Air Compressors Buyer's Guide Part 2

Assessing Your Quality Requirements

Bizarre as it may initially sound, the pressure and rate at which an air-compressor can deliver air are not the only considerations in relation to its output. Depending on the application it can be very important to ensure that the quality of the compressed air delivered meets certain requirements. To this end then; if you are pumping up car tyres, filling scuba tanks or cleaning micro-processors, you will consider different levels of air quality acceptable. Here are the main considerations and ways you can deal with them:

Oil

Like most machinery, air compressors need oil to lubricate their moving parts. Given that the act of compression requires the air to come into contact with at least some of these moving parts; it is not uncommon for the compressed air produced to have traces of engine oil present in it. If you are simply using pneumatic tools then this needn’t be a problem, as they are likely to be tolerant of some oil in their air supply. However, in medical applications for instance, this would be unacceptable and perhaps even dangerous. Given your use then, you have the choice of applying a filter to a regular air compressor or purchasing what is known as an oil-free compressor. The former will clean (to a certain extent) the air produced but when absolutely clean air is required, it is unlikely to suffice. Oil-free or oilless compressors on the other hand, although much more expensive, are the only way to make absolutely sure that your compressed air is free of all oil traces, as they specifically separate the air and oil throughout the compression process.

Moisture

Although some climates are more humid than others, there is always water vapour present in the atmosphere. As ambient air is compressed, this vapour often then condenses into water droplets. Again, with basic applications and machinery this needn’t be a problem. With sensitive equipment, however, prolonged exposure to moisture from your air compressor can cause severe damage. The most effective solution to this problem is the use of an additional drier. Driers can be purchased separately and fitted to most compressors but also come as standard with many industrial air compressors.

Temperature

Anyone familiar with basic physics will be aware that when a gas expands, it cools. The inverse is also true; when a gas is compressed it becomes hotter. As such, the use of compressed air will always be susceptible to temperature fluctuations. The most common temperature related problem is that the air delivered from the compressor, having been rapidly compressed, is too hot for its application. This problem may be solved with a cooler, which works much in the same way as an air-conditioning unit. Coolers can be bought separately or, again, may come as standard on the more heavy-duty types of air compressor.

Noise

It is perfectly acceptable for a mechanics’ garage to be a noisy place. However, the same level of noise would be absolutely unacceptable in a dentists’ surgery. As such, it is important to consider the noise that your air compressor is likely to make and how, if necessary, to minimise it. Due to their design some air compressors, especially rotary or screw compressors, are much quieter. When noise is a serious consideration then, it is wise to consider a screw compressor but you could also get around this problem by housing your compressor in another room or building, should you have the available space.

And Finally...

Although it’s important to correctly asses your needs, you needn’t worry if t you are not absolutely sure which air compressor is right for you. There are plenty of specialist air compressor suppliers out there and with a reasonable understanding of your needs, they should be able to give you some sound advice and perhaps raise some points not discussed in this article.

Air Compressors Buyer's Guide

Determining what your compressed air needs are is very important when choosing the right air compressor. As given the range of types, you can save time and money by considering your requirements in advance. You should see other articles in this section for a fuller description of the various air compressors on the market and how they vary mechanically. In this section however, we will consider the two questions that will govern your choice of compressor; which are: what are your power requirements? And what air quality issues do you need to consider?

Assessing Your Power Requirements

Tools that are powered by compressed air will state either on the tool itself or somewhere in its documentation, what its specific power requirements are. These power requirements will come in the form of a pressure and an airflow rating.

Depending on the country of origin, the pressure rating will be given in either PSI (pounds per square inch) or BAR (meaning [number of] atmospheres; derived from the Greek word báros). These figures denote the recommended pressure that the air is delivered from your compressor to the tool you’re using for optimal performance. On the other hand, the airflow rating will be given in either CFM or LPM, standing for cubic feet or litres per-minute respectively. This measurement will inform you of how quickly the compressed air needs to be delivered to ensure continuous operation.

Having collated the power requirements for all the tools and machinery you will need to use; determine which tools you will be using concurrently and then add together all their respective LPM ratings. The resulting figure is the bare minimum airflow capacity that your compressor will need to run these tools together. Next you should examine the pressure requirements for the tools; you will not need to add them up but you do need to ensure that the tool which requires highest operational pressure will be satisfied (i.e. if your highest rated tool requires 8 Bar, as a bare minimum, your air compressor should be capable of delivering this).

Wastage

Most tools pneumatic, especially if well used, will start to leak some air. As such, if your air compressor is at capacity when delivering the minimum operational requirements for your tools, you are likely to have problems in the future, as your system becomes less-efficient. In light of this then, it is wise to have some spare capacity; especially if you may purchase more equipment in the future. However, you shouldn’t over do it; as you will pay for too much spare capacity in needlessly high running-costs.

