GAS TURBINE ENGINES

How Gas Turbine Engines Work

When you go to an airport and see the commercial jets there, you can't help but notice the huge engines that power them. Most commercial jets are powered by turbofan engines, and turbofans are one example of a general class of engines called gas turbine engines.

An Aircraft Engine

Jet engines on the assembly line.

You may have never heard of gas turbine engines, but they are used in all kinds of un¬expected places. For example, many of the helicopters you see, a lot of s¬maller power plants and even the M-1 Tank use gas turbines. In this article, we will look at gas turbine engines to see what makes them tick!


Types of Turbines

There are many different kinds of turbines:

• You have probably heard of a steam turbine. Most power plants use coal, natural gas, oil or a nuclear reactor to create steam. The steam runs through a huge and very carefully designed multi-stage turbine to spin an output shaft that drives the plant's generator.

• Hydroelectric dams use water turbines in the same way to generate power. The turbines used in a hydroelectric plant look completely different from a steam turbine because water is so much denser (and slower moving) than steam, but it is the same principle.

• Wind turbines, also known as wind mills, use the wind as their motive force. A wind turbine looks nothing like a steam turbine or a water turbine because wind is slow moving and very light, but again, the principle is the same.
A gas turbine is an extension of the same concept. In a gas turbine, a pressurized gas spins the turbine. In all modern gas turbine engines, the engine produces its own pressurized gas, and it does this by burning something like propane, natural gas, kerosene or jet fuel. The heat that comes from burning the fuel expands air, and the high-speed rush of this hot air spins the turbine.


Advantages and Disadvantages of Jet Engines

So wh¬y does the M-1 tank use a 1,500 horsepower gas turbine engine instead of a diesel engine? It turns out that there are two big advantages of the turbine over the diesel:

• Gas turbine engines have a great power-to-weight ratio compared to reciprocating engines. That is, the amount of power you get out of the engine compared to the weight of the engine itself is very good.

• Gas turbine engines are smaller than their reciprocating counterparts of the same power.

The main disadvantage of gas turbines is that, compared to a reciprocating engine of the same size, they are expensive. Because they spin at such high speeds and because of the high operating temperatures, designing and manufacturing gas turbines is a tough problem from both the engineering and materials standpoint. Gas turbines also tend to use more fuel when they are idling, and they prefer a constant rather than a fluctuating load. That makes gas turbines great for things like transcontinental jet aircraft and power plants, but explains why you don't have one under the hood of your car.


The Gas Turbine Process

Gas turbine engines are, theoretically, extremely simple. They have three parts:

• Compressor - Compresses the incoming air to high pressure

• Combustion area - Burns the fuel and produces high-pressure, high-velocity gas

• Turbine - Extracts the energy from the high-pressure, high-velocity gas flowing from the combustion chamber

The following figure shows the general layout of an axial-flow gas turbine -- the sort of engine you would find driving the rotor of a helicopter, for example:



In this engine, air is sucked in from the right by the compressor. The compressor is basically a cone-shaped cylinder with small fan blades attached in rows (eight rows of blades are represented here). Assuming the light blue represents air at normal air pressure, then as the air is forced through the compression stage its pressure rises significantly. In some engines, the pressure of the air can rise by a factor of 30. The high-pressure air produced by the compressor is shown in dark blue.


Combustion Area

This high-pressure air then enters the combustion area, where a ring of fuel injectors injects a steady stream of fuel. The fuel is generally kerosene, jet fuel, propane or n¬atural gas. If you think about how easy it is to blow a candle out, then you can see the design problem in the combustion area -- entering this area is high-pressure air moving at hundreds of miles per hour. You want to keep a flame burning continuously in that environment. The piece that solves this problem is called a "flame holder," or sometimes a "can." The can is a hollow, perforated piece of heavy metal. Half of the can in cross-section is shown below:



The injectors are at the right. Compressed air enters through the perforations. Exhaust gases exit at the left. You can see in the previous figure that a second set of cylinders wraps around the inside and the outside of this perforated can, guiding the compressed intake air into the perforations.


The Turbine

At the left of the engine is the turbine section. In this figure there are two sets of turbines. The first set directly drives the compressor. The turbines, the shaft and the compressor all turn as a single unit:



At the far left is a final turbine stage, shown here with a single set of vanes. It drives the output shaft. This final turbine stage and the output shaft are a completely stand-alone, freewheeling unit. They spin freely without any connection to the rest of the engine. And that is the amazing part about a gas turbine engine
there is enough energy in the hot gases blowing through the blades of that final output turbine to generate 1,500 horsepower and drive a 63-ton M-1 Tank! A gas turbine engine really is that simple.

