Cold chain transport industry
2017/1/17
a) Development survey
The utility model relates to a vacuum insulation board, which is formed by filling the core material with good thermal insulation property into a gas isolating structure, and then sealing them after vacuum. The research and development of VIP board began in the 50s of last century, it has been more than sixty years of history, after the year of 90s, many large companies in Europe and America have carried out the research of the VIP board, and have made rich achievements in scientific research. Now the VIP board at home and abroad has been widely used in refrigerators, freezers, cold storage, refrigerated trucks, refrigerated containers, medical incubator, field wall insulation, such as in Japan, more than 70% of the domestic refrigerator insulation layer using VIP plate, VIP plate has been widely used in Europe and the United States in the frozen ship transport box in.
b) VIP Plate structure
The vacuum insulation panel usually consists of three parts: gas insulation structure, core material and getter, its structure as shown below.
1.Air separation structure 2.Core insulation material 3.Gas adsorption material
Gas insulation structure: usually bag structure, hereinafter referred to as the "barrier bag", is the core material vacuum encapsulated in a vacuum state, so this structure has good gas barrier capability, but also has a certain strength to protect the vacuum chamber can not be damaged. The commonly used air barrier materials are thin steel plate, multilayer polyester film and multilayer polyester film containing metal layer.
The core layer insulation materials: hereinafter referred to as "core", commonly used for porous materials, the first is to play a supportive role, avoid under vacuum conditions in gas insulation structure collapse, followed by preventing heat radiation, in addition to its porous material, the contact surface of small, low thermal conductivity. The commonly used core materials are powder silica, aerosols, polyurethane foam, porous polystyrene foam and superfine glass fiber.
Gas adsorption materials: hereinafter referred to as the "getter", the main role is to absorb the residual air in VIP panel to maintain a certain degree of vacuum. Barrier bag vacuum sealing, there will still be a very small amount of gas will penetrate into the bag, or core material in the use of the process will slowly release some gas, all these gases will destroy the bag vacuum affect the thermal conductivity of VIP plate. Therefore, you must use a special getter absorbs nitrogen, oxygen, carbon dioxide and water vapor gas board of VIP to maintain a high degree of vacuum bag.
c) Adiabatic principle
VIP The plate is made of vacuum thermal insulation principle, that is, under low air pressure, the thermal conductivity of the material will become very small. VIP plate heat conduction can be regarded as one-dimensional plate heat conduction, the thermal conduction process is as follows.
One dimensional heat conduction through VIP plate
According to the structural characteristics of the VIP plate and the thermal resistance formula R= Delta / lambda, the total thermal resistance of the VIP plate can be expressed as the sum of the thermal resistance of the core material and the two sides of the wall
Among
λ by VIP Total thermal conductivity;
λ0 by Apparent thermal conductivity of core material;
λ1，λ2 The thermal conductivity of the two sides of the VIP plate;
δ by VIP Total plate thickness;
δ0 by Core thickness;
δ1，δ2 The thickness of the side wall of the VIP plate;
By formula (1.1) finishing, get
The apparent heat conductivity coefficient 0 of the core material is composed of the following four parts:
λ0=λg+λc+λs+λr
Type，λg Thermal conductivity of air;
λc Convective heat transfer coefficient;
λs Thermal conductivity of thermal insulation materials;
λr Radiation heat transfer coefficient of core material;
Due to the VIP board in the rarefied air, air heat conduction ability is very weak, usually also the core material is a porous material, space partition distance is relatively short, further weaken the heat transfer of air, so the air heat conduction is approximately zero,That is λg=0; 气体被分隔或者密闭在无数微小的空间之内，抽真空后The gas is separated or closed within a tiny space, the mean free path of air molecules inside the bag between the core material is greater than the pores between the vacuum, the heat exchange between gas molecules is almost non-existent, so heat convection can be neglected,That is λc=0.Therefore, the apparent thermal conductivity of the core layer can be expressed as: λ0=λs+λr
The core material is usually porous material, the core material is a point contact or line contact, so the solid heat conduction caused by the core material is also very small,That isλs very small。In addition, the core material powder or fibers increases the radiation and scattering effect in the partition in the,so λr also very small.Therefore, the apparent thermal conductivity of core material λ0 very small.
