A Primer for Instrument Air
How to Design a Compressed Air System, Select the Components, and Get Trouble-Free Performance

Air Quality

The instruments, controls, and equipment determine air quality requirements. An oil-free air system avoids plugging orifices in pneumatic control devices with oil and moisture. On the other hand, pneumatic actuators work better with some oil in the air and need a lubricated compressor.

Investigate individual components to set the system design. The main determinant is the oil concentration in parts per million. Equipment vendors give oil concentration limits either in their operation manuals or by calling the customer service department.

Provide filtration to protect against build-up or erosion caused by particulate matter in the large volumes of air that compressors handle. Moisture in compressed air can lead to scaling, rust, frozen lines, wear, and malfunctioning controls and air logic devices. The pressure dew point defines the amount of moisture removal necessary.

Condensate Disposal

When air is compressed, it is heated. When later cooled, condensate forms. Lubricated compressors leave oil in the condensate. In many areas this condensate is considered hazardous waste so evaluate maintenance and future laws before selecting a water/ oil separator system.

Oil-free systems

Select an oil-free system for applications that cannot tolerate lubricant. An air receiver downstream of the compressor stabilizes system pressure, acts as a demand reservoir, and collects some moisture. Put an air dryer, selected to provide the proper pressure dew point, downstream of the receiver to trap remaining moisture.

A coalescing filter after the dryer provides protection if upstream components fail. For instance, the coalescing filter captures a large portion of moisture traveling downstream of malfunctioning condensation traps.

Installing a "dry" receiver after the coalescing filter further stabilizes pressure and acts as a reservoir for heavy demands.

Lubricated compressors and downstream purification

A modern, lubricated compressor and high-efficiency purification system produce instrument quality air with the minimum of stages of efficient compression. The built-in separator in the compressor removes the bulk of the oil.

This system is similar to the oil-free system with a "wet" receiver, air dryer, and coalescing filter. An activated charcoal filter between the coalescing filter and "dry" receiver removes residual oil vapors.

The following guidelines will assist you on your selection journey.

Air Compressors

The key issues in purchasing a compressor are reliability, cost-effectiveness, ease of operation and maintainability. Compressor reliability is based on the following factors.

Type of control system -- State-of-the-art electronic controls eliminated problems, mechanical switches, and relays. Older pneumatic compressor controls using compressed air taken before the air dryer can prove troublesome because moisture in the air leads to sluggish performance and damage to the compressor. The rubber diaphragms used with these pneumatic control systems area a common weak link in control systems.

Ambient temperatures -- The compressor must be capable of operating in ambient temperatures approaching 110-115EF because compressor rooms are 5-10 degrees warmer than the outdoor temperature. Higher temperature ratings mean longer, more reliable periods between maintenance.

Motor design -- As a minimum, motor insulation must be class F. Temperatures inside the sound attenuating enclosures for motor and compressors are warmer than the ambient air. Summertime operation gives internal temperatures from 110 to 115EF. Standard Class B insulation motors are designed for a maximum installed temperature of only 104EF.

Cooling system -- Compressing air produces heat of compression that must be removed. The compressor oil removes some of heat. Lubricated compressors remove even a higher portion of the heat since the oil is in the compression chamber. The oil is then cooled in a forced draft air-cooled heat exchanger. That portion of the heat remaining is removed in inter-cooler and after-coolers of sufficient capacity to permit continuous, fully-loaded compressor operation in high ambient temperatures. The aftercooler approach temperature, that is, the temperature difference between the compressed air outlet and ambient air temperature, should be in the 15E to 20EF range.

Cost Effectiveness

Power, maintenance, and downtime costs outweigh first cost over the life of the compressor. Since you pay for kW, not horsepower, assemble the data to calculate input kW. Identify all power into the package, including compressor brake horsepower at shaft and motor efficiency at this BHP level, fan horsepower and motor efficiency, oil pump horsepower and efficiency, and so on. Calculate input kW and operational costs by the following formulae:

Input kW =    0.07457 BHP   +    0.07457 Fan HP    + ...
           Motor Efficiency   Fan Motor Efficiency

Operational Cost ($/year) =
Input kW x Power Cost ($/kWh) x operating hours per year.

Insist that vendors supply performance numbers based on the same criteria. Air-end performance testing does not account for losses in the compressor package. Requiring testing in accordance with an industry standards such as acceptance test PN2CPTC2 that is endorsed by the compressed air and gas institute and the European committee of manufacturers of compressors, vacuum pumps, and pneumatic tools insures valid performance comparisons.

Ease of operation

The control system should be easy to use and provide required data. Microcontrollers provide real-time adjustments, but beware of those systems needing arcane codes or hand-held programmers.

The sound level can mean the difference between hearing and not hearing. Sound levels from 75-80 dBA are acceptable, with 85 dBA being the maximum allowable. Avoid unenclosed rotary compressors and others that exceed these noise levels.

Maintainability

The compressor should be easily accessible for maintenance. If enclosed, the panels should be easily removed. Leave at least three feet of clearance around the compressor.

