Adiabatic cooling systems. Application of adiabatic humidification for air cooling Practical aspects of system implementation

02.09.2023

As is known, adiabatic humidification allows not only to increase air humidity, but also to lower its temperature, thereby combining the processes of humidification and cooling. At the same time, to implement adiabatic humidification, practically no energy consumption is required - only water is consumed. Thus, the cost of cooled and humidified air is low, which, if used correctly, can significantly improve the energy efficiency of various systems.

Adiabatic humidification of indoor air

The simplest application of the adiabatic humidification process is the cooling of ventilation air - both supply and recirculation. Cooling occurs without the use of a vapor-compression refrigeration cycle and significant energy consumption. However, the resulting air contains a lot of moisture, and its direct supply into the room will create uncomfortable conditions for humans.

For example, with adiabatic humidification of standard outdoor air for the Moscow region with a temperature of 28 °C and an enthalpy of 54 kJ/kg (relative humidity 43%) to a human-comfortable 22 °C, the humidity will increase to 74%, which is higher than the recommended maximum of 60%.

The situation gets even worse if the outside air is even warmer or humid (adiabatic cooling from 26°C/55% to 22°C will result in 78% output, and from 30°C/40% to 82%).

Thus, direct air cooling by the method of adiabatic humidification is limited by the maximum air humidity of 60%, so it must be considered only as an auxiliary process in creating a comfortable indoor microclimate. One of the ways to create comfortable conditions with the participation of adiabatic humidification - indirect evaporative cooling - was discussed in the article “Calculation of an indirect evaporative cooling system” (“Climate World” No. 71).

Adiabatic air humidification in front of the condenser

Another use of adiabatic humidification is to pre-cool the air that is supplied to the condenser of the air conditioning system. This method is most in demand in the warm season.

In this case, it makes no difference what kind of air conditioning system is being considered - a household split system, a multi-zone system or a chiller-based refrigeration system. The design of the capacitor (built-in or remote) also does not matter, although, of course, such solutions are easier to use in combination with a remote condenser. Moreover, the system in question is suitable for use not only with condensers, but also with dry coolers (drycoolers).

The solution is based on the fact that the temperature of the air cooling the condenser determines the condensation temperature of the refrigerant in the vapor-compression refrigeration cycle, and the lower this temperature, the lower the energy consumption of the cooling system, that is, the higher its energy efficiency.

As is known, a decrease in the condensing temperature by 1 °C leads to an increase in the coefficient of performance by 3%. Based on the ID diagram, we can conclude that adiabatic humidification is quite capable of lowering the condensation temperature by even 10 °C. And this is already a third increase in the energy efficiency of the air conditioning system.

The basic diagram of adiabatic air humidification in front of the condenser is as follows (Fig. 1): water from the water supply source passes through the purification system, then it is pumped by a pump and sprayed through nozzles into the air flow in front of the condenser. The appearance of the installation is shown in Fig. 2.

System composition

In general, the adiabatic air humidification system in front of the condenser consists of the following elements:

control system with built-in regulator;
pipes with custom-made injectors (nozzles) - in Fig. 3, mounted on the air intake side;
electric valve for water drainage;
a reducer with a pressure gauge to set the required water pressure for effective spraying;
Softwater (water softener) is an electronic device that reduces water hardness to prevent * deposits of limescale on the finned surface of the heat exchanger (condenser);

electric valve to control water supply;

thermostat for protection against water freezing in the cold season;
control cabinet protected from water (IP65 version when installed outdoors near the humidification system).

Rice. 3. Appearance of injectors

The effectiveness of humidification directly depends on the degree of water atomization, that is, on the diameter of the resulting droplets. In nozzles used in adiabatic humidification systems, the droplet diameter, as a rule, lies in the range of 0.06-0.08 mm.

Another important characteristic for assessing the flow of a mixture of air and water droplets is the soaring speed of the droplet. If the speed of the drop soaring is less than the speed of the air flow created by the condenser fan, then the drop is carried away by the air. Carrying a drop outside the heat exchanger boundary is obviously undesirable. In table Figure 1 shows the characteristic speeds of drop soaring depending on the diameter.

Table 1. Dependence of the soaring speed of a drop on its diameter

d drops, mm	v VIT, m/s
0,01	0,47
0,05	1,06
0,1	1,48
0,2	2,1
0,3	2,57
0,5	3,32
0,8	4,2
1,0	4,7
2	6,62
3	8,12
4	9,35
5	10,5
7	12,4
8	13,3
9	14,1
10	14,8

To reduce the removal of droplets behind the condenser, it is recommended to limit the air speed to 2-2.3 m/s.

Calculation of an adiabatic humidification system using nozzles

Heat and mass transfer in chambers is characterized by the ratio of real heat transfer to the maximum possible heat transfer in an ideal chamber. This relationship is generally expressed by the formula:

where I 1, I 2 are the initial and final enthalpies of air, kJ/kg; I” v.n. — enthalpy of saturated air at the surface of water at its initial temperature; ΔI, ΔI and are, respectively, the real and maximum (ideal) enthalpy differences.

