SYSTEM START-UP
The Siemens indirectly heated dryer is controlled by a PLC with touch screen controls, color graphics and SCADA interface, and can have different features depending on project requirements.
As a general rule, the operator starts the warm-up process by starting the oil pump and firing the oil heater’s burner (see the description below of the Thermal Fluid System). The thermal transfer fluid used in the hot oil heater requires between 1½ and 2½ hours “heat up” time to reach optimum operating temperature.
The dehydration drum burners (see the description below of the Dehydration Drum) can be started after the thermal transfer fluid has reached 50 percent of its operating temperature.
To ensure the longevity of the thermal transfer properties of the oil and the integrity of the metal being heated, it is important to start up the indirectly heated dryer as detailed in the start-up instructions.
FEED HOPPER
Sludge is placed into the feed hopper by either a loader, a conveyor, or it is directly deposited from the existing mechanical dewatering press. Inside the feed hopper is a wiper bar mechanism and screw auger.
The wiper, positioned directly above the auger located in the bottom of the hopper, maintains a constant feed of wet material to the screw. The auger extends out the bottom of the hopper inside of a feed tube and connects to the “wet” end housing on the sludge dehydrator.
The feed rate (volume) of product fed into the dehydrator and housing is determined by the revolutions per minute of the rotating auger. This speed is controlled from the control panel by the feed auger motor speed controller.
The system is generally set so that the hopper actually feeds product into the end housing at a faster rate than the dehydration auger can move it into the dehydration chamber. This will cause the end housing to become filled with wet product.
To prevent the housing from becoming totally impacted with material, there is a temperature probe located in the hot vapors in the top of the housing that will turn the hopper feed off when it comes into contact with cooler material. When the material level falls several inches, the hopper feed automatically turns back on.
DEHYDRATION AUGER
A thermal fluid pumped through the flights from the thermal fluid heater heats the close-pitched, hollow flight dehydration auger. The temperature of this fluid is maintained at a setting to keep the auger flights hot enough to prevent any sticking of the wet material to the product to the auger on the dry end. The auger rotates and pulls the wet product form the end housing through the heated, counter-rotating dehydration chamber. The wet material starts giving up its moisture as soon as it comes into contact with the heated auger flights. This moisture release is increased as soon as the product is pulled into the heated dehydration chamber.
DEHYDRATION DRUM
Three burners, located below the insulated housing that surrounds the tube, heat the dehydration drum which rotates in the opposite direction of the dehydration auger. Each of the burners controls the temperature in a separate “zone”. The burner that is closest to the wet end housing is generally set to maintain a higher dehydration drum temperature in this zone, because the product is wettest in this area, and can be subject to a higher temperature without the chance of burning or scorching.
As the material is moved into the second, or middle, burner zone, the temperature is decreased because the material has released much of its moisture at this point and the “thermal barrier” (high moisture content) that existed in the first zone is greatly reduced. By the time the product has entered the last zone on the dry end of the dehydration drum, majority of the moisture has been removed. The temperature of this third burner zone is reduced to a setting to ensure the product is not burned or scorched.
The average volumetric reduction of material processed in the indirectly heated dryer is approximately 5:1. Since the hopper feed is generally set to keep the wet end housing completely full of material, the dehydration auger is pulling a full load of product into the dehydration chamber with every revolution, so we can assume that the dehydration chamber is almost 100 percent filled at the feed end.
The volume of sludge at the dry end would be at only 20 percent capacity if it were not for a weir positioned at the end of the dehydration chamber. This weir raises the level of the product on the dry end of the dehydration chamber so that more of the heated surface area of the auger flights is used.
The desired dryness and temperature of the discharged product is based upon three factors:
- The temperature settings of the three dehydration drum burners.
- The temperature setting and flow of the thermal fluid through the dehydration auger.
- The time the product is allowed to remain in the dryer chamber.
Settings depend entirely upon the type of material being processed, the amount of moisture left in the finished product and the required minimum temperature (if any) of the product.
The dehydrated product falls out of the dehydration chamber and into a dry material discharge chamber that has a wiper assembly and screw auger very similar to that in the feed hopper. The discharge auger conveys the dehydrated product out of the dryer and into an airlock chamber.
The discharge port on this chamber is higher than the material inlet port to ensure that the chamber stays full of dry product at all times, preventing any air from being drawn in to the dehydration chamber. Typically, a dry product discharge conveyor is attached to the discharge port of the airlock chamber to convey the material to storage.
THERMAL FLUID SYSTEM
The thermal fluid system is the heat source for the oil that is pumped through the dehydration auger. It is of the triple pass coil heater tube design. The thermal fluid is maintained at a minimum level where it is always visible in the expansion tank sight glass to ensure there is always a positive head of fluid to the suction side of the system pump.
The outlet (pressure side) of the pump is piped directly into the coils on the heater. The fluid is sent through the coils, which are heated by a gas fired burner. The oil is brought up to the set temperature, and then piped to the inlet end (wet end) of the dehydration auger where it passes through a rotary union, flows through the auger flights, exits through another rotary union and then re-enters the pump on the suction side. The fluid loses some degree of heat after it passes through the auger. The amount of heat lost (temperature differential) depends on the oil flow, residence time of the material in the dehydration chamber and volume of material in the dehydration tube.
SCRUBBER / CONDENSER
The product exhaust vapors from the indirectly heated dryer are exhausted from the top of both the wet and dry stationary end housings. Since no outside air is allowed to be introduced to the inside of the dehydration chamber, the exhaust is virtually a pure vapor. The vapors leaving the dehydrator are created from the vaporization of the water contained in the sludge, therefore, the cooling of the vapor to a temperature close to the temperature of the sludge when it was introduced into the dehydrator will virtually eliminate all of the exhaust air.
A certain amount of particulate will be entrained in the exhaust stream. The amount of particulate will depend upon the particulate size of the solids in the product and the dryness of the discharged product. The ducts from both end housings are connected into one line before attaching to the scrubber/condenser tank. Located at various positions along the ducting are water spray nozzles. This water spray contacts with particulate that may be entrained in the vapor stream and flushes it to the condenser. The sprays also help to cool the exhaust stream. The scrubber/condenser system is comprised of a modular system consisting of a quencher stage, venturi stage and a mist eliminator. The remaining small vapor stream is then pulled through a blower and exhausted to the odor control system (by others).
SHUT DOWN
The shut down of the indirectly heated dryer is an automatic process initiated by the operator, or by the depletion of wet sludge cake in the feed hopper. In either case, the indirectly heated dryer is placed into shut down mode through the PLC. The PLC will time the dryer’s operation based on the time the feed hopper was stopped. As the sludge is dehydrated and leaves each zone of the dehydration chamber, the burners are shut off. For example, when there is no longer any sludge in Zone 1, the Zone 1 burner is shut down. This process repeats itself for Zone 2 and then Zone 3.
The thermal transfer fluid continues to circulate through the heater and dryer auger until the fluid reaches a temperature that will allow the PLC to shut down the burner and the oil pump.