Heat and hold techniques. In traditional blanching, products are continuously subjected to the heating medium until a specified product core temperature is reached. In contrast, blanchers using the heat and hold technique expose products to just the minimum amount of steam required for blanching, via the use of a heating section and a holding section. In the heating section, products are exposed to just enough steam to heat the surfaces of the productto the necessary temperature for blanching. The product then proceeds to an adiabatic holding section, in which the product’s surface heat is allowed to penetrate to its core, which raises the entire product to the required blanching temperature without the use of additional steam. Heat and hold blanchers have been reported to reduce blanching times by up to 60% and blanching energy intensity by up to 50% . In 2003, Stahlbush Island Farms, a grower, canner, and freezer of fruits and vegetables in Corvalis, Oregon, replaced an aging and inefficient blancher used for processing pumpkins with an ABCO heat and hold blancher. In addition to heat and hold features, the ABCO blancher also incorporated curtains and water sprays to minimize steam losses, a condensate recovery system, an internal steam recirculation system, a fully insulated steam chamber, bato bucket and programmable logic controls. Stahlbush Island Farms reported annual natural gas savings of 29,000 therms and $16,000 in annual energy savings .
Project costs totaled $202,000, but with an Oregon energy efficiency tax credit of $70,855, the final simple payback period was 8 years. Heat recovery from blanching water or condensate. Heat can be recovered from the discharge water of hot water blanchers via a heat exchanger. Similarly, in steam blanchers were condensate is not recycled internally, it might be possible to recover heat from the hot condensate exiting the blancher. Where fouling is manageable, in both cases heat can be recovered using a heat exchanger and used to pre-heat equipment cleaning water or boiler feed water . Steam recirculation. Some steam blanching systems with forced convection are also capable of recirculating and reusing the steam that does not condensate on the product at first pass, thus reducing the steam inputs into the blanching chamber. The U.S. DOE sponsored the development of the Turbo-Flo blancher, which features a steam recirculation system in addition to hydrostatic seals, a fully insulated steam chamber, and blanching process controls. As of 2002, 40 units have been installed in food processing facilities in the United States. Reser’s Fine Foods, an Oregon based processor of vegetables and specialty foods, has installed five Turbo-Flo blanchers at its processing facilities. According to the company, the Turbo-Flo blancher at its Beaverton, Oregon, facility increased product throughput by 300% while reducing the floor space required for blanching dramatically. At the California Prune Packing Company in Live Oak, California, a TurboFlo blancher installed in 1997 was reportedly four times more efficient than its predecessor .
Estimated payback periods are under two years .Insulation. Any hot surfaces of drying equipment that are exposed to air, such as burners, heat exchangers, roofs, walls, ducts, and pipes, should be fully insulated to minimize heat losses. Insulation should also be checked regularly for damage or decay. Different insulation materials such as mineral wool, foam, or calcium silicate can be applied to various drying system components, depending on temperature . Foam can be used for low temperature insulation while ceramics are useful under high temperature conditions. Mechanical dewatering. Mechanical dewatering of fruits and vegetables prior to drying can reduce the moisture loading on the dryer and save significant amounts of energy. As a rule of thumb, for each 1% reduction in feed moisture, the dryer energy input can be reduced by up to 4% . Mechanical dewatering methods include filtration, use of centrifugal force, gravity, mechanical compression, and high velocity air . At the British Sugar beet factory in Wissington, England, six screw presses were employed to mechanically dewater wet beet pulp prior to dehydration in a rotary dryer. Each screw press had specific energy use of 23 kilojoules /kg of water removed, compared to a specific energy use of 2,907 kJ/kg for the rotary dryer. By using the six screw presses for mechanical dewatering, British Sugar found that its energy costs in drying the beet pulp were 40 times less than they would have been if they had used the rotary dryers alone . Direct fired dryers.
