要求加精(Energy Savings in Drying) [复制链接]

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Energy Savings in Drying

by Paul Tedman, PE

An important key to efficient drying is reducing the energy that is exhausted. This can be done by properly sizing the dryer and the exhaust system. Many dryers can be improved by tuning the exhaust system for the processing conditions.

Return on investment

  Over the last couple of years, a great deal of attention has been given to improving the performance of the dryers that are used in petfood extrusion processing. The return on investment (ROI) is quite favorable for a dryer that produces a product that is not over-dried and is very close to the target moisture. Manufacturers realize the return because they are able to sell more water with the finished product.

At the same time, little attention has been given to drying efficiency. For almost two decades now, we have had relatively low fuel prices in the US. However, with near quadruple increases in the price of natural gas in some geographic areas, the winter of 2000-2001 has reminded many of the US energy crisis of the early 1980s.

The majority of the dryers in the US used in conjunction with petfood extrusion processing are heated with natural gas. As the cost of natural gas approaches US$10 per 1,000 cubic feet, the annual cost for the gas will reach nearly US$250,000.

In recent years, when processors were paying US$2.50-3.00 per 1,000 cubic feet, there were not many savings if the efficiency was improved by 10 to 20%. However, as the price of natural gas increases, the savings become more significant. Based on the data in Figure 1, a 10% improvement in gas consumption can save the processor nearly US$25,000 annually as the price approaches US$10 per 1,000 cubic feet.

While gas prices have subsided somewhat since the cold winter months of 2000-2001, the prices are still higher than what we have become accustomed to in recent years. Many processors are now looking to improve the efficiency of their drying operations in order to lower their operating costs.

Evaluating efficiency

  To evaluate the efficiency of a dryer, one must understand all energy gains and energy losses (energy balance) associated with a dryer. A typical conveyor dryer is shown in Figure 2.

The energy that enters the dryer must equal the energy that exits the dryer. Therefore, the following can be derived:

QiP + QiB + QiMU = QoE + QoC + QoP

      Or

QiB = QoE + QoC + QoP – QiP – QiMU

Energy inputs

  The energy that enters the dryer from the product and associated moisture (QiP) is dependent on the extrusion processing. A significant amount of energy is added during the extrusion process to the product in the form of mechanical and thermal energy. The mechanical energy is added from the friction created in the extruder barrel. The thermal energy is added primarily by saturated steam. These are necessary to cook and expand the product. Although some of the thermal energy is lost as the extrudate expands at the die, some of the energy is retained in the form of sensible heat. This is very beneficial to the drying process, as less energy is needed from the remaining energy inputs (QiB and QiMU).

The amount of energy that enters the system from the make-up air (QiMU) is primarily dependent on the ambient conditions and the operator has little control of this variable. As the ambient air temperature increases, less energy is needed from the burners (QiB). As the ambient air temperature decreases, more energy will be required from the burners.

  Where integral cooling devices are used, additional energy is added to the make-up air (QiMU), further reducing the energy required from the burners (QiB). However, most processors are using separate cooling mechanisms after fat and digest coating, and it is not desirable to reintroduce this energy to the dryer as the air is usually laden with oils that can build-up in the dryer.

  The amount of energy required from the burners (QiB) is dependent on the aforementioned variables, as well as the energy losses that exist in the drying process.             

 Energy outputs

  There are three major energy losses in the drying system. The largest loss is from the exhaust stack (QoE). Every drying process has an exhaust system that is used to extract the air and water vapor mixture that is generated from the drying process. We will discuss this loss in more detail later, as it is the most significant energy loss in the system.

Convective energy losses (QoC) occur in all dryers, although most dryers are insulated very well. The convective losses are greater for dryers in cold ambient conditions compared to those in warm ambient conditions. However, overall convective losses are usually not significant when compared to the other losses.

An energy loss also occurs as the product exits the dryer (QoP). This loss depends on the temperature of the product and its moisture content. Fortunately, this loss is less than the energy gain from the hot, moist product that enters the dryer. The reason for this is that the heat of evaporation temperature is much less than the temperature of the product as it enters the dryer. In addition, a large portion of the moisture is removed from the product in the dryer. Integral cooling devices will help further reduce the energy loss from the product, but these devices are not being utilized as much as they were in the past due to other processing considerations.

The energy loss out of the exhaust stack is by far the most significant energy loss in the system. Referring back to the heat balance equation (QiP + QiB + QiMU = QoE + QoC + QoP), it can be seen that excessive heat loss from the exhaust duct must be equalized on the left side of the equation by an energy gain (See Figure 3). The only variable that can make up the difference is QiB, which is energy input from the burners.

Efficiency

  Dryer manufacturers often express the efficiency of drying based on the amount of energy it takes to evaporate one pound of water, expressed as BTU/lb. of water evaporated. The amount of fuel used over a specific period can be converted to BTU/hour. In addition, by knowing the wet moisture content, the dry moisture content and the production rate, the rate of evaporation can be determined (lb./hour). Dividing the fuel consumption by the evaporation rate will yield the energy required to evaporate one lb. of water. Typical figures for petfood dryers will be in the range of 1,200 BTU/lb. to 1,500 BTU/lb.

