Introduction (What is Plate Heat Exchanger)
A plate heat exchanger is an 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 due to the fact that they need well-sealed gaskets to prevent the fluids from escaping, although modern manufacturing processes have made them feasible.

The concept behind a heat exchanger is the use of pipes or other containment vessels to heat or cool one fluid by transferring heat between it and another fluid. In most cases, the exchanger consists of a coiled pipe containing one fluid that passes through a chamber containing another fluid. The walls of the pipe are usually made of metal, or another substance with a high thermal conductivity, to facilitate the interchange, whereas the outer casing of the larger chamber is made of a plastic or coated with thermal insulation, to discourage heat from escaping from the exchanger.

The plate heat exchanger (PHE) was invented by Dr Richard Seligman in 1923 and revolutionised methods of indirect heating and cooling of 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 gives 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.

Another advantage of the heat exchanger is that it is easily dismantled for inspection and cleaning. The plates are also easily replaceable due the fact that plates can be removed and replaced individually. The main weakness of the plate and frame heat exchanger is the necessity for the long gaskets which holds the plates together. Although these gaskets are seen as a weakness towards this type of heat exchanger, it has been successfully run at high temperatures and pressures.

GASKETED PLATE HEAT EXCHANGERS

	The plate heat exchanger is the most widely used configuration in geothermal systems of recent design. A number of characteristics particularly attractive to geothermal applications are responsible for this. Among these are: 1. 1. Superior thermal performance. 2. 2. Availability of a wide variety of corrosion resistant alloys. 3. 3. Ease of maintenance. 4. 4. Expandability and multiplex capability. 5. 5. Compact design.
	General Capabilities In comparison to shell and tube units, plate and frame heat exchangers are a relatively low pressure/low temperature device. Current maximum design ratings for most manufacturers are: temperature, 400°F, and 300 psig. Above these values, an alternate type of heat exchanger would have to be selected. The actual limitations for a particular heat exchanger are a function of the materials selected for the gaskets and plates; these will be discussed later. Individual plate area varies from about 0.3 to 21.5 ft 2 with a maximum heat transfer area for a single heat exchanger currently in the range of 13,000 ft 2 . The minimum plate size does place a lower limit on applications of plate heat exchangers. For geothermal applications, this limit generally affects selections for loads such as residential and small commercial space heating and domestic hot water. The largest units are capable of handling flow rates of 1636593 liter per hour (6000 gallon per minute gpm) and the smallest units serviceable down to flows of approximately 5 gpm (1363.828 liter per hour ). Connection sizes are available from 3/4 to 14 in. to accommodate these flows.

Materials
Materials selection for plate heat exchangers focuses primarily upon the plates and gaskets. Since these items significantly effect first cost and equipment life, this procedure should receive special attention.

 

Plates One of the features which makes plate‐type heat exchangers so attractive for geothermal applications is the availability of a wide variety of corrosion‐resistant alloys for construction of the heat transfer surfaces. Most manufacturers will quote either 304 or 316 stainless steel a the basic material. or direct use geothermal applications, the choice of materials is generally a selection between 304 stainless, 316 stainless, and titanium. The selection between 304 and 316 is most often based upon a combination of temperature and chloride content of the geothermal fluid. Should oxygen be present in as little as parts per billion (ppb) concentrations, the rates of localized corrosion would be significantly increased (Ellis and Conover, 1981). Should the system for which the heat exchanger is being selected offer the potential for oxygen entering the circuit, a more conservative approach to materials selection is recommended. Titanium is only rarely required for direct use applications. In applications where the temperature/chloride requirements are in excess of the capabilities of 316 stainless steel, titanium generally offers the least cost alternative. The first cost premium for titanium over stainless steel plates is approximately 50%.

	Gaskets As with plate materials, a variety of gasket materials are available. Among the most common are those shown in Table 1.
Table 1. Plate Heat Exchanger Gasket Materials Material Common Name Temperature Limit (°F) Styrene-Butadiene Buna-S 185 Neoprene Neoprene 250 Acrylonitrile- Butadiene Buna-N 275 Ethylene/Propylene EPDM 300 Fluorocarbon Viton 300 Resin-Cured Butyl Resin-Cured Butyl 300 Compressed Asbestos Compressed Asbestos 500
Testing by Radian Corporation has revealed that Viton shows the best performance in geothermal applications, followed by Buna-N. Test results revealed that neoprene developed an extreme compression set and Buna-S and natural rubber also performed poorly (Ellis and Conover, 1981). Although Viton demonstrates the best performance, its high cost generally eliminates it from consideration unless its specific characteristics are required. Buna-N, generally the basic material quoted by most manufacturers, and the slightly more expensive EPDM material are generally acceptable for geothermal applications. Turbinedar company can provide a large range of these gaskets and plates.

