General Dilution Ventilation Systems

Many processes release small amounts of low-toxicity vapours, gases or fumes, such as mild solvents, cleaning agents, adhesives or CO₂ from occupants. Contaminant dilution ventilation brings these concentrations down to a fraction of their occupational exposure limits, protecting workers without the cost of capturing every source individually.

Furnaces, ovens, compressors, motors, steam lines and roof solar gain can push shop-floor temperatures well above outdoor air. A general ventilation system for heat removal replaces hot, humid indoor air with cooler outdoor air, reducing heat stress, condensation and equipment overheating. In humid coastal cities like Mumbai, controlling moisture is just as important as controlling temperature.

Toilets, canteens, kitchens, printing areas, storage rooms and gyms quickly build up odours and stale air. Continuous air changes flush them out, keeping workplaces pleasant and preventing smells from drifting into offices, customer areas or neighbouring units.

Solvent stores, paint stores, fuel handling areas and battery rooms can accumulate flammable vapours or hydrogen. Properly sized dilution airflow keeps concentrations far below the Lower Explosive Limit (LEL), typically at or below 25% of LEL, reducing fire and explosion risk.

Offices, commercial buildings, training rooms and control rooms need a steady supply of fresh outdoor air to keep CO₂, VOCs and odours under control. General ventilation for indoor air quality improves comfort, concentration and productivity, and helps buildings meet ASHRAE 62.1 and NBC 2016 fresh air requirements.

Supply fans push fresh air into the space. Axial ventilation fans suit wall and roof openings, while centrifugal ventilation fans handle ducted systems with higher static pressure. Air handling units (AHUs) add filtration, and cooling where required. Make-up air units replace the air removed by exhaust fans, often with evaporative pre-cooling for Indian factory conditions.

Exhaust fans remove diluted air from the space. Options include wall mounted industrial exhaust fans, louvered exhaust fans with an exhaust fan shutter, roof exhaust fans and roof ventilation fans, and inline exhaust fans fitted within ductwork. Sizes range from a small exhaust fan of 4 or 6 inches for a toilet to a large, heavy duty exhaust fan of 24 inches and above for factory sheds. Spark-resistant or flameproof fans are used where flammable vapours may be present.

Weather louvres, sand-trap louvres, bird screens and gravity dampers let fresh air in while keeping out rain, dust, birds and insects. In Mumbai's monsoon, inlet design is critical to prevent water ingress. For natural systems, ridge vents and roof air vents provide high-level openings.

Many shed-type systems work without ducts. Where air must be carried to or from specific zones, GI HVAC ductwork, grilles, diffusers and ventilation ducting distribute the air evenly. Stainless steel or coated ducts are used for corrosive or humid exhaust. Compress India fabricates ductwork in-house for quality and speed.

Pre-filters and fine filters on the supply side keep outdoor dust, pollen and pollution out of the building. Filter selection balances cleanliness against pressure drop, since every additional filter stage adds to fan energy.

VOC, CO, NO₂, hydrogen and LEL gas detectors, CO₂ sensors, and temperature and humidity sensors measure conditions in real time. They trigger alarms, boost ventilation when needed and let fans slow down when the air is clean.

Control panels with starters, VFDs, timers and interlocks tie the system together. Controls can link with fire alarm systems, gas detection panels and building management systems (BMS) for automatic operation and remote monitoring.

Fans or AHUs push filtered fresh air into the space, and air leaves through relief louvres or openings. This creates positive pressure general ventilation, which keeps dust and unfiltered air out. It suits electrical rooms, control rooms, clean assembly areas and spaces next to dusty zones.

Exhaust fans pull air out, and fresh air enters through louvres or openings. The resulting negative pressure general ventilation stops odours, vapours and hydrogen from escaping into neighbouring areas. It is the preferred choice for toilets, battery charging rooms, paint and solvent stores, and kitchens.

Supply and exhaust dilution ventilation uses fans on both sides, giving full control over airflow paths, mixing and room pressure. A small offset keeps the space slightly negative or positive as needed. Balanced systems with dedicated make-up air are the most reliable choice for large factories and multi-zone buildings.

