Air Dryer High Dew Point at half load 2

[mjpetrag]

I have two air dryers connected to a common suction and discharge header for compressed air. We usually operate one at a time, but when I operate both at the same time the dew point drops off on one of the air dryers. The first air dryer's normal dew point is -30 F and the second is normally around -65 F. When I run both in parallel the dewpoint of the second drops from -65F to around -40. I originally thought the air streams were mixing at the dew point sensor for the second dryer, but the sensor is far upstream of the mixing tee. When I shut the first air dryer off the dew point on the second goes back to normal. I was wondering if an air dryer can see a higher than normal dew point if it is run at half load instead of full load. I don't know the specifics of the alumina desiccant chemistry and how the bead surface reacts with water in the air. It's very counterintuitive.

-Mike

 

[dcasto]

And how do you know thw air splits exactly in half? I'd say you have 10% going through the bed and you can then get channeling which would raise the dew point. At 50% you would not see channeling.

 

[EmmanuelTop]

Also check if you have the same type of desiccant in both dryers. I remember similar situation where, in one dryer, Mol Sieve was loaded by mistake instead of Alumina. This particular bed was capable of achieving lower dew points (when regenerated) but at the same time it would reach the saturation faster than the Alumina bed.

 

[mjpetrag]

It's not exactly in half since they are two different air dryers with different dP's and the secondary one has 2" inlets and outlets while the primary has 3". So there's less flow to the secondary.

Channeling, meaning that flow is constricted to a smaller surface area than evenly distributed throughout the whole bed?

-Mike

 

[CH5OH]

I would suspect the dew point measurement:
the measurement being the difference between moisture entering the sensor and moisture leaving the sensor
this depending on the velocity of the air.
what happens if you choke the air flow going through the dryer?

 

[cheute79]

Mike:

The flow profile will change throughout the system and each dryer when you have one unit on, then another, and then both. Let's look at the situation.

Using English units, @ -65°F dewpoint, you have about 14.9 x 10^-6 grains moisture per lb of dry air, and at -40°F dewpoint, you have about 7.925 x 10^-4 gr/lb (source - Perry's Handbook). The difference is <1.9%. For starters, the absolute difference is very small.

My guess is that the air flow distribution through your 2 dessicant beds is not identical, and the sample point that is intended to give you a "representative" dew point is not in a totally analogous position for each. you also probably have some differences in the beds - age and particle size of the media, particle size distribution within the bed, etc. So, you expect some absolute differences in the capacities of the media and beds in each. The flow profile changes, probably magnifying any dis-similarities in the location of the sample point relative to the flow, when one bed is "on line" vs. two. I'm gonna guess that the sum of the impacts of the changes is probably at least 2%. So, is there really a difference that you can statistically justify?

 

[mjpetrag]

I didn't realize that the difference was that small. It would make sense if the flow profile was screwed up due to reduced flow, causing the air to not be exposed to enough desiccant. Also with less flow I would expect less turbulence = less drying. Thanks for all the replies!

-Mike