Two efficiency coefficients are adopted as characteristics of the efficiency of heat and mass transfer processes:

where t v.n., t v.k. — initial and final water temperatures, °C; t c1, t c2, t m1, t m2 - initial and final air temperatures according to dry and wet thermometers, °C.

Coefficient E' is called universal because experimental testing has shown its suitability for describing and calculating all processes of air treatment with water.

At the same time, we note that in isenthalpic (adiabatic) processes t m2 = t m1 therefore E a = E’.

In calculations of processes occurring with changes in air enthalpy, the heat balance equation between air and water is additionally used:

where B = W / G is the irrigation coefficient.

The coefficients E, E' and E a depend on the diameter of the outlet hole. In particular, with a diameter of 5 mm we have:

where v and ρ are air speed and density, respectively; the formula is applicable for water pressure up to 2.5 bar.

For the wet-bulb temperature range 8 °C - 20 °C, the heat balance equation can be approximately represented as follows:

From equations (1), (2) and (3) we can obtain formulas for determining air and water temperatures:

The combined use of equations describing changes in the coefficients E' and E and the heat balance equation allows you to perform any calculations, including finding unknown final or initial air parameters. The main parameters that should be determined when calculating the adiabatic humidification systems under consideration are the temperature of the humidified air and the amount of water required for humidification.

Practical aspects of system implementation

From a practical point of view, the characteristics of the supplied water are important.

The maximum hardness of water should be in the range of 8-12°Zh (°Zh - degree of hardness, a unit of measurement of water hardness, introduced in Russia since 2005 and corresponding to the concentration of the alkaline earth element, numerically equal to 1/2 of its mole, expressed in mg/ dm 3; 1°F = 1 mEq/l). In other words, the maximum CaCO 3 content is 80-120 ppm.

The pH value (pH is a hydrogen indicator; a value characterizing the concentration of hydrogen ions) of water should be less than 7 to prevent corrosion on the finned surface of the heat exchanger.

For proper operation of the spray system, the excess water pressure in front of the nozzle must be at least 2.5 bar. The water flow rate for one nozzle depends on the specific injector model; at a pressure of 2.5 bar it can range from 1.15 to 1.9 l/min. (69-114 kg/h).

From the point of view of the system layout, it is necessary that sprayed water does not reach the condenser, since its appearance on the surface of the heat exchanger will impair heat transfer and, therefore, complicate the condensation process. Therefore, the recommended distance from the nozzles to the border of the heat exchanger is 20-50 cm.

In addition, we note that in practice it is not always possible to achieve complete evaporation of sprayed water. Therefore, if the installation is located at a height, and the fall of unevaporated water downwards is undesirable, it is necessary to install a tray and drain the drainage into the sewer system. However, most often such schemes are implemented for capacitors located either directly on the ground or on the roof of a building. In these cases, as a rule, a pallet is not required.

Additional benefits

Using an air humidification system before the condenser provides a number of additional benefits. In particular, a dry cooling tower or remote air condenser is selected for use at a lower outside temperature, which makes it possible to reduce the size of the heat exchange surface, and therefore the size of the device itself. We also note the possibility of cooling the liquid at a higher outside temperature. This allows the equipment to be used at external temperatures exceeding the limit allowed by the manufacturer, because colder air is actually supplied, the temperature of which is within acceptable limits.

In addition, the combination of an adiabatic system with an inverter frequency controller for fans makes it possible to reduce the electrical consumption of motors, significantly reduce the sound pressure level and optimize water consumption.

Adiabatic air humidification and recuperation

Another important application of adiabatic humidification is recuperative heat exchangers.

As you know, in the warm season, recuperation is designed to cool the outside, warmer, supply air at the expense of the cooler exhaust air. In this case, the exhaust air is released into the environment, and, therefore, you can do “anything” with it. In our case, it is proposed to humidify it using the adiabatic method, as a result, thanks to the simultaneously obtained cooling, the recovery of heat (or, in our case, cold) will become more effective.

The diagram of the system under consideration is shown in Fig. 4. The exhaust air first enters the humidification section (“1” in Fig. 4), where it is cooled, and enters the recovery section (“2”), in which it cools the warm supply air.

To evaluate the benefits of using an adiabatic humidification section in front of the recuperator, we will calculate this system.

Outdoor air parameters (point “1”, Fig. 5):

Design pressure: P calculated = 0.1 MPa.
Outside air temperature: t out = +28 °C.
Enthalpy of outside air: iad = +54 kJ/kg.
Humidity of outside air (determined by I-d diagram): φad = 43%.

Internal air parameters (point “3”, Fig. 5):

Temperature maintained in the room: t room = 22 °C.
Humidity maintained in the room: φ room = 55%
Enthalpy of air in the room (determined from the I-d diagram): i room = 45.5 kJ/kg.

