Tubular heat exchanger is currently the most widely used heat exchanger in chemical and alcohol production. It is mainly composed of shell, tube sheet, heat exchange tube, head, baffle baffle and so on. The required materials can be made of ordinary carbon steel, copper, or stainless steel. During heat exchange, one fluid enters from the connecting tube of the head, flows in the tube, and flows out of the outlet tube at the other end of the head, which is called the tube side; the other fluid enters through the connection of the shell, and The outflow at the other connection on the housing is called the shell side.
Fixed tube sheet
The tube-and-tube heat exchanger has a simple, compact and inexpensive structure, but cannot be mechanically cleaned outside the tube. The tube bundle of this heat exchanger is connected to a tube sheet, and the tube sheets are respectively welded to both ends of the shell, and a top cover is connected to the top cover, and the top cover and the shell are provided with fluid inlet and outlet connections. A series of baffles perpendicular to the tube bundle are usually installed outside the tube. At the same time, the connection between the tube and the tube sheet and the shell is rigid, while the inside and outside of the tube are two fluids with different temperatures. Therefore, when the temperature difference between the tube wall and the shell wall is large, due to the different thermal expansion of the two, a large temperature difference stress is generated, so that the tube is twisted or the tube is loosened from the tube sheet, and even the heat exchanger is destroyed.
In order to overcome the temperature difference stress, a temperature difference compensation device must be provided. Generally, when the temperature difference between the tube wall and the shell wall is more than 50 ° C, for the sake of safety, the heat exchanger should have a temperature difference compensation device. However, the compensation device (expansion joint) can only be used in the case where the temperature difference between the shell wall and the tube wall is lower than 60-70 ° C and the pressure of the fluid at the shell side is not high. Generally, when the shell-side pressure exceeds 0.6Mpa, because the compensation ring is too thick, it is difficult to expand and contract, and the effect of temperature difference compensation is lost, other structures should be considered.
One tube sheet of the heat exchanger is connected to the shell with a flange, and the other tube sheet is not connected to the shell, so that the tube can expand and contract freely when heated or cooled, but a top cover is connected to this tube sheet, called " "Floating head", so this heat exchanger is called a floating head heat exchanger. Its advantages are: the tube bundle can be pulled out for cleaning; the expansion of the tube bundle is not restricted by the shell, so when the temperature difference between the two heat exchanger media is large, the temperature difference stress will not be generated due to the difference in the thermal expansion of the tube bundle and the shell. . Its disadvantages are complex structure and high cost.
This type of heat exchanger tube bundle can be freely expanded at one end, the structure is simpler than the floating head type, and the cost is lower than the floating head type. However, there is a possibility of leakage of the medium in the shell side, and volatile, flammable, explosive and toxic media should not be handled in the shell side.
U-shaped tube heat exchanger, each tube is bent into a U shape, the two ends are fixed on the same tube plate, each tube can be freely expanded and contracted, thereby solving the problem of thermal compensation. The tube length is at least two, the tube bundle can be pulled out for cleaning, and the tube can expand freely. Its disadvantages are that it is difficult to clean the inner wall of the tube, it is difficult to replace the tube, and there are fewer tubes arranged on the tube plate. The advantage is simple structure, light weight, suitable for high temperature and high pressure conditions.
Eddy current film
The vortex heat film heat exchanger uses the latest vortex heat film heat transfer technology, which increases the heat transfer effect by changing the state of fluid movement. When the medium passes through the surface of the vortex tube, the surface of the tube is strongly washed, thereby improving the heat exchange efficiency. Up to 10000W / m2 ℃. At the same time, this structure realizes the functions of corrosion resistance, high temperature resistance, high pressure resistance, and anti-scaling. The fluid channels of other types of heat exchangers are in the form of fixed-direction flow, forming a flow around the surface of the heat exchange tube, and the convective heat transfer coefficient is reduced.
According to the data of the [Heat Exchange Equipment Promotion Center], the biggest feature of the vortex heat film heat exchanger is the unification of economy and safety. Due to the consideration of the flow relationship between the heat exchange tubes, the heat exchange tubes and the shell, the turbulence is no longer forced by the baffle to force the flow, but instead is replaced by
Tube and tube heat exchanger
The alternating vortex flow is naturally induced between the heat pipes, and the due dithering force is maintained under the premise of ensuring that the heat exchange pipes do not rub against each other. The heat exchanger tubes have good rigidity and flexible configuration and will not collide with each other. This not only overcomes the problem of damage caused by collision between floating coil heat exchangers, but also avoids the problem of easy scaling of ordinary shell and tube heat exchangers.
