if you can't stand the heat
Heat recovery in the food and drink industry could be improved. David Reay from Action Energy looks at the sector and explains how to do soHeat recovery has been a feature of unit operations in the food and drinks sector for at least a century. The Perkins tube (now called a heat pipe) first featured in bread ovens in the late 19th century, in part to separate dirty combustion gases from clean hot air, while plate heat exchangers became common in pasteurisers to improve regeneration efficiency before World War II.
Although 95% of the available heat in these pasteurisers is recovered, the potential for waste heat recovery in a wide range of other unit operations in the food sector is estimated to be over 8PJ/year (>2 x 106MWh). This is similar to the total amount of energy used in the industrial bakery sector. Before describing heat recovery systems, and how they can be applied, it is worth examining the food and drink sector, which has some characteristics that affect heat recovery directly. Influences on heat recovery feasibility
At a recent meeting on innovative processing, Christina Goodacre of DEFRA highlighted some of the main features of the food industry, including: u the microstructure of products - for example, chocolate cannot be cooled too quickly as this interferes with its structure; u manufacturing efficiency - less waste, including energy, means less downtime; and u process flexibility is essential due to short runs and frequent product changes. Some of these sector-specific characteristics have important implications for the use of heat recovery systems. As conserving product structure and process flexibility is critical, it must be considered by companies contemplating the installation of heat recovery equipment.
It is important to avoid adversely affecting the product quality - the installation should not, for example, impose a backpressure on an oven. Equipment design must also avoid making plant more difficult to clean between product changeovers. Heat exchanger fouling/dead-spots need to be minimised - the industry is aiming for a five minutes cleaning/changeover period between products. Other areas of concern When planning the use of heat recovery, it is also important to factor in potential reductions in energy and water use. This not only improves manufacturing efficiency, but reduces costs (heat recovery can be integrated with water conservation).
Pollution control legislation and the effects of the Climate Change Levy (CCL), which are offset to some extent by Enhanced Capital Allowances (ECAs), are also of concern to companies in the food sector.Where does the sector need heat?
The needs for heat in the sector help to identify sources of recoverable energy. It is also relevant to examine cooling duties1, as refrigeration can be provided by waste heat and may be eligible, unlike most heat recovery equipment, for an ECA - if from a combined heat & power (CHP) plant for example.
More specific uses of energy to consider include:
- Biscuit-making/baking - which combines heating and water removal;
- Cooling viscous sauces, baked products etc. Rapid cooling is becoming very important and uses increasing amounts of energy;
- Fruit juice evaporation/water removal, which involves substantial heat inputs, commonly via steam - there are some heat pump opportunities here;
- Phase change in crystallisation of sugars and fats. May involve heating and/or cooling; and
- Fermentation and drying - malt kilns may be viewed in this regard. Warm air is often used to speed up the process, and large quantities of water vapour are exuded.
As well as heat sources within the food making/preparation processes, there are a variety of unit operations which are generic - such as boilers, air compressors, refrigeration plant and prime movers. These should be included in any survey of heat sources carried out on a site.Identification of heat sources and sinks
An audit of sources - which should be sufficiently detailed to give an in-depth understanding of the process operations, is the first step in identifying and implementing waste heat recovery.
Identification and quantification of heat sources permits an assessment of possible uses for the waste heat. Within the food and drinks sector, typical sources include ovens, evaporators, kilns, dryers and wash-water. In addition, generic items of plant may provide useful heat, such as refrigeration.
Estimates of the amounts of heat available require measurements and information on any fouling/corrosive potential. Accurate process data, in conjunction with a full understanding of the process, will help to minimise the aforementioned concerns about the impact of any system on the operation of the process to which it is fitted, as highlighted earlier.Determining the feasibility of heat recovery
Once the process is understood and heat sources are quantified, it is possible to judge whether it is worth examining whether heat recovery is technically and economically feasible.
Uses for the heat often determine economic feasibility - 24-hour operation and a use near the source, preferably on the same process, are ideal. A factory with single shift working that only uses space heating is generally less attractive, if a 2-3 year payback period is assumed.2 Where hot gases/liquids can be directly recycled, a heat exchanger may not be needed. Where the heat source and sink are some distance apart, the run-around coil is a lower cost solution when compared to excessive duct runs.
