Engineered Systems - October 2008 - (Page 58)

Humidification Strategies Action Walking across ungrounded raised floor tile Walking across synthetic carpet Static buildup at 80% rh 250V 1500V Static buildup at 20% rh 12,000V 35,000V TABLE 1. Comparison of static electrical buildups at different rh levels. Equipment or room type Typical IT equipment Wiring closets Computer rooms and data centers Allowable rh range 20% to 80% 20% to 80% 20% to 80% Recommended rh range Check product literature 40% to 55% 40% to 55% Maximum dewpoint Not applicable 70˚F 63˚ TABLE 2. Humidity guidelines for IT equipment, computer rooms, and data centers. WHY WE LOSE AND GAIN HUMIDITY IN THE IT ENVIRONMENT Infiltration, condensation, and ventilation cause changes in IT environment humidity. Humidifiers (add water vapor) and dehumidifiers (remove water vapor) maintain IT environment humidity. Infiltration. If we could place a high-humidity body of air next to a low-humidity body of air, the two would quickly equalize to a humidity level between the high and low levels. When computer rooms are at different humidity levels than the outdoor or office space air that surrounds them, the humidity levels constantly try to equalize between the spaces. Obviously, the walls, floor, and ceiling surrounding the IT environment should stop this equalization, but in many cases they do not. Water vapor can escape or enter through any porous surface or microscopic crack and change IT environment rh. The rate of humidity gain or loss due to infiltration is dependent upon the amount of open area and difference in humidity and temperature between the spaces. For example, let’s assume a small data center (73°, 50% rh) was in a location where the outside weather was 35° and 30% rh. If we open a normal-sized door (an emergency exit door for example) between the room and the outdoor environment, the room’s rh would drop below 50% almost immediately. In less than 12 minutes, the room would be below the minimum recommended rh of 40% (assuming no supplemental humidification in the room). If the data center had supplemental humidification, we would lose 6.1 lb of water for each hour the door was left open. Infiltration problems require extra equipment to regulate humidity levels and in severe cases regulation may be impossible. Condensation. Under some conditions, the process of cooling IT environment air can remove large amounts of water vapor resulting in low rh levels. This occurs when warm data center air is drawn through the cooling coil in a computer room air conditioner (CRAC) or air handler (CRAH). Most cooling coils are maintained at a constant temperature of 43° to 48°. This is usually below the dewpoint of the air in the IT environment, so just as in the example of water droplets forming on the cold drink can, water droplets can form on the cold cooling coil. Very large volumes of air (over 160 cfm for each kW of computing equipment) flow through the cooling coil at high velocity. In instances where the air stays in contact with the cooling coil long enough to be cooled below its dewpoint, condensate forms on the cooling coil. Pumps inside the cooling equipment transport condensate away from the IT environment and into the building drainage system. Humidifiers are used to add needed water vapor back into the airstream exiting the cooling equipment. Humidifiers are a very com58 En gi neer ed S y stem s October 2008 mon option found in CRACs and CRAHs and are discussed in the humidity control section of this article. Ventilation. Fresh outdoor air must be continually introduced into all buildings to supply oxygen to people inside. Fresh air supplied into computer rooms and data centers called makeup air affects rh levels. The amount of fresh air required is determined when the room is designed and calculated by considering the room’s specific use, the number of people that may occupy the room, and the laws in effect at the time of construction. Changes in IT environment humidity due to ventilation issues vary widely based on the amount of outside air introduced into the room and the geography of the building’s location. IT EQUIPMENT AND ENVIRONMENT HUMIDITY GUIDELINES Humidity guidelines for computing equipment are published by most equipment manufacturers. Guidelines for data centers and computer rooms are published by ASHRAE. Typical guidelines are provided in Table 2. Maximum dewpoint temperatures are provided to establish criteria to reduce the chances of condensing humidity, especially when the IT environment is subject to rapid temperature change. HUMIDITY MEASUREMENT The single most important place to maintain proper rh is at the cooling air intake opening on IT equipment. The acceptable temperature and humidity ranges for equipment published by IT manufacturers are based on readings at the point of air intake. Most pieces of computing equipment locate the cooling air intake in the front and the exit in the back as shown in Figure 1. Note the exhaust air exiting the server has a higher temperature and lower humidity but the dewpoint is unchanged. This is because the nature of the heat a server generates raises the temperature of the cooling air but does not change the amount of moisture in the air. The concept of using a single dewpoint setting for data center humidity control is discussed later in this paper. Measurement of humidity levels at the IT equipment cooling air exhaust location is not required and has no effect on equipment availability. Since measurement at every piece of equipment is not normally possible, in environments that use rack enclosures it is minimally acceptable to monitor humidity inside the front door of the enclosure adjacent to the computing equipment itself in the top 1/3 of the rack enclosure. This is the elevation where damaging low humidity conditions at the equipment air intake are most likely to occur. (Prior to measurement, ensure blanking panels are installed where necessary and back-to-front airflow in the rack is minimized.)

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Engineered Systems - October 2008 Contents Editor’s Note Letters Back2Basics HVAC Challenge Case in Point Commissioning Building Automation HydroTech HVACR Designer Tips Special Insert: Engineer's Technical Library Fall 2008 Living Wild! Chilled Beams in Chicago Phoenix Convention Center: On The Rise Cold-Climate HVAC Challenges Humidification Strategies For Data Centers & Network Rooms Special Advertising Supplement: Tabb Talk Computers & Software Issues & Events Products Literature & Catalogs Glossary Classifieds Advertiser Index Tomorrow’s Environment

Engineered Systems - October 2008

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