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«by Tony Evans Contents Executive summary Click on a section to jump to it Introduction 2 As power densities continue to increase in today’s data ...»

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Fully ducted distribution An air distribution or return methodology in which air is directly ducted into or out of the loads.

Glycol A common term for a mixture of ethylene glycol and water (similar to the antifreeze/water combination used in many automobiles) used as a heat removal medium in computer room air conditioners. The glycol mixture is resistant to freezing in cold climates. See “GlycolCooled System”.

Glycol-cooled system A type of precision cooling system widely used in IT environments of all sizes. In a glycol system the air conditioner absorbs heat from the IT room and removes it from the room in the form of heated liquid water/glycol solution. The heated liquid then flows via pumps to an outdoor radiator with a fan where the heat is expelled.

HVAC An abbreviation for Heating, Ventilation and Air Conditioning. Sometimes an “R” is shown at the end to represent refrigeration.

Heat Heat is simply a form of energy. It exists in all matter on earth, in varied quantities and intensities. Heat energy can be measured relative to any reference temperature, body or environment.

Heat exchanger A heat exchanger allows different fluids to transfer heat energy without mixing. It achieves this by keeping the flowing fluids separated by thin tubes or thin metal plates. Heat exchangers are commonly used in the place of condenser coils in water or glycol cooled air conditioning systems.

Heat transfer Heat transfer is the process of an object or fluid losing heat energy while another object or fluid gains heat energy. Heat energy always flows from a higher temperature substance to a lower temperature substance. For example, a cold object placed in a hot room cannot drop in temperature it can only gain heat energy and rise in temperature. The amount of heat transferred can always be measured over a period of time to establish a rate of heat transfer.

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Hot gas line A refrigerant line connecting the compressor to the condensing coil in an air conditioning system. In air-cooled systems the hot gas line may be hundreds of feet in length.

Humidification The process of adding moisture to air. A simple example of the humidification process is when water is boiled and the water vapor produced mixes with the air.

Humidifier The device used to provide humidification in the data center or IT room. Humidifiers either use heat or rapid vibrations to create water vapor. The moisture is usually added to the air stream exiting the air conditioner or air handler.

Latent cooling capacity The fraction of total capacity a computer room air conditioner or air handler uses to condense liquid water from the air stream being cooled. Latent cooling capacity does not contribute to data center or IT room cooling.

Latent heat Heat energy that must be transferred to or removed from a substance to change its state.

For example, energy used to boil water (latent heat energy) cannot raise the temperature of the water beyond 212°F / 100°C. Adding more heat will accelerate the boiling (phase change) but will not raise the temperature of the water.

Latent heat of vaporization A term describing the amount of latent heat transferred during a liquid / vapor phase change for a particular substance.

Liquid line A refrigerant pipe carrying liquid refrigerant connecting the output side of the condensing coil to the input side of the expansion valve. In air-cooled systems the liquid line may be hundreds of feet in length.

Lps The abbreviation for liters per second. Lps is used to measure the flow of air through a delivery system or space. Lps is the metric equivalent to CFM.

Locally ducted distribution An air distribution or return methodology in which air is provided or returned via ducts which have vents located near the loads.

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Make-up air Outside air introduced into the IT room or data center. Make-up air is mandated by building codes primarily to ensure the space is fit for human occupancy.

Microprocessor controller A computer logic based system found in precision cooling systems that monitors, controls and reports data on temperature, humidity, component performance, maintenance requirements and component failure.

Multicool A precision cooling system that combines a chilled water coil and an evaporation coil in the same chassis. Either system can be used. Multicool systems can provide high levels of versatility and redundancy.

Plenum Any dedicated space that is used for the distribution or return of cooling air. The space under a raised floor is an example of a plenum.

Plenum rating A special characteristic of electrical and communication wiring that is used in spaces used to transport conditioned supply or return air. Plenum rated cables have lower flammability and heat release characteristics than standard cables.

Power density Electrical power used in a space divided by the area of the space expressed as watts/ft2.

Alternatively power density can be expressed as the average power per rack in a data center expressed as kW/rack.

Precision air conditioning A term describing air conditioning or air handling systems specifically designed to cool IT equipment in a data center or IT room. Precision air conditioning systems maintain temperature (+/- 1°F) (+/- 0.56 °C) and humidity (+/-4%) within much tighter tolerances than regular air conditioning systems. These systems provide high airflow rates (170+ CFM/kW or 4.8+ Lps/kW), are designed for continuous usage, and provide high levels of air filtration. Precision air conditioners are also engineered to minimize the amount of moisture removed from the air during the cooling process.





Psychometric chart The properties of air and the water contained in it at different temperatures arranged in the form of a chart. In particular it shows the quantitative interdependence between temperature and humidity. It is useful in the planning, specification and monitoring of cooling systems.

Schneider Electric – Data Center Science Center White Paper 11 Rev 3 Explanation of Cooling and Air Conditioning Terminology for IT Professionals Pump package A pump and enclosure used to circulate condenser water or glycol on applicable systems.

Pump packages are specified based on desired flow rate and piping losses for each application.

Radiation A mode of heat transfer in which heat energy is transferred via electromagnetic waves. An item warmed by sunlight is an example of radiant heating. Radiation is one of the three forms of heat transfer, which also include Convection and Conduction.

Refrigerant The working fluid used in the refrigeration cycle is known as the refrigerant. Modern systems primarily use fluorinated hydrocarbons that are nonflammable, non-corrosive, nontoxic, and non-explosive. Refrigerants are commonly referred to by their ASHRAE numerical designation. The most commonly used refrigerant in the IT environment is R-22. Environmental concerns of ozone depletion may lead to legislation increasing or requiring the use of alternate refrigerants like R-134a.

