Problems and Solutions

Principle and Application of Heat Pipe thermostatic Bath

1. Preface
      Heat pipe (gravity heat pipe) is composed of closed pipe, the liquid abs absorb heat and evaporates at the pipe bottom, releases heat and condensates at a part of the above pipe. And then the liquid returns to the bottom abs absorb heat and evaporating, no-stop-running like this. In the process of this circulation work, heat pipe transmits energy from one place to another fast and continuously without any additional power. Heat pipe has high thermal conductivity and excellent performance of mean temperature. It is known as the one of the most effective heat transfer components, and known as the "super heat conductor". In recent years, heat pipe technology has been increasingly prevalent in the application. It has been used in spacecraft, nuclear power plant, chemical industry, metallurgy, power engineering, even in laptop.
       Thermostatic bath is a kind of indispensable equipment in thermal test measurement equipment, widely used in the thermometer, heat resistance, and other measuring instruments’ verification field. Nowadays the most common constant temperature bath are water constant temperature bath and oil constant temperature bath with water or oil as medium. Heat pipe constant temperature bath fabricated by the theory of "heat pipe" is a new type of low and medium temperature constant temperature device. It is suitable for industrial heat resistance, glass liquid thermometer, high precision glass liquid thermometer, cryogenic thermocouple, bimetal thermometer, pressure type thermometer, second-class standard mercury thermometer or other temperature instruments and measuring instruments’ verification or calibration work. It is the ideal substitute of oil or water constant temperature bath. Compared with the traditional water or oil bath, it has a wide temperature range, jack deep, good temperature field uniformity, green environmental protection, automatic cycle and other features.
      The development of the wide temperature field heat pipe constant temperature bath has profound social significance. China's national defense equipment (especially the aircraft, tanks, etc.) uses a lot of short type temperature sense per, short type temperature transmitter, but we are lack of a suitable test with constant temperature bath. It is badly in need of special products on the basis of heat bath technology with wide temperature field.
After many years of efforts, Panran Measurement & Control Science and Technology in Tai’an Co.Ltd developed the PR600 series heat pipe constant temperature bath with wide effective working area, wide working temperature range, small temperature difference, Temperature fluctuation is small, long working life and other advantages. It is in the leading domestic level, by the vast number of customer recognition and praise, and won the national patent (patent number’s 200720028538.9).
2. Working principle
      There are three kinds of heat transfer in the natural way to nature: conduction, convection and radiation. In the gas or liquid, heat conduction and convection occurs at the same time. To solve the actual demands of scientific research, production, people are constantly seeking a more efficient means of heat transfer, one of which is the "heat pipe" technology.
      "Heat pipe" is a kind of high efficient heat transfer component, its thermal resistance smaller than the same size of the metal material several orders of magnitude. It can be called a thermal "superconductors". Heat pipe excellent heat conduction ability at the same time means that it has a very good performance of mean temperature, or "temperature flattening" ability.Its principle schematic diagram as follows:
       Heat pipe constant temperature bath" developed on the basis of the principle of heat pipe is a constant temperature bath. Core part of the heat pipe constant temperature bath is a completely airtight container - heat pipe tube core (see figure 1). There are many different intubations at the top of the tube used to insert the thermometer and standard thermometer. The lower part of the intubation was sealed to ensure the tightness of the tube core. When the external power supply through electric heater heating working medium at the bottom of the tube core, a part of the liquid propellant vaporized after abs absorb a large amount of thermal energy, and the working medium carrying latent heat of steam fills the tube core container. Under the action of gravity,condensed liquid medium returns to the bottom of the tube core, continues to be heated, no-stop-running like this.
      In the process of the above, working medium as the carrier , through the vapor and liquid phase change, unceasingly transmitting energy obtained from the power to parts inside of heat pipe tube where need to make up for the energy, to maintain the tiny tube core’s constant temperature zone dinky temperature difference.
3. PR600 series heat pipe thermostatic bath’s creative characteristics
By improving the heat pipe condensing tube core area, heating area structure, in-depth study of substance to the chemical formula and process, we make the following performance indicators are improved greatly.
 Improve heat pipe dynamic performance. Heat pipe technology is used for thermostatic bath, the need of heat energy transmission power is very small, through the structure improvement, strengthen saturated steam velocity, enhance the capacity of heat transfer, thereby reducing the entire area thermostatic temperature difference.
 Scaling up uniform temperature field area. Before PR600 series heat pipe constant temperature bath, heat pipe constant temperature bath’s effective uniform temperature area is basically from the distance heat pipe port 150mm to 200mm to 450mm in depth. Now after a series of exploration, test and improvement, making effective uniform temperature field range scaling up to no more than 80mm, making effective uniform temperature of heat pipe area covers the area of 80mm to 450mm, and even more wide, meeting the full immersion type glass thermometer, short of sense per, a special sense’s verification or calibration or testing.
