A new internal system for the HVAC units in the Kastor high-rise

Rico Popp

Service Director

Humidity Magazine – 1 December 2025

Dear readers,

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

The focus was on energy efficiency and hygiene. Consequently, not only were the fans and heating and cooling coils replaced, but the humidification systems were also upgraded. The new spray humidifiers operate more hygienically and economically than the air washers previously used.

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

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

The owner of Kastor, alstria office Prime Portfolio GmbH & Co KG, recently commissioned a number of modernisation and refurbishment works on the building services. In addition to improvements such as the replacement of lifts, many measures were implemented that are invisible to most people. For example, key components of the ventilation system, including the humidification units, were renewed.

To ensure a pleasant indoor climate, Kastor is equipped with two large central ventilation systems and one smaller one. 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 situated on one of the top floors; it supplies the offices in the upper half of the building. On the same floor, there is also a smaller HVAC system with a capacity of 6,250 m³/h, which is dedicated to the conference and meeting rooms. These rooms have a higher occupancy density than office floors when in use, but are not constantly occupied; therefore, a separate, demand-driven ventilation system was provided for them.

Refurbishment 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 refurbished. “Replacing the fans, the heating and cooling coils and the humidification units was primarily intended to ensure greater reliability and lower energy consumption,” explains building services engineer Mischa Schneider. He works at Planwerk GmbH in Gießen. “However, particularly with regard to humidification, it made sense to use a different technology to improve both energy efficiency and hygiene.”

The previously installed scrubbers used recirculating water; they pumped the water through the humidifier chamber, producing more of a water spray than a mist. As a result, the air absorbed moisture relatively poorly as it passed through the air washers. There were no elements for post-evaporation. Control was limited to switching the washer unit on or off; its pumps consumed approximately 1.5 kW of electrical power each during operation.

Consequently, the systems operated inefficiently – at least compared to today’s humidifiers.

Technological change: spray humidifiers replace scrubbers

From a hygiene perspective, too, the old solution was not ideal. The scrubber tank held approximately one cubic metre of water, which was recirculated over and over again. It was therefore important to Condair GmbH – the supplier of the new air humidifiers – to propose state-of-the-art technology for the large ventilation systems. Steam humidifiers and adiabatic humidifiers, among others, operate more hygienically. Condair recommended adiabatic low-pressure spray humidifiers for the two large HVAC systems, in this case the Condair DL. In these units, fully demineralised water is finely atomised by a low-pressure nozzle system, after which aerosol-free post-evaporation takes place on ceramic evaporation media. In this way, adiabatic hybrid humidifiers combine the advantages of fine atomisation and high evaporation capacity, whilst avoiding problems that can arise when these technologies are used in isolation.

Adiabatic humidification vs. steam humidification

Adiabatic humidification works by means of evaporation. Moisture can be introduced, for example, via a nozzle system that atomises water. To ensure that the introduced water is absorbed effectively, 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 HVAC systems of the Kastor high-rise.

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

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

due to the high temperatures.

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

The steam humidifier is well suited to low to medium humidification capacity, such as for the smaller HVAC system in Kastor, which is operated as required. Steam humidification can be precisely regulated. Furthermore, these systems are generally easy to retrofit. An electric steam humidifier requires more electricity than an adiabatic humidifier.

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

Adiabatic humidification was chosen for the two large HVAC systems in the Kastor high-rise.

Following an on-site visit together with Planwerk, the Condair team designed the new humidifiers to fit the existing HVAC units. For the large central ventilation units (136,000 m³/h), Condair selected the Condair DL adiabatic spray humidifiers, each with a water treatment system comprising the Condair AT2+ 550 reverse osmosis system and the Condair Soft 120 dual softening system. The Condair AT2+ 550 model is a highly efficient reverse osmosis system designed to reduce the proportion of rinse water. The system features a frequency-controlled pump, achieves a desalination rate of 98 to 99 per cent and produces approximately 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 highest possible air flow rate of 136,000 m³/h. As rotors with humidity recovery are used in the large ventilation systems, some of the humidity 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,

which is installed in the former laundry room.

New humidification system offers efficient part-load control

The output of the active humidification system can be adjusted as required and in line with weather conditions using enthalpy control. “This allows us to adapt the heating output to demand, which saves a lot of energy,” says planner Schneider. He believes the previous humidification system using washers would have consumed considerably more energy. However, it is still too early for concrete comparisons, as the refurbishment was only completed in the spring and humidification requirements are highest during the winter months

Less energy consumption, better 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 unit, the system consumes a maximum of approx. 600 W, which is not even half that of the old system. Furthermore, the humidifier has a frequency-controlled pump. It is controlled via a 0–10 V signal and implements this in a 31-stage control system. This ensures that only the required amount of water is atomised. Thanks to the very fine atomisation, the humidification water is readily absorbed by the air. Furthermore, in hybrid systems, the air absorbs a large proportion of the moisture via the new ceramic surfaces of the evaporation unit.

Due to their design, adiabatic hybrid humidifiers operate more hygienically than washers, as no water is circulated within them. Water treatment and hygiene-related components of the humidifiers, such as conductivity monitoring, sterile filters, silver ionisation and the draining of spray lines during downtime, further contribute to hygiene.

Conversion during the morning and evening hours

The installation of the adiabatic spray humidifiers into the existing empty humidifier sections was carried out by the Condair team, whilst the water treatment system was installed by Klima-Bau Volk GmbH (Wetzlar). As the office building was in use, all work requiring the ventilation system to be switched off had to be scheduled for the early morning or evening hours. Robert Rimer, site manager at Wisag Gebäudetechnik Hessen Mitte GmbH & Co. KG, explains the reason: “Although the retrofit took place during the transitional season, when heating and cooling were not urgently needed, the building’s windows cannot be opened. The occupants were therefore reliant on the air supply from the central systems.” Where a particularly high volume of work and dust was anticipated, the work took place at weekends. This kept the disruption to the office building’s users to a minimum.

The refurbishment of the small ventilation system for the conference rooms was less complex in terms of scheduling. The system is located on the same floor as the large ventilation system above. Important components were also replaced in the small HVAC system, and a contact humidifier was swapped 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 plenum. This humidifier draws fully demineralised water from the same water treatment plant that also supplies the adiabatic air humidifier on the floor.

Successful commissioning in spring 2025

The retrofitted ventilation systems, including the new humidifiers, have been in operation since spring 2025. Site manager Rimer inspects the HVAC systems weekly to check that everything is in order or whether servicing is required. For the humidifiers, he expects maintenance intervals of around six months. Rimer reports: “At first, we had to test how to set the air preheating and the adiabatic humidification,” he says. “The system reacts quite differently from the old system with the heat exchangers.” He and his team now know how much heating capacity is required when using adiabatic humidification to ensure the humidification works optimally. For the people in the offices, the refurbished HVAC technology now provides a very good indoor climate with lower energy consumption and a more hygienic humidification solution.

Humidity is important for health

Alongside fresh air and warmth, relative humidity is crucial for the well-being and health of staff, 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, whilst our immune system is weaker. Air 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. Added to this is a gain in productivity, as air humidification contributes to comfort.