Data Centres. Special Working Group Spin I Relative Humidity in Data Centres

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1 Data Centres Special Working Group Spin I

2 Table of Contents 1. Introduction 1 2. Data Centre Climate Temperature Particles and dust Gases Humidity 3 3. What is Humidity? How can we have water in the air? Absolute humidity and maximum content Relative humidity Relative humidity and heating Dew-point temperature Dry and wet-bulb temperature Energy content in the air (enthalpy) Diagrams Climatic Recommendations ASHRAE Recommendations for Irish data centres Free Cooling Indirect free cooling Direct free cooling Irish Weather and Free Cooling Other Recommendations Water cooling Heat recovery Operation and maintenance 19

3 List of Figures Figure 3.1: Mixing of helium and hydrogen in the same-size container 5 Figure 3.2: Humidity / Temperature Data Sets 6 Figure 3.3: Lines 7 Figure 3.4: Heating Scenario 7 Figure 3.5: Dew Point Scenario 8 Figure 3.6: Dry / Wet Bulb data sets 9 Figure 3.7: Enthalpy Lines 10 Figure 3.8: Changing a Mollier-Ix diagram into a psychometric chart 11 Figure 4.1: ASHRAE temperature and humidity recommendations compared to our recommendations 13 Figure 4.2: Our recommendations shown in a simple two-dimensional diagram and in a Mollier diagram 13 Figure 5.1: Indirect free cooling, with direct expansion (DX), water-cooled condenser, dry cooler and free cooling 15 Figure 5.2: Chiller system with free cooled water circuit 15 Figure 5.3: Ventilation solution with direct free cooling supplemented with mechanical cooling 16 Figure 5.4: Classic DX refrigeration system for cooling below about 20 kw 17 Figure 6.1: Free cooling hours in each month at given cooling temperature, not considering humidity 18 Figure 6.2: Free cooling hours in a full year, at a given cooling temperature, not considering humidity 18

4 1. Introduction The cooling of data centres with outside air ( free cooling ) has become more and more popular, and with this the quality of the outside air has become increasingly important. In the traditional data-centre cooling layout, the recirculated air is not influenced by the outside air conditions, since very little fresh air is brought into the data centre. This document describes what it is important to take into account when designing data centres with free cooling. The main focus will be on direct free cooling, concentrating especially on humidity concerns. Recommendations Data-centre air temperatures should be in the range 24 C 35 C, and the relative humidity be in the range 20% RH 80% RH. If you apply these limits, you can have 6,000 hours of indirect free cooling per year. If you apply the same limits with direct free cooling, there will be very few hours in the year where cooling and humidity control is necessary. But if humidity limits of between 45% and 80% are applied instead, humidification will be necessary for 4,400 hours per year. 2. Data Centre Climate No matter how the data centre is cooled, there are strict requirements on how the climate should be controlled within the centre. There are four climate-related issues to take into account: 1. Temperature of the air 2. Particles (dust) carried by the air 3. Gases (e.g. corrosive) mixed into the air 4. Humidity the water content of the air These issues are important because they can affect the reliability of the servers, measured by the Mean Time Between Failures (MTBF) the average time between each breakdown of a specific piece of electronic equipment. Occupational health and safety concerns, the impact of the building, and fire and explosion hazard topics are also relevant, but not covered in this paper. We first describe the four issues in brief, including recommendations, and then focus specifically on humidity and its relationship to temperature. 2.1 Temperature Room temperature is traditionally considered the most important parameter. If the servers are cooled only by the room air, it is certainly the most important issue. But if, for example, in-rack cooling is used, the room temperature will be of less importance and should be defined by other requirements, such as occupational comfort. 1

