Dry Air Best Practices

Do Refrigerated Compressed Air Dryers Work?

  Refrigerated air dryers typically work well when they are new. However, for a variety of reasons, over time their effectiveness diminishes. These reasons are dust collecting on condenser coils, It's not unusual to find dewpoints in the 50-60F range or higher, especially during the summer months. Our customers have complained of moisture contamination. Most factories live with this problem, thinking that nothing can be done. "Our air lines just have water in them. There is nothing that can be done."

Enter the Parker Membrane air dryers. These dryers are rated for a +35 pressure dewpoint with a 100F saturated feed stream. If these dryers are placed after a refrigerated air dryer, the dewpoint remains a steady +15 and often lower. For more information on these dryers see: http://www.balstonfilters.com/compressed_air_dryers.html

What is the difference between SCFM, and ACFM?

ACFM (Actual Cubic Feet per Minute) is the actual rate of flow of compressed air delivered by the compressor, but expressed in terms of prevailing inlet air conditions. This is also known as Free Air Delivered (FAD).

SCFM (Standard Cubic Feet per Minute) is the measurement of air flow measured at standard conditions.

Since there are different standards in different disciplines, always make sure you understand which standard is being used when discussing compressed air flow. The standard agreed to by the compressed air industry represented by the Compressed Air and Gas Institute is:

68 deg. F at 0% Relative Humidity and 14.5 psig (1 bar)

The geographic location of the compressed air system plays an important role when determining the volume of air that is required. This is the ACFM. There are three location specific components that affect the cfm that is delivered by a compressor. They include heat, humidity and temperature.

To convert the required scfm to the flow that will be required at a specific geographical location (ACFM) this formula can be used:

ACFM = SCFM x (14.5 - 0)/(P1-VPm) x (460 +T1)/(460+68)

Where:
P1 = Absolute Inlet Pressure (psia)
T1 = Inlet Temperature deg. F
VPm = Vapor Pressure of moisture in inlet air (or the Vapor Pressure of the saturated air at inlet temp x Relative Humidity)

Thus, if the Absolute pressure is 14.2 psia, the vapor pressure of the inlet air is zero and the inlet temperature is 35F, 1 scfm is equivalent to:

ACFM = 1 x (14..5-0)/(14.2 - 0) x (460+35)/(460+68) = 1.02 x 0.9375 = 0.96 cubic feet

Thus compressors at altitude typically do not put out as much air as compressors at sea level.

Compressor Output and Using a Receiver Tank for Intermittent Flow

A rule of thumb for the compressor output is approximately 4 SCFM per horsepower.

As posted below, receiver tanks should be 6-8 times the rated flow of the compressor. There are some notable exceptions. Some applications like bag house filters use blasts of compressed air to shake the accumulated dust from the bags. No air is used for possibly 10 minutes or more. During the, one bag at a time, discharge sequence the air usage is high. A large receiver can be recharged during the idle period with a small compressor dryer package. Sizing this tank can be tricky. What methods do you use? Respond by clicking the comments link below or send me an email at dconnaughton@parker.com

Why are Air Receivers Important?

Air receivers are an integral and important part of any compressed air system. Typically they are sized at 6-10 times the flow rate of the system. So, if a compressor has a rating of 25 scfm at 100 psig, their receiver tank should be 150 cubic feet minimum. In a compressed air system, a receiver tank provides the following benefits:

1. The receiver tank acts as a reservoir of compressed air for peak demands.
2. The tank will help remove water from the system by allowing the air a chance to cool
3. The tank minimizes pulsation in the system caused by a reciprocating compressor or a cyclic process downstream.

Much like a water reservoir provides water during times of drought and stores water during the wet times, an air receiver compensates for peak demand and helps balance the supply of the compressor with the demand of the system.

Receiver tanks are required by law to have a pressure relief valve and a pressure gauge. The relief valve should be set to 10% higher than the working pressure of the system.

It is also important to install a drain on the tank either manual or automatic, to remove water from the system. A coalescing filter and air dryer are best placed downstream of the receiver tank.

Dry Air for Laser Purge

Modern Lasers have found widespread use in industry especially for cutting and marking. The laser is produced and then delivered through a mirrored pathway to the material to be cut. This path is purged with dry air to keep a positive pressure and keep out dust and dirt contamination. It is important the the purge be as dry as possible, typically a +35F dew point is acceptable to prevent condensation on the lenses. Contamination of the lenses will reduce laser power at the cutting head.

Explosion Proof method of drying compressed air

Explosive areas that are rich in flammable gases have stringent requirements on electrical circuits and hardware. Among these requirements are "Explosion Proof", "Intrinsically Safe" and "Nonincendive" which are well defined elsewhere on the Internet.

When it comes to drying compressed air in an explosion proof area, membrane air dryers offer considerable benefits. They can operate safely in these environments as they lower the compressed air dew point without any source of spark or energy able to create combustion. Refrig dryers and PSA dryers typically require electrical controls to be enclosed in NEMA explosion proof rated enclosure. These enclosures add greatly to the cost of the dryer. Membrane air dryers are thus the best choice for such environments.

Dewpoint - a definition

The term "Dew Point" can be a confusing term. It is used in the dry air business with the assumption that everyone knows what it means. To confound the confusion we also use the terms Pressure Dewpoint and Atmospheric Dewpoint. Let me try to explain what these terms mean and why they are important to consider in a compressed air system.
Dewpoint can be defined as the temperature at which moisture in the air will begin to condense if the air is cooled at constant pressure. At this point the relative humidity is 100%.
Pressure Dewpoint is the temperature at which water will begin to condense out of air at a given pressure. Pressure dewpoint is important when the goal is to keep moisture out of a piping system that might travel a long distance or outdoors.
Atmospheric Dewpoint is the temperature at which water will begin to condense out of air at atmospheric pressure. This is important when spec'ing an air dryer at the point of use which is usually low pressure.

Since air is a compressible gas, a pressure dewpoint indicates less water presence than an atmospheric dewpoint at a given dewpoint. That is, a pressure dewpoint of 35F at 100 psig holds less moisture than air at an atmospheric dewpoint of 35F. For a given pressure dewpoint, as you lower pressure, you lower the pressure dewpoint of the air.

Dry Air for Plastic Injection Molding

For injection molders, a critical measurement of the manufacturing process is to minimize the cycle times or maximize product throughput. With compressed air used to operate pickers, hoppers and automation associated with the molding process, downtime caused from low quality compressed air can quickly amount to lost productivity and product loss. A compressed air dryers that produce a consistent 35F dew point, placed at the point of use, ensures smooth operation of numerous air valves, cylinders and other sensitive pneumatic components.

Dry Air for Water Borne Paints

VOC requirements are encouraging many companies to switch to Water Borne Paints. This type of paint takes longer to dry than the less environmentally friendly solvent based paints. The quality and dryness of the compressed air used in the process is now, more than ever, a critical factor. Dry air equals faster processing. Be certain to use an appropriately sized compressed air dryer at the point of use to eliminate water contamination and speed the drying time.

Improving reliability of Coordinate Measuring Machines

Oil, moisture and particulate matter can prove to be extremely harmful to the air bearings on Coordinate Measuring Machines (CMM). This contamination often results in drag or resistance in the air bearings and leads to hysteresis. Moisture and oil in the air may also cause condensation and rust, leading to poor machine performance and inaccurate measurements. Costly maintenance repairs and production downtime can be avoided with a proper CMM air dryer.