The primary maintenance items on a compressor are the inlet filter, oil drain, oil fill, motor greasing, condensate traps, and control calibration. Each should be easily accessible. Service indicators help guarantee timely maintenance.

In lubricated compressors, the oil travels downstream and must be replenished regularly. Synthetic lubricants provide superior lubricating characteristics, longer service life, and lower vaporization rates. Polyglycols extend changeout intervals to 8,000 hours, have the lowest vaporization rate, and are biodegradable. Oil-free compressors require limited amounts of lubricant for bearings and gears.

Use SAE O-rings on fittings along with 37 flared connections to avoid oil leaks. Standard pipe fittings will leak in time, given the temperatures and the viscosity of the lubricant.

Air Purification System

Focus on the air purification after selecting the compressor. The air system designer must consider the following:

• Delivering the required air quality
  
• Maintaining air quality during upsets
  
• Minimizing operating costs

Generally, air purification falls into one of three categories: filters, dryers, and receivers.

Compressed Air Filters

Filters remove condensed liquids, particulates, and oil vapors. Coalescing filters to remove water and oils have efficiencies from 99.98% at 0.1 micron particle size to 99.9999% at 0.01 micron. The filters should have a maximum wetted pressure drop of 3 to 3.5 psi. The maximum pressure drop, normally 10 psi, determines the service life of these filters. Expect to replace the filter elements every six to twelve months.

High-performance coalescing filters require changeout every five years. Although these filters have a higher first cost, the lower pressure drop and reduced energy and maintenance costs provide a simple payback of less than one year.

Particulate filters installed downstream of a dessicant dryer should have a different pressure gauge to indicate the condition of filter elements rated for a nominal efficiency of 99.95% at 1 micron particle size and initial pressure drop of 1 psi. Coalescing filters must have a high-quality automatic condensate drains.

Vapor removal and filters absorb oil vapors with activated charcoal. Location and the oil concentration determines filter element life. Normal pressure drop for a vapor removal filter is 1 psi.

Air Dryers

An aftercooler discharging compressed air at 100EF passes 67 gallons of water per 1,000 scfm per 24 hours. Instrumentation fails when water and lubricant condense as the air is further cooled in the piping system or as the air expands through the orifices.

The air exiting the aftercooler is saturated and any further temperature drop results in more condensation. A useful "rule of thumb" states that "a 20 degree reduction in temperature condenses one half the water vapor in saturated air."

Air dryers reduce the moisture content as measured in terms of a pressure dew point (pDp) that is based on a specific set of inlet conditions to the dryer.

Dew point is the temperature at which water vapor condenses -- saturated, 100% relative humidity. Pressure dew point is the dew point of the air at operating pressure. Atmospheric dew point refers to air expanded to atmospheric conditions. To avoid confusion, specify dryer performance in terms of pressure dew point.

Dryer Selection

The instruments and the lowest expected ambient temperature determine the drying method. The most common dryer is a refrigerated unit that cools the compressed air, condenses water and oil vapors, separates them, and drains them from the system. The "dried" compressed air is then fed to the instrument air system.

Dryer performance is specified as a pressure dew point class that is based on a specific inlet and ambient conditions. The lowest pressure dew point class with a refrigerated dryer is Class H. This class delivers a pressure dew point that of 33E to 39EF. Refrigerated dryers should not operate below the Class H range because the water vapor will freeze in the dryer. The highest practical pressure dew point for a refrigerated dryer is 60EF because higher pressure dew points give condensation in downstream piping.

In the United States, most dryer manufacturers base the pressure dew point performance on standard conditions: inlet air flow, 100EF inlet air temperature, 100 psig operating pressure, 100EF maximum ambient temperature (air-cooled units), 85EF cooling water temperature (water-cooled units), and 5 psi maximum pressure drop.

Adjust air dryer sizing to account for deviation from standard conditions. For example, elevating the inlet air temperature 10 degrees increases the load on the dryer by more than 25 percent and raises the outlet pressure dew point above 50EF. Maintaining the original 33E-39EF dew point now requires a dryer 35% larger.

Desiccant dryers give pressure dew points below 33EF if piping is exposed to freezing temperatures. Desiccants dry air through adsorption in which a hydroscopic material -- chemical, alumina, silica, molecular sieve -- removes the water and oil to reduce the dew point to the standard pressure dew point of -40EF. Special designs produce dew points of 100EF or lower.

Standard conditions for rating a desiccant dryer's pressure dew point inlet air flow in scfm, 100EF inlet air temperature, 100 psig operating pressure, and outlet air flow in scfm to account for the inlet air flow used during regeneration.

Dryer Selection Guidelines

Non-cycling and cycling are the two types of refrigerated dryers. On a non-cycling dryer, the refrigeration compressor runs continuously regardless of dryer load. A thermostatic expansion valve and hot gas bypass valve regulate the flow of refrigerant into the heat exchanger to maintain dew point and minimize "freeze-up." Since the unit uses full input power at all times, a non-cycling dryer should be selected for systems with a constant air flow.