Refrigeration cycle Closed cycle of evaporation, compression and condensation that has the net effect of moving heat energy away from an environment and into another environment. Refrigerant changes its physical state from liquid to gas and back to liquid again each time it traverses the various components. As the refrigerant changes state from liquid to gas, heat energy flows into the refrigerant from area to be cooled (the IT environment for example). Conversely, as the refrigerant changes state from gas to liquid heat energy flows away from the refrigerant to a different environment (outdoors or to a water source).

Reheat A heating coil installed in a computer room air conditioner or air handler to assist in dehumidification of the discharge air stream.

Relative humidity The amount of water vapor contained in air relative to the maximum amount the air is capable of holding. Expressed in %.

Return air Air entering an air conditioning system.

Sensible cooling capacity The amount of heat energy the air conditioner can be expected to remove from the IT room or data center. Depending on the operating conditions, this may be less than the air conditioner rating because some of the air conditioner cooling capacity may be used up dehumidifying the air. Dehumidification is typically not desired in a data center but occurs anyway when the return air is low enough in temperature so that the dew point is reached as the air passes Schneider Electric – Data Center Science Center White Paper 11 Rev 3 Explanation of Cooling and Air Conditioning Terminology for IT Professionals through the air conditioner. The resultant condensation of the humidity onto the air conditioner coils represents a loss of cooling capacity. Ideally, the air returning to the air conditioner is a high enough temperature so that the dew point is not reached during cooling, in which case no dehumidification occurs and the full air conditioner rating is used to cool the IT room heat load.

Sensible heat Sensible heat is defined as the heat energy that causes a change in temperature of a substance but does not contribute to a change in state (for example, steam to liquid water) for the substance. The only type of heat energy produced by computers and IT equipment.

Sensible heat ratio The ratio between an air conditioner’s sensible heat removal capacity and its total heat removal capacity. In an IT environment, higher sensible heat ratios contribute to lower operating costs and more effective equipment cooling. Ideally, this ratio is 1, meaning the entire air conditioner capacity is available to cool the IT loads. When this number is less than 1, it means that undesirable dehumidification is occurring in the air conditioner.

Setpoint User-set or automatic thresholds for heating, cooling, humidification, and dehumidification usually measured in the return air stream of the computer room air conditioner or air handler.

Specific heat A term used to describe the relative capabilities of refrigerants and other fluids to transport heat energy. Defined as the quantity of heat required to raise the temperature of a defined amount of a substance one degree.

Split system A computer room air conditioning system (floor, ceiling, or wall mount) where refrigerant is piped to another location for heat to be expelled. The system is said to be split because it consists of two parts: 1) The unit that absorbs the heat from the room (the evaporator) which is connected by refrigerant piping to part 2) The unit that rejects the heat outdoors (the condenser). Most typical built-in air conditioners in a residence are of the Split System configuration, with the evaporator indoors and the condenser outdoors on a ground pad or rooftop.

State change Any change in the properties of a substance among solid, liquid, or vapor.

Supply air Air entering a space from an air conditioner

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Temperature The measurement of heat energy within a body or substance. There are two common scales used to measure temperature, Centigrade and Fahrenheit. The Centigrade scale (also commonly referred to as Celsius) is widely used internationally while the Fahrenheit scale is commonly used in the United States.

Ton (cooling) A measurement of heat energy commonly used historically to measure heat loads in data centers and IT rooms in North America. A ton is equal to 12,000 BTUs and is the amount of heat energy required to melt 2000 pounds (907kg) of ice in one day (24 hours). This is an archaic term typically used to specify heat output when expressed in Tons / day, where the use of the more modern term Watts is the simpler and more universal measure that should be used. Conversions from Tons to Watts are provided at the end of this paper.

Turning vane An air management device installed in many floorstands to assist in redirecting the flow of cooling air from vertical to horizontal as it exits the computer room air conditioner or air handler.

Uninterruptible cooling The process of continued heat removal from the IT room or data center during equipment failure or power failure to preclude thermal damage and data loss due to equipment overheating.

Upflow A term applied to computer room air conditioners and air handlers that discharge air in an upward direction.

Vapor barrier Paint, plastic sheeting, floor or ceiling material specifically designed to minimize the migration of humidity into or out of an IT room or data center.

Water-cooled system A type of precision cooling system widely used in mid-sized to large IT environments. A water-cooled system uses water instead of air as a condensing medium. Condensation takes place in a refrigerant/water heat exchanger typically located within the computer room air conditioner. The water flows in a continuous loop to an outdoor cooling tower where heat is rejected to the outside atmosphere.

Water detector A device used within IT rooms and data centers to sense the abnormal presence of liquid water due to a leak or condensation.

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Water economizer (From from ASHRAE/IESNA Standard 90.1-2007) A system by which the supply air of the cooling system is cooled indirectly with water that is itself cooled by heat or mass transfer to the environment without the use of mechanical cooling.

Watt A measurement of energy commonly used to measure electrical and heat loads in data centers and IT rooms. The wattage consumed by the IT equipment, lights, etc. is the amount of heat energy to be removed from the room by the air conditioning system. This term is becoming more common when specifying cooling systems.

Wet bulb temperature (WB) The temperature of air shown on a wet thermometer as water vapor evaporates from it. The difference between Wet Bulb and Dry Bulb temperatures is a way historically used to determine humidity. Today direct measurement of humidity using electrical sensors causes this terminology to be obsolete.

Working fluid A gas or liquid used to transport heat. In an air conditioning system the working fluid is the refrigerant. In the data center or IT room itself, air is the working fluid used to transport heat energy away from the IT equipment.

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Conclusion A general understanding of the common terms and conversions facilitates more precise communication and fewer mistakes among individuals responsible for planning, managing, servicing and working in IT rooms and data centers.

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