In the case of the turbine used in a tank or a power plant, there really is nothing to do with the exhaust gases but vent them through an exhaust pipe, as shown. Sometimes the exhaust will run through some sort of heat exchanger either to extract the heat for some other purpose or to preheat air before it enters the combustion chamber.

The discussion here is obviously simplified a bit. For example, we have not discussed the areas of bearings, oiling systems, internal support structures of the engine, stator vanes and so on. All of these areas become major engineering problems because of the tremendous temperatures, pressures and spin rates inside the engine. But the basic principles described here govern all gas turbine engines and help you to understand the basic layout and operation of the engine.


Gas Turbine Variations

Large jetliners use what are known as turbofan engines, which are nothing more than gas turbines combined with a large fan at the front of the engine. Here's the basic (highly simplified) layout of a turbofan engine:



You can see that the core of a turbofan is a normal gas turbine engine like the one described in the previous section. The difference is that the final turbine stage drives a shaft that makes its way back to the front of the engine to power the fan (shown in red in this picture). This multiple concentric shaft approach, by the way, is extremely common in gas turbines. In many larger turbofans, in fact, there may be two completely separate compression stages driven by separate turbines, along with the fan turbine as shown above. All three shafts ride within one another concentrically.

The purpose of the fan is to dramatically increase the amount of air moving through the engine, and therefore increase the engine's thrust. When you look into the engine of a commercial jet at the airport, what you see is this fan at the front of the engine. It is huge -- on the order of 10 feet (3 m) in diameter on big jets, so it can move a lot of air. The air that the fan moves is called "bypass air" (shown in purple above) because it bypasses the turbine portion of the engine and moves straight through to the back of the nacelle at high speed to provide thrust.

A turboprop engine is similar to a turbofan, but instead of a fan there is a conventional propeller at the front of the engine. The output shaft connects to a gearbox to reduce the speed, and the output of the gearbox turns the propeller.


Thrust Basics

The goal of a turbofan engine is to produce thrust to drive the airplane forward. Thrust is generally measured in pounds in the United States (the metric system u¬ses Newtons, where 4.45 Newtons equals 1 pound of thrust). A "pound of thrust" is equal to a force able to accelerate 1 pound of material 32 feet per second per second (32 feet per second per second happens to be equivalent to the acceleration provided by gravity). Therefore, if you have a jet engine capable of producing 1 pound of thrust, it could hold 1 pound of material suspended in the air if the jet were pointed straight down. Likewise, a jet engine producing 5,000 pounds of thrust could hold 5,000 pounds of material suspended in the air. And if a rocket engine produced 5,000 pounds of thrust applied to a 5,000-pound object floating in space, the 5,000-pound object would accelerate at a rate of 32 feet per second per second.

Thrust is generated under Newton's principle that "every action has an equal and opposite reaction." For example, imagine that you are floating in space and you weigh 100 pounds on Earth. In your hand you have a baseball that weighs 1 pound on Earth. If you throw the baseball away from you at a speed of 32 feet per second (21 mph / 34 kph), your body will move in the opposite direction (it will react) at a speed of 0.32 feet per second. If you were to continuously throw baseballs in that way at a rate of one per second, your baseballs would be generating 1 pound of continuous thrust. Keep in mind that to generate that 1 pound of thrust for an hour you would need to be holding 3,600 pounds of baseballs at the beginning of the hour.

If you wanted to do better, the thing to do is to throw the baseballs harder. By "throwing" them (with of a gun, say) at 3,200 feet per second, you would generate 100 pounds of thrust.


Jet Engine Thrust

In a turbofan engine, the baseballs that the engine is throwing out are air molecules. The air molecules are already there, so the airplane does not have to carry them around at least. An individual air molecule does not weigh very much, bu¬t the engine is throwing a lot of them and it is throwing them at very high speed. Thrust is coming from two components in the turbofan:

• The gas turbine itself - Generally a nozzle is formed at the exhaust end of the gas turbine (not shown in this figure) to generate a high-speed jet of exhaust gas. A typical speed for air molecules exiting the engine is 1,300 mph (2,092 kph).

• The bypass air generated by the fan - This bypass air moves at a slower speed than the exhaust from the turbine, but the fan moves a lot of air.

As you can see, gas turbine engines are quite common. They are also quite complicated, and they stretch the limits of both fluid dynamics and materials sciences. If you want to learn more, one worthwhile place to go would be the library of a university with a good engineering department. Books on the subject tend to be expensive, but two well-known texts include "Aircraft Gas Turbine Engine Technology" and "Elements of Gas Turbine Propulsion."

ROCKET ENGINE

How Rocket Engines Work
One of the most amazing endeavors man has ever undertaken is the exploration of space. A big p¬art of the amazement is the complexity. Space exploration is complicated because there are so many problems to solve and obstacles to overcome. You have things like:
• The vacuum of space
• Heat management problems
• The difficulty of re-entry
• Orbital mechanics
• Micrometeorites and space debris
• Cosmic and solar radiation
• The logistics of having restroom facilities in a weightless environment
But the biggest problem of all is harnessing enough energy simply to get a spaceship off the ground. That is where rocket engines come in.

