Application Conditions of Direct Combustion Lithium Bromide

Application Conditions of Direct Combustion Lithium Bromide
Refrigerator for Three Purposes

Sun Zhenqi

Architectural Design ＆ Research Institute of Shijiazhuang City, Hebei Province
Add: No.45, Honghua Stree, Shijiazhuang City Zip code:050000
Tel:0311-7022749 　　Fax: 0311-7023414

Abstract Combined with engineering practice, this thesis makes an analysis and study of the features of the direct combustion lithium bromide refrigerator for three purposes and its application characteristics ,on the basis of which concrete conditions for rational use are put forward.
key words Direct combustion Three purposes Simultaneous supply Sanitary hot water Systematic metho Technical and economic appraisal.

　　The main function of a refrigerator is refrigerating and it is used as a refrigeration heat sink in the air conditioning system, no matter which type it is, compression type, absorption type or other types. If the absorption refrigerator changes the refrigerating process flow to make the steam from the high-pressure producer directly enter the evaporimeter for condensing and use the latent heatreleased in the condensing process of cryogen(pyrogen)steam to heat the hot water of airconditioner, the heating cycle can be completed. The kind of heating process has the similar switching funtion of the four-way change valve in an air conditioner of thermal pump type, which turns the evaporimeter of a refrigerator into a condenser. What is different from this also includes the separation type, to which a hot water preparation device for heating is added outside the high and low producing barrel body. For the absorption refrigerator, its general efficiency will be reduced because of the increase of heat transfer links, no matter which heating method is adopted. That is why the absorption lithium bromide refrigerator of steam or hot water type doesn‘t supply air conditioner with hot water. The heating function is fulfilled by other heat transfer equipment.

　　The direct combustion refrigerator is different from the lithium bromide refrigerator of steam or hot water type. Usually a direct combustion refrigerator is used far from urban heating network. Heat sources are lacking but heat has to be supplied in winter, so direct combustion lithium bromide refrigerators all have a heating cycle function(The operate mode is no more than the two kinds mentioned above) and use a set of devices to prepare the cold and hot water of the air conditioner. Generally some public buildings also need sanitary hot water for wash and bath. So in addition to the refrigerating and heating cycle, the direct combustion lithium bromide refrigerator has an additional function of supplying sanitary hot water ,hence the so-called refrigerator for three purposes.

　　Function combination is a method commonly used in product improvement and invitation, the development of this refrigerator for three purposes has put this method into practice. Thesuccess or failure of function combination depends upon whether the combination itself has met the special requirement of the various combined functions . There are two aspects here: one is the service conditions of the product itself, the other is whether the limit of these service conditions has been exceeded in actual engineering practice. Only when these two aspects are combined properly, can the normal use of the project be guaranteed and the functional requirement of the project be met.

　　The writer has published an article in a special discussion column for central hot water equipment of Water Supply and Drainage in its fourth issue for 1996 to analyze the advantages and disadvantages of the direct combustion lithium bromide refrigerator additionally supplying sanitary hot water. The article devotes a discussion on the different service characteristics, safety, economic results and rationality of function combination of air conditioner‘s cold and hot water and sanitary hot water. The article holds that compared with the air conditioner‘s load domestic hot water load forms much smaller proportion except some special projects, which isn‘t sufficient to affect the air conditioner‘s load and that when there is an auxiliary heat source in transition season, they‘d better be separated. The publication of the article evokes strong repercussions in the fratrnity. Water Supply and Drainage Technology and Product Information published an article, "problems and countermeasures of direct combustion lithium bromide refrigerator additionally supplying sanitary hot water"by Xue Chaoying, Architectural Design ＆ Research Institute of PLA General Rear-service Department. He also holds the same views as the writer and suggests that sanitary hot water is prepared and supplied by the secondary heat-exchange equipment because the raw water of sanitary hot water can‘t be softened. More and more people pay attention to and probe the three-purpose problem of the direct combustion refrigerator. This is a matter for rejoicing and no doubt the rational use of three-purpose refrigerator will be promoted.

　　The meaning of three purposes is clear outwardly and it would be superfluous to dwell on it any more. But what is its connotative meaning? Whether the refrigerator can be selected and used in a project according to the various sample data remains to be discussed. The following is the analysis of the sample data from a typical factory.

