Sizing Large Compressed Air Desiccant Dryer

[agcav96]

Hello,

I'm a relatively new engineer working at a manufacturing facility. I'm trying to analyze our current air drying system as we have reoccurring dew point problems, but I haven't seen a lot of information on the internet and there aren't people in my department that can help.

What I'm trying to determine is the total water load that the dryer should be designed for.

Info: 2x 5500 SCFM three stage centrifugal compressors rated at 125 psia discharge. There are two intercoolers and one aftercooler prior to the air dryer with the downstream temperature of the air at 100F. The air dryers are supposed to provide a pressure dew point of -40F. Design weather would be 95F db/ 50%RH and 14.7 psia.

I was trying to follow this website's process:

http://www.drucklufttechnik.de/

http://www/temp/e/drucklfte.nsf/704...1a21affc0574113ac1256625007a40d7?OpenDocument

I don't know where they get the "humidity" values in g/m3. ASHRAE tables use lb_w/lb_da and I've tried converting that into lb_w/ft^3, but the numbers don't seem to agree with the table they use. They also just divide SCFM by the pressure ratio to get ACFM, but I don't believe that is the correct conversion.

Some of the calcs I tried to do:
1) Take inlet conditions and use Ashrae psychrometric table to get humidity ratio. W = Ws*RH.
2) Multiply W by air density to get Lb_w/ft^3. Multiply by ICFM to get inlet water rate. lb_w/min
3) THis is where I run into trouble: with discharge conditions of P=125 psia and T=100F (post- aftercooler), what is the saturation humidity? My idea is calculated the saturation humidity at discharge conditions and the difference between that and our inlet would be the condensation rate of the intercoolers and aftercoolers.
4) The leftover water in the air would be the load on the what the desiccant would need to adsorb. so using BASF-200, 100%RH adsorbs 42% wt H20. so if the dryer load is 1 lb_w/min and the dryer has a 4 hour adsorbing time, I would need around 600 pounds of desiccant?

Sorry for the long winded post. I've had a lot of time to think about this. Thanks in advance if anyone can help or point me to some good resources.

 

[Compositepro]

There are many other design considerations. Often airflow rate vary considerably over time. Depending on bed design and operating conditions, you can get moisture breakthrough long before the bed is fully saturated. The pellets require significant time to absorb moisture, so you will require much more desiccant that an equilibrium equation will give.

 

[LittleInch]

The max humidty they use is at 33C.

Remember the page uses a , as a decimal point.

1m3 of air is about 2.6 lbs of air.

At 100% RH and a dew point of 33C I manage to get pretty close to their numbers using the data here

https://www.engineeringtoolbox.com/water-vapor-air-d_854.html

They are using absolute pressure so actual volume is divided by P2 on the basis P1 is 1 bara, assuming the same temperature

To get to a dew point of -40 you basically need to remove 95% of the water that is present at atmospheric conditions, but some will drop out after the coolers.

But why not ask the vendors? They do this all the time.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[goutam_freelance]

ASHRAE Psychrometric table gives moist air thermodynamic parameters at normal atmospheric pressures. Ideally you should use Psychrometric properties available in Excel. Even there low temperature dew point data is not available.

Depending on calculated amount of water condensed in intercoolers and aftercooler you calculate a certain humidity ratio in compressed air at aftercooler outlet. Say this is x.

Required dew point of -40 deg F is supposed to be pressure dew point. At atmospheric pressure the supercooled water saturation pressure is approximately 0.2 mbar. This is not available in steam tables.

At 125 psia the vapor saturation pressure is Pv= 0.2x(125/14.7)=1.7 mbar

Humidity ratio=0.622(P_v/P_a)=0.622(1.7e2/1.013e5)=0.001044

So the reduction in humidity ratio required is from x to 0.001044.

From the above you can calculate the amount of moisture to be removed.





Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[LittleInch]

Are we not missing something here? If you have an aftercooler can we not just assume that all the free water will be drained off and you end up with 100% RH air at 125 at some max temperature

So figure out the amount of water in the air at that pressure and assume the dryer needs to remove 90% of it.

Sites like this give you data.

https://www.engineeringtoolbox.com/water-content-compressed-air-d_1275.html

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[agcav96]

Thanks for all the replies. I appreciate the help on this