Natural systems rely on wind and the stack effect. Cross ventilation through wall openings, ridge ventilation, roof vents and wind-driven turbo ventilators let hot air escape at high level while fresh air enters low down. Running cost is close to zero, but airflow varies with weather and cannot be guaranteed.

Hybrid systems use natural ventilation whenever conditions allow and switch on mechanical fans when temperature, gas levels or still weather demand it. Roof vent fans, solar-powered roof ventilation fans and thermostat-controlled extractors are common hybrid elements. The result is lower energy use with guaranteed minimum performance.

Heat relief ventilation moves large volumes of outdoor or evaporatively cooled air through hot work areas. Roof extractors remove heat at high level, while make-up air, cooler exhaust fan combinations and air circulation fans deliver air to worker zones. This is the most effective form of industrial heat stress ventilation for sheds without air conditioning.

Demand controlled general ventilation adjusts fan speed to real-time readings from gas, CO₂ or temperature sensors. Gas sensor controlled ventilation in car parks, for example, runs fans at low speed most of the day and ramps up only when CO or NO₂ rises. It delivers the same protection at a fraction of the fixed-speed running cost.

The contaminant should have a relatively high exposure limit, meaning it is harmful only at elevated concentrations. Many common solvents, cleaning chemicals, CO₂ and nuisance odours fall into this category.

Dilution works best when contaminants are released slowly and steadily. Sudden bursts or large releases are hard to dilute quickly enough, so a predictable generation rate makes the dilution airflow calculation reliable.

When emissions come from many small sources spread across a floor, such as drying parts, open containers or occupants, hooding each one is impractical. General ventilation handles all of them together.

The system works best when workers are far enough from emission points for the contaminant to mix and dilute before it reaches their breathing zone.

For heat relief, odour removal and humidity control, dilution ventilation is usually the most economical choice, because the load is spread throughout the space rather than concentrated in one spot.

Substances with very low exposure limits, such as lead fumes, silica dust, isocyanates or carcinogens, would need impractically large airflows to dilute safely. They must be captured at the source.

Grinding, cutting, heavy welding, spray painting, mixing of powders and fume-generating processes release too much, too fast. Diluting them spreads the hazard across the whole building instead of removing it.

If operators work directly over the emission point, they breathe the contaminant before it can mix and dilute. No amount of general ventilation will protect them adequately.

Choose LEV, such as hoods, extraction arms, enclosures, fume cupboards or a paint booth exhaust fan, whenever contaminants are toxic, concentrated, dusty or released close to workers. LEV also saves energy because it moves far less air. Compress India will assess your processes honestly and recommend LEV where it is the safer and more economical solution.

The ACGIH manual, Industrial Ventilation: A Manual of Recommended Practice for Design, is the leading reference for industrial dilution ventilation. It sets out the dilution airflow equations for health protection and for fire and explosion safety, the use of mixing factors, and good practice for supply and exhaust placement.

ASHRAE Standard 62.1 sets minimum outdoor air rates for acceptable indoor air quality in commercial and institutional buildings. Its Ventilation Rate Procedure combines a per-person rate and a per-area rate; for offices, this is 2.5 L/s per person plus 0.3 L/s per m² (5 CFM per person plus 0.06 CFM per ft²). It also gives minimum separation distances between exhaust outlets and fresh air intakes.

The National Building Code of India 2016 covers ventilation in Part 8 (Building Services), including natural ventilation and mechanical ventilation requirements for different occupancies. Part 4 (Fire and Life Safety) addresses smoke ventilation for basements and car parks. Compress India designs to NBC 2016 and local fire authority requirements.

Section 13 of the Factories Act, 1948 requires adequate ventilation by the circulation of fresh air and a reasonable working temperature in every workroom. Section 14 requires effective measures against dust and fumes, including exhaust close to the point of origin where necessary. State factory rules, such as the Maharashtra Factories Rules, add detailed requirements that we account for in every industrial design.

