Month: May 2015
An air receiver is essential to every compressed air system to act as a buffer and a storage medium between the compressor and the consumption system. There are in principal two different air receivers in a compressed air system:
- PRIMARY receiver – located near the compressor, after the after-cooler but before filtration and drying equipment
- SECONDARY receivers – located close to points of larger intermittent air consumptions
The maximum capacity of the compressor in a well designed systems always exceed the maximum mean air consumption of the system (maximum mean air consumption is the mean air consumption over some reasonable time).
Since the maximum capacity of an air compressor also always exceed the minimum air consumption in the system – the compressor must modulate its capacity during normal work, often by using primitive strategies as on/off modulating or more advanced strategies as frequency drives and inverters. Primitive modulating strategies cause more pressure variations in compressed air systems than more advanced strategies.
In addition, the air consumption vary due to the process supported. In shorter periods the demand for compressed air may even exceed the maximum capacity of the compressor. In fact, it is common in well designed systems not to design the compressor for the maximum peek loads.
Air receivers in compressed air systems serves the important purposes of
- equalizing the pressure variation from the start/stop and modulating sequence of the compressor
- storage of air volume equalizing the variation in consumption and demand from the system
In addition the receiver serve the purpose of
- collecting condensate and water in the air after the compressor
Sizing the Air Receiver
The air receiver must in general be sized according
- the variation in the consumption demand
- the compressor size and the modulation strategy
In general it is possible to calculate the maximum consumption in the system by summarizing the demand of each consumer. The summarized consumption must be multiplied with a
- usage factor ranging 0.1 – 1
depending on the system. In practice it is common that the manufacturer use standardized receivers for specific compressor models based on their know-how.
For calculating the receiver, note that it is necessary with a pressure band for the receiver to be effective. If the consumption process requires 100 psigand the compressor is set to 100 psig, there is no storage and no buffer. Any increased demand will make a pressure drop below 100 psig until the compressor responds by increasing the air volume compressed.
If the compressors operates at 110 psig the difference between 110 psig and 100 psig accounts for the air stored in the receiver. If the demand increases, the pressure can drop 10 psig before the minimum requirement is met. Pressure and flow controllers can be used after the receiver for stabilizing downstream pressure to 100 psig and flattening demand peaks. Note that in a compressed air system the pipe work also makes the purpose of a buffered volume.
A commonly used formula to find a receiver size is:
t = V (p1 – p2) / C pa (1)
V = volume of the receiver tank (cu ft)
t = time for the receiver to go from upper to lower pressure limits (min)
C = free air needed (scfm)
pa= atmosphere pressure (14.7 psia)
p1 = maximum tank pressure (psia)
p2 = minimum tank pressure (psia)
Example – Sizing an Air Receiver
For an air compressor system with mean air consumption 1000 cfm, maximum tank pressure 110 psi, minimum tank pressure 100 psi and 5 sec time for the receiver to go from upper to lower pressure – the volume of the receiver tank can be calculated by modifying (1) to
V = t C pa / (p1 – p2)
= (5 sec) (1/60 min/sec) (1000 cfm) (14.7 psi) / ((110 psi) – (100 psi))
= 122 ft3
Read & Follow Your Air Compressor’s Manual
Nothing stops an air compressor faster than an owner who doesn’t read the owner’s manual. There’s going to be some simple tips in there for you that will help you to get a nice long life out of your air compressor – simple stuff for you to do that you would never have thought to do unless you read it. Besides, no one knows your compressor better than the manufacturer.
Check the Oil Level Regularly
If you’re running a compressor that uses oil you should be checking it on a daily basis to make sure that your machine is topped off. Then, every 500-1000 hours (only true of some recip compressors, most synthetics say you can get away with yearly change-out) of use you should be changing this oil to ensure maximum functioning of your air compressor.
3. Drain Traps for Water and Oil Removal
Water and oil removal systems like filters, dryers, tanks, and separators are only efficient if the contaminants are removed from the system. Make sure these contaminant removal devices have proper functioning drains. Press the test button if the drain has one, or better yet, take the time to watch it fill and dump on its own. Open the bypass (your drain does have a bypass, right?) and check for condensate stuck behind a blocked line upstream of the drain.
Find automatic and timer drains now…
4. Inspect & Clean the Air Filter
A dirty air filter makes your air compressor work harder, consume more electrical energy, and runs the risk of contaminating the compressor, particularly if it ruptures. Check your filters regularly and change them if you notice a heavy build up of dust and dirt. Change every three to six months or so if you use it infrequently.
5. Change the Separator Element
The separator element prevents the excessive use of oil, but it has to be replaced periodically. Keep your compressor in top condition by replacing the separator element every 1,000 hours of operation. Remember, every 2 PSI of separator pressure drop can increase your compressor energy costs by 1%. Changing separator elements is very inexpensive by comparison.
6. Stop, Look & Listen
Stop, Look & Listen for any unusual noise, overheating, vibrations or belt slippage and correct before damage of a serious nature develops. Familiarize yourself with proper instrument and gage readings. Check daily for abnormal readings before major damage occurs. Keep a daily record of key readings such as oil pressure, oil temperature, motor amps, and discharge pressure.
7. Check for Leaks Throughout
Leaks in compressed air systems often account for 25% or more of the compressed air generated. Check entire system for air leakage around fittings, connections, and gaskets, using an ultrasonic leak detector. Common areas to check are piping and flexible joint packings, control lines, control line fittings, clamps and connectors, valves, air pressure safety relief valves, and pressure gauge connections.
8. Proper Compressor Temperature
Should be within manufacturer’s limits. Refer to the air compressors manual to make sure you are operating within the proper temperature range. Many of the tips from above can affect temperature if they are not checked regularly.
9. Compressor Cleanliness
Maintain in a clean condition; a compressor should never leak oil. When you’re checking your compressor, remove any dust from the motor, pump and cooling fin area. This will help to keep the pump and motor running cooler and prolong the life of your compressor.
10. Keep Extra Filters and Parts on Hand
One of the main keys to maintaining an efficient compressed air system has been and still is Preventive Maintenance.
- Purchase your maintenance supplies prior to the scheduled date
- Be sure to check all the devices that need maintaining before you purchase your supplies
- Don’t let system failure be your signal that is time to do maintenance
- Plan your preventative maintenance when it is convenient for you-or it will plan itself when it is not convenient for you!