Performance characteristics of vortex hot film heat exchanger:
1. High efficiency and energy saving, the heat transfer coefficient of this heat exchanger is 6000-8000W / m2.0C;
2. Made of stainless steel, with long service life, can reach more than 20 years, free replacement of heat exchanger quality problems within 10 years;
3. The laminar flow is changed to turbulent flow, which improves the heat exchange efficiency and reduces the thermal resistance;
4. Fast heat exchange speed, high temperature resistance (400 ℃), high pressure resistance (2.5Mpa);
5.Compact structure, small footprint, light weight, easy installation, saving civil investment;
6. Flexible design, complete specifications, strong practicality, saving money;
7. Wide application conditions, suitable for large pressure, temperature range and heat exchange of various media;
8. Low maintenance cost, easy operation, long cleaning cycle and convenient cleaning.
9. Adopt nano thermal film technology to significantly increase the heat transfer coefficient.
10. It has a wide application field and can be widely used in the fields of thermal power, factories and mines, petrochemicals, urban central heating, food and medicine, energy electronics, machinery and light industry.
Performance comparison of vortex hot film heat exchanger:
In order to increase the flow velocity of the shell-side fluid, a certain number of baffle baffles that are perpendicular to the tube bundle are often installed in the shell. The baffle not only can prevent the fluid from short circuiting and increase the fluid velocity, but also force the fluid to cross-flow through the tube bundle multiple times according to the prescribed path, which greatly increases the degree of turbulence. There are two types of baffle baffles: disc-shaped and disc-shaped. The former is more commonly used.
The tube-and-tube heat exchanger must consider the influence of thermal expansion on the structure, adopt various compensation methods, eliminate or reduce thermal stress, and take compensation measures based on the temperature difference adopted.
| Nominal diameter || Tube number || Number of tubes || Heat exchange area |
Nominal value / calculated value
| Cross-section area of tube passage |
Tube-side flow rate is
Flow m / hr at 0.5m / sec
| Nominal pressure |
| Tube length (m) || φ25 × 2.5 |
| 1500 || 2000 || 3000 || 4000 || 6000 || φ25 × 2 |
| 159 || 1 || 14 || 1.51.62 || 22.17 || 33.27 || 0.0044 / 0.0049 || 7.92 / 8.82 |
| 219 || 1 || 26 || 3 / 3.00 || 4 / 4.02 || 6 / 6.06 || 8 / 8.1 || 0.0082 / 0.0090 || 14.76 / 16.20 |
| 2 || 26 || 3 / 3.00 || 4 / 4.02 || 6 / 6.06 || 8 / 8.81 || 0.0041 / 0.0045 || 7.38 / 8.01 |
| 273 || 1 || 44 || 5 / 5.08 || 7 / 5.18 || 10 / 10.26 || 14 / 13.72 || 21 / 20.63 || 0.0138 / 0.0152 || 24.84 / 27.36 |
| 2 || 40 || 5 / 4.62 || 6 / 6.19 || 9 / 9.33 || 12 / 12.47 || 19 / 18.76 || 0.0063 / 0.0069 || 11.24 / 12.42 |
| 325 || 1 || 60 || 7 / 6.93 || 9 / 9.28 || 14 / 14.00 || 19 / 18.71 || 28 / 28.13 || 0.0188 / 0.0208 || 33.84 / 37.44 |
| 2 || 56 || 6 / 6.47 || 9 / 8.66 || 13 / 13.05 || 17 / 17.46 || 36 / 26.26 || 0.0088 / 0.0097 || 15.84 / 17.46 |
| 400 || 1 || 119 || 14 / 13.47 || 18 / 18.41 || 28 / 27.76 || 37 / 37.10 || 55 / 55.8 || 0.0374 / 0.0412 || 67.32 / 74.16 |
| 2 || 110 || 13 / 12.70 || 17 / 17.02 || 26 / 25.66 || 34 / 34.20 || 50 / 51.58 || 0.0173 / 0.0190 || 31.14 / 34.20 |
| 500 || 1 || 185 || 45 / 4.15 || 55 / 57.68 || 85 / 86.74 || 0.0581 / 0.0641 || 104.58 / 115.38 |
| 2 || 180 || 40 / 41.99 || 55 / 57.68 || 85 / 86.74 || 0.0283 / 0.0312 || 50.94 / 56.16 |
| 600 || 1 || 269 || 60 / 62.7 || 85 / 83.88 || 125 / 126.13 || 0.0845 / 0.0932 || 152.10 / 167.76 |
| 2 || 266 || 60 / 32.05 || 80 / 82.94 || 125 / 14.72 || 0.0418 / 0.0461 || 75.24 / 83.98 |
| 700 || 1 || 379 || 90 / 88.41 || 120 / 118.17 || 175 / 177.71 || 0.0091 / 0.1313 || 214.38 / 236.34 |