VIP thickness wall and thermal conductivity is usually constant, when the thickness of the core material, by the formula (1.2) shows that the total thermal conductivity of VIP depends on the core material in apparent conductivity λ0，That is λ0The smaller the,λ The smaller.
d) Influence λ Factor
The thermal conductivity coefficient is a parameter which can be used to characterize the thermal conductivity of the material, which is W/m K. The coefficient of thermal conductivity of different materials is different, even if the same material, thermal conductivity is also affected by many factors. The following is a brief description of the vacuum, temperature and moisture content of the core λ impact,It is helpful for us to find the way to reduce the thermal conductivity and improve the thermal insulation performance of VIP board.
(1) Vacuum Degree
VIP The reason why the board has such excellent insulation performance, to a large extent, thanks to the vacuum bag. Because the VIP core material is powder or fiber, gas heat conduction plays a very important role. The relationship between apparent thermal conductivity and residual gas pressure of core material is shown below.
From the above, it is found that when the residual gas pressure drops from atmospheric pressure to 10Kpa, the apparent thermal conductivity remains constant, that is, the thermal conductivity is independent of the gas pressure in the insulating layer. When the pressure is further reduced, that is, when the pressure is 1~ 103Pa, the thermal conductivity is almost linear with the residual gas pressure, and the thermal conductivity decreases with the decrease of pressure. When the pressure is lower than 1Pa, the thermal conductivity is due to the radiation heat transfer and solid heat conduction because of the heat conduction and solid heat conduction of the gas molecules. Therefore, the VIP board production process, the choice of the VIP board vacuum plate generally take less than 1Pa can be further evacuated for reducing thermal conductivity is of no practical significance.
(2) temperature
The temperature has a direct influence on the thermal conductivity of the insulation material, and the thermal conductivity of the material will increase with the increase of temperature. By formula (1.5) knowable λ∝T2，As a result, the role of radiation thermal conductivity increases with increasing temperature,λ will significantly increase. λ can be expressed as:
λ= 4 GdεσT2
In the formula, G is the geometric factor of the stomata, and D is the mean pore diameter (m),εThermal emission rate of pores,σ For the Stephen Pohl Seidman constant, T is the temperature. Therefore, the VIP board in the low temperature environment than in the high temperature environment to achieve better insulation effect.
(3) Moisture content of core material
Core material for porous structure is easy to absorb moisture, aging will also be part of the decomposition of water and gas. The air thermal conductivity is 0.029W/m / K, and the thermal conductivity of water is 0.58W/m / K, is 20 times the air, water and steam and the vacuum deterioration of the VIP board, so the thermal conductivity of core material in water has great influence on the VIP board. The results show that: when the moisture content is greater than 5~10%, insulation material after moisture water occupied the pore space of porous media, due to its thermal conductivity increased rapidly, thermal conductivity of insulation material 0.03W/m K 1% absorption moisture conductivity increased by 25%, and according to the multiple increase. If the insulation material has holes to connect the pores, the water will penetrate into other parts under the action of capillary. Therefore, in the production process of VIP board, the core material must be fully dried.
In addition to the degree of vacuum, temperature and material moisture content, core density, specific heat capacity and the size effect, the type of gas filling and gas barrier structure of gas separation ability were on board of VIP thermal conductivity.
e)VIP Board reliability and life
If you want to ensure that the product factory pass rate, it is necessary to sample the product testing. At present, usually use the following two methods of detection, one is the thermal conductivity using heat flow meter for direct detection of VIP board, this method has the advantages of simple operation and accurate measurement, but each piece of VIP board testing time is about 10 minutes, low detection efficiency should not be used for mass production; the two is the relationship between the number and the degree of vacuum thermal system, through the use of micro vacuum gauge to measure the degree of vacuum in the VIP board to indirectly determine its reliability, this method is convenient and suitable for mass production of online detection, but the detection method is not perfect, and the increase of the pressure sensor will a corresponding increase in product cost, so the current use is not universal.
