SHREE DEVI INSTITUTE OF TECHNOLOGY

Kenjar, Mangalore

(Affiliated to VTU, Belgaum, Approved by AICTE, New Delhi Recognized by Govt.of Karnataka)

SeminarOn

PLATE HEAT EXCHANGER

By

Bhayani Dhaval Satishbhai4SH06ME004

DEPARTMENT OF MECHANICAL ENGINEERING

CONTENTS

• Abstract

• Introduction

• Types of Plate Heat Exchanger

• Design of Plate Heat Exchanger

• Working of Plate Heat Exchanger

• Calculation of Heat Transfer

• Advantages

• Limitations of Operation

• Applications

• References

ABSTRACT

Decreasing size and increasing heat load is the typical feature of the modern day heat exchanger industry. While traditional areas of compact heat exchanger applications such as automotive, aerospace, etc continue to demand for even higher heat transfer with further shrinking of available space, there is large number of new areas coming up in the usage of compact heat exchangers. These include areas such as cooling of electronic equipment, cooling of LASER and related technologies, cooling technology for fuel cells etc. A number of traditional industries have also turned towards compact heat exchangers including chemical process industry, power industry, and food & beverages industry. The usage of compact heat exchangers for multi-phase flow is another area in which a lot of attention has been paid in the recent years.

INTRODUCTION

Heat exchangers are important, and used frequently in the processing, heat and power, air-conditioning and refrigeration, heat recovery, transportation and manufacturing industries. Such equipment is also important in electronics cooling and for environmental issues like thermal pollution, waste disposal and sustainable development. Various types of heat exchangers exists,However, most advanced type of Heat Exchanger is Plate heat Exchanger and has already overcome the initial Shell and tube type of Heat Exchanger.

In Industrial level heat exchangers have played a key role in improving the efficiency of the fuel used in the different machineries and also in reduction of thermal pollution. A plate type heat exchanger is a type of heat exchanger that uses metal plates to transfer heat between two fluids. This has a major advantage over a conventional heat exchanger in that the fluids are exposed to a much larger surface area because the fluids spread out over the plates. This facilitates the transfer of heat, and greatly increases the speed of the temperature change. It is not as common to see plate heat exchangers because they need well-sealed gaskets to prevent the fluids from escaping, although modern manufacturing processes have made them feasible.

Most geothermal fluids, because of their elevated temperature, contain a variety of dissolved chemicals. These chemicals are frequently corrosive toward standard materialsof construction. As a result, it is advisable in most cases to isolate the geothermal fluid from the process to which heat is being transferred.

The task of heat transfer from the geothermal fluid to a closed process loop is most often handled by a plate heat exchanger. The

two most common types used in geothermal applications are: bolted and brazed. For smaller systems, in geothermal resource areas of aspecific character, downhole heat exchangers (DHEs) provide a unique means of heat extraction. These devices eliminate the requirement for physical removal of fluid from the well. For this reason, DHE-based systems avoid entirely the environmental and practical problems associated with fluid disposal.

Shell and tube heat exchangers play only a minor role in low-temperature, direct-use systems. These units have been in common use in industrial applications for many years and, as a result, are well understood. For these reasons, shell and tube heat exchangers will not be covered in this chapter.

Pharmaceutical, biotechnology and specialty chemical companies are challenging the heat transfer community to provide solutions that enable critical processes to operate at extremely cold temperatures.In the past,it was adequate to operate at temperatures as low as -80˚F (-62.2˚C).Now industry continues to push for colder temperatures. Low-temperature heat transfer fluid manufacturers and heat transfer companies are being asked to provide systems that can run reliably at -148˚F to -184˚F (-100˚C to -120˚C).

In many heat exchanger applications, it is desirable to achieve a high, or a very high, design pressure, i.e. to be able to permit a high or a very high pressure of one or both of the media flowing through the plate interspaces. It is also desirable to be able to permit such high pressures in plate heat exchangers of the kind defined above having permanently joined heat exchanger plates, e.g. through brazing. Such high design pressures are difficult to achieve without the provision of external strengthening components.

The cost of heat exchange surfaces is a major cost factor when the temperature differences are not large. One way of meeting this problem is the plate type heat exchanger.

CONSTRUCTION OF PHE

A plate heat exchanger consists of a series of thin, corrugated plates which are mentioned above. These plates are gasketed, welded or brazed together depending on the application of the heat exchanger. The plates are compressed together in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids.

As compared to shell and tube heat exchangers, the temperature approach in a plate heat exchangers may be as low as 1 °C whereas shell and tube heat exchangers require an approach of 5 °C or more. For the same amount of heat exchanged, the size of the plate heat exchanger is smaller, because of the large heat transfer area afforded by the plates (the large area through which heat can travel). Expansion and reduction of the heat transfer area is possible in a plate heat exchanger.

