Building a reliable, cost-effective air system

Both air quality and supply directly impact the paint refinish process. "Re-dos" and "comebacks" increase labor and material costs, and interrupt workflow. And, costly rework due to poor quality paint jobs and work flow interruptions due to inadequate air supply can increase cycle times, causing real losses in hard dollars.

There are three major areas to consider when you are looking at developing the right air system for your shops:

1. Air compressor: selection and sizing.

2. Air treatment.

3. Piping, distribution.

Air compressor: selection and sizing

Avoid a common pitfall — don't over-pressurize. If asked, many shop users say they need 145 or 175 psig compressors. In fact, very few tools require pressures above 100 psig. Refer to the tool manufacturer's manual to determine your shop pressure and flow requirements.

Increasing system pressure will increase the amount of air lost through leaks, wasting both air and electricity. Numerous compressed air industry studies confirm that as much as 35 percent of all compressed air produced is lost through leaks.

Determine flow requirements. Compressor size is not determined by pressure requirements. It is determined by the compressor's output capacity in cubic feet per minute (cfm). To properly size a compressor, find out how much air is needed in terms of volume — not pressure. Some tool and compressor manufacturers publish charts with air consumption rates for many common tools. Adding all of these together for the tools you use yields the total potential flow requirement. However, it does not take into account the percentage of time each tool is used. This requires some study of how the different parts of the repair shop operate throughout the day. Electronic data logging devices are a convenient way to measure and record compressor usage.

Compressor type

Rotary vane and screw compressors have closed circuit, thermostatically-controlled cooling systems that provide a 100 percent allowable duty cycle with operating temperatures of only 170-200°F. This is an important consideration for paint spray booths and other moisture sensitive applications, since moisture vapor content decreases with temperature. An important rule of thumb is that every 20°F decrease in temperature cuts moisture vapor content in half, making it easier to remove moisture from your system.

Compressor location

Compressors are often installed where the noise, vibration and heat will least bother staff and customers, rather than where they'll best perform and be easily serviced.

Rotary compressors offer much lower operating temperatures, up to 200°F cooler than the typical piston, and they offer much lower noise levels, giving the shop owner more flexibility on compressor location.

Maintenance and long-term performance

Maintenance requirements and long-term compressor performance are essential factors to consider. Piston compressors and rotary compressors have different maintenance and service requirements.

However, the pistons, cylinders, rings and valves in reciprocating units wear over time, causing the compressor to deliver less air, and send more lubricating oil past the rings into the compressed air system. Without proper filtration and more frequent filter maintenance, this will cause paint finish problems. Preventive maintenance will slow this process, but rebuilding a piston compressor periodically may be necessary to reduce the oil carry-over and reverse the gradual loss of performance. This is also true of some rotary vane compressors. However, rotary screw compressors are designed so that the rotors do not touch each other or the rotor housing, therefore, performance does not change over time.

Air treatment

Many sources recommend improving paint finish quality by eliminating contamination in the spray booth, but compressed air quality plays an equally important role as well. Here are three basic types of air system contaminants and their effects:

Water in vapor, mist and liquid form may cause defects in finishes as well as excessive wear in tools. For example: On a 75°F day with 75 percent relative humidity, a 10-hp compressor can introduce 7.5 gallons of water into a compressed air system. And on a 90°F day with 90 percent relative humidity, that same compressor will introduce 15 gallons of water in the compressed air system.

Particulates like dirt and dust from ambient air cause defects in paint and finishes, and excessive tool wear. They also build-up in piping and can cause pressure drop. Oil vapors and mists also cause finish defects, and combine with particulates to clog tools and sprayguns.

Dryers remove moisture in a variety of ways. The most popular in spraying and finishing applications are refrigerated dryers.

• Refrigerated dryers use heat exchangers to lower the compressed air temperature well below the ambient temperature, condensing much of the moisture vapor into liquid that can be drained from the system. Refrigerated dryers are designed to produce dew points between 35°F and 50°F at rated conditions and are ideally suited for use with rotary screw compressors.
• High temperature refrigerated dryers include an aftercooler and are primarily used with piston compressors, as the air must be pre-cooled prior to entering the dryer for it to be effective. These dryers are usually designed to achieve 50°F dew points at rated conditions.
• Membrane dryers are a newer technology and work well for select applications. Compressed air passes through a bundle of hollow membrane fibers and the water vapor permeates the membrane walls. Unlike refrigerated dryers, membrane dryers consume compressed air; as much as 30 percent of their rated capacity.
• Desiccant dryers are used in applications that require compressed air at dew points as low as -100°F. Through two identical drying towers, each containing a desiccant bed, air flows alternately. While one tower is on-stream drying, the other is off-stream being regenerated. Purge air is used to regenerate the desiccant. These are more common in special industrial operations where extremely dry air is required.

