Heat transfer fundamentals (5 of 5)

Connections

Connections form an important part of any heat exchanger as they provide the interface between the unit and the system pipework. It is essential for the safe and convenient working of the equipment that the type of connection is chosen carefully to satisfy the safety requirements for the design pressure and temperature and the clients requirements for convenience and suitability for the process. If a heat exchanger is going to be removed for cleaning on a regular basis then quick release clamps may be a more appropriate choice than bolted flanges.

It must be appreciated however that both flanges and the various styles of quick release clamps offered by suppliers are limited in working pressure and temperature. It is essential that whichever style of connection is specified by the end user (or offered by the designer) the temperature and pressure ratings of the connection are checked to confirm acceptability.

In addition to pressure and temperature rating, the process compatibility of the gasket or seal material used for the connection must always be checked. There are a range of materials available for most connection types and if the equipment is to meet the customers’ requirements the one chosen must be appropriate for the working fluids. It must also be appreciated that gasket materials differ not only in chemical composition and therefore process compatibility but also differ in Hardness and Thickness which must be matched to the requirements of the flanges. If the material is too hard on a smooth faced flange it may not seal properly while if it is too soft on a high pressure flange it may not retain its correct position. Gasket materials and thicknesses must be checked with a specialist manufacturer to ensure compatibility with the application.

As important as the type of connection used is the size of the connection. This will often be specified by the installer but the heat exchanger designer must be aware of the implications of the sizes chosen. The latest revision of the HED thermal design programme includes a dialogue box which allows the designer to check the fluid velocities, values of ρv² and nozzle pressure losses when choosing or checking nozzle sizes. It is essential that these values are always checked as an incorrect choice could affect the functionality of the equipment.

Mechanical considerations also affect the choice of nozzle. Essentially the bigger the hole in the shell the weaker the shell will become and under pressure vessel rules the weakening effect must be checked and if necessary reinforcement added in the nozzle area. On the HRS K series units the nozzle sizes chosen as standard are normally the maximum that can be fitted onto the shell. Whatever size is chosen however the effects must be checked for pressure safety.

Bolted flanges normally carry a standard pressure rating based on a working temperature of 100°C and above this temperature the acceptable design pressure is reduced.

Type of Fitting	Maximum Design Pressure	Maximum Design Temperature	Comments
PN6 Flange DIN 2573	6,0 Barg @ 100°C	Refer to DIN 2573	Gasket material must be specified
PN10 Flange DIN 2576	10,0 Barg @ 100°C	Refer to DIN 2576	Gasket material must be specified
PN16 Flange DIN 2502	16,0 Barg @ 100°C	Refer to DIN 2502	Gasket material must be specified
RJT	8,0 Barg	Depending on seal material	Seal material must be specified
IDF	8,0 Barg	Depending on seal material	Seal material must be specified
Tri-clamp	8,0 Barg	Depending on seal material	Seal material must be specified
DIN 11851	8,0 Barg	Depending on seal material	Seal material must be specified
SMS	8,0 Barg	Depending on seal material	Seal material must be specified

Heat transfer fundamentals (4 of 5)

Advantages of corrugation

• Can generate tubeside heat transfer coefficients up to 2½ times greater than the equivalent smooth tube with less than 2½ times increase in pressure loss.
• Does not obstruct the flow area of the tube to a significant extent so they can be used in safety for fluids with high solids or fibre content without fear of blockage.
• Increasing the tubeside heat transfer coefficient brings the temperature of the tube wall closer to the temperature of the bulk fluid on the tubeside this minimising any tendency to cause fouling due to "burn-on", freezing or chemical changes.
• With higher coefficients the heat exchanger size can be reduced and therefore minimises product hold up volumes and residence time within the heat exchanger as well as reducing the overall material content of the heat exchanger. When exotic materials are used this can have a significant effect on the overall cost of the unit and installation.
• The effectiveness of in-situ cleaning processes, Cleaning In Place, is increased because of the increased turbulence generated by the corrugated tube at traditional circulation velocities.
• The higher turbulence created in lower viscosity fluids will minimise any tendency for deposition fouling even at low flow velocities.
• If fouling does occur on the tubeside the deposits will normally be easier to remove as the corrugation leads to an uneven film thickness which experience has shown is less adherent than an equivalent deposit on a smooth tube.

