Fresh water heat exchanger

Published:

1.0 Abstract

1.1 Objective

The objective of this project was to investigate what would be the best choice of materials to construct a Fresh Water Heat Exchanger. The exact question is outlined below:

Materials for a fresh-water heat exchanger

Heat exchangers, typically, consist of a set of tubes through which one fluid is pumped, immersed in a chamber through which the other fluid flows; heat passes from one fluid to the other. The material of the tubing must conduct heat well, have a maximum operating temperature above the operating temperature of the device, not corrode in the fluid, and - since the tubes have to be bent - have adequate ductility.

Material Choice. Typical requirements:

  • Maximum service temperature> 150 C (423 K)
  • Elongation> 20%
  • Corrosion resistance in fresh water: very good
  • As large a thermal conductivity as possible.

1.2 Method

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Research was carried out into the design of heat exchangers and the types of fresh water heat exchangers currently available using the internet as the main tool of choice. Various documents that were appropriate to the design of heat exchangers were selected and studied.

For the purpose of this report we have assumed that Fresh Water will be flowing on both the shell and tube sides of the exchanger.

2.0 Executive Summary

A heat exchanger is a piece of equipment designed so that heat is removed from a hotter stream and transferred to a cooler stream causing the latter to heat up.

There are various different types of heat exchangers available such as:

  • Shell and tube heat exchanger
  • Plate heat exchanger
  • Adiabatic wheel heat exchanger
  • Plate fin heat exchanger
  • Fluid heat exchangers
  • Waste heat recovery units
  • Dynamic scraped surface heat exchanger
  • Phase-change heat exchangers
  • The shell and tube heat exchanger is the most common type of heat exchanger used in the process industry. It consists of a bundle of tubes which are contained within very large diameter pipe called a shell. The two streams are separated by having one fluid flow inside the tube and the other flow outside. Heat is transferred across the tubes from the hot stream to the cold stream.

    Research into the area of Fresh Water Shell and Tube Heat Exchangers highlighted the main area of use to be in marine heating applications. There are 2 main uses for the Fresh Water Heat Exchangers:

  • To integrate the engine's fresh water cooling system with the hot water heating system.
  • To heat domestic water for use in the galley and shower.
  • The fresh water heat exchanger system is used to heat the interior of the boat and heat the domestic water by using the engines waste heat.

    A sample set of heat exchanger design calculations have been outlined in Section 8 of this report, along with the full set of steps used to determine the materials to be used to construct the heat exchanger, using the CES EduPack 2009 software.

    It is our recommendation, after careful design and analysis, that the heat exchanger be constructed as follows:

  • Shell: Stainless Steel
  • Tube Sheets: Stainless Steel
  • Tubes: Copper
  • Baffles: Stainless Steel

Reasoning for these material selections are outlined in Section 10.

3.0 Declaration

Design of a Fresh Water Heat Exchanger

6.0 Introduction

A heat exchanger is a piece of equipment designed so that heat is removed from a hotter stream and transferred to a cooler stream causing the latter to heat up.

There are various different types of heat exchangers available such as:

  • Shell and tube heat exchanger
  • Plate heat exchanger
  • Adiabatic wheel heat exchanger
  • Plate fin heat exchanger
  • Fluid heat exchangers
  • Waste heat recovery units
  • Dynamic scraped surface heat exchanger
  • Phase-change heat exchangers

The shell and tube heat exchanger is the most common type of heat exchanger used in the process industry. It consists of a bundle of tubes which are contained within very large diameter pipe called a shell. The two streams are separated by having one fluid flow inside the tube and the other flow outside. Heat is transferred across the tubes from the hot stream to the cold stream.

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Research into the area of Fresh Water Shell and Tube Heat Exchangers highlighted the main area of use to be in marine heating applications. There are 2 main uses for the Fresh Water Heat Exchangers:

  • To integrate the engine's fresh water cooling system with the hot water heating system.
  • To heat domestic water for use in the galley and shower.

The fresh water heat exchanger system is used to heat the interior of the boat and heat the domestic water by using the engines waste heat.

This report outlines the steps involved in the design of a fresh water shell and tube heat exchanger. The design carried out in this report was based around fresh water flowing on both the shell and tube sides of the exchanger (Liquid on the shell side, steam on the tube side).

Modifications to both the design calculations and the material selection stage limits, within the CES Software, will be required if alternative fluids are selected.

