Glossary

Adiabatic Cooling

What is it?
Lowering the dry bulb air temperature without changing the total amount of heat in the system

Further Explanation
For water in liquid form to evaporate to become a vapour heat is required. This can be done by adding heat as in a kettle, or by allowing the water to absorb heat from air passing over it. In the latter case Evaporative Cooling takes place with, the total heat in the system being unchanged but more as latent heat and less as sensible heat.

 

Air-Side Free Cooling

What is it?
Cooling by bringing outside air into the data centre

Further Explanation
See ‘Fresh Air Free Cooling’

 

ASD

What is it?
Aspirating Smoke Detection

Further Explanation
The generic term for systems such as VESDA, HSSD and HART. These systems provide a very early warning of a potential fire by sampling air through an aspirating system and detecting particles using a laser based system at the incipient (i.e. pre-combustion) stage.

 

Autonomy (battery)

What is it?
The duration, usually expressed in minutes, that a UPS can support its load

Further Explanation
The time for which a UPS can support the load is a function of the power drawn and the battery capacity. Typically in a data centre this will be something like 10 to 15 minutes. It is possible to have longer autonomy times but the cost and bulk of batteries involved becomes prohibitive and generally it is better to install a standby generator.

 

Big Data

What is it?
Big data is a blanket term for any collection of data sets that are extremely large and complex.

Further Explanation
Big Data is used to describe datasets whose volume and complexity is beyond the ability of typical database software tools to capture, store, manage, and analyse.

 

Cab; Cabinet

What is it?
An enclosure for housing computer equipment also often called a rack

Further Explanation
Cabs are usually either 600mm wide for servers or 800mm wide for comms (to allow for the amount of cabling) and 1,000mm deep. Heights are expressed as ‘U’ (1 ‘U’ being 1.75 inches). Typical cab heights are 42U (overall height around 2,000mm) up to 48U (overall height around 2,300mm).

Cabs always have doors (as opposed to “racks”, which can be open). At one time the doors were usually glass but now, with the adoption of front-to-back cooling of servers, the doors should always be perforated.

 

CDM

What is it?
Construction ( Design & Management) Regulations 2007

Further Explanation
Legislation that, put simply, states that every construction project must be designed and managed so as to be safe to build and safe to run. So the design has to take into account both the construction and the on-going maintenance of the completed facility.

It applies to any project lasting longer than 30 days or involving more than 500 person days. This means all but the very smallest server room projects.

All projects to which CDM applies have to be notified to the Health And Safety Executive before any work commences on site.

For further information visit: http://www.hse.gov.uk/construction/cdm/legal.htm

 

CRAC Unit

What is it?
Computer Room Air Conditioning Unit

Further Explanation
An air conditioning unit specifically designed for use in an IT environment . Features which differentiate these from ordinary office air conditioning (comfort cooling) units include: high sensible heat ratio thus making them far more energy efficient; ability to control relative humidity; higher standard of filtration; greater cooling capacities available; designed to run 24 hours x 365 days a year.

 

DCiE

What is it?
Data Centre Infrastructure Efficiency – see also PUE

Further Explanation
This is one of the key metrics used to express the energy efficiency of a data centre. It is the energy supplied to the IT equipment divided by the total energy supplied to the data centre and is the reciprocal of PUE

 

Diesel Rotary UPS (DRUPS)

What is it?
A combined UPS and diesel generator

Further Explanation
A DRUPS comprises an electric motor, a fly-wheel and a diesel generator. Under normal operation the mains power drives the fly-wheel and generator supplying the data centre. In the event of a mains power failure the kinetic energy in the fly-wheel keeps the system running while the diesel engine starts and then this takes over. Advantages of the system are that the data centre supply is always isolated from any disturbances on the mains and there is no change-over required from mains to batteries and back is in a static UPS. Also more efficient than traditional static UPS’s although this is changing with the introduction systems such as Eaton’s EMS.

 

Dry Bulb

What is it?
The temperature of the air measured by a thermometer with a dry bulb (as opposed to a “wet bulb” – see below)

Further Explanation
Whether it is a weather report or the specification for a data centre, unless it specifically states otherwise, a reference to air temperature will mean dry bulb temperature. It is the dry bulb temperature which determines the rate of heat transfer from the hot servers to the air passing through them.

 

Dry Cooler

What is it?
A device comprising a coil and fan(s) by which heat is rejected to the outside air

Further Explanation
In simple terms a dry cooler is a big car radiator placed outside with fans blowing across it. Water (or more often a mixture of water and glycol) from the data centre air conditioning system is pumped through the dry cooler and thus any time the outside air is cooler than the data centre hot aisle air it is possible to get “free” cooling.

The name “dry cooler” originates from the days when cooling towers were in common usage. A cooling tower has water sprayed into the atmosphere whereas a dry cooler does not.

 

DX

What is it?
Direct Expansion (air conditioning unit)

Further Explanation
Type of air conditioning unit where the evaporator coil (i.e. the coil in which the refrigerant gas expands) is positioned directly in the air flow (as opposed to in a chilled water circuit). The heat rejection side of a DX system is also usually directly from the condenser coil to the outside air although increasingly it is to a condenser water circuit – see Water Cooled DX.