Adiabatic humidification will theoretically allow achieving relative humidity up to φ = 100%, but in practice the value of this parameter will be about 90%. Thus, the parameters of the point after the humidifier (point “4”, Fig. 5):

Humidity φ uvl = 55%.
Enthalpy i uvl = 45.5 kJ/kg.
Temperature (determined from the I-d diagram): temp = 17 °C.

To calculate the output parameters, you can use the recovery efficiency parameter (η=30...85% depending on the type of recuperator). For our case, let’s take η=45% and determine the temperature of the supply air after the recuperator t rivers (point “2”, Fig. 5):

Note that the temperature trec can also be determined based on the temperature difference at the cold end of the recuperator (temperature difference between points “2” and “4”). Experience shows that in systems with low temperature differences it is 2-6 °C. In our case, it turned out Δt = t river - t uvl = 28-23 = 5 °C, which correlates well with experimental data.

If there was no section for adiabatic humidification of the exhaust flow before the recuperator, the temperature of the supply air after the recuperator would be:

With a supply air flow rate G air = 10,000 m 3/h, the savings in refrigeration capacity will be:

and its density

On the one hand, this allows you to save on capital costs by choosing a refrigeration unit with a capacity of almost 30 kW less (with a total required cooling capacity of 51.8 kW, a saving of 27.7 kW is more than 50%).

On the other hand, if we consider that the production of 3 kW of refrigeration power requires 1 kW of electricity, a saving of 9 kW of electricity is achieved.

Conclusion

Thus, the cooling effect in the process of adiabatic humidification is difficult to apply for direct cooling of indoor air due to the fact that the resulting air, although it will have the required temperature, but its humidity will significantly exceed the upper limit of the comfortable range.

However, there are a number of possibilities for indirectly using the cooling effect during adiabatic humidification - where the humidity of the resulting air is not important, and only the low temperature is of interest.

This fully applies to the air that cools the condenser or dry cooler of refrigeration units. Thanks to the water spray installation, it is possible to reduce the condensation temperature of the refrigerant by up to 10 °C, and therefore increase the energy efficiency of the air conditioning system by up to 30%.

Another area of application of adiabatic humidification is the cooling of the exhaust flow in front of the recovery section of the air handling unit during the warm season. Due to humidification, colder air enters the recuperator and, therefore, it becomes possible to obtain colder supply air at the outlet.

As practice and calculations show, the introduction of a humidification section in front of the recuperator allows saving more than 50% of the refrigeration capacity required to cool the supply air, which will have a positive economic effect both in terms of capital costs for refrigeration equipment and in terms of operating costs for electricity and power supply for the air conditioning system.

From all of the above, it follows that for energy-efficient solutions in the field of air conditioning systems, you should always keep in mind a tool such as adiabatic air humidification.

Yuri Khomutsky, technical editor of Climate World magazine

The article uses the methodology of the Scientific Research Institute of Sanitary Engineering to calculate an adiabatic humidification system using nozzles.

Up to 35-40% of all energy consumed by a data center is spent on cooling server racks and engineering systems. The adiabatic principle of data center cooling allows for a significant reduction in energy consumption compared to traditional systems. An economical way to cool a data center will be implemented at the DataPro data center in Moscow.

Weather in the data center

In recent years, the density of equipment placement in data centers has increased significantly, and along with it, power costs have also increased. In Russian commercial data centers, one rack on average consumes from 3 to 10 kW - approximately the same amount of heat has to be removed from it. At the same time, the most significant “contribution” to the overall energy consumption landscape is made by cooling systems: their share reaches 35-40%.

In an effort to optimize the traditional design, experts have tried to remove heat by using more efficient refrigerants and by choosing optimal system operating parameters. But these were half measures that did not allow achieving significant savings.

The most energy-intensive component in a traditional cooling circuit is the compressor and condenser units. The elimination of these components in combination with the use of cold outside air (freecooling is the scientific name for the use of free cooling) was the first revolutionary step towards an optimized, low-energy cooling system. This approach has been adopted by many data centers around the world. The principle of free cooling is now widely used in many data centers in Russia - mainly in those regions where the temperature outside the window remains low for many months. Obviously, the use of such technology is quite justified in Murmansk or Norilsk. But is it possible to build an energy-efficient data center in a hot climate? For Russian data centers, this issue is also not idle, since in the summer months in middle and even northern latitudes the air temperature can be quite high.

Hot cooling

Data center "Mercury" eBay company

Paradoxically, there are many examples all over the world of data centers being located in hot climates - in conditions much more extreme than in Russia. For example, eBay built the Mercury data center in the American city of Phoenix, Arizona - in a hot desert, where the thermometer reaches 50 degrees C in summer. And this despite the fact that such factors as continuity and response time of the application are extremely important for eBay's business to the request of users around the world - every second a huge volume of transactions totaling about 2 thousand dollars are concluded on the portal of this company. That is, the reliability of all data center systems is in first place on the list of priorities. It would seem that to cool such a data center, it would be more reasonable to locate it in northern latitudes.