The major advantages can be summarised as follows:

• Reduction in heat exchanger size
• Reduction in product hold up volume
• Reduction in processing time
• Reduction in fouling potential
• Tube wall temperature closer to tubeside fluid
• Increased cleaning potential
• More efficient processing of viscous fluids

Industries in which applications which would benefit in a positive way from any of the advantages listed could become users of the corrugated tube heat exchangers to provide size and weight savings and more effective processes.

Expansion bellows

Most of the standard HRS series heat exchangers are manufactured as fixed tube units and are normally fitted with a thin wall multiconvolution expansion compensator (or Bellows) to allow for the differential expansion between the shell pipe and the tubes. It is vitally important that the bellows unit is designed correctly and the method of design used by HRS Heat Exchangers is that recommended by the "Expansion Joint Manufacturers Association" of America which checks allowable stress values against those produced by the working conditions and gives a prediction of the number of working cycles that the bellows unit will withstand before failure through fatigue. It is important that the worst case conditions of pressure, temperature and differential expansion are identified (which may be a CIP or other non-working condition) for use in the design calculations.

The new Mechanical Design section of the HRS computer software includes these calculations. It must be stressed that the bellows units fitted to HRS heat exchangers are only intended to absorb the differential movement between the shell pipe and the tubes. The absolute expansion of the shell pipe (which can be as high as 20 millimetres on a 6.000 mm unit) must be allowed for by the installer of the equipment by allowing one end of the unit to expand freely with pipework movements being compensated for with an appropriate pipework bellows and sliding supports etc. Failure to do this will quickly damage the heat exchanger.

TEMA stesses that heat exchangers are not intended to act as pipework anchor points. If the pipework designer does not account for the expansions and contractions produced under all operational conditions and allows them to impose external loads onto the heat exchanger connections then both bellows and nozzle pipes can be damaged.

The dimensional standards used by HRS for the multiconvolutional bellows used on our standard fixed tube designs are as shown below. Sizes outside this range or for high pressure/temperature units or for units with very high differential movements are purchased externally from an appropriate manufacturer so dimensional details will sometimes differ from those shown.

Shell Diameter	Shell Thickness	Number of Ply	Ply Thickness
63,5 mm	1,5 mm	1	0,8 mm
76,1 mm	1,5 mm	1	0,8 mm
88,9 mm	2,0 mm	1	0,8 mm
104,0 mm	2,0 mm	1	0,8 mm
114,0 mm	2,0 mm	1	0,8 mm
129,0 mm	2,0 mm	1	0,8 mm
139,7 mm	2,0 mm	1	1,0 mm
154,0 mm	2,0 mm	1	1,0 mm
168,3 mm	2,0 mm	2	0,8 mm
219,1 mm	2,0 mm	2	1,0 mm

Heat transfer fundamentals (3 of 5)

What are corrugated tubes?

When the overall heat transfer rate for a given heat exchanger is limited by the tubeside partial heat transfer coefficient (α inside) the overall surface area of the heat exchanger can usually be reduced if this coefficient is improved in some way. Many methods of artificially enhancing this coefficient have been tried, some successful other less so.

