Convection drying transfers heat energy to the product. The exact amount of energy transferred varies for different products, but it is usually marginal when compared to other energy expenditures within the system. In fact, this transfer of energy is often desirable as it may provide desired chemical or physical enhancement to the product.
With ever-increasing energy costs, however, many organizations are searching for ways to reduce their energy expenditure to maintain profitability. This initiative has led dryer manufacturers to develop energy-efficient technologies that decrease overall consumption within the equipment. So certainly, one way to minimize energy consumption is to specify an energy-efficient dryer design. Aside from technology advancements though, there are several methods to help reduce operating costs when it comes to drying equipment.
Whether your system utilizes a conveyor bed, rotary, fluid bed or other type of drying technology, the techniques to reduce energy usage are similar. To identify the potential for energy reduction in a dryer, you must first understand its balance of energy, which often is spent heating and evaporating water, and heating fresh airstreams. Losses through radiation, conduction leaks, component absorption, and applying heat to the product also must be considered. Examining each of these components individually will provide a glimpse at energy saving opportunities.
Heating and Evaporating WaterWater removal makes up the consumptive “beast” that lives within our drying systems. Within the belly of this beast reside two components: sensible and latent heat. Sensible heat is the energy required to elevate the temperature of the liquid water (1.0 BTU/lb-oF [4.2 kJ/kg- oC] for water), while latent heat is the energy required to change the phase of this water from liquid to vapor (1,000 BTU/lb [2,326 kJ/kg] for atmospheric water). During the drying process, latent heat requirements typically are about 10 times that of sensible heat.
From this component of energy, there are two main observations. First, we cannot reduce the amount of energy required to evaporate the product moisture under conventional, convection dryer conditions. Second, any potential reduction of evaporative load on the drying system can result in large gains in energy savings. Two ways to minimize the evaporative load are to reduce the moisture level of the incoming product and increase the moisture level of the exiting product.
Reduce Incoming Moisture.Look to the feed end of your dryer and the product that is entering. There may be additional moisture entering the system that is not necessary. If so, you may realize immediate energy savings with the proper removal or prevention of this incoming water.
What are possible sources of incoming moisture? The product may receive a rinse before entering the dryer. This adds a surface level of moisture that may be removed easily via air knives or additional conveyance. Perhaps the product is already mechanically dewatered. Check the performance of the dewatering device to ensure it is running optimally, and ask the manufacturer if there are new options that allow further mechanical dewatering of your product.
Excess moisture may not enter through the product alone. You may have online cleaning equipment that cleans the dryer during operation. This is additional moisture that will be evaporated during the drying process. Determine the overall necessity of using this cleaning system during runtime -- it may not be as necessary as originally thought. If it is, consider the frequency of operation and the volume of water used -- perhaps these may be decreased as well.
Increase Exit Moisture.Examine the moisture of the product from the dryer’s discharge end at different locations. It is likely that the overall moisture level of the product is slightly less than the necessary specification. This may be due to many factors, including upstream changes in moisture or poor moisture uniformity.
Make sure that the dryer settings are such that the product is drying to the maximum acceptable moisture setting. This may require some adjustment to ensure good moisture uniformity. This reduction in evaporation will not only save energy but also increase production for weight-based products (table 1).
Heating Fresh (Makeup) AirstreamsIn drying processes, the air that passes through the product gains moisture and loses energy. The humidity level in the dryer is thus increased, and some of the air must be exhausted in order to prevent the process air from becoming saturated. The volume of exhausted air removes the moisture and creates an overall negative system pressure, which is balanced by an equal mass of fresh air introduced to the dryer. The fresh air is ultimately heated to the exhaust temperature before it exits the dryer. This energy demand is typically less than that of the evaporation energy. However, it is where the largest potential area of improvement often exists.
Optimizing Exhaust Humidity.The more air that is removed from the dryer, the more fresh air needs to be heated. Often, the dryer exhaust may remove too much air (overexhausting) in order to ensure proper moisture removal from the product. This may be quite effective for drying, but it can increase energy usage to heat the fresh air entering the system.
