MD 15116 - 2006 Computer Room Air-conditioning Systems

Chapter 2 - Design Requirements

2.1 General

Computer room air-conditioning systems differ from general office air-conditioning systems in many significant ways:

  • Computer rooms have very sensitive electronic equipment.
  • The cooling load is much higher, due to a higher density of heat generating equipment.
  • Temperature and humidity conditions are required to be maintained at the same levels throughout the year; there are no seasonal variations.
  • Most computer rooms require very tight humidity and temperature control, to meet the equipment manufacturers' requirements.
  • There is little moisture addition so that the load consists almost entirely of sensible heat.
  • Ventilation requirements are much lower than for office spaces in the absence of human occupancy.
  • In office buildings, a breakdown in the A/C equipment may cause some discomfort but in a computer room, a breakdown could lead to disruption of essential services.
  • In office buildings, small changes in temperature distribution are tolerable; however, in computer rooms a single localized “hot spot” can lead to equipment failure.
  • Computer room equipment is more expensive than general office equipment.
  • Rate of change in computer technology is very rapid. This will, in general, lead to increasing equipment density, and higher cooling loads during the life cycle of the computer room.
  • Rapid changes in technology in computer rooms can lead to frequent changes in computer room layout.

The computer room air-conditioning system should also provide sufficient flexibility in air distribution to respond to localize hot spots, and to non-uniform distribution of supply air.

Hence, Computer room air-conditioning systems must be independent of all other air-conditioning systems within the building. Also, the system must be capable of adjusting to the frequent changes in layout and computer equipment installed therein, and also capable of operating while changes in layout and equipment are being made. Redundancy must also be provided for mission critical computer applications.

2.2 Design Requirements for Cooling Equipment

The operating characteristics and the environmental conditions required for satisfactory operation of computer equipment will differ with each computer equipment manufacturer.

All characteristics and required conditions shall be considered during the design. These shall include, but not be limited to, the following:

  1. Heat generated by each piece of equipment.
  2. Range of temperature and humidity levels, in accordance with the manufacturers' requirements.
  3. Some components are designed to be cooled by air delivered directly to the racks from below the raised floor, while others may require cooling air at other points. The design should be in accordance with the manufacturers' requirements.
  4. Required environmental conditions at the rack inlet.
  5. The possibility of short-circuiting of hot air from rack outlet with the cold air at the rack inlet.
  6. The possibility of short circuiting between and supply and return air.

2.3 Room Environmental Conditions

2.3.1 Dry Bulb Temperature

  1. For computer rooms at elevations up to 900 metres above sea level, the dry bulb temperature shall be 21plus or minus 2 deg.C, or, as recommended by the manufacturer.
  2. For computer rooms at higher elevations, the effect of altitude on air density should be considered. When air density is reduced due to higher altitude, the heat transfer rate from the equipment to the surroundings is reduced, for the same temperature differential. Hence, in order to maintain the same heat transfer rate, it is necessary to increase the temperature differential, by reducing the ambient air temperature. To accomplish this, the dry bulb temperature should be reduced by 1 deg. C for every 300 metres above 900 metres altitude over sea level. As an example, consider a city located at an altitude of 1500 metres above sea level. Applying this rule, the dry bulb temperature would be reduced by 2 deg. C. Hence, the allowable dry bulb temperature is 19+-2 deg. C

The dry bulb temperature should be measured at the inlet to the computer equipment racks.

2.3.2 Relative Humidity

The Relative Humidity in the computer room shall be 45 plus or minus 5% throughout the year, or, as recommended by the equipment manufacturer.

To maintain this humidity level, a vapour barrier should be provided for the computer room.

2.3.3 Ventilation Air

Ventilation air should be provided in accordance with ASHRAE Standard 62.1 - latest version, based on the occupant density as well as the area of the computer room.

Computer rooms shall be under slight positive pressure relative to the outside and to the remainder of the building at all times

2.3.4 Air Quality

All air entering the computer room shall be filtered to at least MERV 10 in accordance with ASHRAE Standard 52.2 - 1999, Table E-1, corresponding to a dust spot efficiency of at least 50% with an arrestance exceeding 95%.

2.3.5 Vibration and Sound Control

The sound level in Computer Rooms shall meet the requirements of ASHRAE Thermal Guidelines for Data Processing Environments latest edition.

2.3.6 Sensible Heat Ratio

Latent heat gain in computer rooms will generally be very low. The primary sources of latent heat gain in building spaces are people, and, ventilation air. For computer rooms, both occupancy levels and ventilation levels are very low, and the marginal gain in latent heat is offset by the loss of latent heat to surrounding areas that are at lower humidity levels.

Hence the total heat gain is almost entirely sensible heat, and the sensible heat ratio is close to 1. As a result, dry cooling coil operation is required, without any condensation at the cooling coils.

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