Waste Heat Recovery
To evaluate the feasibility of waste heat recovery, in any industry or application, you must first characterise the waste heat source and stream.
Ie. heat producer, together with the air, liquid or gaseous stream to which the recovered heat might be transferred, ie. heat user. It is essential that parameters for these air streams are established at the earliest stage. The parameters required include:- the quantity of heat that might be transferred, the temperature, its composition, the production schedules, operating hours and the phasing of both the heat producer and the heat user.
Operational can then begin to analyse the type of heat recovery system most suited for the particular application. We might propose an air to air system, air to water or thermal oil ‘run-around’ system, liquid to liquid or alternatively a steam to water system. We might choose to utilise heat exchanger design characteristics such as plate, plate coil, finned tube and shell & tube – with materials including construction of steel, stainless steels, hastelloys, non-ferrous metals, glass, plastic or graphite and where necessary associated epoxy coatings.
The overall system should be designed to provide the most practical and energy efficient solution bearing in mind the nature of the heat producer and the heat user streams.
Important factors such as the corrosion resistance of the material used for the system components – including heat exchangers, fans, pumps, pipework and ductwork sections. Whether fouling of the heat transfer surfaces may become a problems due to solids, highly viscous liquid and other deposits. In the event of fouling of the heat exchanger surfaces how do we access these units for cleaning purposes and do we need to incorporate in-situ cleaning systems?
Operational design solutions that will take all of these variables into account in order to harness the waste energy and channel it back into the process for maximum longevity and efficiency. Thus saving you energy and money whilst improving your carbon footprint.
HEAT RECOVERY FROM KILNS & FURNACES
Ceramic Tile Kilns – The ceramic tile making process firstly requires the compression of powdered clay into the tile shape. The tiles are then dried using hot air in a continuous dryer to reduce the water content. After drying the tiles are coated with glaze and patterned. The tiles then enter, typically, a 130 metre long continuous kiln and reach temperatures of up to 1200oC in the centre of this kiln. The high temperature kiln exhaust air can contain solids and potentially corrosive, gaseous and liquid contaminants.
Operational can recover heat from the kiln exhaust air and transfer the recovered heat via a thermal oil circuit to heat exchangers installed in the dryer air recirculation systems. Operational use specially constructed finned and plain tubed air to thermal oil heat exchangers together with a high temperature thermal oil pipework system.
It is estimated that ceramic tile kilns lose around 20% of their heat to the atmosphere via flue gases. We typically assist in recovering between 400 and 600 kWh per kiln, with a return on investment of 2 -3 years.
Brick Kilns – Brick kiln exhaust temperatures will reach 200oC during the initial stage of the drying process before firing up to around 1000oC, with a variety of toxic components in the exhaust gases. These include halogens, particularly chlorides and fluorides, and the potential for material attack in both the gas and liquid phase. Operational’s air to air heat exchangers are constructed from special stainless steels to resist corrosion attack; however, due to the impurities present, control of the heat exchanger internal skin temperatures is important to ensure condensation is minimised. Recovered hot air is re-used as pre-heated make-up air and combustion air for the kiln operation. A return on investment was provided in around 2 years.
Gypsum Calcination Kiln – With calcining temperatures of approximately 180 oC and high particulate dust loading in the exhaust air stream, gypsum calcination kilns require a bag filter upstream of the air to air heat exchangers. By employing air to air heat exchangers to recover heat, our systems will return hot air for use in pre-heated combustion air and as make-up process air. Return on investment is delivered in approximately 18-24 months.
Wire Annealing Furnaces – Operational were consulted and requested to design and supply a system to recover heat from the high temperature exhaust and pre-heat the combustion air to the main pre-mix furnace burners. Annealing of the wires took place at temperatures of between 900oC and 1100 oC and the exhaust gases passed to atmosphere at temperatures of up to 820o We utilised a fully welded air to air plate heat exchanger constructed in stainless steel and high temperature bifurcated fan system capable of recovering 70% of the available sensible heat. With continuous operation of 8400 h/year the return on investment was below 2 years.
