By Benjamin Brits
In the increasing equation of efficiency improvement and integrated building or facility services, making use of every bit of energy will be the order of the day in the near future.
The application of heat exchangers has grown substantially over recent years as well as their genius-deployment to harvest and offset energy towards other needs from several sources. The most known example today is the use of these to supplement water storage for domestic use or cleaning applications that was historically “wasted energy” or “waste heat” commonly referred to in the sector from the plant/equipment cycle. But their potential is much further reaching today – including the creation of novel technologies.
A heat exchanger, in simple terms, is a heat transfer device that exchanges heat (energy) between two or more fluids (generally – from a process function) and has widespread industrial and domestic applications. Many types of heat exchangers have been developed over time for use in energy, chemical processing plants, building heat and air conditioning, transportation, electronics and refrigeration systems, to name but a few.
The “actual design” of a heat exchanger can be a complicated matter and generally involves more than heat-transfer analysis alone. The cost of fabrication and installation, materials, weight and size all play important roles in the selection of the final design from a total cost of ownership point of view – which has become an important factor for clients in today’s environment. In many cases too, although cost is an important consideration, footprint often tends to be the most significant factor in choosing a design as allocated plant areas become smaller and smaller.
Historical information suggests that the most basic examples of heat exchanging devices were found in Europe from the Roman era, where systems referred to today as “recuperators” were used in baths. It is also stated that around the turn of the first millennium, the principles for indirect heat exchange were used in the production of pure alcohol, which still is the major basis of heat exchangers now.
From a formal documentation perspective, heat exchangers can be traced back to around the 1880s, where their major applications were in the food and beverage industries. Interestingly, the first recorded patent for a plate heat exchanger was awarded in 1878, however, the first modern and commercial examples of heat exchangers would only be observed from the early 1900s. This was when the first known designs for tubular heat exchangers was developed catering to the operational demands from power plants, and used as condensers, as well as feedwater heaters (both of which are still common applications for these functions).
Over the time between the late 1880s and 1920s, several developments were made for plate heat exchangers which resulted in many patents covering its different forms, particularly their use in the pasteurisation of milk. As the years passed and technology was improved, heat exchangers started being made out of several different raw materials to accommodate such things as more corrosive media and conditions. Improvements continued through the 1960s with a better understanding of viscous flows and plate exchangers were first introduced to the oil and gas industry during this time. Reports further state that the 1960s also saw the rapid development in the fields of space technology, industrial activity and sciences – combined with improved developments in production techniques, and so heat exchanger manufacturing improved to supply to a variety of applications as well as size requirements.
Today we find that the possibilities with a fluid’s thermal state, be it hot or cold, allow for a vast range of solutions in the world of heat exchangers (HEX) – from simple scenarios to complex processing plants with multiple circuits, and even industrial processing feeding large district systems as seen in several countries around the world.
The technology and design methodology in the various HEX types has therefore continued to develop to offer appropriate solutions for many applications – even insofar as the use of magnets to actively change the geometry inside the exchanger which can produce significant improvement and control of liquid flow or distribution in micro-channel forms. Here a lot of research and development has been pursued at university-levels in the US and UK owing to the growth of these ranges.
In South Africa, a unique HEX design was developed over the Covid pandemic period for the storage of vaccines at ultra-low temperatures and charged with helium gas that can operate independently from any power source – thus making it a perfect stand-alone solution that can also be used in remote areas and easy air freight as light weight storage per capacity. This technology was covered extensively in Cold Link Africa at the time.
Based on the rising demand from various industries and along with an increasing focus on improving efficiency standards, international analysis shows that it is expected that the HEX market will continue to grow exponentially over the next years.
Much of this growth is owed to the favourable conditions in several emerging economies that aim to establish or expand on their manufacturing capabilities – as well as the expected boom in renewable power systems and electric vehicles. These are however only certain segments of growth – much is happening in the micro-technology arena as well as developments with electronics that aim to either manage or harvest even the smallest amounts of energy to control heat load and even create micro-energy-generation systems.
