Abstract
This paper analyzes the characteristics of terminal devices of cold air distribution system, and summarizes the research progress of terminal devices of cold air distribution system in china, points out the key issues to be addressed and possible future research directions. through extensive literature review. The main problems of cold air distribution system’s terminal devices are the condensation of diffuser surface and poor flow in room.At present, the research on cold air terminal devices focused on improving the performance of existing terminal devices, researching and developing new terminal devices, exploring new air supply methods which are suitable for cold air system and so on. It is a good solution to the problem that condensation of outlet’s surface that processing terminal devices by Super hydrophobic surfaces techniques when using conventional outlet. It can get good airflow and prevent condensation on diffuser surface that using the textile air distribution system in cold air distribution system. Exploring the methods that conventional air terminal devices used in cold air distribution system and researching new approach of supply are the future research directions.
Keywords: cold air distribution system terminal devices air conditioning system
research progress
1. Introduction
Cold air distribution conditioning system is different from the conventional air conditioning system, the supply air temperature of conventional air conditioning systems is 12 ~ 16 ℃, while the air temperature of cold air distribution conditioning system is 4 ~ 10 ℃ (Hua Zezhao, Liu Daoping, Wu Zhaolin & Wu Zhimin, 1997). Combined with ice storage technology, cold air distribution system developed rapidly and applied wildly. Ice storage technology and cold air distribution technology are researched in US and Japan earlier, and has been used in a large number of projects. cold air distribution technology is researched in china lately, but after 20 years of development, has made great progress and has been applied in some domestic engineering.
In the cold air distribution system, due to lower supply air temperature, so cold air conditioning system has many advantages compared to the conventional air conditioning system. With the supply air temperature reducing, air supply temperature difference will increase, and room air output will decreases, the capacity and power of the blower can be smaller, the operation cost can reduce. Due to reducing air output, the size of air ducts and equipments can be reduced the initial investment will reduce. it can also save space of buildings and reduce construction costs at the same time. Cold water’s temperature of cold air distribution technology is 1 ~ 4 ℃, it can make that supply air has a lower relative humidity, and make relative humidity of room air to drop, improve the comfort of air-conditioned rooms. However, because of the supply air temperature is generally lower than the dew point temperature of indoor air and the air output is small,the air distribution of air-conditioned room is poor, and it always dews on the surface of outlet. The terminal devices of cold air distribution system is the key to solving these two problems.
The terminal devices used in the traditional cold air distribution system divide into two categories: the mixing box and special diffuser. Through literature review, the author studied the following three questions: 1. What are the problems of conventional terminal devices used in low temperature air supply system? 2. what is the current research situation of terminal devices of cold air distribution system ? 3. What are the new solutions to the terminal devices? This paper analyzed the problems of the mixing box and the current solution to them, and then analyzed the research progress of special diffuser, and last concluded a new solution to the terminal devices of cold air distribution system.
2.Mixing Box
Mixing box is used in combination with conventional outlet. It makes a cold air mixed with induction return air of the conditioned area in the mixing box, and then sent out by the conventional outlet, after reaching the temperature of conventional supply air. Mixing box is divided into the mixing box with a fan and the induced mixing box without fan. According to the arrangement of fan, the mixing box with a fan can also be divided into tandem mixing box and parallel mixing box.
2.1 The mixing box with a fan
Series hybrid box makes a cold air mixed with induction return air of the conditioned area, and then sent it to the interior conventional diffusers through the fan of mixing box. The structure shown in Fig.1. However, a cold air do not through the fan of mixing box in parallel hybrid box. And after the interior induction return air which was sucked by mixing box fan mixed with a cold air, it would be sent to interior through the conventional diffusers. The structure shown in Figure 2. The mixing box with a fan is often used in low temperature air supply system because it’s easy to use, easy to adjust the air volume, etc. But the use of mixing box with a fan is restricted because of its disadvantages.
The main disadvantages of the mixing box with a fan are: 1. using the fan, the motor efficiency is not high and high energy consumption. 2. need to be installed in the ceiling, occupied building space. 3. the fan noise, affect the indoor environment. 4. repair and maintenance workload, and the high cost.
