Heat Transfer White Paper

Heat TransferModern solutions for optimum efficiency

www.spiraxsarco.com/uk

E X P E R T I S E S O L U T I O N S S U S T A I N A B I L I T Y

H e a t T r a n s f e r

1.0 Executive Summary

2.0 Effective heat transfer is at the heart of almost every process

3.0 Types of heat exchanger 3.1 Shell and tube 3.2 Plate and frame heat exchangers 3.3 Plate and shell heat exchangers 3.4 Corrugated tube heat exchangers 3.5 Shell and coil heat exchangers 3.6 Heat Pipe heat exchangers

4.0 Controlling steam heat exchangers 4.1 Condensate control maximises heat transfer 4.2 Tackling heat exchanger stalling

5.0 Spirax Sarco heat transfer solutions 5.1 Plate and shell heat exchangers (PSHE) 5.2 Exhaust vapour condensers (EVC) 5.3 EasiHeat™ heat transfer systems 5.4 Related systems support effective heat transfer

6.0 Conclusion

Contents


Effective energy transfer is a basic prerequisite for the success of many industrial processes, as well as underpinning building services for space heating and hot water. The foundation of most energy transfer applications in industry is the heat exchanger.

Steam is the most efficient and flexible energy transfer medium and there is a range of heat exchanger technologies available that can provide reliable service across a wide variety of different applications. What’s more, the exchangers at the heart of energy transfer are increasingly supported by advanced controls and other innovations such as pre-fabricated systems that make it far easier to optimise their performance.

However, it’s important to have a sound grasp of the basics of energy transfer and steam plant operation in order to specify systems correctly and prevent problems cropping up later. In the case of users for whom steam lies outside the scope of their core activities, bringing in expertise from external steam specialists can help to ensure that their heat transfer installations provide optimised, reliable service for years to come.


1.0 Executive Summary


Whether energy is needed for an industrial process or for space heating, effective energy transfer underpins the operations of almost every organisation.

At its most basic, heat transfer is concerned with two things: temperature and the flow of thermal energy from a heat source to a heat sink. On top of this, there are two requirements that any successful heat transfer technology must provide: efficiency and control.

Energy efficiency determines what proportion of the energy entering the process ends up where it’s needed. It’s key to minimising the carbon footprint of the process and reducing operating costs.

Meanwhile, effective control enables the thermal energy transferred to vary flexibly to match the changing demands of the process. Not only does this link closely with energy efficiency, but it can also affect production efficiency by impacting on parameters such as product quality and waste.

Thermal energy can be transferred via convection (e.g. air currents), radiation (e.g. from a flame or other radiant element) and conduction (e.g. from steam to hot water via a pipe wall or plate). Of these, conduction is the most widespread and versatile approach and can be applied using a number of different heat exchanger technologies.

2.0 Effective heat transfer is at the heart of almost every process

The heat transfer coefficient is the proportionality coefficient between the heat flux (the rate of transfer of heat across a surface) and the thermodynamic driving force for the flow of heat (i.e. the temperature difference).

whereh : heat transfer coefficient (W/m2.oC)q : heat flux (W/m2)ΔT : difference in temperature between the heat source and sink (oC)

The best heat exchangers will have a high heat transfer coefficient determined by several factors, including the construction material and the flow regime of the fluids.

Heat transfer: the basics

h =qΔT



There is good reason why steam is the heat transfer vehicle of choice across a vast range of industrial sites and other establishments, such as hospitals.

It’s cost efficient. For example, electricity currently costs roughly three times more per kilowatt hour in the UK and Ireland than natural gas. By choosing the right boiler, steam users have the flexibility to use the fuel supply of their choice to match their needs.

Water has a very high specific heat capacity, making steam one of the most efficient energy-carrying fluids, able to transport a large amount of energy in a small mass. As a comparison, steam at 6 bar g would need only a 40 mm bore pipe to carry the same amount of energy as an 80 mm bore pipe carrying low temperature hot water (LTHW) with an input/output temperature drop of 11°C. To put this into perspective, 20 m of 80 mm bore pipe will contain 100 kg of water, while the weight of 6 bar g steam filling the same pipe bore and length would be about 0.37 kg, or 99.63% less weight than water. The demands on mechanical installation of plant are much less.

