Frequently Asked Questions

The A to Z of air conditioning

STULZ pride ourselves as industry experts in datacenter cooling, however we understand that some terms can be confusing. That's why we've created an FAQ to help answer all your datacenter cooling questions.  

  • A

    Adiabatic humidification
    Technical term for humidifying systems that manage without external, primary heat energy. The water is atomised in a fine mist and absorbed by the air.

    Air Conditioning
    Air handling technology
    Air-conditioning systems belong to the field of air handling technology. Air handling technology is dedicated to controlling the climate in closed rooms.


    Air Conditioning
    "AER" stands for Airflow Efficiency Ratio and is dependent on two operating points, airflow and external pressure. the AER (measured in W/m3/hr.) describes the ratio of the fans power consumption to the airflow rate of an air conditioning unit. As such this value measures the efficiency of the function i.e. the movement of air through the data center, irrespective of how much cooling capacity is achieved with this airflow.

  • B

    Air Conditioning
    BMS – Building management system
    Building management systems (BMS for short) represent the management level of building automation and as such control and monitor building services. In modern air-conditioning systems, the control software has open interfaces to communication standards for building management systems, such as OLE for Process Control (OPC), Building Automation and Control Networks (BACnet) and Local Operating Networks (LON).

  • C

    Precision Air Conditioning
    Chiller, Chilled water generator
    Chillers (or chilled water generators) produce cold water for CW air-conditioning systems (see 'Cooling systems'). Chillers function on the basis of the DX principle (see 'DX'). Heat is dissipated by means of an air or water-cooled condenser. With a suitable control system and the integration of free cooling, large heat loads, in particular, can be cooled especially effectively using chillers. Another important advantage is that chillers are compact and come ready for connection.

    Air Conditioning
    Cooling capacity
    The cooling capacity indicates the extent to which your air conditioner can cool the room. In other words, it specifies the maximum amount of heat the unit can take in and convey to the outside. Cooling capacity is measured in kilowatts (kW).

    Precision Air Conditioning
    Cooling system
    Technical method of generating cold. STULZ precision air-conditioning systems are available with the following cooling systems:

    • A: Cooling system based on the DX principle, in which the heat removed from the room is discharged via an air-cooled condenser.
    • G: Cooling system based on the DX principle, in which the heat removed from the room is discharged via an air-cooled plate-type condenser in the air conditioner.
    • GE: Hybrid cooling system, comprising a DX system with integrated indirect free cooling.
    • CW: Cooling system in which the heat from the room is absorbed by a water-cooled direct cooling unit (CW 2 has two redundant refrigerant circuits).
    • ACW: Two independent cooling systems (CW and A) in one air conditioner. If the main cooling system (CW) fails, the A system ensures that air conditioning continues without interruption.
    • GCW: Two independent cooling systems (G and CW) in one air conditioner. If the main cooling system (CW) fails, the G system ensures that air conditioning continues without interruption.
  • D

    Precision Air Conditioning
    DFC - Dynamic Free Cooling
    DFC is a control concept for the GE cooling system developed exclusively for the CyberAir 2, STULZ’s hybrid precision air-conditioning system. The DFC automatic air-conditioning system ensures precise and efficient interior climate control with minimal energy consumption. DFC combines compressor cooling and free cooling in four stages in all, and automatically searches for the most economical operating mode: when the outside temperature is low, DFC cools with thrifty indirect free cooling, whereby the energy-intensive compressors remain switched off. If the outside temperature rises, DFC first selects extended free cooling mode, then switches the compressors on step by step. With the help of standby units, the automatic control balances all the various units in energy-saving partial load mode.

    Precision Air Conditioning
    Direct free cooling
    Air-conditioning systems with direct free cooling use cold outside air for cooling when outside temperatures are low. The incoming flow of air is regulated by an air flap. When the outside temperature rises, the air-conditioning system gradually switches to compressor cooling (mixed mode) and then to pure closed-circuit cooling (DX mode), in which the air is simply recirculated internally. Energy consumption is considerably higher in closed-circuit mode, which is why it is only employed on especially hot days. Direct free cooling is used in ICT facilities with lower heat loads – in base stations for mobile phone networks, for example (also see 'Indirect free cooling').