 

EEV

What is it?
Electronic Expansion valve

Further Explanation
The cooling in a compressor driven air conditioning unit relies on the cooling effect which occurs when a gas is allowed to expand. The rate of expansion has to be controlled and this is achieved with an expansion valve. Traditionally these have been mechanical devices known as TEVs (Thermostatic Expansion valves). EEVs, being electronically controlled, are better at maintaining the optimum pressure and temperature with a marked improvement in efficiency.

 

EN1047-2

What is it?
British Standard for fire protection of data storage and IT equipment

Further Explanation
The normal fire ratings of buildings are designed primarily to protect life and as such are concerned with limiting the spread of fire and preventing the early collapse of structures which could then impede escape. The temperatures allowed would very quickly destroy storage media cause damage to IT equipment.

EN1047 limits the temperature rise across the relevant structure (wall, ceiling, floor etc) to 50°C for the stated time (as opposed to 180°C) which will give time for a fire outside the data centre to be extinguished.

 

Evaporative Cooling (also see Adiabatic Cooling)

What is it?
Cooling air by passing it over a wet surface

Further Explanation
Passing air over a wet surface causes the dry bulb temperature of the air to fall and so can be used to assist with data centre cooling. It is achieved by passing air through a fibrous matt which is continually wetted with water. It can only be used on full fresh air systems otherwise the RH of the air will quickly rise to the point where no more evaporation takes place. It is only effective when the RH of the incoming air is low, and it causes the RH to rise which means air at high RH is delivered to the servers which can be problematic.

 

Free Cooling

What is it?
Cooling without the operation of a compressor

Further Explanation
Many older air conditioning systems have to run the compressors even when the outdoor temperature is well below the temperature of the data centre. ‘Free cooling’ systems operate without the need to run the compressors, bringing considerable savings in energy use – see ‘Fresh Air Free Cooling’ and ‘Indirect Free Cooling’.

 

Fresh Air Free Cooling

What is it?
Cooling by bringing cool outside air into the data centre

Further Explanation
Cooling using outside air which is filtered, and, if required, tempered by mixing with return air and humidified using an evaporative humidifier.

 

HART

What is it?
Proprietary ASD system

Further Explanation
See ASD

 

HSSD

What is it?
High Sensitivity Smoke Detection

Further Explanation
See ASD

 

Hybrid UPS

What is it?
A UPS comprising more than one means of energy storage e.g. fly-wheel +batteries

Further Explanation
A number of companies now produce hybrid UPS’s which combine batteries with fly-wheels, super capacitors or compressed air.

 

Indirect Free Cooling

What is it?
Free Cooling using a water circuit – also called ‘Water-side free cooling’

Further Explanation
System using a dry cooler or cooling tower to provide compressor-free cooling via a chilled water coil.

 

Intelligent PDU

What is it?
PDU which allows remote switching and/or monitoring

Further Explanation
The term ‘Intelligent PDU’ is usually only applied to ‘Rack PDU’s’. Remote switching and monitoring can be per PDU or per individual outlet.

 

kVA

What is it?
Kilovolt amp (i.e. Volts/1000 x amps)

Further Explanation
Kilovolts x amps which if the Power factor (see below) is 1 will be the same as kW.

 

kW

What is it?
Kilowatt

Further Explanation
Kilowatts are the actual power used. Whatever is delivered in kW to the room comes out in heat so has to be removed by the air conditioning system.

 

Latent Heat

What is it?
Change in thermal energy due to a change of state (e.g. water to steam)

Further Explanation
In the context of air conditioning, Latent Heat is heat removed in order to reduce the humidity of air.

 

Modular Room

What is it?
A room constructed from pre-formed insulated steel panels

Further Explanation
Modular rooms generally give a higher for rating better protection against water ingress than other forms of construction.

They are also faster to build and avoid the need for any “wet trades”.

The insulation can be mineral wool or foam.

Some, but not all, meet the requirements of EN1047-2

 

MPDU

What is it?
Main Power Distribution Unit

Further Explanation
A panel comprising an incoming isolator and outgoing circuit breakers supplying all of the equipment in the data centre such as the air conditioning, lighting and ancillary equipment as well as the IT load ( via the UPS system) – see also PDU

 

PDU

What is it?
Power Distribution Unit

Further Explanation
A panel comprising an incoming isolator and outgoing circuit breakers supplying the IT load and fed from a UPS (see also ‘MPDU’ and ‘Rack PDU’).

 

PF

What is it?
Power Factor

Further Explanation
Power factor expresses the difference between kW and kVA expressed as a ratio i.e. if the data centre is drawing 100kVA with an 80kW load the power factor is 0.8.

Power factors can be lagging (inductive load) which was normal for IT equipment unit a few years ago or leading (capacitive) which is becoming more common particularly for blade servers. A purely resistive load (e.g. an electric fire) has a power factor of 1.0 (i.e. it draws the same in kW as in kVA).

