5 ENERGY SAVING APPROACHES IN DRYERS

Typical values of dryer efficiencies and specific energy consumption are given below in table 5.1.

Table 5-1: Expected Dryer Efficiencies

Dryer group and type	Typical Heat loss sources	Typical specific energy consumption, MJ/kg of water	Typical efficiency
Rotary
·         Indirect Rotary	Surface	3.0 to 8.0	28 – 75%
·         Cascading Rotary	Exhausts, leaks	3.5 to 12.0	19 – 64%
Band, Tray & Tunnel
·         Cross circulated tray/oven/band	Exhaust, surface	8.0 to 16.0	14 – 28%
·         Cross circulated shelf /tunnel	Exhaust, surface	6.0 to 16.0	14 – 38%
·         Through circulated tray / band	Exhaust	5.0 to 12.0	19 – 45%
·         Vacuum tray / band / plate	Surface	3.5 to 8.0	28 – 64%
Drum	Surface	3.0 to 12.0	19 – 75%
Fluidised / Sprouted bed	Exhaust	3.5 to 8.5	28 – 64%
Spray
·         Pneumatic conveying/Spray	Exhaust	3.5 to 8.0	28 – 64%
·         Two stage	Exhaust, surface	3.3 to 6.0	38 – 68%
·         Cylinder	Surface	3.5 to 10.0	23 – 64%
Stenter	Exhaust	5.0 to 12.0	19 – 45%

The main categories of energy saving approaches in Industrial Dryers are as follows:

5.1 Evaluation of Energy Efficiency and Diagnostics

This approach is of fundamental importance in identifying areas of wastage and in deciding needs for improvement in operational practices, retrofits modifications and changes in technology. The primary requirement is for quantification through appropriate measurements. Often a heat balance approach is useful as an analytical tool. Comparison with already established industrial standards or norms is useful provision of certain minimum level of instrumentation can help In-House Audit.

The importance of time utilisation, efficiency and machine production efficiency in energy conservation is often not evident to users. However, technologies leading to higher rates of drying in a shorter time and aids, which reduce energy consumed during machine stoppage, also contribute significantly to energy saving. Automatic controls can eliminate manual dependences and enhance production efficiency.

In terms of retro-fit modifications, different methods of heat recycling especially In- situ Heat Recovery enables quick return on investments.

5.2 Increasing the Temperature Differential

The higher the temperature differential (dT) across the dryer, the more efficient the operation, the higher the energy transfer, and the greater the productivity of the unit. In many instances, users may have concerns about operating temperatures that are unfounded, and these temperatures can be adjusted without a detrimental effect. Even a small adjustment can result in a much-improved yield.

Increasing the temperature differential may increase the inlet temperature or reduce the exhaust temperature -- optimally, it will affect both. Some of the primary concerns regarding increasing the dT are:

• Damaging the product (overheating, discoloring, modifying the particle characteristics, skinning, cracking).

• Increasing the humidity of the exhaust stream, potentially causing a moisture block.

• Creating condensation problems related to the exhaust humidity.

• Causing thermal expansion of the dryer due to the higher temperatures.

• Exceeding the physical limitations of the materials of construction.

• Increasing heat losses due to inadequate insulation and leakage.

The process of drying imparts various energies to the feed, including sensible heat and latent heat of vaporization. Sensible heat raises the temperature of the feed and the fabric of the dryer to the operating condition, and no more. Water molecules that evaporate from the product being processed retain the latent heat as they leave the product mass and hence, reduce the energy of the mass. This reduction in energy, in the form of heat, will promote the phenomenon of evaporative cooling and will keep the product mass at a reasonably constant temperature for the bulk of the drying process. Testing often reveals that this temperature is substantially lower than the temperature at which damage would occur to the product.

Similarly, it is preferable to maintain the exhaust above the dew point temperature. In many instances, there is a conservatism that is applied to this aspect. Once again, testing the actual condition will provide a potential opportunity.

5.3 Reduce Moisture Loading.

Moisture is introduced to the dryer by the feed, the process air and, in certain instances, by reaction, such as combustion. Reducing this loading allows the energy to be better utilized on the drying process.

Mechanically dewatering: Energy used in mechanical dewatering is only 1% of the energy used for evaporate the same quantity of water. Wherever possible, mechanical dewatering techniques -- filtration (vacuum, pressure, membrane, etc.), concentration, air knives, centrifugation, etc. -- should be employed. Also, it may be advantageous to change your current mechanical dewatering system to a more efficient method. For instance, concentrates can be dewatered on vacuum filters to approximately 25% moisture (wet basis). Membrane pressure filters can achieve final moistures below 10% for the same concentrate.