And yet, eBay built its data center in Arizona, and did not fail. It would seem that using external air was out of the question. But, after analyzing all the available options for reducing energy consumption, eBay experts came to the conclusion that free cooling would best provide the required efficiency of a new data center in the desert. The secret is that adiabatic humidification was used in combination with free cooling at this facility.

Wind was blowing from sea

It has long been noticed that the air coming from the sea is cooler than the steppe wind blowing in the direction of the water area. In ancient Rome, houses were cooled in this way: under the open windows there was a pool with a fountain: passing over the water, the air cooled as a result of its evaporation.

Wet cooling towers are also based on this principle, one of the oldest cooling methods that is actively used in production. The operating principle of these systems is based on cooling water by a stream of air blown through its surface.
A more advanced version of this process is used in adiabatic air cooling systems.

Economics of the issue

Adiabatic cooling of a data center is an inexpensive and reliable system that does not require complex units and does not require redundant components. To implement adiabatic humidification, virtually no electricity is required - only water is consumed. Thus, the cost of cooled air is low, which, if used correctly, can significantly improve the energy efficiency of air conditioning systems.

In general, the equipment of modern data centers can withstand both higher temperatures and increased air humidity well. The parameters recommended by ASHRAE (American Society of Heating, Refrigerating and Air Conditioning Engineers) are used as acceptable limits. The first edition of these recommendations, published in 2004, set the upper limit at 25 degrees Celsius at 40% humidity, while the second (2008) set the upper limit at 27 degrees Celsius at 60% humidity. In the 2011 recommendations, two new classes of equipment for data centers appeared - A3 and A4 with a temperature range of up to 40 and 45 degrees. Although such “hot” cooling is not yet widespread, innovation lovers are actively starting to use it. This allows us to significantly expand the geography of application of “green” cooling.

Adiabatic cooling is not always required - only in the hottest months. During the cold season, cooling occurs using external air. Not long ago, adiabatic cooling systems were mainly used in regions with dry and hot climates. But recent developments by climate control equipment manufacturers have shown great potential for the use of adiabatic cooling systems in European regions with temperate climates.

It should be noted that neither the initial water temperature nor the air temperature have practically any effect on the process, unlike humidity, explains Mikhail Balkarov, technical expert at Emerson Network Power. - So if the data center is located in the desert, but at the same time has a source of water, the result is a completely effective system. But if it rains at an air temperature of plus 25 degrees Celsius, then, alas, it will not be possible to extract any cooling from the system, since during rain the humidity of the outside air is close to 100%.

Mikhail notes that it is necessary to take into account local humidity anomalies that occur near large bodies of water. In addition, in Russian regions with changeable weather, it may be necessary to have two systems at the same time - traditional and alternative, which will significantly increase the size of capital investments and may nullify all attempts to save money.

The disadvantage of the adiabatic cooling method is also the increase in air humidity. There may be concerns that humidity will pose a threat to sensitive electronic equipment in a data center. One example of such an incident is discussed below (see section “Facebook in the rain”).

Among other disadvantages of the adiabatic cooling system, the expert notes the water consumption and the need to prepare this water. “Water is consumed about 2 l/h per 1 kW/h at peak consumption and about 0.3 l/h on average during the warm season,” says Balkarov. “This is significant money, and considering the costs of cleanup, it is even more significant.”

It is necessary to purify water, Mikhail Balkarov emphasizes, because upon evaporation all minerals end up in the air in the form of fine dust. “And if for cooling towers this is a fairly cheap process associated with rough cleaning - cleaning is mainly intended to prevent scale - then nozzles in an adiabatic system require microfilters and osmotic filtration,” explains the expert. So not only the cost of the system, but also the operating costs increase.”

When using adiabatic cooling, you should remember that you will also have to solve issues of water supply, water disposal and water treatment, which, in turn, will flow into problems of architecture and building structures. Don't forget about the cost of water. While its price is not comparable with the cost of electricity, it is constantly growing.

WUE coefficient

The use of adiabatic cooling systems reduces PUE and energy consumption, but the water consumption can be very high. Therefore, the Green Grid organization in March 2011 introduced another parameter characterizing the useful water consumption in a data center - the WUE (Water Usage Effectiveness) coefficient. The coefficient is calculated using the formula:

WUE = annual water consumption / IT equipment power

The unit of measurement for WUE is l/kWh.

Facebook became the first data center operator to openly share the meaning of WUE. In the data center located in Prineville in the second half of 2011, this parameter was 0.22 l/kWh.

In general, the use of adiabatic cooling makes it possible to achieve high energy efficiency of the data center: the PUE coefficient can reach a value of 1.043, due to the fact that auxiliary equipment, including the cooling system, even in summer consumes only about 4% of the data center energy, and in winter - even less (in winter PUE period is about 1.018). The efficiency of compressor-condensing systems based on chillers or DX air conditioners is significantly lower; for them, PUE = 1.3 is an excellent result.

The Mercury data center mentioned at the beginning of the article, with an area of 12,600 square meters and a capacity of 4 MW, has been operating for more than a year. The use of free cooling in conjunction with adiabatic evaporative cooling in this data center has proven its effectiveness.