Aluminium Smelting Furnaces – An aluminium smelting process exhausting hot combustion gases from a natural gas fired combustion chamber. Heat is recovered from the hot combustion gases at temperatures of up to 600 oC and transferred, via an air to thermal oil, stainless steel heat exchanger, to a thermal oil ring main circuit. This thermal oil ‘run-around system’, operating at 270 oC, is then used to transfer heat to pre-heat the combustion air for the furnace gas burner system. A 2 year return on investment was achieved.
HEAT RECOVERY FROM DRYERS & OVENS
Bakery & Food Ovens – Operational have installed many oven heat recovery systems to harness the waste heat in the exhaust gases leaving large travelling bread ovens in plant bakeries. The exhaust air leaving these ovens varies from 120oC up to approximately 300 oC and often contains copious amounts of moisture due to the steaming of bread at the inlet of the ovens. Accordingly, large amounts of sensible and latent heat are available for recovery and re-use throughout the Bakery.
Many challenges have been overcome such as the fouling of heat exchangers from the ‘tincol’ oils used in the production process to the corrosion caused by the products of combustion.
The heat recovered from the oven exhaust gases is transferred into a low/medium pressure hot water ring main from where the recovered heat is transferred to a number of energy users throughout the bakery.
Typically, this recovered heat would be used to heat the initial and final provers, the hot air blast systems for skinning of the dough pieces and for the pre-heating of make-up water to the steam boilers, the basket washers and calorifiers.
Typical return on investments of between 2 and 3 years is achieved.
Textile – non-woven materials
Continuously rising energy usage and costs in the technical construction materials market led a leading manufacturer of non-woven glass fibre veil to consult Operational in order to evaluate the viability of heat recovery from his production line. Previous attempts to recover energy had failed due to fouling of the heat exchanger heat transfer surfaces and blockage of the air channels.
Operational investigated the various processes and developed a bespoke solution for the matching of energy supply and demand in their production processes. Fouling issues were addressed by the incorporation of an innovative in-situ cleaning system. Advanced PLC controls, providing user friendly interfaces, optimised the amount of heat recovered and re-utilised.
Our client’s production director commented….”Our financial administration department has clearly identified the moment that Operational’s heat recovery system came on-line. A massive reduction in our gas bill!”
Energy Recovery and re-utilisation of 800 kWh was achieved, with cost savings of €220,000 per annum giving a return on investment of less than 12 months.
Wallcoverings, Floor-coverings & Carpets
A leading producer of tufted carpets was running three drying ovens that were operating inefficiently and using increasingly more energy. They had previously installed a heat recovery system but this had failed due to fouling and ultimate blocking of the heat exchanger airways with fibres and condensed oils.
Operational’s solution was to install a fully integrated heat recovery system using a low pressure hot water run-around system to transfer the heat from the hot exhaust gases to the incoming make-up air. The system incorporated a ‘state of the art’ integrated cleaning system to prevent build-up of fibres and condensed spinning oils.
Our system has reduced natural gas consumption by 800kWh and has improved the dryer performance through more efficient hot air distribution within the dryers – leading to higher production operating speeds.
Energy savings of 800kWh were achieved with a return on investment of 1.3 years.
Paper & Tissue – Vast amounts of energy and water are used in the manufacture of tissue paper. The fibrous substrate formed from pulp stock is passed over a Yankee cylinder, operating at 180°C for drying. The hot air impingement hoods, operating at up to 400°C, accelerate the drying process with the exhaust air vented to atmosphere.
Waste energy is presently used to heat pre – combustion air for the Yankee hood burners and the pre-heating of process water streams. However, many of the existing heat recovery systems are inefficient and Operational’s detailed surveys have identified energy can be more efficiently recovered to replace live steam and other process hot water uses within the mill, including:-
-The utilization of a waste heat steam generator; hot water to pre-heat combustion air; hot gas to water and water to water heat recovery.
-Steam generation plant installed between the Yankee dryer and the existing heat exchangers with a condensing heat exchanger replacing the existing under-performing installation
Benefits – 1300 kWh of steam generated at 12 bar and 1000 kWh of hot water at 70°C. Potential Energy savings of £ 420,000 per annum with a Return on Investment of approximately 2 years.
HEAT RECOVERY FROM OTHER SOURCES
Fish Processing Plant