| 2 || 358 || 85 / 83.51 || 110 / 111.62 || 165 / 167.85 || 0.0562 / 0.0620 || 101.16 / 111.60 |
| 800 || 1 || 511 || 120 / 119.20 || 160 / 159.16 || 240 / 239.60 || 0.1605 / 0.1770 || 288.90 / 318.60 |
| 2 || 488 || 115 / 113.83 || 150 / 152.16 || 230 / 228.81 || 0.0767 / 0.0845 || 138.06 / 152.10 |
| 900 || 1 || 649 || 150 / 151.39 || 200 / 202.36 || 305 / 304.3 || 0.2039 / 0.2248 || 367.02 / 404.46 |
| 2 || 630 || 145 / 146.96 || 195 / 196.44 || 295 / 295.40 || 0.0990 / 0.1091 || 178.20 / 196.38 |
| 1000 || 1 || 805 || 185 / 187.78 || 250 / 251.00 || 375 / 377.45 || 0.2529 / 0.2788 || 455.22 / 501.74 |
| 2 || 792 || 185 / 184.75 || 245 / 246.95 || 370 / 371.36 || 0.1244 / 0.1374 || 223.92 / 246.96 |
Leakage of heat exchangers is the most common equipment management problem in the use of heat exchangers. Leakage is mainly caused by corrosion, and a small part is due to heat exchanger selection and defects in the manufacturing process of the heat exchanger itself. There are basically two forms of corrosion of the device: electrochemical corrosion and chemical corrosion. In the manufacture of tube-and-tube heat exchangers, the manual welding of tubesheets and tubes is usually performed by manual arc welding. There are different degrees of defects in the shape of the weld, such as depressions, pores, slag inclusions, etc., and the stress distribution of the weld is uneven. In use, the tube sheet part is generally in contact with industrial cooling water, and impurities, salts, gases, and microorganisms in the industrial cooling water will cause corrosion to the tube sheet and the weld. This is what we often call electrochemical corrosion. Studies have shown that whether industrial water is freshwater or seawater, there will be various ions and dissolved oxygen. The changes in the concentration of chloride ions and oxygen play an important role in the shape of metal corrosion. In addition, the complexity of the metal structure also affects the corrosion morphology. Therefore, the corrosion of the weld between the tube sheet and the tube is mainly pitting and crevice corrosion. Viewed from the outside, there will be many corrosion products and deposits on the surface of the tube sheet, with pits of various sizes distributed. When seawater is used as a medium, galvanic corrosion will also occur. Chemical corrosion is the corrosion of the medium, and the heat exchanger tube sheet will be corroded by the chemical medium when it contacts various chemical mediums. In addition, there will be some bimetal corrosion between the heat exchanger tube sheet and the heat exchanger tube. Some tubesheets are also under the erosion of corrosive media for a long time. Especially for the fixed tubesheet heat exchanger, there is also temperature difference stress, and the joint between the tubesheet and the heat exchange tube is easy to leak, resulting in the failure of the heat exchanger.
In summary, the main factors affecting the corrosion of the heat exchanger tube sheet are:
(1) Medium composition and concentration: The effect of concentration varies. For example, in hydrochloric acid, the larger the concentration, the more severe the corrosion. Carbon steel and stainless steel have the most severe corrosion in sulfuric acid with a concentration of about 50%, and when the concentration increases to more than 60%, the corrosion decreases sharply;
(2) Impurities: Harmful impurities include chloride ions, sulfur ions, cyanide ions, ammonia ions, etc. These impurities can cause severe corrosion in some cases
(3) Temperature: Corrosion is a chemical reaction. For every 10 ° C increase in temperature, the corrosion rate increases about 1 to 3 times, but there are exceptions;
(4) ph value: Generally, the smaller the ph value, the greater the corrosion of the metal;
(5) Flow rate: In most cases, the greater the flow rate, the greater the corrosion.
