Steam is a highly effective medium for industrial heating and cooling,
but in order to get the best from a system, users need to devote
proper care and attention, reports Brian Attwood.
Steam heating has been a feature of industrial processes in the UK since James Watt and his collaborator Matthew Boulton pioneered its use for domestic and factory heating. Robertson Buchanan wrote the first book on the subject in 1810 and the UK and USA ensured its primacy in the workplace, not least after the invention of vacuum return steam heating.
It remains a key heat transfer medium and has significant advantages in many respects over fluid alternatives, says Matt Hale, international sales manager at HRS Heat Exchangers.
Plus points
“Steam is a very good heat transfer fluid due to the latent heat of condensation, it requires no pumping, [and] is usually at a higher temperature compared to water, so usually requires less heat transfer surface area,” says Hale.
Steam can also be employed in cooling. In previous decades, it was a widely used method for air conditioning – both in buildings and on railways – via steam jet cooling.
In an industrial context cooling provides a useful means of reusing steam produced for other applications. It can also be employed in vacuum steam, absorption chillers and quenching.
Consulting and engineering services (CES) manager for steam specialists TLV, Sakis Palavratzis, spells out the reasons why steam methods continue to play an indispensable part in so many industrial processes:
- It has a high energy per unit mass content
- It is relatively efficient and cost-effective to produce
- It will release heat at constant temperature
- It is clean and odourless
- It is easily controlled and distributed to point of use • It does not require pumps and three-way valves
- It remains environmentally friendly, being non-toxic, non-flammable and also sterile.
Despite these advantages, the usefulness of steam in industry has too often been overlooked. The situation has been exacerbated in recent years by the recruitment problems that have plagued British engineering.
While these problems of supply and demand affect the engineering sector generally (and, without remedy, will continue to do so for a decade), steam engineering’s lack of profile has been a contributing factor, reckons Palavratzis.
As more of the experienced workforce of engineers retired, the knowledge gap has widened, he warns: “Nowadays most sites are under-staffed and, when combined with a skill shortage, this creates conditions that will affect processes and their efficiency.
“In most cases the solution that many sites see is to outsource certain areas of responsibility to external specialist companies and work in partnership and trust to overcome this obstacle and stay competitive to a continuously changing and demanding environment.”
The main issue or challenge with steam systems today is that, in most cases, sites don’t pay too much attention to their steam and condensate systems due to insufficient resources, lack of engagement by employees, inadequate understanding of the potential benefits…
Sakis Palavratzis, consulting and engineering services manager, TLV
There is, he says, a frequent failure to perceive steam systems as an asset that needs to be properly managed.
“The main issue or challenge with steam systems today is that, in most cases, sites don’t pay too much attention to their steam and condensate systems due to insufficient resources, lack of engagement by employees, inadequate understanding of the potential benefits and challenges etc.”
Other methods may be employed – water and oil heating are, of course, widely used. Yet heat transfer from steam condensation has out-performed that achieved by convection from liquid mediums; sensible heat from the latter mediums cannot match the energy release achieved in a shorter time by the use of latent heat from a steam medium.
The smaller heat transfer surface area means lower equipment outlay, better temperature control, and greater productivity is possible using steam.
Steam used in heating may be provided as positive pressure steam, commonly used in food and drink and chemicals production, where it is provided at temperatures in excess of 100°C. Vacuum saturated steam has been increasingly employed for temperatures below this.
Given that steam can produce appreciable benefits, what is available and, if used, how can a system be maintained at maximum effectiveness without excessive reliance on troubleshooting and outsourcing?
Where heating systems are concerned, the main division is between direct and indirect steam heating.
Issues arising
The former mixes hot and cold streams – live steam injected into cold water via a temperature sensing control valve. The latter involves no direct contact between the two streams but the transfer of heat via a heat exchanger.
Comparing the characteristics of the two methods, says Palavratzis [pictured above], it must be noted that the direct system involves no condensate return, thus the volume of steam is added to the final mixture volume. It also utilises the total energy of steam used, both latent and sensible heat.
In the case of an indirect steam heating system, the process will produce condensate return. Most of the steam energy is used while the remainder – the sensible heat/energy within the condensate – can be recovered back to the boiler house, improving efficiency.
Whatever their virtues though, all systems, steam or otherwise, do present individual challenges, warns Hale.
“Condensate drainage has to be well designed to stop condensate flooding occurring in systems,” he says, adding that this problem can cause uneven thermal expansion across the tube bundle.
“Typically steam systems operate at a higher temperature, which can cause burning/fouling in high sugar or protein fluids.”
