The A to Z of air conditioning

Please find all relevant terms below.

  • BTU

    A unit of energy, the British Thermal Unit is the amount of heat energy required to raise the temperature of one pound of water by one degree Fahrenheit.

    A unit of power, btuh/hr is the rate of energy per unit time. Commonly referred to as Btuh. 

  • Chilled Water (CW) Unit

    Air handler with a water cooled coil that is connected to a chiller.

  • Colocation Data Centers

    A data center that is built by a Colocation provider for end users. The Colocation provider usually provides the physical building, cooling, power, bandwidth, and physical security, while the end user populates that space with their servers and storage. Colocation can be broken down into Wholesale and Retail. Wholesale providers rent out the entire facility, while retail rents out by the rack.

  • Common Energy Metric Terms: SCOP, EER, SEER

    There are multiple energy metric terms seen around the HVAC world in total. The efficiency metric that is mandated in ASHRAE 90.1 for "Air Conditioners and Condensing Units Service Computer Rooms" is the Sensible Coefficient of Performance (SCOP). The SCOP is defined in ASHRAE 127 (Method of Testing for Rating Computer and Data Processing Room Unitary Air Conditioners) as the quotient of the total net sensible cooling divided by the total power required to produce that cooling with all values in like terms (commonly calculated with both parts in kW). SCOP was chosen specific to the computer room due to the unique nature and operation of these spaces. The Energy Efficiency Ratio (EER) was deemed inadequate because it is a metric that is commonly used for comfort cooling units, which commonly must be designed for partial latent cooling. Since the efficient design target for a computer room is a Sensible Heat Ratio (SHR) of one, the EER was deemed not to be an appropriate or useful metric. The Seasonal Energy Efficiency Ratio (SEER) also gives credit for latent cooling and ignores the fact that the cooling load in a computer room is not seasonable, but more or less consistent year-round. For these reasons, SEER and EER are both deemed to be not applicable for the computer room air conditioning units.

  • Containment

    Room design used to separate the return and supply air paths by preventing mixing of the hot and cold air. Containment greatly increases the cooling system efficiency by ensuring higher return air temperatures to the CRAC and CRAHs.

    There are three types of containment; hot aisle containment, cold aisle containment, and rack based containment.


    Computer Room Air Conditioner. Usually seen as a Direct Expansion (DX) unit. 

    Computer Room Air Handler. Usually seen as a Chilled Water (CW) unit.

  • Direct Expansion (DX) Unit

    Two phase refrigerant based cooling unit which takes advantage of the latent heat vaporization of the refrigerant fluid exchanging its energy with the air passing over an evaporator coil.  DX units are differentiated by their means of heat rejection for the condenser portion of the DX circuit, which there are three types: air cooled, water cooled, and glycol cooled.

  • Downflow

    Defines the direction of the supply air path in an air conditioning/air handling unit. The air enters the top of the unit and continues down through the bottom of the unit, into the floor or into an enclosed floor stand. The air may also enter the top of the unit and exit out the front of the unit in designs where no raised floor is present.

  • Upflow

    Defines the direction of the airflow in an air conditioning/air handling unit. Most upflow units will be either front free return or rear return units. Cooled air exits the top of the CRAC or CRAH and may either be ducted or non-ducted.

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