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- HEAT EXCHANGER
- COOLING TOWER
- Transfer of heat from one place to another place, Which occurs as a result of a temperature difference
- Heat transfer in bodies due to fixed molecules.
- This take place is solid bodies.
- Heat transfer in bodies due to moving molecules.
- This take place in flawed liquid of gas
- q conduction = -kA x (?T /? x )
where A the cross- sectional area
?T temperature difference ( T1-T2)
between the two surfaces separated by a distance ?x
- is the heat transfer process where the heat transfer takes place due to the natural temperature difference or density difference and no external forces are employed for the fluid movement.
heat transfer process ( Name the law )
- q emitted = es . AT4
where A is the surface area, T is the temperature of the body, s is a constant called Stefan-Boltzmann constant, equal to 5.67×10-8
W/m2K4, and eis a material property called emissivity.
The emissivity has a value between zero and 1.
- Laminar flow: which the fluid particles move in regular path.
- Turbulent flow: is the one in which the fluid particles move in zig-zig path
- Parallel flow: is the one where the 2 fluid steams move parallel to each other.
- Counter flow: the one where the fluid steams move opposite to each other.
- Parallel flow heat exchanger.
- Recuperation heat exchanger.
- Plate heat exchanger.
- Counter flow
- Shell& tube heat exchanger.
- Cross flow heat exchanger.
- Direct& indirect contact
- Double pipe heat exchanger
- Log-mean temperature difference (?Tlm )
- Plate exchangers are limited when high pressures, high temperatures, or aggressive fluids are present.
- Because of this problem these type of heat exchangers have only been used in small, low pressure applications such as on oil coolers for engines.
- rear header
- front header
- tubes sheet
A heat exchanger is a device that transfers heat from one fluid to the others. A condenser is simply a heat exchanger. It compresses refrigerants
into a hot gas to then condense them into a liquid. Majorly condenser is used to cool the required vapor and heat exchanger is used to heat the
Diffusion heat transfer is due to random molecular motion. Neighboring molecules move randomly and transfer energy between one another - however there is no bulk motion. Radiation heat transfer, on the other hand, is the transport of heat energy by electromagnetic waves. All bodies emit thermal radiation. In particular, notice that unlike diffusion, radiation heat transfer does not require a medium and is thus the only mode of heat transfer in space. The time scale for radiative heat transfer is much smaller than diffusive heat transfer.
In natural convection, the movement of the fluid is due entirely to density gradients within the fluid (e.g. hot air rises over cold air). There is no external device or phenomenon which causes fluid motion. In forced convection, the fluid is forced to flow by an external factor - e.g. wind in the atmosphere, a fan blowing air, water being pumped through a pipe. Typically heat transfer under forced convection conditions is higher than natural convection for the same fluid.
15. Define a black surface.
A black surface is defined by three criteria:
• it absorbs all radiation that is incident on it
• it emits the maximum energy possible for a given temperature and wavelength of radiation (according to Planck's law)
• the radiation emitted by a blackbody is not directional (it is a diffuse emitter)
A black surface is the perfect emitter and absorber of radiation. It is an idealized concept (no surface is exactly a black surface), and the characteristics of real surfaces are compared to that of an ideal black surface.
The emissivity e ranges between 0 and 1.
A gray surface is defined as one for which the emissivity (e) and the absorptivity (a) are independent of wavelength (l).
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