Which is more essential? An aftercooler or refrigerated drier?

[chopengnr]

I've got a small plant compressed air system (33cfm) that has no drier or aftercooler. As expected we have a problem with water in the lines. According to my Fluid Power Handbook, a good aftercooler can remove up to 90% of the moisture.

The way I understand it, you should install the aftercooler first, then if more drying is still required, install a seperate drier. Is this generally the case?

 

[DLANDISSR]

Chopengnr:

An after cooler can only remove cause the water to condense from the compressed air stream down to the temperature at which the after cooler can cool the air. If you use cooling tower water little water will be removed. IF you use chilled (~40Deg F water) you can remove much more water and near to what a refrigerated dryer can remove.

You are correct usually an after cooler is used first and then a refirgerated dryer. Some systems use a receiver which will allow some of the water to condense and collect.
This water is then drained off with at trap. Then a refrigerated dryer is used.

 

[quark]

Chopengnr!

Yes, the scheme is after cooler and dryer. The use of dryer depends upon the lowest temperature present in compressed air system. (this will be the saturation temperature corresponding to the lowest pressure in the system)

Note that pressure dew point is always higher than atmospheric dew point. So dryer is not always compulsory but depends upon your system.

Some questions.

1. What is the pressure of compressed air?
2. What is the ambient DBT and RH?
3. What is the minimum temperature of cooling water you can
provide?

These things will be helpful in evaluating your system.

Regards,

 

[chopengnr]

Quark:

1. Compressed Air is 140-150 psi
2. Ambient DBT (dry bulb temp?) is basically the outdoors ambient temperature. I'm in NC so it can range from ~30F to ~100F. The same goes for RH, ranging from 50% to 99%.
3. I doubt that I can provide any cooling water at all. I'm looking into the air cooled aftercoolers. I realize I will only be able to cool the compressed air to around 10 degrees above the ambient temperature. If i put the aftercooler in the plant, I can count on a constant cooling air temp of 70F.

I'm sort of approaching this from the "something is better than nothing" viewpoint. This facility has been operating without any type of drying unit for a couple of years, so convincing management to invest in anything is going to be a tough sale.

 

[chicopee]

Do you have an air receiver on the outlet of the air compressor? Such air tank will remove a large amount of moisture. Are your air lines slightly pitched to drain condensation away from point of useage? An air drier or an aftercooler for a 33 scfm does not seem to warrant the expenditure until you try out the above measures.
You are not telling us anything about the duty cycle of the air compressor and the application of the compressed air.

 

[BRT549]

Installing an aftercooler will reduce the size of the dryer. That is true for any style dryer (refrigerated, heated regenerative, or desiccant).

I would suggest the aftercooler first, since you'll probably see the most benefit for the least cost. Make sure you put an automatic blowdown valve on the separator - don't rely on an operator to do it manually.

A desiccant pack after the aftercooler is also a low cost alternative, especially with the size (33 acfm)of your system. This should work for almost any air application except instrument air (-40 dewpoint). VanAir makes both of these units, and I've had pretty good luck with them.

 

[chopengnr]

chicopee:

yes, we have a reciever, which does remove some moisture. however, we still have a lot of condensation in the lines. The lines are not sloped, and there are no drains. I've already included this in my recommendation, but thanks for the thought. In all actuallity, i believe sloping the main lines and installing drain traps will remove a good part of the remaining water in the line. However, the usage lines are fed off side and bottom of the main rather than off the top, so in order for sloping the line to be effective i'd have to refit every feed tap.

The compressor cycles 5 times per hour. The air is used to run wire cutting and termination equipment.

As a side note:
Schedule 40 PVC was used to run the airline, which we expreienced first hand is a bad idea. We had a 4 inch main literally explode about a month ago. Luckily it happened over the weekend and no one was hurt. I've read up on PVC all the reasons not to use it for compressed air. (OSHA even released a Hazard Bulletin in 1988 warning against it)
Thats what got me started on all this in the first place.

I realize this is a comparatively small system, and I'm doing a lot of this for my own knowledge. We are very financially strained (currently under new management and trying to recover from a forclosure under the old management in December). I'm sure very little, if any money will be spent on system improvement.

If I can get the piping replaced with metal, I'll definatley be sure have it slightly pitched, with drains, etc. In the meantime, I'm just trying to check out all the options.

 

[automatic2]

A desiccant dryer should come in at the lowest cost and will easily handle your volumes.

 

[quark]

Chopengnr!

Presuming 36 cfm free air delivery I did some calculation. When you compress the air and cool it down to 110 ⁰F, your air is cooled down to an atmospheric dew point of 35 ⁰F. (are you at present using air at high temperature directly?) This may cause condensation in if you are drastically reducing air pressure at some application downstream.

As suggested by majority, desiccant dryers are good and cheaper. You should have to go for after cooler before treating your air with dryers because at high temperatures air may not shed much moisture and capacity of desiccant also reduces.

Secondly you have to waste minimum 7% and maximum 10% of dryed air for regeneration. First better check with an after cooler.

Regards,

 

[TD2K]

This has already been pretty well covered but I'll add in a few comments.

1. Compressed air will always, for practical purposes, be saturated when you take atmospheric air and compress it up 100 psig or higher and then allow it to cool down to ambient temperatures or close to it. You can see this with a copy of the steam tables and looking at the volumetric change as you compress that air.

2. I disagree with DLANDISSR in that if you cool air down to say 100F with CW, the bulk of the water will not drop out. Yes, using chilled water will definitely remove more water but air has progressively less and less capacity to hold moisture as it is cooled. By the time it is at 100F and 100 psig, less than 1% by volume of the air will be water vapor and the majority of the inlet water has condensed out unless you have very dry ambient air. At 40F, it's down to 1/6th or so of what it could hold at 100F. However, the air leaving the aftercooler in either case is still saturated so unless you chill it BELOW atmospheric temperature before distributing it, you will still have water condensing out in your distribution system as it cools off. For many industry air systems, that is why dessicant driers which can reach very low dewpoints (-40F at pressure or lower) are commonly used.

Basically, a cooler/receiver by itself will reduce the amount of water removed in your air lines but won't eliminate it. To supply dry air, you need to either install a chiller as DLANDISSR suggests or a packaged instrument air system. For 36F, unless you have chilled water available, a packaged PSA system would make sense. However, those usually want fairly cool inlet temperatures for the dessicant to work so you are back to a packaged cooler, receiver, drier system.

Find out who does compressed air packages in your area and talk to a couple of vendors to see what they suggest. Let them know what your budget is so they can concentrate on realistic options. If you are on a tight budget, I think your option is to install a small air cooler as you indicated with a receiver to remove the mositure that condenses at that point and then well trap your distribution headers to remove water as it condenses out along the line.

 

[chopengnr]

thanks for the help everyone.
i decided on a packaged drier unit designed for small units with no aftercoolers. I'm waiting on a couple of more quotes but i think this will be the best bang for the buck.

thanks again.