Direct fired dryers are generally more energy efficient than indirect heated dryers, because they remove the inefficiency of first transferring heat to air and then transferring heat from air to the product. Direct fired dryers can reduce primary fuel use by35% to 45% compared to indirect heating methods . Exhaust air heat recovery. A simple form of heat recovery in retrofit applications is to utilize the exhaust air of a dryer to preheat the inlet air stream, thereby saving energy. The success of this measure depends on the available space for additional duct work near the dryer . Either the exhaust air can be directly injected into the inlet air stream, or a recuperation system can be employed to indirectly heat the inlet air stream using exhaust air . In the former approach, the saturation of the exhaust air might limit the effectiveness of heat recovery . If there isn’t sufficient room for additional duct work around the dryer, heat can be recovered from exhaust gases using “run-around coils,” which contain a heating medium such as water to transfer heat to the inlet air stream via a heat exchanger . Using dry air. The use of dry air reduces the amount of moisture in the air that requires heating and vaporization. Thus, by removing this moisture, the heating load on the dryer is reduced. Air can be dried using desiccants or dehumidifying techniques, but, in general, this measure is only practical for dryers with small volumes of air . Heat recovery from the product. In cases where products are deliberately cooled using forced air after drying, it might be feasible to recycle the resulting warm air, either directly into the dryer or through a heat exchanger to preheat the inlet air stream . However, for products that don’t require cooling, the cooling fan and heat recovery system cost might be greater than the energy cost savings associated with the recovered heat . Process controls. Process controls, such as feedback controllers, feed forward controllers, and model-based predictive controllers, can help to minimize dryer energy consumption by more precisely controlling energy inputs to meet the needs of the product being processed. Common sensors used in drying process control include thermocouples and resistance thermometers , infrared pyrometers , and wet-bulb and dry-bulb thermometers, resistance sensors, and absorption capacitive sensors . At the British Sugar beet sugar factory in Wissington, England, sugar is extracted from the beets and the remaining spent beet pulp is dried using rotary dryers to produce cattle feed. The company chose to install a model-based predictive control system to more accurately control the process performance of its rotary dryers. Following installation, the company reported saving £32,900 per year , dutch bucket hydroponic which was comprised of £18,900 in dryer energy savings and £14,000 per year in downstream energy cost savings . Furthermore, increased yields boosted savings by another £61,600 per year, enabling a payback period of just 17 months.Multiple effect evaporators. In general, significant energy efficiency gains can be realized by using multiple effect evaporators instead of single effect evaporators, where economically feasible. In multiple effect evaporators, the hot vapors that “boil” out of the liquid in one evaporator are used as the heating medium in another effect, which is operated at a lower pressure. By using multiple effects, the amount of water evaporated per pound of steam supplied to the evaporator system can be greatly increased. For example, a typical single effect evaporator will evaporate around 0.95 pounds of water per pound of steam input ; steam economy rises to around 1.8 for a double effect evaporator system, and to 2.6 for a triple effect evaporator system . There is a tradeoff between energy savings and the added capital costs of additional evaporator effects.
Furthermore, there is practical limit to the number of effects that can be used for any given product application; in practice, up to five effects might be feasible for evaporator systems used in food processing . Vapor recompression. In general, energy efficiencies higher than that of multiple-effect evaporator systems can be realized using vapor recompression systems, in which the vapors exiting the evaporator are compressed and reintroduced into the evaporator as a heating medium. There are two types of vapor recompression systems available: thermal vapor recompression systems and mechanical vapor recompression systems.In TVR systems, the vapors exiting the evaporator are compressed in a steam ejector using high pressure steam and the mixture is reintroduced into the same evaporator unit as a heating medium. Part of the vapors exiting the evaporator must be removed in order to maintain the proper mass balance of steam entering the evaporator unit. In MVR systems, the vapors exiting the evaporator are compressed mechanically and then reintroduced into the evaporator unit as a heating medium. A small amount of heating steam is added to the system to make up the condensate formed during compression of water vapors . The steam economy of MVR systems can range from 10 to 30, while TVR systems are less energy efficient and have a typical steam economy in the range of 4 to 8. Because of compression limitations and the high costs of evaporation under vacuum, vapor recompression units are mainly applicable where the product is not too concentrated and can be boiled under atmospheric or moderate vacuum conditions . Thermal recompression systems are most economical when high-pressure steam is available at low cost, while MVR systems are most economical when electricity is available at low cost . Sunmöre Meieri, a dairy processor based in Norway, opted for an MVR evaporator system to concentrate the basic ingredients of brown cheese from 11% dry matter to 55% dry matter. Because membrane concentration does not require a phase change , it is a more energy-efficient option for water removal than traditional steam-based evaporation methods. Membrane filtration systems have been successfully applied to the concentration of fruit and vegetable products, both in producing finished concentrated products directly and in pre-concentrating products prior to evaporation. The latter approach reduces the moisture content of the evaporator feed stream and thus reduces the energy requirements of the evaporator. The most common types of membrane filtration systems used in the food processing industry are reverse osmosis systems and ultra-filtration systems . At Golden Town Apple Products, a manufacturer of peeled apples and apple juices based in Canada, a combination of ultra-filtration and reverse osmosis has been used for apple juice concentration. In this process, the juice is heated to about 140°F and afterwards passed through a reverse osmosis membrane and an ultra-filtration membrane to produce apple juice concentrate. The system has maximum capacities of 3,000 liters per hour for feedstock, 1,500 liters per hour for final concentrate, and 1,500 liters per hour for water removed by reverse osmosis. The energy savings associated with this system were estimated at 66% compared to a traditional evaporation process. Additionally, the volume of equipment required for concentration was reduced by 50%. The payback period for the system was estimated at 2.5 years .Freeze concentration. For certain types of fruit juices and extracts, freeze concentration can offer a more energy-efficient concentration option than traditional evaporation methods. In freeze concentration, fruit juices are concentrated using a combination of freezing and mechanical separation. First, fruit juices are frozen to produce a slurry of frozen fruit liquids and ice crystals. Next, a separation device is used to separate the ice crystals from the fruit liquids. Energy savings are due to the fact that crystallizing a pound of water requires only about one-eighth the energy required to vaporize the same amount . In addition to energy savings, freeze concentration is said to produce fruit juice concentrates without appreciable loss in taste, aroma, color, or nutritive value, and to result in less equipment corrosion as a result of the low operating temperatures of the process . However, the high capital and refrigeration costs associated with freeze concentration might make it attractive for only high-value juices and extracts . To date, freeze concentration has been successfully applied in the making of fruit juices, beer, wine, vinegar, milk, and coffee .