«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 ...»
Explanation of Cooling and
Air Conditioning Terminology
for IT Professionals
White Paper 11
by Tony Evans
Executive summary Click on a section to jump to it
As power densities continue to increase in today’s data
centers, heat removal is becoming a greater concern HVAC terms 2
for the IT professional. Unfortunately, air conditioning terminology routinely used in the cooling industry is Conversion tables 15 unnecessarily complicated. This complexity makes it Conclusion 17 difficult and frustrating for IT professionals to specify cooling requirements and even makes it difficult to Resources 18 discuss current cooling system performance with contractors, engineers, and maintenance personnel.
This paper explains cooling terms in common language, providing an essential reference for IT professionals and data center operators.
white papers are now part of the Schneider Electric white paper library produced by Schneider Electric’s Data Center Science Center DCSC@Schneider-Electric.com Explanation of Cooling and Air Conditioning Terminology for IT Professionals Rising power densities in today’s IT environment have driven the need for IT managers to Introduction understand the planning, purchasing, operation and maintenance of cooling solutions.
Unfortunately the Heating/Ventilation/Air Conditioning (HVAC) industry routinely uses complicated and highly redundant terminology. Apart from the terms related to mainframedriven cooling methodologies, there is an influx of new terms related to the server and rack enclosure architectures common in today’s IT environment. This unnecessarily complex terminology makes it difficult for IT managers to effectively communicate requirements to cooling professionals, which may lead to suboptimal cooling solutions.
The complex jargon used to describe cooling systems can be easily explained in simple terms of heat, humidity, temperature, pressure, and flow. Cooling-related units of measure
ASHRAE TC9.9 Technical Committee for Facility and Equipment Thermal Guidelines for Data Center and other Data Processing Environments. This is a consortium of IT users and manufacturers creating common guidelines for the standardization, layout, testing and reporting of IT rooms and data centers.
BTU The abbreviation for British Thermal Unit. A measurement of heat energy commonly used to measure heat loads in data centers and IT rooms in North America. A BTU is defined as the amount of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit in one hour. This is an archaic term typically used to specify heat output when expressed in BTU/Hr, where the use of the term Watts is the simpler and more universal measure. Conversions from BTU to Watts are provided at the end of this paper.
Ceiling mount A small precision air conditioner hung from, or suspended above, a ceiling. This type of air conditioner comes in many designs, but usually is connected to a heat rejection unit on an outdoor pad or rooftop via refrigerant or water lines.
CFM The abbreviation for cubic feet per minute. CFM is used to measure the flow of air through a delivery system or space.
Chilled water system A type of precision cooling system widely used in mid-sized to large IT environments. A chilled water system uses water as a cooling medium. Cold water is pumped from a chiller to computer room air handlers designed to cool the space. A chilled water air conditioner can be thought of as similar to a car radiator with a fan, with hot air being cooled by being blown through a cool radiator. In a chilled water system cooling an IT facility, the chilled water may be provided as a utility in the building, or special dedicated water chillers may be installed.
Chiller A device used to continuously refrigerate large volumes of water. A chiller uses the refrigeration cycle to produce large volumes of chilled water (typically at 45-48°F / 7-9°C) that is distributed to Computer Room Air Handlers (CRAH) units designed to remove heat from the IT environment.
Clean room A room that is virtually free of dust or bacteria; used in laboratory work and in assembly or repair of precision equipment. Clean rooms usually use precision air conditioning.
Comfort air conditioning Common air conditioning systems designed for the comfort of people. When compared to computer room air conditioning systems, comfort systems typically remove an unacceptable
amount of moisture from the space and generally do not have the capability to maintain the temperature and humidity parameters specified for IT rooms and data centers.
Compressor The compressor is an essential component in the refrigeration cycle that uses mechanical energy to compress or squeeze gaseous refrigerant. This compression process is what allows an air conditioner to absorb heat at one temperature (like 70°F / 21°C) and exhaust it outdoors at a potentially higher temperature (like 100°F / 38°C).
Condensate The water that results as a by-product of dehumidification. Condensate is usually pumped out of the IT room or data center (via a condensate pipe) into the building drainage system.
Since maintaining humidity is a desired goal of a computer room air conditioning system, dehumidification is typically not a desired function. However, dehumidification and the resultant production of condensate commonly occur as a result of sub-optimal design.
Condenser coil A condenser coil is one means of heat rejection commonly used in an air conditioning system. It is typically located on an outdoor pad or on a rooftop and looks like an automobile radiator in a cabinet. It is usually hot to the touch (120°F / 49°C) during normal use. Its function is to transfer heat energy from the refrigerant to the cooler surrounding (usually outdoor) environment. The related Dry Cooler or Fluid Cooler serves the same purpose of heat rejection and physically appears similar, with the difference that the condenser coil uses hot refrigerant which changes from a gas to liquid as it move through the coil, whereas the Fluid Cooler uses hot liquid such as water or a water-glycol mix.
Conduction A mode of heat transfer in which heat energy is transferred within an object itself or between objects in contact. When a cold spoon is left in a pot of boiling water, the spoon eventually becomes hot. This is an example of conduction. Conduction is one of the three forms of heat transfer, which also include Convection and Radiation.
Convection A mode of heat transfer in which heat energy is transferred from an object to moving fluid such as air, water, or refrigerant. The heat sink of a computer processor is an example of heat transfer by convection. Convection is one of the three forms of heat transfer, which also include Conduction and Radiation.
Cooling tower A heat rejection method that transfers heat energy from a data center or IT room to the outside atmosphere via the evaporation of water. In a cooling tower, water is sprayed onto a high surface-area packing material as large volumes of air are drawn across through the structure. The net effect of this process is that a small portion of the water circulated through the cooling tower evaporates into the outside atmosphere. The remaining water (now cooler) is collected at the bottom of the cooling tower.