This all has led to the fact that technological advancements coupled with constant efforts and investments into product innovation, and research & development, are expected to increase competitiveness over the coming years for manufacturers. The large-scale adoption of some of the novel techniques in the production of heat exchangers is likely to further complement the expected growth.
Understanding basic principles
The basic rule in heat transfer theory is that heat will always be transferred from a hot medium to a cold medium, until equilibrium is reached. There needs to be a temperature difference between the two mediums for heat transfer to take place and the heat “lost” by the hot medium is equal to the amount of heat gained by the cold medium, except in cases of losses to the surroundings. To continually use this transferred energy between two mediums, heat exchangers are used.
The two main types of devices are direct and indirect HEX. In a direct HEX, both mediums are in direct contact with each other and with the indirect HEX both mediums are separated by a barrier through which heat is then transferred.
Outside of a HEX, the predominant heat transfer means is air while internally it ranges from water, with or without additives, to natural and synthetic refrigerants, steam and even oils, to name the most common.
All air-conditioning and refrigeration applications make use of HEX. When determining the right HEX per application, there are a number of factors to consider. Some of the most common factors include:
- flow rates
- maximum pressure inside of the HEX
- pressure drop
- temperature parameters
- system pressures
- liquid viscosity and concentration
- system upset conditions (start-up/shut-down)
- space availability
- future expansion plans
- life cycle costs
- maintenance requirements
Also, consideration needs to be given to whether the application will endure continuous or cyclical conditions. All of the various HEX options typically have different detail requirements, as well as preferable refrigerants per application. Each HEX type is therefore designed to perform under a particular application’s criteria.
HEX sizing is also a function of this application, and this point particularly affects every aspect of a HEX coil. Other crucial considerations not already mentioned include tube sizing, fin spacing and the overall construction methodology. As part of the initial design choices, different material types also suit different environments. Typical commercial applications will utilise copper tube and aluminium fins, while NH₃ and CO₂ plants will use stainless steel tubing with aluminium fins.
Avoidance of dissimilar metals in the HEX unit also removes risks such as galvanic corrosion in applications like marine vessels. Offering the multiple material alternatives allows manufacturers to provide the market with a HEX suitable for a wide range of conditions and satisfying all specifications.
Some of the most important factors considered for an efficient heat exchanger solution are the overall heat transfer coefficient; pressure drop across the plates and material of construction as mentioned. Overall heat transfer coefficient is a measure of resistance to heat flow. The resistance is caused by the plate material, fouling nature of fluids and type of HEX. Pressure drop (∆P) is the price paid for high heat transfer. The higher ∆P, the higher turbulence and the thinner laminar film, resulting into an efficient heat transfer and a compact heat exchanger.
A balance between a compact unit with smaller surface area and electricity cost need to be worked out since the higher ∆P gives a higher pumping cost. It is also very important to select a compatible material for the application. Plates unlike tubes, are made of a thin material with no allowance for erosion or corrosion. The selection of heat exchanger can either be fusion bonded or semi-welded heat exchangers, depending on the type of a coolant or refrigerant, capacity and fatigue sensibility.
The biggest challenge of designing an efficient heat exchanger as noted by manufacturers is to improve the medium’s flow, at the same time to optimise the pressure drop utilisation and minimise the fouling.
Pressure drop and flow rates
Plant designers should know and use ‘real’ parameters, even if they will vary during the year (or day/operation cycles). Designing on the actual operating flow rate will ensure that the channel velocity and wall shear are kept high and that the pressure drop is fully utilised. Flow rate or temperature can be very different inside of the same unit. Designers should therefore include all of the different operating modes to enable suppliers to design the most appropriate unit to meet specifications.
Refrigerant flows are most readily calculated from a pH diagram at the relevant system conditions. This method enables the mass flow required to be determined simply and quickly while pressure drops are highly design-dependent and require the use of dedicated correlations to suit each application.