Steam systems can have high velocities inside the heat exchanger, which must be accounted for and designed correctly
Palavratzis is forthright about the need to be aware of the problems presented by condensate water and wet or poor steam quality that can compromise equipment, efficiency and health and safety.
He cites four other issues common to steam systems: poor insulation; wasted energy – usually through flash steam loss and wasted condensate; inadequate steam trap management; and poor design by piping contractors who lack steam system knowledge.
Hale shares the concern for ensuring proper design by experts in theis area.
“Steam systems can have high velocities inside the heat exchanger, which must be accounted for and designed correctly,” he cautions.
Uneven thermal expansion also needs to be countered with some basic care taken regarding the exchanger used.
“Mounting a shell and tube heat exchanger vertically stops this uneven thermal expansion and results in a much longer life for the unit,” advises Hale.
Checklist it
Palavratzis recommends a hit list to ensure system effectiveness. First, the corrective actions: insulate against heat losses, identify sources of steam leaks.
To put the latter in perspective he points out that a 1mm hole in a user’s steam main and a 2-inch isolation valve leaking steam could cost an operation £500 and £750 respectively each year Second, make efficient use of resources: “Condensate is valuable and should be recovered as it is hot and is already processed ready to be used again.”
Thirdly, take a strategic approach that ensures maximum efficiency becomes the norm, with problems identified before issues become risks and sources of unnecessary downtime and expense.
Palavratzis emphasises steam metering is an essential: “If you can’t measure, you can’t improve it.”
Likewise, steam trap management needs a monitoring programme that ensures sustainable asset management with appropriate action taken in a timely manner to avoid problems arising from leaking and blockages.
Steam-using equipment should be sourced from a reputable manufacturer and the installation should be carried out by experienced subcontractors
Sakis Palavratzis, consulting and engineering services manager, TLV
Finally, in order to ensure that company-wide overview, establish full system visibility.
“Steam-using equipment should be running with clean, dry steam supplied at the specified conditions. The equipment should be sourced from a reputable manufacturer and the installation should be carried out by experienced subcontractors,” advises Palavratzis.
“In the design stage the heating system should be accurately assessed for stalling conditions and, if found to operate in such a state, then an active condensate removal device should be installed to alleviate the issue.”
Even without the benefit of specialist assistance, regular maintenance of equipment to manufacturer recommendations and testing of procedures can provide substantial steps towards ensuring best practice.
Process Engineering asked Sakis Palavratzis for his choice of noteworthy innovations in steam technology for heating and cooling:
Pressure reducing valves with built-in strainer, separator and steam trap. In this way you combine essentially a complete pressure reducing station in one robust space-saving unit, simplifying system layout, pipping and maintenance.
Wireless continuous monitoring steam trap testing. Use of wireless ultrasonic trap testing modules, located at strategically selected critical applications to ensure continuous and reliable system operations. This avoids expensive downtime and energy loss, increasing system safety and process reliability.
Handheld steam trap diagnostic instrument and specialised software. These instruments compare ultrasonic and temperature readings with known laboratory data to determine whether a steam trap is operating as it should be. The major benefit of such an instrument is that it doesn’t rely on the surveyor’s objectivity and an accurate percentage of leaking steam can be identified. It is advisable to seek a devise which is certified by a reputable third party for its judgement accuracy.
Flow meter with dryness fraction measurement. In order to ensure optimal system performance, integrity and safety, it is highly recommended to use clean dry steam. Having a device which can measure the quantity and quality of the supplied steam can be a valuable tool in optimising your steam system and its performance by being able to respond in any changes in a timely manner.
Vacuum steam low temperature heating systems used at temperature ranges of 30 to 100°C, increasing productivity and product quality. Raising temperatures of sensitive mediums up to 100°C is usually done in two stages, utilising hot water brought to temperature with steam. Using vacuum steam (<1 bar abs.) is an innovative method that can offer precise control, reduced heating time by 25% or more, and even product heating in a more compact unit.
Thermal compressor units. Low pressure waste steam can be recovered and boosted up to a usable pressure using available high pressure site steam. The benefits of such a devise is that by keeping the low pressure steam in a low pressure (typically ~0.5barg) more flash can be generated and less back pressure is applied to the steam traps.
Low grade steam absorption chillers, utilising venting flash steam to satisfy site cooling requirements.
Steam system asset management programs and sustainable steam system optimisation programs. Based on best practices when done correctly, they can provide regular health checks and offer continuous improvement of the steam and condensate system.