The process of using VIP plate, due to the external gas through the gas insulation structure slowly penetrate into the VIP plate and the core material in the process of using internal slow release gas, will destroy the vacuum plate, resulting in thermal conductivity of VIP plates increased gradually lead to product failure. This is related to the life of the product. The VIP board products there is no national standard, the following definitions of VIP plate service life, in the standard conditions of environmental temperature of 24 DEG C and a relative humidity of 50%, with thermal conductivity of time gradually increased to 0.010W/m VIP in K (super thermal insulation material thermal conductivity, the upper limit) we think the end of VIP plate life. Experienced this time is the service life of VIP. Now VIP plate life is generally 10~15 years, gas phase method of silica core material for up to 50 years.
The service life of the VIP board is directly related to the vacuum in the board. Therefore, in order to extend the service life of the product, it is necessary to improve the vacuum. We are from the following two aspects: to improve the vacuum structure, improve the barrier properties, barrier bag material increases the getter; process, keep the production environment clean and dry, fully drying the core material, appropriate to extend the exhaust time.
Two)vacuum insulation board application research
a) Insulation effect
In order to accurately evaluate the thermal insulation effect of vacuum insulation panel, we made a set of molds to form the square box, and compared the thermal insulation performance of the vacuum insulation board and the rigid polyurethane foam.
Two boxes of the same size, the size of 40cm*40cm*44cm, the insulation layer thickness of 4cm, the box cover wall thickness of 7cm, which is a pure PU foam, another VIP plate is inlaid on the inner wall thickness of 1cm PU foam. During the experiment, the two boxes contained exactly the same cold source. Put it in the constant temperature and humidity testing machine, testing machine set temperature is 40 DEG C, now 50% of humidity, then change the temperature curve of continuous as follows:
Can be seen from the above picture
(1) 48 hour observation period, with the box plate cooling rate of VIP than pure PU box and the box fast, the lowest equilibrium temperature is much lower than that of pure PU, this experiment box using VIP plate, can greatly enhance the speed of cooling to reduce job waiting time, and achieve the same insulation effect the cold, Yuanyuan than not using VIP plate less.
(2) the use of the VIP board is slower than the case of pure PU, which means that the use of the VIP board can prolong the cooling time.
Therefore, the use of VIP board to speed up the cooling rate, reduce the waiting time of the work, reduce the temperature, prolong the cooling time; ensure the same insulation effect, can greatly reduce energy consumption.
Many foreign companies continue to develop VIP board products, improve the quality of the VIP board at the same time, also carried out a large number of in-depth application research. As the Dow Chemical Company has made the following experiment, quantitative analysis of the economic effect of using VIP plate.
Cold insulation box with the same insulation,
(1) when using PU foam, the dimension of the 57cm*50cm*50cm, the insulation layer thickness of 3.8cm, the built-in 21.8kg coolant, holding time is 120h;
(2) when the VIP board is used for heat insulation, the dimension of the 7kg is 33cm*46cm*28cm, the thickness of the insulation layer is 2.5cm, and the coolant is 200h;
With the VIP board, the volume of the heat preservation box is reduced by 70%, the coolant is reduced by a factor of 68%, and the holding time is prolonged by a factor of 66%. Reduce the freight, improve the transport distance.
SIEMENS is the volume of 120L, the internal size of 45cm*50cm*53.5cm, the thickness of the refrigerator 5cm thickness of the experiment, with 20mm VIP board insulation of 60% of the insulation. Keep the thickness of the insulation layer is constant, can save energy by 23%, while maintaining the same energy consumption, the insulation layer thickness can be reduced to 30mm, an increase of refrigerator volume of 22%.
The domestic and foreign research shows that when the VIP board is used as insulation material (in panel form)
(1) the same cooling requirements, can reduce the box wall thickness, increased internal volume, but also can reduce weight.
(2) if the wall thickness of the insulation box is the same, if the VIP plate is used in the tank wall, the cooling time can be prolonged, and the energy consumption can be saved.
b)Application process
Although the VIP board has excellent thermal insulation performance, but because of its own structural characteristics and production processes and other factors, VIP board in practical application has the following characteristics.
