Experimental study of dew point evaporative cooling air-conditioning unit by inner loop for data-centers buildings

. According to the climate characteristics, a new dew point evaporative cooling air conditioning unit was design and analysis. Through take return air of the data-center into the dew point evaporative cooling air conditioning unit, then it in dry channel was cooled by wording air come from fresh air at wet channel. The work performance of the unit was test at different outdoor climate conditions of refrigeration performance parameters, and take a data-center project of Lanzhou as an example, the potential of energy saving and economic were analysis. Result showed the energy saving effect was significant and economic recovery within 1.5 years.


Introduction
With the rapid development of the Internet plus information technology, the construction of large data center showed a rapid growth trend, financial, Reduce data center energy consumption, research and development of air conditioning equipment applicable to the data center, has become a hot spot for the development of the industry.

Characteristics of cooling load and common cooling schemes 2.1 Characteristic of cooling load
Due to the electronic equipment heat release a large amount at work, in order to guarantee the normal operation of the equipment, must be cool to cool the interior and equipment. Because indoor few personnel, so the load characteristic of data center is: a, big load density, year-round operation need cooling; Second, give priority to with sensible heat, usually without moisture load, and even in part time if need to add wet in winter.

Design of the dew point evaporative cooling air conditioning unit
The inner loop mode, that is, let indoor return air flows through the evaporative cooling equipment, and after cooling to continue into the interior, because the return is not mixed with air, it guarantee the indoor air quality is not affected by outdoor air; At the same time, there is no change in the moisture content of air return, will not lead to equipment failure due to high humidity.  When the fresh air through the filter into the refrigeration core body; Core body after evaporation liquid water to form water film and take away heat, fresh air will become the work flow of low temperature and humidity air; Work in low temperature air return through the tube wall heat exchange with high temperature, return air is damp or wet but, become output flow of low temperature, to use place, as shown in Fig.2.

Test parameters and test platform
According to the working principle of evaporative cooling air conditioning unit, air return type developed corresponding unit, the unit of structure diagram as shown in Fig.3 and Fig.4.

The analysis of experimental results and discussion
According to the result of the return air temperature and humidity measurement, it can be seen that indoor air parameters of Chambers can better maintain the dry bulb temperature 24 ℃, relative humidity 60%, wet bulb temperature at about 18.50 ℃. When the outdoor climate parameters change, the temperature and humidity of the air supply air flow (output) have great changes, the related refrigerating capacity and can effect comparing also there is a big change. When outdoor temperature is higher, the dry bulb temperature 30.70 ℃, the wet bulb temperature 20.80 ℃, for example, due to air dry bulb temperature and wet bulb temperature difference only 3.3 ℃. The experimental results show that the output air dry bulb temperature can drop to 20.80 ℃, at this point, the return air generating condensate, therefore all refrigerating capacity for sensible heat capacity, can effect comparing is 3.76, higher than general precision air conditioning around 3.0.
When outdoor temperature is low, such as outdoor air dry bulb temperature at around 9 ℃, back to the dry bulb temperature can drop to 9.8 ℃. And as a result of the air temperature is lower than the return air dew point temperature (15.7 ℃), so the return air through the unit with condensed water, refrigerating capacity, including sensible heat capacity and latent heat of the refrigerating capacity. Under the working condition of two points, the sensible heat refrigeration can effect comparing, respectively up to 15 or so, the whole thermal capacity can reach more than 22.
Circulating power of dew point evaporative cooling air conditioning equipment is two main fan and a water pump, fan of the test machine power is about 0.41 kW; Export pressure at about 2.5 Pa.
Circulating within the analysis results show that mentioned above dew point evaporative air conditioning unit in the outdoor climate is not at the same time, the refrigeration capacity and eer is different. To find out the comprehensive energy saving potential of circulating dew point evaporative air conditioning unit, it is necessary to analyze its usage for the whole year.

Output air temperature and humidity change rule
Proven circulating within the energy conserving potential of the dew point evaporative air conditioning unit and its influencing factors, is conducive to the application of engineering design and operation control. Effect in circulating dew point evaporative air conditioning unit energy saving potential under different climate conditions are the main factors of output of air temperature and humidity and refrigerating capacity. For year-round climate output of air temperature and humidity data, the following will be on the basis of the experimental data fitting to get the corresponding empirical formula, and then according to the import and export of enthalpy difference refrigerating capacity. tsd = 0.015txs 2 + 0.3txs + 7.5 0 txs≤6 (1) tsd = 0.013txs 2 + 0.34txs + 7.33 6 txs≤ 12.2 (2) tsd = 0.071txs 2 -1.3txs + 18. 86 12.2 txs≤34 (3) Under different wet bulb temperature cooling effect is also different. According to the comparison of fitting results and the experimental results, the above formula can be drawn from the error is within 10%.

Engineering case analysis
In a data center room project as an example, in Lanzhou internally circulating the energy conserving potential of the dew point evaporative air conditioning unit are analyzed. The project for small data center, precision air conditioner and circulate in evaporative cooling air conditioning unit according to 1:1 configuration.
Distribution of all-year hourly wet bulb temperature interval in lanzhou, hours, corresponding to the unit under the condition of refrigeration capacity are shown in table 2, and output of return air it for fixed air volume for 1500 m3 / h, fixed return air conditions for 24 ℃ dry bulb temperature, wet bulb temperature of 18.5 ℃, the dew point temperature 15.7 ℃, the absolute moisture content of 11.3 g/kg.
Consider below 0 ℃ water condensation phenomenon will happen, so set preconditions for the working temperature of wet bulb temperature greater than 0 ℃. According to the all-year hourly climatic data in Lanzhou, wet bulb temperature greater than 0 ℃ time number is 6102, the highest temperature of 21.5 ℃. In fact, when the outdoor air wet bulb temperature is lower than 0 ℃, dew point evaporative cooling unit can still as air -air heat exchanger, cold quantity for the data center. Did not limited to, below are analyzed.
From tab.2, you can see that under different outdoor conditions, the refrigeration capacity of the unit is different. With the increase of outdoor dry wet bulb temperature, output air temperature will increase; When outdoor wet-bulb temperature above 18 ℃, refrigerating capacity becomes a negative, that there is not only not refrigeration, instead of return air heating. With the outdoor air enthalpy value is greater than the return air enthalpy. It also can be seen that some climate throughout the year, unfavorable open the inner loop of indirect evaporative cooling unit. 2) when the outdoor air wet bulb temperature is greater than 20 ℃, the low efficiency of circulating dew point evaporative cooling air conditioning, strong suggestion unit; 3) the climate in Lanzhou area as an example, the use of circulating inside the dew point evaporative cooling air conditioning unit, energy-saving effect is remarkable, the economic recovery period in 1.5 years.