Raising a dryer’s operating temperature and increasing the dryer size are two strategies that can help reduce energy usage in convection dryers.

As I write this, the price of a barrel of oil has been flirting with $100 again, and the price of a gallon of gasoline is well above $3 in most parts of the country. While the current energy environment is unprecedented, it is doubtless just a harbinger of the challenges to come in the near future.

One of the most energy-intense unit operations in a processing plant is that of moisture removal through thermal drying. Raising the temperature and increasing the dryer size are two approaches that can be taken to reduce the energy usage in a convection-type dryer.

Figure 1. At higher temperatures, the exhaust rate per pound of evaporation is already low, which leads to diminishing returns. Therefore, the greatest gains in making running a dryer at a slightly hotter temperature can be achieved with those dryers operating at a relatively low process temperature such as 212oF (100oC).

Make It Hotter

To improve an air dryer’s energy efficiency, raise the temperature. Yes, that is correct: As counter-intuitive as this may seem at first, there is a sound thermodynamic basis for this advice that can be easily demonstrated with a psychrometric chart. Most dryers do not run at maximum possible capacity on all the products being run through them. Instead, to prevent overdrying -- an obviously inefficient practice -- the operating temperature usually is turned down well below the maximum value allowable for acceptable product quality.

While this approach does reduce energy use somewhat, it is not the most effective means. Instead of being reduced, temperature should be increased as much as product quality will allow. Concurrently, the exhaust should be reduced to raise the humidity to a level that slows down the drying rate enough to produce the desired final moisture in the product. This new operating condition will likely use less energy than before because the reduction in exhaust flow more than offsets its increased temperature.

The actual savings achievable depend upon the type of dryer and the details of each application but, in general, this method works best for lower temperature drying, becoming less effective for exhaust temperatures much above 212oF (100oC). Of course, in addition to product quality, there may be other concerns -- controlling pressure inside the housing, or condensation in the ductwork, for example -- that limit how much the exhaust rate can be reduced.

This concept can be demonstrated by the following simplified example of a through-circulation dryer operating in the constant rate drying region at 200oF (93oC) dry bulb (DB) with exhaust conditions at 150oF (66oC) DB and 110oF (43oC) wet bulb (WB). If you raise the operating temperature to 250oF (121oC) and reduce the exhaust flow until the wet bulb is 160oF (71oC), you can produce product at approximately the same moisture level as before while increasing efficiency.

How is this possible? You first must understand the concept of wet bulb depression (WBD), which is a measure of the difference between DB and WB. The drying rate is roughly proportional to the wet bulb depression, which in the example has remained the same prior to and following the change. Because the evaporative rate is unchanged, the DB temperature drop through the product bed will be the same 50oF (27oC), making the exhaust temperature 200oF (93oC) DB.

In order to achieve the new operating point, the mass flow of the exhaust will have been cut back so much that, even though the temperature is higher, the overall energy expended will be less. You can get a sense of this by looking up the absolute humidity values for each of the two exhaust conditions; remember, exhaust rate is inversely proportional to humidity.

The first point is 0.04924 pounds of water vapor per pound of dry air (lb-W/lb-DA) having an enthalpy of 91.554 BTU/lb-DA. The second point is 0.28433 lb-W/lb-DA with an enthalpy of 374.601 BTU/lb-DA.

Dividing each enthalpy by its associated humidity, you get the relative amount of energy ideally required to evaporate a pound of moisture at each operating condition: 1,859 BTU/lb-W for the first and 1,317 BTU/lb-W for the second, higher temperature operating point.

Figure 1 illustrates this concept for an actual dryer. At higher temperatures, the exhaust rate per pound of evaporation is already low, which leads to diminishing returns. In addition, other increasing heat losses, like those through the insulation or end leakage, eventually overcome the savings from diminishing the exhaust rate.