Ying Xiaoer developed a new mixing box with fan. This mixing box has double fans, the one is a low temperature blower and the other is indoor induced return fan. The low temperature blower sucks the cold air which was treated by the combined air-conditioning. Induced return fan absorbs indoor recirculated air, mixed with the cold air, and then sent to the air conditioning area. The structure shown in Fig 3. Performance test results showed that this new hybrid energy efficient than series hybrid box (Ying Xiaoer, Chen Yonglin & Ye Shuiquan, 2006).
Fig.1 Tandem mixing box Fig.2 Parallel mixing box
Fig.3 Mixing box with double fans Fig.4 The induced type mixing box
2.2 Induction type mixing box
Induction type mixing box based on ejector principle, the low temperature air supply will induce the return air to the mixing box, and then sent to the indoor diffuser after mixing. the structure as shown in figure 4. This kind of mixing box has the advantages of low energy consumption, no moving parts, repair workload of the small, low noise. It can increase the air conditioning load because the mixing box induced draft in the ceiling. and because the injection wind around needs certain space, it brings certain difficulties to other equipment installed in the ceiling.
Yu Tingting simulated the flow and heat transfer of internal air of induction type mixing box, and studies the flow law of internal gas and its induced performance, drawed the conclusion as follows:
(1) the air flow pattern of the mixed box is in line with the basic law of the air jetmixing. The change of the speed of supply air and the change of sizes of the air inlet had no effect on flow law of internal gas of the mixing box.
(2) with the increase of primary air velocity, the induction ratio is increasing, mainly due to the small wind speed is easy to produce a wind short-circuit phenomenon, affecting. with the increase of wind speed, the induction ratio will goes to be stabilized.
(3)when the sizes of outlet and inlet changed, the induction ratio of mixing box will also change. the induction ratio will increase continually with the increase of port area of air outlet or inlet. The change of air outlet’s size has a great influence on the induction ratio (Yu Tingting & Wang Yi, 2012).
3.Special air diffuser
Special air diffuser for low temperature air supply is specially designed for low temperature air supply diffuser. Special diffuser has the advantages of no dew, satisfy indoor comfort requirements, low energy consumption. It is the most commonly used terminal device. But its high production costs of cold air distribution system. currently mainly imported low-temperature diffuser in china, as it’s price is too high, utilization of cold air distribution technology in practical engineering restricted.
In 1990s, American scientists have developed for nozzle type air diffuser and scroll diffuser, the structure of them as shown in Fig. 5 and fig. 6. In recent years, the domestic scholars have conducted many studies on special diffuser, and achieved certain results (Hua Zezhao, Liu Daoping, Wu Zhaolin & Wu Zhimin, 1997).
In 1997, Cai Deyuan has developed windmill-centrifugal fan induction-diffuser (as shown in Fig. 7). Based on the analysis of the test results and calculated, this model induced by type end has high induction ratio, and the indoor temperature and velocity distribution is uniform and stable, and there is no draft sensation and the noise is low in indoor (Cai Deyuan & Liu Yingqing, 1997).
In 2005, Shi Chuping ,who developed a swirling type cold air supply air diffuser (as shown in Fig.8), and tested and analyzed the performance of samples. The experimental study shows that, swirling type cold air diffuser has unique air supply mode, has a high induction ratio, can form the airflow uniform distribution and temperature distribution in air conditioning room. Used in conjunction with conventional air adjusting device, it can achieve good temperature regulating effect and energy saving effect. The wind temperature of the diffuser outlet is slightly higher than that of conventional air conditioning, it can avoid air condensation phenomenon caused by cold air. This kind of air diffuser has the advantages of simple structure, production, cost far less than induced fan box type terminal air supply device, it can be long-term trouble free operation, easy for users to accept (Shi Chuping, Xia Qingmei & Xiao Biao, 2006).
Fig.5 nozzle type diffuser Fig.6 vortex type diffuser
Fig.7 windmill-centrifugal fan induction-diffuser Fig.8 swirling cold air diffuser
4.Other solutions
In addition to improving the performance of mixing box and of exploiting new special diffuser, Chinese researchers also try to apply conventional outlet in the cold air distribution system by some technical means, and scholars have tried to use terminal devices that is made of special material in the cold air distribution system.