System limits are also a factor and when suppliers know any such limits, they are able to include them in evaluations. There must be a system balance to ensure an efficient cooling or heating solution is delivered. Other factors in addition to pressure drop are the type of refrigerant, capacity and the use/application of refrigerant. Pump capability is also important and plays a role in pressure drop too.
Maintenance and best practices
As is common in South Africa (and many other countries), maintenance regimes are typically an afterthought, however every site will need some form of maintenance to ensure the proper continuing functioning of the plant and sub-systems. Proper maintenance guidelines and horror maintenance stories could cover vast articles, but the reality is that the time for neglect – which essentially translates into waste – is fast reaching its end as facility managers and owners place increasing value onto efficiency themselves, or, will in the near future be mandated to comply with certain energy efficiency standards.
This becomes quite impactful with HEX units, particularly fin and tube designs, that are by far the most commonly deployed type in the region and also form part of several unitary ranges as well as air handling units.
As this journal has reported on in quite a few prior issues to date, cleaning of coils has become imperative and with the advancement of cleaning solutions and environmentally friendly chemicals – the use of water can also be reduced significantly thus producing more savings or facility efficiency.
In the case of coil HEX units, monthly visual inspections of the units should be carried out to check that there are no line blockages and all defrost processes are working. Water build-up in drip trays can cause spillages and product damage as well as becoming a health and safety risk. If units do not defrost properly, performance will definitely deteriorate sharply and in certain cases and conditions can result in irreparable damage.
It is recommended that every three to six months units should be cleaned, all electrical connections checked and tightened, motor integrity checked, fan blades inspected for abnormal wear patterns and insulation integrity confirmed. In the plant room, all electrical and mechanical equipment should be checked and all preventative maintenance carried out timeously.
Serious problems can often be avoided if routine checks are done and any abnormal system behaviours are identified, correctly diagnosed and rectified. This may also include damage to any heat exchanger or cracks thus forming leaks. It is recommended that damaged HEX be replaced timeously as associated risks for overall plant equipment, users and related persons may be high.
In cases of plate-type HEX units, general cleaning can be carried out in two ways. Either by removing the pack and mechanically clean each plates, or through the clean in place (CIP) process which is the circulation of an appropriate chemical in the HEX to remove any fouling. Both methods are effective when carried out correctly. Manufacturers’ first calls would be to try and design a unit to minimise any need for maintenance.
Common design errors
Common issues seen from a supplier’s point of view include incorrect pipe sizing, incorrect expansion valve sizing and fitment (especially the bulbs), incorrect placement of coils (for example fitted above doorways) or too close to walls. Another critical consideration in unit performance is the air distribution in a room which is closely linked to fan selection and coil placement.
Other experiences related to design mistakes start with the “actual” versus the “designed” parameters where this mostly results in an over specified system because of high margins added at a feed stage. This includes provision for future expansions. If suppliers have all the information up front, they can size the HEX very accurately with the product of less problems.
Another major design error is the flaw of assuming fluids have the same properties or will act like water. A simple example is a brine solution, although this may look similar to water, its thermal properties are vastly different. It is important to provide information on the fluids to ensure that the correct physical properties are used and compatible material is selected.
Trends and new technology
Product footprint continues to be a significant factor as allocated plant room space is just far smaller than what was seen in previous eras owing to the nature of facility aesthetics, but also general building and specialist costs associated with projects. Designers therefore are able to specify several ranges of products to maximise space utilisation on sites. These could be in the form of dedicated containerised plant rooms or outdoor racks with the addition of integrated condensers which provide clients flexibility and cost benefits against the rising building costs traditionally relied on.
System refrigerant charge is another notable trend that design teams have focused on while taking a wider view, trends have moved towards customers requiring more end-to-end solutions that minimise points of responsibility and localise all project elements. This approach means that each individual design decision and implementation is made with cognisance of its impact on the whole project, and therefore the best overall outcomes can be achieved while limiting expenses and errors.