• Wire or strip inserts are sometimes used pushed into each tube to stir the boundary layer liquid away from the tube wall into the bulk of the fluid. This type has the disadvantage of a substantial increase in pressure loss per unit length and any particles or product pieces entrained in the fluid render them useless.
• Internal ribs or fins along the length of the tube which are designed to increase the internal surface area per unit length of tube. Even moderate viscosity fluids can bridge the gap between the fins to give a layer of cold static fluid which negates any benefits that may be present.
• Deformed tubes where the tube is flattened to reduce the effective hydraulic diameter and increase the coefficient accordingly. Once again any particles or product pieces entrained in the fluid render them inadvisable as blockage can easily occur.
• The HRS Heat Exchanger' type of shallow deformation which causes the boundary layer to be disrupted without too large a reduction in flow area. Providing the working diameters are chosen carefully this does not prevent the free passage of particles and pieces and does not allow even the most viscous fluids to bridge adjacent corrugations.

Why corrugated tubes were developed?

An increasing requirement for food products to be pasteurised for long term storage and general hygiene led to the realisation that the heat transfer characteristics of a large proportion of food products were poor. In addition to this they commonly contain pieces of fruit, vegetable or meat which must maintain their integrity during processing to keep the quality of the product at an acceptable level. Without enhancement the heat exchangers required for processing even modest quantities of some food products can be unrealistically large and expensive.

Because of these inherent operational requirements, methods of reducing size and cost for this type of heat exchanger led to the development of tubes which can be used with fluids containing large size particles but still increase the rate of heat transfer by disrupting the boundary layer to give values of heat transfer coefficient higher than would normally result from the flow conditions being used.

Experimentation with different styles and types of tube deformation led to a general form consisting of a shallow spiral deformation down the length of the tube which has been optimised by further experimentation by HRS Heat Exchangers into our "Hard" and "Soft" standard types. Whilst being deep enough to disrupt the boundary layer they are not deep enough to constitute a barrier for any solids content which could cause blockage.

Our two standard corrugations differ only in the angle which the spiral indentation takes down the tube, the "Hard" being a steeper angle to the longitudinal centre line of the tube.

Experimentation with high viscosity food products has led to further development as an intermittent indentation which has proved more effective at values of Reynolds number between 40 and 200 and is now used when high viscosity causes these low Re values. The so called "dimple" corrugation is shown in the image.

Heat transfer fundamentals (2 of 5)

  Fluids

The fluids for both product and Service systems with which the heat exchanger designer has to work are as varied as the processes which use heat exchangers. They can however be classified into two very broad categories:

• Newtonian - Where the inherent property defined as viscosity is independent of the rate of shear within the fluid.
• Non-Newtonian - Where the inherent property defined as viscosity is dependant on the rate of shear within the fluid.

In simple terms, the effective viscosity of a Newtonian fluid does not depend on the velocity with which it flows through a pipe or tube but for a Non-Newtonian fluid it does.

As well as the viscosity of the working fluids four other properties are of major importance when modelling heat transfer performance.

• Density - the mass of the fluid per unit volume which directly affects the velocity with which the fluids flow through a system.
• Specific heat - the amount of heat which a given mass of a fluid requires for the temperature to be changed by 1°.
• Thermal conductivity - the rate at which heat can flow through a fluid.
• Latent heat - the amount of heat which a given mass of a substance requires to change state - that is to melt if it is a solid, freeze if it is liquid, evaporate if it is a liquid or condense if it is a gas.

Equally important from the operational point of view are the corrosion characteristics of the fluid which influence the final choice of materials of construction that the designer must use.

It is particularly important to identify fluids which are known to be high in Chlorides as these can lead to stress corrosion cracking in some grades of stainless steel but any high acidity or alkalinity fluids should be checked with an expert metallurgist to confirm material suitability. In applications such as exhaust gas cooling it is important to check for condensation on the tube wall and the composition of the gas (or fuel) to check if any acids will form as the gas is cooled. If condensation is confirmed and the gas or fuel contains any Sulphur compounds then the assistance of an expert metallurgist should again be sought for advice on suitable materials.

