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Computer Room Air Conditioning

1. Introduction

   Precision or close control air conditioning is that branch of air conditioning, which provides the environmental conditions required by computer rooms, telecommunications rooms and other critical applications.

   These applications have a mandatory requirement for the precision control of temperature, humidity, air movement and air cleanliness.

   Comfort air conditioning standards will not satisfy the needs of Precision Cooling. The systems must be designed to meet the needs of the electronic hardware. ‘Hardware comfort’ determines the basis of the system design.

2. Design Criteria
   1. Temperature and Humidity:
      In most applications the design temperature & humidity conditions are 22° C +/-1°C and 50% RH +/- 5% RH.
   2. Room Load
      The typical room load of 400-800 W/m2 is much higher than normal comfort applications. Internet facilities may have up to 1400 W/m2
   3. Sensible Heat Ratio:
      The total cooling load in any room consists of two factors:
      
      • Sensible cooling which affects the dry bulb temperature
      • Latent cooling which is the moisture removal load required
        
      The sensible heat ratio expresses the amount of sensible cooling as a percent of the total cooling (sensible + latent).
      In comfort applications, latent cooling is greater due to the contribution of the higher number of people and additional ventilation air; the sensible cooling is relatively low and typically in the range 60-70% of the total load.
      In precision air conditioning applications, the heat generated is mostly dry heat and the moisture due to people and outside air is very low, resulting in sensible heat ratios in the range of 85-95%. For this reason precision air conditioning equipment must be selected to provide cooling at that same ratio. Standard comfort air conditioners cannot properly condition the room. They will dehumidify excessively causing low moisture levels and excessive humidification requirements, which in turn wastes energy and causes additional maintenance issues.
   4. Flexibility
      Rooms are always expanding and the precision air conditioning equipment must be flexible in meeting these changing needs; these needs can include equipment relocation, facility expansion or changes in heat output. If flexibility is not built into the system, then poor air distribution and hot spots can occur.
   5. Air Distribution
      Precision air conditioning equipment normally has 2 airflow options - Downflow and Upflow:
      Downflow systems are preferable and more common for the following reasons:
      · Air is returned through the top of the unit and supplied out the bottom into a raised floor plenum permitting higher air quantities without disturbing the occupants.
      · Perforated floor outlets can be easily moved giving flexibility in room layout and close matching of the load.
      · The units themselves also form part of the floor grid and can also be easily moved. Upflow systems are used in instances where there is no raised floor. They are typically used for smaller rooms when the air is distributed via a ductwork system or outlet plenum. The upflow systems do not offer the flexibility of the downflow systems when it comes to room reorganization.
   6. Redundancy
      Equipment has become highly reliable; however, it designing a critical application, it must be assumed that all equipment will fail. All systems should be designed with redundancy to provide full cooling continuously. The degree of redundancy should be weighted against the consequences of equipment downtime. A minimum of N+1 redundancy is recommended but future expansion and the degree of backup required should also be considered.
   7. Vapor Barriers
      As rooms must be held at a relative humidity of 50% +/-5% it is very difficult and costly to maintain these conditions throughout the year without a vapour barrier. It is not uncommon to build computer and telecomm rooms in existing office areas with suspended ceilings and to forget to use a vapour barrier over the tile ceiling. Also room partitions are often not extended past the false ceiling, which allows migration of moisture to and from adjoining areas. The impact of a poor vapour barrier may be mitigated by the use of a pressurized system.
   8. Outside Air
      Generally a maximum of 8 - 10 l/s per person of fresh air is required. Ensure that this method of calculating the fresh air is used as the fresh air also has an effect on the room latent load. Avoid using an air change rate per hour, as this will have a detrimental effect on the room Latent load. If there are no occupants then fresh air should be minimized. The design should therefore seek to minimize the level of contaminants, reduce the outside air heat load, reduce the dehumidification load and seek to preserve the target room condition.
   9. Circulated Air Quality
      Comfort air conditioning conditions require the delivery of 600-700m3/hr per ton (200-250m3/hr per kW) of cooling. Precision air conditioning requires a higher quantity of circulated air, usually 1000-1600m3/hr ton (350-400m3/hr per kW) of cooling. This higher air quantity also keeps the SHR high over the cooling coil.
3. Precision Air-Conditioning
   1. Criteria
      Systems selected must meet the specific design requirements including:
      • High sensible cooling capacity
      • Cool, heat, humidity and dehumidify
      • Filtration EU4 and above
      • Full microprocessor control
      • Low noise levels
      • High reliability
      • Built in redundancy with multiple circuits
   2. Basic Types Of Precision Air Conditioning Systems
      For precision air conditioning systems the cooling media available are air, water/glycol and chilled water. The basis of selection depends on the project size, site location, services available and the summer and winter ambient conditions.
      Air Cooled Systems are chosen in 90% of cases. The system is stand-alone system with no dependence on other services for its reliability. The system consists of an indoor evaporating unit and an outdoor condenser. These units can generally operate in most ambient extremes.
      Water / Glycol Systems units are used particularly when the outside ambient in winter drops below about –25oC. The system consists of an indoor evaporator and condenser and an outdoor drycooler or cooling tower. These systems have the added advantages that they are easy to install and also have a free cooling option when low ambient conditions prevail.
      Chilled Water Systems are generally used for very large rooms or when there is a readily available guaranteed supply of chilled water. The system consists of an indoor unit with its own chilled water coil, fan and controls.
   3. Common Problems and Solutions
      Room will not hold Temperature and Humidity.
      This is usually the result of having too much outside air coming into the room. This can be verified when the humidity increases when it rains or decreases during very cold conditions. If the outside air system is properly, then look for (a) little or no vapour barrier used during construction; (b) air passing into the suspended ceiling and into the chamber above where it is being dissipated, or (c) holes through the walls or floors, such as unsealed cable entries, exhaust fans, etc. Using building central air can cause problems due to improper moisture content - too low or too high.
      Room Temperature has been rising steadily with time.
      This may be due to the installation of additional equipment over time with an increasing sensible cooling load, which the cooling equipment cannot satisfy. Not only must more cooling be supplied but also this means that you don’t have enough cooling equipment to provide back-up in the event of cooling equipment failure. Redundancy is needed - always.
      Humidity too low.
      The humidifiers may not have been maintained or a sufficient number installed to meet the humidification loads. If the humidity is too low all time, it is possible that the humidifier needs maintenance, or that the cooling coil is taking out more moisture than the humidifier can handle. As more equipment is added to the data center, the cooling equipment operates at full capacity more often with an increased humidification load. Additional process cooling may be required.
      The room has hot spots.
      Some air re-balancing is necessary to provide air outlets at the heat producing equipment. Usually relocation of the floor grilles or perforated tiles will solve the problem. If overhead distribution is being used, the outlets cannot easily be relocated, a change in the air outlet quantity may be required. We hope our brief introduction to Precision Air Conditioning Systems has helped – if you need any additional information, contact us or any of our Distribution support team.