New interior for the HVAC systems in the Kastor high-rise building

Rico Popp

Service Director

Magazine Humidity - December 1, 2025

Dear readers,

Approximately 25 years after the building was first occupied, the ventilation systems in the Kastor high-rise were renovated.

The focus was on energy and hygiene aspects. Therefore, not only were the fans and heating and cooling coils replaced, but also the humidification systems. The new spray humidifiers are more hygienic and economical than the air washers used previously.

Since 1997, the Kastor and Pollux high-rise buildings have flanked the Platz der Einheit in Frankfurt am Main.

Designed by New York-based architectural firm Kohn Pedersen Fox Associates, the buildings are named after the Dioscuri of Greek mythology. Pollux rises 130 m with 33 floors, while Kastor, standing 70 m away, is slightly lower at 95 m, but with almost 30,000 m² of office space on its 22 floors, it offers almost as much space as its counterpart.

The owner of Kastor, alstria office Prime Portfolio GmbH & Co KG, recently had some modernization and renovation work carried out on the building services. In addition to improvements such as the replacement of elevators, many measures were implemented that are invisible to most people. For example, important components of the ventilation system, including the humidification systems, were renewed.

Two large central ventilation systems and one smaller system work to maintain a pleasant indoor climate in Kastor. One large system is located in the basement and supplies the lower floors with up to 136,000 m³/h of air. An equally large ventilation system is located on one of the top floors; it supplies the offices in the upper half of the building. On the same floor, there is a smaller HVAC system with a capacity of 6,250 m³/h, which is assigned to conference and meeting rooms. These rooms have a higher occupancy density than office floors when in use, but are not in constant use, so separate, demand-oriented ventilation was provided for them.

Renovation for greater reliability and efficiency

As more than 25 years had passed since the building was first occupied, alstria office decided to have all ventilation systems thoroughly renovated. “The replacement of the fans, heating and cooling coils, and humidification units was primarily intended to ensure greater reliability and lower energy consumption,” explains TGA planner Mischa Schneider. He works for Planwerk GmbH in Giessen. “But when it came to humidification in particular, it made sense to use a different technology to increase both energy efficiency and hygiene.”

The previously installed scrubbers used circulating water; they pumped the water through the humidifier chamber, creating more of a water surge than a mist. As a result, the air absorbed relatively little moisture as it passed through the air washers. There were no elements for post-evaporation. The only control option was to switch the scrubber unit on or off, with its pumps consuming approximately 1.5 kW of electrical power during operation.

The systems were correspondingly inefficient – at least in comparison to today's humidifiers.

Technology change: Spray humidifier replaces scrubber

The old solution was also less than ideal from a hygiene perspective. The scrubber tank held approximately one cubic meter of water, which was recirculated over and over again. It was therefore important to Condair GmbH – the supplier of the new humidifiers – to propose a modern technology for the large ventilation systems. Steam humidifiers and adiabatic humidifiers, among others, are more hygienic. Condair recommended adiabatic low-pressure spray humidifiers for the two large HVAC systems, in this case the Condair DL. In these units, fully desalinated water is finely atomized by a low-pressure nozzle system, followed by aerosol-free post-evaporation on ceramic evaporation bodies. Adiabatic hybrid humidifiers thus combine the advantages of fine atomization and high evaporation performance and avoid problems that can arise when the technologies are used in isolation.

Adiabatic humidification vs. steam humidification

Adiabatic humidification works by means of evaporation. Moisture can be introduced, for example, by a nozzle system that atomises water. In order for the introduced water to be absorbed properly, the air usually needs to be heated (red line in the diagram). The effect of evaporative cooling causes the air to cool down again during humidification (blue line). In modern adiabatic humidification systems, the air is heated in a preheating coil, as is also installed in the air handling units of the Kastor high-rise building.

The power consumption of humidifier pumps is very low in low-pressure humidification systems.

When humidifying the air with steam, the air temperature remains almost constant. Steam is generated using electricity, natural gas boilers or humidifiers draw steam from a steam network. Because this humidification process does not cause any cooling (a quasi-isothermal process), no additional preheating of the air is necessary. Humidification with steam is

hygienically safe due to the high temperatures.

For Kastor, neither a steam humidifier supplied by a steam network nor a natural gas-powered steam humidifier was an option. For the air handling units, the choice was therefore between electric steam humidification and adiabatic humidification.

The steam humidifier is well suited for low to medium humidification capacity, such as for the smaller air handling unit at Kastor, which is operated according to demand. Steam humidification can be precisely regulated. In addition, these systems are usually easy to retrofit. An electric steam humidifier requires more electricity than an adiabatic humidifier.

An adiabatic humidifier provides high to very high humidification capacity efficiently and meets medium to high requirements for control accuracy. This requires preheating of the air, but the power consumption is much lower than with electric steam humidification.