7.0 What is a Heat Exchanger?

A heat exchanger is a piece of equipment designed so that heat is removed from a hotter stream and transferred to a cooler stream causing the latter to heat up.

There are various different types of heat exchangers available such as:

  • Shell and tube heat exchanger
  • Plate heat exchanger
  • Adiabatic wheel heat exchanger
  • Plate fin heat exchanger
  • Fluid heat exchangers
  • Waste heat recovery units
  • Dynamic scraped surface heat exchanger
  • Phase-change heat exchangers

For the purpose of this report we will be concentrating specifically on the shell and tube type of heat exchanger.

7.1 Shell and Tube Heat Exchanger

The shell and tube heat exchanger is the most common type of heat exchanger used in the process industry. It consists of a bundle of tubes which are contained within very large diameter pipe called a shell. The two streams are separated by having one fluid flow inside the tube and the other flow outside. Heat is transferred across the tubes from the hot stream to the cold stream. Again there are various different types of shell and tube heat exchangers available, some of which are detailed below:

7.1.1 Concentric Tube Heat Exchanger

This is the most basic type of shell and tube heat exchanger and consists of an inner tube surrounded by and outer tube.

Flow if the hot and cold streams can be either co-current (in the same direction) or counter-current (opposite direction). This type of heat exchanger is often used at the lab scale or pilot scale of the process industry. Baffles are sometimes placed in the inner tube to encourage turbulence which in turn increases the inner heat transfer co-efficient (h) and hence the overall transfer coefficient (U) and hence the rate of heat transfer (Q).

7.1.2 Multiple Tube Heat Exchanger

This type of heat exchanger consists of multiple inner tubes which are surrounded by and outer tubes. The multiple tube heat exchangers can be single pass or multiple pass. In a single pass heat exchanger the fluids will only flow past each other once ie: the fluid will flow in one end and out the other end. In a multiple pass heat exchanger, in order to increase the surface area for convection relative to the fluid volume, the heat exchanger is designed with "U Tubes". The fluid inside the tubes flows in and out at the same end of the exchanger. This causes the fluid to flow co-current in one region and counter-current in the other. This type of heat exchanger is called a 1-2 pass heat exchanger. This indicates that the shell side fluid passes through the unit just once and the tube side passes through the unit twice.

There are many different variations available, such as 1-2, 1-4, 1-6 etc.. and 2-4, 2-6, 2-8 etc... The number of shell side passes is always listed first.

When designing a heat exchanger a different temperature correction factor is required, depending on the number of shell and tube passes in the particular unit being designed. The tables below show the temperature correction factors to be used.

7.2 Heat Exchanger Components

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Heat exchangers are the most important class of heat transfer equipment available in industry because they can be constructed with very large surfaces in a relatively small volume, they can be fabricated from alloy steels to resist corrosion, and they can be used for heating and condensing all kinds of fluids.

A typical heat exchanger consist of the following items

  • Tubes
  • Shell
  • Baffles and Support Plates for Tubes
  • Tie Rods
  • Tube Sheets/Plate

7.2.1 Tubes

The tubes are usually 19 mm or 25 mm outside diameter (do). The pitch of the tubes will be dictated by the necessity for cleaning the outside of the bundle. If the fluid is likely to cause scaling a square pitch is preferred as this permits cleaning round the tubes; the minimum spacing is 1.25do between centres. If space is at a premium, or if the fluids are very clean, a triangular pitch is used with a 6 mm space between the tubes, as this enables considerably more tubes to be put into a shell.

7.2.1.1. Tube arrangements

The tubes in a heat exchanger are usually arranged in an equilateral triangular, square, or rotated square pattern. The triangular and rotated square patterns give higher heat-transfer rates, but have a higher pressure drop than the square pattern. A square, or rotated square arrangement, is used for heavily fouling fluids, where it is necessary to mechanically clean the outside of the tubes. The recommended tube pitch (distance between tube centres) is 1.25 times the tube outside diameter; and this is what has been used for the purpose of the calculations in Section 7.