Steam is produced by the evaporation of water, which is a relatively inexpensive and plentiful commodity that is environmentally-friendly.

It’s also flexible. Modern steam distribution systems can supply steam to even the most inaccessible places on site, delivered at a temperature and pressure to suit the process. Its temperature can be adjusted accurately by the control of its pressure.

Steam: an ideal energy transfer medium


Heat exchanger technology has evolved substantially in recent years and is now available in a range of configurations to suit different applications. Heat exchanger types can be divided broadly as follows.

3.0 Types of heat exchanger

Tube bundle

Secondary fluid in

Secondary fluid out

Steam in

Condensate out

3.1 Shell and tube Shell and tube heat exchangers (or calorifiers) used to be the norm in most applications and are still very common in heavy industries such as oil, gas and petrochemicals, where they are well-suited to high-pressure applications. They are also used widely in hospitals to provide heating and domestic hot water services. However, they are gradually being superseded by more compact and energy efficient alternatives.

The shell is a pressure vessel containing a bundle of tubes. One fluid flows through the shell and the other through the tubes. Each fluid enters the exchanger at a different temperature and heat passes between them through the tube walls as they flow through the process.

The minimum practical temperature difference achieved in a typical shell and tube exchanger by the time the fluids exit the unit (known as the approach temperature) will not usually be less than 5°C.

The shell is typically pressurised and will therefore require an annual insurance inspection involving a comprehensive and time-consuming strip-down. This also applies to all the related technologies listed below that include a pressurised shell.

Shell and tube



3.2 Plate and frame heat exchangers Standard plate heat exchangers (PHEs) are increasingly popular in applications that transfer heat between medium- and low-pressure fluids, although more specialised welded, semi-welded and brazed versions can be used with high-pressure fluids.

In place of tubes passing through a shell, PHEs are built from a series of corrugated metal plates that are held together to form channels through which the two heat transfer fluids flow in alternating layers of the ‘sandwich’. A standard plate stack is spaced using a series of gaskets, which are usually the limiting factor when it comes to high-temperature/high-pressure applications.

The plates produce an extremely large surface area relative to physical size, which promotes very effective heat transfer. Each chamber is only a few millimetres across so the majority of the volume of each liquid contacts the plate. The corrugated troughs also promote turbulent flow. Both of these features encourage greater heat transfer, even at low flow rates, and help prevent fouling.

The approach temperature achievable with PHEs can be as low as 1°C. They are also far more compact than shell and tube exchangers when performing the same duty. This means that they suffer lower heat losses, boosting energy efficiency by as much as 6% compared to shell and tube heat exchangers. Sub-cooling of the condensate can increase energy efficiency even further.

There is no pressurised shell and the volume of liquid in a PHE is very low, so this is one type of exchanger that does not generally require an annual insurance inspection as long as it is fitted with a spray/splash guard as the system is self-relieving, which significantly reduces downtime and maintenance costs.

3.3 Plate and shell heat exchangersA plate and shell heat exchanger combines PHE and shell and tube heat exchanger technologies.

The heart of the exchanger contains a fully welded circular plate pack, with nozzles carrying flow in and out (the ‘plate side’ flow path). This assembly sits in an outer shell that creates a second flow path (the ‘shell side’).

This plate and shell combination offers high heat transfer, compact size, low fouling and a close approach temperature. It is also able to cope with high pressures and temperatures without leaking, thanks to the all-welded plate pack that eliminates the use of gaskets that may leak under high pressure. As this type of heat exchanger includes a pressurised vessel, it would typically need annual inspections.

Plate and frame heat exchangers Plate and shell heat exchangers


3.4 Corrugated tube heat exchangers These are a variation on traditional shell and tube exchangers. They have corrugated tubes to create greater turbulence and this delivers a substantial increase in heat transfer compared to smooth tube heat exchangers. Improved heat transfer translates into a smaller heat transfer area for the same duty, which in turn means more compact heat exchangers. In fact, some manufacturers claim that the performance of a two- or four-pass smooth tube design can be achieved in a single pass in a corrugated tube exchanger.

Shorter tubes and/or fewer passes also result in a lower pressure drop across the exchanger, which saves on pumping costs, while the increased turbulence also makes corrugated tube exchangers more resistant to fouling than smooth tube versions. This helps to maintain the heat exchanger’s efficiency and can reduce maintenance costs.