 

PUE

What is it?
Power Utilisation Effectiveness – see also DCiE

Further Explanation
This is one of the key metrics used to express the energy efficiency of a data centre. It is the total energy supplied to the data centre divided by the energy supplied to the IT equipment and is the reciprocal of DCiE

 

Rack PDU

What is it?
The power distribution strip within a rack

Further Explanation
Strip of socket outlets mounted within the rack – minimum two per rack fed from separate circuits on the PDU(s). Sockets are usually 10amp IEC but can be others particularly for high power computers and blade servers. See also ‘Intelligent PDU’

 

Rack; Server rack; Comms rack

What is it?
An enclosure for housing computer equipment also often called a rack – also see “cab”

Further Explanation
The term “rack” is used interchangeably with “cab” or “cabinet” but the term originates in the telecommunications industry and strictly speaking refers to an open frame.

 

Relative Humidity (RH)

What is it?
The amount of moisture in air expressed as a percentage of the total moisture it could hold (at its present temperature of pressure)

Further Explanation
As air temperature rises its capacity to hold water vapour increases, hence the Relative Humidity falls even though the amount of moisture remains the same.

 

Rotary UPS

What is it?
A UPS which uses a rotating mass to store energy

Further Explanation
See ‘DRUPS’ and ‘Hybrid UPS’

 

Sensible Heat

What is it?
A change in thermal energy without a change of state

Further Explanation
In the context of air conditioning, Sensible Heat is the heat removed in order to reduce the temperature of air without adding to or reducing the amount moisture held.

 

SHR

What is it?
Sensible Heat Ratio

Further Explanation
The sensible cooling duty as a proportion of the total cooling duty. Ideally in a computer room the SHR should be 1.0. A typical ‘comfort cooling’ air conditioner will have an SHR of about 0.7 i.e. only 70% of its power consumption is going into cooling the room the other 30% is condensing water vapour out of the air, resulting in low R.H.

 

Static UPS

What is it?
A UPS which uses batteries to store energy and therefore has no moving parts (as opposed to a Rotary UPS)

Further Explanation
A piece of equipment comprising a rectifier, batteries and inverter which maintains its power output in the event of a failure in the incoming supply. The time for which it can maintain the supply is determined by the capacity of the batteries and is referred to as the ‘autonomy’.

 

TIA

What is it?
Telecommunications Industry Association

Further Explanation
US based trade association for the telecoms industry.

 

TIA 942

What is it?
Telecommunications Infrastructure Standard for Data Centres

Further Explanation
The most commonly used standard for specifying data centre infrastructure. Categorises by ‘Tiers’ from 1 to 4 with Tier 1 being the lowest and Tier 4 the highest.

 

Tier

What is it?
Specification of data centre infrastructure from TIA 942 or The Uptime Institute

Further Explanation
The TIA “Tiers” run from Tier 1 to Tier 4 and define all the parameters of a data centre from its location in relation to transport and urban centres to the number of power supplies. Tier 1 is the lowest and almost any computer room will meet this, up to Tier 4 which requires 2N mechanical and electrical systems and dual incoming mains supplies.

 

Tier Certification

What is it?
Specification of data centre infrastructure

Further explanation
There are two organisations, both based in the USA, offering standards for data centre infrastructure. These are the TIA and The Uptime Institute. Both rate data centres on their ability to cope with events such as power outages and component failures using a grading system from Tier 1 (lowest level) to Tier 4 (highest level). The TIA standard also covers a wide range of other criteria including such things as location in relation to transport and urban centres.

 

Total Cooling Capacity

What is it?
Sensible cooling + latent cooling

Further Explanation
Air conditioning units are usually rated by their total cooling capacity whereas is a data centre it is only the sensible cooling that matters (see SHR). With comfort cooling units the difference is significant with a unit rated at say 10kW (total cooling) only providing 7kW of useful cooling in data centre.

 

UPS

What is it?
Uninterruptible Power Supply

Further Explanation
A piece of equipment in the event of a failure in the incoming supply.

 

VESDA

What is it?
Very Early Smoke Detection Apparatus

Further Explanation
See ASD

 

VIEW

What is it?
Very Early Intelligent Warning (smoke and fire detection)

Further Explanation
An alternative to ASD – uses a large number of laser detectors distributed around the room as opposed to drawing the air to a central sampling unit.

 

Water Cooled DX

What is it?
DX air conditioning system where heat rejection is by a water cooled condenser

Further Explanation
The main advantage of these systems and the reason for their increasing popularity is that they allow ‘free cooling’ during low ambient conditions by having a chilled water coil as well as the DX coil.

 

Water side free cooling

What is it?
Free Cooling using a water circuit

Further Explanation
See ‘Indirect Free Cooling’

 

Wet Bulb

What is it?
The temperature of the air measured by a thermometer with a bulb which is wetted by a wick

Further Explanation
When air passes over a wet surface it causes some of the moisture to evaporate. This change of state from liquid (water) to gas (water vapour) requires energy which comes from the air. This results in a fall in temperature which is how adiabatic (evaporative) cooling works. The rate of evaporation is dependent on the relative humidity (RH) and so the difference between the wet bulb and the dry bulb temperatures, known as the wet bulb depression, can be used to measure RH.