    Precision Air Conditioning
    DX – Direct Expansion
    Traditional compressor-type air-conditioning system, which generates cold via a closed refrigerant circuit with an evaporator, compressor, condenser and expansion valve. The evaporator evaporates the refrigerant, absorbing heat as it does so. The compressor sucks up the refrigerant gas and compresses it under high pressure. The refrigerant becomes liquefied again in the condenser, emitting heat in the process. The expansion valve allows the refrigerant to expand, and the cycle starts again from the beginning. We distinguish between different cooling systems depending on the type of condenser (also see 'Cooling systems').

  • E

    Air Conditioning
    The efficiency indicates how much cold or heat energy the air-conditioning system is producing from the energy used (electricity or gas). Professionals recognise the level of efficiency from the COP value. Devices with energy efficiency class A, for example, have a COP of at least 3.2; in other words, they produce a cooling capacity of at least 3.2 kW from 1 kW of electrical power. In partial load, the COP even achieves between 4 and 5, as the power input is even lower in relation to the output.

    Air Conditioning
    Energy efficiency class
    Many consumers are already familiar with energy efficiency classes from washing machines and other household appliances. The energy efficiency class indicates the efficiency with which your air conditioner converts electricity into cooling power. The less electricity is used to generate a given cooling capacity, the more economical and ecological the air conditioner is in terms of energy. The classes range from A to G. Class A means that the air conditioner is especially energy efficient.

    Air Conditioning
    "EER" stands for energy efficiency ratio, and is used to rate the efficiency of mechanical chilling systems. The EER describes the ratio of the cooling capacity (in kW) to the power input (in kW). So, the higher the EER figure, the more efficiently the machine is working. EER data is mostly dependent on both operating points and on standard design parameters defined by the manufacturers. This means comparison is difficult, especially since some manufacturers use "total" cooling capacity as their point of reference, whereas others use "sensible" cooling capacity.

    Air Conditioning
    "ESEER" stands for European Seasonal Energy Efficiency Ratio and describes the annual energy rate of a chiller. The ESEER is calculated by combining the EER full load and partial load values (for units up to 600kW). It focuses specifically on the machine in partial load mode, and takes into consideration the fact that the chillers only have to achieve full cooling capacity for a few hours a year, as in most applications the required power is in the partial load range between of between 25% and 75% for the vast majority of the time. The calculation on which the ESEER is based is a European compromise based on extremely diverse load requirements and outside temperature profiles. The ESEER is therefore a comparative value that has little in common with a real-life load profile. Consequently, the calculation is based on different heat loads and various outside and chilled water temperatures, which are then weighted. The basis for calculation is a temperature of 7°C exiting the chiller, this makes an ESEER value redundant for any free cooling chiller. The temperature difference however through the chiller is variable, depending on the load.

    Air Conditioning
    Equivalent Length
    The equivalent length is the calculation of the pipe length + straight length of pipe bends (indicated in meters in the manual), i.e. the straight length of the pipe bends must be added to the pipe length.

  • F

    Air Conditioning

    Free Cooling

    The term free cooling refers to an economical method of using low external ambient air temperatures to cool a medium to a desired set point, which can then be used for industrial processes, or air conditioning systems. The medium can either be used immediately or be stored for the short or long-term.

    Although the name would indicate that this mode of cooling is free, it’s actually not. In a free cooling system you still need to use a number of components that will require power to enable the system to function. However the system itself has a drastically reduced power consumption over the traditional DX systems so it is dubbed “Free Cooling”

  • G

  • H

    Air Conditioning
    Heat pump
    A heat pump is an additional option in high-quality air-conditioning systems. These systems can heat as well. The heat pump reverses the principle of cold generation: the "refrigerant" removes heat from the outside and conveys it to the inside. On mild winter days and other in-between times, heat pumps are a valuable, cost-efficient alternative to conventional heating, e.g. for weekend retreats and holiday apartments.

    Humidifying systems control the air humidity. They safeguard production processes, preserve materials, prevent static electricity and ensure that people in closed rooms can breathe healthy air. Depending on the system used, humidifiers produce either hot steam (isothermal humidification) or a cold mist from energy-efficient atomisation (adiabatic humidification).




  • I

    Precision Air Conditioning
    Indirect free cooling
    In contrast to direct free cooling, in which “direct” use is made of outside air for cooling, indirect free cooling transfers the cold by means of a pipe system containing a water-glycol mixture. A recooling heat exchanger cools the mixture with outside air. Air-conditioning systems with indirect free cooling can be configured with great flexibility, and are especially energy efficient (also see 'Direct free cooling').


