Introduction to Heat Exchanger & Its all types

Heat Exchanger

Introduction:

Heat exchanger is a device built for efficient heat transfer from one medium to another, whether the media are separated by a solid wall so that they never mix, or the media are in direct contact. They are widely used in space heating, refrigeration, air

conditioning, power plants, chemical plants, petrochemical plants, petroleum refineries,

and natural gas processing. One common example of a heat exchanger is the radiator in

a car, in which a hot engine-cooling fluid, like antifreeze, transfers heat to air flowing

through the radiator. To design a heat exchanger, many criteria have to be taken before

making any decision. The important parameters of heat exchangers are collected and put

a major consideration on it. In this project, the software needed to configure all the

parameters to put in. From there, the software runs and the main point is to create a new

windows program that is similar to another existed software of calculating heat transfer

in market. The major purpose is to obtain a desired output by using this program. Thus,

the ideal shell and tube heat exchanger is produced at the end.

Heat exchanger is one of devices that is convenient in industrial and household application. These include power production, chemical industries, food industries,

electronics, environmental engineering, manufacturing industry, and many others. It

comes in many types and function according to its uses.

So what exactly heat exchanger is? Heat exchanger is a device that is used to

transfer thermal energy between two or more fluids, between a solid surface and a fluid

at different temperatures and in thermal contact. There are usually no external heat and

work interactions. In most heat exchangers, heat transfer between fluids takes place

through a separating wall or into and out of a wall in a transient manner. (Shah R.K.,

2003)

This chapter will discuss about the uses and application of shell and tube heat

exchanger, type of heat exchangers, and shell and tube heat exchanger.

2.2 USES AND APPLICATIONS OF HEAT EXCHANGER

Heat exchangers are used to transfer heat from one media to another. It is most

commonly used in space heating such as in the home, refrigeration, power plants and

even in air conditioning. It is also used in the radiator in a car using an antifreeze engine

cooling fluid. Heat exchangers are classified according to their flow arrangements

where there are the parallel flow, and the counter flow. Aside from this, heat exchangers

also have different types depending on their purpose and how that heat is exchanged.

But the fact is that there are heat exchangers even in the circulation system of fishes and

whales. The veins of these animals are intertwined such that one side is carrying cold

blood and the other has cold blood. As a result, these species can prevent heat loss

especially when they are swimming in cold water. In some whales, the heat exchanger

can be found in their tongues. When it comes to the manufacturing industry, heat

exchangers are used both for cooling and heating. Heat exchangers in large scale

industrial processes are usually custom made to suit the process, depending on the type

of fluid used, the phase, temperature, pressure, chemical composition and other

thermodynamic properties. (Kakac, S. 2002)

Heat exchangers mostly can be found in industries which produce a heat stream.

In this case, heat exchangers usually circulate the output heat to put it as input by

heating a different stream in the process. The fact that it really saves a lot of money

because when the output heat no longer needed then it can be recycled rather than to

come from an external source as heat is basically recycled. When used in industries and

in the home, it can serve to lower energy costs as it helps recover wasted heat and

recycle it for heating in another process. Typically, most heat exchangers use fluid to

store heat and heat transfer can take the form of either absorption or dissipation.

             

         For instance, heat exchangers are used as oil coolers, transmission and engine

coolers, boiler coolers, waste water heat recovery, condensers and evaporators in

refrigeration systems. In residential homes, heat exchangers are used for floor heating,

pool heating, snow and ice melting, domestic water heater, central, solar and geothermal

heating. Of course, heat exchangers have different designs which depend on the purpose

it is intended for. Brazed heat exchangers, a collection of plates which are brazed

together, are used for hydronic systems like swimming pools, floor heating, snow and

ice melting. The shell and coil heat exchanger design is best for areas with limited

spaces as it can be installed vertically. Of course, for the highly industrial process, the

shell and tube heat exchanger is the perfect solution.

2.3 TYPE OF HEAT EXCHANGERS

In industries, there are lots of heat exchanger that can be seen. The types of heat

exchanger can be classified in three major constructions which are tubular type, plate

type and extended surface type.

2.3.1 Tubular Heat Exchangers

The tubular types are consists of circular tubes. One fluid flows inside the tubes

and the other flows on the outside of the tubes. The parameters of the heat exchanger

can be changed like the tube diameter, the number of pitch, tube arrangement, number

of tubes and length of the tube can be manipulate. The common type of heat exchangers

lie under this categories are double-pipe type, shell-and-tube type and spiral tube type.

