Condensation

Condensation occurs when a vapor is cooled and changes its phase to a liquid. Condensation heat transfer, like boiling, is of great significance in industry. During condensation, the latent heat of vaporization must be released. The amount of the heat is the same as that absorbed during vaporization at the same fluid pressure.

There are several types of condensation:

Homogeneous condensation, as during a formation of fog. Condensation in direct contact with subcooled liquid.

Condensation on direct contact with a cooling wall of a heat exchanger: This is the most common mode used in industry:

o Filmwise condensation is when a liquid film is formed on the subcooled surface, and usually occurs when the liquid wets the surface.

o Dropwise condensation is when liquid drops are formed on the subcooled surface, and usually occurs when the liquid does not wet the surface.

Dropwise condensation is difficult to sustain reliably; therefore, industrial equipment is normally designed to operate in filmwise condensation modes.

Condensation phenomena occur in many industrial applications. There are two idealized models of condensation (i.e., filmwise and dropwise). The former occurs on a cooled surface which is easily wetted. The vapor condenses in drops which grow by further condensation and coalesce to form a film over the surface, if the surface-fluid combination is wettable; if the surface is non-wetting rivulets of liquid flow away and new drops then begin to form. The phenomena of dropwise condensation results in heat transfer coefficients which are often an order of magnitude greater than those for filmwise condensation.

APPARATUSAlthough the differences in the modes of boiling heat transfer are generally recognised many students are not aware that two distinct modes of condensation can occur. The H911 is a self-contained bench top unit that allows the investigation of heat fluxes and heat transfer coefficients during the condensation of steam.

The considerable difference between the appearance and heat transfer rates of dropwise and filmwise condensation are clearly demonstrated. Heat fluxes approaching 1000 kW/m2 can be obtained.

A vacuum pump is fitted to the unit so that the effect of non-condensable gases in condensers may be investigated. A computer upgrade allowing all temperatures, pressures and flow rates to be automatically recorded is available.

Experimental Capabilities

Visual observation of filmwise and dropwise condensation, and of nucleate boiling.

Measurement of heat flux and surface heat transfer coefficient in both filmwise and dropwise condensation at pressures up to astmospheric.

Investigation of the saturation pressure/temperature relationship for H2O between about 20oC and 100oC.

Demonstration and investigation of the effect of air in condensers.

Demonstration of Daltons Law.

REFRENCE:

p-a-hilton.co.uk homebuilding.co.uk

Kidsgeo.com

http://wins.engr.wisc.edu/teaching/mpfBook/node9.html

HE Free & Forced Convection Heat Exchanger has been designed for students to study the phennomena of natural .

HE Free & Forced Convection Heat Exchanger has been designed for students tostudy the phennomena of natural (free) and forced convection. The unit consists of mainly a bench mounted vertical air duct and a control panel. Demonstration of convection is achieved in this apparatus by studing temperature profiles and heat flux in the air duct with three alternative heat transfer surfaces, i.e. vertical flat plate, array of cylindrical pins and finned surface.Each of these surfaces may be installed separately in the wall of the vertical duct. The surfaces incorporate an electrical heating element, with protective thermal cut-out, and a temperature sensor for accurate temperature measurement. An acrylic window is provided in the duct wall opposite the mounted exchange surface to allow flow pattern visualisation. For forced convection, a variable speed fan at the top of the duct draws ambient air upwards through a flowstraightener and over the exchange surface. The air velocity, whether occuring by free or forcedconvection, is indicated on a velocity detector probe of which is inserted in the tunnel up-stream of the exchanger. A separate temperature probe measures the in-going and out-going air temperature of the selected exchanger.An instrument panel provides for:a) Variable control of power input to the exchangersurface with direct readout in Watts.b) Direct readout of temperature in Deg C.c) Fan speed variation and control.

