Compressed air receivers are an important part of every compressed air system.
Although it looks like they don't do much, they are actually very important.
Air receivers act as a buffer: they stabilize the air pressure and they are a backup storage for peak demands.
The easiest way to understand what happens in your compressed air system is to compare it to a water tower.
Just like our compressed air system, the system has a supply (a water pump), storage (the water tower) and demand (you taking a shower).
If we pump in water, the water level in the tower will rise.
When we use water, the water level will fall.
Why is an air receiver important?
The main functions of an air receiver are:
- Make the system stable (slow)
- Storage for peak demands
- Other functions: helps cool down air, reference pressure
A stable system
A compressor doesn't just output a certain pressure. It outputs a certain flow of compressed air (the compressor capacity) up to a certain pressure (the compressor's rated pressure).
The pressure in a compressed air system is the result of the amount of air we have pushed into the system, and the amount of air we have taken out.
Most air compressors work with a load/unload capacity control.
That means they either put out 100% of their capacity, or nothing.
If we don't have any buffer in our compressed air system, the pressure would rise very quickly as soon as the compressor starts pumping air.
It's similar to a very narrow water tower. If we start the pump, the water level will shoot up.
Then, if the compressor stops (because the pressure shot up, so we reached our desired pressure), the pressure falls down fast as well.
Now compare this to a compressed air system with a large buffer tank.
When the compressor starts pumping air, it takes some time before the required pressure is reached – which is a good thing!
It's like a big, wide water tower. We need to pump in a lot of water to make the water level rise.
In a typical compressed air system, it's normal this takes a few minutes, at least (with no air consumption).
The bigger the buffer is, the longer it takes to fill up, but when we use air, it also takes longer before the pressure drops.
And that's great!
It makes the whole system slow.
Since cycles are longer, the compressor starts and stops less frequently, which reduces wear on the air compressor.
With a big storage, changes in pressure are very slow. This allows you to set a small pressure differential (difference between load and unload setpoint) and also allows you to set the overall pressure lower – since the compressor has plenty of time to start up before the pressure drops too low.
There's also less running unloaded (if the buffer is big enough and the compressor has a chance to completely stop) and it also makes sure that a second (trim) compressor only starts up when really needed, and not every time the pressure dips for a few seconds.
Storage for peak demands
Most compressed air systems don't have a steady air usage/demand.
Air demand fluctuates throughout the day. For long periods of time of high air demand, we need a compressor with a big enough capacity to handle those loads.
But for short spikes in compressed air usage, an air receiver of big enough size is usually enough to supply air during these spikes.
During these peaks, compressed air is supplied from the receiver.
Since the air usage is higher than the air supply during these moments of high demand, air pressure will drop. But if we make the air receiver big enough, it holds enough air to supply the excess use.
The bigger the air receiver is, the slower the pressure will drop, and the higher the chance that the peak demand stops before the pressure drops too low.
Other functions
The air receiver fulfills more functions. For example, it helps cool down the air (and remove moisture that way) and it functions as a reference pressure for the air compressors.
Sizing an air receiver
It's a big tank/receiver, usually between 500 and 5000 liters, depending on the size of the compressed air system.
The rules of thumb for sizing an air receiver:
US units
1 gallon for each ACFM (Actual Cubic Feet per Minute)
So a 200 acfm system must have a 200 gal air receiver.
SI units
1 m3 for every 7.5 m3/min capacity
or
1 m3 for every 450 m3/hr capacity
For example, a 90 kW air compressor has a capacity of around 1000 m3/hr.
This compressor would need a 1000/450 = 2.2 m3 air receiver.
Bigger is better.
While it might be tempting to look at the price – bigger receivers are of course more expensive – it's almost guaranteed that you will make back that money in reduced energy costs, reduced maintenance and better air quality.
Actually, air receivers can't be big enough.
The worst you can have is a compressed air system with a too small air receiver.
Wondering which size is right for you? Get the bigger one!
Wet or dry receiver?
The receiver is always located in the compressor room, close to the air compressor.
There are two basic setups possible:
- Dry receiver (installed after the air dryer)
- Wet receiver (installed before the dryer and filters)
Both have their pros and cons.
Wet receiver:
- pro: direct pressure feedback to compressor
- pro: cools down the air – knocks out water condensate
- con: during demand peaks, air velocity through filters and dryer could be too high
Dry receiver:
- pro: no problems with demand spikes
- con: pressure difference with compressor – possible control issue
- con: higher load on air dryers (warmer air, more water vapor present)
Think of it this way: you are either storing 'dirty' air or 'clean' air, ready for use. In case of a sudden spike in air demand, a dry receiver can instantly supply the required compressed air.
With a wet receiver, a demand spike can create a huge surge of air through the air dryer and air filters. This creates a huge pressure drop and will also influence the air quality negatively.
In bigger systems, you will often see two air receivers: a wet receiver and a dry receiver – this is always the best solution.
In practice
A few practical points from the field:
- Air receivers usually have connections at the bottom and at the top. It's best to use the bottom one for incoming air (from the air compressor) and the upper one as supply to the piping system. This minimizes the amount of water and dirt in the compressed air going out.
- Air receivers should only be installed outside if the temperature never drops below 0 degrees Celsius.
- Drain condensate water from the receiver daily, or check that the automatic condensate drain is working properly.
- There's no real running cost and no specific maintenance required – but check the condition of the receiver periodically, and test the safety valve.
An adequately sized air receiver (bigger is better) will save you money in the long run.