Jiabin LIN, Zhongkai ZHOU, Zhu QIN, Hui LI, Yuxin TANG, Hongru GU, Jie YANG, Chi TANG, Jun CHEN, Gang YU*

1. Institute of Agricultural Facilities and Equipment, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China;

2. Institute of Animal Science, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China

Greenhouses play an important role in the rapid development of facility agriculture. The greenhouses have been expanded rapidly in the planting and breeding industries.However,the high temperature inside greenhouses in summer is a popular problem,which has not been addressed effectively. The high temperature is undoubtedly adverse to the growth of animals[1]. Awnings, light-reflecting sheds and other covering materials can effectively shade. In addition, fans can also enhance the air circulation. However, the cooling effects of the facilities above are relatively weak[2-3]. Although the cooling effect of the wet curtains is relatively good, they have a strict requirement for the airtightness of sheds.Air conditioners consume too much energy.While underground cooling devices require a large investment in the very early period.Even worse,their cooling abilities are limited[4-5].

In recent years,the high pressure spray cooling method has started to be applied in summer. During the Shanghai World Expo,the spray cooling systems in the open squares left the visitors a deep impression. The mechanism of spray systems is same with that of wet curtains. The vaporation of water molecules will absorb the heat in surrounding air, thereby decreasing the ambient temperature. In the spray cooling system, the highpressure water is jetted into air though spiral nozzles,and thus the droplets of several microns are formed, increasing the surface area and accelerating the vaporization rate of water. A smaller gravity also prolongs the residence time of the water droplets in theair, and water stains are substantially not formed on the ground. The vaporization can occur in anywhere inside the greenhouse. The spray cooling system has a higher efficiency.For the uneasily-sealed flexible plastic sheds,the high pressure spray cooling system is almost currently the only effective cooling method[6-7].On the basis of available spray cooling products in the market, the energy-storing high pressure spray cooling system was researched and developed.The ordinary water pump is used as the source of high-pressure water.The partial kinetic energy was stored in the energy-storing tubes. When the water pump is stopped, the energy produced by releasing the compressed air can still be used to maintain the spray. And thus the use-cost and the systematic wear are reduced. The cooling system only requires 1 unit of electricity per day.The energy-storing high pressure spray cooling system is economical and practical cooling equipment that is suitable for the planting and breeding in greenhouses.

Constitution of Energystoring High Pressure Spray Cooling System

The energy-storing high pressure spray cooling system consists of water supply device, high pressure pump,program controller, energy-storing tube, horizontal water pipe and nozzle(Fig.1). The water supply device consists of valve, tank, float in the tank and filter screen. The tap water and deep-well water all can be used as the water source. Under the control of float, the tank can rise automatically,and thus the sediments and other macromolecules in water can be trapped with the filter screen. The water supply device is connected to the high pressure pump. The high pressure pump is connected to one end of the horizontal pipe through a fiber-reinforced high-pressure rubber pipe.The connection is installed with water pressure sensor. The pressure sensor can communicate with the program controller. Thus the high pressure pump is under control. The other end of the horizontal water pipe is connected to the vertical energy-storing water pipe though elbow. The top of the energy-storing water pile is sealed with cap.

Design of Energy-storing System

Water and air are the only media inside pipes of the high pressure spray system. It is well known liquid is substantially incompressible, whereas air has a strong compressibility. During the process of gas compression, the work done by external force will be transformed into energy that is stored in compressed air. The common air compressors, pneumatic picks and pneumatic drills all work based on this principle[8-9]. The energy storage in the spray system is also realized by compressed air. However, the second high-pressure air pump is not added to reduce costs and enhance the security. We noted that in a vertical container of which the top is sealed,a certain amount of air always exits in the top part of the container if the highpressure water is jetted to the lower part of the container. When the highpressure water flows in the container from the lower part,the air in the upper part will be compressed. Thus the energy is stored in the form of compressed air. Once there is a way out,the water will spray out, releasing the stored energy. Therefore, a vertical energy-storing tube is installed in the pipeline of the system.The high pressure water pump only intermittently pressurizes water to the system.When the pressure reaches a certain value, the pump will stop working. Instead,the compressed air starts to expand,releasing the stored energy.And thus a continuous spray is maintained.