Theoretical energy requirements can be obtained from a saturated steam table. At an atmospheric pressure of 14.696 pounds-per-square inch (PSI), the total required energy to evaporate one lb. of water is 1150 BTU. This includes both the sensible heat and the latent heat of vaporization. In the extrusion process, the product enters the dryer at elevated temperatures, sometimes as hot as 200ºF. This means that a significant amount of the sensible heat remains in the product.

Therefore, it is probably more appropriate to use only the latent heat of vaporization in the efficiency calculation, which would be 970 BTU/lb. The true efficiency can be calculated by dividing the latent heat of vaporization (970 BTU/lb.) by the actual energy needed to evaporate one lb. of water. So if it takes 1200 BTU/lb., then the true efficiency would be (970/1200) X 100% = 81%. At 1500 BTU/lb., the efficiency would be 65%.

Improving efficiency

Most modern dryer designs recycle a portion of the exhaust air (See Figure 4). This is achievable because there is a large volume of air that passes through the product, and it is usually not completely saturated. The recycle air is mixed with make-up air and reheated. Therefore, energy is conserved. The key to this process is understanding how much air to exhaust and how much to recycle.

The amount of air to be exhausted depends on the temperature of the exhaust air. Exhaust air temperatures typically range from 140-190 ºF for most petfood dryers. The amount of water vapor that can be carried by the air at these temperatures also varies. Efficient dryers will exhaust air that is moderately saturated. Completely saturated air can create condensation problems along the ceiling and in the ductwork.

As the temperature increases from 140 to 190 ºF, the specific humidity increases from .060 to 0.129 lb. of water/lb. of dry air. This simply means that less air needs to be exhausted at higher temperatures, which in turn, improves efficiency (See Figure 5). Many dryers have been oversized in the past to ensure that there will be sufficient retention time to dry the product. If the dryer is larger than necessary, then it is likely that the unit is operated at low temperatures. This will result in low exhaust temperatures, and in turn, the efficiency will be poor.

Some operations have multiple extruders feeding a single dryer. At times, only one extruder may be in operation, which essentially results in an oversized, inefficient drying operation. In addition, some extrusion operations today process a multitude of different products. A single line may produce some products at 4-ton/hr and produce other products at 8-ton/hr. In addition, the wet and dry moisture contents may vary significantly. These types of processing conditions make it difficult to have efficient drying operations. Those processors that produce products that are very similar will likely have more success in improving efficiency than those that process a wide range of products.

Overall, the key to an efficient drying process is to reduce the major energy loss as much as possible, which is the energy that is exhausted out of the stack (QoE). This can be done by properly sizing the dryer and the exhaust system. Many dryers in operation today can be improved by simply evaluating the exhaust system and tuning it for the processing conditions. Careful consideration should be given to all processing variables when planning a new installation. It is important not to undersize a dryer from a production rate standpoint. However, from an efficiency standpoint, it is critical not to oversize the dryer             

  Most modern dryer designs recycle a portion of the exhaust air (See Figure 4). This is achievable because there is a large volume of air that passes through the product, and it is usually not completely saturated. The recycle air is mixed with make-up air and reheated. Therefore, energy is conserved. The key to this process is understanding how much air to exhaust and how much to recycle.

The amount of air to be exhausted depends on the temperature of the exhaust air. Exhaust air temperatures typically range from 140-190 ºF for most petfood dryers. The amount of water vapor that can be carried by the air at these temperatures also varies. Efficient dryers will exhaust air that is moderately saturated. Completely saturated air can create condensation problems along the ceiling and in the ductwork.

As the temperature increases from 140 to 190 ºF, the specific humidity increases from .060 to 0.129 lb. of water/lb. of dry air. This simply means that less air needs to be exhausted at higher temperatures, which in turn, improves efficiency (See Figure 5). Many dryers have been oversized in the past to ensure that there will be sufficient retention time to dry the product. If the dryer is larger than necessary, then it is likely that the unit is operated at low temperatures. This will result in low exhaust temperatures, and in turn, the efficiency will be poor.

Some operations have multiple extruders feeding a single dryer. At times, only one extruder may be in operation, which essentially results in an oversized, inefficient drying operation. In addition, some extrusion operations today process a multitude of different products. A single line may produce some products at 4-ton/hr and produce other products at 8-ton/hr. In addition, the wet and dry moisture contents may vary significantly. These types of processing conditions make it difficult to have efficient drying operations. Those processors that produce products that are very similar will likely have more success in improving efficiency than those that process a wide range of products.

Overall, the key to an efficient drying process is to reduce the major energy loss as much as possible, which is the energy that is exhausted out of the stack (QoE). This can be done by properly sizing the dryer and the exhaust system. Many dryers in operation today can be improved by simply evaluating the exhaust system and tuning it for the processing conditions. Careful consideration should be given to all processing variables when planning a new installation. It is important not to undersize a dryer from a production rate standpoint. However, from an efficiency standpoint, it is critical not to oversize the dryer