CLASSIFICATION OF HEAT EXCHANGERS BASED ON CONSTRUCTION

Classification of heat exchangers based on construction

Plate-Type Heat Exchangers

• Plate-type heat exchangers are usually built of thin plates (all prime surface). 
• The plates are either smooth or have some form of corrugation, and they are either flat or wound in an exchanger. 
• Generally, these exchangers cannot accommodate very high pressures,temperatures, or pressure and temperature differences. 

• Plate heat exchangers (PHEs) can be classified as gasketed, welded (one or both fluid passages), or brazed, depending on the leak tightness required. 
• Other plate-type exchangers are spiral plate, lamella, and platecoil exchangers.

Extended Surface Heat Exchangers

• The tubular and plate-type exchangers are all prime surface heat exchangers, except for a shell-and-tube exchanger with low finned tubing. 
• In some applications, much higher (up to about 98%) exchanger effectiveness is essential, and the box volume and mass are limited so that a much more compact surface is mandated.
• Also, in a heat exchanger with gases or some liquids, the heat transfer coefficient is quite low on one or both fluid sides. This results in a large heat transfer surface area requirement.
• One of the most common methods to increase the surface area and exchanger compactness is to add the extended surface (fins) and use fins with the fin density ( fin frequency, fins/m or fins/in.) as high as possible on one or both fluid sides, depending on the design requirement. 
• Addition of fins can increase the surface area by 5 to 12 times the primary surface area in general, depending on the design. 
• The resulting exchanger is referred to as an extended surface exchanger. 
• Flow area is increased by the use of thingauge material and sizing the core properly.
• The heat transfer coefficient on extended surfaces may be higher or lower than that on unfinned surfaces. 

• Plate-fin and tube-fin geometries are the two most common types of extended surface heat exchangers.

Regenerators

• The regenerator is a storage-type heat exchanger. 
• Regenerator, is a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. 
• To accomplish this the hot fluid is brought into contact with the heat storage medium, then the fluid is displaced with the cold fluid, which absorbs the heat.

• The operation of regenerative heat exchangers is cyclic.
• In the first cycle hot gases/fluids passing through the media heat up the media. In the following cycle, the cold gases pass through the media and they are heated by the already hot media.
• Typical medias used are packed towers of ceramic material with required gaps for the gases to pass through them.
• In regenerative heat exchangers, the fluid on either side of the heat exchanger can be the same fluid. 

CLASSIFICATION OF HEAT EXCHANGER BY CONSTRUCTION

Heat exchangers are frequently characterized by construction features into four major construction types  as follows

1. Tubular heat exchangers
2. Plate-type heat exchangers
3. Extended surface heat exchangers
4. Regenerative heat exchangers

Tubular heat exchanger

• These exchangers are generally built of circular tubes, although elliptical, rectangular, or round/flat twisted tubes.
• Tubular exchangers can be designed for high pressures relative to the environment and high-pressure differences between the fluids.
• Tubular exchangers are used primarily for liquid-to-liquid and liquid-to-phase change (condensing or evaporating) heat transfer applications. 
• They are used for gas-to-liquid and gas-to-gas heat transfer applications primarily when the operating temperature and/or pressure is very high or fouling is a severe problem on at least one fluid side and no other types of exchangers would work. 
• These exchangers may be classified as 

1. Shell-and tube heat exchangers
2. Double-pipe heat exchangers
3. Spiral tube heat exchangers

Shell-and-Tube Exchangers. 

• This exchanger, is generally built of a bundle of round tubes mounted in a cylindrical shell with the tube axis parallel to that of the shell. 
• One fluid flows inside the tubes, the other flows across and along the tubes. 

• A variety of different internal constructions are used in shell-and-tube exchangers, depending on the desired heat transfer and pressure drop performance and the methods employed to reduce thermal stresses, to prevent leakages, to provide for ease of cleaning, to contain operating pressures and temperatures, to control corrosion, to accommodate highly asymmetric flows, and so on. 
• The three most common types of shell-and-tube exchangers are 

1. Fixed tubesheet design
2. U-tube design
3. Floating-head type. 

The major components of this exchanger are tubes (or tube bundle), shell, frontend head, rear-end head, baffles, and tubesheets 

Tubes. 

• Round tubes in various shapes are used in shell-and-tube exchangers. 
• Most common are the tube bundles with straight and U-tubes used exchangers. 
• In most applications, tubes have single walls, but when working with radioactive,reactive, or toxic fluids and potable water, double-wall tubing is used. 
• In most applications, tubes are bare, but when gas or low-heat-transfer coefficient liquid is used on the shell side, low-height fins (low fins) are used on the shell side. 
• Tubes are drawn, extruded, or welded, and they are made from metals, plastics, and ceramics, depending on the applications.