Selection of Main Parts of Spray System

High pressure pump

The spray systems in the market generally adopt professional high pressure spray piston pump, which is connected to 3-phase motor with power higher than 2 kW. Thus the longterm uninterrupted work is realized.Since the piston pump maintains a stable water pressure through discharging high-pressure water, part of the energy will be wasted. Even worse, the device’s weight and size are all large, and it is also expensive.It is not economical if the high pressure spray cooling system (piston pump) is only used in summer[10]. We selected the household car wash pump as a power source for the spray system. Its main technical parameters are as follows: input voltage, 220 V;power, 1 200 W; work pressure, 60 kg/cm2; flow, 12 L/min; weight, 23 kg.The household car wash pump is inexpressive, easy to purchase accessories,and easy to maintain.However,it cannot work continuously for a long time. Moreover, it can only start under low water pressure.

Program controller

The spray system is installed with both semi-automatic and automatic program control accessories to meet different actual needs.The semi-automatic controller adopts the digital display multi-period cycle time relay, and its start and end are controlled by hand. The operator should carefully set the 2 times to keep the water pressure within the set range and prevent the damages to pipeline caused by too long working time or too high pressure. The life of the mechanicalrelay contact is 105 times. If there is 1 time of press on the contact within 1 min and the relay works 12 h per day, the working times of the contact are 720 per day and the contact can work 139 d (four and a half months).Since the time relay only allows a low current through, a high-current solid time relay is installed in the output section. The solid relay is a kind of non-contact electronic switch, and its work-life is not limited. The solid time relay will significantly improve the reliability of the control system.

The automatic controller adopts the 32-bit embedded system. It is assembled with color touch screen and a plurality of temperature, humidity and wind speed sensors. The startup and shutdown times can be reserved. In addition,a series of control on multiple criteria PC programs can also be achieved. For example, the fans can be first started to ventilate and cool;then the spay system is started; and then the spray system is shut down when the humidity reaches a certain value.

Anti-blocking design

The anti-blocking design of the high pressure spray system is very important. The aperture of the nozzle outlets ranges from 0.01 to 0.03 mm,so the tiny impurities in the water all can cause clogging. In the practical application of the spray system,well or reservoir water can only be used in some cases. Therefore, the requirements for anti-blocking design are relatively restrict. In the energy-storing high pressure spray cooling system,the filter of commercial water purifier is installed at the intake to intercept sediments and other large particles of impurities. The filter has a transparent housing, so the amount of dirt can be observed visually. Its pressure limit is 3 kg/cm2.The filter can be directly connected to water pipe, so the water pressure can be used to filter and add water. In addition, the self-cleaning dirt-decomposing anti-blocking device,a patented technology, is installed inside the nozzle at the output to prevent the blocking of nozzle caused by small impurities and algae that are remained in water[11-12].

Selection of key valves

Non-return valve is an essential part for the spray system. When the pressure inside the pressure-adding water pump is larger than that the energy-storing system can bear,the nonreturn valve will be opened to import water to the system; when the water pump is stopped, the water pressure inside the system is larger than that in the pump circuit, so the non-return valve will be shut down automatically,preventing the accumulated energy discharged from the pump pipeline.

Circuit fault of the control system can lead to the pump pressure exceeding the preset value, resulting in the burst of the pipeline. To prevent the phenomenon above from happening, a safety valve is set up in the system. When the water pressure is larger than the limit, the safety valve will be turned on spontaneously to discharge pressure and alarm simultaneously.

If the spray system is shared by several greenhouses, some diverter valves should be installed in the system, providing water to each branch pipe respectively as needed.