Leak test method
Nondestructive testing of tube heat exchangers
In the fault detection, especially the heat exchanger, professional knowledge and instruments can be used to detect the cause of the corrosion phenomenon. Here we take the technical product of Mika Hua as an example to understand the function of the nondestructive testing equipment:
1) Inspection of the inner surface of the tube sheet by a visual endoscope;
2) customized problem research and quotation;
3) APR (Acoustic Pulse Reflection Method), an innovative non-destructive testing technology, based on the analysis of multi-dimensional sound waves generated in the tube sheet;
4) Non-destructive testing of straight and curved heat exchanger tubes made of magnetic and non-magnetic materials;
5) Fast detection: less than 10 seconds per pipe;
6) Detection of leaks, full and partial blockages, erosion and pitting;
7) Suitable for oval, square, spiral, finned tubes and bends from 9/16 "diameter;
8) Immediate visual results;
9) Digital storage for later inspection and comparison;
10) Customized problem research and cost estimation .
Water leak test method
Traditionally, we use water to leak test the heat exchanger, which is to inject water into the heat exchanger, fill it with water, and then press the water in the heat exchanger with a water pump to make it reach a certain pressure for heat exchange. Leak detection. After the leak detection is finished, mark and release the pressure and drainage, and then stop the leak. If the heat exchanger is large and the leakage is serious, leakage will occur when the water pressure is low, and it will be necessary to drain and plug. After the plugging is completed, fill with water and charge pressure and test leaks repeatedly. Increase the heat exchanger maintenance time. . Because it takes a lot of time to fill and suppress water, if the leaked parts need to be repaired and leaked by hot ignition, the leaked parts must be dried, otherwise the quality of repaired leaks will be affected. If there is a flammable medium inside the leaked heat exchanger, it must be replaced with nitrogen, and the welding repair and leakage can be performed only after passing the qualification, otherwise it will cause fire and explosion, endangering the safety of personnel and equipment.
Method for leak test of nitrogen
After the tube-type heat exchanger leaks, nitrogen is used for pressure test and leakage, which is relatively fast. When using nitrogen to plug the heat exchanger for leakage and fire operation, it is not necessary to perform replacement treatment, which saves the time for maintenance and leakage elimination.
Use of nitrogen in leak test of synthetic ammonia heat exchanger
(1) Because it is high-pressure nitrogen (pressure 9.5 MPa), two or more people must operate the valve to open the valve. One pressure gauge is installed between the two valves. One monitors the pressure gauge and the other opens the valve. , To ensure that the pipeline pressure is within the allowable range, to prevent overpressure and cause pipeline or personnel injury.
(2) The heat exchanger EA103 is a floating head heat exchanger, and a leak test head needs to be installed on the heat exchanger when leak testing.
(3) Commissioning steps: Fully open 1 and 3 valves, as shown in Figure 1, and then slowly open 2 valves to charge the heat exchanger. At the same time, pay attention to the pressure gauge pressure, which is within 3.0MPa.
(4) The pressure-relief step is to fully close 1 and 2 valves. After the pressure-relief valve is opened, 3 valves are to be fully closed.
Comparison of two test leak methods
From the above usage situation, using nitrogen leak test saves a lot of maintenance time and maintenance costs. See Table 1 for a comparison of water and nitrogen leak test methods.
Safety precautions for nitrogen leak test
(1) Because nitrogen has the risk of suffocation, it is necessary to pay attention to the safety of personnel when using it to prevent poisoning.
(2) Prefabrication of pipelines is required in advance when nitrogen is used.
(3) When nitrogen is used, a pressure gauge should be added to the temporary pipeline. Two people use an intercom to open the valve to prevent overpressure.
(4) After the nitrogen is used, the pressure must be released and removed in time.
(5) When using nitrogen, more than two people must use the two-way radio to contact the filling and depressurizing operations to prevent overpressure in the pipeline, damage the pipeline, and prevent people from suffocating and causing personal injury.