While the approach of simply raising the temperature and cutting back the exhaust rate is one of the simplest energy-saving strategies, it has one obvious potential problem. The product temperature goes up and, at some point, the loss of product quality prevents any further raising of the temperature. When this point is reached, another strategy must be used to make further gains on energy efficiency.  

Figure 2. In this example drying application, energy savings are plotted against size relative to a base. (The base represents the minimum size that can evaporate a given amount of water from a product at typical exhaust humidity conditions.) For each operating temperature, the capacity is held constant as the size of the dryer is increased and the exhaust rate is decreased.

Make It Bigger

To improve an air dryer’s energy efficiency, if there is excess, unused drying capacity, the exhaust can be reduced to save energy.

The easiest time to implement this strategy is when purchasing a new dryer. Unfortunately, while energy consumption figures are routinely included in quotations, they rarely spark a dialog about the tradeoffs between operating and capital costs. Consequently, most purchase decisions seem to be made solely on an equipment cost basis. This compromises energy efficiency. It would provide a more complete and long-range picture if the vendor were given the expected cost of energy and required rate of return, and asked to provide an alternate, lower energy option. Keep in mind that the purchasing department needs to understand and support the bigger, long-range picture for this approach to yield useful results.

Figure 2 provides an example where energy savings are plotted against size relative to a base. The base represents the minimum size that can evaporate a given amount of water from a product at typical exhaust humidity conditions. For each operating temperature, the capacity is held constant as the size of the dryer is increased and the exhaust rate is decreased.

For existing installations, adding to the physical size of the dryer can be problematic even if space is available. Before this is contemplated, a thorough evaluation of the dryer and its operating parameters should be made. The first step towards energy conservation is making sure the drying process itself is optimized. From an energy point of view, increasing the drying efficiency is the same as adding drying area. Once the drying capability is increased, the exhaust rate can be reduced to save energy.

The first thing to do is make sure that the dryer and related equipment have been properly cleaned and maintained. Begin with general “housekeeping,” such as ensuring that all heaters, conveyor perforations and air-distribution devices are clean. Then, move on to the basics checklist, which includes:
  • Adjusting spreaders to provide an even load across the conveyor.
  • Adjusting conveyor seals.
  • Verifying that all fans are running and that they are rotating in the correct direction.
  • Evaluating whether the proper overlap of the fan and inlet cones exists.
  • Recalibrating temperature sensors and controllers.
Once the mechanical aspects of the dryer are addressed, process optimization can begin.

First, bed depth is an important drying parameter, and one of the easiest to optimize. All that is required is to slowly vary the speed of the conveyor while monitoring the product moisture at the dryer outlet. Slower and deeper will result in more evaporation until the increasing resistance of the product bed reduces airflow to the point that the humidity of the air leaving the bed begins to retard the drying. This can be done while the equipment is in operation. Make small incremental changes so the flow downstream is not upset. Make sure that the dryer curtains have enough flex to allow the product to pass freely.

Second, the ability to run deeper loads is enhanced with better airflow. Consider adding more fan capacity.

Third, make sure that the product entering the dryer is as uniform as possible in size, free flowing and as free as possible from fines. If the particle size can be reduced without unduly restricting airflow, the drying rate usually will be increased.

As evidenced by a reduction in discharge moisture, with each improvement in the dryer operation, the exhaust can be cut back to bring the product moisture back into specification, saving energy in the process. Check the exhaust system to make sure that the humidity has not become high enough to cause condensation. Insulate if necessary.

An energy monitor at the control panel provides documentation of the savings and, hopefully, motivation to the operator to continue operating the dryer efficiently. Alternatively, product moisture and exhaust humidity sensors in combination with model-based computer control systems can be used to automatically take energy usage into account while controlling output moisture.

One final additional advantage to saving energy by making the dryer “bigger” through process optimization or increased size is that this approach provides a latent reserve of drying capacity that can be tapped, as needed, simply by increasing the exhaust rate. So bigger and hotter really often are better.