How to eliminate air partial pressure in the hotte

2022-08-15
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How to eliminate the partial pressure of air in high temperature cooking experiment Abstract: This paper discusses the temperature accuracy of high temperature (back pressure) cooking pot, which is the high temperature resistance test instrument of cooking bag, and puts forward two feasible solutions

key words: cooking bags, high temperature (back pressure) cooking pots, temperature accuracy

high temperature cooking food has the advantages of being able to store food at room temperature and having a long storage period. It can pack both cold food and cooked food. It is easy to eat, easy to carry, and can be used as tourist food. In recent years, it has developed rapidly at home and abroad because of the establishment of the strategic alliance. Its special packaging -- cooking resistant composite film and bag has also become an important part of flexible packaging production

cooking bags can be divided into: (1) 100 ℃ boiling water resistance; (2) Resistant to 121 ℃ cooking and sterilization (sterilization at 121 ℃ for 40min.); (3) It can withstand high temperature cooking and sterilization at 135 ℃ (sterilization at 135 ℃ for 30~40min.). Because the latter can basically kill all bacteria, it can achieve the purpose of long-term preservation of food under non refrigerated conditions. In the manufacturers of cooking resistant bags, how to detect the packaging of the above temperature state has always been a problem that the industry pays more attention to

I. causes of error

at present, the high-temperature (back pressure) cooking pots on the market are mostly developed based on medical high-temperature sterilization containers. Medical sterilization containers, most of which were first proposed by HRA British modern medicine, are based on the theory that objects disinfect surgical instruments by killing Salmonella under the action of closed high-temperature steam. According to the principle of thermal engineering, under the standard state, the absolute temperature (k, -273.15 ℃) of the substance in the confined space and the absolute pressure of the vapor of the substance can be regarded as a linear proportional relationship, as shown in the figure below. The higher the temperature, the greater the pressure of the system. The temperature displayed by the high temperature (back pressure) cooking pot corresponds to the pressure of saturated steam. Generally, the following four pressure levels are set according to the British system for medical disinfection:

when the relative pressure is psi10 pounds/inch, the corresponding temperature is 115 ℃

when the relative pressure is psi15.6453 lb/in, the corresponding temperature is 121 ℃

when the relative pressure is psi20 lb/in, the corresponding temperature is 126 ℃

when the relative pressure is psi30 lb/in, the corresponding temperature is 135 ℃

it can be seen that the gas in the working high-temperature (back pressure) cooking pot must be saturated steam to form a saturated steam pressure

in addition to the actual operation of the cooking bag test in the laboratory, we must open and close the working chamber door of the high-temperature (back pressure) cooking pot when taking the sample, so that part of the air will remain in the working chamber when closing the door before the test, and the mixture of air and steam will be formed in the chamber after heating, At this time, the pressure measured by the pressure (temperature) gauge is the pressure of mixtures with different molecular weights -- including the partial pressure formed by air under high pressure and the common pressure with the partial pressure of water vapor, and the temperature results displayed by it have inevitable errors

if air is regarded as an ideal gas with an average molecular weight of 29, according to Dalton's law, the absolute pressure (P) of the mixed ideal gas is equal to the sum of the absolute partial pressure of water (P water) and the absolute partial pressure of air (p air):

P = P water + p air

for the absolute pressure in the working chamber of the high-temperature (back pressure) cooking pot, it contains an air partial pressure of 0.1013mpa. With the increase of temperature, the pressure P and P water also increase, while the P gas remains unchanged, and the influence of air on the total pressure becomes smaller and smaller

for such a problem, for general surgical disinfection, the sterilization purpose can also be achieved by prolonging the sterilization time, and the sterilization time specified in medical treatment greatly exceeds the bacterial death time. However, for the precise temperature detection of soft packaging cooking bags, this has become a fatal problem. No matter how accurate the external thermometer is, the error between the temperature displayed on the thermometer and the actual value, and the actual detection conditions are completely lower than the standard value, which may mislead the enterprise, and the heat-resistant temperature of the detected ink, adhesive and film will produce a series of errors

the above temperature difference, according to the detection of authoritative departments, at 135 ℃, the maximum actual temperature is only 133.9 ℃, and the temperature difference reaches 1.1 ℃

second, scheme 1 eliminates the influence of air through gas replacement

in fact, this temperature difference can be basically eliminated through effective gas replacement