(6) When the nitrogen is not in use, the valve of the pipeline must be hung with a "forbidden movement" sign to prevent personnel from malfunctioning.
Long-term operation of the tubular heat exchanger will cause the equipment to be blocked by scale, which will reduce efficiency, increase energy consumption, and shorten life. If the scale cannot be removed in time, there is a danger of equipment maintenance, downtime or scrap replacement. For a long time, traditional cleaning methods such as mechanical methods (scraping, brushing), high-pressure water, and chemical cleaning (pickling) have caused many problems when cleaning the heat exchanger: the deposits such as scale cannot be completely removed, and the equipment is corroded. The residual acid causes secondary corrosion or sub-scale corrosion of the material, which eventually leads to equipment replacement. In addition, the cleaning waste liquid is toxic and requires a large amount of funds for wastewater treatment. Enterprises can use high-efficiency environmentally friendly cleaning agents to avoid the above situation. It has the characteristics of high efficiency, environmental protection, safety and non-corrosion. It not only has a good cleaning effect, but also has no corrosion to the equipment, which can ensure the long-term use of the air compressor. Fustek cleaning agent (specific addition of wetting agent and penetrant) can effectively remove the most stubborn scale (calcium carbonate), rust, oil scale, slime and other deposits generated in water equipment, and will not cause human body Injury will not cause corrosion, pitting, oxidation and other harmful reactions to steel, copper, nickel, titanium, rubber, plastic, fiber, glass, ceramic and other materials, which can greatly extend the service life of the equipment.
Case description: Taiwan-funded Fusheng brand screw air compressor of a large petrochemical power plant. The heat exchanger is oil-water heat exchanger, the inner tube is copper tube, the heat exchange area is 28 square meters, the normal working temperature is 77 ℃ -93 ℃, cleaning The previous working temperature was 92 ° C. About 15 kg of Fustek cleaning agent was used, mixed with about 40 kg of water, and the circulating pump pressure was 3 kg. The cycle was repeatedly cleaned for 6 hours, and various scales, rust scales, slime and other substances were washed out. 1 kg, the working temperature after startup is 81 ° C, which effectively guarantees the normal production and operation of the equipment.
Generally, the practice of most companies is to purchase high-quality heat exchangers as much as possible. After careful maintenance, the life of the heat exchanger is extended as much as possible. After the inevitable leakage, it will be forced to stop the surfacing welding, 2 to 4 people. It takes a few days to complete the repair. If the senior welder hired by the company can guarantee that the heat exchanger will continue to be used for a period of time, if the welder's technology is ordinary, it will cause more leaks and even scrap, and the company will have to replace This is due to the various disadvantages caused by this traditional method, which can not guarantee the safe and continuous production of enterprises. Therefore, many enterprises actively seek new technologies to solve the problem of heat exchanger leakage. Corrosion resistance and erosion resistance of the material. By protecting the new heat exchanger in advance, this not only effectively treats the weld seam and trachoma problems of the new heat exchanger, but also avoids chemical substances from corroding the metal surface of the heat exchanger after use. And welding points, in the future regular maintenance, you can also apply Fu Shilan polymer composite material to protect the bare metal; After the emergence of leakage can be repaired in time by blessing the world Blue technology, to avoid a long period of welding repairs affecting production. It is because of this refined management that the probability of leakage of the heat exchanger is greatly reduced, which not only reduces the equipment purchase cost of the heat exchanger, but also guarantees product quality, production time, and improves product competitiveness.
High-thickness coatings for tube sheet:
1) Restore damaged tubesheets to prevent corrosion caused by cavitation or electrical erosion;
2) Can be applied to tanks, pipes, valves, coating thickness suitable for many important application environments;
3) Restore the integrity and impermeability of the tubesheet due to corrosion or damage caused by incorrect expansion operations;
4) The tube sheet is insulated from the coolant, interrupting the circuit caused by corrosion;
5) Streamlined inlet treatment surface can reduce cavitation and promote the performance of continuous cleaning system;
6) Solve problems related to excessively fast flow rates, cavitation, turbines and suspended wear particles;
7) Restore the inlet to the best hydraulic conditions;
8) The reduced coating thickness of castings does not present problems with plastic and metal-based inserts (cavitation at the ends of the inserts; resistance).