5 It is important to understand that room air temperature cannot be described as a single temperature, and that it is closely related to the airflow. These matters are described in detail in A Guide to Energy Savings in Data Centres, published by SEAI. Essentially, if the temperature of the air that meets the individual internal components (electronic and mechanical) gets too high, these components will eventually show increasing frequency of failures (can be self-healing), which will result in irreversible breakdown. The optimal operating temperatures of each of the individual components are not necessarily the same, but the manufacturer of servers and other electronic equipment will define the combined limits, and in some cases also define the airflow necessary to maintain the correct temperature all the way through the equipment. Recommendations The temperature of the cooling air going into the server should not exceed 24 C. As a general rule, the MTBF decreases by 3% 7% for every degree the temperature is raised above 24 C. The MTBF also decreases if the temperature drops below 19 C. The temperature should not exceed 35 C anywhere locally or inside the server. At temperatures above 35 C, there is a risk that the equipment will not get enough cooling and will break down. The temperature should not vary by more than 1.5 C per hour. Rapid temperature changes can result in mechanical strain and stress between joints and contacts, and result in a lower MTBF. 2.2 Particles and dust There are many reasons to avoid particles and dust in the air, not only for occupational comfort but also for reasons of MTBF: Particles will stick to surfaces of the electronic components (heat sinks) and reduce the ability to get rid of the heat, both by radiation and convection to the cooling air. Particles will stick to the blades, both of the internal fans in the servers and in the main cooling fans, and reduce the fan efficiency. Particles will clog filters locally in servers, reduce the airflow and thereby decrease the cooling ability. Particles will clog filters in the main cooling units and increase fan losses. Particles can clog sensors and reduce control quality. Particles can ruin open mechanical components by degrading lubrication and stopping small parts from moving freely. Open contacts can be disrupted by dust, and connectors can lose contact after insertion. Dust can absorb moisture and water from the air, and create leaking currents or short-circuits, especially if high voltages are present. If particles are conductive this can happen even without moisture. This being said, electronic equipment has become increasingly immune to dust, compared to older equipment where many contact (e.g. keyboards) and mechanical parts were open and bigger in number. However, types of equipment vary a lot in their ability to function in a dusty environment. (The author of this report once serviced a mini-computer that was completely clogged with iron particles and dust, and yet it still worked perfectly well.) Recommendations If you use outdoor air for cooling, it is in most cases necessary to filter the air. Outdoor air should be filtered through a fine filter class F7, but finer filters may be needed depending on ambient air particulate matter. A class F7 filter separates the bulk of larger particles, dust and spores. 2

6 The air should be taken from a place where dust, leaves, insects, snow, rain, etc are known not to be present. This simple precaution is often forgotten. Internally circulating air does not generally need to be filtered, but local conditions may still make it necessary. The finer the filter, the greater the pressure drop, and the higher the electricity consumption needed to transport the air. Do not use filters that are finer than necessary. 2.3 Gases Gases are an issue not often addressed when server and control rooms are planned and built. It is important to investigate the risk of presence of corrosive gases, especially in industrial and other control applications. Examples of corrosive gases are: Ammonia, for example, leaking from cooling systems will corrode certain types of contact materials and reduce the MTBF of electrical and electronic installations. Industries using ammoniafilled cooling systems (such as breweries) will often prescribe special types of wire assembly blocks and cables/wires. Sulphuric gases corrode electrical installations. (The author once serviced a server- and control room in a large sewage treatment plant that had been badly damaged by gases emanating from underlying sewage lines.) Recommendations For direct free cooling, the air intake should be taken from a place where corrosive gases are never present. Also data-centre planning and design must mitigate the risk of corrosive gases on site. 2.4 Humidity Correct humidity, like temperature, is an important parameter in obtaining a high MTBF for data-centre equipment. Humidity must be neither too low nor too high. Low humidity Static electricity can damage electronic components. Static high voltages can build up unattended between any stationary equipment parts, building features, and moving equipment/personnel. When the static voltage gets high enough (kilovolts!) a current pulse will flow, usually via a spark. This current pulse is often destructive to electronics, since it will be many magnitudes higher than the extremely small currents flowing inside and between the electronic components. Furthermore, the combination of low humidity and static electricity ensures that particles are more easily attracted to electrical parts, especially if they are carrying higher voltages (such as power supplies). This dust will, as mentioned earlier, decrease the MTBF. Static electricity damage is especially risky when servicing equipment, where arcing and high-pulse currents can be induced by the service technician or his tools, and connection cables which can be charged to several thousand volts. Therefore, service personnel should always be grounded with a conductive wristband. Interconnecting and grounding equipment properly is always a necessity, but the risk can also be minimised by keeping the humidity above a certain limit. The electrical conductivity of atmospheric air 3