Facebook data centers

Adiabatic cooling system in Facebook data center

Another striking example of the use of new cooling technologies is Facebook data centers. Facebook built its first data center in the American town of Prineville in 2010. A year later, a second, redundant data center was built in Forest City, North Carolina. These sites have PUEs of 1.07 for the Prineville data center and 1.09 for the Forest City data center. This was achieved only due to reduced losses during transmission and conversion of electricity, as well as higher operating air temperatures inside the data center (+35 °C allowed in racks in the cold aisle).

Data centers have a traditional cooling system, but it is used only in emergency situations. The main air conditioning system is direct free-cooling with several air preparation chambers through which outside air passes.

Initially, air from outside is taken by air intakes on the second tier and enters the preparation chamber, where it is filtered and mixed with hot air. The air then passes through the refrigeration panels. They are a humidification chamber with a large number of pipes that spray distilled water with nozzles under high pressure, thereby increasing humidity and lowering the temperature of the blown air. To prevent finely dispersed moisture from conducting electricity, distilled water is used. Further along the air path there are membrane filters that separate large moisture particles. The air is then directed by powerful fans into the machine room. Waste water is collected in a special tank and purified.

Facebook in the rain

One day, a cloud of moisture formed inside the refrigerated room of Facebook's data center in Prineville, which literally covered the server rooms along with their (excuse the pun) “cloud” computing.

In 2001, this data center encountered a control system problem that caused the temperature of the air used to cool the servers to reach more than 26 degrees Celsius and the humidity to exceed 95%. As a result, condensation began to accumulate and a rain cloud formed, filling the entire space with computing equipment. It was impossible to believe what was happening. We started calling our colleagues at the problem escalation center, but for a long time they could not figure out what kind of rain cloud we were talking about? It was easier to convince them that apple trees bloomed on Mars than to convince them of a fairy tale about rain.

To save electricity, Facebook used outside air to cool its data center instead of a traditional system. But after the control system failed, heated air with low moisture levels began to be recirculated through a cooling system based on a water evaporator.

This led to the fact that the air became very humidified and a cloud formed, which caused a lot of trouble. Some servers were completely out of order: those specialists who were in the data center could watch the servers spark and agonize. It was impossible to imagine anything worse. However, the incident did not happen again: Facebook specialists carefully isolated the contacts where the servers were connected to power supplies, protecting them from moisture.

What about in Russia?

Adiabatic cooling systems are not yet very popular in Russia, but experts believe that in the coming years, data center designers will be increasingly interested in them. The reason for this is Federal Law FZ-261, which sets strict limits for energy consumption and requires an increase in energy efficiency by 40% by 2020. The only possible scenario that will satisfy such requirements is the transition to free cooling in combination with adiabatic cooling. And the first examples of such implementations already exist. In particular, this cooling principle will be used in the new DataPro data center under construction in Moscow.

The design of this site involves the use of a cost-effective solution to ensure the necessary climatic conditions - the EcoBreeze modular system manufactured by Schneider Electric. The DataPro company plans to implement the largest installation of this system in Europe in its own mega-data center in Moscow on Aviamotornaya Street - a facility with an installed capacity of 20 MW. The EcoBreeze system is built using the wet cooling tower principle (a form of adiabatic cooling technology) combined with free cooling, which is discussed in this article. In Moscow, where electricity tariffs are high, the use of this system will allow for significant savings in operating costs in the data center.

“Technical solutions using adiabatic cooling cannot be called innovative, since they are successfully used in many data centers abroad,” explains Alexey Soldatov. - But the use of this principle in Russian data centers is a rare phenomenon. The installation of EcoBreeze at our Moscow site is one of the first implementations.”

But at another facility, in the data center of the DataPro company in Tver, the traditional principle using freon routes is used to cool server rooms and electrical equipment, which is due to low capital costs and low electricity tariffs.

At the facility in Tver, another version of the adiabatic principle is used - isothermal humidification to maintain the required level of humidity in server rooms, we will talk about it in our next article.

Operating principle
Mikhail Balkarov. Excerpt from the book “Cooling server and data centers. Fundamentals,” 2011.

The operating principle of an adiabatic cooling system is to atomize water in the form of tiny droplets that are injected into hot air. (The water must be cleared of all impurities.) Water evaporating in the air can cool it to a temperature close to the temperature of the wet thermometer.

Strictly theoretically, the cooling limit in this process is noticeably lower and equals the dew point temperature. To realize this possibility, it is enough to cool part of the initial air to the wet-bulb temperature by evaporating water, and then use it to cool the remainder without humidifying it. Further, the cold air is also humidified, acquiring a lower temperature. The process can be repeated again with part of the air, reaching a temperature close to the dew point. The only obvious technical difficulty in achieving the minimum possible temperature is the severalfold increase in the required volumes of supplied air and the heat exchanger area.

Such systems are made either on the principle of wet cooling towers - that is, they use a large surface of plates covered with a thin film of water - or they spray water under pressure of several hundred atmospheres, through micron nozzles, in very small drops directly into the air ducts.