7.2.2 Shells

These are commonly made of carbon steel, and standard pipes are used for the smaller sizes and rolled welded plate for larger sizes (0.4-1 m). The thickness of the shell can be calculated from the formula for thin-walled cylinders, but a minimum of 9.5 mm is used for shells over 0.33 m outside diameter and 11 mm for shells over 0.9 m outside diameter. Unless the exchanger works at very high pressure, the calculated thickness will usually be less than these figures, but a corrosion allowance of 3 mm is commonly added to all carbon steel parts. There are various different shell configurations such as:

  • One Pass Shell
  • Split Flow
  • Divided Flow
  • Two Pass Shell with Longitudinal Baffle
  • Double Split Flow
  • Kettle Type Re-boiler
  • Cross Flow

7.2.3 Baffles and Support Plates for Tubes

As mentioned earlier, these are fitted to increase the rate of flow over the tube bundle. The most common type is the segmental baffle with about 25 percent cut. The diameter is dictated by the fit of the bundle in the shell but should be not more than 5-10 mm for vapours, and 2.5-5 mm for liquids to allow for good work clearance between the baffle and the inside of the shell. Baffles are not normally spaced closer than one-fifth of the shell diameter; very close spacing is avoided because the increase in heat transfer is then small compared with the increase in pressure drop. Support plates to hold the tubes in position are similar to the baffles, but the holes for the tubes are only 0.4 mm greater than the tube diameter, whereas for baffles the clearance may be twice this. These support plates are fitted at least every 1 m for 19 mm tubes and every 60 tube diameters for larger tubes.

7.2.4 Tie Rods

In order to keep the tube bundle straight, tie rods are fitted to the fixed tube sheet and to the baffle nearest the floating tube sheet. Usually between four and six rods of thickness 9.5 - 13 mm are necessary. These can be used to hold the baffles and support plates in position if sleeves are fitted over the rods between the baffles.

7.2.5 Tube Sheets

To allow sufficient thickness to seal the tubes the tube sheet thickness should not be less than the tube outside diameter, up to about 25 mm diameter. The thickness of the tube sheet will reduce the effective length of the tube slightly, and this should be allowed for when calculating the area available for heat transfer. As a first approximation the length of the tubes can be reduced by 25 mm for each tube sheet.

For a more detailed calculation of tube sheet thickness, the thickness of the fixed tube sheet is often calculated from the formula below:

Where dG is the diameter of the gasket on the tube sheet, P the design pressure, f the allowable working stress, and dt the thickness of the sheet measured at the bottom of the partition baffle grooves. The floating tube sheet may be made times as thick.

For the purpose of this report we will be using the standard 25 mm diameter method of calculating effective length.

8.0 Heat Exchanger Design Calculations & Material Selection

8.1 Sample Design Calculations

This section outlines the steps involved in designing a shell and tube heat exchanger.

This heat exchanger was designed to completely convert dry saturated steam to liquid (Water), while keeping the liquid exiting the exchanger at the same temperature as the steam which entered.

Fresh water was used as the coolant and I have assumed a counter current flow on the exchanger.

The following values were used in the design:

  • Saturated Steam Flow Rate - 2kg/s
  • Saturated Steam Pressure - 3 Bar
  • Saturated Steam Temperature (and liquid temperature) - 133.5°C
  • Fresh Water Temperature (entering the exchanger) - 15°C
  • Water Flow Rate - 4kg/s
  • Latent Heat of Vaporisation () - 432.8kJ/kg [Saturated Steam @ 3Bar]
  • Heat Transfer Co-Efficient (U) for Exchanger - 1500W/m2K
  • Specific Heat Capacity of Water - 4.18kJ/kg K

Assumption:

No losses to environment from exchanger

8.1.1 Draw a Schematic of the Temperature Profiles:

8.1.4 Calculate Log-mean Temperature Difference:

The cooling water has a mass flow rate of 4 kg/s, enters 15°C and exits at 66.8°C. The log-mean temperature difference has been calculated based upon counter-current flow.

8.1.5 Calculate the surface area and diameter of the exchanger:

The overall transfer coefficient, U, for the condenser was found in Coulson & Richardson Vol. 6, which states that U lies in around 1500 W/m2K. This figure must be assumed so that the maximum surface area of the condenser can be calculated. The following formula was used to calculate the surface area of the exchanger:

8.1.6 Calculate the number of tubes in the exchanger:

This Effective Heat Transfer Area then has to be divided into tubes within the Heat Exchanger. Standard tube sizes are an outside diameter (OD) of 20 mm and an internal diameter (id) of 16 mm. Tubing was taken to be 1.83 m long (6 Feet), which gives an effective length of 1.78 m (Coulson & Richardson et al., 2004). Calculating the surface area of one tube can be done using the following equation:

8.1.7 Calculate bundle diameter and exchanger diameter:

Dividing this by the required surface area of the condenser, 57 tubes were needed. This was increased to 60 tubes to add a degree of safety. A 1.25 triangular pitch was employed as the tube arrangement in the condenser. From this the bundle diameter could be calculated using the following equation:

Thus, using a split-ring floating head unit, diametrical clearance between the shell and tubes of a standard of 85 mm, the shell diameter is 315 mm.