Performance

Superior thermal performance is the hallmark of plate heat exchangers. Compared to shell-and-tube units, plate heat exchangers offer overall heat transfer coefficients 3 to 4 times higher. These values, typically 800 to 1200 Btu/-hrúft2 °F (clean), result in very compact equipment. This high performance also allows the specification of very small approach temperature (as low as 2 to 5°F) which is sometimes useful in geothermal applications. This high thermal performance does come at the expense of a somewhat higher pressure drop. Selection of a plate heat exchanger is a trade-off between U-value (which influences surface area and hence, capital cost) and pressure drop (which influences pump head and hence, operating cost). Increasing U-value comes at the expense of increasing pressure drop.

Fouling considerations for plate heat exchangers are considered differently than for shell-and-tube equipment. There are a variety of reasons for this; but, the most important is the ease with which plate heat exchangers can be disassembled and cleaned. As a result, the units need not be over-designed to operate in a fouled condition. Beyond this, the nature of plate heat exchanger equipment tends to reduce fouling due to:

 

:: High turbulence,

Shell and tube heat exchanger

	A shell and tube heat exchanger is a class of heat exchanger designs.[1][2] It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher-pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle, and may be composed by several types of tubes: plain, longitudinally finned, etc.
Selection of tube material
	To be able to transfer heat well, the tube material should have good thermal conductivity. Because heat is transferred from a hot to a cold side through the tubes, there is a temperature difference through the width of the tubes. Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresses occur during operation. This is in addition to any stress from high pressures from the fluids themselves. The tube material also should be compatible with both the shell and tube side fluids for long periods under the operating conditions (temperatures, pressures, pH, etc.) to minimize deterioration such as corrosion. All of these requirements call for careful selection of strong, thermally-conductive, corrosion-resistant, high quality tube materials, typically metals. Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid cross-contamination and possibly loss of pressure.
Design Considerations for Shell and Tube Heat Exchangers
1.  Is there a phase change involved in my system?     A quick look at the boiling points compared with the entrance and exit temperatures will help you answer this question. 2.  How many "zones" are involved in my system?     "Zones" can best be defined as regimes of phase changes where the overall heat transfer coefficient (Uo) will vary.  Using T-Q (Temperature-Heat) diagrams are the best way to pinpoint zones.  The system is defined as co-current or countercurrent and the diagram is constructed.  The diagram on the left illustrates the use of T-Q diagrams. These diagrams should accompany your basic (input-output) diagram of the heat exchanger.   Chemical #1 enters the shell at 2000C as a superheated vapor. In Zone 1, it releases heat to the tubeside chemical (Chemical #2).  Zone 1 ends just a Chemical #1 begins to condense.  The tube side (Chemical #2) enters as a liquid or gas and does not change phase throughout the exchanger.  Chemical #1 leaves Zone 1 and enters Zone 2 at its boiling temperature, Tb1.  T* marks the temperature of Chemical #2 when Chemical #1 begins to condense.  In Zone 2, Chemical #1 condenses to completion while Chemical #2 continues to increase in temperature.  The temperature of Chemical #2 when Chemical #1 is fully condensed is denoted at T**.  Finally, in Zone 3, both chemicals are liquids.  Chemical #1 is simply liberating heat to Chemical #2 as it becomes a sub cooled liquid and exits the shell at 100 0C.     Defining zones is one of the most important aspects of heat exchanger design.  It is also important to remember that if your process simulator does not support zoned analysis (such as Chemcad III), you should model each zone with a separate heat exchanger.  Thus, the previous illustration would require 3 heat exchangers in the simulation.  BUT, do not draw 3 exchangers on your PFD (Process Flow Diagram).  This is all happening in one exchanger.