Shells

• The shell is a container for the shell fluid.
• Usually, it is cylindrical in shape with a circular cross section, although shells of different shapes are used. 
• Although the tubes may have single or multiple passes, there is one pass on the shell side. 
• To increase the mean temperature difference and hence exchanger effectiveness, a pure counterflow arrangement is desirable for a two-tube-pass exchanger.  

Nozzles. 

• The entrance and exit ports for the shell and tube fluids, referred to as nozzles
• These are pipes of constant cross section welded to the shell and channels. 
• They are used to distribute or collect the fluid uniformly on the shell and tube sides. 

 Front- and Rear-End Heads. 

• These are used for entrance and exit of the tube fluid.
• The front-end head is stationary, while the rear-end head could be either stationary (allowing for no tube thermal expansion) or floating, depending on the thermal stresses between the tubes and shell. 
• The major criteria for selection of the front-end head are cost, maintenance and inspection, hazard due to mixing of shell and tube fluids, and leakage to ambient and operating pressures. 
• The major criteria for selection of the rear-end head are the allowance for thermal stresses, a provision to remove the tube bundle for cleaning the shell side, prevention of mixing of tube and shell fluids, and sealing any leakage path for the shell fluid to ambient. 

Baffles. 

• Baffles may be classified as transverse and longitudinal types. 
• The purpose of longitudinal baffles is to control the overall flow direction of the shell fluid such that a desired overall flow arrangement of the two fluid streams is achieved.
• Single- and double-segmental baffles are used most frequently due to their ability to assist maximum heat transfer (due to a high-shell-side heat transfer coefficient) for a given pressure drop in a minimum amount of space. 
• Triple and no-tubes-in-window segmental baffles are used for low-pressure-drop applications. 
• The choice of baffle type, spacing, and cut is determined largely by flow rate, desired heat transfer rate, allowable pressure drop, tube support, and flow-induced vibrations. 

Tubesheets. 

• These are used to hold tubes at the ends. 
• A tubesheet is generally a round metal plate with holes drilled through for the desired tube pattern, holes for the tie rods (which are used to space and hold plate baffles), grooves for the gaskets, and bolt holes for flanging to the shell and channel. 
• To prevent leakage of the shell fluid at thetubesheet through a clearance between the tube hole and tube, the tube-to-tubesheet joints are made by many methods, such as expanding the tubes, rolling the tubes, hydraulic expansion of tubes, explosive welding of tubes, stuffing of the joints, or welding or brazing of tubes to the tubesheet. 
• The leak-free tube-to-tubesheet joint made by the conventional rolling process.

Double-Pipe Heat Exchangers

• This exchanger usually consists of two concentric pipes with the inner pipe plain or finned.
• One fluid flows in the inner pipe and the other fluid flows in the annulus between pipes in a counterflow direction for the ideal highest performance for the given surface area

• However, if the application requires an almost constant wall temperature, the fluids may flow in a parallel flow direction. 
• This is perhaps the simplest heat exchanger. 
• Flow distribution is no problem, and cleaning is done very easily by disassembly. 
• This configuration is also suitable where one or both of the fluids is at very high pressure, because containment in the small-diameter pipe or tubing is less costly than containment in a large-diameter shell. 
• Double-pipe exchangers are generally used for small-capacity applications where the total heat transfer surface area required is 50m2 (500 ft2) or less because it is expensive on a cost per unit surface area basis. 
• Stacks of double-pipe or multi tube heat exchangers are also used in some process applications with radial or longitudinal fins. 

Spiral Tube Heat Exchangers. 

• These consist of one or more spirally wound coils fitted in a shell. 
• Heat transfer rate associated with a spiral tube is higher than that for a straight tube.
• In addition, a considerable amount of surface can be accommodated in a given space by spiraling. 
• Thermal expansion is no problem, but cleaning is almost impossible

HEAT EXCHANGER

Heat exchanger

• Heat exchanger are thermal devices that transfer or exchange heat from one fluid stream to one or more others.
• Heat exchanger transfer heat by one of three ways

1. By recuperation, or recovery, of heat from hot stream to a cold stream
2. By regeneration, as the hot and cold stream alternatively flow through a matrix
3. By direct contact of one fluid stream with another

Classification of heat exchangers

• Heat exchanger are classified either by flow arrangement, by construction or by their degree of compactness

Classification by flow arrangement

• Heat exchanger can be classified according to flow arrangement as follows

Parallel flow heat exchanger

• In a parallel flow (also referred to as cocurrent or cocurrent parallel stream) exchanger, the fluid streams enter together at one end, flow parallel to each other in the same direction, and leave together at the other end

• In a parallel flow exchanger, a large temperature difference between inlet temperatures of hot and cold fluids exists at the inlet side, which may induce high thermal stresses in the exchanger wall at the inlet.