    Isothermal humidification
    Technical term for humidifying systems that produce steam by heating water.

  • J

  • K

  • L

  • M

  • N

  • O

  • P

    Precision Air Conditioning
    Power Usage Effectiveness (PUE)
    The international IT consortium "The Green Grid" uses PUE to deliver a cross-border comprehensive approach in determining the total energy efficiency of a data centre. This index has become a globally recognised standard and serves to highlight the success of optimisation processes which reduce the energy consumption of data centres. PUE is calculated by dividing the total power consumption of the data centre (in kW/h) by the IT equipment consumption (servers, storage systems and network hardware). The figure that results from this calculation is generally known as the PUE value and should be somewhere between 1.0 and 2.0. A value of 2 means that the operation of air conditioning and UPS equipment (physical infrastructure) consumes twice as much energy as the IT equipment used (IT load).

    Precision Air Conditioning
    Precision air-conditioning systems
    Highly specialised air-conditioning systems, adapted to the requirements of information and communication technology. Precision air-conditioning systems keep the room temperature constant to the last degree, precisely regulate the relative air humidity, filter the air and distribute the cooled air around the room thoroughly and evenly. They are designed for failure-free continuous operation – 24/7, 365 days a year. In contrast to conventional comfort air conditioners, precision air-conditioning systems have a considerably better ratio of total cooling capacity to sensible cooling capacity – in other words, up to 100 % of the energy consumed is utilised to cool the room temperature.

  • Q

  • R

    Air Conditioning
    Recooling heat exchanger
    Ready-to-connect air/water heat exchanger for cooling water-glycol mixtures. The brazed plate heat exchanger of the air conditioner and the externally fitted recooling heat exchanger are connected via a closed water-glycol circuit. The recooling heat exchanger discharges the heat from the mixture into the ambient air.

    Air Conditioning
    The refrigerant is a standard component of every air-conditioning system. As a heat-transport fluid, it conveys the heat in the room from the indoor to the outdoor unit. Modern air-conditioning systems contain CFC-free refrigerant R 410 A or R 407 C. R 410 A also increases the efficiency of air conditioners and enables them to be especially compact in design. The refrigerant circulates in a closed circuit, and therefore cannot escape. During installation, a qualified cooling system engineer expertly fills your air conditioner with refrigerant and checks the system for leaks. If the unit has to be removed, the cooling system engineer siphons the refrigerant into recycling bottles for environmentally friendly disposal.

    Air Conditioning
    Refrigerant pipe
    The refrigerant pipe is similar to a heating pipe. It connects the outdoor unit to one or more indoor units. The diameter and length of the refrigerant pipe vary depending on the air-conditioning system and the refrigerant used. Thanks to its high heat absorption, refrigerant R 410 A enables especially long, slim piping to be installed. In addition to the refrigerant pipe itself, the system has separate lines for the electrical control system and the removal of condensed water.

    Air Conditioning
    Remote control
    High-quality air-conditioning systems can be controlled by remote control - both centrally and as individual units. Depending on the type of unit, remote controls can regulate the room temperature, mode (cooling, heating, ventilating, dehumidifying, automatic), fan stage and direction of air. Time switches are often incorporated for programming the output. Wired, infrared and central remote controllers are available, and can be operated via console, touchscreen, PC software or the Internet.




  • S

    Air Conditioning
    Sensible cooling capacity
    The cooling capacity generated by air conditioners is partly latent, partly sensible. The sensible cooling capacity lowers the temperature, while the latent capacity dehumidifies the air. Comfort air-conditioning units use up to 50% of their energy for dehumidification, while precision units convert more than 95% of the energy used exclusively to cooling power. The technology required to achieve this pays off quickly through lower running costs.

    Air Conditioning
    Sound pressure level
    The sound pressure level is stated in decibels, or dB(A), and is a measure of the running noise. The lower the value, the quieter your air-conditioning system is running. In particularly quiet room air conditioners, the sound pressure level is only slightly higher than the rustling of paper, at approx. 25 dB(A).



  • T

    Air Conditioning
    The thermostat switches the air-conditioning system on or off as the temperature becomes too high or too low. In especially sophisticated air-conditioning systems, the thermostat is replaced by energy-saving inverter technology. In this case, the air-conditioning system is not simply switched on and off, but regulated gradually by the ramped slow-down and acceleration of the compressor.

  • U

  • V

  • W

  • X

  • Y

  • Z

Free Cooling

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