HEAT EXCHANGERHEAT EXCHANGER

GUIDED BY:GUIDED BY: PREPARED BY: PREPARED BY:Prof. S.C. SharmaProf. S.C. Sharma Aayush Toshniwal Aayush Toshniwal

Harsh ChudgarHarsh Chudgar Nikhil LeleNikhil Lele

FOULING FACTORFOULING FACTOR

The performance of heat exchanger usually deteriorates with time as a result of accumulation of deposits on heat transfer surfaces. The layer of deposits represents additional resistance to heat transfer and causes the rate of heat transfer in a heat transfer to decrease. The net effect of these accumulation of heat transfer is represented by fouling factor which is a measure of thermal resistance induced by fouling.

Fouling should be considered in the design and selection of heat exchanger. In such application it may be necessary to select a larger and expensive heat exchanger.

The periodic cleaning of heat exchanger and the resulting down time are additional penalties associated with fouling.

The fouling factor depends on operating temperature and the velocities of fluids and length of service. Fouling increases with increasing temperature and decreasing velocities.

Fouling Factor

Precipitation of solid deposits

Corrosion and Chemicals fouling

Biological fouling

PRECIPITAION OF SOLID PRECIPITAION OF SOLID DEPOSITESDEPOSITES

These is specially found in the areas where the water is hard. The scales of such deposits come off by scratching and the surfaces can be cleaned of such deposits by chemical treatment.

To avoid these problem, water in power and process plants is extensively heated and its solid contents are removed before it is allowed to circulates through the system.

The solid ash particles in the flue gases accumulating on the surfaces of air pre-heaters create similar problems.

CHEMICAL FOULINGCHEMICAL FOULING

In this case surfaces are fouled by the accumulation of the products of chemical reactions on the surfaces. This form of fouling can be avoided by coating metal pipes with glass or using plastic pipe instead of metal ones.

BIOLOGICAL FOULINGBIOLOGICAL FOULING

Heat exchangers may also be fouled by the growth of Heat exchangers may also be fouled by the growth of algae in warn water. This type of fouling can be algae in warn water. This type of fouling can be prevented by chemical treatment.prevented by chemical treatment.

Analysis of heat exchangerAnalysis of heat exchanger

For analyzing of heat exchanger we will discuss the two For analyzing of heat exchanger we will discuss the two methods.methods.

Log Mean Temperature Difference Method (LMTD)Log Mean Temperature Difference Method (LMTD)

effectiveness- NTU methodeffectiveness- NTU method

Heat exchangers usually operate for long periods of time Heat exchangers usually operate for long periods of time with no change in there operating condition. Therefore, with no change in there operating condition. Therefore, they can be modeled as they can be modeled as study flow device.study flow device.

Assumptions Assumptions

The mass flow rate of each fluid remain constant.The mass flow rate of each fluid remain constant.

The fluid properties such as temp. and velocity at inlet and outlet The fluid properties such as temp. and velocity at inlet and outlet remain same.remain same.

The fluid stream experienced little or no change in their velocities The fluid stream experienced little or no change in their velocities and elevations, and thus the and elevations, and thus the kinetic and potential energykinetic and potential energy changeschanges are negligible.are negligible.

The specific heat of the fluid, in general, changes with temp. But, in The specific heat of the fluid, in general, changes with temp. But, in a specified temperature range it can be treated as a constant at a specified temperature range it can be treated as a constant at some average value with little loss in accuracy. some average value with little loss in accuracy.

Axial heat conductionAxial heat conduction along the tube is usually insignificant and can along the tube is usually insignificant and can be considered negligible.be considered negligible.

Finally, the outer surface of the heat exchanger is assumed to be Finally, the outer surface of the heat exchanger is assumed to be perfectly insulatedperfectly insulated..

Under these assumptions, the first law of thermodynamics requires that the rate of heat transfer from the hot fluid be equal to the rate of heat transfer to the cold one.That is,

Principle of heat exchangerPrinciple of heat exchanger

Counter flow Parallel flow

HC CmCm HC CmCm

LMTD methods.LMTD methods.The curse of the non-linear behaviorThe curse of the non-linear behavior

Due to the nonlinear behavior of the temperature difference cross the heat exchanger. An appropriate average temperature difference has to be adopted

The lmtd definitionThe lmtd definition

The correction factor F for multi-pass The correction factor F for multi-pass and and

cross-flowcross-flow

The standard lmtd formulation is limited to The standard lmtd formulation is limited to the simple cases of parallel and counter the simple cases of parallel and counter flow configurations.flow configurations.

In more complex cases as cross flow and In more complex cases as cross flow and multi-pass the correction factor F has to be multi-pass the correction factor F has to be considered.considered.

F factors for various flow configurationsF factors for various flow configurations

F factors for various flow F factors for various flow configurationsconfigurations

SELECTION OF HEAT EXCHANGERS

Heat exchangers are complicated devices and the result obtained with the simplified approach presented should be used with care.

It is natural to tend to overdesign the heat exchanger in order to avoid unpleasant surprises.

Engineers in industry often find themselves in a position to select heat exchangers to accompany certain heat transfer tasks.

Heat transfer enhancement in heat exchanger is usually accompanied by increased pressure drop and thus higher pumping power. Therefore any gain from enhancement should be weighed at the cost of the accompanying pressure drop.

Some thought should be given to which fluid should pass through the tube side and which through the shell side. Usually the more viscous fluid is suitable for shell side (larger passage area and thus lower pressure drop) and fluid with higher pressure for the tube side.

THE PROPER SELECTION DEPENDS ON FOLLOWING FACTORS:

•HEAT TRANSFER RATE

•SIZE AND WEIGHT

•COST

•PUMPING POWER

•MATERIAL

HEAT TRANSFER

The heat exchanger should be capable of transferring heat at the specified rate in order to achieve the desired temperature change of the fluid at the specified mass flow rate

SIZE AND WEIGHT

Normally the smaller and lighter heat exchanger is the better one. This is especially in the case of automotive and aerospace industries.

Larger heat exchangers carry higher price tag. The space available for the heat exchanger in some cases limits the length of the tube that can be used.

COST

Budgetary limitations plays an important role in the selection oh heat exchangers, except where money in not so important.

An Off-the-shelf heat exchanger has a definite cost advantage over those made to order as in the cases where heat exchangers are integral part of the overall device to be manufactured.

The operating and maintenance costs of the heat exchanger are also important considerations in assessing the overall cost

OPERATING COST = (PUMPING COST, kW) X (HOURS OF OPERATIONS, hrs) X (PRICE OF

ELECTRICITY, Rs/kWh)

PUMPING POWERPUMPING POWER

In heat exchangers, both fluids are usually forced to flow by pumps or fans that consume electrical power.

Pumping power is the total electrical power consumed by the motors of the pumps and fans.

MATERIALS

A temperature difference of 50oC or more between the tubes and the shell will probably pose differential thermal expansion problems and need to be considered. In case of corrosive fluids, we may have to select expensive corrosion resistance materials such as stainless steel.