For each 1% reduction in feed stock moisture content, the dryer input can be reduced by 4%.

Using Dry Air. Using dry air for the process air reduces the quantity of moisture in the air that requires heating and vaporization. For small volumes of air, using desiccant or dehumidifying techniques will reduce air moisture levels effectively, but for larger volumes, this becomes impractical. In very humid environments, however, conditioning of the air will reduce the energy

An example of this technique would be the case of kaolin dryer with a duty to produce 50,000 lb/h (12,727kg/h) of solids with 1% moisture from a feed of 99,000 lb/h (45,454kg/h) of material at 50% moisture. Typically, this duty would be performed in a large spray dryer. However, if the solids content of the feed material can be increased from 50% to 60% by evaporation, the amount of water to be evaporated in the spray dryer is reduced by 33%.

Note that in a large system, it is possible to evaporate 7 or 8 mass units of water for 1 mass unit of steam supply. Mechanical recompression evaporation can be even more energy efficient. A typical dryer does not even evaporate 1 mass unit per 1 mass unit of steam.

5.4 Good House Keeping & Miscellaneous Measures

Good house keeping includes:

 Reduce Losses. Energy losses to the atmosphere -- whether caused by surface radiation, leakage of process air, product discharge temperature being too high, or exhaust temperature being too high -- are to be avoided.

 Prevent Leakage. Leaks reduce the operation's effectiveness. Ingressive leaks dilute the air and expend valuable energy on heating up this additional air and any moisture in it. Exfiltration result in the loss of process air and will decrease the unit's performance.

 Insulation. Insulation will contain the energy for the process. All surfaces should be insulated appropriately -- with the correct material, thickness and installation quality -- to restrain heat from being lost. The thickness of insulation varies from50mm to 200mm.Different insulation materials like Glass, Mineral wool, Foam, Calcium Silicate etc. is applied to different parts of dryers like burner, heat exchanger, roofs, walls and pipes etc. The insulation areas differ and range from 50-100 m2 . Temperatures ranges from 100-750 deg C. Foam is used for low temperature at near ambient conditions and ceramics are useful for high temperatures.

 Maintain Utility Supply Lines. Utilities such as steam, fuel, compressed air, etc., should be regularly maintained to control losses. These losses are unrecoverable and will contribute to the overall operating cost of the system.

 Avoiding steam leaks and regular steam trap checking

 Avoiding air leaks and repair of doors and seals

 Cleaning of filters at fans

 Checking of belt slippage and fan speeds

 Cleaning of heaters

 Avoiding fouling and pressure drop at heaters

 Monitoring heat transfer efficiency

 Checking burners/ combustion efficiency

 Improving insulation efficiency at burners compartments, heat exchangers, duct work and the body of dryer itself

5.5 Instrumentation and Control

Air temperature can be measured using either a thermocouple or a resistance thermometer. Resistance thermometers are more expensive but accurate. The surface temperature of solids can be measured using infrared pyrometer. The internal temperature of solids is difficult to measure.

Air humidity can be measured buy wet-bulb and dry-bulb thermometers. Resistance sensors, which consists of an absorbent material whose resistance changes with moisture content.

Absorption capacitive sensors consist of a parallel plate capacitor whose dielectric is sensitive to humidity. Material used is usually aluminum oxide doped with lithium chloride.

The commonly used control methods are discussed below. In manual control systems, at some point downstream of the dryer exit, the operator measures the moisture content of the material and compares the same with desired value.

Then the energy input/feed rate is adjusted to get desired quality of drying. This type of manual feed back control is seen in many plants, they are simple and less expensive. But they are not effective especially when good control is required. If the adjustments to energy input/feed rate etc are made automatically in a closed loop control scheme, the variations in moisture contents can be limited.

The above control systems (manual and automatic) do not effectively tackle the disturbances at the input. For example, a 1 Tph dryer suddenly operates at 50% of the load and if the inlet moisture content is higher, in the above control systems, though work hard to give desired moisture content, the energy consumption is not optimised. Hence a feed forward control system which measures all the above parameters is used when lot of variations are expected.