ROCKET

Rocket engines are, on the one hand, so simple that you can build and fly your own model rockets very inexpensively (see the links on the last page of the article for details). On the other hand, rocket engines (and their fuel systems) are so complicated that only three countries have actually ever put people in orbit. In this article, we will look at rocket engines to understand how they work, as well as to understand some of the complexity surrounding them.

When most people think about motors or engines, they think about rotation. For example, a reciprocating gasoline engine in a car produces rotational energy to drive the wheels. An electric motor produces rotational energy to drive a fan or spin a disk. A steam engine is used to do the same thing, as is a steam turbine and most gas turbines.

Rocket engines are fundamentally different. Rocket engines are reaction engines. The basic principle driving a rocket engine is the famous Newtonian principle that "to every action there is an equal and opposite reaction." A rocket engine is throwing mass in one direction and benefiting from the reaction that occurs in the other direction as a result.

This concept of "throwing mass and benefiting from the reaction" can be hard to grasp at first, because that does not seem to be what is happening. Rocket engines seem to be about flames and noise and pressure, not "throwing things." Let's look at a few examples to get a better picture of reality:

• If you have ever shot a shotgun, especially a big 12-gauge shotgun, then you know that it has a lot of "kick." That is, when you shoot the gun it "kicks" your shoulder back with a great deal of force. That kick is a reaction. A shotgun is shooting about an ounce of metal in one direction at about 700 miles per hour, and your shoulder gets hit with the reaction. If you were wearing roller skates or standing on a skateboard when you shot the gun, then the gun would be acting like a rocket engine and you would react by rolling in the opposite direction.

• If you have ever seen a big fire hose spraying water, you may have noticed that it takes a lot of strength to hold the hose (sometimes you will see two or three firefighters holding the hose). The hose is acting like a rocket engine. The hose is throwing water in one direction, and the firefighters are using their strength and weight to counteract the reaction. If they were to let go of the hose, it would thrash around with tremendous force. If the firefighters were all standing on skateboards, the hose would propel them backward at great speed!

• When you blow up a balloon and let it go so that it flies all over the room before running out of air, you have created a rocket engine. In this case, what is being thrown is the air molecules inside the balloon. Many people believe that air molecules don't weigh anything, but they do (see the page on helium to get a better picture of the weight of air). When you throw them out the nozzle of a balloon, the rest of the balloon reacts in the opposite direction.

Next, we'll look at another scenario that explains action and reaction: the space baseball.

Action and Reaction: The Space Baseball Scenario
Imagine the following situation: You are wearing a space suit and you are floating in space beside the space shuttle; you happen to have a baseball in your hand.

If you throw the baseball, your body will react by moving in the opposite direction of the ball. The thing that controls the speed at which your body moves away is the weight of the baseball that you throw and the amount of acceleration that you apply to it. Mass multiplied by acceleration is force (f = m * a). Whatever force you apply to the baseball will be equalized by an identical reaction force applied to your body (m * a = m * a). So let's say that the baseball weighs 1 pound, and your body plus the space suit weighs 100 pounds. You throw the baseball away at a speed of 32 feet per second (21 mph). That is to say, you accelerate the 1-pound baseball with your arm so that it obtains a velocity of 21 mph. Your body reacts, but it weighs 100 times more than the baseball. Therefore, it moves away at one-hundredth the velocity of the baseball, or 0.32 feet per second (0.21 mph).

If you want to generate more thrust from your baseball, you have two options: increase the mass or increase the acceleration. You can throw a heavier baseball or throw a number of baseballs one after another (increasing the mass), or you can throw the baseball faster (increasing the acceleration on it). But that is all that you can do.

A remote camera captures a close-up view of a Space Shuttle Main Engine during a test firing at the John C. Stennis Space Center in Hancock County, Miss.

A rocket engine is generally throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction in order to get a reaction in the opposite direction. The mass comes from the weight of the fuel that the rocket engine burns. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The fact that the fuel turns from a solid or liquid into a gas when it burns does not change its mass. If you burn a pound of rocket fuel, a pound of exhaust comes out the nozzle in the form of a high-temperature, high-velocity gas. The form changes, but the mass does not. The burning process accelerates the mass.

Let's learn more about thrust.

