How to Calculate Dehumidifier Capacity for an Industrial Space

Quick answer: Dehumidifier capacity is the moisture entering your space every hour, plus a safety margin. Add up the water brought in by outside air, people and process work, then multiply by 1.15 to 1.25. Compare that figure, in kg of water per hour, with a unit's rating at your own room conditions.

Excess humidity does real damage in industrial buildings. It rusts machined parts, cakes powders, puts condensation on packaging, grows mould on stored goods and causes static problems in electronics. In a pharma area it can push a batch out of specification. A dehumidifier only helps if it matches the load it actually faces.

An undersized dehumidifier runs around the clock and never reaches its set point. An oversized one costs more to buy and, in some designs, cycles badly. Sizing it properly takes a tape measure, a psychrometric chart and about twenty minutes. This guide walks through the method, with a worked example for a plant in Delhi NCR.

Step 1: Set the conditions you need to hold

Decide the indoor temperature and relative humidity (RH) your product or process needs. Then find the worst outdoor condition at your site. In North India that is the monsoon, often 32 to 35 °C at 70 to 80% RH.

Treat these as starting ranges. Your product specification or applicable standard always overrides them.

Next, convert each condition into a humidity ratio, which is grams of water per kg of dry air. A psychrometric chart or any online calculator does this in seconds. The gap between outdoor and indoor values is the water every kilogram of incoming air carries into the room.

Step 2: Add up the moisture load

Four sources matter.

1. Outside air. This covers leakage through doors, gaps and openings, plus any fresh air you supply on purpose. It is usually the largest share. For leakage, a rough planning figure is 0.25 to 0.5 air changes per hour in a reasonably tight building, and 1 or more where loading doors open often.
2. People. Allow about 0.05 to 0.15 kg/h per person, depending on how hard they work.
3. Process. Open tanks, washing, steam, wet products and drying all add water. Get these numbers from your process team, because this is the load most often forgotten.
4. Building ingress. Vapour passing through walls and roofs is small in insulated rooms. In an uninsulated shed it can be significant, so raise your margin there.

The outside-air load uses this formula: Load (kg/h) = airflow (m³/h) × 1.2 × humidity ratio difference (g/kg) ÷ 1000

Step 3: A worked example

Take a packing hall measuring 20 m × 15 m × 5 m, which is 1,500 m³. The target is 24 °C and 50% RH, a humidity ratio of 9.3 g/kg. The monsoon outdoor condition is 32 °C and 70% RH, or 21.1 g/kg. The difference is 11.8 g/kg.

The hall needs roughly 28 kg of water removed per hour. Since 1 kg of water is about 1 litre, that is close to 670 litres per day. These figures are illustrative, so use your own airflow and process data.

Step 4: Check the rating at your conditions

Brochure ratings are usually quoted at a warm, humid test point such as 30 °C and 80% RH. At 24 °C and 50% RH the same machine can remove well under half as much. Ask the manufacturer for output at your design condition, not the headline number.

If fresh air is pre-treated in an air handling unit, the load on the room dehumidifier falls. Our guide on what an air handling unit is explains how a cooling coil and fan section handle that job.

Step 5: Plan placement and controls

Sizing is only half the job. Put the unit where it can draw from the dampest part of the room, and make sure its airflow reaches every corner so no dead zones form. Seal gaps and fit strip curtains or air curtains at loading bays to cut the leakage you just calculated. Mount the RH sensor away from the unit's outlet and from doors, so it reads the room and not a local draught. Condensate from a refrigerant unit needs a proper drain, and a desiccant unit's regeneration exhaust should be ducted outside.

Refrigerant or desiccant: which type fits?

• Refrigerant (condensing). Cools air below its dew point so water condenses on a coil. It suits rooms above about 20 °C that need 45 to 60% RH, and it uses less energy per kg removed.
• Desiccant. Draws moisture into a rotating wheel, which is dried with heated air. It holds 20 to 40% RH and works in cold rooms where a coil would frost.

The packing hall above, at 24 °C and 50% RH, normally suits a refrigerant unit. Had the target been 30% RH, a desiccant unit would be the safer choice.

Sizing mistakes to avoid

• Using average weather instead of the peak monsoon condition.
• Ignoring loading bays, shutters and doors that stay open.
• Leaving process moisture out of the total.
• Trusting the nameplate figure instead of output at room conditions.
• Skipping RH sensors and controls, so the unit cannot hold the set point.

Frequently asked questions

How do I calculate dehumidifier capacity for a factory?

 Find the humidity ratio difference between outdoor and indoor air, multiply it by the air entering per hour, and add people and process moisture. Apply a 15 to 25% margin. The result is in kg/h.

How many litres per day is 1 kg/h?

 About 24 litres per day, because 1 kg of water is roughly 1 litre. Our example load of 28 kg/h works out to around 670 litres per day.

Is a bigger dehumidifier always better? 

No. Oversizing raises purchase and running cost and can make some units cycle on and off. A 15 to 25% margin over the calculated load is usually enough.

Can I size a dehumidifier from floor area alone?

 No. Floor area gives only a rough idea. The load comes from air leakage, people and process moisture, so two rooms of the same size can need very different capacities.

Which type should I use for 30% RH? 

Use a desiccant dehumidifier. Refrigerant units struggle to hold humidity that low, especially in cooler rooms.

Get your space sized

If you would rather have an engineer check the numbers, send your room dimensions, target RH and process details to AirTree HVAC. You can see the full dehumidifier and air handling range on our products page, and find more HVAC guides on the AirTree blog.