HEAT TRANSFER AND CIRCULATION SYSTEM

Presented byV.VASANTH KUMAR

What is heat transfer

Heat transfer can be defined as transmission of energy from one region to another region due to temperature difference.

In Boiler heat energy is released from the combustion of fossil fuels and heat is transferred to different fluids in system and part of its lost or left out as unutilized.

As a result, there is a essential to study modes of heat transfer in boiler.

HEAT TRANSFER MODES IN BOILER

CONDUCTION HEAT TRANSFER

The heat transfer from one part of a body to another part of a same body or one body to another in physical contact without appreciable displacement of particles of a body. The heat flow rate q can be expressed as , q = KS ( t1 - t2 ) / l Where , K = Thermal conductivity K.cal/m^2/hr/deg.cel S = Heat surface in m^2 t1 t2 = Temperature diff. C l = Length or thickness of a plate m

CONVECTION HEAT TRANSFER The heat transfer from one to another with in a fluid ( gas or liquid ) by the mixing of one part with another due to movement of fluids. The heat flow rate q can be expressed as Qc = Uc S t Where Qc = Heat flow by convection in k.cal / hr Uc = Convection film conductance in k.cal / M^2 / hr / c t= Temperature difference in C.

FREE CONVECTION A fluid at rest, If heated at one point the temperature that location raises and causes density difference with respect to other location and this causes movement of circulation of fluid with in fluid itself The Equation can be cal. Of free convection are Ufc = ( t )^1.3

FORCED CONVECTION In this heat transfer the flow of fluid is produced by external force like fan, pump, stack, change of state., etc.., Most of heat transfer in boiler is transferred by this method. Some of dimensionless number which are more frequently used in this heat transfer are Reynolds no, Prandtl no, Nusselt no, Stanton no.

RADIATION HEAT TRANSFER Radiation is the transfer of heat energy from one body to another by electromagnetic waves which can travel even through vacuum.

When radiation impinges on a body, some of radiant energy will be re radiated, some its will be transmitted through the body and the reminder will absorbed.

Radiation emitted by black body depends upon its surface area and temperature. The relation between is given by Stefan bolt man law. Q = S T^4 Where = Stefan bolt man constant T = Absolute temperature in deg.cel Q = Heat flow and S = surface area.

HEAT TRANSFER ARRANGEMENT There are three general kinds of arrangements of heat transfer surface as relative flow of fluid is concerned as ,

PARALLEL FLOW COUNTER FLOW

CROSS FLOW

PARALLEL FLOW Flow path of two fluids are in same direction

COUNTER FLOW Flow path of two fluids will opposite to each other.

CROSS FLOWFlow path of two fluids are perpendicular to each other.

LMTDTemperature difference between hot and cold fluid in heat exchanger varies from point to point.Therefore , based on concept of appropriate mean temperature difference ,also called as LMTD , the total heat transfer rate in heat exchanger is expressed as, Q = U A(T)m where U = Overall heat transfer coefficient A = area in M^2 ( T)m= LMTD

LMTD for parallel flow is, ( T)m= [(T1-t1)-(T2-t2)] / ln [ (T1-t1)/(T2-t2)]LMTD for counter flow is, ( T)m= [(T1-t2)-(T2-t1)] / ln [ (T1-t2)/(T2-t1)] where T1 = entry temp. of hot fluid , T2 = exit temp. of hot fluid t1 = entry temp. of cold fluid ,t2 = exit temp. of cold fluid

OTHER HEAT TRANSFER ASPECTS IN BOILEREffect of lanes

Insulation and heat loss

Convection and radiation superheater rate for load variation

Effect of exess air on heat transfer in boiler

Effect of heat transfer due to internal deposit

HEAT TRANSFER IN BOILER PRESSURE PARTS RADIATIONCONVECTION & RADIATIONCONVECTION & CONDUCTIONCONVECTION

DOWN COMERS

WATER WALL FRONT

WATER WALL REAR

PLATEN SUPERHEATER

REHEATER FRONT

REHEATER REAR

FINAL SUPERHEATER

WATER WALL BOTTOM RING HEADER

SCW SH FRONT

SCW SH REAR

DRUM

CRH IN

REAR ROOF

WATER INLET

SH STEAM OUTLET

EXTENDED WW

EXT SCW SH

FRONT ROOF

HRH OUT

LTSH

Economiser

BOILER TEMPERATURES IN DIFFERENT REGIONSSec Air NozzlePrimary Air with coal

**CIRCULATION SYSTEMBelow 175 kg/cm2 is natural circulation.Above 175kg/cm2 to 200kg/cm2 is controlled circulation.Beyond the critical pr. (225kg/cm2) is combined circulation.Circulation ratio = wt of water steam mixture/ wt of steam generated.For utility it is 6 to 9.For industrial 8 to 30.

**

CIRCULATION THEORYNUCLEATE BOILING :

when the bulk of water reaches saturation temperature the bubbles do not collapse. This condition is known as nucleate boiling.

Beyond nucleate boiling region the bubbles coalesce to form a film of superheated steam over part or all heating surface. This condition is known as film boiling.

The point , beyond which film boiling occurs in known as departure from nucleate boiling (DNB).

Till the occurrence of DNB, metal temp.is slightly above the water temperature .

When water starts boiling the metal temperature is slightly above the saturation temperature .

But DNB Occurs , the metal temperature increases much higher than the saturation temperature.

FACTOR AFFECTING CIRCULATIONHeat input to the water walls

Operating pressures

Disturbance in the system

Pattern of heat heat loading

THANK YOU

All industrial are working under natural circuation principle. Y axis water, x axis steam in density graph.Water having more density steam having less density. *