Next, either the temperature is exchanged with what needs to be cooled, or the moist air is directly used to cool the equipment. Water consumption is about 2 kg per 1 kW/h of heat removed. Since most of the water evaporates, the requirements for its chemical composition increase accordingly, which requires the use of ion exchange filters or reverse osmosis filters.

When using injectors, strict requirements are imposed on mechanical contamination; the installation of microfilters after the high-pressure pump is required. These complications are due to the fact that, starting from a certain droplet size, the evaporation process occurs very quickly, and due to this, the size of the irrigation chamber is significantly reduced.

The use of larger diameter nozzles, medium and low pressure, is easier from the point of view of the operation of the nozzles and the water treatment process. But at the same time, part of the water does not participate in the process and is drained (the drops do not have time to evaporate completely), in addition, the size of the humidification chambers becomes comparable to the rest of the system.

One of the effective ways to increase energy efficiency in a data center is to use adiabatic air cooling, which is based on the unique properties of water.

As is known, to assess the efficiency of energy use in data centers, the PUE (Power Usage Effectiveness) indicator is used - the ratio of total energy consumption to the energy consumption of the data center’s IT equipment. There is also an inverse indicator - DCE (Data Center Efficiency). PUE values from 1.5 to 2.0 are considered typical; the latter means that IT equipment consumes only 50% of the energy consumed (DCE = 0.5). In the case of traditional mechanical cooling systems using specialized CRAC (Computer Room Air Conditioner) air conditioners, they usually account for approximately 35-40% of the total energy consumption.

But there is an approach that allows much more efficient use of energy in a data center - adiabatic air cooling.

Principle of the method

Adiabatic cooling is due to the unique properties of water, which has one of the highest latent heat of vaporization values among liquids (584.8 kcal/kg). Its principle is to spray water in the form of tiny droplets - from an energy point of view, this is much more effective than mechanical cooling (the same principle is also found in natural phenomena). Under adiabatic conditions, in which the total energy content of the medium (expressed as enthalpy) remains unchanged, with the evaporation of 1 liter of water per hour, 680 W (584.8/0.86, where 0.86 is the conversion factor kcal/W) of sensible heat contained in air and characterized by its temperature, turns into latent heat contained in the resulting water vapor. When using spray-type air humidifiers, external energy consumption is relatively small, their typical value is only 4 W per 1 liter of sprayed water, which is due to the relatively low surface tension of water. Thus, the efficiency of the adiabatic cooling process as a whole is characterized by the ratio 680/4 = 170.

Direct and indirect cooling

There are two methods of adiabatic cooling: direct DEC (Direct Evaporative Cooling) and indirect IEC (Indirect Evaporative Cooling); the diagram of their constructive implementation is shown in Fig. 1. Direct cooling is carried out by spraying water on the inflow side. The supply air, cooled by the evaporation of water droplets suspended in the air, is supplied directly to the internal volume of the serviced object. With indirect cooling, water is sprayed on the exhaust side. The cooled air enters a plate heat exchanger, where sensible heat is exchanged with approximately 65% efficiency without transferring latent heat concentrated in water vapor, which is formed due to the evaporation of sprayed water in the hood.

Terms of Use

Both methods have certain limitations in use depending on the heat and humidity characteristics of the atmospheric air. At relatively low temperatures and low atmospheric humidity, direct adiabatic cooling DEC significantly expands the capabilities of the popular free cooling method, or free cooling (FC), carried out without spraying water both in the supply and exhaust. Free cooling is possible provided that the ambient air temperature does not exceed the temperature inside the serviced facility. In the case of DEC, due to the adiabatic evaporation of atomized water, the inlet air temperature is further reduced relative to the ambient air temperature. Thus, natural cooling is provided, without the use of mechanical cooling, at atmospheric air temperatures slightly higher than the temperature inside the serviced object. However, there is a limitation associated with the saturation of air with water vapor. The accompanying increase in enthalpy should not exceed values that correspond to the required temperatures and relative humidity inside the serviced object.

In contrast, adiabatic IEC cooling is only possible when the temperature and enthalpy of the air inside the object being served is lower than the temperature and enthalpy of the ambient air.

It should also be borne in mind that free cooling, in addition to the above temperature limitation, is possible only under the condition that the absolute humidity (moisture content) of the atmospheric air does not exceed the value corresponding to the required temperature and relative humidity inside the serviced facility.

Hence, the share of mechanical cooling (Mechanical Cooling, MC) remains only such a combination of heat-humidity characteristics of atmospheric air, when at the same time both its temperature and absolute humidity exceed the values corresponding to the required values of temperature and relative humidity inside the serviced object.

The optimal values of temperature and relative humidity in data centers are set by ASHRAE TC 9.9 recommendations (2008 edition) and are 230°C and 60%, respectively. In Fig. Figure 2 shows an i-d diagram reflecting the limitations listed above, taking into account these values, which clearly shows the areas of preferential use of various methods of cooling data centers.