Schneider Electric – Data Center Science Center White Paper 11 Rev 3 Explanation of Cooling and Air Conditioning Terminology for IT Professionals CRAC The abbreviation for Computer Room Air Conditioning unit. A device usually installed in the data center that uses a self-contained refrigeration cycle to remove heat from the room and send it away from the data center through some kind of cooling medium via piping. Must be used with a heat rejection system which then transfers the heat from the data center into the environment. The heat rejection system typically takes one of the following forms: condensing unit, fluid cooler, or cooling tower to discharge heat to the outdoor atmosphere.
CRAH The abbreviation for Computer Room Air Handling unit. A device usually installed in the data center or IT room that uses circulating chilled water to remove heat. Must be used in conjunction with a chiller.
CHR The abbreviation for chilled water return, the term used for all piping intended to return chilled water from the computer room air handlers to the chiller.
CHS The abbreviation for chilled water supply, the term used for all piping intended to deliver chilled water from the chiller to the computer room air handlers.
CWR The abbreviation for condenser water return, the term used for all piping intended to return condenser water from the chiller to the cooling tower.
CWS The abbreviation for condenser water supply, the term used for all piping intended to deliver condenser water from the cooling tower to the chiller.
Dehumidification The process of removing moisture from air. In the data center or IT room, most dehumidification occurs as moisture-laden air flows across the cold evaporator coil. I basic example of the dehumidification process is when a cold soda can is left outdoors. The water moisture in the air is removed by condensing on the surface of the can as water droplets.
Design condition The desired properties for an environment expressed in dry bulb temperature, wet bulb temperature and relative humidity. Design conditions are commonly used during the planning stages of a data center or IT room as a basis to aid in the specification of air conditioning systems. Cooling equipment manufacturers normally published performance data of air conditioning systems at several design conditions.
Dew point (DP) The temperature at which water vapor begins to condense. On a hot summer day, a cold soda can is below the dew point which causes condensation on the surface of the can.
Direct expansion systems (DX) A general term applied to computer room air conditioning systems that have a self-contained refrigeration system and are air, glycol, or water-cooled.
Downflow A term applied to computer room air conditioners and air handlers that discharge air in a downward direction. Typically used to feed air to a raised floor, but also can distribute air at floor level if the air conditioner is placed on an elevating stand.
Dry bulb temperature (DB) The temperature of air shown on a standard thermometer.
Dry cooler See “Fluid Cooler”.
Economizer coil The term applied to an additional cooling coil installed into glycol-cooled computer room air conditioning units to provide free cooling in cold climates. The economizer coil contains cold glycol circulating directly from the fluid cooler when atmospheric conditions allow.
EER Abbreviation for energy efficiency ratio, a measurement quantifying the performance of a compressor relative to its energy consumption. A higher number is generally better.
Enthalpy The total quantity of energy used to heat or cool a substance between two temperatures including the energy used to change the state of the substance if applicable. For example, if we heat a sample of water at normal atmospheric pressure from 33°F to 275°F (1°C to 135°C), the enthalpy is the sum of the sensible heat energy added (from 33°F / 1°C to 212°F / 100°C and from 212°F / 100°C to 275°F / 135°C) and the latent heat energy added (state change from liquid to vapor at 212°F / 100°C).
Evaporation The process of a liquid becoming a vapor. If a cup of water were boiled for long enough, all the water would be gone. By adding heat, all the water becomes a vapor and mixes with the air.
Evaporator coil The evaporator coil is an essential component used in the refrigeration cycle. It looks like an automobile radiator. This is the part of the system that gets cold to the touch (about 45°F / 7°C for air conditioning systems) during normal use. Its usually found inside the space we need to remove heat from. Cold-feeling air that exits an air conditioner has just transferred some heat energy to the flashing refrigerant as it passed through the evaporator coil.
Expansion valve The expansion valve is an essential component used in the refrigeration cycle. It regulates the flow of high-pressure liquid refrigerant into the evaporator coil. It is designed to open just enough to let refrigerant flow while maintaining a high pressure differential from its inlet to its outlet. The pressure at the exit of the expansion valve is low enough that it initiates a phase change in the liquid refrigerant to a vapor. A pressurized spray can is an example of how an expansion value works. If you spray a can of butane fuel for a few seconds, the can will become colder as the pressure inside decreases.
Firestat A device located in the air conditioner that warns of a fire and initiates unit shutdown when return air temperatures exceed a pre-set threshold.
Flash A term used to describe the change in state of refrigerant from a liquid to a vapor inside the expansion valve and evaporator coil of a computer room air conditioning unit.
Flooded distribution An air distribution or return methodology in which the computer room cooling system and IT equipment eject or draw in bulk air from the room without any special ductwork between them.
Floorstand A device used to raise the height of the computer room air conditioner or air handler to match the height of the raised floor and manage the flow of air exiting the unit.
Fluid cooler A device consisting of coils and fans to transfer heat energy from a flowing glycol stream to the outside atmosphere.
Fluid regulating valve A device, often controlled by an electric motor, to regulate the flow of water or glycol through the coil and / or heat exchanger in a computer room air conditioner or air handler.
Free cooling A practice where the outside atmosphere is used to directly cool the IT room or data center.
There are two common types of free cooling. Airside free cooling introduces cold outside air directly into the IT room or data center when atmospheric conditions allow. Waterside free cooling uses an additional cooling coil containing cold glycol circulating directly from the fluid cooler when atmospheric conditions allow. There are building codes for areas in the Pacific Northwest that mandate free cooling for all data centers.