3.5 Shell and coil heat exchangersShell and coil heat exchangers are built from circular layers of helically corrugated tubes inside a compact shell. The fluid in each layer flows in the opposite direction to the layer surrounding it, producing a criss-cross pattern. The large number of closely packed tubes creates a significant heat transfer surface, while the alternate layers create a swift uniform heating of fluids and increase the total heat transfer coefficient. The corrugated tubes produce a turbulent flow, which improves heat transfer and resists fouling. This helps to maintain the heat exchanger’s efficiency and can reduce maintenance costs.

As this type of heat exchanger includes a pressurised vessel it would typically need annual inspections.

Service

Product

Corrugated tube heat exchangers

Shell and coil heat exchangers



3.6 Heat Pipe heat exchangers Heat Pipes work on a different principle from the other heat exchange technologies already discussed. However, recent advances in their design and manufacture mean that they are starting to make inroads into certain applications, such as heat recovery from exhaust gases.

Heat Pipes are sealed vacuum tubes with one end in the ‘hot’ stream and other in the ‘cold’ stream. They contain a working fluid, and it’s the constant cycle of evaporation and condensation as the working fluid moves around the sealed tube that transfers thermal energy from one stream to the other. The Heat Pipe is normally positioned vertically (but can operate effectively at 4° from the horizontal) with the lower end of the tube sitting in the hot stream. The working fluid evaporates and rises to the top of the tube, which sits in the cold stream. The working fluid then condenses, giving up heat to the cold stream and running back down the vacuum tube to begin the cycle again. The working fluid is chosen to suit the temperature range for the particular application.

The big advantage of heat pipes is their great efficiency in transferring heat. For example, a heat pipe can transfer up to 1,000 times more thermal energy than copper, the best known conductor, with a temperature drop of less than 17°C along a 30 cm length.

Vacuum tube

Condensation

Evaporation

Heat transfer fluid

Heat out

Separation plate

Heat in

Heat Pipe heat exchangers


With the exception of heat pipes, all the different types of exchanger described are most often controlled in a similar way, by sensing the temperature of the secondary fluid (often water) emerging from the unit and using a valve to modulate the primary fluid (the incoming flow of steam) to the exchanger accordingly.

A fully independent high-limit cut-out should be fitted in accordance with Health & Safety Executive recommendations to protect people or equipment should the water temperature exceed a set limit.

The transfer of energy from the steam routinely results in the generation of condensate in the heat exchanger, and under normal operations this is removed via a steam trap.

4.1 Condensate control maximises heat transferAn alternative control method to modulating the steam flow into the heat exchanger is condensate control, which keeps the input steam pressure constant and instead adjusts the flow of condensate coming out of the exchanger. This varies the amount of condensate inside the exchanger to control its heat transfer area and hence its heat transfer rate.

Condensate control allows the condensate to be maintained at a sub-cooled temperature of 95°C to extract the maximum amount of useful heat from the steam and avoids any potential flash steam plumes.

Plate and frame, vertical shell and coil, and plate and shell heat exchangers can all be used with condensate control. However, shell and tube heat exchangers are not well suited to condensate control because their rigid construction makes them susceptible to the thermal stresses caused by the temperature difference between the hot incoming steam and cooler condensate held inside. This can lead to thermal fatigue and heat exchanger failure.

Condensate control also cannot be used in applications with quickly varying demand, such as domestic hot water (DHW) systems. It is more suited to applications that require a small rate of change of flow such as Low Temperature Hot Water (LTHW). Also, demand on the secondary side should never fall below 20% of the design flow conditions.

Heat exchangers controlled in this way generally offer reliable, trouble-free operation, but there are some issues common to all types of heat exchanger that operators should look out for. 4.2 Tackling heat exchanger stallingHeat exchangers of any type can stall when the condensate is not removed effectively and builds up internally. Typical symptoms of heat exchanger flooding include banging and crashing noises coming from shell and tube heat exchangers caused by waterhammer. Plate heat exchangers are more resilient with only a modest risk of waterhammer occurring. Other symptoms could include erratic temperature control and corrosion caused by condensate collecting inside the unit, leading to leaks.