　　
The data in column1-4 of the table are taken directly from the sampple book, first type (smallest)is taken from each type series. Measurement unit remains the same as the sample book for easy comparison. The data in column5-10 are calculated according to the sample data. The hot water heating quantity of column9(i.e. heat quantity needed by sanitary hot water from the heating sample) is divided into two kinds, sample condition and actual condition. The former is the condition of the sample . The water temperature is 44℃ before heating and 60℃ after heating.The temperature difference is 16‘C. In reality, it is almost impossible for the temPerature ofrunning watr to reach 44℃ before heating. If that were true, there would be no need to heat rawwater again. After some cold watCr is added or the temPerature drops to a certain degree, it can beused for bath and wash. The cold water calculation temperature in the design manual varies from 4℃ to 10‘C with different regions. The hot water heating quanity of actual condition in the table is calculated under the condition that temperature rises from 10‘C to 60‘C. Under actualconditions the heat quanity needed by sanitary hot water is nearly three times the total heat value of fuel. If only sanitary hot water needs to be supplied without considering air conditioner‘s load,the heating capacity of the refrigerator is still far from enough. This must arouse much attention ofusers.
　　
The data of column 6, 7 and 8 are in accordance with that of a common direct combustion refrigerator for two purposes. Under nominal conditions, fuel consumption matches refrigefating output or heat output. In column 9 under sample condition the hot water heating quantity is in conformity with heat output on the whole. From this a conclusion can be deduced that the refrigerating output, heat output and hot water quanity from the sample are independent from each other, i.e. the refrigerator can guarantee any one of the three functions. In other words, if the refrigerator guarantees refrigerating or heaing, it is impossible for it to supply sanitary hot water in addition. When equipment in both heating and ventilation and water supply and drainage is selected and if the personnel in each field only take what they needs, the system operation is doomed to failure. We should take warnings from previous mistakes of this kind in actual engineering. If the big difference between sample condition and actual condition is ignoredz when model is selected, the serious consequence is self-evident.

　　In order to achieve the three purposes of a refrigerator and supply necessary sanitary hot water while heating and refrigerating are going on, the usual way is to select larger number and high-pressure generator to increase heat capacity. If the generator is one number larger, the rated heat output can increase by 20%. From the angle of economic gains it can be seen that it is irrational to select excessively large generators. To make the bor worse, this will cause other technical problems (for example, automatic conirol). Of course, a larger direct combustion lithium bromide refrigerator can also meet the requirement of three purposes, but technical and economic rationality will be out of the question. Therefore usually it is inadvisable for hot water load to exceed 20% of the rated heat output of the air conditioner. What should be emphasized here is the heat output, not the refrigefating output mentioned in some literature. The air conditioner‘s hot Water and sanitary hot water have the same heat exchange process, so the efficiency is all less than 1. But the refrigefating process has a different workng principle, so the energy efficiency ratio is over 1 (see co1umn 6 and 7 of the table). Under the same condition of heat input, the heat output is only 80% of the refrigerating output (0.76 in the table). So it is rational and practical to take the rated heat output as the base of calculation.

　　There is no lack of successful precedents of using direct combustion lithium bromide refrigerator as air conditioner‘s heat source tO scyply sanitary hot water. The project of Yutong intemaional sports center of Shijiazhuang city is an example. The project is a commercial and sportS building with an area of 72000m2, including 27000m² sperts Part (mainly standard foothall field and track and field sports field) and 45000m² commercial part (25000m2 supermarket). It is designed by Architectural Design & Research Institute of Hebei Province. The project is far from urban heat network. The air conditioner‘s refrigeration duty is 8588KW and the heating load 4695KW. The food and beverage department and sportS cener (gymnasium, bowling alley, saunabath and hotel etc.) need some sanitary hot water. Considering the time difference of peak values between conditioner load and sanitary hot water load, the designers in the fields of heating and ventilation and water supply and drainage made a rational combination of loads. They didn‘t simply add these two kinds of loads together. The aim of one refrigerator with three purposes was achieved through concrete technical measures. Up to now it still has a smooth operation. The project adoptS three direct combustion cold and hot water sets, including one set of 3489KW and two sets of 2908KW The high-pressure generators for the two sets of 2908KW are one number larger Six heaters of the three sets supply 90-65℃ air conditioner water and 60℃ sanitary hot Water. In the operation season of air conditioners the load peak of sanitary hot water staggers the time of air conditioner‘s load peak (The supermarket has closed business), the total load isn‘tobviously increased. Because the air conditioner system needs humidified steam and supplies part of domestic steam, two steam boilers (2t/h) are equipped. In transition season the air conditioner system stops and no humidification is needed, sanitary hot water is supplied by boiler steam through heater and the direct combustion refrigerator stops operating. Not only has the problem of the air conditioner‘s load and hot water supply of the whole year been solved but also the heat network construction cost has been saved. The investment saved reaches 5,000,000 yuan. Together with the heating collective money saved (120 yuan / m²) the economic benefits are remarkable.

