By Benjamin Brits
Heat recovery, like many aspects of the HVAC&R world, is not a new element, however its use today is on the rise as design techniques improve, and the development of up-cycling technology is driven to create ways to lower facility energy demand.
Heat recovery can simply be described as the conversion of waste heat into a useful form that can be applied to multiple applications. Its incorporation in today’s systems is said to be a ‘no brainer’ in projects as the drive for efficiency and carbon footprint reduction is attracting global pressure to accelerate (even more so in the local context). A little research took me back over 20 years where a number of published works already existed around the subject.
New innovative methodologies throughout the HVAC&R sectors are being developed that open up another level of efficiency as more and more clients are realising not only the importance of operational costs over the lifetime of their systems, but their moral obligations toward future generations of humanity (and thus better use of resources) is top of mind.
Considering the simple idea of heat recovery, integrating it into a system to actually add significant value is quite a complex matter and requires a number of considerations and calculations, much like a normal refrigeration system. Some engineers are even of the opinion that heat recovery will become so important in future that even the smallest recovery achieved will be useful in an overall system design, supplemented for example by various heat pumps and thermal storage vessels. Heat recovery units could also be installed as a retrofit on existing systems in future on the local front.
By utilising a resource that would otherwise go to waste, overall energy consumption is reduced, and therefore operational costs and environmental impact too. The process of implementing heat recovery, may in some cases, also result in smaller capacity equipment being required.
An element of many facilities, whether industrial or commercial, that is often overlooked or miscalculated when considering efficiency, is the quantity of heat generation that is actually used or required. In the local context, services are generally compartmentalised, and so there is ‘siloed work’ and overall facility efficiency through system integrations is not as common as it could be. All you need to do is look at the appointed list of professionals to any project and you will rarely find that the same company handles all of the services. Understandably, each company and engineering team will have their own methodology and preferences that then may not align to the next professional team.
In a food or beverage plant as an example, a significant amount of heat is required in the production processes. The heat that is applied to the product is then ultimately removed by the refrigeration system prior to storage or dispatch. By utilising the waste heat from the refrigeration systems and integrating the heating and cooling processes, significant opportunities are created for further efficiency. This is more difficult and could become costly as an afterthought or retrofit when opportunities are only identified far down the line in a project.
Considerations for heat recovery inclusion
The start of the process would be to determine the client requirements or application of the recovered heat, and then to determine if the need will be practically sourced from the refrigeration or air conditioning system. It must be noted up front that some applications, particularly in processing or industrial applications, cannot be met by heat recovery alone. In those cases, a combination of heat reclaim system with alternative heating systems such as boilers become attractive.
“If a client wants to wash down their facility as an example with 70°C water, and the (ammonia) refrigeration plant is located 400 meters from the factory floor, it’s going to be energy and capital intensive trying to reach those temperatures that may require high discharge pressures and high pumping cost. That in turn makes the system very inefficient so you defeat the purpose of trying to harness that energy,” says Petrie van der Merwe, senior mechanical engineer – industrial refrigeration at Energy Partners Engineering.
In most cases, each installation and system would be unique and so the considerations would be varied. In order to get to the required end result, each design would have to consider options in refrigerant choice, suction and discharge pressures, use of high quality – and low-quality waste heat and account for different heat losses in the system.
Van der Merwe continues, “The other major considerations in heat recovery are to evaluate and understand the profile of the heat load that the client requires for their facility, timelines and volumes. This could be a continuous draw-off as you find in a hospital or hotel, a batch production application where you use 100 kilolitres of water over a specific time of the day, or a washdown cycle. The load profile and the logical interfaces for heat reclaim will then determine the system design. In an industrial refrigeration system, the heat reclaim interfaces would (mostly) be from two areas, being the de-superheating of the compressor discharge or the compressor oil cooling circuits (if present).”
Heat recovery can be achieved through several types of heat exchangers based on system design characteristics. Some of the more common types include shell and tube, plate, condensers, evaporators, and even boilers. Depending then on the type, heat transfer can occur in gas-to-gas, liquid-to-gas, or liquid-to-liquid forms. Other important factors of the particular heat exchangers design characteristics include material types, transfer mechanisms and flow configurations.
When installing a heat recovery device, placement is extremely important too. “Considering de-superheating as the source (which is a relatively simple process), heat recovery devices need to be placed at the very highest point in system piping. If the discharge gas starts to condense in the heat exchanger, it is critical that the liquid drains to the condenser rather than the compressors, which will obviously have serious implications. The circulation on the secondary side of the heat exchanger depends on the system design – the secondary pumps could simply circulate through some type of buffer tank, circulate directly to the site or alternatively tie into a second heat exchanger, with tertiary pumps connected to a process in the facility,” van der Merwe adds.