Comparative analysis of energy consumption

We conducted a comparative assessment of energy consumption when using various methods of cooling data centers (the results of these calculations are summarized in the table). It was assumed that CRAC air conditioners used in the mechanical cooling system have a COP (Coefficient of Performance, characterizes the ratio of cooling capacity to power consumption) value of 2.8, like most models of devices on the market. The energy consumption of reverse osmosis (RO) units used in water treatment systems is assumed to be 2.4 W/(l/h), which corresponds to typical values.

Examples of data centers where adiabatic cooling has been successfully used include HP Wynyard Park (Middlesbrough, UK; operational since April 2009, achieved PUE value of 1.2) and the Fujitsu data center (Nuremberg, Germany; active since February 2010 ., PUE value reached 1.25). In both cases, the reduction in energy costs for the needs of data center cooling systems was about 95% (i.e., actual costs are about 5% of those for mechanical cooling), which in the first example provided annual savings of $4.16 million. And these numbers speak for themselves.

Weather in the data center

Hot cooling

Data center "Mercury" eBay company

Wind was blowing from sea

A more advanced version of this process is used in adiabatic air cooling systems.

Economics of the issue

Adiabatic cooling is not always required - only in the hottest months. During the cold season, cooling occurs using external air. Not long ago, adiabatic cooling systems were mainly used in regions with dry and hot climates. But recent developments by climate control equipment manufacturers have shown great potential for the use of adiabatic cooling systems in European regions with temperate climates.

It is necessary to purify water, Mikhail Balkarov emphasizes, because upon evaporation all minerals end up in the air in the form of fine dust. “And if for cooling towers this is a fairly cheap process associated with rough cleaning - cleaning is mainly intended to prevent scale - then nozzles in an adiabatic system require microfilters and osmotic filtration,” explains the expert. So not only the cost of the system, but also the operating costs increase.”

WUE coefficient

WUE = annual water consumption / IT equipment capacity

The unit of measurement for WUE is l/kWh.

Facebook became the first data center operator to openly share the meaning of WUE. In the data center located in Prineville in the second half of 2011, this parameter was 0.22 l/kWh.

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Adiabatic cooling system in the data centerFacebook

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What about in Russia?

Operating principle

Mikhail Balkarov. Excerpt from a book "Cooling of server rooms and data centers. Basics.", 2011

Description:

It is obvious that even within just one zone where people engage in physical exercise, air conditioning systems should be designed taking into account the fact that within such a zone, separate areas are allocated for various types of physical activity and air treatment for them should be organized in a special way.

Climatic comfort in fitness centers

Adiabatic cooling with heat recovery

Fitness centers are separate establishments or are part of various multifunctional complexes (swimming pools, hotels, etc.). In recent years, quite large areas (up to 5,000 m2) have increasingly been allocated for fitness centers. Fitness centers include not only gyms, but also swimming pools, relaxation areas with hydromassage facilities, solariums, saunas, Turkish baths, as well as restaurants and bars.

Typically, such a division is carried out already at the stage of drawing up a general plan of the facility, since some types of physical exercise are simply not compatible: for example, aerobics, where there are many people in a relatively small room, and exercises on sports equipment, which take place in more spacious halls, because, in addition to the space for those exercising, space is required to accommodate the exercise machines themselves. Another specific type of exercise is exercise on exercise bikes, where the main problem is moisture removal, taking into account the large volume of latent heat from athletes.

Design data

Each zone of the fitness center is characterized by different occupancy rates and types of physical exercise, which affects the calculated microclimate parameters. In Fig. Figure 1 shows the dynamics of air temperature fluctuations depending on the type of physical activity and clothing of athletes with a thermal insulation index of 0.1 clo (very light), 0.5 (light) and 0.9 (heavy) (abbr. clo - unit of thermal insulation of clothing).

The thermal load created by a person is also determined by the physical exercises performed. The table shows the parameters of the average metabolic index (Met) (human heat output) during various types of exercise. The value of 1 Met corresponds to 58 W/m2. In addition to the type of physical exercise, heat generation is also determined by the intensity of the exercise. In people who are untrained and not accustomed to heavy loads, heat generation usually approaches the maximum - the body releases the greatest amount of heat, mostly in latent form (in the form of sweating), which is thermal compensation and utilization of the increase in temperature caused by muscle tension. As a rule, exercises that require extreme tension are not long and should be alternated accordingly throughout the session. If we take, for example, an exercise bike room, where the average duration of classes ranges from 20 to 40 minutes, then the period of maximum stress, when the greatest amount of heat is released, lasts no more than 5–10 minutes.

The efficiency of physical heat removal, in particular latent heat, is largely determined by the level of relative humidity in the room. As a result, with equal physical stress, a person who is in a room where the relative air humidity is lower sweats less than someone who works out in a room with higher air humidity, since in the first case the air is less saturated and is more inclined to absorb water vapor, secreted by human skin.

In these circumstances, regulating the humidity level in the gym is of particular importance.