Shell & Tube heat exchangers

When marketing surveys of heat exchanger usage are carried out they usually report that the most common type of heat exchanger in use in most industries is the shell and tube type. Other types are used when operational or process circumstances dictate, for example air cooled units when there is no source of cooling water readily available or plate types where weight and space are of paramount importance and adequate servicing facilities are available, but the highest proportion of units are still shell and tube types.

Shell and tube heat exchangers can be classified into two broad categories:

• Fixed tube units, where the tubes are fixed rigidly into the pressure retaining envelope of the heat exchanger.
• Removable tube units where the mechanical design allows the tube bundle to be removed from the pressure envelope for inspection and cleaning purposes.

In terms of initial cost the fixed tube type of heat exchangers is usually the least expensive but as the outer tube surfaces are not readily accessible for cleaning and inspection purposes they should only be used in circumstances where the shell side fluid is non fouling or where fouling deposits can be removed by chemical methods.

Removable tube units are made in a variety of different styles to suit the application but all have gasketted or "O" ring sealed joints in contact with the fluids which have to be sealed against the working pressures and temperatures and still be compatible with the application in terms of chemical resistance, approval for food industry use etc. The cost per square metre of heat transfer surface for these units is higher than the equivalent fixed tube design, the level of cost depending on mechanical design considerations.

As a very general rule, a shell and tube unit with a removable tube bundle will increase the cost of a heat exchanger designed for a specific thermal performance by approximately 30%.

An important factor to be considered is that the design and manufacture of shell and tube heat exchangers falls into two very distinct phases.

• The first phase is the completion of the thermal sizing calculations, where the skilful designer can use the enhanced performance characteristics of the corrugated tube to his advantage in reduction of size, weight, hold up volumes etc.
• The second phase is the identification of the appropriate design conditions, temperature, pressure etc. and the mechanical design of the unit where the designer has to follow a rigid set of rules for the pressure envelope of the heat exchanger. This may involve compliance with one or more of the international pressure vessel codes to satisfy the requirements of the purchaser or more frequently the purchasers Insurance Company and National legislation. Equally important is the selection or confirmation of the nozzle sizes, connection types and bellows design which are appropriate for the application and correct for the design conditions.

The designer has to work within an environment which may control his actions and reduce the opportunity of using the advantages of the product to full effect.

Heat transfer fundamentals (1 of 5)

Introduction

A large number of production facilities in many industries use processes in which heat is transferred between different fluids. The basic principle of heat transfer is extremely simple, two fluids at different temperatures are placed in contact with a conductive barrier (the tube wall) and heat is transferred from the hotter fluid to the colder fluid until they reach the same temperature level. In industrial processes this is carried out in heat exchangers of various types and styles usually purpose built for the process and site conditions of the application.

The driving force for heat transfer is the difference in temperature levels between the hot and cold fluids, the greater the difference the higher the rate at which the heat will flow between them. With complex processing sequences the designer must optimise the temperature levels at each stage to maximise the total rate of heat flow.

A second factor controlling the transfer of heat is the area of the conductive barrier provided for heat flow. The greater the area the larger the amount of heat that will flow in a given time with a given temperature difference. The designer has to minimise this area to provide cost effective solutions to his client and with skill the amount of area can be minimised and configured to reduce the containment volume and overall cost.

The third and perhaps the most important factor controlling the transfer of heat is the rate at which the heat flows into or out from each of the fluids. A high resistance to heat flow in either fluid will produce a slow overall rate of transfer. The level of resistance to heat flow results from many different factors including the inherent thermal characteristics of the fluids but can be influenced by the designer in a very positive way by the generation of turbulence within the fluids to prevent the creation of a thermally resistant static "boundary layer" of fluid in contact with the heat transfer surface.

The fourth factor, also under the control of the designer, is the flow of heat through the conductive barrier between the fluids. The material chosen has to be compatible with the fluids of the process, it must not corrode or contaminate a food product, it must have an appropriate level of mechanical strength to withstand working temperatures and pressures and it must have a low resistance to heat flow so that it does not become the overriding factor in the heat transfer process.

