DATA CENTRES
because of issues such as obstructions, vortices, pressure differentials, air pockets etc. that cause cold air and warm air to mix. Similarly, the temperature of the air leaving the IT equipment is frequently cooler by the time it enters the coolers. This is usually because cooled air mixes with the warmed air for a variety of reasons; all of which indicate inefficiency in air flow management.
So, the four delta Ts are the temperature
differences: Q Before and after the IT equipment Q Before and after the coolers Q Between air leaving the coolers and air enter- ing the IT equipment
Q Between air leaving the IT equipment and air entering the coolers
By accurately monitoring these four delta Ts, data center managers can gain a better understanding of the factors affecting cooling inefficiency, which can then inform mitigation and improvement measures to fine tune data center performance. In dry climates evaporative cooling is effective at dissipating heat. In cold climates direct cooling with dry, cold air can be used. In recent years, liquid cooling solutions have become popular because they are more effective at removing heat. To support this trend, Vaisala has developed a new high-quality sensor for measuring cooling / heating liquid temperatures. The Vaisala TMI110 is an immersion temperature transmitter offering a fast response with high levels of accuracy. TMI110 is an addition to the extensive HVAC product offering, including e.g. the ever-popular HMD60 for air ducts, HMT120 for indoor air measurements, and state of the art Indigo platform for most precise measurements in data centers.
Humidity
IT equipment can also be adversely affected by humidity; low levels increase the risk of static electricity, so spray or evaporative humidifiers may be necessary. However, high levels of humidity are also to be avoided because they can result in condensation, and the corrosion of metallic equipment.
In cooler climates it may be possible to utilize colder outside air to cool data center equipment in a process known as economization. The absolute water content of this outside air may be low, and since the relative humidity of air decreases when it is heated, the humidity level may fall below acceptable levels unless appropriate controls are in place. Different types of humidity measurement
are required, depending on the location and application. For example, relative humidity and dew point can be measured in rooms, spaces, ducts and outdoors, whereas wet bulb temperatures would be necessary in cooling towers and evaporative humidifiers, and enthalpy sensors may be required for airside economizers. Enthalpy expresses the total heat energy (sensible heat and latent heat) involved with thermodynamic changes. These calculated humidity parameters are typically available directly from advanced humidity sensors, such as Vaisala’s. The three most common sensor configurations for airside economiser control are dry bulb, single enthalpy, and dual enthalpy. While dry bulb control is the simplest method, it may miss potential energy savings by not opening the economizer when the ambient temperature is slightly warmer but relatively dry. Dual enthalpy works in a similar way, except that two enthalpy sensors are employed; one monitoring outdoor air, and the other return air. The economizer will run if the outdoor air enthalpy is less than the return enthalpy. Vaisala’s transmitters are designed specifically for applications such as air conditioning and economization; measuring humidity and temperature, with derived outputs for dew point, wet bulb temperature and enthalpy. Temperature measurements do not generally suffer from drift, but traditional humidity sensors do, so Vaisala’s humidity sensors employ HUMICAP® technology which delivers long- term stability and insensitivity to interferences such as dust and condensation. These thin- film capacitive humidity sensors have become the industry standard in a wide variety of applications where long-term accurate, reliable,
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maintenance-free humidity measurements are required. Air flow in ducts allow sensors to respond quickly to changing conditions, whereas air movement in rooms and other spaces can be slow, so some data centers prefer to use dew point temperature as a humidity control parameter because it does not depend on the temperature of the sensor. Other sensors that may be deployed at data centers measure differential pressure in ducts and between hot and cold aisles, as well as additional outdoor meteorological parameters such as air pressure, rainfall, wind speed and direction. These measurements can be undertaken by an automatic weather station, by individual sensors, or by one of Vaisala’s WXT530 instruments, which utilize solid state technologies to minimize operating and maintenance costs. Outdoor sensors should be placed in a location with free airflow, away from any surfaces that might radiate heat and disturb measurements. Naturally, outdoor measurements must be as accurate and reliable as indoor measurements, so Vaisala’s world- leading meteorological instruments provide the long-term reliability that is required in even the harshest environments. Investments in accurate, stable measurement systems are negligible in comparison with the value of the systems and information held by data centers. Add the financial and environmental costs of energy inefficiency to the equation, and you have a hugely compelling case for a comprehensive monitoring and control system utilising the most accurate and reliable sensors.
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www.vaisala.com
www.acr-news.com • November 2022 27
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