Occupational exposure limits define the maximum airborne concentration a worker may be exposed to. References include ACGIH Threshold Limit Values (TLVs), OSHA Permissible Exposure Limits (PELs), and the permissible levels in the Second Schedule of the Factories Act. Occupational exposure limit ventilation is designed to keep concentrations well below these values, not just at them.

For flammable vapours and gases, dilution airflow is sized to keep concentrations at or below 25% of the LEL, and often lower. Hydrogen, for example, has an LEL of 4% by volume, so battery rooms are ventilated to keep hydrogen far below that level. LEL gas detectors typically alarm at 10% LEL and trigger higher-level actions at 20–25% LEL.

The starting point is how much contaminant enters the air. We review material consumption records, safety data sheets, process temperatures, evaporation rates, occupancy, vehicle movements and battery charging currents. For example, a process that uses 1 kg of solvent per 8-hour shift releases about 35 mg of vapour every second.

At steady state, the required dilution airflow is:

Q = (G × K) ÷ C

where Q is airflow (m³/s), G is the contaminant generation rate (mg/s), K is the mixing factor, and C is the target concentration (mg/m³). For solvent evaporation, the ACGIH form is Q (CFM) = (403 × 10⁶ × SG × ER × K) ÷ (MW × TLV), with ER in pints per minute and TLV in ppm.

*Worked example:* 35 mg/s of vapour, a target concentration of 100 mg/m³ and K = 3 gives Q = (35 × 3) ÷ 100 ≈ 1.05 m³/s, or about 3,800 m³/h (2,200 CFM). A separate LEL-based calculation is done for flammable vapours, and the higher airflow governs.

Real rooms never mix perfectly, so the calculated airflow is multiplied by a mixing factor (K).

• K = 1: Perfect mixing, a theoretical ideal
• K ≈ 1.5–2: Well-designed supply and exhaust positions with good air throw
• K ≈ 2–3: Average mixing in typical industrial spaces
• K ≈ 3–5 or higher: Poor mixing, obstructions, high racking or risk of short-circuiting; some guidance uses values up to 10

Good air distribution lowers K, which directly reduces fan size and running cost.

Air changes per hour (ACH) express airflow relative to room volume: ACH = airflow (m³/h) ÷ room volume (m³). A 30 m × 20 m × 8 m shed (4,800 m³) at 10 ACH needs 48,000 m³/h, or about 28,250 CFM. The ranges below are typical starting points; the final design always comes from the contaminant or heat load calculation.

Where heat is the main load, airflow is calculated from the heat to be removed and the allowable temperature rise:

Q (m³/s) = Heat load (kW) ÷ (1.2 × ΔT)

In imperial units, Q (CFM) = BTU/hr ÷ (1.08 × ΔT in °F). Removing 60 kW of machine heat with a 5 °C rise needs 10 m³/s, or 36,000 m³/h (about 21,200 CFM). Because outdoor air in Mumbai often reaches 33–35 °C, ventilation alone cannot bring indoor temperature below outdoor; evaporative pre-cooling or spot cooling is added where lower temperatures are needed.

Fresh air should reach workers first, then pass over emission sources, then leave through exhausts located close to those sources. Heavier-than-air vapours need low-level exhaust, while hydrogen and heat need high-level exhaust. Intakes are kept well away from exhaust outlets, DG stacks and loading bays, using ASHRAE 62.1 separation guidance and prevailing wind direction.

Short-circuiting happens when supply air flows straight to the exhaust without sweeping the room. Dead zones form behind racking, in corners and under mezzanines. We prevent both with correct spacing, adequate air throw, directional louvres, and air circulation fans or jet fans where needed.

Every cubic metre of air exhausted must be replaced. Without planned make-up air, exhaust fans lose performance, doors become hard to open, and air is pulled in from toilets, drains or dusty areas. Dilution ventilation with makeup air is sized so supply is usually 5–10% less than exhaust for negative pressure zones, or slightly more for positive pressure zones.

Computational fluid dynamics (CFD) simulates air velocity, temperature and contaminant concentration throughout the space before anything is built. Dilution ventilation CFD reveals dead zones, short-circuiting and hot spots, so fan positions can be optimised on screen instead of corrected on site.

