CO-CURRENT vs. COUNTER-CURRENT DRYERS
Counter-Current Flow Rotary Direct heat Dryer:
The counter-current flow dryer operates by having the burner on one end and material is fed to the dryer from the opposite end. Therefore, the material is advancing towards the flame. This method is utilized on many of the aggregate dryers used in the asphalt industry when there is a batching system.
The perceived advantage is that the material advancing towards the hot end of the dryer can be heated to a discharge temperature which is not related to the discharge gas temperature. Stated another way, the material can leave the dryer hotter than the temperature of the exhaust gas. This provides a “perceived” reduction in heat energy due to lower stack loss.
The primary reason for this arrangement is that the material in the counter-current flow dryer is being heated after drying in order to accomplish another purpose. In the asphalt dryer example, the material may be combined with recycled asphalt product (RAP) which is the material stripped from old roads. This material cannot be processed in the dryer because it is coated with Bitumen. So the rock is superheated in order to carry enough heat energy to dry the RAP during mixing, which follows the dryer.
The disadvantages of the counter-current flow dryer are:
• Since there is no relationship between the exhaust gas temperature and the material temperature, the burner must be controlled by the temperature of the material exiting the dryer. If material temperature drops, the burner heat is increased and vise verse. But the change in process conditions is based on the incoming material, not the exiting material. Therefore the response in the burner is just a guess: the result will not be known until the dryer has cycled through the residence time for the material in the unit. This would be several minutes.
• Since the feed material is entering on the cool wet end of the dryer, the quick flash of evaporation usually occurs somewhere towards the middle of the drum instead of near the feed end. It is not unusual for a cake ring to form on the shell just prior to this spot.
• Since there is limited control of the discharge gas temperature, there is a real danger of the exhaust gas temperature dropping below the dew point, especially in the winter. This increases the risk of mudding the bag filters in the dust collector.
• Since the control has a long lag time, most operators will over dry and over heat the material so the system will run smoother. So the lower stack loss is eaten up by unnecessary work being performed for built in tolerance.
For special circumstances, the disadvantages of the Counter-Current flow may be out ranked for the following reason. The operator may have to heat rock to say 350-400 OF for the added mixing process and cannot tolerate the high gas temperatures in the baghouse. So the counter-current flow dryer has a very good reason for existence.
Co-Current Flow Rotary Direct heat Dryer:
The Co-Current flow dryer operates by locating the burner and feed system on the same end. The material and the exhaust gas exit on the opposite end. This method is utilized on all direct heat dryers processing heat sensitive material and most ore and aggregate dryers where the goal is to dry and not to add heat to the dried product.
The primary advantage of a Co-Current flow dryer is:
• Product dryness has a direct relationship with temperature of the exhaust gas. The exhaust gas will exit the dryer at some amount greater than the product temperature. The spread is dependent on the efficiency of the dryer and the extent of dryness of the processed material. This allows the burner control to operate on exhaust gas temperature. The gas molecules travel through the dryer in only a few seconds. Therefore, changing feed conditions can be compensated for in seconds instead of minutes.
A typical drying curve is located below for both Co-Current flow and Counter-Current flow rotary dryers. The reference line would be representative of a bench mark temperature for each system. The TM curve represents the temperature of material. The TG line represents the gas temperature.