As can be expected, certain HEX types have grown in popular in a variety of applications due to the potentially lower purchasing cost or their performance above other solutions. These products, such as plate heat exchangers, have new features that ensure higher heat transfer efficiency, better reliability and greater serviceability.
“Certain specialist suppliers are also setting new standards for thermal efficiency with their products.”
Certain specialist suppliers are also setting new standards for thermal efficiency with their products, raising the bar when it comes to producing reliable, serviceable and energy efficient HEX technology for their customers. This means that results are bold and innovative new features and technical concepts that have not been seen before are being released.
These solutions are now available in the world’s most modern gasket plate heat exchangers and the smallest of design change has significant results. This could be improvements as an increased thermal surface area which can boost efficiency across the whole plate pack. In some cases, it even means that fewer plates are required per pack without losing out on performance.
New designs also incorporate asymmetric channels that optimize thermal efficiency, resulting in up to 30% higher efficiency and reduced fouling. In other words, it helps provide long-term, cost-effective, and quality performance.
Further innovations come in the form of plate patterns which improves flow and distribution, so the risk of fouling and unplanned downtime is considerably reduced. This saves customers on time, energy and maintenance costs. Moreover, new welding techniques and flow assistance designs have been developed.
CO2 systems in HEX industry
By Marius La Grange
In the last 10 years the “CO2 Equator” has shrunk dramatically thanks to ever increasing operating pressures and new components becoming freely available to the commercial refrigeration sector. The “CO2 Equator” is significantly smaller than before with trans-critical systems capable of operation in extremely warm ambient temperatures at the cost of a lower COP.
The operating COP thus being a deciding factor and much ongoing development going towards improving all important COPs at extreme ambient conditions. The competitive COP’s trans-critical systems being a driving factor towards the rapid application rate. Fewer discussions revolve around COP, but the installation cost remains higher for R744 trans-critical systems when compared to similar HFC applications.
Many retailers in SA have made the switch to trans-critical systems for new installation and this clearly indicates that the operational benefits outweigh the premium in most applications. The very competitive retail sector being a fertile soil for innovation and application trails, and of course a big consumer of energy.
To newcomers, we first need to answer the question “what is a trans-critical” refrigeration system?” Figure 1 will assist in order to get a better grasp of these applications. CO2 or R744 has a comparably low critical temperature of 31°C. The critical temperature for most HCFC & HFC refrigerants being 30 to 60°C warmer. R22 has critical temperature of 96°C and for R404A if is 72°C. So, what does this mean? If you needed to condense the discharge gas from a R404A system and the ambient air available (the cooling medium) was 48°C (much lower than the critical temperature for R404A of 72°C) it is hopefully immediately apparent that it is feasible since the ambient air is very warm to any human but significantly cooler than the refrigerant’s critical temperature. The same would not be true for an R744 system.
Rejecting heat energy and condensing the R744 would not be possible with ambient air of 48°C in a similar process. This is the great problem solved in the last 20 years with the development of trans-critical technologies. One being that traditional air-cooled condenser was developed into a device capable of much higher operating pressures. Hence from a distance they seem very similar to traditional air-cooled condensers.
In case you are not familiar with concept of the critical point – it is the temperature above which it is not possible to liquefy (condense) the refrigerant. Why is this relevant? The short answer is that a refrigeration expansion process requires the refrigerant at high pressures and in a liquid state. Heat absorption/cooling (with the refrigerant gaining enthalpy/boiling off) takes place with the liquid refrigerant changing phase and “evaporating” as the main purpose for having a refrigeration installation offering the required cooling effect with vapour drawn back the system’s compressor.
The aspect of CO2 gas cooler design, function and installation will be covered in a technical article in the following issue of RACA Journal.
- Alfa Laval
- Food Processing History
- HC Heat Exchangers
- Johnson Controls
- Market Reports
- Oak Ridge National Laboratory
- Science Direct