4 replies to this topic

[harshad katre]

Dear All,

 

I am attaching general schematic of Compressed Air Systems with Heat of Compression Dryers for which I want to know the calculations for water to be removed after dryer.

 

My current inlet conditions are 37 Deg C, 50% RH and 4000 ACFM. The compressors are maintained for delivery pressure of 7 barG. HOC driers contain 1580 kgs of Alumina each tower. The circulating flow rate of cooling water through aftercooler is 90 m3/hr. -4 Deg C dew point at the outlet is to be maintained.

 

Now the question is how to calculate water removed in dryers and after cooler.

 

Please correct me if I am wrong in below fundamentals.

 

From the fundamentals of dewpoint, it is known that with increase in pressure, saturation vapour pressure increases proportionally. Also, moisture contents at 37 deg C, 50% RH remains constant after compression too.

 

 

Waiting for your positive reply.

 

 

Regards,

Harshad Katre

Attached Files

•  Cmp Pipe Lay.png   28.64KB   3 downloads

[abubakarce19]

Dear

 

1- Water is not removed in inter stage coolers. It reduces temperature and moisture is removed in separator / knockout normally located after cooler. 

2- Alumina balls works as desiccant and and regenerated after a period of time. Amount of water removed can be calculated from the initial and final moisture level of streams, 

 

You can also look for vendor manual for those dryers. More information to be provided for better estimation

 

 

Abubakar

[srfish]

The efficiency of the Alumina in  the dryer is a big problem. To find out more about the subject of dryers, check out chapter 3.13 of the Heat Exchanger Design Handbook.

[Art Montemayor]

Harshad Katre:

 

You are a student and therefore you are missing some practical details in what you are presenting.  An adsorption dryer removes water from compressed air by attracting the water molecules to the surface of an adsorbent (in your case, activated alumina).

 

Additionally, before introducing the humid compressed air into the adsorbent bed(s), the compressed air is cooled using the compressor's intercoolers (in the case of a multi-staged compressor) and after-cooler.  As abubakarce19 points out, as the air is compressed and cooled, liquid water is formed in the product air because it is in excess to the amount of water vapor required for 100% saturation.  This liquid water is separated and drained in vapor liquid separators after the inter- and aftercooler of the compressor, prior to entering the adsorption unit.  In your supplied sketch, you show a so-called "HOC" (Heat of Compression) adsorption scheme.  This is a simplification of the entire process and only shows that the heat of the compressed air exiting the compressor is apparently used to regenerate the spent adsorption bed (there is a minimum of two beds used in an adsorption dryer).  After using some of the compression heat to pre-heat the spent bed, the exiting humid air is further cooled and the resulting liquid water condensed is drained out.  The remaining, cooled, saturated air is sent to the drying adsorption bed being used.  Your sketch is faulty.  There is no "moisture separator" used after the adsorption dryer unit, simply because there is no practical liquid moisture left in the dried air product.  So you have supplied a sketch of something that doesn't apply or simply doesn't make sense - or doesn't have appropriate identification of the vessels being used and the process employed.

 

You simply want (or need) to make a heat and material balance around the compressed air process - even though you don't state it as such.  Therefore, make a material balance first: find the amount of water in your compressor suction inlet, in your compressor intercooler traps (both liquid and vapor), in your compressor aftercooler (both liquid and vapor), in your adsorber inlet (vapor), and in your adsorber outlet (vapor).  The water amount difference between the working adsorber inlet and outlet is the amount left in the adsorbent bed to be regenerated later.  You find the amounts of water vapor existing in the compressed air by referring to water content tables or charts.  Today, you may even use a simulation program to calculate the amounts.

 

Refer to the attached workbook for examples of how to find water content in air.

 

 Air Moisture Content Conversion.xlsx   497.26KB   43 downloads

[harshad katre]

Thanks for all of your reply Sir.

 

I had forgotten the effect because of intercoolers thanks for that. 

 

I agree to what Art Sir is saying about heat and material balance inside the system. Also I can understand the difference in water removal rate and adsorption phenomena happening inside these dryers.

 

Art Sir: At the same time is seems to be difficult to find the amount of water in compressor intercooler traps (both liquid and vapor), in compressor aftercooler (both liquid and vapor), in adsorber inlet (vapor), and in adsorber outlet (vapor) etc as said by you. I know inlet compressor suction conditions of Air as 37 deg C and 50 % RH with volumetric flow rate of 4000 ACFM, the dew point of -4 Deg C is to be maintained with compressor delivery at 7 barg currently.  How this data will help me to solve the problem to next level as guided by you? Also please let me know what more relevant data is needed to calculate the same if any.

 

 

@ Fish Sir: Are you referring to 2nd Edition of Heat Exchanger Design Hand book by Kuppan Thulukkanam?

 

Thanks once again to all.