　　From the above analysis it can be seen that the direct combustion refrigerator is mostly used in places far from Urban heat network and under the condition that the supply of heat, electricity and refrigerating is difficult to be achieved. Of course, there are Other conditions, under Which direct combustion refrigerators are adopted. For example, heat price is too high and collective money too big so it doesn‘t pay. Under these conditions if the supply of sanitary hot Water is needed, heat source also isn‘t guaranteed. So it can be understood that conthetion units are interested in the refrigerator with three purposes, In order to rationally use direct combustion refrigerator, guaranee the normal operation of the system and avoid falling into the same old trap again, the writer holds that a good job must be done in the following aspects:

　　1.Make a careful analysis of the sample data and working conditions, especially parameters and supply conditions of sanitary hot water. See if refrigerating, heating and supply of sanitary hot water can be achieved simultaneously. Don‘t apply it mechanically. The personnel in the fields of heating and ventilation and water supply and drainage should act in close coordination and don‘t only take What they themselves need so as to avoid foture troubles.

　　2.When the air conditioner load and sanitarp hot water load are calculated, it isn‘t suitable to add their Peak values together directly. Otherwise excessively large model may be selected and the invetment in equipment, construction and Power.etc. would be wasted and problems like automatic control etc. would come out. The loads should be arrange and compared in order of time so that rational values closest to acttal condition will be acquired. The Peak load values of heating and ventilation and water supply and drainage calculated ascording to the design manual hava big safety margin and they don‘t appear in the same time interval. Direct addition will result in a decided depwture from actual load.

　　3. Direct combustion refrigerator has complex fonctons and is accurately manufactured. Its manufacture cost is much higher than that of a common hot water heater and its efficiency is also lower than that of a common hot Water heater with direct heat conduction because of the secondary heat transfer When the refrigerator is used for three purposes in order to simplify the system, the sanitary hot water load should be confined within 20% of the heating load. Under this condition the generator one number larger is enough. The selection of larger generators than that is economically and technically irrational and is quite different from the original intention of three purposes.

　　4. In transition season there is no air conditioner‘s refrigerating and heating load and only sanitary hot water is supplied. Usually smaller combustor nozzle is adopted for normal combustion and lower energy consumption. In reality the air conditioner system haS a humidification requirement especially in northern area in winter, and usually the boiler supplying humidified steam is equipped and it can be used as a heat source to heat sanitary hot water in transition season. Obviously there are a lot of means of humidification, for example, ultrasonic humidification, high-pressure water spray etc. and at this time there is no steam available. In order to meet the demand for a small amount of hot water other auxiliary measures can be adopted, for example, electfical heating or solar heater etc. In short in transition season usually you‘d better stop the direct combustion refrigerator.

　　5. Sanitary hot water belongs to a straight-flow system, raw water continuously emters the heating equipment of the system. So under the same conditions the heat exchange surface is encrusted much more seriously than the air conditioner system, especially in northem area where the Water is much harder. Limited by the functional requirement of bath water, raw water can‘t be softened and can only be handled through the simple physical method. The heating process of the sanitary hot watr of the direct combustion refrigerator is different form that of the heat exchanger The hot Waer of the heat-exchanger is usually heated in the shell path but the heating Process of the direct combustion refrigerator is conducted in the pipe path. The inWall of the heat-exchangin pipe is encrusted and Washed with much more difficult than the outer wall. Sanitary hot Water at most times is in a low load and low flow condition, which accelerates the encrustment. In order to protect the key equipment of the air conditioner system, it is necessary to add a water-to-Water heat exchanging equipment to make the sanitary hot water heaing of the direct combustion refrigerator become a soft Water circulation system.

　　6. When the three-purpose system is selected for a project, you‘d better not select a ready-made model in the sample book. You should give your technical requirements to the manufasturer, according to which its technical department makeS special designs and manufactures it in a special way in order to meet the requirement of the project. Some manufacures also Put forwar such suggestions and it should be said that they are very conscientious for doing so.

It is less than ten years since the coming out of the three-purPose refrigerator. There have been both successful and unsuccessful examples. According to one‘s experience the writer has written the above thesis for the purpose of exchanging views with the fraemity. The use of it should be rationally determined on the basis of technical and economic appraisal. Limited by one‘s level the writer would be glad to be the first speaker on this issue by way of breaking the ice.

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