Stalling happens when the pressure in the heat exchanger is less than or equal to the back pressure on the steam trap, often occurring when demand from the heating process falls due to a change of flow rate. When this happens, the control valve reduces the steam pressure accordingly and this may reach a level that’s too low for the steam trap to clear the condensate effectively. The risk of stalling is increased when the condensate is discharged against a lift in the pipework after the steam trap.

4.0 Controlling steam heat exchangers



Controlling steam heat exchangers

Secondary flow out

Secondary flow in

Control valve

Steam in

Heat exchanger

Vacuum breaker

Steam trap

Controller

Condensate dischargeagainst a lift or backpressure

Such variable loads can occur in batch industries like food, brewing, pharmaceuticals and fine chemicals. In a hospital, a heat exchanger for space heating may be rated to keep the wards warm in the coldest winter, so it spends much of its life running at relatively low loads. The load on an exchanger serving a domestic hot water system will also vary according to demand.

The best protection against stalling heat exchangers is prevention by good system design and by fitting measures to solve the problem. For example, a vacuum breaker could be fitted, but this works by allowing air into the system, which increases the risk of corrosion. A better solution is to fit an automatic pump trap to ensure condensate is always cleared under even the most demanding conditions – see panel “Automatic pump traps solve the issue of heat exchanger stalling.”


Automatic pump traps from Spirax Sarco use plant steam to provide the motive power to pump out condensate. Condensate enters the trapping chamber and, if there is no back pressure, it flows freely through the chamber and into the condensate return system.

However, if back pressure prevents the condensate from leaving normally, the pump trap’s condensate outlet closes. Condensate continues to flow into and fill the chamber. A mechanical float rises with the condensate level until a snap action mechanism opens a steam inlet valve. The resulting steam pressure in the chamber forces out the condensate and the float falls until it re-engages the pump mechanism. This closes the steam inlet and the cycle is repeated.

Automatic pump traps solve the issue of heat exchanger stalling

Spirax Sarco offers a range of heat transfer solutions.

5.1 Plate and shell heat exchangers (PSHE)The PSHE range of heat exchangers provides high heat transfer rates with a high pressure and temperature operating range. They have no gaskets, are compact, low fouling and can operate at close approach temperatures.

The PSHE has a very wide operating range with capacities of up to 100 MW, and can operate at pressures up to 100 bar and temperatures up to 400°C.

5.2 Exhaust vapour condensers (EVC)The Spirax Sarco EVC, based on the Turflow heat exchanger, is a compact, corrugated tube heat exchanger and uses flash steam from discharge and exhaust vent pipework to pre-heat make-up or process water thereby recovering valuable heat energy that would otherwise be lost to atmosphere.

The heat-exchange surface is made up of straight corrugated tubes designed to generate turbulent flows in low-viscosity fluids.

Exhaust

Condensateinlet

Condensateoutlet

Exhaust

Condensateinlet

Condensateoutlet

5.0 Spirax Sarco heat transfer solutions



5.3 EasiHeat™ heat transfer systemsThe core business of most steam system operators is not, in fact, steam. For users such as hospitals, steam is simply a way to heat their premises and generate the domestic hot water they need. The straightforward approach of the EasiHeat is ideal for this type of user.

Spirax Sarco EasiHeat systems are built around compact plate heat exchangers and are supplied skid-mounted, complete with everything they need to work as efficiently as possible. Buying a complete system saves time and effort during the specification and design stages of a new installation, as well as ensuring that all the components are designed to work optimally together. Offsite, factory construction

and quality testing also translate into minimal onsite disruption and rapid commissioning.

For users who might previously have relied on shell and tube exchangers, the ability of EasiHeat systems to deliver heating and hot water on-demand and without the need for hot water storage also offers several advantages:• Ensuring a reliable supply of hot water at all times

– including challenging peak demand periods• Eliminating hot water storage promotes significant

energy savings through reduced heat losses• Eliminating hot water storage also helps protect

against the possibility of a health and safety issue with Legionella

• St George’s Hospital in Tooting is saving £45,000 per year by upgrading the heating and hot water systems in one of its plant rooms to Spirax Sarco EasiHeat systems. The savings arise from a combination of improved energy efficiency and reduced maintenance.