In a feed forward control system, it is necessary to include a feed rate sensor/transmitter and an inlet moisture content transmitter/transmitter. From the sensed parameters, the controller calculates the material and energy balance and estimate the quantity of water to be evaporated and the fuel quantity required. The estimated fuel quantity requirement is compared with the actual fuel flow rate and this difference is used to correct the moisture content. Costs of these systems would be 3 to 4 times cost of a manual control system. The measurement of feed rate and inlet moisture content is necessary because the actual moisture content of the material inside the dryer is not generally available. This value, if measured would be representative of the inlet variations, for a given energy input.

A relatively advanced control-Delta T Dryer Control- has temperature probes continually measure the moisture content of the product inside the dryer during the drying cycle and readjust the time and temperature of the dryers accordingly.

The control variable is delta T. The exact definition depends on the type of drying. It is usually defines as the change in temperature of air before and after contact with product. In batch drying, it may be defined as the temperature of entering hot air minus the temperature air leaving the dryer. The dryer works using a mathematical equation to continually adjust the temperature based on information provided by the temperature probes. Customized control mechanisms have been created to work within the wide variety of dryers in manufacturing including conveyor, rotary, flash, fluidized bed, and rotary louver.

A schematic of hot air drying in carpet drying is given below in fig 5.1. RTDs are used to measure hot end temperature (T-hot) and cold end temperature (T-cold). The resulting temperature drop is used as a process measurement to relate to moisture content. A change in conveyor speed or energy input can be made based on the temperature drop.

Figure 5-1: Delta T control system for hot air drying

5.6 Technical Modification / Selection of drying method

Direct Heating: Direct fired dryers are more efficient than indirectly heated dryers. Direct heating can reduce using steam/ thermic fluid about 35 to 45% of the primary fuel requirement. Apart from use of hot combustion gases exhaust of gas turbine from combined heat power project or gas fired infrared heating can also be used. The application will depend on retrofit modification needed in existing dryer and nature of the material to be dried.

Drying and curing using natural gas direct firing with individual zone control, in place of steam system in a stenter is an example of using direct heating. Using CHP exhaust gases in fluidized bed drying directly is also done to utilise direct heating principles.

Electro-Magnetic Heating: Some of the material take long drying time because of their bulk and thickness. Sometimes there is a possibility of non-uniform drying or damage. In such cases a targeted drying of moisture in the material results in faster and more efficient drying and better audit of product. Infrared heating, induction heating and dielectric heating (Radio Frequency and Microwave Drying) can be used in such cases for direct delivery of electromagnetic energy to the solid or moisture.

5.7 Use the Exhaust Air Effectively.

The humidity of exhaust air is well below its equilibrium value, in relation to the moisture content of the material being dried. This means that it has removed less water that material than it can and that more heat is used to heat the air than necessary. For example, suppose if the equilibrium humidity content of exhaust air is 0.1 kg/kg dry basis, but the actual humidity of exhaust air is 0.02 kg/kg of dry air. Then for a flow rate of 50 kg/s of dry air, the same rate of water removal could be theoretically achieved with 10 kg/s of dry air. The remaining 40 kg/s is not needed for drying the material. Energy used for heating this air is wasted.

However, it is always not possible to do that, because the rate of drying is proportional to the difference between the equilibrium and the actual humidity.

Heat recovery is the simplest method of retrofit modification of dryer to enhance its efficiency. The major methods are as follows:

 Recycling of exhaust air

 Use of recuperators, heat wheels, plate heat exchangers run-around coils

Heat recovery is used with rotary, spray, fluid-bed and conveyor/band dryers in chemicals, mineral and food industries. These are also used in textile dryers like stenters and paper machine dryers.

Recycling. Recycling the air within the dryer reduces the sensible requirements to heat the air from its atmospheric condition to the operating condition. Recycling involves redirecting the exhaust air or a portion thereof, back into the process. Limiting factors for recycling will include saturation of the gas and depletion of the oxygen content of the gas (for direct-fired applications). They can be overcome by controlling the percentage recycle.

Recuperation. The use of recuperation to preheat the feed product, inlet air or combustion air offers additional advantages. This same concept also could be used as the source of energy to preheat the product. Recuperators can be air-to-air, air to solid, or air to liquid units. Some recuperators may be relatively large and will absorb a certain amount of power (from the fans) to overcome losses associated with the equipment.

5.8 Final Moisture Content Specification.

In many instances, the product's final moisture content can be increased without any detrimental effect on the post process. Easing this requirement can significantly improve the overall production. As an alternative, it may be advantageous to install a second dryer to remove the last, small fraction of moisture. Frequently referred to as two-stage drying, this approach offers benefits in both energy consumed and production due to reduced airflows and heat requirements for such a small fraction of moisture removal.