Etiket Arşivleri: FOOD OPERATION LABORATORY
The objective of this experiment is to study the working principles of a concentric tube heat exchanger operating under parallel and counter flow.
Up to this point we have learned how to analyze conduction and convection heat transfer in various systems with different geometries. This information, however, is not very useful unless it can be applied to practical situations. For this reason we shall devote this experiment to a prototypical application of heat transfer analysis known as a heat exchanger.
A heat exchanger is a device that efficiently transfers heat from a warmer fluid to a colder fluid. A device we are probably all familiar with is the automobile radiator. Other applications for heat exchangers are found in heating and air conditioning systems. Heat exchangers are categorized in many ways, but the two most common practices are, by the method of construction, and by the flow arrangements. The analysis for designing an effective heat exchanger is very important; after all who’d want to be caught on the side of a deserted desert road with an overheated engine!
In this experiment we studied a concentric tube heat exchanger with parallel and counter flow. For the analysis of this heat exchanger we needed to find important quantities such as the heat transfer coefficient, power emitted, absorbed, and lost, the log mean temperature difference, and the overall efficiency to compare the two types of flow.
The aim of the this experiment was to learn working principles of batch type of tray dryer its application in drying of wet marrow to obtain information about falling rate period, constant rate period, equilibrium moisture content, free moisture content of drying periods, and total drying time was calculated according to results which measured during the experiment on the tray dryer.
A range of technologies are used for food drying which include tray and tunnel dryers, spray, roller and freeze dryers. With the exception of tray dryers none of these are appropriate, in terms of cost and output, for use by small and medium enterprises.
While sun drying on trays or in solar dryers can be considered as tray drying the term is normally applied to small industrial systems with some form of air heater and a fan to pass air over the product being dried. While small tray dryers are available from Europe and the USA, where they are used in pilot plants and Universities, their cost makes them unaffordable and un-economic for producers in developing countries.
In the early 1980’s; the need for small, controllable, powered tray dryers capable of producing high quality products that could be constructed by engineers in developing countries to a great extent from locally available materials. The required basic development work was carried out and there are now tray dryers, based on the principles developed by some company, in some eight countries. The greatest up-take of the technology has been in Latin America where probably over 100 units are now operational. The key point to bear in mind when considering the local construction of such a dryer is to understand the basic principles involved and adapt them to local conditions such as the dimensions of local plywood sheet, common stock steel sizes, social conditions and fuel availability.
Shell and tube heat exchangers (STHE’s) are apparatuses in which the heat exchange between hotter and colder fluid is done. Fluid flowing through tubes is called – tube fluid, and fluid flowing around tube bundle is called – shell side fluid. Baffles, placed in shell side space, are providing the cross flow direction of shell side fluid and so the more intensive heat exchange between fluids could be realized. Besides, baffles are carriers of tube bundle, which helps them to decrease the deflection in horizontal and vibrations in horizontal and vertical units. STHE’s usually have combined fluid flow, which means that there is parallel in one, and counter flow in other part of the exchanger. These apparatuses are usually denoted as m-n STHE’s, where m is the number of fluid passes through the shell, and n is the number of fluid passes through the tube bundle [9, 14]. If the STHE is with so called “full tube bundle”, the shell side fluid flows through baffle cuts along the tubes. On the shell side, there is not just one stream, beside a main cross-flow stream the four leakage or bypass streams exist as a result of design type: baffle to tubes, baffle to shell and tube bundle to shell gaps (tube – to – baffle hole leakage stream, bundle bypass stream, pass – partition bypass stream and baffle – to – shell leakage stream).
One of STHE manufacturer’s main goals is to improve their exploitation reliability and efficiency. Two approaches in STHE design improving are possible: experimental investigation and numerical investigations. Experimental investigations are very expensive and long lasting, because of shell side complex geometry. Numerical simulations can be used to check the old design and to develop a new more efficient STHE design. Shell side flow is almost always turbulent since tube bundle and baffles are very nice turbulent promoters.
Character of flow around some tube rows in tube bundle is strongly influenced by tube layout (square, rotated square, triangular, rotated triangular, circular).
Character of flow around the tubes has a direct influence on heat exchange between fluids.
Problem is more complex if heat transfer is simultaneous with phase change. Since the detailed measurements of turbulent characteristics of shellside fluid flow are almost impossible, the calculated fields of pressure, velocity, temperature as well as turbulent characteristics are of great significance in explaining very complex thermal and flow processes in STHE’s.
Basically, one can conclude that heat transfer between fluids in STHE’s is highly influenced not only by thermal and flow quantities, such as inlet temperatures and velocities, but also with baffle cut size, baffle spacing, size of inlet and outlet zones and number of baffles.
To investigate influence of mentioned parameters, thermal, flow and geometric, or by other words, to find the “apparatus response” to thermal and fluid quantities and shell side geometry, in steady regime, by experimental and numerical methods, it was necessary to conceive of one compact experimental STHE.
Heat exchangers are devices used to transfer heat between two or more fluid streams at different temperatures. Heat exchangers find widespread use in power generation, chemical processing, electronics cooling, air-conditioning, refrigeration, and automo- tive applications.The simple type of heat exchanger is double pipe heat exchanger. In this experiment, the working princples a concentric tube heateexchanger operating under parallel and counter flow conditions was observed and the effect of hot water flow rare and hot water temperature variation the performance characteritics of a concentric heat exchanger were observed.