Heat Transfer White Paper

www.spiraxsarco.com/uk

Heat Transfer Modern solutions for

optimum efciency

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

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Contents 1.0

Executive Summary

2.0

Effective heat transfer is at the heart of almost every process

3.0

Types of heat exchanger

4.0

5.0

6.0

E X P E R T I S E

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

Controlling steam heat exchangers 4.1

Condensate control maximises heat transfer

4.2

Tackling heat exchanger stalling

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

Conclusion

S O L U T I O N S



1.0

Executive Summary

Effective energy transfer is a basic prerequisite for

However, it’s important to have a sound grasp of the

the success of many industrial processes, as well as

basics of energy transfer and steam plant operation

underpinning building services for space heating and

in order to specify systems correctly and prevent

hot water. The foundation of most energy transfer

problems cropping up later. In the case of users for

applications in industry is the heat exchanger.

whom steam lies outside the scope of their core activities, bringing in expertise from external steam

Steam is the most efcient and exible energy transfer

specialists can help to ensure that their heat transfer

medium and there is a range of heat exchanger

installations provide optimised, reliable service for

technologies available that can provide reliable

years to come.

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.

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Effective heat transfer is at the heart of almost every process

Whether energy is needed for an industrial process or

Meanwhile, effective control enables the thermal

for space heating, effective energy transfer underpins

energy transferred to vary exibly to match the

the operations of almost every organisation.

changing demands of the process. Not only does this link closely with energy efciency, but it can also affect

At its most basic, heat transfer is concerned with two

production efciency by impacting on parameters

things: temperature and the ow of thermal energy

such as product quality and waste.

from a heat source to a heat sink. On top of this, there are two requirements that any successful heat

Thermal energy can be transferred via convection

transfer technology must provide: efciency and

(e.g. air currents), radiation (e.g. from a ame or

control.

other radiant element) and conduction (e.g. from steam to hot water via a pipe wall or plate). Of these,

Energy efciency determines what proportion of

conduction is the most widespread and versatile

the energy entering the process ends up where it’s

approach and can be applied using a number of

needed. It’s key to minimising the carbon footprint of

different heat exchanger technologies.

the process and reducing operating costs.

Heat transfer: the basics The heat transfer coefcient is the proportionality coefcient between the heat ux (the rate of transfer of heat across a surface) and the thermodynamic driving force for the ow of heat (i.e. the temperature difference). h=

q ΔT

where h : heat transfer coefcient (W/m2.oC) q : heat ux (W/m2) ΔT : difference in temperature between the heat source and sink (oC) The best heat exchangers will have a high heat transfer coefcient determined by several factors, including the construction material and the ow regime of the uids.

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Steam: an ideal energy transfer medium 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 efcient. 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 exibility to use the fuel supply of their choice to match their needs. Water has a very high specic heat capacity, making steam one of the most efcient energycarrying uids, 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 lling 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 i s environmentally-friendly. It’s also exible. 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.

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3.0

Types of heat exchanger

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

3.1

Shell and tube

Shell and tube heat exchangers (or caloriers) used

The minimum practical temperature difference

to be the norm in most applications and are still very

achieved in a typical shell and tube exchanger by the

common in heavy industries such as oil, gas and

time the uids exit the unit (known as the approach

petrochemicals, where they are well-suited to high-

temperature) will not usually be less than 5°C.

pressure applications. They are also used widely in hospitals to provide heating and domestic hot

The shell is typically pressurised and will therefore

water services. However, they are gradually being

require an annual insurance inspection involving

superseded by more compact and energy efcient

a comprehensive and time-consuming strip-down.

alternatives.

This also applies to all the related technologies listed below that include a pressurised shell.

The shell is a pressure vessel containing a bundle of tubes. One uid ows through the shell and the other through the tubes. Each uid enters the exchanger at a different temperature and heat passes between them through the tube walls as they ow through the process. Secondary fluid out Steam in

Tube bundle

Condensate out

Secondary fluid in Shell and tube

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There is no pressurised shell and the volume of liquid

Standard plate heat exchangers (PHEs) are

in a PHE is very low, so this is one type of exchanger

increasingly popular in applications that transfer heat

that does not generally require an annual insurance

between medium- and low-pressure uids, although

inspection as long as it is tted with a spray/

more specialised welded, semi-welded and brazed

splash guard as the system is self-relieving, which

versions can be used with high-pressure uids.

signicantly reduces downtime and maintenance

3.2

costs. In place of tubes passing through a shell, PHEs are built from a series of corrugated metal plates that are

3.3


held together to form channels through which the two

A plate and shell heat exchanger combines PHE and

heat transfer uids ow in alternating layers of the

shell and tube heat exchanger technologies.