He continues, “Heat recovery gets a bit more intricate in oil cooling circuits. This method can be used when the plant is equipped with screw compressors. The oil cooling circuit is connected to a cooling tower where the heat is rejected to atmosphere. Temperature is controlled via a three-way bypass valve on the supply and return headers to the cooling to maintain a minimum return temperature to the oil cooling heat exchanger on the compressor. This maintains the oil in the optimum temperature range, protecting the compressor. The reclaim heat exchanger is installed in the discharge header out of the oil cooling circuit, before the bypass valve. The oil cooling circuit typically achieves heat recovery of lower temperatures – between 35°C and 55°C. If this temperature range is not high enough, the reclaimed energy can be fed into an augmenting device such as a heat pump that upcycles the product to higher temperatures. It is important to consult the equipment supplier in this process to ensure that the heat reclaim system will not be harmful to the compressor.”
The design parameters would also need to consider any leaks and how this would affect the overall operations and safety aspects. For example, risk of contamination may occur in a line feeding a food processing section, or the supply of hot water to a building may become compromised.
In addition to this, there are other associated aspects of legislation that come into the realm of heat recovery, particularly if one is going to tie into water system or if domestic hot water reticulation is involved. SANS10252 – related to hot water generation must be applied as well as various requirements of SANS10400. With hot water systems, Legionnaires disease also becomes a consideration. 35°C to 40°C is the ideal temperature for legionella bacterial growth, so if the heat reclaim system that can only generate water at these temperatures, it needs to include an electric element, steam line, a heat pump or like device. This will then be used to periodically raise the temperature above 50°C to kill off any potential bacterial growth.
“Control methodology is also extremely important when looking at heat recovery and you would need to understand the capacities of each heat exchanger type. Heat exchangers are typically designed and manufactured to accommodate various degrees of hot water, but if you reach boiling temperatures, steam arises, and this may cause damage to the product. Protection in this regard is therefore critical in the strategy and the placement of sensors too. One would also need to understand the differences between stand-alone heat recovery systems and systems directly integrated,” says Alistair Bell, technical support engineer – refrigeration and air conditioning at Danfoss.
Further considerations would include the ability to bypass the heat recovery unit so as not to have a total system shutdown should something go wrong on that particular component, whether a closed or open circuit method will be used, and corrosion protection that can experience different outcomes based on temperatures. Further understanding a system’s total load profile and future expansion plans become important when looking at how heat recovery will affect the overall system in terms of generation capacity or pressure drops.
“In addition, here, and depending on the application, if you are including water storage, you need to have a trustworthy tank. We have moved over to using mild steel tanks with a rust-proof coating applied on the inside. This particular tank is manufactured locally and has a five-year warranty. The biggest enemy, however, to any mild steel tank is rust that we have to deal with. We have tried fibreglass type tanks, but these prove to be too sensitive to pressure and temperature differences and are therefore limited. Also, any heat recovery or heat pump will always be more efficient than a direct element solution,” says Cules Smit, general manager at Matador Refrigeration.
“Adding onto what Petrie and Cules have said, safety should always be a priority, particularly as heat recovery is typically best suited to larger installations. Here it is common to come across CO₂ that operates at high pressures and ammonia that is toxic. All of the correct stops, warning aspects and incident apparatus have to be included without question,” Bell stresses.
Can heat recovery work with any system?
With the inclusion of new technology and design methods, the short answer is yes. As already mentioned, any savings that are possible will soon be seen as essential. Savings already become possible by closing the gap on supply and demand temperatures, even if this is only a couple of degrees. This not only has an impact on operational costs, but peak energy demand can also significantly be reduced as well.
“Although you can recover heat from any refrigeration system, plant scale is something we need to keep in mind because in theory anything can work. Heat recovery works well on large plants that typically use CO₂ or ammonia – these plants normally have stable operating conditions and accessible interface junctions. With these conditions, more heat is available with even part load conditions yielding useful amounts of recovered energy. Heat recovery always seems attractive, but as mentioned if you have to pump hot water through a long circuit to get it to the point of use, the pumping costs increase, reducing the energy savings. It then becomes more practical to consider installing an alternative solution. In my view heat recovery is then best suited for large commercial and industrial scale systems with continuous operations, where the heat source and points of use are close to each other. Heat reclaim systems are not particularly well suited to plants with seasonal operations,” van der Merwe notes.
What can you practically do with recovered heat?
Although the most commonly known function of heat recovery is to generate hot water for a building, many other applications are in fact possible, and this range is being expanded at a fast rate with the development of new technologies such as heat pumps that now are able to generate temperatures of up to 95°C. It is now possible to generate steam at 5 bar (145°C) from a heat pump using Pentane as refrigerant. It is still in the testing phase but could be an important addition to industrial applications in the near future.