The tubular heat exchangers can be designed for high pressure relative to the

environment and high pressure difference between the fluids. These exchangers are used

for liquid-to-liquid and liquid-to-vapor phase. But when the operating temperature or

pressure is very high or fouling on one fluid side, it will used gas-to-liquid and gas-togas

heat transfer applications.

2.3.1.1 Double-Pipe Heat Exchanger

According to Sadic Kakac, a double-pipe heat exchanger consists of smaller and

larger diameter pipe where the smaller pipe fitted concentrically into the larger one in

purpose to give direction to the flow from one section to another. One set of these tubes

includes the fluid that has to be cooled or heated. The second fluid runs over the tubes

being cooled or heated in order to provide heat or absorb the heat. A set of tubes is the

tube bundle and it can be made up of several types of tubes such as longitudinally plain,

longitudinally finned, and more. If the application requires an almost constant wall

temperature, the fluids may flow in a parallel direction. It's easy to clean and convenient

to disassemble and assemble. The double-pipe heat exchanger is one of the simplest.

Usually, it is used for small capacity applications because it is so expensive on a cost

per unit area basis. Figure 2.1 presents the model of double-pipe heat exchanger.

Figure 2.1: Double-pipe heat exchanger

Source: Ritai China

2.3.1.2 Shell-and-Tube Heat Exchanger

This exchanger is built of a bundle of round tubes mounted in a large cylindrical

shell with the tube axis parallel to the shell to transfer the heat between the two fluids.

The fluid flows inside the tubes and other fluid flows across and along the tubes. But for

baffled shell-and-tube heat exchanger the shell side stream flows across between pairs

of baffles and then flows parallel to the tubes as it flows from one baffle compartment to

the next. This kind of exchanger consists of tubes, shells, front-end head, rear-end head,

baffles and tubesheets. The different type of shell-and-tube heat exchangers depends on

different application. Usually in chemical industry and process application, it is used as

oil-coolers, power condensers, preheaters in power plants and also steam generators in

nuclear power plants. The most common types of shell-and-tube heat exchanger are

fixed tubesheet design, U-tube design and floating-head type. Cleaning this heat

exchanger is easy. Instead of easily cleaning, it is also low in cost. But among all tube

bundle types, the U-tube is the least expensive because it only needs one tube sheet.

Technically, because of its construction in U shape, the cleaning is hardly done in the

sharp bend. An even number of tube passes only can be achieved. The figure 2.2 shows

the type of shell-and-tube heat exchanger.

Figure 2.2: U-tube shell-and-tube heat exchanger

Source: API Heat Transfer

2.3.1.3 Spiral-Tube Heat Exchanger

A spiral heat exchanger is a helical or coiled tube configuration. It consists of

spirally wound coils placed in a shell or designed as co-axial condensers and co-axial

evaporators that are used in refrigeration systems. The heat transfer coefficient is higher

in a spiral tube than in a straight tube. Since the cleaning is impossible, the spiral tubes

are suitable for thermal expansion and clean fluids. The biggest advantage of the spiral

heat exchanger is its efficient use of space. A compact spiral heat exchanger can lower

costs, while an oversized one can have less pressure drop, higher thermal efficiency,

less pumping energy, and lower energy costs. Spiral heat exchangers are frequently used

when heating fluids that have solids and therefore often foul the inside of the heat

exchanger. Spiral heat exchangers have three types of flow arrangements. Firstly, the

spiral flow and cross flow has one fluid in each. The spiral flow passages are welded at

each side and this type of flow is good for handling low density gases which pass

through the cross flow. This can be used for liquid-to-liquid applications if one fluid

has a much greater flow rate than the other. A second type is the distributed vapor and

spiral flow. The coolant moves in a spiral and exits through the top. The hot gases that

enter will leave as condensate out of the bottom outlet. The third type is the

countercurrent flow where both of the fluids will flow in opposite directions and are

used for liquid-to-liquid applications. The spiral heat exchanger is good for

pasteurization, heat recovery, digester heating, effluent cooling, and pre-heating. Figure

2.3 presents the spiral-tube heat exchanger.