‘sandwich’. A standard plate stack is spaced using a series of gaskets, which are usually the limiting factor

The heart of the exchanger contains a fully welded

when it comes to high-temperature/high-pressure

circular plate pack, with nozzles carrying ow in and

applications.

out (the ‘plate side’ ow path). This assembly sits in an outer shell that creates a second ow path (the

The plates produce an extremely large surface

‘shell side’).

area relative to physical size, which promotes very effective heat transfer. Each chamber is only a few

This plate and shell combination offers high heat

millimetres across so the majority of the volume of

transfer, compact size, low fouling and a close

each liquid contacts the plate. The corrugated troughs

approach temperature. It is also able to cope with high

also promote turbulent ow. Both of these features

pressures and temperatures without leaking, thanks

encourage greater heat transfer, even at low ow

to the all-welded plate pack that eliminates the use

rates, and help prevent fouling.

of gaskets that may leak under high pressure. As this type of heat exchanger includes a pressurised vessel,

The approach temperature achievable with PHEs can

it would typically need annual inspections.

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 efciency by as much as 6% compared to shell and tube heat exchangers. Sub-cooling of the condensate can increase energy efciency even further.



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These are a variation on traditional shell and tube exchangers. They have corrugated tubes to create Service

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 Product

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 efciency and can reduce maintenance costs. 3.5


Shell and coil heat exchangers are built from circular layers of helically corrugated tubes inside a compact shell. The uid in each layer ows in the opposite direction to the layer surrounding it, producing a criss-cross pattern. The large number of closely packed tubes creates a signicant heat transfer surface, while the alternate layers create a swift uniform heating of uids and increase the total heat transfer coefcient. The corrugated tubes produce a turbulent ow, which improves heat transfer and resists fouling. This helps to maintain the heat exchanger’s efciency and can reduce maintenance costs. As this type of heat exchanger includes a pressurised vessel it would typically need annual i nspections.

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3.6


Vacuum tube

Heat Pipes work on a di fferent principle from the other heat exchange technologies already discussed. However, recent advances in their design and

Heat out

manufacture mean that they are starting to make inroads into certain applications, such as heat Condensation

recovery from exhaust gases. Heat Pipes are sealed vacuum tubes with one end in

Separation plate Evaporation

the ‘hot’ stream and other in the ‘cold’ stream. They contain a working uid, and it’s the constant cycle of evaporation and condensation as the working uid moves around the sealed tube that transfers thermal

Heat in

energy from one stream to the other. Heat transfer uid

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 uid evaporates and rises to the top of the tube, which sits in the cold stream. The working uid then condenses, giving up heat to the cold stream and running back down the vacuum tube to begin the cycle again. The working uid is chosen to suit the temperature range for the particular application. The big advantage of heat pipes is their great efciency 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.


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Controlling steam heat exchangers

With the exception of heat pipes, all the different types

Condensate control also cannot be used in

of exchanger described are most often controlled

applications with quickly varying demand, such as

in a similar way, by sensing the temperature of the

domestic hot water (DHW) systems. It is more suited

secondary uid (often water) emerging from the unit

to applications that require a small rate of change of

and using a valve to modulate the primary uid (the

ow such as Low Temperature Hot Water (LTHW).

incoming ow of steam) to the exchanger accordingly.

Also, demand on the secondary side should never fall below 20% of the design ow conditions.

A fully independent high-limit cut-out should be tted in accordance with Health & Safety Executive

Heat exchangers controlled in this way generally offer

recommendations to protect people or equipment

reliable, trouble-free operation, but there are some

should the water temperature exceed a set l imit.

issues common to all types of heat exchanger that operators should look out for.

The transfer of energy from the steam routinely

Tackling heat exchanger stalling

results in the generation of condensate in the heat

4.2

exchanger, and under normal operations this is

Heat exchangers of any type can stall when the

removed via a steam trap.

condensate is not removed effectively and builds up internally. Typical symptoms of heat exchanger

Condensate control maximises heat transfer

ooding include banging and crashing noises coming

An alternative control method to modulating the steam

waterhammer. Plate heat exchangers are more

ow into the heat exchanger is condensate control,

resilient with only a modest risk of waterhammer

which keeps the input steam pressure constant and

occurring. Other symptoms could include erratic

instead adjusts the ow of condensate coming out of

temperature control and corrosion caused by

the exchanger. This varies the amount of condensate

condensate collecting inside the unit, leading to leaks.

4.1

from shell and tube heat exchangers caused by

inside the exchanger to control its heat transfer area and hence its heat transfer rate.

Stalling happens when the pressure in the heat exchanger is less than or equal to the back pressure

Condensate control allows the condensate to be

on the steam trap, often occurring when demand

maintained at a sub-cooled temperature of 95°C to

from the heating process falls due to a change of ow

extract the maximum amount of useful heat from the

rate. When this happens, the control valve reduces

steam and avoids any potential ash steam plumes.

the steam pressure accordingly and this may reach a level that’s too low for the steam trap to clear

Plate and frame, vertical shell and coil, and plate

the condensate effectively. The risk of stalling is

and shell heat exchangers can all be used with

increased when the condensate is discharged against

condensate control. However, shell and tube heat

a lift in the pipework after the steam trap.

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.