Tube surface inner coating protection of heat exchanger and condenser:
1) Tube sheet coating is a fast and durable solution for any tube sheet inner surface problem;
2) Prevent damage caused by dirt, various corrosion, erosion and cavitation;
3) Suitable for soaking in various water service environments;
4) It is 100% non-volatile organic compounds and certified to be used in drinking water;
5) Fast application (20 m / s) from 30 microns to 250 microns by using our patented semi-automatic equipment;
6) Different coating thicknesses according to customer requirements;
7) The inner direct range is from 15 to 80mm and the pipe length is up to 25m;
8) Application time: during maintenance shutdown;
9) Endoscope control and DFT ensure complete coating efficiency;
10) Research and tests show that the coating improves heat transfer rate. A heat exchanger protected with the coating maintains a high heat transfer efficiency, while requiring less cleaning intervention and less prone to leakage.
10 Reasons to Use Mika Hua's Inner Tube Sheet Coating System:
1) Immediately terminate various corrosion processes;
2) Long-term solutions for new and in-service heat exchangers;
3) A variety of feasible coatings can extend the effective service life;
4) Increase heat exchange efficiency and condenser vacuum;
5) Comprehensively increase power output by improving plant performance;
6) Improve dirt resistance by reducing dirt;
7) Cold work technology is safe for heat exchangers, factories and personnel;
8) It is a cost-saving option for the recovery tube board-50% faster and 30% cheaper;
9) Field application;
10) Tested to ASTM D5162-28 to ensure 100% surface coverage.
At the beginning of the 21st century, Gongyi City, Henan Province, China finally developed a silicon carbide tube-type ceramic heat exchanger. The ceramic heat exchanger has been well developed under the limitations of the use of metal heat exchangers, because it better solves the problems of corrosion resistance and high temperature resistance, and has become the best heat exchanger for recovering high temperature waste heat. After many years of production practice, it shows that the ceramic heat exchanger works well. Its main advantages are: good thermal conductivity, high temperature strength, good corrosion resistance, oxidation resistance and thermal shock resistance. Long life, small maintenance, reliable and stable performance, and easy operation. It is the best device to recover high temperature flue gas waste heat.
CCTV reports that for the first time in China, traditional metals and other traditional materials are easily damaged under high temperatures or harsh environments. A new heat exchanger technology that uses ceramics to replace metals for the first time has been included in the national torch plan. This new technology turns the cold air used in industrial kiln into hot air, which not only improves work efficiency but also saves a lot of energy. Since ceramic heat exchanger is one of the main equipments to improve energy utilization rate, it has a wide range of industrial uses, so its application prospect is very promising.
Ceramic heat exchangers can be used in the main thermal kiln of metallurgy, non-ferrous metals, refractory materials, chemicals, building materials and other industries, and they have made great contributions to the world's energy conservation and emission reduction.
Selection of hot and cold fluid flow channels
In a tube-and-tube heat exchanger, the hot and cold fluid flow channels can be selected based on the following principles:
(1) Liquids that are not clean and easy to scale should go through the tube, because the tube is easy to clean;
(2) Corrosive fluids should go through the tube to prevent the tube bundle and the shell from being corroded at the same time;
(3) It is advisable to go through the pipe when the pressure is high, so as to avoid pressure on the shell;
(4) Saturated steam should go through the shell side. Because saturated steam is relatively clean, the convective heat transfer coefficient is independent of the flow rate and the condensate is easily discharged;
(5) The cooled fluid should go through the shell side to facilitate heat dissipation;
(6) If the temperature difference between the two fluids is large, for a heat exchanger with a rigid structure, it is appropriate to pass a fluid with a large convective heat transfer coefficient through the shell side to reduce thermal stress;
(7) Fluids with small flow and high viscosity should go through the shell;
Determination of fluid inlet and outlet temperature
If the heat exchanger's inlet and outlet temperature for the purpose of cooling the hot fluid has been determined by the process conditions, the outlet temperature of the cooling medium needs to be selected. If you choose a higher outlet temperature, you can choose a small heat exchanger, but the flow rate of the cooling medium must be increased; otherwise, you must choose a low outlet temperature, the flow rate of the cooling medium is reduced, but you must choose a large heat exchanger, so the cooling medium The exit temperature should be determined by weighing the investment of both.