An Introduction Heat exchange's and types

An Introduction Heat exchange's and types 

INTRODUCTION
Heat exchangers transfer thermal energy from one medium to another. Industry typically uses “two-fluid” exchangers. Both fluids can be part of the process stream. If only one process stream fluid is involved, the other fluid is usually steam for heating, or water or air for cooling.

HEAT EXCHANGER TYPES

Heat exchangers permit exchange of energy from one fluid to another, usually without permitting
physical contact between the fluids.
The following configurations are commonly used in the power and process industries.
Types of Heat Exchangers by construction
1-Shell and Tube Heat Exchangers
Shell and tube heat exchangers normally consist of a bundle of tubes fastened into holes, drilled in
metal plates called tubesheets. The tubes may be rolled into grooves in the tubesheet, welded to the
tubesheet, or both to ensure against leakage. figure 1 as shown straight type 

FIG 1 

When possible, U-tubes are used, requiring only one tube sheet. The tube bundle is placed inside a large pipe called a shell, see Fig. 2. Heat is exchanged between a fluid flowing inside the tubes and a fluid flowing outside the tubes in the shell.

FIG 2 

The Tubular Exchanger Manufacturers Association (TEMA) provides a manual of standards for
construction of shell and tube heat exchangers,1 which contains designations for various types of
shell and tube heat exchanger configurations. The most common types are summarized below.
figure 3

FIG 3

2- Plate-Type Heat Exchangers
Composed of a series of corrugated or embossed plates clamped between a stationary and a movable
support plate, these exchangers were originally used in the food-processing industry. They have the
advantages of low fouling rates, easy cleaning, and generally high heat-transfer coefficients

FIG 4 

3- Spiral Plate Heat Exchangers
These exchangers are also becoming more widely used, despite limitations on maximum size and
maximum operating pressure. They are made by wrapping two parallel metal plates, separated by
spacers, into a spiral to form two concentric spiral passages
The most common uses are for difficult-to-handle fluids with no phase change

FIG 5

4-Air-Cooled Heat Exchangers
It is sometimes economical to condense or cool hot streams inside tubes by blowing air across the
tubes rather than using water or other cooling liquid. They usually consist of a horizontal bank of
finned tubes with a fan at the bottom (forced draft) or top (induced draft) of the bank

FIG 6

5- Compact Heat Exchangers
The term compact heat exchanger normally refers to one of the many types of plate fin exchangers
used extensively in the aerospace and cryogenics industries. The fluids flow alternately between
parallel plates separated by corrugated metal strips that act as fins and that may be perforated or
interrupted to increase turbulence. Although relatively expensive to construct, these units pack a very
large amount of heat-transfer surface into a small volume, and are therefore used when exchanger
volume or weight must be minimized.

FIG 7 

Types of Heat Exchangers by  TRANSFER PROCESSES

Heat exchangers are classified according to transfer processes into indirect- and direct contact
types

1- The direct contact type heat exchangers transfer heat from one fluid to another by their physical contact in which heat and mass is transferred as the fluids mix. These types are used to transfer the unused energy into the environment. in direct-contact heat exchangers,

(1) very high heat transfer rates are achievable, 

(2) the exchanger construction is relatively inexpensive,

(3) the fouling problem is generally nonexistent, due to the absence of a heat transfer surface (wall) between the two fluids.

These exchangers may be further classified as follows.

FIG 8

1-1 Immiscible Fluid Exchangers. In this type, two immiscible fluid streams are brought into direct contact. These fluids may be single-phase fluids, or they may involve condensation or vaporization. Condensation of organic vapors and oil vapors with water or air are typical examples.

FIG 9

1-2 Gas–Liquid Exchangers In this type, one fluid is a gas (more commonly, air) and the other a low-pressure liquid (more commonly, water) and are readily separable after the energy exchange. In either cooling of liquid (water) or humidification of gas (air) applications, liquid partially evaporates and the vapor is carried away with the gas. In these exchangers, more than 90% of the energy transfer is by virtue of mass transfer (due to the evaporation of the liquid), and convective heat transfer is a minor mechanism. A ‘‘wet’’ (water) cooling tower with forced- or natural-draft airflow is the most common application. Other applications are the air-conditioning spray chamber, spray drier, spray tower, and spray pond.

