Many dried goods producers do not clearly understand how energy is used in their convective drying processes. Additionally, some producers may be expending considerably more fuel than is required. Failure to understand the concepts of dryer energy usage and energy balance within the dryer is the leading cause of incorrect or ineffective dryer adjustments, which inevitably lead to process inefficiencies.
To avoid energy mishandling, dry goods producers should devote at least one resource to dryer efficiency; specifically, they should designate a dryer energy guru. Having an in-house expert for your drying system can help your facility save money due to reduced energy consumption. Also, your guru will develop a sense for budding in-process issues that can improve day-to-day product quality. Finally, by instituting the methods suggested in this article, a dryer energy guru will be able to collect invaluable baseline dryer energy data. This data -- in concert with data about other factors such as evaporation rate, production rate, and product and process temperatures -- will help identify the constituents that lead to your current inefficiencies. Using this data, your guru can help guide you past the deterrents that stand in the way of optimal efficiency.
In this article, I will discuss two methods a dryer guru can use to quantify a dryer’s energy usage: via a flowmeter device, or via calculations, determined empirically as a sum of its quantifiable components.
Figure 1 illustrates graphically the individual energy components at work during the drying process. The control volume, shown drawn around the dryer, identifies the boundary through which energy and matter will pass. From this, the three main energy components (excluding losses) are found:
- Evaporating water.
- Heating air.
- Heating the product.
Understanding these components will help you take back control of your dryer’s energy usage.
A Fuel Flowmeter Provides Advantages
For the lucky few that have a dedicated dryer fuel flowmeter, the learning curve to becoming a dryer energy guru is greatly reduced. Because most dryers are either steam or fuel heated, a flowmeter can be utilized to provide a direct reading of the dryer’s energy consumption. This online fuel measurement data then can be recorded and utilized to improve dryer efficiency.
One way to provide instantaneous energy usage feedback is to integrate the data found by your flowmeter back into your dryer’s human-machine interface (HMI). How will this help a dryer guru? By having a good backlog of product-specific consumption data at hand, a dryer guru can judge day to day if the dryer is performing efficiently.
For example, if your cleaning crew has left out (or open) an internal process-air blank-off -- creating a major airflow short circuit -- your energy usage will be considerably higher than normal. With the flowmeter data, the dryer energy guru would notice the abnormally high energy usage and begin the process of troubleshooting. Without the backlog data to compare statistics day to day, this situation might go undetected.
Why must a dryer guru understand the components of energy usage if they have a dedicated flowmeter on their dryer? Because understanding how energy is used -- not just how much -- will help them to diagnose and repair energy inefficiencies.
No Flowmeter? No Problem
Understanding how the energy is used in the dryer -- to evaporate water, heat air and heat the product -- can help any dryer guru. By examining each component, any dryer energy guru will be able to find potential pathways to unrealized efficiencies.
Component 1: Evaporating Water. The fundamental goal in all drying processes is to feed the dryer with a high-moisture-content product and subsequently produce a low-moisture-content product. Therefore, it is important that you realize that the dryer’s energy usage is largely dependent on its evaporative processing requirements.
Two types of heating contribute to the energy required to evaporate water: sensible and latent. Sensible heating raises the temperature of the water that is to be evaporated. Latent heating is the heat required to change the state of water from a liquid to vapor. Drying gurus take note: latent heating is responsible for the majority of the dryer’s energy consumption.
The energy required to evaporate water is equivalent to the mass flow of water evaporated (evaporation rate), multiplied by the enthalpy difference of the water vapor exiting the dryer exhaust and the liquid water present in the product at the dryer entrance.
- To calculate the evaporative load in the dryer, subtract the mass of product entering from the mass of product exiting. This assumes that the dry solid rate is consistent throughout, and that no additional water mass is added during the drying process.
- Liquid water enthalpy is based upon the temperature of the product entering the dryer, and the water vapor enthalpy is based upon the temperature of the exhausted air. To find the enthalpy of liquid water and water vapor, reference a thermodynamics textbook for a “Properties of Saturated Water” (or steam table). This table will equate the liquid and vapor enthalpy values for saturated water at given temperatures.