In cycling dryers, the refrigerant cools an intermediate fluid that cools and "dries" the air. During low-load operation, the refrigeration circuit stops its compressor and restarts it when the fluid temperature rises. The cycling type dryer conserves energy and minimizes dryer freeze-up making cycling dryers the choice with fluctuating air flow and inlet temperatures. Over-sized cycling dryers provide additional drying capacity for future air system upgrades.

Review the subsystems when selecting the refrigerated dryer.

Refrigerant system -- Look for:

• Low input power (Kw) refrigeration compressor (ignore compressor horsepower, you pay for Kw);
• Hermetic compressors above 2,500 scfm; below, use semi-hermetic with valve unloaders;
• Refrigerant HFC-134A on dryers to 100 scfm, HCFC-22 above 100 scfm;
• Refrigerant pressure below 100 psig for 100 scfm and smaller dryers to increase compressor reliability; and
• Air-cooled refrigerant condenser designed for 130EF maximum ambient temperature to assure trouble-free operation during hot summers.

Air system -- Look for:

• Precooler/reheater to remove up to 65% of the heat from the compressed air to allow using a smaller refrigeration compressor
• Smooth copper tubes on heat exchangers to reduce maintenance and eliminate prefiltering air entering the dryer
• Water/polypropylene glycol solution as the intermediate fluid in cycling dryers

Instrumentation and controls -- Look for:

• easy operation monitoring with parameters displayed digitally
• simple, manual adjustment of pressure dew point in cycling dryers
• controls that sense ambient temperature to maintain dew point suppression.

Desiccant Dryers

There are two designs -- heatless and heated. Heatless dryers provide a consistent pressure dew point with minimal maintenance and maximum desiccant life. However, the air compressor must deliver excess flow to compensate for the 13-plus percent of the inlet air flow consumed for desiccant regeneration. If the desiccant absorbs oil vapor then it must be replaced so desiccant life is lower on lubricated systems.

Use a heated dryer when the compressor cannot deliver the required excess flow. The four types of heated dryers are internally heated, externally heated, blower purge, and heat of compression. Both the internally and externally heated designs use a heater and a low-rate air purge to regenerate the desiccant.

The blower purge design uses a heater and a 3 psig blower instead of compressed air for regeneration. The heat of compression dryer, specifically designed for use with an oil-free compressor, uses the hot compressed air to regenerate the desiccant and yields the lowest utility costs.

Be sure to check temperature limits on instrumentation. Heated dryers produce a spike in dew point and a 180 to 200EF temperature spike immediately after regeneration.

Other things to look for are:

• Vessels that avoid fluidizing the desiccant while drying;
  
• ASME coded vessels for quality and safety;
  
• Easily accessible low-maintenance valves with externally mounted valve actuators to
  permit cool operation;
  
• Energy saving control systems to match purge consumption and heater usage to
  actual dryer load; and
  
• Purge mufflers to reduce depressurization noise.

Review the application with a reputable manufacturer because desiccant dryer selection can be a time consuming and tricky process.

Air Receivers

The final components needed, the air receivers:

• provide storage capacity to prevent rapid compressor cycling
• reduce wear and tear on compression module, inlet control system, and motor
• eliminate pulsing air flow
• avoid overloading purification system with surges in air demand
• damp out the dew point and temperature spikes that follow regeneration.

A rule of thumb is to provide a minimum of one gallon of receiver capacity for each cubic foot of compressor flow.

The Engineered Instrument Air System

The preferred instrument air systems are shown in figures 1 and 2. Both systems meet the designers goals by:

• using a combination of dryers and filters to provide the required air quality;
• maintaining the desired air quality even if the drain valve on the dryer plugs or malfunctions by locating the coalescing filter downstream of the refrigerated dryer;
• protecting the desiccant and final air quality by placing the coalescing filter ahead of the desiccant dryer; and
• minimizing operating costs by eliminating the need for redundant and ineffective filtration.

Low pressure drop is important. One "rule of thumb" states that for every 1 psi increase in pressure drop, the compressor uses 0.5% additional power. In other words, 1 psi of pressure drop on a 200 hp air system will cost approximately $500 more per year.

Conclusion

Instrument air systems provide reliable, high quality compressed air if the designer properly selects the components and system layout. Selecting the suppliers for the system can be the difference between a good installation and one which never quite works. Look for the following when selecting a vendor:

• Supplier's qualifications and references to confirm expertise with the system components;
• Extensive product knowledge to assist you select components;
• Ability to supply the system components for a cohesive fit;
• One source of warranty support to eliminate the finger pointing among multiple vendors;
• Factory trained and certified service technicians during installation and system start- up;
• A supply of consumables like filter elements, intake filters, and lubricant; and
• Preventative maintenance contracts.

Through proper definition of system requirements and vendor and component selection, the modern instrument air system can be as easy to design and maintain as those of years gone by.