Suomeksi
The winter season is severe for the Caravelle’s interior. Especially the high humidity increases the risks of mould and damages the surfaces. Temperature changes cause the indoor air humidity to condense on surfaces which are not insulated.
Cold weather and temperatures below zero Centigrade are not a risk as such. There are no parts or structures in the aircraft which could be broken or frozen in low temperatures. In really cold weather the outdoor air doesn’t contain much water.
Humidity control in winter 2024–2025
In autumn 2024 the passenger seats and their textiles had not yet been assembled in the Caravelle’s cabin. The interior, however, with the wall and ceiling insulation and its covering materials was almost ready in the front part of the cabin.
That winter we reduced the indoor air humidity by placing 5-6 bags of salt evenly around the cabin. The salt bags remove water from the humid indoor air by absorbing it into the salt, which melts and drips into a bucket below. The method is cheap and easy to assemble, but to ensure optimal results it needs regular monitoring, emptying the drip containers, and maintenance of the salt bags. We had to replace the salt bags once during the winter. We also had borrowed a small condensing air dryer which operated only in temperatures above zero.
In spring we noticed, to our dismay, small new spots of mould on the new wall surface materials which had been installed before the winter. It was clear that before the following winter we had to find a more efficient way to control the indoor air humidity.
Alternatives
In early autumn 2025 Erja Reinikainen scanned the alternatives we could use for humidity control, and the following three options were chosen for detailed analysis.
1) Air-to-air heat pump
The air-to-air heat pump operates by transferring thermal energy from one place to another via its indoor and outdoor units, utilizing the state changing features of a refrigerant. In winter it transfers heat from outdoor air heating the indoor space where the indoor unit is. In summer the process is reversed, and the heat pump transfers heat out from the indoor space, cooling it. The dehumidifying function utilizes the refrigerant and the cold surface of the indoor unit coil, where the indoor air is cooled and its humidity condenses on the coil. The condensed water is drained from the indoor unit.
Advantages
- Purchase cost 0 €, we have at our disposal a 16-year-old second-hand air-to-air heat pump (with one indoor unit and an outdoor unit)
- The equipment is available without delay
- The heat pump could be used for heating and cooling the cabin and cockpit when there is somebody working inside
Questions
- How to get a contractor to install an old heat pump unit and what is the installation cost?
- Is the refrigerant piping ok and without leaks?
- Will there be problems in the assembly?
- Will the old controls and electronics work reliably in humidity control?
Disadvantages
- Dehumidification will produce condensing water in the indoor unit, the water must be led out from the aircraft
- The heat pump will not dehumidify indoor air in temperatures below +10°C
- The heating capacity of such an old unit may not be sufficient when the outdoor temperature is below -10°C
- There are EU restrictions for using the R410A refrigerant in new and existing installations. Furthermore, there may be problems in purchasing the refrigerant in the future and/or its cost may be high
2. Desiccant dehumidifier
A desiccant dehumidifier has a rotating wheel, often referred to as a desiccant rotor, which is made from layers of fibrous material impregnated with a moisture-absorbing substance such as silica gel. As the wheel slowly turns, it divides the airflow through the unit into two streams: the process air and the regeneration air. The process air, which contains excess humidity from the room, passes through the section of the wheel dedicated to moisture removal. Its water vapour is adsorbed onto the desiccant material, resulting in dry air that is then circulated back into the indoor environment. Meanwhile, the regeneration air stream is heated before it passes through a separate sector of the wheel. This warm air drives off the accumulated moisture from the desiccant material, effectively "recharging" it for further use. The now humid regeneration air is expelled outside, usually via a dedicated hose. This continuous cycle allows the dehumidifier to efficiently extract moisture from the air, even at low temperatures, without adding heat to the room.
Advantages
- Ready to use, no installation, plug in
- A compact unit, no piping, no water in the unit
- Dehumidification works up to -10...-20°C temperatures
Questions
- How to duct the humid regeneration air from the aircraft
- Where is the extract air hose stored when the dehumidification is not needed?
- How to clear the equipment away if there are visitors coming to see the aircraft during winter months?
Disadvantages
- The purchase cost is high
- Munters units are more than 2 000 euros + VAT, Trotec TTR 300 has similar capacity and is 1780 euros + VAT
- Are there cheaper models, rented units, second-hand units?