Wear and Tear

Most air compressors should have an operational life of at least 10 – 15 years. Like most machinery, however, an air compressors’ lifespan will ultimately be determined by how heavily it’s used and how well it’s maintained. As such, when choosing your compressor ensure it can comfortably cope with its workload, even if that means spending more on your initial purchase. An overworked compressor will need to be replaced or repaired sooner and thus, could prove to be more expensive in the long-run.

This article was taken from http://www.approvedindex.co.uk/indexes/AirCompressors/articles/aircompressorsbuyersguide.aspx

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Gas Compressors and Systems

Gas compressors and systems are used to pressurize and circulate gas through a process, enhance conditions for chemical reactions, provide inert gas for safety or control systems, recover and recompress process gas, and maintain correct pressure levels by either adding and removing gas or vapors from a process system. Gas compressors work in two stages. In the first stage, gas flows through the inlet check valve and fills the larger diameter first-stage cylinder. Pressurized hydraulic fluid, acting on the hydraulic piston, strokes the piston assembly to the left compressing the gas in the first-stage cylinder. Gas in the first-stage cylinder flows through the check valves into the smaller diameter second-stage cylinder.

At the end of the first stage, the four-way valve changes position and directs pressurized hydraulic fluid to the left side of the hydraulic piston. The piston assembly moves to the right compressing gas in the second-stage cylinder. Gas flows out of the second-stage cylinder into the discharge gas line. The piston assembly reverses direction at the end of the second-stage stroke and the cycle repeats.

There are four broad categories of compressor types. There are many variations within each type: reciprocating compressor, fan / blower compressors, rotary compressors, and ejector compressors.

In reciprocating compressors, the thrust of a positive displacement pump, within the cylinder, moves the gas through the system. This thrust enhances both the pressure and the density of the gas being transported. Examples of reciprocating gas compressors include piston compressors, lubricated and non-lubricated, and metal diaphragm compressors. In general, reciprocating pumps offer low rational and piston speed, leading to a high reliability factor with a minimum of maintenance.

Fan / blower compressors contain high-speed impellers through which a dynamic head is imparted to the gas. This category of gas compressors includes axial flow, radial, centrifugal and fan-blower compressors. They may also be referred to as turbomachinery.

Rotary compressors and rotary screw compressors move gas through the system by the positive displacement of two rotating lobes or by oscillating vanes confined in an eccentric cylinder. Rotary screw compressors are well known for their robustness, compactness and reliability. They are designed for long periods of continuous operation, needing very little maintenance. The smooth running action of the rotors enables the screw compressor to handle the most difficult of gases, contaminants, or liquid slugs without vibration.

Ejector compressors move gas via kinetic energy induced through high-velocity nozzles. The advantage of ejector gas compressors over mechanical pumping machinery is that it has no moving parts and as such requires very low maintenance.

Storage Tanks and Process Tanks

Storage tanks and process tanks are general purpose industrial containers. Storage tanks and process tanks can have many configurations depending upon dimensions, orientation, placement, and wall configuration. Materials of construction will dictate the application that is suitable for the tank. Storage tanks and process tanks are used in a number of applications including short term storage, long term storage, mixing, blending, metering and dispensing.

The most important parameters to consider when specifying storage tanks and process tanks are their capacity and dimensions. The capacity of the storage tank or process tank is the internal volume available for the storage of materials. The diameter of the tank is typically expressed in feet units. The length of the tank is measured in feet. The orientation of the tank can be vertical or horizontal. Vertical tanks stand vertically and typically have access ports on the bottom. Horizontal tanks are often mounted on stands or saddles and can have access ports on the bottom or top. The placement of tanks is typically either above ground or underground, depending on the construction. Portable tanks can be moved from one place to another, via wheels or other moving device. The wall construction of the tank may dictate the application that the tank is suitable for. Single wall tanks are common for various applications. Double wall tanks are used in applications where higher-pressure considerations are necessary.

Materials of construction for storage tanks and process tanks include fiberglass FRP, galvanized steel, plastic, stainless steel, steel, and titanium. Fiberglass reinforced polyester (FRP) is made of a series of long glass fibers embedded in a resin. It can be formed into almost any shape before curing. Once cured it is light in weight, very strong material that has excellent corrosion resistant properties. In some cases, fiberglass is used along with the plastic in the body material of the tank. Galvanized steel is cold rolled steel that has been surface treated with a layer of zinc. Stainless steel is a type of metal that resists corrosion. Steel is a ferrous-based metal having a variety of physical properties depending on composition. Steel used in tank applications is typically rolled sheet steel. Titanium is a lightweight, very strong metal used in applications where there are temperature extremes or extraordinary stresses. Storage tanks and process tanks with special linings are constructed of special materials for corrosive or other special processes. Considerations might also include glass lined or special coatings.

Common industries and applications that use storage tanks and process tanks include chemical processing, cosmetics processing, food and beverage processing, oil and fuel processing, paper and pulp processing, pharmaceutical processing, plastic processing, power generation and energy processing, and water applications.