Heat Recovery
How heat is wasted and how it can be captured and reused to reduce commercial energy consumption.
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Hi I’m Phil Noel and this short video is about heat recovery for small businesses I’m going to explain where heat recovery might be possible and the kind of benefits it may have in terms of cost savings and reducing carbon Emissions because no processes 100 100% efficient wherever energy is used some of it is converted to heat and escapes into the environment even when a process requires heat it can’t all be contained perfectly so there is always some level of waste historically energy from fossil fuels has been so cheap that this wasted
He was not worth worrying about some processes such as Refrigeration or electricity generation actually need heat to be rejected from them to operate correctly so this kind of waste heat has generally just been accepted as a cost of operations it’s only been relatively recently as fossil fuel prices have
Increased and there’s been a growing need to reduce carbon emissions that waste heat has been something worth taking Seriously in some situations instead of wasting this energy it’s possible to capture some of it to use again either by returning it to an earlier stage of the process or sending it elsewhere to serve a completely separate purpose this is what we call heat Recovery wherever there is a nearby need for heat as much of it as possible should be brought from the waste source to where there is demand this avoids using additional energy to deliver the heat requirement through more typical means resulting in an overall energy Saving to see if there is potential for heat recovery within a business a sitewide assessment should be made to identify all the major heat inputs and outputs this means finding where heat is needed and where it is escaping to give you a few ideas of what heat recovery looks like and where op
Unities might be found let’s take a quick look at a few Examples one use of heat recovery that’s familiar to a lot of people is actually found in their home’s gas boiler most modern boilers are of the condensing type which means that the steam produced when gas is burned is made to condense within the boiler rather than being
Allowed to escape from the flu pipe the heat that’s given off as this Steam condenses is then captured through a heat exchanger and transferred into the heating system capturing the heat that would otherwise escaper steam can improve boiler efficiency by around 10 to 12% it’s common practice to try and minimize heat loss from a building by eliminating drafts as much as possible in large commercial buildings however this air tightness means that some form of powered ventilation is then needed to ensure enough fresh air is supplied to maintain good air quality in large
Buildings this is usually done with an air handling unit which combines balanced flows of extraction and fresh air ventilation as well as potentially heating Andor cooling of the incoming air to stop energy being wasted when heated air leaves the building via the ventilation system there are a number of
Ways to transfer the heat from the outgoing air to the incoming air so that the heat is retained within the building without limiting the air quality this can be done with a fixed plate heat exchanger which passes one Airstream over the other with a thin walled
Barrier in between them to keep the two airstreams separate alternatively a rotating thermal wheel can be used and this continuously absorbs heat on the areas that are passing through the extract air and then releases the heat as that part of the wheel passes through the colder in Inc coming air this can be
Very efficient but it does result in some mixing of the two airst streams which could be undesirable in cases where it’s important to maintain air Purity if the supply and extract air flows are not running alongside each other water can be used as an intermediary for transferring the Heat
This system of pipes and heat exchanges that do this is often called a runaround coil this can be more convenient but it’s generally less efficient than the other two options Air handling units are usually specified and installed when the building is first constructed and retrofitting this kind of system is not often feasible due to the cost and complexity of the installation work rather than using an air handling unit for the whole building several simpler heat recovery ventilation systems maybe more straightforward to
Retrofit and these can be small enough to fit in ceiling voids or roof spaces but serve much the same purpose as the larger Air Handlers including having a fixed plate heat exchanger to retain the thermal energy within the building these kind of units although simple can be
Expected to recover about up to 80% of the heat that would otherwise be vented from the Building cooling is a very common process that’s used in almost every industry whether that’s to keep food fresh to stop staff from overheating or as part of an industrial process regardless of the reason it’s usually done using a refrigerant system which comprises a heat exchanger to absorb heat called the evaporator a compressor
And then another heat exchanger where the heat is rejected called the condenser it’s the heat rejection at the condenser that’s responsible for the warmth felt at the back of a fridge or freezer for example so it’s therefore at the condenser that some heat might be recovered this can be done directly by