If the heat sink is at a higher temperature than the source, a heat pump may be attractive. The case for any installation is aided if other benefits, such as water recovery or improved working conditions, can be claimed.Some heat recovery options
Heat exchangers: Tubular heat exchangers are sometimes used in the food industry to recover heat in areas where the plate heat exchangers are inappropriate. A study by the Campden & Chorleywood Food Research Association Group, in conjunction with Tetra Pak, has examined such an exchanger to recover heat using medium viscosity products.A hot processed product is cooled down in the tubes by the cold unprocessed p
roduct, which in turn is heated in the shell, thus recovering heat that might otherwise be dissipated in cooling water. To maximise heat transfer efficiency, the product in the tubes flows in a counter-current direction to the product in the shell.
One purpose of the study was to demonstrate that heat recovery was economically viable. This was achieved using starch solutions of varying viscosity to represent the flow behaviour of foods that could be processed in tubular heat exchangers. Determination of the maximum product viscosity allowable in existing commercial exchangers, and the necessary redesign of the shell-side flows to prevent stagnation (dead spots) and poor flow distribution around tube supports, were two outcomes of the work.
Calculations showed that potential energy savings of 50-70% could be achieved with heat recovery over a wider range of foods than is currently commercially possible. The general rule was that higher savings were achieved with lower viscosity foods because of improved flow conditions and heat transfer.
For example, for a food with a viscosity equivalent to a 2wt% starch solution (a thin soup for instance), the savings were of the order of 58%. These equated to annual energy savings of £6,707 for a single factory on conversion of existing tubular heat exchangers to heat recovery and £16,288 on conversion of existing batch systems to tubular heat recovery exchangers. Additional annual savings were also achieved on reduced water use, estimated for cooling water and effluent treatment as £8,043 and £5,712 respectively for a single factory.
The equipment supplier involved with this project, Tetra Pak, is undertaking work to design a 6m long multi-tube exchanger that will eliminate dead spots. If successful, this will complement their existing range of tubular heat exchangers. A follow-up government LINK project is under way.
Heat pumps: Pure Malt Products produces a wide range of malt extracts at its Haddington factory near Edinburgh. These extracts find many applications in the food and beverage industries.
Evaporation is a key operation in the manufacturing process, and Pure Malt Products uses single and multiple effect evaporators designed to provide the optimum processing conditions for each product. Until recently, all the evaporators were heated by steam and this represented the major use of steam onsite.
Continuing expansion created a need to increase evaporation capacity, but the existing boiler was already operating at full load on occasions. After considering several options, the company decided to install a new falling film evaporator that operates on the mechanical vapour recompression (MVR) principle - a heat pump. Heat pumps are able to raise the useful temperature of waste heat. The new unit is a single effect, three-stage system, installed by Beedes.
The evaporator was commissioned in January 1999, and careful monitoring of utilities consumption has confirmed the anticipated savings. At an evaporation rate of 4,000kg/h, the new evaporator uses a total of 85kW for the compressor (250-325mbar abs. pressure rise) and all pumps, about 40kg/h of low-pressure steam, and a small quantity of cooling water for the vent condenser.
When compared with the operating costs for the steam-heated evaporator used previously, the company found that the running costs were reduced by at least £2/t of water, which was lost to evaporation. It is estimated that the payback time of a comparable installation, compared with a steam-heated evaporator and additional boiler plant, would be 3-4 years.
At Pure Malt Products, the compressor takes vapour from the evaporator, raises its temperature and pressure, and uses the energy to produce high-temperature steam for further evaporation.
The success of this plant has encouraged the company to plan a much larger effluent evaporator, using the same MVR method which can recover up to 12,000kg/h of water and, using reverse osmosis treatment, will nearly eliminate the need for mains water at the plant.Moving forwards
Heat recovery is a viable option in the food industry. Its attractiveness is enhanced with accompanying water recovery and/or effluent reduction. Some food processors may be concerned that heat recovery might adversely affect the quality of their products, but with sensible precautions, potential pitfalls can be avoided through good design and installation.
Additionally, companies have a social responsibility to minimise waste in all its forms, and this should be included in the payback equation as a positive return.Notes
1 Cooling is a growing feature of the whole food chain - even refrigerated transport could benefit from heat recovery.
2 Short payback periods are difficult to achieve when energy prices are low. The Climate Change Levy helps by raising energy costs, and Enhanced Capital Allowances (ECAs) encourage investment in energy saving. However, ECAs are not generally applicable to heat recovery at this stage.