Fan power varies with the cube of fan speed. Running a fan at 80% speed uses only about half the power of full speed. Variable frequency drives (VFDs) let fans run at the speed actually needed, rather than full speed all day, and provide soft starting that reduces wear.

Gas, CO₂ and temperature sensors tell the system how much air is needed at any moment. Fans slow down when the air is clean and ramp up only when readings rise. In car parks, offices and intermittently used plant rooms, this substantially reduces fan running hours and energy.

In air-conditioned offices and clean commercial spaces, an energy recovery ventilator (ERV) or heat recovery wheel pre-cools and dehumidifies incoming fresh air using the outgoing exhaust. An ERV system is not suitable where exhaust carries flammable, corrosive or toxic contaminants; in those cases, run-around coil systems that keep the airstreams fully separate may be considered.

Direct or indirect evaporative cooling can lower make-up air temperature by several degrees at a fraction of the cost of air conditioning. It is most effective in drier months; during Mumbai's humid monsoon, indirect or two-stage evaporative cooling works better. On some sites, earth-air tunnels, a simple form of geothermal HVAC, can also temper incoming air.

Ridge vents, roof ventilation systems, turbo ventilators and cross ventilation can handle base loads for free. Mechanical fans then run only when conditions demand. Choosing high-efficiency EC or BLDC exhaust fan models for smaller duties also lowers exhaust fan wattage across dozens of units.

VOC detectors monitor solvent areas, printing units and chemical stores. CO and NO₂ detectors are standard in basement car parks and loading bays, staging fans up in steps as vehicle exhaust levels rise.

Catalytic or infrared LEL detectors continuously measure flammable vapour and hydrogen levels. At the first alarm, ventilation boosts to high speed; at higher levels, the system can raise alarms, shut down charging or process equipment, and alert the control room.

CO₂ sensors are a reliable indicator of occupancy and fresh air adequacy in offices, meeting rooms, gyms and halls. IAQ sensors that also measure PM2.5, humidity and VOCs give a complete picture of indoor air quality.

Thermostats and temperature sensors stage roof extractors and supply fans on and off, or modulate VFD speed, as shop-floor, DG room or electrical room temperatures change. This keeps conditions safe without running every fan at full speed through cooler hours.

General ventilation BMS integration over BACnet or Modbus provides remote start and stop, fan status, alarms, sensor trends, filter pressure drop and energy data. Interlocks with fire alarm and gas detection panels ensure the system responds correctly in emergencies.

With fewer components than a hood-and-duct network at every machine, dilution systems are quicker to design, install and commission. For large, open buildings, the initial cost per square metre is usually much lower than LEV.

One system controls vapours, heat and odours from many small sources at once, including sources that move, change or cannot be hooded.

Continuous air movement and fresh air reduce heat stress, humidity and stuffiness. Comfortable workers are more alert, make fewer errors and are more productive through long shifts.

Because the system serves the whole space rather than individual machines, you can rearrange production lines, racking or workstations without redesigning the ventilation.

Fans, louvres, filters and sensors are straightforward to service. With a planned AMC, a dilution ventilation system runs reliably for many years with minimal downtime.

Substances with very low exposure limits would need impractically large airflows. These must be controlled at the source with LEV or process enclosure.

Diluting contaminants across a whole space means moving large air volumes, which adds fan energy. VFDs, demand control and efficient fans reduce, but do not eliminate, this cost.

By design, contaminants mix into the room air before they are exhausted, so workers near sources may still be exposed. Source location and airflow paths must be planned carefully.

Poorly placed supply and exhaust points create dead zones where concentrations stay high. Air distribution design matters as much as fan capacity.

If the space is air-conditioned or heated, every cubic metre of outdoor air must be cooled, dehumidified or heated, which significantly raises HVAC energy use. Heat recovery helps where contaminants allow it.

Dust and heavy particles settle and re-suspend rather than dilute evenly. Dusty processes need capture at the source, with dilution only handling residual background levels.