 Improve the uniformity of the temperature field. In the above effective uniform temperature field, we further improved the uniformity of the temperature field in order to meet temperature field indicators which is needed by first, second-class standard glass thermometer.
 Improve the technical index of the lower temperature limit. Now, the heat pipe constant temperature bath is near the lower limit working temperature. Because of the lack of working fluid steam pressure resulting in insufficient heat exchange capacity, directly influence the temperature field uniformity index and temperature fluctuation degree index, should be improved. That has the same meaning like reducing the working temperature limit of heat pipe constant temperature bath.
 Increase fast cooling function. When necessary, through external addition condensation tube in the heat pipe radiator and other measures under the premise of each index, increase the cooling rate, improve the efficiency of the upper and lower stroke (cycle) test.
4. Quality assurance measures
1). The research, improvement and test on key technology and tooling
Heat pipe production process mainly includes: preparation, welding, cleaning, drying, leak detection, evacuating, filling, sealing, electrical instrumentation installation, inspecting, testing and a series of processes, each working procedure is very strict. In the process of project development, by repeated trial and grope, we summarized a lot of heat pipe production experience, developed the detailed file and the strict process as the basis of the guide for future production. The process problems mainly solved in the process of project development has:
 Prevent the sense per intubation shrinkage cavity in welding process.
 The introduction of helium mass leak detector instead of bubble method of leak detection, the leakage rate of the detection sensitivity increased 3 ~ 4 orders of magnitude, ensure the heat pipe air tightness, thus ensuring the service life of the heat pipe products.
 The quality control of evacuation.
 Special technology to ensure tightness of sealing.
2). The further research of chemical refrigerant formula and technology
Heat pipe thermostatic bath used for measurement or calibration is strict and special in the demands on working medium purity. Through the further study on the temperature period of heat pipe working medium formula, based on the theory of data and the existing experimental data, using orthogonal test, regression analysis and other scientific methods, finding out the best proportion of refrigerant and preparation process make the heat pipe working temperature range more wider, and temperature field indicators more higher.
5. The comparison of performance between heat pipe thermostatic bath and conventional water thermostatic bath
 When the constant temperature tank works, medium will evaporate, smoke, pollutes environment, affect the staff health. Heat pipe constant temperature working medium was fully sealed to avoid the above pollution, belongs to the green environmental protection product. So it has a broad market prospect.
 Trough restricted by heating medium, own material and structure, oil constant temperature bath’s highest working temperature is generally no more than 300℃. Heat pipe constant temperature bath’s working temperature is 300℃~500℃ or even more higher, temperature field uniformity is better than 0.05℃. The other thermostatic equipment cannot achieve that, and it filled a void in this temperature.
6. The Comparison of performance between heat pipe constant temperature bath and stem body check bath.
    What is worth mentioning,please do not confuse the heat pipe constant temperature bath with the "stem body check bath". Stem body check bath relies on the slot metal "thermostatic" heat conduction uniform temperature, the biggest advantage is portable, and it is mainly used for the temperature calibration. Limited by working principle and volume, stem body check bath generally do not emphasize the temperature field uniformity, the vertical temperature difference in the 40mm range from the lower part of the jack is generally in the range of 3℃ to 4℃; And for the heat pipe constant temperature bath, the temperature field uniformity is the most important index. Because the movement speed of the working fluid in the heat pipe is thousands of heat conduction velocity of the same material metal block, the heat pipe constant temperature bath has the better temperature field uniformity. It’s more suitable for laboratory calibration or calibration work. The only common advantage of both is no pollution.
7. The comparison of domestic similar products 
      Main performance and parameter comparison of PR600 series heat pipe thermostatic bath and others produced by our company adopting new technology, new material and new technology is as follows:
NO. Items Technical index
Ordinary heat pipe thermostatic bath PR600 series heat pipe thermostatic bath
1 Uniform temperature field width From 150mm or 200mmapart from heat pipe upper port to the depth of 450mm From 80mm or 100mmapart from heat pipe upper port to the depth of 450mm
2 Temperature field uniformity 0.03℃~0.12℃ 0.01℃~0.06℃
3 Temperature fluctuation degree (0.05℃~0.3℃)/10min (0.01℃~0.05℃)/10min
4 Working temperaturelower limit characteristic Large temperature fluctuation degree, and bad temperature field uniformity The lower limit temperature generally moves down, the working temperature range widens, the temperature fluctuation reduces, and temperature difference in working area reduces
5 Service life Dramatic variation in longevity More than 10 years
6 Rate of heating and cooling Slow Fast
7 Product standards No Yes
8 Appearance Simple and crude Artistic
 Summary
PR600 series of heat pipe thermostatic bath using a number of new technologies, with a series of advantages, such as advanced design, reasonable structure, good uniform temperature field, environmental protection, and accurate and reliable value of a quantity transfer, can fully meet requirement of temperature instrument verification and calibration, which reach the advanced level in China. It is suitable to enterprise and verification or calibration laboratory for various types of thermometer and thermal resistance of the test.