Cover Story: Top Ten Air System Mistakes
Compressed air systems can cause huge headaches for many maintenance teams. There are a few myths and problem areas that are common to most air systems. Here are the top 10 mistakes people make regard...
April 1, 2003 | By MRO Magazine
Compressed air systems can cause huge headaches for many maintenance teams. There are a few myths and problem areas that are common to most air systems. Here are the top 10 mistakes people make regarding air systems, and some ideas for making improvements in your plant.
1. Believing compressed air is cheap
Air compressors powered by electric motors will use a surprisingly large amount of energy each and every year of operation. The annual power cost to operate a compressor can equal the initial cost of the unit.
The initial purchase price of a 100-hp air compressor can range between $30,000 and $50,000, depending on the type and options. The same 100-hp compressor operating 6,000 hours a year (a power rate of $0.07 per kW and a motor efficiency of 0.90) will have an annual power cost of $34,800.
You can determine the annual electric power cost of your compressors with this formula. The first step is to multiply the horsepower of the compressor times 0.746 times the hours of operation times your power rate (HP x 0.746 x hours x power rate). Then, divide that number by the motor efficiency.
Everyone in the plant should be made aware of the total power cost for operating the compressors. This is especially important for anyone that works with air-operated equipment.
2. Air leaks
The most common opportunity to recover energy cost is to control the compressed air leaks.
Effective leak control in an air system can pay huge dividends. A 1/4-in. leak in a 100-psi system will pass about 100 CFM of compressed air. This is approximately $12,000 in annual wasted power cost based on 24 hour-a-day compressor operation with a power rate of $0.07 per kW.
The process of detecting and monitoring leaks should focus on more than the basic header and piping system. The fact is that you will find a majority of your leaks at valves, fittings, connections, tools and at the point of use.
It is important to remember the interdependent relationship between air system components. Controlling air leaks will not translate into reduced energy cost unless the compressor controls and air delivery system are in proper working condition.
3. Compressor controls
There are several types of control systems in operation for reciprocating, rotary screw and centrifugal compressors. It is common to find a variety of these in use in the same plant.
Compressor controls make it possible for you to translate lower air usage into lower energy cost. However, the typical plant is operating multiple compressors at part load, which is inefficient and expensive.
4. Pressure loss in the piping system
It is important to minimize pressure drop throughout the compressed air system. This prevents you from producing pressure that never reaches the demand point, which is a direct waste of energy.
Every pound of increase or decrease in pressure requires a half per cent increase or decrease in power. Therefore, a 10 psig decrease can save you 5% in power costs.
This amounts to $1,740 in annual savings for the 100-hp compressor in the earlier example. The savings adds up because most industrial plants have more than one 100-hp compressor.
The pressure of an air system is often raised to overcome pressure drop. The cause is usually found to be shortcomings in the piping system and pressure loss at the filters and dryers. Each of these problem areas will cost you money on an annual basis.
The following are some common problem areas:
a) Delivering air to the point of use in pipe that is too small. An example; using 30 ft or more of 3/8-in. rubber hose, rather than 1-in. pipe with a short hose at the tool.
b) Using “tee” pipe connections rather than 30- or 45-degree angle entry connections when introducing air into a flowing stream of air.
c) Saving money by using undersized filters and dryers that have a higher pressure drop.
5. Contamination from the piping system
The following are two common mistakes made during the installation of a piping system.
a) Dirt, rust and liquids are commonly found in the piping of a compressed air system. This interrupts the supply of air and results in the need for maintenance.
The amount of these contaminants that are carried along with the air stream will increase when the air velocity increases. Air velocity increases as the pipe size goes down.
It is acceptable for the interconnecting pipe (from compressors, dryers and inline filters to the main header) and main air header pipe to have a different air velocity specification than the piping from the main header to the points of air usage.
The interconnecting pipe and main header should have an air velocity between 20 and 30 ft per second (not to exceed 30 ft per second). The air lines running to the points of air usage should not exceed 50 ft per second.
You can calculate the air velocity of your system. The formula is Flow in CFM divided by the Compression Ratio in the pipe divided by the Area of pipe divided by 60. This will give you the velocity in ft per second.
b) The piping system should always take the air off the top of an air line when running a line from a header to the point of air usage. This will prevent condensation and trash from migrating to the air usage equipment.
6. Poor condensation management
Condensation is the moisture that drops out of an air flow as it cools. The condensation in a compressed air system is a constant threat to cause expensive problems. The following are a few examples:
a) Moisture washes lubrication from air tools and production equipment, causing downtime and maintenance.
b) An inconsistent supply of dry air causes production quality problems.
c) Excessive rust and scale often form in the air distribution system.
d) Water can back up into the compressor and wreck the machinery.
e) Air dryers can become overloaded.
f) In-line filters can be destroyed.