A plate heat exchanger consists of several gasketed metal plates that are clamped between a stationary head and follower plate by tie bolts. The plates are rectangular with circular ports at each corner in which the fluids may enter and exit. A specially designed corrugated surface is stamped onto the thin walled plates. Numerous plates are arranged with gaskets that cause the two fluids to be directed through alternating spaces between the plates.

A single unit can use up to 700 plates giving an overall surface area of 2500m2.

The plate heat exchanger (PHE) is a specialized design well suited to transferring heat between medium- and low-pressure fluids. Welded, semi-welded and gasketted heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required. In place of a pipe passing through a

chamber, there are instead two alternating chambers, usually thin in depth, separated at their largest surface by a corrugated metal plate. The plates used in a plate and frame heat exchanger are obtained by one piece pressing of metal plates. Stainless steel is a commonly used metal for the plates because of its ability to withstand high temperatures, its strength, and its corrosion resistance. The plates are often spaced by rubber sealing gaskets which are cemented into a section around the edge of the plates. The plates are pressed to form troughs at right angles to the direction of flow of the liquid which runs through the channels in the heat exchanger.

The plates produce an extremely large surface area, which allows for the fastest possible transfer. Making each chamber thin ensures that the majority of the volume of the liquid contacts the plate, again aiding exchange. The troughs also create and maintain a turbulent flow in the liquid to maximize heat transfer in the exchanger. A high degree of turbulence can be obtained at low flow rates and high heat transfer coefficient can then be achieved.

A plate heat exchanger consists of a series of thin, corrugated plates which are mentioned above. These plates are gasketed, welded or brazed together depending on the application of the heat exchanger. The plates are compressed together in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids. As compared to shell and tube heat exchangers, the temperature approach in a plate heat exchangers may be as low as 1 °C whereas shell and tube heat exchangers require an approach of 5 °C or more. For the same amount of heat exchanged, the size of the plate heat exchanger is smaller, because of the large heat transfer area afforded by the plates (the large area through which heat can travel).

CONSTRUCTIONAL DETAILS

The nature of fluid flow through the plate heat exchanger. The primary and secondary fluids flow in opposite directions on either side of the plates. Water flow and circuiting are controlled by the placement of the plate gaskets. By varying the position of the gasket, water can be channeled over a plate or past it. Gaskets are installed in such a way that a gasket failure cannot result in a mixing of the fluids. In addition, the outer circumference of all gaskets is exposed to the atmosphere.

Types of Plates

The most commonly used plate metal is stainless steel, although other materials such as Hastelloy, Incoloy, titanium, nickel and tantalum can be used as well. In order to achieve market success, each plate pattern must undergo extensive research, as well as technical and commercial reasoning.The plates are mass-produced in many different sizes,shapes and corrugation patterns. The two main corrugation patterns used are the intermating or ìwashboardî type and the chevron or ìherringboneî type. Herringbone Type

The herringbone type is most commonly used and is shown in Figure. The corrugations are pressed to the depth of the plate spacing. This means that the two adjacent plates will have numerous contact points and will produce a more turbulent flow. Also, with the inclusion of contact points the structure yields a higher strength. This enables it to withstand higher pressures.

Washboard Type

The washboard type operates only at lower pressures and requires a heavier plate. Transverse corrugations are pressed to a depth larger than the plate spacing. As a result, a means of maintaining the plate spacing must be established.

This is accomplished by dimples that are pressed on to adjacent troughs and crests. These dimples contact one another to keep the desired spacing.

WORKING

The plate heat exchanger (PHE) is a specialized design well suited to transferring heat between medium- and low-pressure fluids. Welded, semi-welded and brazed heat exchangers are used for heat exchange between high-pressure fluids or where a more compact product is required. In place of a pipe passing through a chamber, there are instead two alternating chambers, usually thin in depth, separated at their largest surface by a corrugated metal plate. The plates used in a plate and frame heat exchanger are obtained by one piece pressing of metal plates. Stainless steel is a commonly used metal for the plates because of its ability to withstand high temperatures, its strength, and its corrosion resistance. The plates are often spaced by rubber sealing gaskets which are cemented into a section around the edge of the plates. The plates are pressed to form troughs at right angles to the direction of flow of the liquid which

runs through the channels in the heat exchanger. These troughs are arranged so that they interlink with the other plates which forms the channel with gaps of 1.3–1.5 mm between the plates.

The plates produce an extremely large surface area, which allows for the fastest possible transfer. Making each chamber thin ensures that the majority of the volume of the liquid contacts the plate, again aiding exchange. The troughs also create and maintain a turbulent flow in the liquid to maximize heat transfer in the exchanger. A high degree of turbulence can be obtained at low flow rates and high heat transfer coefficient can then be achieved.