3. Humidity controlled ventilation
The indoor humidity is controlled with mechanical ventilation (supply and extract). The air change rate in the cabin is once in 2…4 hours. Heated supply air is blown in through one end of the cabin, and the humid extract air is taken out from the other end. The fan operation (or air flow) is controlled by measuring the indoor humidity.
Advantages
- Purchase cost for fans, ducts, controls and cables, etc. is difficult to estimate
- Requires a lot of installation work by the Caravelle team, maybe
- Takes time to plan, purchase and install
Questions
- Is the system sufficient for dehumidification?
- What is the energy consumption?
Disadvantages
- Electricity use for heating supply air may be high
- The ductwork in the cabin is a permanent installation – or then assembly and disassembly is needed every year
Selection of dehumidification system
The selection between the three alternatives had to be made considering ease of assembly, reliability and economy. Looking at the purchase costs and electricity consumption there were significant differences.
The used air-to-air heat pump we were offered was considered too old and a new one too expensive, and the dehumidification in low temperatures wasn’t reliable. The assembly would have required changes in the aircraft to bring in the refrigerant pipes. The installation of the outdoor and indoor units would have been permanent and would have changed the appearance of the aircraft.
The ventilation option wasn’t encouraging. Heating the supply air with electricity during the long winter months would have been too expensive for our limited budget. This option would also have required proper design and planning before buying the equipment. The assembly work would have taken time too.
This is how we ended up choosing the desiccant dehumidifier. An interesting leasing option was found but eventually we found suitable second-hand devices in an online auction, on a reasonable price. The Aviation Museum Society bought two Trotec TTR 250 dehumidifiers. One of them stayed in Turku to be installed in the Caravelle and the other was taken to Tuusula, to the new premises of the Tuesday Club, to be used for dehumidification in one of their sea containers.
Installation
We decided to place the dehumidifier in the middle of the open area in the rear cabin. The dry air is led through two ducts to both ends of the aircraft, i.e. to the cockpit and the rear galley. The dehumidifier will take in the humid indoor air from mid-cabin, causing an air circulation inside the aircraft. The humid regeneration air is exhausted from the cabin where the dehumidifier is located.
We found a good and elegant solution for the exhaust air hose as we had spare exit hatches available. One of the exit hatches was brought in from the storage container and a through hole was drilled in the middle of its triangular window. The exhaust air hose is led through the hole in a duct bend, which is fastened tightly on the hole. When the dehumidification period is over in spring, the dehumidifier and its ducts and the exit hatch with the hole in its window can be disassembled and taken to storage. An original exit hatch will be put back into place for the summer when there will be visitors coming to see the Caravelle. With this arrangement there won’t be any extra installations, equipment or through holes visible during the summer season.
The installation can be seen in the picture. The dehumidifier had to be placed on a bench which is on a table, because the exhaust air hose for the warm and humid air has to slope downwards so that there won’t be water pockets in it. Through these it might be possible to have the water leak back into the dehumidified space.
In the photo below the humid air intake is behind the dehumidifier and can’t be seen. The dry air is blown through the duct back into the cabin. The T-joint in the duct divides the air into the cockpit and the rear galley. The duct is 100 mm in diameter and 10 m in length to both directions. Towards the cockpit the last 5 metres is made of plastic land drainage pipe with holes in it. This distributes the supply air evenly into the front part of the cabin where the fabric-covered passenger seats are.
The humid exhaust air is blown out of the cabin through the hose which is led through the hole in the exit window as seen in the picture. We also made sure that there is a sufficient flow of make-up air through the aircraft’s own ventilation ducts, even with natural ventilation without fans.
We asked comments from Sweden how they control the humidity in their museum aircraft which are stored outside. Flygvapenmuseum from Linköping replied that in their Caravelle they use Munters desiccant dehumidifier, i.e. similar to ours. They were pleased with it and said it works very well also in winter conditions.
After a few weeks’ experience we can say that the results are good, the dehumidifier keeps the relative humidity in the cabin about 15-20 percent points lower than the outdoor relative humidity. It is too early to say how well the dehumidifier will work through the winter, but we will collect data during the winter months to ensure that this solution was a good choice.
Photos: Jouko Tarponen
Translation to English: Erja Reinikainen