Heat energy transferred between a surface and a moving fluid at different temperatures is known as convection.In reality this is a combination of diffusion and bulk motion of

molecules. Near the surface the fluid velocity is low, and diffusion dominates. Away from the surface, bulk motion increase the influence and dominates.Convective heat transfer may take the form of either

forced or assisted convection natural or free convection

Forced or Assisted Convection

Forced convection occurs when a fluid flow is induced by an external force, such as a pump, fan or a mixer.

Natural or Free Convection

Natural convection is caused by buoyancy forces due to dens ity differences caused by temperature variations in the fluid. At heating the density change in the boundary layer will cause the fluid to rise and be replaced by cooler fluid that also will heat and rise. This continues phenomena is called free or natural convection.Boiling or condensing processes are also referred as a convective heat transfer processes.The heat transfer per unit surface through convection was first described by Newton and the relation is known as the Newton's Law of Cooling.The equation for convection can be expressed as:

q = hc A dT        

where

q = heat transferred per unit time (W)

A = heat transfer area of the surface (m2)

hc= convective heat transfer coefficient of the process (W/m2K or W/m2oC)

dT = temperature difference between the surface and the bulk fluid (K or oC)

Applications of Convection Currents of Heat:

1. The air, which we breathe out, is warmer and lighter. It moves up in the room to go out of the ventilators, near the top of side walls. The fresh air comes through the windows and doors. It is cool and takes the place of exhaled air rising upward.

2. In winter, hotels and other buildings are heated centrally on the principle of convection currents.

3. Land and sea breezes are convection currents. During the day the land absorbs the heat of the sun more quickly than the sea. As a result, the temperature of land rises much higher than that of sea water.

4. Convection of currents causes trade winds. The sun heats up land and water which receive same amount of heat for equal areas. This heating is greatest at the tropics. The heated earth heats the air coming in coming with it. The hot air expands and becomes less dense than the air from the temperate and polar regions. This hot air rises up and cool air from the temperate and polar regions rushes in to take its place. This convection of air is the cause of (trade) winds.

5. Convection currents of heat cause oceanic currents. Water near the equator gets heated up, whereas water near the poles is comparatively cold. The warm water is lighter than cold water. The cold water near the poles sinks and the surface water flows to take its place. So convection currents are produced by warm water flowing from equator towards the poles. Convection current of cold water flows from poles towards the equator below the surface of oceans. These convection currents are called oceanic currents. They control the temperature of ocean.

REFRENCES:

ww.efunda.com

WWW.engineeringtoolbox.com

WWW.ALIBABA.COM

www.preservearticles.com

What is conduction? heat is transferred in several different ways: conduction, convection, and radiation. Regardless of the method of transfer, only heat can be transferred because cold is the absence of heat. Conduction is the transfer of heat from one molecule to another through a substance. Not all substances conduct heat at the same speed. Metals and stone are considered good conductors since they can speedily transfer heat, but wood, paper, air, and cloth are poor heat conductors. Various materials are often researched for their conductive properties. The materials are given numbers that tell their relative rates of conduction. Materials are compared to silver(coefficient of heat conduction of 100). The coefficient of some other products are copper(92), iron(11), water(0 .12), wood(0.03). A perfect vacuum has a conduction coefficient of zero.

Application of conduction in real life

One of the examples of conduction in real life is cooking, something essential in every household.

Cooking From the stove, heat is generated by the fire. The heat from the stove will then be transmitted through the pot/pans to the oil and finally to the food. This process is conduction,e.g.

Pan-frying a fish fillet. The heat of the flame is transferred (on a molecule to molecule basis)first through the pan, then through the thin oil layer, and finallyHowever, speed of conduction varies from matetials. Metal conducts heat more quickly and on the other hand, wood and plastic, slower. Thus, metal is used to make cooking pots and pans while wood and plastic are used to make pot handles and cooking spoons so that our hand will not feel as much heat.Another example will be barbeque.The

grate or grill upon which the meat rests, having a higher specific heat than the meat, conducts heat to the meat and that is why, when the grill is right, beautiful brown stripes will magically appear upon the surface of the steak.(brown strips)

Then the exterior of the meat conducts heat to the interior, molecule by molecule, cooking it.Another illustration is to put a metal spoon in a hot cup of coffee. At first the utensil's handle is cool. It then grows warm, and eventually hot. Conduction is the process that explain this illustration.