7 increases with the water content in the air. A high relative humidity will help to lower the ionisation in the air and lessens the risk of static electricity. Low humidity also causes deterioration of some plastics and rubber materials (they become brittle). High humidity If the water content of the cooling air is too high, there is a risk of condensation on cold surfaces, which can create corrosion on metal parts and faults in electronics. Electronic faults are typically due to leak currents (worst case, short-circuit), through the condensed water on surfaces, moist dust, or hygroscopic materials (hygroscopy is the ability of a substance to attract and hold water molecules), which have absorbed water. This risk is lower if the data centre is sited in a warm place where the intake air is almost always heated. High humidity also creates a risk of micro-organic growth in ducts and air-handling equipment. As well as corrosion, this growth can cause bad odours and, more seriously, cause disease. Recommendation The relative humidity should be in the range 20% 80%. Servers have the best performance and longest MTBF within a relative humidity of 45% 55%. 3. What is Humidity? The growing interest in cooling data centres with outside air has resulted in an increased interest in the practical physics of humid air. The following sections explain in plain language how to interpret the data and characteristics covered in articles and data-sheets. 3.1 How can we have water in the air? The atmospheric air used as cooling air in a data centre is a mixture of different gases, and each gas has different characteristics. These characteristics include the pressures and temperatures at which a gas changes state, e.g. from gas to liquid. All matter can exist in different states: solid, liquid and gaseous. For example, H 2O changes state from solid (ice) to liquid (water) at 0 C, and from liquid to gaseous (steam) state at 100 C. These changes in state represent much more energy than an ordinary change in temperature. However, H 2O also can exist as a gas at a much lower temperature than 100 C, depending on the pressure. At atmospheric pressure, H 2O will become a gas (steam) at 100 C. But even though the pressure of the air mixture is atmospheric, the partial pressure of the H 2O gas in the air is much lower. This is why H 2O gas can still be present in the air below 100 C, and even below 0 C. Simply explained, H 2O has its own unique partial pressure within air, as do the other components of air such as nitrogen and oxygen. However, since the amount of H 2O in atmospheric air is low compared to those of nitrogen and oxygen, the partial pressure of H 2O is also low. The figure below shows an example of mixing helium and hydrogen in the same-size container, to illustrate how both partial pressure and mass are simply added: 4

8 2.9$atm 7.2$atm 10.1$atm 0.6$mol$H 2 1.5$mol$He 2.1$mol$gas Figure 3.1: Mixing of helium and hydrogen in the same-size container Thus, the water in atmospheric air is in a gaseous state (steam), unless there is too much water and it forms small droplets. How much is too much? It depends on the temperature and the pressure. In this paper, we will only look at air under atmospheric pressure (1,013 mbar or 101,300 Pascal). The rule is that the higher the temperature, the more H 2O it will be able to contain before the air becomes saturated. This is why a hair-dryer works so well: the heated air can contain relatively much more H 2O, and the H 2O in your hair will therefore evaporate and disappear. The many terms associated with air humidity, such as absolute humidity, relative humidity, dew-point temperature, dry-bulb temperature, wet-bulb temperature and so on, have been derived experimentally over the years. The relationships between these terms can be expressed as formulas. However, the easiest way to understand and use these terms is through the use of a chart. Once understood, the charts facilitate the understanding and use of the different aspects of humidity. In the following section, we explain the various terms, one at a time, and show their relationships with one another, using simple sketched charts (Mollier-type). 3.2 Absolute humidity and maximum content The absolute humidity (sometimes called humidity ratio) is measured, at a given temperature, in kilogrammes of H 2O per kilogramme of dry air (i.e. air with no H 2O). This data is then plotted on a chart where one axis is humidity and the other axis is temperature. The higher the temperature, the higher is the maximum H 2O content. At any air temperature, we know from experiments how much H 2O gas can be absorbed before it is saturated and becomes fog, droplets or dew: At 20 C the maximum possible H 2O content is kg/kg_dry, and the content can be anything between this value and down to 0.0 kg/kg_dry. At 25 C the maximum possible H 2O content is 0.02 kg/kg_dry. 5