Filtration

Compressed air filters can produce air that is up to 250,000 times cleaner than the air we breathe. However, if air is to be used with breathing air apparatus or respirators, the Occupational Safety and Health Administration (OSHA) specifies Grade D air. More information on the specific requirements can be obtained through the Compressed Gas Association, OSHA or the National Institute for Occupational Safety and Health (NIOSH).

Condensate drains and traps

The drain trap is a critical but often over-looked component of the compressed air system. If the separated contaminants are not drained from tanks, dryers and filters, they build up over time and find their way back into the air system.

Liquid accumulation in tanks gradually eliminates air storage capacity, causing periods of inadequate air flow/pressure. They should be used on air receivers, standalone aftercoolers, refrigerated dryers, separators, filters and header piping.

Piping and distribution

As compressed air piping is responsible for actually delivering the compressed air to the point of use, its material, age and condition also impact both system reliability and air quality. Iron piping will rust and corrode over time, creating buildup on the interior and reducing the functional diameter.

This buildup will create a pressure drop in the system and contribute to poor air quality. Appropriately sized copper or aluminum piping offers the best performance over time. Easily installable, modular piping products are available and are ideally suited for shop upgrades, as well as new facilities.

Avoiding pressure drop

Restrictions in airflow create air turbulence that results in a reduced system pressure. This occurs in many components, including the dryer, filters, valves and piping. The degree depends on the choice of material and pipe size. Pressure drop can be greatly reduced with proper system design and maintenance, but there will always be some. Be sure to account for the total pressure drop when selecting the compressor's operating pressure.

Pipe size has a major impact on system performance. Pressure drop changes exponentially with pipe diameter. Bigger is better. Look ahead when planning a system and allow for business growth. It is time consuming and expensive to install a larger distribution system later. There are standard charts published that provide guidelines. Check with a compressed air specialist or contact the Compressed Air and Gas Institute (www.cagi.org).

For example, the pressure drop of 40 cfm (from a 10-hp compressor) through 500 feet of straight ¾-inch smooth pipe would be about 8 psi. If the shop added another 10-hp compressor, the total flow would be about 80 cfm and the pressure drop would increase to 32 psi. An increase to 1-inch diameter pipe would change these numbers to 2 psi and 9 psi, respectively. Pressure drop increases with the number of turns in the system and with rough interior surfaces.

Piping recommendations:

• Plan for future growth and install the largest pipe diameter feasible.
• Minimize the use of pipe "T"s and right angles.
• Install a flexible hose between the compressor or tank and the piping to eliminate stress on pipe connections caused by compressor vibration.
• Provide adequate bracing/support when hanging pipe from ceilings or walls.
• Use only full flow ball valves to minimize pressure drop.
• Loop distribution to balance pressure and flows at all points of use.
• Connect point of use pipe drops to the top of the header to reduce moisture carry-over.
• Install drip legs at each point of use to capture residual moisture.

Leaks

An important rule of thumb is that every 2 psi increase in pressure increases energy consumption 1 percent. The higher the system pressure, the greater the volume lost through leaks. A 1/16-inch leak loses 7-8 cfm at 120 psig. At 150 psig, it loses 9-10 cfm. A 1/8-inch leak loses 30 cfm at 120 psig and nearly 38 cfm at 150 psig. Thirty-eight cfm is more than many 10-hp compressors can produce.

Storage and control — Receiver tanks provide the first stage of moisture separation and store air for later use. Tanks stabilize system pressure and provide an "air buffer" to compensate for fluctuations in air demand. Regulators placed at the point of use will further "regulate" the air pressure for the specific tool. For example, spray guns using regulators have a steady stream of air resulting in a more consistent spray pattern.

Guidelines for tanks — The pressure rating must exceed the highest possible system pressure; must have a safety relief valve, pressure gauge and drain to remove liquids; and must meet American Society of Mechanical Engineers (ASME) or other required code (check with local authorities).