• Switching its domestic hot water and heating from shell-and-tube calorifiers to EasiHeat systems has delivered energy savings of at least 15% at Aberdeen Royal Infirmary. Replacing the hospital’s ageing shell and tube systems has also saved up to three weeks of maintenance work each year, since the old systems had to be stripped down for regular insurance inspections.

• Catalent Pharma Solutions has been gradually replacing its old heat exchangers over the past five years, and now has nine EasiHeat systems in place. Spirax Sarco calculated that Catalent could save £6,500 a year in energy costs by replacing a single 540 kW calorifier with an EasiHeat.

• A single EasiHeat system replaced two bulky shell and tube heat exchangers at Murex Biotech’s site in Dartford, which produces diagnostic tests designed to protect the blood supply from infectious agents. The change delivered reduced maintenance and more space in the plant room.

• Chocolate manufacturer Bendicks (Mayfair) Ltd. now has an unlimited supply of hot water for washing down its equipment, thanks to the installation of an EasiHeat system. The new system has eliminated the disruptions that were caused by an unreliable hot water supply.

EasiHeat systems: straightforward solutions


5.4 Related systems support effective heat transfer The Spirax Intelligent Monitoring System (SIMS™) is a control platform that enables EasiHeat performance monitoring, diagnostics and communications. The technology delivers meaningful energy management and system performance data to the user via a mobile device (SMS or e-mail), remotely over the Internet, or through compatibility with existing onsite communication systems such as Ethernet, BACnet, Modbus, Profibus, CANopen, EtherCAT, DeviceNet and others.

FREME (Flash Recovery Energy Management Equipment) is an innovative packaged system that recovers all the energy in condensate and flash steam and uses it to preheat the feedwater to the boiler. FREME is a closed, pressurised system that enables returned condensate to be fed into the boiler at much higher temperatures than a conventional system that is open to atmosphere. This reduces the amount of work the boiler needs to do to raise steam, reducing energy costs considerably.

A Spirax Sarco flash steam recovery system is saving Cambridgeshire-based Jardin Corrugated Cases nearly £40,000 per year and has reduced the company’s CO2 emissions by 282 tonnes per year.

In its ongoing efforts to save energy and become a more environmentally-friendly operation, Jardin Corrugated Cases decided to tackle an issue it had with flash steam escaping from its 11,000 m2 manufacturing site in Ely. Not only did the escaping plume of steam waste energy, but it potentially created a poor environmental image.

Spirax Sarco was called in to help and designed an advanced flash steam recovery system, which it then supplied, installed, commissioned and project managed.

At the heart of the new installation is a Spirax Sarco FREME system that ensures all the usable heat in the condensate from the steam system can be returned to the boiler.

Spirax Sarco also provided automatic boiler blowdown controls with full heat recovery, boiler feedtank insulation and a packaged pump system to ensure all condensate from the main corrugator is returned for recovery.

As well as the savings in energy and carbon dioxide emissions, the system is saving water and water treatment chemical costs for Jardin Corrugated Cases and has eliminated the need for boiler operators to manually blow down the boilers each day saving manpower resources.

Corrugated cardboard maker recovers flash steam for £40,000 per year cost saving



Energy transfer is one of the fundamental applications underpinning many industrial processes, as well as providing building services such as heating and hot water. Steam is the most widespread and cost-effective medium for transporting heat around many facilities, and the latest heat exchange technologies are significantly improving energy efficiency, raising the reliability of hot water supply, reducing maintenance and protecting against Legionella.

Most heat exchangers that use steam as the primary heating fluid are sturdy and reliable, although problems can arise if a lack of understanding leads to poor system design or incorrect specification. This may be a particular issue for steam users who do not have a high level of steam expertise within their organisation, which is often the case where steam systems are outside the scope of their core activities. Hospitals are a good example.

Spirax Sarco is the world leader in steam and energy solutions. We can support steam users and help them optimise their systems to achieve the best possible energy and process efficiencies.

Find out moreTo find out more about Spirax Sarco heat transfer solutions:

e: [email protected]: www.spiraxsarco.com/ukt: 01242 521361

6.0 Conclusion

Charlton House, Cheltenham,Gloucestershire GL53 8ERTel: 01242 521361 Fax: 01242 573342 E: [email protected]/uk



Documents

Heat Transfer White Paper