Factory dilution ventilation removes process heat, low-level vapours and stale air from production halls. Warehouse dilution ventilation uses roof extractors, industrial ventilation fans and make-up air louvres to control heat under metal roofs, protect stored goods and keep pickers and forklift operators comfortable.

Maintenance shops, assembly areas and light fabrication units benefit from workshop dilution ventilation using a shop exhaust fan network and fresh air inlets. Where continuous welding, grinding or soldering takes place, we add LEV at those benches and use dilution for the background.

Commercial buildings need fresh air supply for occupants, exhaust for toilets and pantries, and coordination with HVAC air conditioning systems. We provide treated fresh air units, bathroom ventilation systems with ceiling ventilation fans, canteen kitchen ventilation and kitchen exhaust systems, and plenum ventilation designs that integrate cleanly with AC ventilation.

Basement car parks need continuous dilution of CO, NO₂ and vehicle fumes, plus smoke extraction during a fire. We design jet fan and ducted basement ventilation systems with CO and NO₂ sensors and fire-mode operation to NBC 2016 and fire authority requirements.

Lead-acid and other vented batteries release hydrogen during charging. Battery room dilution ventilation uses high-level exhaust, low-level fresh air inlets, spark-resistant fans and hydrogen detectors interlocked with the chargers, keeping hydrogen far below its LEL.

Solvent vapour dilution ventilation for paint and chemical stores typically uses low-level and high-level exhaust, flameproof motors and LEL monitoring. Spray painting itself needs a dedicated paint booth exhaust fan system, which we can also provide.

Solvent-based inks, cleaning solvents and press heat make printing units a classic dilution ventilation application. We combine general ventilation with local extraction at presses and ink stations for complete VOC control.

Industrial and hotel laundries generate heat, moisture and lint. Dilution ventilation removes heat and humidity from the workspace, while dryer exhaust is ducted separately to prevent lint build-up and fire risk.

Spinning, weaving and processing departments produce heat and fibre fly and need controlled humidity. Dilution and supply systems in textile mills remove heat, work alongside humidification plants, and use filtration to manage airborne fibres.

DG rooms need combustion air plus enough airflow to carry away engine and radiator heat. Pump rooms and electrical rooms need heat removal to protect motors, panels and transformers, often with filtered positive-pressure supply to keep dust out.

High occupancy and physical activity raise CO₂, humidity and odours quickly. CO₂-based demand-controlled ventilation keeps gyms and sports halls fresh during peak hours while saving energy when they are quiet.

Kruger is one of the most widely specified fan brands in Indian HVAC projects. Its centrifugal, axial and inline fans are commonly used in AHUs, ducted supply and exhaust systems, and basement ventilation.

Swedish ventilation group Systemair offers a broad range of inline duct fans, roof fans, axial fans, car park jet fans and air handling units, with EC motor options for energy-efficient ventilation.

Nicotra Gebhardt is known for centrifugal fans, plug fans and blowers used in air handling units and ducted ventilation systems, with a wide selection of sizes and pressure ranges.

US-based Greenheck manufactures roof exhausters, upblast fans, inline fans, louvres and dampers. Its products are widely used in commercial kitchen exhaust, industrial exhaust and general ventilation.

German specialist ebm-papst is a leader in EC fans and motors. Its high-efficiency plug fans and axial fans are a popular choice for EC fan dilution ventilation and for retrofits that replace older AC motor fans.

Indian manufacturer Almonard produces industrial axial fans, wall and roof exhaust fans, air circulators and man coolers. Its products are widely used for factory heat relief and shed ventilation.

Other options include Östberg for compact inline duct fans, Soler & Palau for a wide range of residential and commercial fans, and Indian brands such as Havells and Crompton for small exhaust fans in toilets, pantries and utility rooms. We choose from the full market so your system is never tied to a single brand's limitations.

We check fan speed, motor current, vibration, belt tension and alignment, bearing condition and lubrication, and tighten electrical terminations. Early detection prevents unexpected breakdowns during production.

Supply filters are inspected, cleaned or replaced based on pressure drop and dust load. Clean filters protect airflow and indoor air quality while keeping fan energy in check.