Counter flow heat exchanger

• In a counterflow or countercurrent exchanger, the two fluids flow parallel to each other but in opposite directions within the core.

• The counterflow arrangement is thermodynamically superior to any other flow arrangement.
• It is the most efficient flow arrangement, producing the highest temperature change in each fluid compared to any other two-fluid flow arrangements for a given overall thermal conductance (UA), fluid flow rates (actually, fluid heat capacity rates), and fluid inlet temperatures.

• The maximum temperature difference across the exchanger wall thickness (between the wall surfaces exposed on the hot and cold fluid sides) either at the hot- or cold-fluid end is the lowest, and produce minimum thermal stresses in the wall for an equivalent performance compared to any other flow arrangements.

Cross flow heat exchanger

• In this type of exchanger, the two fluids flow in directions normal to each other.
• In a crossflow arrangement, mixing of either fluid stream may or may not occur, depending on the design.

CLASSIFICATION OF HEAT EXCHANGER BY CONSTRUCTION

TYPES OF PUMP

Pump

• A pump is a device that moves fluids (liquids or gases), or sometimes slurries, by mechanical action.
• The basic purpose of pump is to transfer fluid or liquid or gases or slurries from a lower level to higher level.
• The pumps increase the mechanical energy of the liquid, increasing its velocity, pressure or elevation or all three
• Pump are widely used in variety of application
• Pump exist in a variety of shapes and sizes, depending on their intended function.
• When the flowing fluid is a gas, the pump is typically referred to as a compressor.
• There are two major classes of pump

1. Positive displacement pump
2. Centrifugal pump

Positive displacement pump

• It is the first major class of pump. 
• In this pump, a definite volume of liquid is trapped in chamber, which is alternately filled from the inlet and emptied at a higher pressure through the discharge.
• Positive displacement units apply pressure difference directly to the liquid by a reciprocating piston, or by rotating members which form chambers alternately filled by and emptied of the liquid. 
• Positive displacement pumps are a category of pumps designed to move fluid at a steady rate through a system. 
• These pumps are able to handle viscous fluids, which flow at lower speeds and create more resistance, more efficiently than kinetic (dynamic) pumps. 
• There are two sub classes of positive displacement pumps.

1. Reciprocating pumps
2. Rotary pumps

Reciprocating pumps

• In reciprocating pumps, the chamber is a stationary cylinder that contain a piston or plunger or diaphragm.
• They utilize a piston, plunger or diaphragm which draws fluid in (upstroke) and pushes it out (downstroke), using check valves to regulate and direct flow through the system.
• In a reciprocating pump, a volume of liquid is drawn into the cylinder through the suction valve on the intake stroke and is discharged under positive pressure through the outlet valves on the discharge stroke. 
• The discharge from a reciprocating pump is pulsating and changes only when the speed of the pump is changed. This is because the intake is always a constant volume. 
• Often an air chamber is connected on the discharge side of the pump to provide a more even flow by evening out the pressure surges. 
• Reciprocating pumps are often used for sludge and slurry.
• There are three example of reciprocating pumps

1. Piston pumps
2. Plunger pumps
3. Diaphragm pumps

Piston pumps

• In piston pump, liquid is drawn through an inlet check valve into the cylinder by the withdrawal of a piston and then is forced out through a discharge check valve on return stroke.
• Most piston pumps are double acting with liquid admitted alternately on each side of the piston so that one part of the cylinder is being filled while the other is being emptied.
• The piston may be motor driven through reducing gears , or a steam cylinder may be used to drive the piston rod directly
• The maximum discharge pressure for commercial piston pumps is about 50 atm. 

Plunger pumps

• For high pressure plunger pumps are used 
• A heavy walled cylinder of small diameter contains a close fitting reciprocating plunger, which is merely an extension of the piston rod
• At the time of stroke plunger fills nearly all space in the cylinder.
• Plunger pumps are single acting and usually are motor driven
• They can discharge against a pressure of 1500 atm or more.

Diagram pumps

• In a diagram pump, the reciprocating member is a flexible diaphragm of metal, plastic or rubber
• This eliminates need for packing or seals exposed to the liquid being pumped, a great advantage when handling toxic or corrosive liquid
• Diagram pumps handle small to moderate amount amounts of liquid, up to 100gal/min, and can develop pressure in excess of 100 atm 

Rotary pumps

• A wide variety of rotary positive displacement pumps are available.
• They bear such names as gear pumps, lobe pumps, screw pumps, cam pumps, and vane pumps.
• Rotary pumps do  not contain check valve.
• Rotary pumps operated best on clean, moderately viscous fluid, such as light lubricating oil.
• In rotary pumps discharge pressures up to 200 atm or more can be attained.