9 These two data-sets are plotted in the figure below: Mollier Ix C x kg/kg_dry Figure 3.2: Humidity / Temperature Data Sets 3.3 Relative humidity It is useful to express humidity as a value relative to the maximum humidity for a given temperature, because practical issues such as drying processes and static electricity are related to how much H 2O the air can contain, rather than the absolute H 2O content. This relative value is expressed as % RH. Relative humidity expresses in % the actual H 2O content compared to the maximum possible content. Diagrams such as the European Mollier-Ix diagram and the US psychometric chart show the relationship between temperature, relative humidity and absolute H 2O content. One axis is the temperature and the other is H 2O content. The relative humidity is shown as curved lines: At 25 C and 100% RH, the absolute humidity is 0.02 kg/kg_dry, and if the humidity is 50% RH, the absolute humidity will be kg/kg_dry. The reason that the absolute humidity at 50% RH is NOT (50% of 0.02) is that relative humidity is calculated from the partial pressure. The figure below shows the relative humidity lines, and the data-sets from the example: 6

10 Figure 3.3: Lines 3.4 Relative humidity and heating Relative humidity expresses in % the actual H 2O content compared to the maximum possible content at the same temperature. The higher the temperature, the higher is the maximum possible H 2O content. If you have a volume of air with a fixed H 2O content (measured in kg/kg_dry), and heat it up while keeping the absolute content constant, the relative humidity will drop: A fixed 20 C air volume with 50% RH equals a H 2O content of kg/kg_dry At 20 C the maximum possible H 2O content is kg/kg_dry (= 100% RH). If this is heated up to 25 C, a H 2O a content of kg/kg_dry corresponds to 37% RH. At 25 C the maximum possible H 2O content is 0.02 kg/kg_dry (= 100% RH) The figure below shows the heating scenario from the example: Figure 3.4: Heating Scenario 7

11 3.5 Dew-point temperature The lower the temperature, the lower is the maximum possible H 2O content. If there is a volume of air with a fixed H 2O content (measured in kg/kg_dry), and it is cooled down while keeping the content constant, the H 2O will start to condense when reaching the temperature where the actual content equals or exceeds the maximum possible content. This temperature is the dew-point temperature: A fixed 20 C air volume with 50% RH equals a H 2O content of kg/kg_dry. At 20 C the maximum possible H 2O content is kg/kg_dry (= 100% RH). This volume is cooled down. At 9.3 C the maximum possible H 2O content is kg/kg_dry (= 100% RH). The dew-point temperature is 9.3 C. General observations: If the dew-point temperature is close to the dry air temperature, then the relative humidity is high. If the dew-point temperature is far below the dry air temperature, then the relative humidity is low. The dew-point temperature of the cooling air is important to know in order to avoid condensation. If the dew-point temperature of the air is equal or lower than the temperatures of the hardware surfaces, dew will form on these surfaces. The figure below shows the scenario from the example: Figure 3.5: Dew Point Scenario 3.6 Dry and wet-bulb temperature Before the electronic age, temperatures were measured with liquid-filled (mercury or alcohol) glass thermometers with a bulb in the end as the measuring point: The dry-bulb temperature is the air temperature measured with a normal thermometer. The wet-bulb temperature is the temperature measured with a thermometer that has a wet measuring point (= bulb). The wet bulb will be cooled from the evaporation of the water on the bulb. This amount of cooling is determined by the humidity of the air. 8