8.2 Material Selection

Now that we have calculated the number of tubes and the diameter of the heat exchanger we need to work out the material of construction for the exchanger. As mentioned earlier this exchanger has fresh water flowing on both the shell and tube sides. The design criteria states that the material of the tubing must conduct heat well, have a maximum operating temperature above the operating temperature of the process, not corrode in fresh water, and - since the tubes have to be bent - have adequate ductility.

Material selection was carried out using the "CES EduPack 2009" material selection software and the procedure is shown in the following sections.

8.2.1 Material Selection Procedure

8.2.1.1. Limit Stage:

  • Maximum service temperature > 150 C (423 K)
  • Thermal Properties: Set minimum value of maximum service temperature = 150C
  • Elongation > 20%
  • Mechanical Properties: Set minimum elongation = 20%
  • Corrosion resistance in fresh water to be very good
  • Durability: Water and Aqueous Environments: Tick excellent for durability in "Fresh Water"

This gives the following materials as results:

  • Age Hardening Wrought Al-alloys
  • Brass
  • Bronze
  • Commercially Pure Titanium
  • Copper
  • Nickel
  • Nickel based Super Alloys
  • Nickel Chromium Alloys
  • Non Age Hardening Wrought Al-Alloys
  • Polyetheretherketone (PEEK)
  • Polytetrafluoroethylene
  • Silicone Elastomers
  • Stainless Steel

8.2.1.2. Graph Stage

To reduce the list of results further, another requirement needed to be considered at this point

  • The heat exchanger is required to have as large a thermal conductivity as possible.

To do this we set up the graph stage with thermal conductivity on y-axis and picked the materials with the highest thermal conductivity.

The graph stage results show that the 4 materials with the highest thermal conductivity are:

  • Copper has a thermal conductivity of between 160 - 390 W/m.K
  • Non Age-hardening Wrought Al-Alloys have a thermal conductivity of between 119 - 240 W/m.K.
  • Age-hardening Wrought Al-Alloys have a thermal conductivity of between 118 - 174 W/m.K.
  • Brass has a thermal conductivity of between 100 - 130 W/m.K.

9.0 Market Analysis

9.1 Fresh Water Heat Exchangers

Research into the area of Fresh Water Heat Exchangers has shown that the main area of use is in marine heating applications. There are 2 main uses for the Fresh Water Heat Exchangers:

  • To integrate the engine's fresh water cooling system with the hot water heating system.
  • To heat domestic water for use in the galley and shower.

The fresh water heat exchanger system is used to heat the interior of the boat and heat the domestic water by using the engines waste heat.

9.2 Existing Systems/Design Requirements

The results showed that Copper and Aluminium Alloys were the top two materials to construct a Fresh Water shell and tube heat exchanger, as they had the highest values for thermal conductivity.

9.2.1 Alfa Laval

Alfa Laval, who are the world leader in heat transfer, centrifugal separation and fluid handling, currently have a Shell and tube Heat Exchanger for fresh and sea water applications for sale which has the following properties:

  • Shell: Carbon Steel
  • Tube Sheets: Carbon Steel
  • Tubes: Integrally Finned Thick Wall Copper Tubing
  • Baffles: Teflon

As you can see this matches the results that have been obtained from the CES software, ie: Copper.

The main difference between this unit and the unit that has been designed as part of this report is that R134a refrigerant is used as the cooling liquid as opposed to water.

9.2.2 Tubular Exchanger Manufacturers Association, Inc. (TEMA)

According to the TEMA the material for the tubes of water-cooled exchangers should be selected as follows:

  • If carbon steel is suitable for the process fluid (shell side) and the water is stated as being suitably treated so that it is not corrosive to carbon steel, carbon steel tubes and tube sheets are acceptable. The use of carbon steel tubes for other conditions shall be proposed for agreement by the company.
  • Where the water is not specified to be treated as above the following materials shall be used for tubes subject to process (shell) side acceptability.
  • Admiralty brass for fresh and recirculated waters.
  • Aluminum brass for salt water and other corrosive water.
  • Copper-Nickle tubes for sea water.

