By Benjamin Brits
When managing or changing a fluid’s thermal state, the world of the heat exchanger is vast – with many solutions to fit.
The basic rules in heat transfer theory is that heat will always be transferred from a hot medium to a cold medium, until equilibrium is reached. There must always 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 for losses to the surroundings.
To continually transfer heat between two mediums, heat exchangers (HE) are used. Two main types of heat exchangers are the direct and indirect heat exchangers. In a direct heat exchanger, both mediums are in direct contact with each other and the indirect heat exchanger, both mediums are separated by a wall through which heat is then transferred.
Outside of a HE, 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.
{os-gal-152} Images credit: Danfoss
All air-conditioning and refrigeration applications make use of HEs and they extend to other industrial applications such as mining, power generation, military, transport and agricultural cooling.
To start, when determining the right HE per application, there are a number of factors to consider. “Some of the most common factors include flow rates, maximum pressure inside of the HE, pressure drop, temperature parameters, system pressures, liquid viscosity and concentration, system upset conditions (start up/shut down), space availability, expansion plans, life cycle costs and maintenance requirements. Also consideration needs to be given to whether the application will endure continuous or cyclical conditions,” says Zaur Kutelya, business development manager for the Danfoss Heating Team.
All of the various HE options typically have different detail requirements, as well as preferable refrigerants per application. Each HE type is therefore designed to perform under a particular application’s criteria.
HE sizing is also a function of this application, and this point particularly, affects every aspect of a HE 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. Variations to these combinations are designed to enhance the life and/or performance of the unit in its particular environment. The impact of global warming too has focused design considerations around a refrigerant’s ozone depletion potential (ODP) and global warming potential (GWP) values. These are helpful metrics but we also believe in the value of further considerations such as total equivalent warming impact (TEWI) calculations, to ensure the full system is the most sustainable solution possible,” adds Gert van Rooyen, product engineer at HC Heat Exchangers.
Further, avoidance of dissimilar metals in the HE unit removes risks such as galvanic corrosion in applications like marine vessels. Offering the multiple material alternatives allows manufacturers to provide the market with a HE suitable for a wide range of conditions, satisfying all specifications.
{os-gal-153} Images credit: Alfa Laval
“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 up by plate material, fouling nature of fluids and type of heat exchanger. 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 very important to select a compatible material for the application. Plates unlike the tube, are made of a thin material with no allowance for and 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. It all depends on the plate design, and the biggest challenge of designing an efficient plate heat exchanger is to improve the mediums flow, at the same time as you optimise the pressure drop utilisation and minimise the fouling,” explains Moses Modibela, heat exchanger product and application manager at Alfa Laval.
Pressure drop and flow rates
Plant designers should know and use ‘real’ parameters, even if they will vary during the year (or day). According to Kutelya, “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 include all of the different operating modes to enable the suppliers to design the most appropriate unit.”
Refrigerant flows are most readily calculated from a pH diagram at the relevant system conditions. Van Rooyen adds, “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 here and when we know any limits, we can work around them. 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,” notes Modibela.
Maintenance and best practices
Maintenance regimes are always site-specific, but every site will need some form of maintenance to ensure the proper continuing functioning of the plant. Proper maintenance guidelines would essentially fill several articles just like every topic we cover, but covering the basics the following can be considered.
“In the case of coil HEs, monthly visual inspections of the units should be carried out to check that there are no drain 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 sharply degrade, and if ice build-up isn’t stopped, it will cause irreparable damage to a unit. Every three to six months the 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. Serious problems can often be avoided if routine checks are done and any abnormal system behaviours are identified, correctly diagnosed and timeously rectified,” says van Rooyen.
Kutelya adds, “On our plate heat exchangers, general cleaning can be carried out in two ways. Either by removing the plate pack and mechanically clean the plates, or through the clean in place (CIP) process which is the circulation of an appropriate chemical in the HE to remove the fouling. Both methods are effective when carried out correctly. Another way to clean an HE when you have a mechanical blockage is to dismount the HE and take out the plates to clean with water, re-assembling again.”
“In these aspects, our first call would be to try and design a unit to minimise any need for maintenance. Our strategy is to use our global experience that exceeds more than 50 years and also get the user to pass onto us the type of challenges they have encountered with their existing system to enable us to see what design changes can be implemented or how the product can further be optimised to avoid or minimise failures in the future. This could be an efficiency challenge or a material type failure. Secondly training on any HE product is critical because most HEs look so simple, so users may attempt maintenance without being familiar with a particular unit, causing irreparable damage,” Modibela says.