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Controller

Control valve

Secondary flow out

Vacuum breaker

Steam in

Condensate discharge against a lift or backpressure

Heat exchanger

Secondary flow in

Steam trap

Controlling steam heat exchangers

Such variable loads can occur in batch industries like

The best protection against stalling heat exchangers

food, brewing, pharmaceuticals and ne chemicals. In

is prevention by good system design and by tting

a hospital, a heat exchanger for space heating may

measures to solve the problem. For example, a

be rated to keep the wards warm in the coldest winter,

vacuum breaker could be tted, but this works by

so it spends much of its life running at relatively low

allowing air into the system, which increases the risk

loads. The load on an exchanger serving a domestic

of corrosion. A better solution is to t an automatic

hot water system will also vary according to demand.

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.”

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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 ows 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 ow into and ll the chamber. A mechanical oat 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 oat falls until it reengages the pump mechanism. This closes the steam inlet and the cycle is repeated.

Exhaust

Exhaust

Condensate

Condensate

inlet

inlet

Condensate

Condensate

outlet

outlet

5.0

Spirax Sarco heat transfer solutions

Spirax Sarco offers a range of heat transfer solutions.

5.2

Exhaust vapour condensers (EVC)

The Spirax Sarco EVC, based on the Turow heat 5.1

Plate and shell heat exchangers (PSHE)

exchanger, is a compact, corrugated tube heat

The PSHE range of heat exchangers provides

exchanger and uses ash steam from discharge

high heat transfer rates with a high pressure and

and exhaust vent pipework to pre-heat make-up

temperature operating range. They have no gaskets,

or process water thereby recovering valuable

are compact, low fouling and can operate at close

heat energy that would otherwise be lost to

approach temperatures.

atmosphere.

The PSHE has a very wide operating range with

The heat-exchange surface is made up of straight

capacities of up to 100 MW, and can operate at

corrugated tubes designed to generate turbulent

pressures up to 100 bar and temperatures up to 400°C.

ows in low-viscosity uids.

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5.3

EasiHeat™ heat transfer systems

The core business of most steam system operators is

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

not, in fact, steam. For users such as hospitals, steam is simply a way to heat their premises and generate

For users who might previously have relied on shell

the domestic hot water they need. The straightforward

and tube exchangers, the ability of EasiHeat systems

approach of the EasiHeat is ideal for this type of user.

to deliver heating and hot water on-demand and without the need for hot water storage also offers

Spirax Sarco EasiHeat systems are built around

several advantages:

compact plate heat exchangers and are supplied

•

– including challenging peak demand periods

skid-mounted, complete with everything they need to work as efciently as possible. Buying a complete

•

Eliminating hot water storage promotes signicant energy savings through reduced heat losses

system saves time and effort during the specication and design stages of a new installation, as well as

Ensuring a reliable supply of hot water at all times

•

Eliminating hot water storage also helps protect

ensuring that all the components are designed to

against the possibility of a health and safety issue

work optimally together. Offsite, factory construction

with Legionella

EasiHeat systems: straightforward solutions • 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 efciency and reduced maintenance. • Switching its domestic hot water and heating from shell-and-tube caloriers to EasiHeat systems has delivered energy savings of at least 15% at Aberdeen Royal Inrmary. 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 ve 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 calorier 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.

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Related systems support effective heat transfer 5.4

The Spirax Intelligent Monitoring System (SIMS™) is

FREME (Flash Recovery Energy Management

a control platform that enables EasiHeat performance

Equipment) is an innovative packaged system that

monitoring, diagnostics and communications. The

recovers all the energy in condensate and ash steam

technology delivers meaningful energy management

and uses it to preheat the feedwater to the boiler.

and system performance data to the user via a

FREME is a closed, pressurised system that enables

mobile device (SMS or e-mail), remotely over the

returned condensate to be fed into the boiler at much

Internet, or through compatibility with existing onsite

higher temperatures than a conventional system that

communication systems such as Ethernet, BACnet,

is open to atmosphere. This reduces the amount of

Modbus, Probus, CANopen, EtherCAT, DeviceNet

work the boiler needs to do to raise steam, reducing

and others.

energy costs considerably.

Corrugated cardboard maker recovers ash steam for £40,000 per year cost saving A Spirax Sarco ash 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 ash 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 ash 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 al l 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.

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6.0

Conclusion

Energy transfer is one of the fundamental

Spirax Sarco is the world leader i n steam and energy

applications underpinning many industrial

solutions. We can support steam users and help them

processes, as well as providing building services

optimise their systems to achieve the best possible

such as heating and hot water. Steam is the most

energy and process efciencies.

widespread and cost-effective medium for transporting heat around many facilities, and the

Find out more

latest heat exchange technologies are signicantly

To nd out more about Spirax Sarco heat

improving energy efciency, raising the reliability

transfer solutions:

of hot water supply, reducing maintenance and protecting against Legionella.

e: [email protected] w: www.spiraxsarco.com/uk

Most heat exchangers that use steam as the primary heating uid are sturdy and reliable, although problems can arise if a lack of understanding leads to poor system design or incorrect specication. 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.

t:

01242 521361


Charlton House, Cheltenham, Gloucestershire GL53 8ER Tel: 01242 521361 Fax: 01242 573342 E: [email protected] E X P E R T I S E

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