Again we can see that Brass, Aluminium and Copper are mentioned as suitable materials for the construction of a Fresh Water Heat Exchanger.

10.0 Results

It is our recommendation, after careful design and analysis, that the heat exchanger be constructed as follows:

  • Shell: Stainless Steel
  • Tube Sheets: Stainless Steel
  • Tubes: Copper
  • Baffles: Stainless Steel

10.1 Shell

Stainless Steel been selected for the shell because it meets the requirements with regards to service temperature, elongation and corrosion resistance but at the same time has a lower Thermal Conductivity (12 - 24 W/m.K).

While we want the shell to possess the same characteristics as the tube, with regards to service temperature, elongation and corrosion resistance, we do not want the shell to be as thermally conductive.

This decision has been taken from a safety perspective. There is no need for the shell itself to be conductive. If the shell is highly conductive it could cause problems with scalds and burns from people accidently rubbing off the shell. It is safer to use a lower thermally conductive material.

Stainless Steel is also an extremely durable and is a clean metal which is used extensively in the process industry.

10.2 Tubes

Copper Tubing has been selected for the tubes because it meets the requirements with regards to service temperature, elongation and corrosion resistance and has the highest Thermal Conductivity (160 - 390 W/m.K) of all the materials.

Also, as can be seen from the market analysis in Section 9 copper is currently both, used in and recommended for use in, fresh water heat exchangers by the world leaders in heat transfer, Alfa Laval, and the world leaders in quality assurance standards for Shell and Tube Heat Exchangers, TEMA.

The high Thermal Conductivity of copper makes it an ideal material to use in the tubing in the heat exchanger.

While we have selected copper for the purpose of our design, aluminium alloys and brass are also perfectly acceptable materials to use for the tubing in a Fresh Water Heat Exchanger.

  • Non Age-hardening Wrought Al-Alloys have a thermal conductivity of between 119 - 240 W/m.K.
  • Age-hardening Wrought Al-Alloys have a thermal conductivity of between 118 - 174 W/m.K.
  • Brass has a thermal conductivity of between 100 - 130 W/m.K.

10.3 Tube Sheets & Baffles

Stainless Steel will also be used for the Tube Sheets and Baffles based on its durability and its cleanliness.

11.0 Conclusion

The material selection results shown in Section 8 show that Copper and Aluminium Alloys and Brass were the 3 best materials to construct a the tubing for a fresh water heat exchanger from, as they have the highest values of thermal conductivity, while also meeting the design criteria outlined at the start of this report:

  • Maximum service temperature> 150 C (423 K)
  • Elongation> 20%
  • Corrosion resistance in fresh water: very good

For the purpose of this report we have chosen the following materials for our design:

  • Shell: Stainless Steel
  • Tube Sheets: Stainless Steel
  • Tubes: Copper
  • Baffles: Stainless Steel

Brass and Aluminium Alloys are also perfectly acceptable materials which could have been used in the tubes.

The design carried out in this report was based around fresh water flowing on both the shell and tube sides of the exchanger. Modifications to both the design calculations and the material selection stage limits will be required, if other fluids are required to be used.

12.0 References

  • Coulson & Richardson, (1999), Chemical Engineering Design Vol. 1, 3rd Edition
  • Coulson & Richardson, (2004), Chemical Engineering Design Vol. 6, 4th Edition
  • Dr. Edmond Byrne, (2005), Heat Transfer & Applied Thermodynamics
  • Barry Ronan, (2005), Heat Transfer & Applied Thermodynamics, Class Notes (UCC)
  • TEMA (Tubular Exchanger Manufacturers Association), (1994), Engineering and Material Standard for Shell and Tube Heat Exchangers, Original Edition July 1994
  • Alfa Laval, Water cooled condensers, Shell and tube condensers for fresh and sea water applications Available at: http://www.alfalaval.com/solution-finder/products/fresh-water-condenser/Documents/ERC00042EN.pdf [Accessed on 27th February 2010]
  • Shell and Tube Heat Exchanger Available at: http://en.wikipedia.org/wiki/Shell_and_tube_heat_exchanger [Accessed on 27th February 2010]