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WHAT IS HX……???

Heat exchangers are equipment that transfer

heat from one medium to another.

E.g.

From hot water to cold water,

From hot steam to cold water,

From hot gas to cold water,

From hot water or unsaturated steam to cool air.

WHAT IS HX…..???

A heat exchanger is a component that allows the transfer of

heat from one fluid (liquid or gas) to another fluid.

Reasons for heat transfer include the following:

1. To heat a cooler fluid by means of a hotter fluid

2. To reduce the temperature of a hot fluid by means of a cooler

fluid

3. To boil a liquid by means of a hotter fluid

WHAT IS HX….???

4. To condense a gaseous fluid by means of a cooler fluid

5. To boil a liquid while condensing a hotter gaseous fluid

Regardless of the function the heat exchanger fulfils, in order

to transfer heat the fluids involved must be at different

temperatures and they must come into thermal contact.

Heat can flow only from the hotter to the cooler fluid.

TYPES

Although heat exchangers come in every shape

and size imaginable, the construction of most

heat exchangers fall into one of two categories:

tube and shell,

or plate.

TUBE AND SHELL

The most basic and the most common type of heatexchanger construction is the tube and shell, as shown inFigure.

This type of heat exchanger consists of a set of tubes in acontainer called a shell. The fluid flowing inside thetubes is called the tube side fluid and the fluid flowing onthe outside of the tubes is the shell side fluid.

At the ends of the tubes, the tube side fluid is separatedfrom the shell side fluid by the tube sheet(s). The tubesare rolled and press-fitted or welded into the tube sheetto provide a leak tight seal.

Shell & Tube HX

TUBE AND SHELL

In systems where the two fluids are at vastly different

pressures, the higher pressure fluid is typically directed through

the tubes and the lower pressure fluid is circulated on the shell

side.

This is due to economy, because the heat exchanger tubes can

be made to withstand higher pressures than the shell of the heat

exchanger for a much lower cost.

The support plates shown on Figure also act as baffles to direct

the flow of fluid within the shell back and forth across the

tubes.

PLATE HX….?

A plate type heat exchanger, as illustrated in Figure 2, consists ofplates instead of tubes to separate the hot and cold fluids.

The hot and cold fluids alternate between each of the plates. Bafflesdirect the flow of fluid between plates.

Because each of the plates has a very large surface area, theplates provide each of the fluids with an extremely largeheat transfer area.

Therefore a plate type heat exchanger, as compared to a similarly sized tube and shell heat exchanger, is capable of transferringmuch more heat.

This is due to the larger area the plates provide over tubes.

Plate Type Heat Exchanger

FLOW OF ARRANGEMENT

1. Parallel Flow

2. Counter Flow

3. Cross Flow

In parallel-flow heat exchangers, the two fluids enter the

exchanger at the same end, and travel in parallel to one

another to the other side.

In counter-flow heat exchangers the fluids enter the

exchanger from opposite ends. The counter current design

is most efficient, in that it can transfer the most heat.

Parallel Flow, Counter Flow, Cross Flow Types of HX

FLOW OF ARRANGEMENT

In a cross-flow heat exchanger, the fluids travel

roughly perpendicular to one another through the

exchanger.

The exchanger's performance can also be

affected by the addition of fins or corrugations in

one or both directions, which increase surface

area and may channel fluid flow or induce

turbulence.

REGENERATIVE HEAT EXCHANGER

A third type of heat exchanger is the regenerative heat

exchanger.

In this, the heat from a process is used to warm the fluids to be

used in the process, and the same type of fluid is used either

side of the heat exchanger (these heat exchangers can be either

plate-and-frame or shell-and-tube construction).

These exchangers are used only for gases and not for liquids.

The major factor for this is the heat capacity of the heat transfer

matrix.

Regenerative Heat Exchanger

Recuperator type Heat Exchanger

RECUPERATORS

A fourth type of heat exchanger uses an intermediate fluid or

solid store to hold heat, which is then moved to the other side

of the heat exchanger to be released.

Two examples of this are adiabatic wheels, which consist of a

large wheel with fine threads rotating through the hot and cold

fluids, and fluid heat exchangers.

This type is used when it is acceptable for a small amount of

mixing to occur between the two streams.

LMTD

The log mean temperature difference (LMTD)

is used to determine the temperature driving

force for heat transfer in flow systems (most

notably in heat exchangers).

The LMTD is a logarithmic average of the

temperature difference between the hot and cold

streams at each end of the exchanger.

For Counter current flow (i.e. where the hot stream, liquid or gas,goes from say left to right, and the cold stream, again liquid or gasgoes from right to left), is given by the following equation:

And for Parallel flow (i.e. where the hot stream, liquid or gas, goesfrom say left to right, and so does the cold stream), is given by thefollowing equation:

T1 = Hot Stream Inlet Temp.

T2 = Hot Stream Outlet Temp.

t1 = Cold Stream Inlet Temp.

t2 = Cold Stream Outlet Temp.

NTU METHOD

The Number of Transfer Units (NTU) Method is used to calculatethe rate of heat transfer in heat exchangers (especially countercurrent exchangers) when there is insufficient information tocalculate the Log-Mean Temperature Difference (LMTD).

The method proceeds by calculating the heat capacity rates (i.e. flowrate multiplied by specific heat) Ch and Cc for the hot and cold fluidsrespectively, and denoting the smaller one as Cmin.

value of qmax is the maximum heat which could be transferred between the fluids.

EFFECTIVENESS OF HX

E is then defined in terms of that maximum:

E can be calculated using correlations in terms of the

'heat capacity ratio‘

and the number of transfer units, NTU

PURPOSE OF THE PERFORMANCE TEST

To determine the overall heat transfer coefficient

for assessing the performance of the heat

exchanger.

Any deviation from the design heat transfer

coefficient will indicate occurrence of fouling.

CALCULATIONS

Step A - monitoring and reading the steady state

parameters like temperature and pressure (inlet , outlet,

hot and cold)

Step B – with monitored data the physical properties of

stream is determined like

1. Density

2. Viscosity

3. Specific heat etc.

CALCULATIONS

Step C - The thermal parameters are calculated

and tabulated like the temperature and pressure

range.

CALCULATIONS

Step D – finally all the thermal parametrs are

evaluated like,

1. Capacity

2. Effectiveness

3. NTU

4. LMTD

5. U = overall heat transfer by the use of various

heat transfer formulaes.