“Mirror, mirror on the wall, who makes the most efficient dryer of all?”
Good question. Any self-respecting manufacturer or sales rep is likely to give you essentially the same answer: “We are the energy leaders. No dryer performs like ours. My dryer is the most efficient dryer of all.”
Obviously, not everyone can be the most efficient. Let’s take a closer look.
There are two major consumers of energy in a resin dryer: the process circuit and the regeneration circuit.
- Heating element. Dryers can provide heat through means of gas burners or electric heating elements. Electric heaters are more efficient than gas heaters and they are generally a better choice except in regions where gas prices are very low. Among modern dryers that have similar heating capacity, I doubt that you’ll find a significant difference in energy usage between different heating elements although heater type can significantly impact its overall life span and serviceability. Tube style heaters with solid state relay control are much more reliable than other alternatives and hold up the best under the constant cycling required.
- Open or closed loop. If you’re running a dryer that has a single blower that powers both the process and regeneration air, your system is open loop, which means that it is continually drawing and mixing a quantity of fresh, relatively moist air into the drying circuit. This is necessary by design, but it is not efficient, since all that moist air now needs to be dehumidified. Closed loop systems, such as that used in the Conair Carousel Plus™, do not admit outside air. This means the system continues to recirculate the same filtered, low dew point air, which requires much less energy for dehumidification and is immune to ambient conditions.
- Airflow/airflow monitoring. Drying occurs when resin is at the proper temperature for a designated time and airflow is the means through which heat is transferred to the pellets and moisture is transferred away from the pellets. Dryers and hoppers are sized by application experts at Conair to make sure the proper ratio of heat and airflow is properly met. In more advanced systems, some monitor heat transfer by taking the temperature at one or two positions in the hopper. Conair takes this one step further with its Drying Monitor™. With Drying Monitor™, we observe the entire temperature profile at six points within the column of pellets to make sure that the heated airflow is adequate. Conair’s EnergySmart® dryers use Drying Monitor™ information to automatically reduce the airflow to maintain good drying conditions in the hopper using the lowest amount of heated air possible. By sensing the entire hopper temperature profile, we ensure that you are getting the right amount of pellet time at temperature.
Precise temperature/airflow monitoring can also be a helpful feature for processors who change molds regularly. If you’re running a dryer at anything less than 100% capacity, accurate monitoring allows the dryer to automatically “dial down” the airflow input, enabling you to realize substantial energy savings as you reduce the amount of air that needs to be heated. For example, if you’re running 750 lbs/hr in a dryer with a 1000 lbs/hr capacity, accurate monitoring provides the data needed to dial back heated airflow, reduce consumption and save energy, all while maintaining required pellet airflow and temperature. In most other dryers, you’re stuck running full capacity for partial loads.
Regeneration circuit. Regeneration is the process by which the captured moisture from the pellet is removed from the system so that the desiccant can once again be used to dehumidify the process air. This is done in different ways depending on manufacturer, but the goal is the same.
- Blower size. Blower size and energy consumption are determined by work requirements. For example, pushing air through one wedge of a desiccant wheel is easier than pushing it through a tall canister of densely packed desiccant beads, due to its smaller mass and open channels. So, desiccant wheel dryers can use a smaller regenerative blower, while desiccant canister dryers require much larger regenerative blowers that consume more energy.
- Regenerating desiccant requires heating for moisture removal, followed by cooling to bring the desiccant back to working temperature. Desiccant wheels have considerably less mass to heat, so regeneration heating/cooling (which are continuous) takes far less energy overall. Large canisters of loose desiccant beads have far greater mass so, although they are regenerated less frequently, the process requires considerable amounts of energy to heat them up for regeneration and to cool them for use. A typical tower dryer regenerates at 550°F to 600°F while max regeneration temperatures on wheel dryers are about 350°F. However, all Conair dryers included a dew point control feature that further reduces energy consumption by lowering the regeneration temperature when possible. It is not uncommon to see a Conair dryer making a -40°F dew point with a regeneration temperature well under 300°F.
- To adsorb moisture, any desiccant must be maintained at a reasonable operating temperature. Therefore, many desiccant dryers are equipped with aftercooling circuits that pull heat from the process line and dissipate using a water-cooled or air-cooled heat transfer unit. In addition to improving desiccant performance, aftercooling also improves filtering efficiency, particularly for “volatiles” and other impurities that are common when drying recycled materials. Filtering out these impurities improves material quality and prevents them from contaminating or reducing the efficiency of the desiccant.
Aftercooling is most commonly needed when processing materials require high drying temperatures and when dryers have comparatively less desiccant mass, such as desiccant wheel dryers. Some processors who utilize desiccant canister dryers, particularly large ones, don’t worry as much about aftercooling and filtration, since they have a large mass of desiccant that must be periodically replaced anyway.
Quick Summary. I hope that this discussion sheds light on the practical factors that contribute to energy efficiency in dryers. And, while the debate about who’s got the most efficient dryer is likely to go on, you can see where I stand, based on the quick summary below:
|Factors that favor higher efficiency||Factors contributing to reduced efficiency|
|Closed loop air circuit||Open loop air circuit (admits humid fresh air)|
|Multi-point, in hopper temperature profile monitoring||Single or dual-point temperature/airflow monitoring (less accurate)|
|Adjustable airflows (use less heated air for smaller pellet loads)||Fixed airflows, regardless of pellet load|
|Smaller regeneration blower||Larger regeneration blower|
|Smaller amount of desiccant to regenerate (heating, cooling, aftercooling)||Larger amount of desiccant to regenerate|