One of the first known applications of recovered heat was to increase a building’s temperature in colder climates. Today for commercial applications another well known technology that is widespread is variable refrigerant flow (VRF) systems that use the same methodology on a smaller and more controlled scale.
“Temperatures can be varied through the heat recovery process by adjusting how the system operates, speeding up certain components or slowing things down. This obviously has a number of impacts or influence on how a system needs to be designed. However, it must be noted that heat recovery is actually a secondary function to the primary function of any refrigeration or air conditioning system and therefore any heat recovery applied should not affect the main system’s operation. This is most commonly seen when COPs drop and this then essentially creates inefficiency rather than savings,” says Bell.
Other applications of recovered heat in a single stage use include washdown water that is used in facilities with fatty or oily residue where the requirement is just enough to ‘melt’ such products – usually around 45°C and clean in place (CIP) that can be anything from 65-70°C. With the addition of industrial heat pumps, steam boilers or electrical elements, recovered heat can be upscaled to reach temperatures suitable for pasteurisation, scalding, and other processes requiring 120-150°C temperatures.
“As an example of an in-use installation, we use between 10% and 25% of the total heat of rejection generated by the refrigeration plant. We achieve this by using a plate heat exchanger that we run the refrigeration plant’s hot gas line through (which is roughly between 65°C and 95°C). Water is then circulated out of an insulated hot water tank (similar to a geyser) through another plate heat exchanger. The water is then directed through a different ring main circuit throughout the facility and then tees-off to different departments. The water temperature is regulated by an actuator valve in the hot gas line which is controlled by a temperature sensor inside the water tank. The hot water in this example is kept between 55°C and 60°C for the application,” says Smit.
Smit continues, “For an average-sized supermarket, a 500-litre insulated hot water tank is generally adequate and with the ring main an insulated 22mm copper tubing circuit is preferable. If you had to heat up the water in that 500-litre water tank using an electrical heating element, you would have used about 9kW of energy. With heat reclaim from a refrigeration plant, you use zero extra energy. Similarly, if you use heat reclaim off a refrigeration plant for air handling, you also eliminate a lot of electrical heating elements which will show a huge cost saving.”
Heat recovery has been popular in comfort heating of spaces including commercial, residential and retail applications in European countries for a long time and here too, larger installations are able to push back heat from plants tying into district systems.
“Further applications of heat recovery include underfloor heating and hot gas defrost cycles. All of these solutions are an efficient way of harnessing an essentially free resource that can reduce the client’s carbon footprint,” adds Bell.
Practicality as a retrofit solution
As a number of plants in the country age and are known to be inefficient, or owners are trying to stretch their assets and are continuously looking at part-replacement of components, a simple way to generate better efficiency is to strategically retrofit certain aspects.
“If the system in question comprises a multiplex plant, retrofitting can be done by use of a plate heat exchanger as illustrated earlier or alternatively a heat pump can be installed to increase efficiency,” Smit comments.
Van der Merwe advises, “Although it is possible to retrofit older systems, you would need to evaluate this on a case-by-case basis and not just assume that it is possible for all the systems that are out there. There are a number of other aspects that need to be considered for example: if the piping profile is suitable, how the piping to the condensers has been installed, what changes would be required to the system, would rebuilding certain sections be required, and so on. If you change the parameters of some older systems, it often becomes critical to evaluate the original design and consult the equipment suppliers to make sure that additions will not be harmful to the installed system.”
Looking forward in heat recovery
Historically the drive to incorporate techniques such as heat recovery has been lacking because there is a general consensus that carried from the past, the cost of electricity was always extremely low and so there was no incentive to look at these sorts of solutions. Further, to incorporate other techniques was burdensome due to additional design work, calculations, and on site the installation of separate pipe systems and components was just extra effort.
Today and looking forward, harnessing free energy and utilising it in a number of applications will not only be practical, but it will also become critical as the world drives requirements to combat climate change, and so even locally we will not be protected from this push. We are likely to continue to see increased pressure to reduce carbon footprints and in South Africa this is directly related to our reliance on fossil fuels as the major energy provider.
Payback periods of capital investments are also reducing at a very fast pace, given the increasing operational costs facility owners have to keep in mind, and where every savings opportunity now has to be evaluated. With technology and advanced techniques across the board, plants can in future have a much smaller footprint too making them more affordable but more powerful at the same time.
“The world is getting very close to that point of no return in terms of global warming. It is therefore sensible for everyone to start looking towards the future of what we call home and really start thinking more seriously about lowering our carbon footprints – exactly like harnessing the free (or very low cost in the bigger picture) resources. These include natural refrigerants – ceasing blowing off harmful refrigerants into the atmosphere and green energy. Global warming is going to affect everyone – even those who dismiss it because of placing profits first, and we need to take into account future generations,” Bell concludes.