Heat Exchangers

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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 heat exchanger construction is the tube and shell, as shown in Figure.



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



At the ends of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheet(s). The tubes are rolled and press-fitted or welded into the tube sheet to 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 of plates instead of tubes to separate the hot and cold fluids.



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



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



Therefore a plate type heat exchanger, as compared to a similar ly sized tube and shell heat exchanger, is capable of transferring much more heat.



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

Plate Type Heat Exchanger

FLOW OF ARRANGEMENT 1. 2. 3.

Parallel Flow Counter Flow 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 gas goes from right to left), is given by the following equation:



And for Parallel flow (i.e. where the hot stream, liquid or gas, goes from say left to right, and so does the cold stream), is given by the following 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 calculate the rate of heat transfer in heat exchangers (especially counter current exchangers) when there is insufficient information to calculate the Log-Mean Temperature Difference (LMTD).



The method proceeds by calculating the heat capacity rates (i.e. flow rate multiplied by specific heat) Ch and Cc for the hot and cold fluids respectively, 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 Density Viscosity Specific heat etc.

1. 2. 3.

CALCULATIONS 

Step C - The thermal parameters are calculated and tabulated like the temperature and pressure range.

CALCULATIONS 

1. 2. 3. 4. 5.

Step D – finally all the thermal parametrs are evaluated like, Capacity Effectiveness NTU LMTD U = overall heat transfer by the use of various heat transfer formulaes.