Table 1 Bearable pressure limits of selected PPR pipes in the spray system kg/cm2

Pressure Limit Test of Energy-storing System

The safety of the system should be ensured since the pipeline of the system is long maintained at a high pressure.At the same time, the economy and operability of selected materials should also be taken into account due to the practicability of the system.The PPR hot water pipe is selected as an alternative piping material for the safety studies[13]. The outside diameter and thickness of the energy-storing pipe are 63 and 8 mm, respectively,and of the horizontal pipe are 20 and 3 mm, respectively. PPR pipe is most commonly used in recent years. It is mainly used in the water system of home decoration. The PPR pipe adopts the hot melt connection. It has complete accessories.The market has a large demand for PPR pipe. Moreover, the PPR pipe is cheap and easy to operate. However, the nominal value of bearable pressure of PPR pipe is only 14 kg/cm2. Therefore, we conducted a series of extreme tests to examine the actually bearable pressure of the selected PPR pipe. Thus whether the selected PPR pipe can be used in the spray system is judged.To test the bearable pressure limit of the whole system, 4 kinds of same-model PPR pipes and accessories were selected. They were connected and installed in the system with hot melt way.Then the pressure inside the system was added constantly by the electric pressure detector until the pipe burst.The determined bearable pressure limits of the selected pipes are shown in Table 1.

In the pressure limit tests, the burst all occurred in the upper part of the energy-storing pipe.The horizontal pipe, elbow, hot melt connection and non-return valve were all not damaged. The results indicated the energy-storing pipe is the weakness of the bearable pressure of the entire spray system. The bearable pressure of the energy-storing pipe determines the bearable pressure of the entire spray system. If the working pressure of the spray system is assumed to be 25 kg/cm2, the radially withstood pressure of the energy-storing pipe is 369 kg/cm(25 kg/cm2(pressure)×4.7 cm (inner diameter)×3.14),and of the ho-rizontal pipe is 109.9 kg/cm (25 kg/cm2(pressure)×1.4 cm (inner diameter) ×3.14). The calculation results showed the withstood pressure of the energystoraging pipe, of which the inner diameter is largest,is largest,and this is also consistent with the test results.Therefore, in order to maintain the spray pressure at a certain level, the diameter of the energy-storing pipe should not be reduced. The actual burst pressure of the PPR pipes wasaround 60 kg/cm2, which was almost twice the normal working pressure(10-30 kg/cm2) of the spray system.Therefore, the system is theoretically safe.A 3-year actual application of the spray system indicates the energystoring pipe will not be damaged if the circuit is smooth and the working program is normal.

Length of Energy-storing Tubes

The main purpose of energy-storing system design is to reduce operating costs and to reduce the starting load through reducing the working hours of the pump, thereby reducing electricity consumption and prolonging the life of the machine. If the stored energy per time is too little, the machine should be started frequently. In that case, our design intent is not fully reflected. From the analysis above,under constant pressure,the larger the inner diameter of the pipe is,the larger the radial pressure withstood by perunit pipe is. According to the test results, it can be concluded the upper limit of inner diameter of the energystoring PPR pipe is 47 mm. Then the length of the energy-storing pipe should be determined further to ensure a reasonable interval for the work of the pump. The working interval of the pump is closely related to the water flow from the nozzle and the energystoring capacity of the energy-storing pipe.The water flow from the nozzle is determined by machining precision,wear and tear, smooth or blocking of the water circuit and water pressure.So the water flow from the nozzle cannot be estimated in theory. But it can only be determined practically. In the actual running of the spray system,when the systematic pressure declines to 10 kg/cm2, the pump will be started;when the systematic pressure rises to 30 kg/cm2, the pump will be stopped,and the stored energy is used to maintain the continuous spray. It is indicated the working pressure of the spray system ranges from 10 to 30 kg/cm2.In tests, 20 kg/cm2is treated as the constant pressure.Thus the water flow of a single nozzle is determined to be 30 ml/min. If the nozzle within a spray system is assumed to be 20,the water flow of the entire spray system is 600 ml/min.However,in actual application,the actual water flow of the spray system is less than 600 ml/min.This is because the pressure is a non-linear variable. With the decrease of the pressure, the water flow trends to decrease nonlinearly.

