Does a higher outlet (usage) dew point design (maintained) consume more energy?
The outlet dew point quality needs to be discussed in two ways:
I. Optimal Dew Point
II. Switching Dew Point
I. Optimal Dew Point
- When the adsorbent has adsorbed a certain amount of moisture, it starts to fail to completely intercept the moisture, causing some water vapor to escape to the outlet. At this point, the dew point value of the outlet compressed air begins to deteriorate until the dew point value drops to the set value, at which time the dryer performs a switching process. The compressed air will be directed to another adsorption tank that has completed regeneration and drying for adsorption again.
The original adsorption tank then undergoes a regeneration process of moisture desorption, which is to dehydrate and dry the wet adsorbent for reuse.
Regardless of the form of regeneration, theoretically, the adsorbent after regeneration should be completely free of moisture. Therefore, when the newly regenerated adsorption tank starts to operate, theoretically, the outlet dew point quality should be very high.
Theoretically, the dew point value of completely dry air is about -273℃, or at least the optimal dew point should be maintained below -100℃.
However, in reality, the optimal outlet dew point of general adsorption dryers rarely reaches below -100℃, and even below -70℃ is often unattainable. - The main reason for the above is that the pollution is not completely isolated during the desorption and regeneration of the adsorbent:
1.External Air Heating:
The ambient air is heated and its temperature is raised by a blower. The ambient air itself contains moisture. The higher the heating temperature, the lower the probability of this moisture remaining inside the equipment. However, the heating temperature needs to consider the heat resistance of the equipment. Due to this design limitation, even though only a very small amount of moisture remains during external air heating, the outlet dew point quality will still be affected.
In comparison, using CDA heating regeneration can further reduce the concern of contamination.
2. Circulation Cooling:
A blower is used in conjunction with a heat exchanger to cool the adsorbent using a closed loop circulation.
Because it is a rigid closed space, when the temperature drops, the circulating environment will create a vacuum, which will test the tightness of pipes and valves. There are also concerns about leakage from heat exchangers and heaters, and the largest source of leakage is the "blower".
Due to leakage, ambient moisture will quickly diffuse into the dry interior of the equipment. Therefore, moisture contamination is difficult to avoid in a circulation cooling design. As a result, manufacturers will use a "heating and cooling reversal" method to move the source of contamination to the upstream of adsorption to minimize the impact on the outlet dew point quality. However, no matter how it is avoided, the problem of moisture contamination still exists. - Using CDA cooling can significantly reduce concerns about cooling contamination, but the relative cost will definitely be higher, and its use must be evaluated for its pros and cons.
- The above can be summarized as the first stage conclusion:
Regardless of whether the heating or cooling process uses compressed dry air for regeneration, the outlet dew point quality will be better than using a blower for regeneration.
However, considering the cost of gas production, the manufacturing cost differs by 15 to 25 times between a compression ratio of 8 and 1.1 (comparing an air compressor outlet of 7 kgf/cm2 with a blower's gauge pressure of 1000 mmH2O). Therefore, the common thinking is that "as long as the outlet quality meets the requirements, pursuing a high dew point will consume more energy." However, this overlooks the important point of "adsorption efficiency." For the same batch of the same adsorbent, if one side is -60℃ after regeneration and the other is -100℃, and they are made to adsorb saturated compressed air with the same inlet conditions, switching at -40℃ will result in different switching (adsorption) times. If one side has an adsorption time of 6 hours and the other has 8 hours, the adsorption efficiency differs by 25%.
Another way to express the above is that a -60℃ dryer needs to regenerate 4 times a day, while a -100℃ dryer only needs to regenerate 3 times a day. - If the "adsorption efficiency" factor is included in the evaluation, we can find that there is still much room for discussion, and we must not equate high dew point with high energy consumption.
II. Switching Dew Point: Which is more energy-saving to switch between -40℃ and -70℃?
- Undoubtedly, -40℃ is more energy-saving than -70℃. But who can provide operational data to understand the energy-saving ratio of the two modes (%?)?
When the dryer is operated for a long time in a high-quality (high dew point) state, the adsorption efficiency will increase, the standby time will be longer, and the machine will be relatively stable. Conversely, the operating conditions will not be as good as the former. - If the actual energy-saving efficiency differs by more than 20% as mentioned above, the effect is certainly remarkable.
However, if factors such as operational stability, adsorption efficiency, the impact of compressed dry air quality on pneumatic systems and product yield are added, or if the actual energy saving is within 10%, careful evaluation and major decisions must be made between energy saving and system stability.
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