From thermodynamics, we find that the energy required to evaporate water, at standard conditions, remains constant. However, the amount of water evaporated is a process-dependent variable. Hence, a dryer energy guru will work to minimize the amount of water evaporated in their dryer.
Component 2: Heating Air. Fresh air must be heated because all convective dryers, regardless of shape, design or function, have a means to introduce fresh intake air due to the negative pressure developed by the exhaust fan. As discussed previously, evaporation over time is a mainstay of all drying processes; therefore, to maintain an optimal, steady, internal humidity, a constant mass of air must be removed from the system. Likewise, for a balanced system, that constant exhausted mass of air must be replaced by an equivalent mass of fresh air.
The energy required to heat air is equivalent to the mass flow of intake air (or exhaust, in a balanced system) multiplied by the specific heat of the intake air, multiplied by the temperature difference of intake and exhaust air.
- Unfortunately, directly measuring the mass flow of intake or air exhaust is quite difficult. A simpler method is to directly measure the volume of air entering the system. From a dry and wet bulb measurement taken in the same stream, the air density and specific heat can be found using psychrometric properties.
- To calculate intake mass flow, multiply the air volume measurement by the air density.
- Lastly, with the use of a thermocouple and handheld digital thermometer, the exhaust and intake air temperatures can be found.
Typically, the exhaust and fresh air mass exchange will yield large energy savings if optimized. A dryer guru should examine the amount of exhausted air with respect to exhaust air humidity. If humidity levels are low, mass flow can be reduced.
Also, a dryer guru could use cooling air, which has passed through hot product, as input air to the dryer. This reduces the temperature difference between the exhaust and intake air. Both of these strategies provide additive energy savings.
Component 3: Heating the Product. It is obvious that energy from the dryer will go toward the heating of the product. However, it may not be so evident that this amount of energy increases throughout a product’s retention time.
The reason for this is simple: As the moist product enters the dryer, water is easily removed, and the product is cooled via this rapid evaporation. As the product becomes drier, the evaporative cooling effect is lessened, causing the product temperature to rise.
The energy required to heat the product is equivalent to the mass flow over time of product discharging the dryer, multiplied by the specific heat of the product, multiplied by the entrance and exit temperature difference of the product. Collecting the data required to calculate this portion of the dryer’s energy consumption is relatively simple.
- Dryer discharge mass flow, in a majority of processes, typically is monitored and should be easy to find.
- Product specific heat, by definition, is the amount of heat required to raise one unit of mass by one unit temperature. Most product specific heat values can be found in a heat transfer textbook.
- Finally, product temperature at dryer exit and entrance can be measured with a handheld infrared or contact thermometer.
Why is this important to a dryer energy guru? Typically, the product will exit the dryer’s control volume at a higher temperature than when it entered. Reducing this product temperature difference will reduce the overall consumption.
Putting Your Data to Work
Now that you’ve gone through the effort of understanding your dryer’s energy streams and collecting valuable baseline process data, how much energy should an efficient industrial dryer consume?
Based on field experience with convective dryer energy optimizations, most optimized dryers will consume less than 1,500 BTU/lb (3,489 kJ/kg) of water evaporated. However, it is not uncommon to find dryers in the field operating at 2,500 BTU/lb (5,815 kJ/kg) of water evaporated. What would it mean to your bottom line if you could save 66 percent of your dryer’s current energy consumption?
To calculate the energy use per mass of water evaporated in your scenario, divide the energy consumption value, either found from your flowmeter or calculated based on a component analysis, by your dryer’s evaporation rate. If these calculations are followed, and you find your dryer is consuming much more energy per unit mass than this value, I’d suggest using your new found dryer energy understanding to make educated modifications to your drying process.
Finally, remember to always enlist the help of your dryer manufacturer. They may provide you with an innovative pathway to understanding and maximizing your dryer’s performance. Furthermore, they may offer training to further enhance your guru ego.
Editor's Note: This article was originally published with the title, "How to Become an Dryer Energy Guru" in the June 2009 issue of Process Heating magazine.