Figure 2.3: Spiral tube heat exchanger

2.3.2 Plate Heat Exchangers

A second type of heat exchanger is a plate heat exchanger. It has many thin

plates that are slightly apart and have very large surface areas and fluid flow passages

that are good for heat transfer. This can be a more effective heat exchanger than the

tube or shell heat exchanger due to advances in brazing and gasket technology that have

made this plate exchanger more practical. Large heat exchangers are called plate and

frame heat exchangers and there allow for periodic disassembly, cleaning, and

inspection. There are several types of permanently bonded plate heat exchangers like

dip brazed and vacuum brazed plate varieties, and they are often used in refrigeration.

These heat exchangers can further be classified as gasketed plate, spiral plate and

lamella.

2.3.2.1 Gasketed Plate Heat Exchangers

A gasketed plate heat exchanger consists of a series of thin plates that have wavy

surface which function as separating the fluids. The plates come with corner parts

arranged so that the two media between which heat is to be exchanged flow through

interchange exclaim spaces. Appropriate deisgn and gasketing permit a stack of plates

to be held together by compression bolts joining the end plates. Gaskets prevent leakage

to the outside and direct the fluids in the plates as desired. The flow patern is generally

chosen so that the media flow countercurrent to each other. Since the flow passages are

quite small, strong eddying gives high heat transfer coefficients, high pressure drops,

and high local shear which minimizes fouling. These exchangers provide a relatively

compact and lightweight heat transfer surface. Gasketed plate are typically used for heat

exchange between two liquid streams. This type can be found in food processing

industries because of the compatibility to be cleaned easily and sterilized as it

completely disassembled. Figure 2.4 presents the gasketed plate heat exchanger.

Figure 2.4: Gasketed plate heat exchanger

2.3.2.2 Spiral Plate Heat Exchanger

Spiral heat exchangers are formed by rolling two long, parallel plates into a

spiral using a mandrel and welding the edges of adjacent plates to form channels. The

distance between the metal surfaces in both channels is maintained by means of distance

pins welded to the metal sheet. The two spiral paths introduce a secondary flow,

increasing the heat transfer and reducing fouling deposits. These heat exchangers are

quite compact but are relatively expensive due to the specialized fabrication. The spiral

heat exchanger is particularly effective in handling sludges, viscous liquids, and liquids

with solids in suspension including slurries.

The spiral heat exchanger is made in three main types which differ in the

connections and flow arrangements. Type Ι has flat covers over the spiral channels. The

media flow countercurrent through the channels via the connections in the center and at

the periphery. This type is used to exchange heat between media without phase changes

such as liquid-liquid, gas-liquid, or gas-gas. One stream enters at the center of the unit

and flows from inside outward. The other stream enters at the periphery and flows

towards the center. Thus the counterflow is achieved.

          Type ΙΙ is designed for crossflow operation. One channel is completely sealwelded,

while the other is open along both sheet metal edges. The passage with the

medium in spiral flow is welded shut on each side, and the medium in crossflow passes

through the open spiral annulus. This type is mainly used as a surface condenser in

evaporating plants. It is also highly effective as a vaporizer. Two spiral bodies are often

built into the same jacket and are mounted below each other.

             Type ΙΙΙ, the third standard type is in principle similar to type Ι with alternately

welded up channels, but type ΙΙΙ is provided with a specially designed top cover. This

type of heat exchanger is mainly intended for condensing vapors with sub-cooling of

condensate and noncondensable gases. The top cover, therefore, has a special

distribution cone where the vapor is distributed to the uncovered spiral turns in order to

maintain a constant vapor velocity along the channel opening. The figure 2.5 and 2.6

presents the types of spiral plate.

Figure 2.5: Spiral plate heat exchanger Type Ι

Figure 2.6: Spiral plate heat exchanger Type ΙΙ

2.3.2.3 Lamella Heat Exchangers

The lamella type of heat exchanger consists of a set of parallel, welded, thin

plates channels are lamellae placed longitudinally in a shell. It is a modification of the

floating-type shell-and-tube heat exchanger. These flattened tubes, called lamellae are

made up of two strips of plates, profiled and spot- or seam-welded together in a

continuous operation. The forming of the strips creates space inside the lamellae and

bosses acting as spacers for the flow sections outside the lamellae on the shell side. The

lamellae are welded together at both ends by joining the ends with steel bars in between,

depending on the space required between lamellae. Both ends of the lamella bundle are

joined by peripheral welds to the channel cover which at the outer ends is welded to the

inlet and outlet nozzle. The lamella side thus completely sealed in by welds. At the

fixed end, the channel cover is equipped with an outside flange ring which is bolted to

the shell flange. Figure 2.7 presents the lamella heat exchanger