12 If the wet and dry-bulb temperatures are equal, the humidity of the air is 100%RH. At this humidity the wet bulb will not have any evaporation. If the wet-bulb temperature is far below the dry-bulb temperature, the humidity of the air is low. At this humidity the wet bulb will not have a high evaporation and therefore will not be cooled a lot. The humidity of the air can be determined from the wet and dry-bulb temperatures by using the Mollier-Ix diagram or psychometric chart (mentioned earlier). The wet-bulb temperatures are shown as diagonal lines going from top left corner and down (on the Mollier Ix). Where the wet-bulb temperature crosses the drybulb temperature (read from one axis), the humidity can be read either from the water content (at the other axis) or from the relative humidity shown by the curved lines. Note that not all new charts have the wet-bulb temperature lines, but a fairly good approximation can be made by drawing a line from the crossing of a dry-bulb temperature with the 100% RH curve and diagonally up parallel to the enthalpy lines (see section 3.7 below) that are always shown. A 20 C dry-bulb temperature and a 13.5 C wet-bulb temperature will equal 50% RH and a H 2O content of kg/kg_dry. The figure below show the data-sets from the example: Figure 3.6: Dry / Wet Bulb data sets NOTE: Neither the dry nor the wet bulb lines are exactly parallel, as the figures may indicate, but the skew is very limited and it is easier to explain the relations in the charts when parallel lines are assumed throughout. 3.7 Energy content in the air (enthalpy) Working with heating, cooling, dehumidification and humidification of air will always involve some kind of energy change. Engineers will want to know how much energy is needed to change the condition of the air, and some control systems also use the energy content for regulation purposes. The total energy content of air is called enthalpy (I or h), and the unit is usually kj/kg. By convention, the enthalpies of both water and dry air are defined to be 0.0 kj/kg at 0 C 9

13 Enthalpy consists of three quantities: the amount of energy needed to vaporize the H 2O the amount of energy needed to warm the H 2O gas to the required air temperature the amount of energy added to the other components of air (mostly nitrogen and oxygen) These quantities are computed using formulas based on experimental data. The enthalpy lines are similar to wet-bulb temperature lines, being diagonal lines going from top left corner to bottom right corner (on the Mollier Ix). The enthalpy lines are all parallel and evenly spaced. General observations: The enthalpy (not surprisingly) increases if the temperature is increased and the H 2O content is kept constant. The enthalpy increases if the H 2O content is increased and the temperature is kept constant. The enthalpy increases even more if both H 2O content and temperature are increased. At a constant enthalpy, if H 2O content is increased the temperature will decrease. A practical example is spray humidification cooling the air. At a constant enthalpy, if H 2O content is decreased the temperature will increase. A practical example is dehumidification with desiccant dryers. The figure below show the enthalpy lines going through 50% RH and 100% RH for air at 25 C. The arrow A shows dehumidification with desiccant dryer, and the arrow B shows humidification with spray: Figure 3.7: Enthalpy Lines 3.8 Diagrams The Mollier-Ix diagram commonly used in Europe and the psychometric chart commonly used in the US both show the relationship between temperature (dry bulb), relative humidity, absolute H 2O content, dewpoint temperature and energy content. One axis is the temperature and the other is H 2O content (x). The relative humidity is showed shown as curved lines. The enthalpy lines and the wet-bulb temperature lines are shown as diagonal lines. 10

14 The difference between the two types of diagram is that they are mirrored and turned 90, and usually temperatures in the Mollier are in C while the temperatures in the psychometric chart are in F. Differences between the other units can also occur: energy can be in kj or BTU, and water content can be in mass units other than kg. The figure below shows an example of how a Mollier-Ix diagram can be transformed into a psychometric chart: Mollier Ix US psychometric chart x C dry bulb I kj/kg_dry wet bulb C Mirror around a vertical line, then rotate 90 and change units I BT U/lb_dry grains/ lb_dry wet bulb F x kg/kg_dry dry bulb F Figure 3.8: Changing a Mollier-Ix diagram into a psychometric chart One major difference is that the energy lines have different offsets, by convention: In the Mollier diagram the enthalpies are defined to be 0.0 kj/kg at 0 C. In the psychometric chart the enthalpies are defined to be 0.0 BTU at 0 F. As long as calculations are made relatively (such as energy savings), normal conversion factors between units can be used, but if, for example, weather enthalpy for two different climates are to be compared, it is important to apply the correct offset. A few useful conversions: BTU/lb_dry = 7,68 + 0,4299 * kj/kg_dry F = 32+ 1,8 * C Lb = 7,000 grains 11

