It is actually concerned with hood tubes used for cooling basic oxygen furnace gas in LD(Linz Donawitz)-vessel.hood tubes have water flowing at a certain rate on inside and one outer side hot gas flows at high temp. of 1450 deg c. The temp varies on both sides of tubes and cooling of hot gas is done by water. Heat transfer modelling needs to be done.inner tube diameter= 29mm ,no. of tubes-282,
The common lapse in questions is the use of abbreviations or jargon that make it that much more difficult to understand the question and provide a suitable answer. I had to Google "bof gas" and LD vessel to understand the question. Not everyone may have so much time.
Another difficulty with the question is specifying volume flow rate (1200m3/hr). If the velocity and water flow duct dimension had been given, a more meaningful answer is possible. If the gas velocity too had been given, it might have been even better.
I will now try to give an answer that might make sense for you. The heat transfer coefficient of gas (even hot gas) will be in the order of several tens of W/m2K, while that of water will be in the thousands. So, it is safe to expect that the thermal resistance on the gas side to be two orders of magnitude higher than for water. Even with the maximum temperature of gas (1450 deg C) and minimum cooling water temperature (say, 30 deg C), it is reasonable to expect the inner temperature of the wall to be well under 100 C, possibly even as low as 50 C. In the event, properties, and therefore Prandtl no., of water are unlikely to vary much. I encourage you, as a first approximation, to ignore the effects of temperature on 'h' of cooling water and go ahead with your water inlet temperature.
The convective heat transfer coefficient depends also on the flow behavior (laminar, transitional, turbulent) and if it is fully developed. This also has to be in the modelling. Regarding the temperature dependance of the h, supposing your problem is steady, the film temperature may be used as an estimation for the h. Indeed, h depends on flow properties (thermal conductivity, viscosity) that are functions of the temperature for themselves. Unless you know how these properties depend on the temperature, the film temperature works well as an approximation.
convective heat transfer coefficient (hf) can be represented as represented in term of Reynold's 'Re' and Prandtl number 'Pr' i.e.: hf = 0.332*(k/t) * (Re)^0.33* (Pr)^0.33 where t= thickness and k = thermal conductivity.
To get the heat transfer coefficient in such system you should modeling the situation. Look, for example, on my book "Conjugate Problems in Convective Heat Transfer". Ask questions if you need help. Approximate value you may get by formula which recommended Pankaij Biswas. Other expressions of this type you may find also in indicated book
The common lapse in questions is the use of abbreviations or jargon that make it that much more difficult to understand the question and provide a suitable answer. I had to Google "bof gas" and LD vessel to understand the question. Not everyone may have so much time.
Another difficulty with the question is specifying volume flow rate (1200m3/hr). If the velocity and water flow duct dimension had been given, a more meaningful answer is possible. If the gas velocity too had been given, it might have been even better.
I will now try to give an answer that might make sense for you. The heat transfer coefficient of gas (even hot gas) will be in the order of several tens of W/m2K, while that of water will be in the thousands. So, it is safe to expect that the thermal resistance on the gas side to be two orders of magnitude higher than for water. Even with the maximum temperature of gas (1450 deg C) and minimum cooling water temperature (say, 30 deg C), it is reasonable to expect the inner temperature of the wall to be well under 100 C, possibly even as low as 50 C. In the event, properties, and therefore Prandtl no., of water are unlikely to vary much. I encourage you, as a first approximation, to ignore the effects of temperature on 'h' of cooling water and go ahead with your water inlet temperature.
Sometimes the question seems very simple, but it is not. The matter of convective heat transfer is not easy. Although we have numerous correlations between Nu as a function of Re and Pr, each correlation has conditions to be used. We must care about the flow regime and its developing or being fully developed. The boundary later growth is the enemy of convective heat transfer coefficient. Also, you should care about the flow properties if they vary with the temperature or are constant. In many situations, we may seek local variation over tube length, then integrate the variation to obtain an average convective heat transfer coefficient.
I think the question should be addressed as follows: Is there any phase change during the cooling process? Then the answer to the question can be organised easily. Could you please tell me more about the problem? I will be happy to help.
Heat transfer coefficient will be greatly dependent on the temperature, if there is phase transitions, as you have already informed colleagues.
If the surface temperature reaches the boiling point of water, the koefiitsient teploodachi increase from about 1,000 W / m2K to about 10,000 W / m2K or more.
If the temperature reaches about 110 ° C, it will film boiling and the heat transfer coefficient drops to 100 ... 1000 W / m2K.
Moreover, the surface may reach a temperature of 100 ... 110 ° C even at a water temperature of 10 ° C. This depends on the heat flux.
That's what burn through heat exchangers.
I highly recommend you look for sources on the "boiling crisis" - this will help clarify your calculations.
And in any case it is impossible to calculate the heat transfer coefficient on the flow of the medium, as you asked. Perhaps calculate only medium speed.
I am pleased to answer your questions, please contact!
It was mentioned above jn several answers that the calculation of the water-side heat transfer coefficient requires the knowledge among others the Nu and Pr which are functions of the pressure and temperature (and the geometry and the velocity, of course). In the attached document (pages 10-11) a short description is given of the Fortran 95 module h2o, the functions of which are needed in the calculations. The package is freeware, the download addres is given there too. The article [52] is included in the package. (Hungarians live over the world, therefore interpreters can be found easily.)
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