The first question most people ask about a dehumidifier is how many square metres it covers. It is the wrong question, and the answer to it is the reason so much of this equipment underperforms.
A dehumidifier does not dry a floor area. It removes a quantity of water per hour. Two identical warehouses with identical footprints can have moisture loads that differ by a factor of five, entirely because of how often the doors open.
What you are actually measuring
A dehumidification load is a rate: kilograms of water per hour, or litres per day, that has to be removed to hold the space at its target.
That means relative humidity on its own is not enough information. Relative humidity is relative to temperature, so 60% RH in a 30 degree warehouse and 60% RH in a 5 degree cold store contain very different amounts of water. The calculation works in absolute moisture content, grams of water per kilogram of dry air, and converts to a percentage only at the end.
The practical consequence: always state the target humidity together with the temperature it applies to. A specification that says keep it at 50% without a temperature is not a specification.
Where the moisture comes from
Infiltration, usually the largest source. Every time a door opens, outside air comes in carrying its moisture with it. In a Saudi distribution centre with loading bays working through the day, this is normally the dominant load and frequently larger than everything else combined. It is also the load most often left out, because it depends on how the building is used rather than on anything visible in the drawings. Estimating it needs the door sizes, how often they open, how long they stay open, and whether there is a pressure difference driving air through the opening.
Evaporation from open surfaces. Any exposed water evaporates continuously. Pool surfaces, wet product, washdown water left on floors, curing concrete in a new building. Pool halls are the clearest case: the water surface is a permanent moisture source running day and night whether the building is occupied or not.
The product itself. Materials brought in wet, or materials that release moisture as they acclimatise, are part of the load. Timber, paper, produce and freshly manufactured goods all give up moisture to the surrounding air, sometimes for days after they arrive.
People. Each occupant adds moisture through respiration and perspiration. Negligible in an empty warehouse, significant in a busy production hall, and it varies with how hard people are working.
Fresh air ventilation. Separate from infiltration and often much larger. Where the air handling system deliberately brings in outside air, that air arrives at outdoor moisture content and has to be dried along with everything else.
Putting the load together
The method is the same whatever the building. Establish the target: humidity, the temperature it applies to, and the tolerance. Holding 50% RH within plus or minus 10 is a different system from holding it within plus or minus 2. Establish the design outdoor condition, which is not the annual average but the condition you still expect the system to cope with. Calculate each source as a moisture rate, add them, then apply a realistic operating margin rather than a defensive one.
That last step is where the same trap sits as in boiler sizing. Add a margin to every individual source, then a contingency to the total, and you arrive at equipment far larger than the building needs, which will short cycle and control the space badly. The same logic is set out in our boiler sizing guide.
Which technology the answer points to
Once the load and the target are known, the technology follows from the temperature and how dry the space has to be.
Refrigerant dehumidifiers pass air across a cold coil so moisture condenses out. They are efficient at normal ambient temperatures and are the right answer for most warehouses, production areas and pool halls. Their limitation is physical rather than a matter of quality: the coil has to be colder than the dew point to condense anything, so as the space gets colder there is less temperature difference to work with, capacity falls away, and eventually the coil ices up instead of draining.
Desiccant dehumidifiers pass air through a rotor coated with a moisture-absorbing material, continuously dried by a separate regeneration air stream. Because nothing has to condense, they keep working at low temperature and reach much lower humidity.
As a rough guide, refrigerant suits spaces at normal temperatures with moderate targets. Below roughly 10 to 15 degrees, or where the target is drier than about 40% RH, desiccant is usually the only thing that will hold it. The crossover depends on the specific combination, so it is worth calculating rather than assuming.
The cold store case, and why it is different
Cold storage is the hardest dehumidification problem in the Gulf and the one most often solved with the wrong equipment.
Warm humid air enters every time a door opens. In a cold space it cannot hold the moisture it arrived with, so the moisture leaves the air immediately: fog in the aisles, ice building on evaporator coils, frost on the floor near the doorway. Iced coils then stop transferring heat properly, so the refrigeration plant works harder to hold temperature and the defrost cycles get longer.
Refrigerant dehumidification cannot fix this, because at cold store temperatures it is operating in exactly the conditions where it loses effectiveness. This is desiccant territory, and it has to be sized against the door opening rate rather than the volume of the store.
If the fog clears slowly after a door closes, the load is larger than whatever is currently installed can handle.
What to send when you ask for a specification
- Space volume, and the ceiling height
- Target humidity, and the temperature it applies to
- Tolerance on that target
- Operating temperature of the space
- Door and loading bay sizes, and honestly how often they open
- Whether the space is mechanically ventilated, and the fresh air rate if so
- Any open water surfaces, and their area
- What is stored or produced, and whether it arrives wet
- Typical occupancy
- What is installed now, and in what way it is falling short
The last line is often the most useful. A description of how the current setup fails tells us more than most of the figures above.
Send the figures and we will size against them
Hi-Best specifies and installs industrial dehumidification across Saudi Arabia. Tell us the space, the target and how the building is used, and we will calculate the load rather than quote from floor area.
Call +966 54 433 2843 or request an assessment. You can also see our full dehumidification range.