15 4. Climatic Recommendations 4.1 ASHRAE The American Society of Heating, Refrigerating and Air-Conditioning (ASHRAE) has prepared guidelines for indoor climate and air quality in data centres. ASHRAE treats data-centre air quality similarly to that for cleanrooms used in the semiconductor or pharmaceutical industry, and has transferred guidelines from cleanrooms standards (such as EN14644) to create guidelines for data centres. For example, ASHRAE recommends that data centres satisfy class 8 air quality requirements. This requirement is stricter than our recommendation of a class F7 filter, which gives enough purity to meet the requirements of office areas. In the figure below, ASHRAE temperature and humidity recommendations are compared to our recommendations. Our requirements have been prepared in collaboration with a number of professional partners, and they differ only slightly from ASHRAE requirements. ASHRAE permits temperatures up to 27 C in a data centre, which means that free cooling in many climates can be used virtually the whole year. Whether your actual equipment can operate safely at these higher temperatures should always depend on approval from the equipment supplier. 4.2 Recommendations for Irish data centres Recommendations 1. Temperature of the cooling air going into the server should not exceed 24 C. 2. Temperature should not exceed 35 C anywhere locally or inside the server. 3. Temperature should not vary by more than 1.5 C per hour. 4. Outside fresh air should be filtered through a fine filter class F7. 5. Outside fresh air should be taken from a place where dust, leaves, insects, snow, rain, etc are known not to be present. 6. Outside fresh air should be taken from a place where corrosive gases are never present. 7. The relative humidity be in the range 20% 80% RH. 12

16 MAX. MAX. MAX. MAX. Temperature Relative humidity ASHRAE Our recommendations ASHRAE Our recommendations Figure 4.1: ASHRAE temperature and humidity recommendations compared to our recommendations The figure below shows our recommendations both in a simple two-dimensional diagram and in a Mollier diagram. The light-green area below is where the air in the server room should always be: Figure 4.2: Our recommendations shown in a simple two-dimensional diagram and in a Mollier diagram 13

17 5. Free Cooling The term free cooling is misleading (since nothing is free) but commonly accepted as the name for a cooling method where outside air at a low temperature is used to cool something inside a building. Free cooling is usually classified in two groups: Indirect During cold periods, the outside air is used to cool the cooling system, without the aid of a chiller. Outside air is isolated from the data centre and cools indirectly via a cooling liquid. Humidity considerations and control will be the same as with traditional mechanical cooling. Direct The outside air is used to cool the room directly, and is not isolated from the room. Humidity considerations must be taken since the data centre is no longer isolated from the influence of the weather. Both types of system will always require a chiller system that is big enough to cool the data centre in a traditional fashion, in the event that the free cooling system fails or when the weather gets too hot. Whether using direct or indirect free cooling, the cooling temperature for the data centre is chosen as high as possible. The higher the temperature, the more hours over the year that the outside temperature is low enough to cool, and the bigger the benefit from the high energy efficiency ratio (EER) of a free cooling system. EER is the Energy Efficiency Ratio, and is defined as the ratio between the transported energy (in this instance the cooling power) and the energy needed for the transport (in this instance the power to chillers, pumps and fans): EER = kw cooling / kw chillers+pumps+fans 5.1 Indirect free cooling There are two common types of indirect free cooling system: 1) Direct expansion (DX) water-cooled condenser, dry cooler and free cooling Indirect free cooling supplements the mechanical cooling. In this case the cooling coil in the indoor unit is used when water from the dry cooler is cold enough to cool the cold air supply. This solution is also simple to implement in existing data-centre cooling systems. The figure below shows a set-up with direct expansion (DX), water-cooled condenser, dry cooler and free cooling. Red arrows show heat flow, and blue represents cold: 14