Ding Tao research the performance of the conventional double deflection grille, when it is applied to the cold air distribution system. he found that it can produce satisfactory results by choosing appropriate wind speed of air supply, when the conventional double deflection grille is used in cold air distribution system (Ding Tao & Li Rui, 2002).
Zhang Zhili has carryed out experimental study of condensation characteristics of the three kinds of common outlet at low temperature, analyzed the mechanism of dewing of various air outlet according to the experimental phenomenon, and made soft-start control strategy for the use of the ordinary air outlet at low temperature. if achieving the soft-start, it can avoid condensation occurs in the surface of ordinary outlet by controlling the temperature of air supply according to air temperature (Zhang Zhili & Wang Dijun,2004).
Yin ping,who researched the condensation characteristic of conventional diffuser which was processed by super hydrophobic surface treatment technology. it shows that using the super hydrophobic materials processed conventional diffuser were not appear dew under various conditions (Yin Ping, Shen Guoli & Wang Hua, 2006).
Ji Zhaoliang studied the application characteristics of the fabric type terminal air supply device in cold air distribution system, and pointed out that the fabric type terminal air supply device can well solve the problem of draft sensation and the condensation. at the same time, the device also has the performance of environmental protection and other advantages. it is a very good terminal device for cold air distribution system (Ji Zhaoliang & Jiang Jian, 2009).
5.Conclusions
In this article, through reading and analyzing the literature about the terminal devices of low temperature air supply air conditioning system, I draw the following conclusions:
(1) Since supply air temperature of cold air distribution system is low, generally lower than the dew point temperature of the indoor air, it will dew on the surface of the outlet and affecting the indoor environment, when conventional terminal device is applied to cold air distribution system. In addition, since temperature of the supply air decreases, the volume of supply air is less than conventional air conditioning, it will make air change rate of the air conditioned room to reduce and the air to get worse, and affects quality of indoor air. Furthermore, because of the large temperature difference, it will happen that cold air sinks fast or hot air floats quickly when the supply air gets into the air-conditioned room. Distribution of cold and hot is uneven and the thermal comfort will go bad indoors. Improving air distribution and preventing condensation on outlet’s surface is the key to cold air distribution system can be widely applied, and air terminal devices is a key factor affecting the indoor air distribution. Therefore, the study of terminal devices suitable for low-temperature air supply system is very important to the promotion of ice storage and cold air distribution conditioning system .
(2) At present, the domestic researches for terminal devices of cold air distribution conditioning system mainly focus on the following several aspects:
a. Improving mixing box
Enhancing the performance of mixing box and reducing its energy consumption.For instance, it will improve the efficiency of the mixing box by Changing the single fan to double fan. It can not only reduce energy consumption of the mixing box, but also improve the mixing box’s ability to regulate the amount of supply air, when replacing the ordinary fan by frequency conversion fans .
b. Developing new special air diffuser for cold air distribution system.
Domestic researchers had developed many practical special air diffuser for cold air distribution system through researching the characteristics of the jet, such as windmill-centrifugal fan induction-diffuser and swirling cold air diffuser and so on.At the same time, researchers also studied the performance of existing special air diffuser and found the way to improve it.
c. Finding the ways to use conventional terminal devices in cold air distribution system.
Conventional air terminal devices have many advantages, such as mature technology, low price, etc. Chinese researchers are seeking the possible ways to apply it to cold air distribution system. For example, it can avoid surface condensation on outlet to achieve soft-start when using conventional terminal devices in cold air distribution system, processing surface of the outlet by super hydrophobic surface treatment also can. However, researches are still in the basic stage in this area, more researches needs to be further expanded. Such as developing the method of soft-start strategy to cold air distribution system, improving treatment process,and the strength of the hydrophobic surface and so on.
d. Exploring the new way of air supply
In recent years, some scholars have proposed that the textile air distribution system is applied to cold air distribution conditioning system, and studied the performance of the textile air distribution system which is used in the cold air distribution system. And they found that it can solve the problems of cold air distribution system well. It is very different from the traditional way of air supply, it can not only make indoor air distribution better, but also prevent condensation phenomenon does to occur. particularly suited to open space without the ceiling. It is one of the future development direction of the main air-conditioning technology.
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