Beijing landmark company solves this problem in this way. In the process of heating the high-temperature (back pressure) cooking pot from room temperature to 135 ℃, they carry out gas replacement with the increase of temperature and pressure in the working bin. 95 ℃ was an open design before, making full use of steam to drive out the air, and began to heat up at 95 ℃. After that, four deflating processes were designed, which were deflated at 108 ℃, 112 ℃, 116 ℃ and 120 ℃ respectively, and the air was driven out by the pressure of steam. After several times of effective gas replacement, at last, when the pressure is 0.2192mpa, less than 1% of the air left in the bin, basically eliminating the influence of air partial pressure on temperature. The high temperature (back pressure) cooking pot with gas replacement developed by them has been tested by the Thermal Energy Laboratory of Xi'an Jiaotong University. At 135 ℃, the actual temperature is 134.98 ℃, which fully meets the specified design standard

III. scheme II separates the thermometer from the pressure gauge and sets a separate temperature control display

to completely eliminate the influence of air pressure, separate the temperature display from the pressure gauge display, set a separate temperature sensor in the work bin and connect it to the temperature display outside the equipment, so that the displayed temperature is not affected by the air partial pressure

for the working procedure of the back pressure cooking pot, the parameter of the control part in the process of heating up and maintaining the temperature is the temperature. In the process of cooling down, the parameter of starting the external air source and making pressure supplement (back pressure) is the pressure. Take the ts-19c back pressure cooking pot of Rand MEC company as an example, its 135 ℃ back pressure cooking computer program is set as follows:

the sample is put into the working chamber, heated to 135 ℃ (during which there is sufficient gas replacement), kept warm for 30 minutes, and in the process of natural cooling to 40 ℃, the pressure of the stable system is between 0.215-0.22 MPa (self-set interval, the pressure is lower than 0.215 MPa, the supplementary air supply starts, and the supplementary air supply stops when it reaches 0.22 MPa), Until it drops to 40 ℃

is gas replacement necessary for high temperature (back pressure) cooking pots with separate temperature control? We think it should be replaced. If gas replacement is not set up to meet the requirements of PE double arm corrugated pipe, winding pipe and various pipe standards, the mixed gas pressure in the working chamber will increase additionally at 135 ℃, which is easy to cause unsafe factors of pressure equipment

for flexible packaging, cooking bags belong to products with high requirements, high investment and high added value. Compared with ordinary products, in addition to the testing of routine performance indicators of packaging bags should be more strict, the following testing and evaluation should be carried out for raw materials and final products used in production according to the principle of quality key control points:

ink, is it high temperature resistant ink? If yes, what is the temperature resistance

adhesive, is it a high temperature resistant adhesive? If yes, what is the temperature resistance

whether each layer of structural film is high temperature resistant? If yes, what is the temperature resistance? Can its layer affect the final product

whether the final product can withstand high temperature according to the customer's requirements. If so, under what state? What is the temperature resistance

in view of the high risks brought by high investment and high efficiency, as a cooking bag manufacturer, it is undoubtedly the right choice to try not to let the problem products leave the factory. This requires flexible packaging manufacturers to strengthen the detection level, not only to consider the accuracy of the corresponding test instruments when selecting them, but also to test according to the standard procedures in the process of operation, in order to achieve the best results

(author: Beijing Rand Mecca softening point experimental machine adopts computer control technology, and Wang Jiaju of windows Chinese (English) interface packaging materials Co., Ltd.)

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