Decreasing this energy usage requires that you optimize the exhaust humidity level. The ideal humidity level varies for different products and can be provided by your dryer manufacturer. Using humidity probes or dry- and wet-bulb thermometers, the exhaust stream’s humidity level should be checked and adjusted to the design humidity level. On simple systems, just reducing the amount that an exhaust damper is open can increase the humidity level. More advanced systems may require control system adjustments such as reducing an inverter signal to slow an exhaust fan. If an automatic humidity-control system -- intended to adjust the exhaust system to ideal conditions -- is in place, check it for proper operation and calibration.
Consider Your Fresh Air Source.Another consideration is the source of fresh air entering the dryer. Air from a cold, outdoor climate may cause significant demands on the heat energy required to reach the desired temperature. Consider whether it is possible to collect fresh air from inside the building. If this air is warmer due to equipment heat loss indoors, it may be a great means of energy recovery and net savings.
Likewise, if the product is cooled after drying, think about collecting the spent air from the cooling process and using this as makeup air for the dryer. It will be at a higher temperature than ambient air, and it will contain very little additional humidity from passing through the product for cooling.
Heat Recovery Systems.Given the right conditions, it may be possible to incorporate a heat recovery system with your dryer to capitalize on energy remaining in the exhaust stream. One of the most common means to accomplish this is through a heat exchanger.
There are many types of heat exchangers such as air-to-air and air-to-water. Although the principle is the same for all systems -- recapture the waste heat in the exhaust stream to provide higher energy makeup air -- the proper solution will depend on your drying environment. Many dryer manufacturers can advise which system would be best for your application.
Dryer Operation. Improper dryer operation such as a short retention time or airflow short circuits will result in excessively high operating temperatures, including exhaust. By keeping the exhaust temperatures low, there is less energy leaving the dryer, resulting in net savings.
Check the dryer design to ensure that all exhaust air is coming from the optimal location: after it has passed through the product. Eliminate any areas where process air may “short-circuit” the desired path and enter the exhaust stream. Finally, reduce the operating temperature of the dryer as much as possible, compensating typically by increasing retention time and airflow.
Other LossesLosses due to radiation and conduction losses, component heating and air leaks, although undesired, are inevitable. However, it is important to reduce the magnitude of these losses when possible. This opportunity is found primarily in leak reduction.
External doors and gaskets should be maintained at regular intervals. Older doors may be replaced with better-insulated doors that help reduce heat radiation to the atmosphere. Furthermore, seals in other areas where product enters or exits should be properly installed and maintained to avoid energy leaks.
So, how much can you really save by implementing some of the suggested changes? Actual savings depend on many factors, including the evaporation rate, your current process settings, and the production schedule for the dryer. Some values from a typical application example are shown in table 2.
Thermodynamics tells us that a perfectly efficient dryer typically requires, at a minimum, about 1,200 BTU/lb (2,800 kJ/kg) of water evaporated. Experience tells us that some improperly functioning and inefficient driers can consume up to 4,000 BTU/lb (9,304 kJ/kg) of water evaporated. A more common situation is to see an inefficient dryer using approximately 2,500 BTU/lb (5,815 kJ/kg); after optimization, that same dryer can reach reasonably “good” levels of 1,500 BTU/lb (3,489 kJ/kg).
Now consider (table 2) these different energy levels for a feed-drying operation producing approximately 15 ton/hr (13,608 kg/hr) at an evaporation rate of approximately 6,000 lb/hr (2,722 kg/hr).
If your dryer’s operation is in a sad state of affairs, there may be a great deal of savings from these energy consumption improvements. Because many of the improvements require simple adjustments and low up-front costs, why not push for ideal conditions even when you are already somewhat efficient?
Follow these simple tips to achieve energy savings in your drying systems. Enlist all of the tools as your disposal, including a consult with your dryer manufacturer. Simple adjustments such as these just might provide a larger payback than you expect.