3.  What are the flow rates and operating pressures involved in my system?     This information is critical in establishing the mass and energy balance around the exchanger.  Operating pressures are particularly important for gases as their physical properties vary greatly with pressure.   4.  What are the physical properties of the streams involved?     If you're using a process simulator, obtaining the physical properties of your streams should be just a click of the mouse away.  However, if performing the calculation by hand, you may have to do some estimating as the streams may not be of pure substances.  Also, you should get the physical properties for each zone separately to ensure accuracy, but in some cases it is acceptable to use an average value.  This would be true of Chemical #2 in the tubes since it is not changing phase or undergoing a truly significant temperature change (over 1000C).  Physical properties that you will want to collect for each phase of each stream will include:  heat capacity, viscosity, thermal conductivity, density, and latent heat (for phase changes).  These are in addition to the boiling points of the streams at their respective pressures. 5.  What are the allowable pressure drops and velocities in the exchanger?     Pressure drops are very important in exchanger design (especially for gases).  As the pressure drops, so does viscosity and the fluids ability to transfer heat.  Therefore, the pressure drop and velocities must be limited.  The velocity is directly proportional to the heat transfer coefficient which is motivation to keep it high, while erosion and material limits are motivation to keep the velocity low.  Typical liquid velocities are 1-3 m/s (3-10 ft/s).  Typical gas velocities are 15-30 m/s (50-100 ft/s).  Typical pressure drops are 30-60 kPa (5-8 psi) on the tubeside and 20-30 kPa (3-5 psi) on the shellside.   6.  What is the heat duty of the system?     This can be answered by a simple energy balance from one of the streams.   7.  What is the estimated area of the exchanger?     Unfortunately, this is where the real fun begins in heat exchanger design!  You'll need to find estimates for the heat transfer coefficients that you'll be dealing with.  These can be found in most textbooks dedicated to the subject or in Perry's Chemical Engineers' Handbook.  Once you've estimated the overall heat transfer coefficient, use the equation Q=UoADTlm to get your preliminary area estimate.  Remember to use the above equation to get an area for each zone, then add them together. 8.  What geometric configuration is right for my exchanger?     Now that you have an area estimate, it's time to find a geometry that meets your needs.  Once you've selected a shell diameter, tubesheet layout, baffle and tube spacing, etc., it's time to check your velocity and pressure drop requirements to see if they're being met.  Experienced designers will usually combine these steps and actually obtain a tube size that meets the velocity and pressure drop requirements and then proceed.  Some guidelines may be as follows:  3/4 in. and 1.0 in. diameter tubes are the most popular and smaller sizes should only be used for exchangers needing less than 30 m2 of area.  If your pressure drop requirements are low, avoid using four or more tube passes as this will drastically increase your pressure drop.  Once you have a geometry selected that meets all of your needs, it's on to step #9.   9.  Now that I have a geometry in mind, what is the actual overall heat transfer coefficient?     This is where you'll spend much of your time in designing a heat exchanger.  Although many textbooks show Nu=0.027(NRE)0.8(NPR)0.33 as the "fundamental equation for turbulent flow heat transfer", what they sometimes fail to tell you is that the exponents can vary widely for different situations.   For example, condensation in the shell has different exponents than condensation in the tubes.  Use this fundamental equation if you must, but you should consult a good resource for accurate equations.  I highly recommend the following:  Handbook of Chemical Engineering Calculations, 2nd Ed., by Nicholas P. Chopey from McGraw-Hill publishers (ISBN 0070110212).  Also, don't forget to include the transfer coefficient across the tube wall and the fouling coefficient.  These can be very significant!

10.  What is the actual area of the exchanger using the 'actual' heat transfer coefficient?
    If you recall, you used estimated heat transfer coefficients to get an initial area.  Now it's time to recalculate the area.

THE LOOP
   Now you're on your way, pick a new geometry corresponding to your new ("actual") area, check the velocity and pressure drop, calculate the overall heat transfer coefficient again.  How does it compare with the previously calculated value?  If it is not within 5-10%, recalculate the process over and over (using your new value for Uo) until it does!  Sounds like alot of work.  Add in the fact that some of the individual heat transfer coefficients require iterative solutions and it's not hard to see why people usually use a complex spreadsheet or a program to do this.  You can save some time by using estimates that you've undoubtedly seen, however you must realize that each time you estimate, you're losing accuracy. 
    Remember two main items:   
1.  ZONED ANALYSIS
2.   ACCURACY OF INITIAL OVERALL HEAT TRANSFER COEFFICIENT
The zoned analysis is the key to starting the process correctly.  The accuracy of the initial overall heat transfer coefficient will in part determine how many time you will be going through the calculation.

Other Considerations:

• Materials of Construction

• Ease of Maintenance

• Cost of Exchanger

• Overall Heat Integration

For Designing the various types of heat exchangers  you can trust Turbinedar  engineers and our huge background in this field.