7 × 20 + 0.3 × 8 = 16.4°C.
• For comparison: mixing alone without the exchanger would give 0.7 × 20 + 0.3 × (–10) = 11°C, and 100% outdoor air through the same exchanger – 8°C.
• Power needed to heat 3000 m³/h of supply air from 16.4°C to 20°C: about 1.0 kg/s × 1.005 kJ/(kg·K) × 3.6 K ≈ 3.6 kW. With 8°C supply it would be about 12 kW.

These numbers show where the savings come from: limiting outdoor air to the necessary minimum (CO₂ control) and recovering heat from the part that has to be exhausted anyway. Heat losses through the building envelope still have to be covered separately – in fan coil mode the unit can take over that role too.

Simulator: see for yourself how the unit works in the heating season

Below is a simplified physical model of the unit. The injector is calculated from a momentum balance, the ducts and the intake from their resistance curves, and the temperatures from a heat balance. Change the outdoor temperature (0–10°C), fan speed, damper opening, exchanger efficiency and the number of people in the hall. You will see how much fresh air the injector entrains, the temperature after mixing, how much power the heater and the fan need, and how this compares with classic units. Note one thing: good entrainment needs a high nozzle velocity, and that costs fan pressure.

Where this principle already works

Induction – drawing in air with a motive jet – is a proven technique in ventilation, just usually at a different scale:

• active chilled beams and induction units – primary air from the AHU leaves the nozzles and induces room air through a water coil,
• jet fans in car parks and tunnels – a fast jet entrains the surrounding air and moves it towards the extract without ducts,
• swirl and long-throw diffusers – use induction to mix supply air quickly with room air,
• “bladeless” fans – a narrow slot ejects air that entrains many times more air from the surroundings.

The injector air handling unit brings the same idea inside the unit: the extract air jet draws in the fresh air.

Recirculation, regulations and hygiene

The solution relies on recirculation, so it does not fit everywhere. Building regulations generally exclude recirculation in rooms with emissions of harmful substances or unpleasant odours, explosion risk, pathogens, or gas-fired appliances. They also require heat recovery in mechanical ventilation above a certain capacity. The outdoor air flow must follow from the number of people and the purpose of the room – a CO₂ sensor helps to adjust it in real time, but it must never drop below the design minimum (see e.g. EN 16798-3 and your national regulations).

Good candidates are therefore large, uniform volumes: retail and warehouse halls, sports halls, production halls without harmful emissions. Poor candidates: kitchens, laboratories, sanitary rooms, offices with many zones with different needs.

Summary

• One fan handles extract, supply and fresh air intake thanks to the Venturi effect.
• The energy savings come mainly from CO₂-controlled recirculation and heat recovery from the exhaust air, not from the “single motor” itself.
• The price is lower aerodynamic efficiency, jet noise in the nozzle and sensitivity to system resistance – which calls for good control and silencers.
• It is a concept for uniform halls where recirculation is allowed; the device still needs development and testing.

Polish version: Centrala inżektorowa – nawiew i wyciąg na 1 silniku.