Experienced auditors become wary when they see a desiccant dryer installed in a customer's facility. This type of dryer is required when a plant requires high quality compressed air or when compressed air piping is exposed to low temperatures. However, although desiccant dryers can perform this function, the energy cost of reducing the dew point of the compressed air from 35OF to -40OF is high. In order to achieve low dew point of air at low cost, blast heat regenerative adsorption dryer came into being.
This article describes three commonly used types of desiccant dryers and some experience with blast heat regenerative desiccant dryers, both good and bad.
Types of Desiccant Dryers
Before introducing the blast heat regenerative adsorption dryer, it is necessary to understand the common types of adsorption dryers, starting with the simple heatless regenerative adsorption dryer. While there are other, less common types of desiccant dryers, they are not discussed in this article. All typical desiccant dryers have two pressure vessels filled with desiccant granules and a valve control system that directs the flow of compressed air from one pressure vessel to the other. Desiccant particles have a limited ability to remove water vapor from the compressed air (adsorption) before saturation. Once the desiccant is saturated, the moisture in the desiccant must be removed during the regeneration cycle, otherwise the dryer will not be able to absorb moisture from the air. Moisture, which lowers the dew point of the compressed air to a lower level.
Before the desiccant in one pressure vessel is saturated, the valve control system will send the compressed air that is not completely dried to another pressure vessel full of desiccant. At the same time, place the previous pressure vessel under atmospheric pressure, pass through the dried compressed air, take away the moisture in the saturated desiccant, and regenerate the desiccant. Because compressed air is at atmospheric pressure, its dew point will decrease, so its ability to remove moisture is stronger. The desiccant in the two pressure vessels is alternately dried and regenerated in this way, and the cycle is usually 10 minutes. This cyclic regeneration process consumes about 15% to 20% of the dryer's rated capacity of compressed air.
It should be noted that the 15% to 20% here is 15% to 20% of the rated output of the dryer. In some cases, the dryer will not work at full capacity. For example, when the dryer is operating at half load, the compressed air used to remove the moisture from the desiccant here will be 30% to 40% of the output. Such dryers themselves will use up most of the dried compressed air in the process of drying the air, which is why auditors look down on these compressors.
In order to make up for this shortcoming, designers have proposed different versions of adsorption dryers, one of which is the method of adsorption drying with thermal regeneration. Similar to the heatless regenerative adsorption dryer, the already dried compressed air is still used to dry the saturated desiccant, but the compressed air will be heated by an electric heater before it is passed into the pressure vessel.
This design change allows less compressed air to be consumed, approximately 7.5% of the dryer's rated capacity, freeing up more compressed air to meet demand and reducing overall electrical operating costs. This type of dryer is slightly larger than the heatless regenerative dryer, and the cycle time becomes 4 hours.
A further design resulted in a blast heat regenerative adsorption dryer. It uses a blower to drive ambient air through a heating element and then into a pressure vessel to remove moisture from the desiccant, a process called endothermic dehydration. This type of dryer does not consume the dried compressed air at all, and completely supplies the dried compressed air to the factory. But the model still has problems drying the cold air stream.
The desiccant that has just been heated cannot effectively absorb moisture from the cold air. Therefore, after the endothermic dehydration process is completed, a large amount of dried compressed air needs to be used to take away the heat of the desiccant. This process is called cooling regeneration. During the cooling regeneration of a desiccant dryer with thermal regeneration, the amount of dry compressed air that needs to be used is approximately 7.5% of the dryer's rated capacity, typically taking 1 hour out of a 4 hour cycle. For blower heat regenerative desiccant dryers, the cooling regeneration process typically requires 2% of the dryer's rated compressed air production. But when we read the details here, we will find that this 2% is actually the average flow of each hour in the four-hour period, and the average flow of four hours is 8%. This 8% may have unintended consequences when the air compressor reaches a peak demand for greater power, and may even require additional compressed air.
Newer blower dryers use a built-in closed-loop cooling circuit, where a blower blows internally circulating air into a pressure vessel to remove heat, rather than using compressed air for cooling. In this model, some kind of heat exchanger will be used, using the surrounding air or cooling water to remove heat.
Operating costs
Because compressed air is very expensive, the cost of using compressed air to produce dry compressed air is high. In addition, if the compressed air usage of the dryer reaches 15% to 20% of the rated output of the compressor, in order to meet the production demand, it is necessary to purchase a compressor with a rated output 15% to 20% higher than the existing compressor. With heaters, less compressed air is used, which reduces operating costs.
It should be noted that the premise of the calculation results is to assume that the dryer is 100% loaded. Assume that the air compressor has a specific power of 0.2kW/scfm, 10 cents per kWh, and operates 8,760 hours per year. This table shows that using a more sophisticated desiccant dryer can save you considerable money each year.
important features
In actual operation, you don't usually see a 100% loaded air dryer with an input pressure of 100psi and an input temperature of 100OF or ambient temperature. This is because the lighter the load, the lower the cost.
For example, when the inlet temperature of the dryer is 80OF, the load is 75% of the full load, so it can only absorb 40% of the designed water absorption. This is because the moisture content of air 20OF lower than the design temperature is only 50% of that of air at 100OF. For a heatless regenerative adsorption dryer with a fixed cycle, the low temperature and low humidity conditions do not affect the operating cost, and the same amount of compressed air will still be consumed to regenerate the desiccant. But the heat regenerative dryer is different, because the desiccant contains less water, and the heater is controlled by temperature (to avoid overheating of the desiccant), so the heating time of the heating element will be reduced, saving operating costs.
For dryers that are not fully loaded, using the compressed air dew point to control the system can result in significant cost savings. A detector is used to measure the dew point of the compressed air produced by the dryer. When the dew point is lower than the set value, the system will delay the opening time of the cooling regeneration cycle. The cooling regeneration cycle will not start until the detector detects that the dew point of the exhaust air is higher than the set value. For dryers under low load, this reduces energy consumption. But different types of dryers will respond differently at low loads.
For heatless regenerative adsorption dryers, the reduction of compressed air used in the cooling regeneration process is often only related to the reduction of the drying gas of the dryer, and is proportional to it, and has nothing to do with the lower humidity of the gas to be dried caused by the lower inlet temperature . But for heat regeneration adsorption dryers, on the one hand, the reduction of the drying gas flow rate of the dryer will reduce the amount of compressed air used in the cooling regeneration cycle; on the other hand, the lower inlet temperature makes the humidity of the gas to be dried lower, It also results in a reduction in the use of compressed air in the cooling regeneration cycle. That is to say, the method with heat regeneration is cheaper than the method without heat regeneration. Following the previous example, the heatless regenerative dryer will save 20% to 25% of the cost, while the heat regenerative dryer can save 50% to 60% of the cost after using the dew point control system.
How to calculate compressed air dryer energy consumption ~ refrigerated & desiccant
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