18 CHILLER ROOM SERVER ROOM HOT RETURN AIR OUTSIDE PART FREE COOLING VALVE DRY COOLER WATER CIRCUIT FRESH AIR EXPANSION VALVE PUMP CONDENSER CHILLER COOLING AGENT CIRCUIT COLD AIR TO ROOM Figure 5.1: Indirect free cooling, with direct expansion (DX), water-cooled condenser, dry cooler and free cooling When the outdoor temperature is low, the data centre is cooled by indirect free cooling only. In an intermediate period, cooling the data centre must combine mechanical cooling and indirect free cooling. In hot times of the year, the data centre is cooled by mechanical cooling only. 2) Chiller system with free cooled water circuit This type of indirect free cooling system is essentially a traditional mechanical cooling system, where the water circuit that brings the cooling from the chiller to the data centre is cooled by the outside air when the right conditions are present. This is a larger and more complex solution that is best suited for the design of new data centres or major refurbishments of existing buildings. CHILLER SERVER ROOM CONDENSER CHILLER EVAPORATOR 3-WAY VALVE PUMP HOT RETURN AIR FRESH AIR COOLING AGENT CIRCUIT COOLING COIL EXPANSION VALVE DRY COOLER COLD AIR TO SERVER ROOM FRESH AIR Figure 5.2: Chiller system with free cooled water circuit 15

19 Sometimes a design with a heat exchanger instead of the three-way valve is chosen. In this instance, it is important to use a heat exchanger that has sufficient size to transport cooling at low temperature differences. This is essential to take full advantage of free cooling. When the outdoor temperature is low, the data centre is cooled only by indirect free cooling. In an intermediate period, cooling the data centre must combine mechanical cooling and indirect free cooling. In hot times of the year, the data centre is cooled by mechanical cooling only. 5.2 Direct free cooling There are two common types of direct free cooling systems: 1) Ventilation-type systems The solution is based on a ventilation solution with direct free cooling supplemented with mechanical cooling. The figure below shows the principles: HEATING RECOVERY TO OTHER PARTS OF BUILDING HOT AIR EXHAUST HOT AIR FROM SERVER ROOM AUTOMATED DAMPERS HUMIDIFIE R FILTER MEASURING POINT FOR HUMIDITY AND TEMPERATURE FRESH AIR INTAKE COLD AIR TO SERVER ROOM COARSE FILTER COOLING Figure 5.3: Ventilation solution with direct free cooling supplemented with mechanical cooling The relative humidity and temperature are measured just before the inlet to the data centre. The measurements determine whether the air can be used directly from outside, or if it must be conditioned and/or recirculated. A cooling coil is supplied with cold water from a traditional mechanical chiller system. Supply air temperature is regulated to a level of 24º C. If the relative humidity is too low (<20%), supply air temperature is lowered gradually. When the supply air temperature is down to 19ºC and relative humidity still too low, the humidifier is used via recirculation. Temperature must not drop faster than 1.5ºC per hour. The relative humidity in the supply air must be kept between 20% and 80%, to keep absolute water content at 2.7 to 15.0 g water per kg air at respectively 19ºC and 24ºC. The refrigeration unit and heat exchanger must be designed to provide all the cooling of the data centre on a hot and humid summer day. 16

20 2) DX evaporator with injection This solution is designed as a simple solution where two separate commercial products refrigeration equipment and a free cooling unit are combined into one comprehensive solution. The principle is basically the same as the indirect DX mentioned earlier, but the solution is simpler and can be easily implemented in an existing small data centre. However, there is no option for humidification. When humidity is too low, the system must run with full recirculation. The solution outlined is a classic DX refrigeration system for cooling below about 20 kw. The figure below shows the main principles: HOT ZONE IN SERVER ROOM COLD ZONE IN SERVER ROOM MEASURING POINT FOR HUMIDITY AND TEMPERATURE STANDARD DX COOLING UNIT EVAPO- RATOR MIXING BOX FREE COOLING UNIT EXTRACT AIR FILTER FRESH AIR Figure 5.4: Classic DX refrigeration system for cooling below about 20 kw 17