A plate heat exchanger consists of a series of thin, corrugated plates which are mentioned above. These plates are gasketed, welded or brazed together depending on the application of the heat exchanger. The plates are compressed together in a rigid frame to form an arrangement of parallel flow channels with alternating hot and cold fluids.

As compared to shell and tube heat exchangers, the temperature approach in a plate heat exchangers may be as low as 1 °C whereas shell and tube heat exchangers require an approach of 5 °C or more. For the same amount of heat exchanged, the size of the plate heat exchanger is smaller, because of the large heat transfer area afforded by the plates (the large area through which heat can travel). Expansion and reduction of the heat transfer area is possible in a plate heat exchanger.

CALCULATIONS

The performance of heat exchangers operating under forced flow conditions is defined by the amount of heat transferred between the two fluid streams and is characterized by the UA value or the dimensionless factors: the effectiveness,ε, or number of transfer units (NTU’s), and the capacity ratio,Cr,

ADVANTAGES

Advantages that exist with the use of plate heat exchangers are as follows:

Compactness: The units in a plate heat exchanger occupy less floor space and floor loading by having a large surface area that is formed from a small volume. This in turn produces a high overall heat transfer coefficient due to the heat transfer associated with the narrow passages and corrugated surfaces.

Flexibility: Changes can be made to heat exchanger performance by utilizing a wide range of fluids and conditions that can be modified to adapt to the various design specifications. These specifications can be matched with different plate corrugations.

Low Fabrication Costs: Welded plates are relatively more expensive than pressed plates. Plate heat exchangers are made from pressed plates, which allow greater resistance to corrosion and chemical reactions.

Ease of Cleaning: The heat exchanger can be easily dismantled for inspection and cleaning (especially in food processing) and the plates are also easily replaceable as they can be removed and replaced individually.

Temperature Control: The plate heat exchanger can operate with relatively small temperature differences. This is an advantage when high temperatures must be avoided. Local overheating and possibility of stagnant zones can also be reduced by the form of the flow passage

Disadvantages

A flat plat heat exchanger also has some disadvantages in comparison with other types of heat exchangers as follows:

1. Potential for leakage - Although plate and frame heat exchangers are designed to allow the plates and the gaskets between them to be firmly clamped together, there is still a greater potential for leakage than with either shell and tube or spiral heat exchangers.

2. Higher pressure drop - The narrow passageways for fluid flow, which lead to a high overall heat transfer coefficient, also lead to a higher pressure drop, and thus a higher cost for pumping, than shell and tube heat exchangers.

3. Not good for large fluid temperature differences - A flat plate heat exchanger does not work as well as a shell and tube heat exchanger for cases where there is a large temperature difference between the two fluids.

4. Doesn't work well with very high fluid temperatures - The gaskets may impose temperature limitations for plate and frame heat exchangers.

APPLICATIONS

Plate heat exchangers were introduced in1923 for milk pasteurIzation applications and now find major applications in liquid–liquid (viscosities upto10 Pas ) heat transfer duties.They are most common in the dairy, juice, beverage, Alcoholic drink, general food processing, and pharmaceutical industries, where their ease of cleaning and the thermal control required for sterilization/pasteurizationmake the mideal. They are also used in the synthetic rubber industry, papermills, and in the process heaters, coolers, and closed-circuit cooling systems of large petro chemical and powerplants. Here heat rejection to sea water or brackish water is common in many applications, and titanium plates are then used.

Plate heat exchangers are not well suited for lower-density gas-to-gasapplications.They are used for condensation or evaporation of non-low-vapordensities. Lower vapor Densities limit evaporation to lowe routlet vapor fractions. Specially designed plates are

Now available for condensing as well as evaporation of high-density vapors such as ammonia, propylene, and other common refrigerants, as well as for combine devaporation/condensation duties, also at fairly low vapordensities.

References:

1. www.Alfa-laval.com 2. http://www.tranter.com 3. http://en.wikipedia.org/wiki/Plate_heat_exchanger 4. http://books.google.co.in/books?

id=P3gTR8YHLHgC&pg=PA10&lpg=PA10&dq=classification+of+plate+heat+exchanger&source=bl&ots=5bJSFagJ6m&sig=ilpzDkTciV19yH4iNDKP7D_-eFE&hl=en&ei=7C9jTdPcGYaGrAegpIHVAg&sa=X&oi=book_result&ct=result#v=onepage&q=classification%20of%20plate%20heat%20exchanger&f=false

5. Plate heat exchangers: design, applications and performance   By L. Wang, Bengt Sundén, R. M. Manglik

Patent Referred:

Vladimir L. Goldstein. Corrugated Plate Heat Exchanger. CA 1315558, filed Aug. 11 1987, issued Apr. 6, 1993.