FIG 10 

1-3 Liquid–Vapor Exchangers. In this type, typically steam is partially or fully condensed using cooling water, or water is heated with waste steam through direct contact in the exchanger. Noncondensables and residual steam and hot water are the outlet streams. Common examples are desuperheaters and open feedwater heaters (also known as deaeraters) in power plants.

FIG 11

2- Indirect-Contact Heat Exchangers

In an indirect-contact heat exchanger, the fluid streams remain separate and the heat transfers continuously through an impervious dividing wall or into and out of a wall in a transient manner.

2-1 Direct-Transfer Type Exchangers. In this type, heat transfers continuously

from the hot fluid to the cold fluid through a dividing wall. Although a simultaneous flow of two (or more) fluids is required in the exchanger, there is no direct mixing of the two (or more) fluids because each fluid flows in separate fluid passages.

FIG 12

2-2 Storage Type Exchangers. In a storage type exchanger, both fluids flow alternatively through the same flow passages, and hence heat transfer is intermittent.

FIG 13

2-3 Fluidized-Bed Heat Exchangers. In a fluidized-bed heat exchanger, one side of a two-fluid exchanger is immersed in a bed of finely divided solid material, such as a tube bundle immersed in a bed of sand or coal particles

FIG 14

Types of Heat Exchangers by flow 

1- crossflow heat exchanger

In this type of Heat Exchanger. The both fluids flow perpendicular to each other. one fluid flow in the tubes and other flows across the tubes at 90 angle. This type of heat exchanger is used where one fluid changes its state. Steam condenser is an example of this heat exchanger in which the steam exiting from turbine, enters in shell side and cold water is in tubes that absorb the heat of stem and converts it into the water

FIG 15

2-  parallel flow type heat exchangers, the tube side and shell side fluids flow in

parallel and in the same direction. These types of heat exchangers are inefficient and not commonly used because the amount of surface area for a given heat load must be larger than for other types.

FIG 16

3- In counterflow heat exchangers, the tube side and shell side fluids flow in parallel, but in opposite directions. Generally, this is the most desirable arrangement, because a higher average temperature difference between the two fluids is maintained. For equally sized heat exchangers (those with the same amount of surface area), counterflow heat exchangers provide more heat transfer capabilities. The shell side flow in counterflow heat exchangers may also be in a crossflow direction because of the shell side baffles.

FIG 17

Single pass Heat Exchanger

Heat Exchanger in which the fluid passes to each other only once is called single pass. Fluid enters in the tubes & shell once and exits from other end.

FIG 18

Multi pass Heat Exchanger

In multi pass Heat Exchangers, the fluids are directed to reverse within the Tubes or Shell. One method is to bend the tubes in U-shape to provide the flow back and forth across the length of heat exchanger.

Second method is to inserts the baffles in shell side of heat exchanger. These baffles are direct the shell side fluid to allow back and forth across the tubes and to achieve the effect of multi-pass.

FIG 19 

Working Principle

For heat transfer there must be one fluid temperature should be differ from another one. Heat is always transferred from hotter fluid to cooler fluid.

FIG 20 

Heat Transfer Calculations

The heat exchanger calculations covered in this Module are for heat exchangers that have single phase fluids (liquid) on both the shell and tube sides.

The basic heat transfer relation that describes the heat exchanger thermal duty, or heat transfer rate Q

Q =U A 𝜟Tlm

where

Q       is rate of heat transfer BTU / hr

U       is the overall heat transfer coefficient in BTU / hr-ft² - °F   figure 21

A        is the effective heat exchanger heat transfer surface area in ft²

ΔTlm   is the logarithmic mean temperature difference (LMTD) in °F

FIG 21