(1) VIP plate structure is special, the application process can not be cut into a variety of specifications and shapes, can only be determined according to the application of the product specifications and dimensions
(2) VIP plate air separation structure usually adopts multi-layer polyester based film, which is easy to scratch, so the VIP plate is usually used in the practical application of protective devices
(3) after the VIP plate barrier film is strongly stretched, the film becomes thinner and thinner, or the sealing edge is damaged, so the VIP plate can not be bent or used on the curved surface with small curvature radius;
(4) VIP plate edge is usually not very smooth, so the board and the board when splicing has a large gap, so often as VIP plate panels and conventional insulation materials such as rigid polyurethane foam used to fill the splicing gap to prevent leakage of heat;
(5) the production equipment restrictions, the current domestic VIP board product area within 22 m, applied to the cold storage, refrigerated containers, walls and other wall under the condition of large, often need to link connections into large plane;
In view of the above defects of VIP plate, the corresponding protective measures must be adopted in practical application:
(1) the VIP plate with adhesive paste in the refrigerated space inside or outside of the insulation box, and then pouring polyurethane foam insulation to space overall foam, each space foam filled cavity insulation, and tightly wrapped in VIP. (2) the VIP plate is placed in the rectangular box mold, injection liquid polyurethane foam molding, so, polyurethane foam tightly wrapped around the VIP board, after opening the formation of massive composite plate polyurethane with VIP plate;
Three) the application of VIP board in cold chain system
Refrigerated foods in the production, storage, transportation, sales, to the consumer before each link is always under the low temperature, so the super insulation material VIP board can be applied to every link in cold chain system. Whether it is cold storage, refrigerated trucks, refrigerated container, or the refrigerator insulation space is mostly rectangular structure, here in a refrigerated truck compartment insulation to illustrate the practical application of assembly process of the VIP board, VIP board applied in other fields with similar refrigerated vehicles, can reference.
The refrigerator is usually made up of the top plate, the bottom plate, the left and right side walls, the front wall, the rear door frame and the door. Due to the requirements of carriage with heat insulation function, so its skeleton also played the dual role of bearing and heat, which is called the main bearing for the purpose of skeleton skeleton, metal material with high stiffness and a selection of strength, and to heat for the purpose of skeleton known as auxiliary skeleton, a selection of non metal materials, installation in the main skeleton outside or inside. Inside and outside the skin compartment is connected with the main frame and the auxiliary frame of the rivet connection, forming a space filled with heat insulation materials. The outer skin is mostly flat, but the left and right walls of the outer surface of the atmospheric pressure into a corrugated or semi-circular, Inner Mongolia skin is generally flat shape, inside and outside the skin commonly used steel plate, aluminum alloy plate, stainless steel plate, glass plate, etc..
According to the different structure type of insulated car, the process flow is different, and the use of VIP board is different.
a) integral structure heat insulation carriage. The carriage is a traditional structure which is developed on the basis of a common carriage, and the reinforcing plate is welded at the intersection of the framework to increase the overall strength and rigidity.
According to the different foaming technology, the whole structure can be divided into two types: field spraying foam and field injection foaming.
(1) the spraying process is: foam parts production -- each piece of metal frame assembly, carriage assembly, metal frame riveting outer skin - install auxiliary frame of spray foam insulation, processing were brush glue, paste VIP board - brush glue, installation of Inner Mongolia leather and accessories with layering compartment with the back door assembly
(2) injection process of foaming is integral forming - carriage rain test - Inner Mongolia leather paste VIP board - install car Inner Mongolia leather - injection foaming material - installation box accessories installation carriage back door external compartment
The forming process of spraying foam hard polyurethane foam insulation materials insulation field, easy to fill the whole car space heat insulation, the heat insulation performance is good, but not easy to control the thickness of the insulating layer uniform, install the VIP board before the insulation layer inner surface after processing shall be taken. The air pollution is serious and the labor intensity of workers is large, so it is not used in this field.
Injection foaming process is first made of the overall skeleton type carriage and reserved insulation space, then the overall injection of polyurethane rigid foam, the rapid expansion of the foam filled the air space and fills the gap between the board of VIP wrapped VIP plate. As the car has a complete insulation layer, so the thermal insulation performance is good, but the foam has a lot of expansion force, it is necessary to use a large fixture positioning, and the wall roughness is also poor. Such as the use of the components of polyurethane foaming degree before injection, then injected into the cavity insulation foam stereotypes, namely "two foaming process", does not need the large fixture, but two times the foaming process is difficult to grasp.
b) Split assembled heat insulation carriage. The utility model is characterized in that the top plate, the bottom plate, the left and the right side walls, the front wall, the back door frame and the door are respectively formed into a heat insulation layer of the car wall, and then the car body is assembled into a complete compartment.