Table 1

Type of physical exercise	Met (1 Met = 58 W/m2)
Training apparatus	3–4
Modern and folk dances	4–5
Physical training	4–6
Tennis	5–7
Aerobics	6–8
Running 15 km/h	9
Running 12 km/h	8
Running 9 km/h	7
Martial arts, boxing	7–9
Exercise bike	8–10

Another important factor that should be taken into account is air speed, since it determines the rate of heat exchange between the human body and the air in the room, taking into account the type of physical activity. In this regard, it is advisable to use the evaluation criterion proposed by P. Ole Fanger, a professor at the Technical University of Denmark, who, in particular, notes: “The state of comfort directly depends on the average temperature of the skin and the thermal power given off by the body in the form of fluid secretion , occurring mainly through the mechanism of sweating.”

The total heat generation of a person exercising in appropriate sportswear is 390 W, of which 135 W is sensible heat and 255 W is latent heat (Fig. 2). Considering that the evaporative heat is 2,501 J/g, a value of 255 W corresponds to the release of water vapor in a volume of 367 g/h per person.

Design parameters

Based on the above and taking into account the purpose of individual halls allocated for various sports, it is possible to determine the minimum design parameters for volumetric air flow for individual rooms. When calculating air exchange, you should take into account the amount of water vapor created by sweating, the number of people exercising and the specific type of exercise. Calculating volumetric flow only on the basis of data on the required air exchange (usually from 60 to 120 m 2 / h per person) is not enough here, since corrections are necessary for moisture removal and heat demand. After determining the total volume of moisture released in the room (q mv, expressed in g/h), the volumetric air flow rate required to remove moisture from the air is determined by the difference between the absolute humidity of the internal and supply air and is calculated by the formula:

q ma = q mu / x a – x m, kg/h,

V a = q ma / p a, m 3 / h.

The amount of air required to neutralize the physical heat load (q s) is determined by the difference between the temperature of the internal and supply air and is calculated by the formula:

V a = q s (physical heat load) / 0.34 ∆t, m 3 /h.

It should be noted (by the way, this circumstance is very often overlooked) that the human body, during prolonged physical exercise, consumes oxygen from the air in noticeable amounts. Thus, the more intense the sport for which the room is intended, the more important it is to ensure the required air exchange, regardless of how much the actual heat and humidity parameters of the room satisfy regulatory requirements or calculated data. In order to provide the necessary comfort, the premises of fitness centers in operating mode must be continuously provided with a constant flow of outside air.

Special equipment

For air conditioning fitness centers, specially designed air treatment systems are of particular interest. This equipment has a number of distinctive design features:

Provides cooling power and moisture removal in the volumes required for a specific type of physical exercise;

It is possible to precisely adjust the microclimate parameters depending on the physical exercises performed, when the values of volumetric air flow and heat and humidity parameters of the supply air are set depending on the sensible and latent heat to be removed.

This equipment is characterized by reduced energy consumption thanks to two modern technologies:

Heat recovery from exhaust air using two cross-flow heat exchangers, which are installed in line and operate in counterflow;

Adiabatic cooling system combined with a refrigeration cycle based cooling system.

The air flow rate of this equipment varies from 1,200 to 27,000 m 2 /h, the total refrigeration load (adiabatic system plus refrigeration unit) ranges from 6.6 to 159 kW.

We are talking about completely independent systems supplied complete with electrical equipment and an automatic control system. Supply and exhaust fans have a high-performance free-rotating impeller with curved blades, mounted directly on the electric motor shaft, the rotation speed of which is controlled by a separate inverter. The operation of the system as a whole is regulated by special vibration sensors. Bag filters (EU4 class) are installed on the suction (outside air and exhaust air), are easily removed, maintenance intervals are observed according to the readings of the differential pressure sensor located on the main electrical panel.

The exhaust air heat recovery system is based on two cross-flow plate heat exchangers mounted in line.

The regeneration unit allows for extremely low load loss while increasing the heat transfer coefficient and energy recovery efficiency up to 75%. The condensate collection pan with forced drain is made of polypropylene. Adiabatic air cooling occurs by spraying water over the surface of water exchangers and produces a temperature drop of about 10 °C. The installation is equipped with nozzles, a water level adjustment system, a water supply and drainage valve, a recirculation pump, a filter, a water change system, and an automatic washing cycle.

Operating modes

In Fig. Figures 3–7 show the operating modes of this equipment at different times of the year. In Fig. Figure 3 shows a mode with complete heat recovery, providing summer cooling or winter heating of indoor air. During the transition period, the installation can be started in mode with partial heat recovery by bypassing (bypassing) a certain volume of air from the heat exchanger (Fig. 4) or in full natural cooling mode during the transition or night period through complete bypass (without heat recovery) when the largest volume increases air flow up to 10% (Fig. 5).

In summer, an adiabatic cooling system is used (Fig. 6), which, at high outside temperatures, can be integrated with a cooling and dehumidification system through a refrigeration cycle (Fig. 7).

Translated with abbreviations from the RCI magazine.

Translation from Italian S. N. Bulekova.