21 6. Irish Weather and Free Cooling The potential for cooling with outdoor air depends on the requirements for temperature and humidity. The figure below shows how many hours can be used for free cooling in each month at a given cooling temperature, not considering the humidity. The warmer the supply air temperature, the more hours available, especially during summer: Hours Free-cooling hours pr month Based on average of last five years temperatures in Dublin Hours below 22 grc Hours below 20 grc Hours below 19 grc Hours below 18 grc Hours below 16 grc Hours below 14 grc January February March April May June July August September October November December Hours below 12 grc Hours below 10 grc Hours below 8 grc Figure 6.1: Free cooling hours in each month at given cooling temperature, not considering humidity The figure below shows how many hours can be used for free cooling in a full year, at a given cooling temperature, not considering the humidity. The warmer the supply air temperature, the more hours: Free-cooling hours in a year Based on average of last five years temperatures in Dublin Hours Hours below 22 grc Hours below 21 grc Hours below 20 grc Hours below 19 grc Hours below 18 grc Hours below 17 grc Hours below 16 grc Hours below 15 grc Hours below 14 grc Hours below 13 grc Hours below 12 grc Hours below 11 grc Hours below 10 grc Hours below 9 grc Hours below 8 grc Figure 6.2: Free cooling hours in a full year, at a given cooling temperature, not considering humidity The two temperature diagrams can be used to read how many free cooling hours are available with indirect free cooling. Taking the desired cooling temperature (preferably 24 C as recommended) and depending on the size of the dry cooler (and maybe heat exchanger), subtract the differential temperature that is needed to drive the heat transport out. This differential temperature is normally around 10 C in a 18

22 well-designed system. In this example, you end up with (24 minus 10 =) 14 C. According to the diagram above, the data centre will benefit from free cooling for 6,000 hours per year. As a rule of thumb, calculate the energy consumption for cooling using EER=3 when the cooling is with chiller, and EER=25 when using indirect free cooling. If the free cooling is direct, the temperatures in the table can be used without any subtraction. If the recommended supply air temperature of 19 to 24 C is used, practically all the year is available for free cooling, but the humidity of the incoming air must be considered. For example, if using the humidity limits as recommended, between 20% and 80%, there will be very few hours in the year where humidity control is necessary. But if humidity limits between 45% and 80% are used instead, humidification will be necessary 4,400 hours per year. Dehumidification will rarely be necessary, simply because the power from the data equipment will heat the air and decrease the relative humidity. In the example above, even with a 55% upper limit, no dehumidification will be necessary. Still, one must be careful in each individual case to ensure that dewpoint temperature of supply air is never higher than the coldest surface in the data centre, to prevent condensation. 7. Other Recommendations 7.1 Water cooling Cooling with groundwater and seawater is often used as a good alternative to mechanical cooling, indirect free cooling and direct free cooling with outside air. If there is easy access to open water, it may be used to cool the data centre all year round. Groundwater cooling is more difficult and expensive (drilling) to implement, but has the same advantages. 7.2 Heat recovery The hot exhaust air from the data centre may have a temperature of about 35 C or more. It can be used to preheat the fresh air going into office areas, for example. Heat recovery is typically done in one of the following ways: Cross exchanger, where the intake air is heated in a box where the hot air from the data centre passes, separated only by thin heat-conductive plates. Fluid-coupled, where the intake air to the building is heated with a liquid-filled heating coil connected via pipes and a pump to a similar ( cooling -) coil placed in the exhaust air stream. Heat pump, similar to the liquid coupled type, but here a compressor circuit serves to raise the temperature taken from the exhaust air to a level more suitable for heating. 7.3 Operation and maintenance It is very important to perform operations and maintenance, focusing on energy consumption, even when data centres and cooling systems are designed for energy efficiency. All types of plant will eventually deteriorate, and without effective maintenance, energy consumption will increase. Remember: Carry out maintenance systematically, and document all events and actions. 19

23 Keep the condensing temperature in the cooling system as low as possible by periodically cleaning cooling and condenser coils. Dust and dirt on the coils raise the condensing temperature and increase electricity consumption. Check the filters in the cooling system every month and change filters once every quarter. Clogged filters reduce cooling performance considerably and require extra power. Check air filters in cooling units each month, and remember to change them according to the manufacturer s instructions. Clogged filters require extra power to the fan if flow has to be obtained. Make sure the belt drives for fans have the right tension, and remember to change them, according to the manufacturer s instructions. Regularly check that the cooling system set-points (CTS) are appropriate for the task and that they have not been erroneously changed. Also check that all the metrics are within their required limits. New trends in the cooling of data centres continuously evolve. Check new opportunities when you must rebuild, replace equipment or design a new data centre. Check the type of refrigerant. The refrigerant R22 is being phased out (from 2010). Chillers will in future have small charges of refrigerant. 20

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