Air Dryer Location Relative to Receiver 2

[dave2109]

In a compressed air system that has a compressor, dessicant dryer and air receiver, where is the best location for the dryer?
i.e. should it be upstream or downstream of the receiver.

I've always placed the dryer downstream of the receiver simply based on previous projects our company has performed.

I just recently worked at a plant where the dryer was placed upstream of the receiver. This plant had constant problems with getting water into the air system even though the dryers were supposed to lower the dew point to -40 deg F. Could this problem be due to the dryer/receiver configuration or is it more likely a compressor or dryer malfunction? The air system was fairly new (< 2 years) and apparently has had water problems for most of that time.

 

[MintJulep]

Compressor > after-cooler > drier > Receiver > distribution.

Placing the drier upstream of the receiver makes is muct less likely that LIQUID water will form in the receiver. If you go the other way around you are almost guaranteed that you will get liquid in the tank.

 

[quark]

I had a big and long discussion on this topic (and a fight with my boss) with dryer and compressor manufacturers. Both options have their own advantages and disadvantages.

Few points to consider are,

1. If the compressor and dryer capacity is same and you have less load than compressor discharge, I prefer compressor--->Dryer--->Receiver, because as the compressor is loaded and unloaded based on receiver pressure you can stop dryer whenever the compressor gets unloaded. (for this you need constant dew point system, i.e hot gas bypassing)

2. If you place dryer after receiver, moisture accumulates by coalescing in the receiver and load on dryer decreases.

3. If you don't have a constant dew point dryer (hot gas bypass arrangement) then as the load varies, power consumption by dryer varies (ofcourse, marginally) if you have dryer after receiver.

My preferred choice for tight control of dew point is Compressor----->Receiver---->Dryer (and I did 10 installations this way for pharmaceutical plants)

Regards,

 

[quark]

Oops, I forgot to mention (actually) about desiccant dryer. As the air cools a little bit further in receiver better to place the dryer after receiver. (one more advantage is that your dryer always operates at a constant pressure, more or less)

 

[evelrod]

In general, quark is right on. Most reciprical compressors are integral with their receivers and make it extremely difficult to interpose a mechanical dryer. In those cases the dryer is almost always after the receiver. In the case of large capacity non-recip type compressors it is possible to place the dryer between the pump and the receiver but in my personal experience, this results in inefficiency in the overall production process, ie, you still get water in the outlet air! Obviously, the ultimate layout is not always possible but, in all the various layouts, any water accumulating in the receiver is automatically drained periodically by means of a simple, inexpensive, timed auto drain. The use of intercoolers (recips) and aftercoolers on rotary comps before or after the receiver but before the refrigerated dryer has been benificial in several installations. In the installation of coalescing filters, dessicant dryers/filters, downstream of all the mechanical bits and before final application has proven to work quite well.

Rod

 

[automatic2]

I like the dryer after the reciever for all those reasons.
- act of compression condenses water, a simple 'y' with auto drain discards this.
- reciever allows cooling and further condensate to be removed (along with any bypassing oil). Again an autodrain removes this.
- Dryer can be sized and located for specific demand, rather than whole load, if allowed.

 

[Aldream]

dave2109:

What the gents wrote in regards to the location of the receiver is pretty good. However, you mentioned that you still have water in your air. This can be contributed to one of two reasons:

Either your dried air is subjected to cold temperatures that are lower than the dew point of your dry air. A rule of thumb is to design your driers for a dew point at least 10 to 20 degrees below that of the lowest ambient temperature your air will be subjected to.

The other reason is that your driers are not drying the air upto the spec. You should monitor the moisture content on the outlet of each dryer. A very important factor you should check is the flow going into each drier. Make sure it is not exceeding the flow it is designed for. If this happens, you will saturate the desiccant bed with moisture and eventually this will raise the moisture content in your air. Hope this helps

 

[pablo02]

I've had this same question after being assigned to a plant with the dryers before the (station air) receiver (instrument air receiver downstream of dryers) which had nothing but headaches with water in the air -- all of the other plants had the receivers downstream of the station air receiver...

Newer (combustion turbine) plants I am familiar with only one receiver have the dryers upstream...

I always assumed that allowing the air to cool off in the receiver reduced the moisture load on the dryer -- we put auto-blowdowns on the receiver drains...

I prefer to have the dryer downstream (if two receivers are used) -- but if given only one receiver, I'd put the dryer in front...

As for your problems, there are a number of items to check: make sure the aftercooler is piped in correctly and you have adequate temperatures and flows to knock a lot of the water out -- check the aftercooler drain pot and piping to make sure it drains properly... also, check the dryers -- are they heating up and cycling properly (I worked at one plant where they never checked this; the dryer had failed and they wondered why their instrument air system froze up ???) check the dryer capacity vs. the air usage -- it may be that the dryer is sized too small to handle the flow quantities...

(what type of compressor are you using?)

 

[sbriley]

Thermodynamics favor water absorption at higher pressures. In fact, that's how a &quot;pressure swing&quot; dryer system works--absorb water from high pressure air, then drop the pressure to desorb the water from the dessicant, then switch again. Most IA systems will have two drying beds for that reason, located after the compressor so as to take advantage of the pressure. I can't understand why a dryer would ever be placed on the suction side of the compressor; the dryers would be huge (expensive) and only briefly effective after each regeneration.

The receiver is often located between the compressor and the dryers so that it can behave as a water k/o vessel, usually outfitted with an automatic drainer. However, it's best to have an air cooler and a coalescing filter immediately on the discharge of the compressor to knock out most of the water. Also, many air compressors place some oil in the air, so the cooler/coalescer system is very important for removing the oil. Note: a dry filter should be located after the dryer; the dessicant will continuously emit dust which can plug downstream equipment.

Another consideration must be the &quot;shutdown&quot; behavior of the entire system. When shutdown of the compressor occurs you want the air to continue flowing forward through the beds, not backward. Backward flow will usually damage the beds. Therefore, placing the receiver ahead of the dryers helps. Good soft-seat check valves also help.

You should also review the OSHA standards for air compressor systems (

http://www.osha.gov).

And, finally, NEVER use air from a system such as this for breathing air. Breathing air systems are further regulated and must be designed according to a totally different set of standards.

 

[witchdoc]

There is one additional consideration when using a dessicant or deliquescent type dryer. When the receiver is placed between the compressor and dryer, the potential instantaneous flowrate from the receiver can be high enough to damage or blow out the dessicant bed. I usually use a backpressure regulator on these system to prevent this from happening. The most common source of instantaneous surges in such a system results from using an air lance.