The mathematical equations which describe the process of heat transfer are fairly simple:

Where:

• Q is the amount of heat transferred, W
• A is the area for heat transfer, m²
• ∆T is an effective temperature difference, °K
• U is the overall heat transfer coefficient, W/m².°K

The value of U is slightly more complex to calculate:

Where:

• h1 and h2 are the partial heat transfer coefficients, W/m².°K.
• Rw is the thermal resistance of the wall, m².°K/W.
• Rf1 and Rf2 are the fouling factors, m².°K/W.

While the values for Rf are usually specified by the client, the values of h and Rw can be influenced directly by the designer by the choice of tube size and thickness and the materials of construction. The values of the partial heat transfer coefficients h depend greatly on the nature of the fluids but also, crucially, on the geometry of the heat transfer surfaces they are in contact with. Importantly the final values are heavily influenced by what happens at the level of the boundary layers, the fluid actually in contact with the heat transfer surface.

Heat Transfer Processes

Most of the academic research taking place into heat transfer processes concentrates on ways of predicting with accuracy the precise values of the boundary layer resistance and on ways of affecting the values without paying too high a penalty in terms of increased pressure losses.

Many techniques to reduce the tubeside boundary layer resistance have been tried including various styles of tube "inserts" which take the form of complex wire shapes or flat twisted strips fitted inside the tubes and various styles of tube deformation. Most have the disadvantage of increasing the resistance to fluid flow, the pressure loss, at a rate which increases more rapidly than the decrease in boundary layer resistance. One technique which does not have this disadvantage however is that of deforming the tube with either a continuous spiral indentation (corrugated) or an intermittent spot indentation (dimple). Our own research has shown that by choosing the depth, angle and width of the indentation carefully, the rate of decrease in boundary layer resistance can exceed the rate of increase in pressure loss. This is the form chosen for HRS Heat Exchangers' units.

The continuous disturbance of the boundary layer of the tubeside fluid increases the amount of turbulence within the fluid as described mathematically by the "Nusselt number" and, providing the tubeside fluid has the higher resistance to heat flow, will increase the overall rate at which heat is transferred.

Fouling Factors

These are normally specified by the client based on his experience of running his plant or process but if not restricted to proper levels can totally negate any benefits generated by skilful design. They represent the theoretical resistance to heat flow due to a build up of a layer of dirt or other fouling substance on one or both of the tube surfaces but are often overstated by the end user in an attempt to minimise the frequency of cleaning. In reality they can, if badly chosen, lead to increased cleaning frequency.

Fouling mechanisms vary with the application but can be broadly classified into four common and readily identifiable types.

Types of Fouling

• Chemical fouling where chemical changes within the fluid cause a fouling layer to be deposited. A common example of this phenomenon is scaling in a kettle or boiler caused by calcium salts depositing onto the heating elements as the solubility of the salts reduce with increasing temperature. This is outside the control of the heat exchanger designer but can be minimised by careful control of the tube wall temperature in contact with the fluid.
• Biological fouling caused by the growth of organisms within the fluid which deposit out onto the surface. This is outside the control of the heat exchanger designer but it can be influenced by the choice of materials as some, notably the non-ferrous brasses, are poisonous to some organisms.
• Deposition fouling where particles within the fluid settle out onto the surface when the velocity falls below a critical level. This is to a large extent within the control of the designer as the critical velocity for any fluid/particle combination can be calculated to allow a design to be drawn up with minimum velocity levels higher than the critical level. Mounting the heat exchanger vertically can also minimise the effect as gravity would tend to pull the particles out of the heat exchanger away from the heat transfer surface.
• Corrosion fouling where a layer of corrosion products builds up on the surface of the tube forming an extra layer of, usually, high thermal resistance material. By careful choice of materials of construction the effects can be minimised as a wide range of corrosion resistant materials based on stainless steel are now available to the heat exchanger manufacturer.