However, cooking not only involves conduction, it also involve the other two processes, convection and radiation.

Cross Flow Heat ExchangerCross Flow Heat Exchangers are one of the most common types of heat exchanger used in countless applications such as engine radiators, air heaters, refrigeration evaporators and condensers, super-heaters and economisers.

The unit is designed to enable students to investigate steady state rates of free and forced convective heat transfer at various air velocities and is supplied complete with a separate instrumentation console and a variable speed fan as standard.

A single plain tube plate, six row plain tube bundle, four row finned tube plate, local heat transfer cylinder and mounting plate are available as individual optional extras for detailed investigation. In addition a flat plate, pinned plate and finned plate are available as well as a heat pipe evaluation plate for entry level students.A computer upgrade covering all optional cross flow heat exchanger modules can be supplied together with menu driven Windows software for computerised data acquisition. The upgrade may be user or factory fitted. Data files are easily exported to Excel or other spread sheet formats for evaluation and presentation.

APPLICATIONS:Air Cooled Cross Flow

Car radiators are air-cooled cross flow heat exchangers. One type of cross flow heat exchanger uses air as the cooling medium. Typically, a hot liquid or gas flows through tubes while air is forced past the tubes transferring heat from the liquid or gas, through the tube material and into the air. The air is forced over the tubes by fans that either push or pull the air mechanically. Examples of air-cooled cross flow heat exchangers are air conditioner evaporator coils and car radiators. An air conditioner transfers heat through evaporator coils from a refrigerant. The refrigerant is used to pull heat from a space or provide heat, depending on the design. A car radiator uses a fan to push air across tubes with heat transfer fins. The coolant in the tubes transfers heat picked up from the automobile engine

Cross flow heat exchanger found aplications where one of the fluids changes states (2phase flow).an example is a steam system condenser in which steam exiting the turbine enters the condenser shell side and the cool water flowing in stainless steel tubes absorbs heat from steam,condensing it into water. Large volume of vapour may be condensed using this type of heat exchanger flow

Currently used in these Industries:

• Automotive• Commercial and Residential Heating/Cooling• Aircrafts• Manufacturing• Cooling Electronics

REFRENCES: www.stainless-steel-tube.org

http://me1065.wikidot.com/micro-scale-heat-exchangers

www.sciencedirect.com

www.ehow.com

p-a-hilton.co.uk

What is conduction? heat is transferred in several different ways: conduction, convection, and radiation. Regardless of the method of transfer, only heat can be transferred because cold is the absence of heat. Conduction is the transfer of heat from one molecule to another through a substance. Not all substances conduct heat at the same speed. Metals and stone are considered good conductors since they

can speedily transfer heat, but wood, paper, air, and cloth are poor heat conductors. Various materials are often researched for their conductive properties. The materials are given numbers that tell their relative rates of conduction. Materials are compared to silver(coefficient of heat conduction of 100). The coefficient of some other products are copper(92), iron(11), water(0 .12), wood(0.03). A perfect vacuum has a conduction coefficient of zero.Factors Influencing Conductive Heat Transfer The factors influencing conduction are temperature difference, length (or thickness), cross sectional area, and the thermal conductivity of the conductor.Thermal conductivity is the measure of the quantity of thermal energy which flows through a conductor. In addition to form, there are a number of factors influencing thermal conductivity of materials including molecular bonding, structure, and density. Units of measure for conductivity must account for the amount of energy transferred in a given amount of time, thickness (or distance), and temperature difference. The SI units of measure for thermal conductivity are Watts per Kelvin per Meter. When the temperature of one surface of a solid material is higher than another, heat will move through the material. Depending on the characteristics of the material, this conductive heat transfer may be slow or it may occur quickly. The rate of heat transfer is defined by the coefficient of thermal conductivity.