{os-gal-154} Images credit: HC Heat Exchangers
Common design errors
“Common issues seen are 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. A further critical consideration in unit performance is the air distribution in a room which is closely linked to fan selection and coil placement,” says van Rooyen.
According to both Modibela and Kutelya, their experience related to design mistakes start with the actual versus the designed parameters. Modibela states that this, “mostly ends up with an over specified system because of high margins added at a feed stage. This includes provision for future expansions. If we have all the information up front, we can size the HE very accurately with the result of less problems from an HE point of view”.
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
Product footprint has become a significant factor in recent years as available plant room space becomes smaller. Suppliers now offer several products to maximise the usage of floor space on sites. “Our range now includes dedicated containerised plant rooms to outdoor racks with the addition of integrated condensers. These give our clients flexibility and offer large cost benefits against building the traditional dedicated plant room,” notes van Rooyen.
Further, market design trends are focusing on the use of natural or low GWP synthetic refrigerants, with significant attention being given to low charge systems. In the wider market, the trends have moved towards customers preferring full end-to-end solutions that minimise points of responsibility and localise all project design considerations. 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.
Kutelya adds, ”Plate HEs are becoming increasingly popular in a variety of applications due to the potentially lower purchase cost, a high heat recovery (up to five times more efficient than a shell and tube application), a close temperature approach (up to 1 C), small installation footprint, the flexibility of using various types (gasketed, semi-welded, welded, spiral), and ease of maintenance.
“Developement of a completly new range of gasket plate heat exchangers (GPHE), where the latest innovation is offered on the heat exchanger market is part of new trends. New GPHE has a combination of new features that ensures higher heat transfer efficiency, better reliability and greater serviceability. Environmentally-friendly refrigerants are also big elements of the trends we see today as products are driven to be as compact as possible.
The new heat exchangers are now also supplied with a U-turn separator. The U-turn separator was developed to ensure efficiency and to achieve smaller overall dimensions for easier and more compact installations. This new concept in flooded evaporators, offers enhanced separation efficiency via gravity and centrifugal effects, insignificant internal pressure losses, compact design, low ammonia charge and reduced hold-up, short vertical ammonia driving columns to allow for small temperature approaches and high system efficiency and reduced height and compact profile allow for this packaged evaporator system to be installed in one piece in narrow spaces,” says Modibela.
What is a plate HE?
A compact heat exchanger (plate HE), depending on the technology and design parameters, is a set of thin heat transfer plates compressed together to form a plate-pack which forms the heat transfer area. Each compact technology has headers and followers which hold the plate packs together. The inner working of a plate heat exchanger is to transfer thermal energy between two fluids, without the fluids mixing together.
“As for gasketed HEs, there are different numbers of plates according to the required flow rate and pressure drop. Hot mediums go through one pair of plates (between the first and second plate), and cold mediums go through the second and third plate, and so on – so each alternating plate thereafter will be hot and cold respectively. Cold and hot mediums exchange heat through a thin plate that is most commonly 0.5mm thick. This same technology works for welded units, but these don’t have gaskets between the plates. These plates are welded by a laser machine,” Kutelya notes further.
Within a gasketed HE, the plate material is thinner so heat transfer is higher, thus resulting in a high heat transfer coefficient. Each channel plate has an efficient distribution area and plates chevrons that utilise the pressure drop fully. The result is high turbulence which results in high wall shear. The additional benefit of turbulent flow is minimised fouling on the plate surface.
Why plate HEs were developed
According to Kutelya, plate heat exchangers have been around since the 1920s and revolutionised methods of indirect heating and cooling of fluids. They were developed as an alternative to the shell and tube technology in some industries and applications.
There are also different tasks for different industries and this technology was designed for a corresponding solution. In these different industries, varieties in parameters also differ between countries. For instance in most countries, the quality of water is completely different. Flow rates and system pressure also depends on a facility’s capacity which can be different at every location and so plate HE offered a new method.
Plate HEs, having a now wider application use are also generally smaller and so transportation and installation cost became lower, maintenance is easier and they are much higher in efficiency to the traditional shell and tube system.