Adiabatic humidification was chosen for the two large air handling units in the Kastor high-rise building.

After an on-site appointment with Planwerk, the Condair team designed the new humidifiers for the existing air handling units. For the large ventilation centres (136,000 m³/h), Condair selected Condair DL adiabatic spray humidifiers, each with a water treatment system consisting of a Condair AT2+ 550 reverse osmosis system and a Condair Soft 120 double softening system. The Condair AT2+ 550 model is a highly efficient reverse osmosis system for reducing the amount of rinse water. The system has a frequency-controlled pump, achieves a desalination rate of 98 to 99 per cent and produces around 80 litres of permeate from 100 litres of raw water.

With a maximum of 440 kg/h (including 29 kg/h of rinse water), the humidification capacity is sufficient to achieve the desired humidity even at the maximum possible air flow rate of 136,000 m³/h. Since rotors with moisture recovery operate in the large ventilation systems, some of the moisture contained in the exhaust air is transferred back to the supply air stream. This reduces the humidification requirement. The treated water is introduced at low pressure via a nozzle system

installed in the former laundry room.

New humidification system offers efficient partial load control

The output of the active humidification system can be adjusted as required and depending on the weather using enthalpy control. ‘This allows us to adapt the heating output to demand, which saves a lot of energy,’ says planner Schneider. He believes that the previous humidification system with scrubbers consumed considerably more energy. However, it is still too early for concrete comparisons, as the renovation was only completed in the spring and the humidification demand is highest in the

winter months.

Less energy consumption, more hygiene

One thing is certain: the power consumption of the Condair DL humidifier is significantly lower than that of the old scrubber. Including the control system, the system consumes a maximum of approx. 600 W, which is less than half of the old system. In addition, the humidifier has a frequency-controlled pump. It is controlled with 0–10 V and converts the signal into a 31-step control system. This means that only the amount of water that is needed is sprayed. The very fine mist allows the humidification water to be easily absorbed by the air. In addition, in hybrid systems, the air absorbs most of the moisture via the new ceramic surfaces of the evaporation unit.

Due to the nature of the system, adiabatic hybrid humidifiers are more hygienic than scrubbers because no water is circulated in them. Water treatment and hygiene-related components of the humidifiers, such as conductivity monitoring, sterile filters, silver ionisation and spray line drainage during shutdown, further contribute to hygiene.

Conversion in the morning and evening hours

The Condair team installed the adiabatic spray humidifiers in the existing empty humidifier compartments, while Klima-Bau Volk GmbH (Wetzlar) installed the water treatment system. As the office building was in use, all work that required the ventilation system to be shut down had to be carried out in the early morning or evening hours. Robert Rimer, property manager at Wisag Gebäudetechnik Hessen Mitte GmbH & Co. KG, explains the reason: ‘Although the conversion took place during the transition season, when heating and cooling were not urgently needed, the windows of the building cannot be opened. This meant that users were dependent on the air supply from the central systems.’ When a particularly high workload and amount of dirt was to be expected, the work was carried out at weekends. This minimised the restrictions for the users of the office building.

The renovation of the small ventilation system for the conference rooms was less complex in terms of working time planning. The system is located on the same floor as the large ventilation system above. Important components of the small air handling unit were also replaced and a contact humidifier was exchanged for an electric steam humidifier with a capacity of 60 kg/h. In this case, Condair recommended the RS model and the Optisorp steam distribution system.

This steam distribution system enables steam to be introduced into the existing short humidifier tank. This humidifier obtains fully demineralised water from the same water treatment system that also serves the adiabatic air humidifier on the floor.

Successful commissioning in spring 2025

The converted ventilation systems, including the new humidifiers, have been in operation since spring 2025. Project manager Rimer inspects the HVAC systems weekly to check that everything is in order or whether servicing is required. He expects maintenance intervals of around six months for the humidifiers. Rimer reports: ‘At first, we had to test how to adjust the air preheating and adiabatic humidification,’ he says. ‘The system reacts very differently than it did with the washers.’ Now, he and his team know how much heating power is required when using adiabatic humidification so that the humidification works optimally. For the people in the offices, the renovated HVAC technology now provides a very good indoor climate with lower energy consumption and a more hygienic humidification solution.

Humidity is important for health

In addition to fresh air and warmth, relative humidity is crucial for the well-being and health of employees, as dry heated air irritates the eyes, skin and mucous membranes and also increases the risk of infection. At a relative humidity of less than 40 per cent, viruses remain active for longer and linger in the air – the risk of infection is higher and our immune system is weaker. Humidification therefore serves to protect health.

And it pays off: if absences due to respiratory illnesses are reduced by just a few hours per employee per year, the humidification solution will quickly pay for itself. In addition, there is a gain in productivity because humidification contributes to comfort.