Price and true cost

No one wants to spend more on compressed air equipment, but calculate the cost of refinishing one "fish eye." The labor, material and workflow interruption costs far exceed the price of investing some thought and money in the compressed air system. A thorough system analysis goes a long way in building a reliable, cost-effective system. Carefully consider each system component and its impact on the application. Remember, value is more than initial price. Selecting quality equipment now will save time and money for years to come.

The following is courtesy of the Prevost Corporation.

Streamline your air lines

The cost to generate and maintain compressed air in a shop is often a company's second largest energy expense. A new efficient piping system can usually produce enough annual savings to pay for itself in less than two years.

Since we all can relate to cars and driving in traffic, think of an air piping system as a highway and air molecules as cars in heavy traffic that are pressured to get somewhere fast. An efficient highway must have enough lanes to effectively move the anticipated volume of traffic. Similarly, the header pipe of an air piping system must be large enough (in diameter) to distribute the anticipated volume of airflow at a specified pressure throughout a shop.

Generally, highway systems that loop around a metropolitan area are the most efficient. The header pipe or ringmain is also most efficient if designed as a closed-loop system.

A highway should remain smooth, void of potholes and able to handle weather and traffic conditions for prolonged periods of time. The most efficient piping systems are constructed with non-corrosive piping materials that have low coefficient of friction on the pipe's inside surface.

A highway should be kept free of debris and standing water. An efficient piping system should properly treat the compressed air so it is free of contamination and condensation.

Here are guidelines that can help optimize your compressed air 'highway' system:

Material selection — The four piping materials most often used for compressed air are black iron pipe, copper, plastic and aluminum.

• Black iron pipe is the cheapest material, but it's also the most expensive over the life of the shop, due to installation and repair costs, inherent leaks and inefficient energy consumption after it begins to corrode.
• Copper pipe has excellent resistance to corrosion and also a lower coefficient of friction on the inside surface. This translates to less work that the compressor must do to supply the required volume of air at the necessary pressure throughout the shop. Drawbacks with copper are using open flames to sweat the pipe, the extensive labor hours required to install it and the very high cost of the copper piping.
• Plastic pipe is being used less often due to the fact that systems using improper materials can literally explode, causing a serious safety hazard. The PPI (Plastic Pipe Institute) and the Occupational Safety and Health Administration (OSHA) recommend against the use of PVC for use with compressed air systems. Certain advanced compressor lubricants can also attack the glues, causing pipe failure.
• Aluminum pipe has gained significant popularity over the last 10 years. It offers excellent resistance to corrosion, the lowest coefficient of friction and is very easy to install. It also require no open flames, threading machines or special swaging or crimping equipment. It can be installed quickly and easily, and even be dismantled and reassembled if the shop is moved to another location.

Sizing the system — Make sure your piping system is properly sized to supply any anticipated demand of air volume that your shop will need during peak operation, including possible future shop expansions.

System design — Follow these tips to have a safe and efficient system:

• Properly size all piping components from the ringmain to each drop.
• A looped ringmain with a minimum number of elbows (especially around obstacles such as support columns) will provide the most efficient system.
• Install the pipe straight with adequate support for each section. This will ensure both optimum efficiency and safety.
• Install drain legs in strategic places in order to remove any moisture that might accumulate in the system

Accessory products — Selecting quality accessory products, such as quick couplings and hose assemblies, can dramatically improve a shop's safety, productivity and, most importantly, energy consumption, due to pressure drops and air leaks. Consider these points when selecting components for a safe, efficient, and leak-free shop:

• Quick couplings — Specify "venting-action" safety quick couplings at the air supply manifold. Safety couplings eliminate the risk of both personal injuries and costly damage to nearby vehicles caused by the "hose whip" that occurs when disconnecting a pressurized air hose. Also, select couplings that will last in the tough conditions of a body shop or garage. There are many styles to choose from, including impact-resistant, composite quick couplings that are easier to operate with a simple push of a button and will last longer than conventional, manual sleeve couplings.
• Hose assemblies — Install either self-coiling hoses or retractable hose reels in areas where there is considerable activity or traffic in order to keep hoses off of the floor and to provide your employees with safe and efficient work areas.
• Swivel adapters — Install free-angle, swivel couplings onto hoses where workers do extensive work with air tools, including grinders in prep areas and HVLP spray guns in paint booths.
• Blow guns – Although this is a minor area, there are ergonomic and safe blow guns that operate effectively at full pressure and are OSHA-compliant.

So change your compressed air piping system from an expensive pay-as-you-go toll road into an efficient super highway that will get you to a more profitable destination.

Courtesy of the Prevost Corporation