The problems get worse if you operate lubed reciprocating or oil-flooded rotary screw compressors, which is just about everyone. Compressor oil makes its way into the distribution system with the compressed air. The mixture of oil, water, dirt and heat tends to build up a sludge that will ultimately jam or clog production equipment, air tools and drains.
The situation is further complicated by climate and seasonal weather changes. This is because the amount of condensation generated will change according to changes in the temperature and the relative humidity of the inlet air.
Consider that a 200-hp compressor operating in a climate of 60F with 40% relative humidity will generate approximately 50 gallons of condensate a day. However, that same compressor operating in a climate of 90F with 70% relative humidity will generate approximately 260 gallons of condensate a day.
The typical compressed air system is designed to remove condensation at strategic locations. This means there are drains at the aftercooler separator, receiver tank, air dryer, in-line filters and at drain points in the piping.
The problem is that there are shortcomings with the products being used to drain the condensation. A condensation drain should automatically remove condensate when it appears at the drain without wasting air or clogging. [We have found only one drain that meets this criteria. If you want details, send an email to email@example.com with your mailing address. We will have the appropriate information sent to your attention.]
7. Supplying higher pressure air than is needed
The first consideration is to determine the specific pressure required for all the air requirements in your plant. Plants that have studied this issue often found that they were producing high pressure air for the entire plant just to satisfy an isolated high pressure requirement.
They solved this problem by installing a small departmental compressor designed to handle the higher pressure. This allowed them to lower the pressure requirement of the main compressors and immediately save energy.
8. A lack of air system training
The operation and man
agement of a compressed air system takes the efforts and talents of many people. A decision to work towards energy savings will require all of these individuals to be part of the process. However, they cannot be effective if they don’t understand the cost of compressed air and the interdependency of the components of an air system.
The companies that have trained their people on the importance of saving energy have reaped the biggest savings. This type of training has a very fast payback.
The energy savings will go directly to the bottom line and can make a difference in the profitability of any company.
9. Lack of troubleshooting information
Temperature and pressure readings can organize your troubleshooting efforts when dealing with a problem in the compressed air system. Specifically, you can use normal condition readings as a reference point in order to isolate the cause of a problem.
The normal condition readings are taken on a regular basis for historical reference and to observe any trends that indicate the beginning of a problem. The readings are usually taken at locations before and after the air equipment, including, among others, the compressors, aftercoolers, dryers, receivers, air tools and filters.
a) Increasing temperature in a compressed air system is one of the best indicators of a problem. If you monitor temperatures over time, you can build a base line for normal conditions and create a model for predicting when you will have trouble with your compressed air equipment.
The most useful tool for this application is an infrared thermometer. This is a hand-held device that gathers temperature readings by aiming it at an object.
There are a couple of points to keep in mind on this issue. The first is to be aware that you are measuring the surface temperature, not the temperature of the oil, air or water inside the object. This method is not as accurate as putting a probe in the oil, air or water. However, it is a practical way to get information that can be used in troubleshooting.
The second point is to always take temperature readings when the compressor is at full load. This gives you meaningful information that can be compared to the design standards for the equipment.
b) Monitoring pressure is another useful troubleshooting tool.
The best idea we have observed was the use of a single gauge that was adapted to fit in an air line quick connection. The operator simply inserted the gauge into quick connections that were mounted in key locations on the air system piping.
Accuracy is critical when comparing readings and trying to isolate a problem. It is important that you use a high quality gauge.
10. Cookie cutter approach to oil and filter changes
A preventive maintenance program is essential for maximizing the service life of a compressor. The key is to make sure your program matches your application.
This means monitoring filter and lubricant condition by measuring pressure drop and by using a regular oil analysis. This will help you create a schedule for change intervals that will provide the best protection for your compressor.
a) Oil analysis can be a helpful tool for compressor diagnostics. Quarterly samples of the oil appear to be more than adequate to keep an eye on normal wear. There are many good labs available for oil analysis in most areas.
b) The best way to determine when to change your inlet air filters is to measure the restriction in the piping between the filter and the intake of the compressor. This restriction can be monitored by a water manometer, an intake filter indicator or a dial gauge that is calibrated in inches of water.
The idea is to service the filter when the monitor reaches a certain level of restriction. This specification can vary between 10 and 20 inches of water because of the different designs of filters. It’s best to get guidelines about this from a filter vendor that is knowledgeable about compressors.
A final thought: Don’t rely solely on the advice of equipment vendors when considering making improvements to your compressed air supply. Get a second opinion.
This report was prepared by the consultants at CompressorWise.com. The organization, which does not sell equipment, offers a newsletter and a website (www.compressorwise.com) dedicated to helping industry with information about how to get the most from compressors and compressed air systems.