Sending the warmed air to where it can be used for things like space heating alternatively it can be done by rejecting the heat to water which is used as a medium for storing recovered heat for use at another time or for transporting the recovered heat to a different Location industrial ovens and dryers can consume enormous amounts of heat energy and often waste a great deal of it as well to remove fumes from combustion or from the materials being heated as well as to remove moisture that’s evaporated there is usually some kind of flu or extraction system
Involved due to the high temperatures involved and the high flow rate required through the extraction system large quantities of heater often allowed to escape into the outdoor air it’s increasingly common though for these facilities to have some kind of heat recovery because the heat is often of a high temperature and it’s channeled
Into a single duct normally rather than being emitted at multiple locations around the plant this resource can be used in the same ways as other low grade heat I.E for space Heating and hot water but occasionally the temperatures might be high enough to create Steam for industrial heating or to generate electricity Where temperatures are high enough and there is a large enough quantity of waste heat available electricity generation might be feasible the high cost of electricity could make this a very attractive option but it’s likely to have a very high upfront cost that needs to be paid back by a consistent
And reliable source of waste heat normally electricity is generated from a heat Source by creating high temperature super hea steam at over 400° C waste heat is rarely available at this kind of temperature meaning a different technology is required this is most commonly the organic ranking cycle or orc which works
In basically the same way as a steamed turbine generator but rather than using water or steam it uses an organic fluid with a lower boiling point than water in this kind of generator only a fraction of the Heat going into the generator is converted to electricity which means
That most of the original wasted heat still goes unused and may be even harder to recover that means that before choosing to install a waste heat electricity generation system it’s essential to compare the benefit of converting a small percentage of heat to electricity with recovering a much
Higher percentage but for a lower value purpose such as hot water for cleaning for example Another very common cause of waste heat in industry is from air compressors these are found all over the place in most factories and workshops as they provide a very versatile form of mechanical Power they are however quite energy inefficient typically they only convert around 10 to 12% of the
Electricity used to power them into actually compressing the air with the remaining 88 to 90% being wasted as heat compressors often use oil both to lubricate the system and to draw excess heat away which is then fed through a radiator or heat exchanger to allow the heat to escape it’s therefore quite
Straightforward to replace this heat exchanger with an oil to water version so that again the waste heat can be stored and piped to where it’s Useful as we’ve seen most waste heat is considered low grade meaning it’s at a temperature below the boiling point of water and so can’t be used to either create steam or to generate useful amounts of electricity this kind of heat on its own is therefore only suitable for things
Like space heating domestic hot water cleaning and drying processes lowgrade heat can however be used to preheat something that needs to be brought to a higher temperature making a contribution towards that heating process for example feeding waste heat into a boiler’s return pipe would reduce the temperature
Increase that the boiler has to achieve alternatively if waste Heat’s only available at a low temperature of around 20 or 30° but there is a very large volume of it being produced its temperature could be upgraded via a heat pump to the point where it can serve a
Useful purpose the heat pump option is a lot less straightforward to design and Implement than a simple heat exchanger system though so a heat pump engineer with specialist knowledge of these kind of scenarios must be consulted to ensure that an effective system is Designed it should perhaps go without saying that modifying any system that involves high temperatures should not be carried out without properly understanding the system and assessing the health and safety risks involved a suitably qualified engineer should be appointed to design a complete system that will integrate with the existing equipment without any adverse
Effects on its operation or the site’s health and safety wherever heated water is stored at temperatures of around 20 to 40° C there is an increased risk of harboring or proliferating Legionella bacteria these systems must therefore have controls in place to reduce any of these bacteria to safe levels most often
This comprises an electric heater and a mixing pump to raise the water to a sufficiently high and uniform temperature alternatively if this water cannot be increased in temperature then a biocide can be used to serve the same purpose so I hope you found this video useful um you can find more information
About heat recovery by downloading our fat sheet or if you’re interested in any other energy saving Technologies we have several more videos available but thanks very much for watching