Louvres, bird screens and inlets collect dust, leaves and nests that choke airflow. Regular cleaning, especially before and after the monsoon, keeps fresh air flowing freely.

Gas detectors and sensors drift and must be bump-tested and calibrated with certified test gas at the intervals recommended by the manufacturer. We maintain calibration records for audits and inspections.

Ducts that carry grease, lint, dust or condensate are inspected and cleaned to restore airflow, reduce fire risk and prevent odours and microbial growth.

Cause: Supply and exhaust points are poorly spaced, or racking, mezzanines and machinery block airflow.

Solution: Relocate or add inlets and exhausts, use directional louvres, or add air circulation fans or jet fans. CFD analysis confirms the fix before work begins.

Cause: Supply and exhaust openings are too close together, so fresh air leaves without sweeping the room.

Solution: Separate supply and exhaust across the space, redirect supply air, or reposition exhaust fans.

Cause: Airflow is too low, fans are under-performing, generation has increased, or a source needs LEV.

Solution: Measure actual airflow, restore fan performance, recalculate dilution airflow, and add LEV for major sources.

Cause: Insufficient airflow, hot air recirculating near the roof, or no make-up air.

Solution: Add roof extractors, provide adequate make-up air, add evaporative pre-cooling, and direct air to worker zones.

Cause: High-velocity supply air aimed directly at workers or poorly selected grilles and louvres.

Solution: Change diffusers or louvre angles, reduce outlet velocity, or rebalance airflow with VFDs.

Cause: The space is at positive pressure, or doors and openings let contaminated air escape.

Solution: Increase exhaust relative to supply to create negative pressure, and seal openings between zones.

Cause: Fixed-speed fans running at full speed all day, inefficient old motors, or over-sized systems.

Solution: Add VFDs and sensor-based demand control, replace old fans with EC fans, and right-size airflow to the actual load.

If one or two processes dominate contaminant levels, adding local exhaust at those points removes the bulk of the load. The general ventilation can then run at lower airflow, saving energy while improving air quality.

Retrofitting VFDs and gas, CO₂ or temperature sensors to existing fans converts a fixed-speed system into demand-controlled ventilation. It is one of the quickest-payback upgrades available for car parks, factories and commercial buildings.

When a system moves enough air but still leaves hot spots or high readings, the problem is usually distribution. A CFD study identifies where to relocate inlets, add jet fans or redirect supply air to fix dead zones without adding fan capacity.

Converting plain outdoor air intakes to evaporatively cooled make-up air lowers shop-floor temperatures by several degrees in dry weather, improving heat relief without the cost of full air conditioning.

Old belt-driven fans with standard AC motors lose efficiency over time. Replacing them with direct-drive EC fans, or BLDC exhaust fan models for smaller duties, reduces power consumption, eliminates belt maintenance and adds built-in speed control.

Airflow is the biggest cost driver, and it is set by room volume and the required air changes or dilution calculation.

Simple exhaust-only systems cost the least per CFM. Ducted, filtered and conditioned systems cost more but deliver far better control.

• Fans: Wall-mounted industrial exhaust fans of 18–36 inches typically cost about ₹6,000 to ₹30,000 each, with flameproof models costing more. Standard motorised roof extractors are commonly in the ₹15,000 to ₹45,000 range, while branded, high-capacity or centrifugal units cost more.
• Ductwork: GI ductwork is priced by duct surface area, typically around ₹70 to ₹150 per sq ft fabricated and installed. Insulation, stainless steel and coated ducts add to this.
• Controls: Cost depends on the number of gas detectors and sensors, hazardous-area ratings, VFD sizes and whether BMS integration is required.

• Required airflow and number of fans
• Hazard level, including flameproof or spark-resistant equipment
• Ductwork length, material and insulation
• Make-up air treatment, such as filtration or evaporative cooling
• Number and type of sensors, and BMS or SCADA integration
• Structural work, roof openings and access at height
• Electrical supply, cabling distances and panel requirements
• Testing, commissioning and CFD scope

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