According to the different adding process, the thermal insulation compartment can be divided into two types: rigid polyurethane injection foam type and sandwich board prefabricated bonding type.
(1) hard polyurethane foam injection process is: making parts -- all skeleton welding -- the VIP plate was fixed between the main and auxiliary frame riveting inside and outside the skin - injected polyurethane foam material to form a heat insulating layer, the pieces are assembled into joint injection compartment: foaming material and apply sealant - installed car internal connectors and accessories: install the back door and accessories.
The process is, in the structure, the key is to deal with the seam stitching, each piece between the seal and the "broken bridge" arrangement. In the process, we must first ensure that the hard polyurethane and VIP board, both inside and outside the skin and skeleton and reliable bonding; secondly, polyurethane foaming uniform requirements; finally, the carriage assembly after the polyurethane foam material into joints, sealing tape or glue coated sealing paste, to increase the sealing performance, reduce heat conduction.
If the car frame is made of polyvinyl chloride plastic material can greatly simplify the assembly process, the profile is each piece of plate frame, is the connection between the plates, inside and outside the skin directly inserted into the plastic material, in the special press on the pressure plate made of injection foaming agent. The carriage assembly will board frame plastic material is inserted into the plug together, apply sealant, the carriage assembly is completed. This not only simplifies the assembly process, improve production efficiency, but also can reduce the leakage defects produced by cold seam.
(2) the process of "sandwich" plate pre bonding:
Skin material, cleaning, the skin in which a polished surface roughness, remove surface dust, primer spraying - into similar shape without cover rectangular box mold, paint the surface of spray glue - up mucus and VIP block placed polyurethane plate composite plate, block and block joint coating composite plate is sprayed on the mucus - mucus - placed on the skin, pay attention to the treatment of skin surface toward the rubber vacuum pressure to eliminate mucus - pressure, curing, the process is the forming process of each plate, the whole car assembly process with polyurethane foam injection process, namely, the pieces are assembled into the seam into the compartment: foaming material and coated with sealant installation car fittings and accessories to install the back door and accessories.
The process is, without making the overall injection of large fixture required foam type, but in order to improve the "sandwich" the intensity of the board, shall be between the insulation blocks increases non metal material reinforcement, in addition, the VIP board due to technology and equipment is limited, usually smaller plates and when the large span when the occasion requires the use of auxiliary connecting mechanism between the board of VIP. It is proved by the practice of some refrigerated trucks and refrigerated containers that the advantages of the sandwich board:
1.appearance of the car body is smooth and smooth, and the weight of the car body is light;
2.structure of the car body is simple, the process is simple, and is suitable for batch and multi variety production;
3.carriage for thermal design is more reasonable;
4.carriage of heat insulation, good sealing performance, thermal indexes reach the national standard a professional index.
Regardless of the overall structure or assembling cars, keep the car size under the same conditions, the use of VIP insulation board can enhance the wall of the box, and the assembly process of change is very small, only is inserted into the fixed link VIP board based on the current process; the premise can keep the insulation effect unchanged next, to reduce the thermal insulation layer with the thickness of the VIP plate to increase the volume of cars, reduce the weight, then the assembly process remains the same, but the size of the processing parts to make corresponding adjustment.
The characteristics of the assembly process of the VIP board in the refrigerated compartment are different, but can be used alone or mixed. The use of the above VIP board is not only in the refrigerated car compartment, but also for other cold chain project construction.
At present, the world is facing energy crisis, energy saving and emission reduction has become the focus of attention of the world, the development of new energy, new technologies and new materials is imperative. Because of the super thermal insulation performance of VIP board, it has attracted the attention of the world in the field of refrigeration and heat preservation. In foreign countries, VIP board has been widely used in the field of home appliances, cold chain and wall insulation. In the domestic food safety gradually become the focus of people, cold chain standards have been put on the schedule, but due to the lack of industry standards and management experience and development of energy consumption high cost limits of cold chain in china. This paper discusses the foreseeable, as VIP board application of new insulation materials to the cold chain system construction can greatly reduce energy consumption, reduce operating costs and application prospect in the cold chain industry is very broad, will contribute to the development of cold chain industry.
