heat exchangers principle

7/29/2019 heat exchangers principle

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By

R.Kargen

Pgia/11/8700


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Heat and Temperature

Often make the mistake of interchangeablyusing the terms heat and temperature

Heat - energy in transit

Temperature - measure of the amount ofenergy possessed by the molecules of a

substance

Temperature is an intensive property, whileheat is an extensive property

Is really temperature a measure of heat?


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Heat exchanger principle

Heat transfer from one medium to another

Start with the determination of the overall heattransfer coefficient

Heat transfer depends on Type of materials

Thickness

Surface area

Type of fluid

Flow rate of fluids

Etc.

How to calculate?


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Wall heat exchange


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Inclusion of surface fouling and fin (extended

surface) effects, the overall heat transfer

coefficient change as


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Physical situation Btu/h ft2 F W/m2 C

Brick exterior wall, plaster interior, 0.45 2.55uninsulated

Frame exterior wall, plaster interior:Uninsulated 0.25 1.42With rock-wool insulation 0.07 0.4Plate-glass window 1.10 6.2Double plate-glass window 0.40 2.3Steam condenser 200-1000 1100-5600Feedwater heater 200-1500 1100-8500

Freon-l2 condenser with watercoolant 50-150 280-850

Water-to-water heat exchanger 150-300 850-1700Finned-tube heat exchanger, water in

tubes, air across tubes 5-10 25-55Water-to-oil heat exchanger 20-60 110-350Steam to light fuel oil 30-60 170-340

Steam to heavy fuel oil 10-30 56-170Steam to kerosone or gasoline 50-200 280-1140

Finned-tube heat exchanger, steam in

tubes, air over tubes 5-50 28-280Ammonia condenser, water in tubes 150-250 850-1400Alcohol condenser, water in tubes 45-120 255-680Gas-to-gas heat exchanger 2-8 10-40

Approximate values for overall heat transfer coefficient


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Fouling factor

Due to various deposits, corrosion

Represents an additional resistance to the heat

flow

Decreased performance

Overall effect is usually represented byfouling


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Normal fouling factors


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What is a heat exchanger?

A piece of equipment built for efficient heat

transfer from one medium to another

May be separated by a solid wall, so that they

never mix, or they may be in direct contact

Used in space heating, refrigeration, air

conditioning, power plants, chemical plants,

petrochemical plants, petroleum refineries,

natural gas processing, and sewage treatment


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Heat exchanger analysis

Log mean temperature difference

Relate the total heat transfer rate to quantities

such as the inlet and outlet fluid temperatures, ,

the overall heat transfer coefficient, and the totalsurface area for heat transfer

Applying overall energy balances


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i -fluid enthalpy

subscripts h and c refer to the hot and cold fluids

i and o - fluid inlet and outlet conditions


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If the fluids are not undergoing a phase change

and constant specific heats are assumed


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Parallel-Flow Heat Exchanger


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The heat exchanger is insulated from its

surroundings, in which case the only heat

exchange is between the hot and cold fluids. Axial conduction along the tubes is negligible.

Potential and kinetic energy changes are

negligible.

The fluid specific heats are constant.

The overall heat transfer coefficient is constant.


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Counter flow Heat Exchanger

Provides for heat transfer between the hotter portions

of the two fluids at one end, as well as between thecolder portions at the other


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The Effectiveness-NTU Method

Depends on maximum possible heat transfer rate

and many more Use when only inlet temperatures are known


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Compact Heat Exchangers

Used when a large heat transfer surface areaper unit volume is desired and at least one of

the fluids is a gas

Heat transfer correlated in terms of theColburn j factor jH = St Prm and the Reynolds

number, where both the Stanton (St = h/Gcp)

and Reynolds (Re = GDh/u) numbers are based

on the maximum mass velocity


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Types of heat exchangers

1. Shell and tube heat exchanger

consist of a series of tubes

One set contains the fluid that must be either

heated or cooled

second fluid runs over the tubes

typically used for high-pressure applications


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2. Plate heat exchanger

composed of multiple, thin, slightly-separated

plates with very large surface areas fluid flow passages for heat transfer

stacked-plate arrangement can be more effective


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3. Plate fin heat exchanger

"sandwiched" passages containing fins to

increase the effectiveness of the unit

include crossflow and counterflow coupled with

various fin configurations

used for low temperature services such asnatural gas, helium and oxygen liquefaction

plants, air separation plants and transport

industries such as motor and aircraft engines


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4. Phase-change heat exchangers

heat a liquid to evaporate (or boil) it or used as

condensers to cool a vapor and condense it to aliquid


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5. Spiral heat exchangers

pair of flat surfaces that are coiled to form the

two channels in a counter-flow arrangement highly efficient use of space


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6. Screw heat exchanger


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Selection

High/ Low pressure limits

Thermal Performance

Temperature ranges

Product Mix (liquid/liquid, particulates or high-solids liquid)

Pressure Drops across the exchanger

Fluid flow capacity

Cleanability, maintenance and repair

Materials required for construction

Ability and ease of future expansion


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heat exchangers principle

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