When the systematic pressure declines from 30 to 10 kg/cm2, the volume of increased air inside the system can be calculated with the ideal gas equation:

Wherein, P refers to pressure; V refers to the volume of gas; n refers to the molar volume of gas; R refers to the proportional constant; T refers to the absolute temperature.Since the increased temperature of the gas inside the pipe, brought by adding pressure,is absorbed by surrounding water and pipe wall,the absolute temperature(T)can also be treated as a fixed value.In addition,

Wherein, S refers to cross-sectional area of the pipe; h refers to the height of the compressed air. When the variation of gas volume in the same-segment of the energy-storing pipe is studied, S is also a constant.The formula can be simplified as:

Wherein,P0refers to the standard atmospheric pressure; h0refers to the height of gas under normal atmospheric pressure; P1refers to the gas pressure when the pump is stopped;h1refers to the height of compressed air when the pump is stopped;P2refers to the gas pressure when the pump is restarted to add pressure; h2refers to the height of gas when the pump is restarted. The variation of water level in the energy-storing pipe is shown in Fig.2. Under normal pressure, the energy-storing pipe and the water pipeline are filled with air; when the spray system starts to work, the air inside the water pipeline and the energystoring pipe are all compressed to the energy-storing pipe. So under normal pressure,the total volume of air inside the system refers to the sum of air inside the energy-storing pipe and the water pipeline. The standard length of the energy-storing pipe is 4 m. In order to minimize the use of the materials, the length of the energy-storing pipe is shortened to 2 m. The length of the water pipeline is 30 m. If the air inside the water pipeline is all compressed to the energy-storing pipe,the height of the compressed gas will be 266.2 cm ((1.4/2)2×3 000/(4.7/2)2).Therefore, under normal pressure, the total length of air inside the energystoring pipe is 4.66 m. When the pressure inside the spray system is 30 kg/cm2, the h1will be 15.53 cm (P0h0=P1h1). In that case, the volume of the compressed air is 269.30 cm3(V1=(4.7/2)2× π × 15.53). Then the pressure inside the energy-storing pipe will be released gradually. When the systematic pressure is decreased to 10 kg/cm2,the h2will be 46.6 cm (P0h0=P2h2). Then the volume of the compressed air is 808.07 cm3(V2=(4.7/2)2×π × 46.6). In order to meet the systematic water flow of 600 ml/min, the working interval of the pump should be set around 1 min. The nominal flow rate of the pump is 12 L/min. It requires only 3 s to inject 600 ml of water to the system. However, since the injected water should overcome the pressure resistance, the injection time will be slightly prolonged. Combining the mechanical properties of the water pump,the rhythm of 3-5 s of work,along with 1 min of interval is more reasonable. Therefore, 2 m of energystoring pipe can basically meet the working requirements.

The developed energy-storing high pressure spray cooling system(20 nozzles, the spacing between 2 adjacent nozzles was 1. 33 m) was ever installed in a deep-litter breeding shed (length, 30 m; width, 10 m) located in Luhe Animal Base of Jiangsu Academy of Agricultural Sciences.The start-up time and the working intervalof the water pump were set as 5 s and 1 min, respectively. The spraying effects were good. The power of the pump motor was 1 200 W.If the working time of the system was assumed to be 12 h, the daily actual working time of the pump was 1 h (5 s/min × 60 min/h × 12 h = 3 600 s). So the daily power consumption was only 1.2 kilowatt hours. The practices proved that the power-saving effect of the spray system was remarkable.

Conclusions

The developed energy-storaging high pressure spray cooling system is characterized by small size, low cost,low energy consumption, and simple maintenance. It has been widely used in unsealed breeding sheds in summer. After a continuous improvement,its functions have been extended to disinfecting, removing dust, humidifying, immunizing and cooling squares,venues and streets. The energy-storing high pressure spray cooling system has a broad application prospect.

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