Thrust

The "strength" of a rocket engine is called its thrust. Thrust is measured in "pounds of thrust" in the U.S. and in Newtons under the metric system (4.45 Newtons of thrust equals 1 pound of thrust). A pound of thrust is the amount of thrust it would take to keep a 1-pound object stationary against the force of gravity on Earth. So on Earth, the acceleration of gravity is 32 feet per second per second (21 mph per second). If you were floating in space with a bag of baseballs and you threw one baseball per second away from you at 21 mph, your baseballs would be generating the equivalent of 1 pound of thrust. If you were to throw the baseballs instead at 42 mph, then you would be generating 2 pounds of thrust. If you throw them at 2,100 mph (perhaps by shooting them out of some sort of baseball gun), then you are generating 100 pounds of thrust, and so on.

One of the funny problems rockets have is that the objects that the engine wants to throw actually weigh something, and the rocket has to carry that weight around. So let's say that you want to generate 100 pounds of thrust for an hour by throwing one baseball every second at a speed of 2,100 mph. That means that you have to start with 3,600 1-pound baseballs (there are 3,600 seconds in an hour), or 3,600 pounds of baseballs. Since you only weigh 100 pounds in your spacesuit, you can see that the weight of your "fuel" dwarfs the weight of the payload (you). In fact, the fuel weights 36 times more than the payload. And that is very common. That is why you have to have a huge rocket to get a tiny person into space right now -- you have to carry a lot of fuel.

You can see the weight equation very clearly on the Space Shuttle. If you have ever seen the Space Shuttle launch, you know that there are three parts:

• The Orbiter
• The big external tank
• The two solid rocket boosters (SRBs)
The Orbiter weighs 165,000 pounds empty. The external tank weighs 78,100 pounds empty. The two solid rocket boosters weigh 185,000 pounds empty each. But then you have to load in the fuel. Each SRB holds 1.1 million pounds of fuel. The external tank holds 143,000 gallons of liquid oxygen (1,359,000 pounds) and 383,000 gallons of liquid hydrogen (226,000 pounds). The whole vehicle -- shuttle, external tank, solid rocket booster casings and all the fuel -- has a total weight of 4.4 million pounds at launch. 4.4 million pounds to get 165,000 pounds in orbit is a pretty big difference! To be fair, the orbiter can also carry a 65,000-pound payload (up to 15 x 60 feet in size), but it is still a big difference. The fuel weighs almost 20 times more than the Orbiter [source: The Space Shuttle Operator's Manual].

All of that fuel is being thrown out the back of the Space Shuttle at a speed of perhaps 6,000 mph (typical rocket exhaust velocities for chemical rockets range between 5,000 and 10,000 mph). The SRBs burn for about two minutes and generate about 3.3 million pounds of thrust each at launch (2.65 million pounds average over the burn). The three main engines (which use the fuel in the external tank) burn for about eight minutes, generating 375,000 pounds of thrust each during the burn.

In the next section, we'll look at the particular fuel mixture in solid-fuel rockets.

Solid-fuel Rockets: Fuel Mixture

Solid-fuel rocket engines were the first engines created by man. They were invented hundreds of years ago in China and have been used widely since then. The line about "the rocket's red glare" in the national anthem (written in the early 1800's) is talking about small military solid-fuel rockets used to deliver bombs or incendiary devices. So you can see that rockets have been in use quite awhile.

The idea behind a simple solid-fuel rocket is straightforward. What you want to do is create something that burns very quickly but does not explode. As you are probably aware, gunpowder explodes. Gunpowder is made up 75% nitrate, 15% carbon and 10% sulfur. In a rocket engine, you don't want an explosion -- you would like the power released more evenly over a period of time. Therefore you might change the mix to 72% nitrate, 24% carbon and 4% sulfur. In this case, instead of gunpowder, you get a simple rocket fuel. This sort of mix will burn very rapidly, but it does not explode if loaded properly. Here's a typical cross section:

A solid-fuel rocket immediately before and after ignition

On the left you see the rocket before ignition. The solid fuel is shown in green. It is cylindrical, with a tube drilled down the middle. When you light the fuel, it burns along the wall of the tube. As it burns, it burns outward toward the casing until all the fuel has burned. In a small model rocket engine or in a tiny bottle rocket the burn might last a second or less. In a Space Shuttle SRB containing over a million pounds of fuel, the burn lasts about two minutes.

Solid-fuel Rockets: Channel Configuration
When you read about advanced solid-fuel rockets like the Shuttle's solid rocket boosters, you often read things like:

The propellant mixture in each SRB motor consists of an ammonium perchlorate (oxidizer, 69.6 percent by weight), aluminum (fuel, 16 percent), iron oxide (a catalyst, 0.4 percent), a polymer (a binder that holds the mixture together, 12.04 percent), and an epoxy curing agent (1.96 percent). The propellant is an 11-point star-shaped perforation in the forward motor segment and a double- truncated- cone perforation in each of the aft segments and aft closure. This configuration provides high thrust at ignition and then reduces the thrust by approximately a third 50 seconds after lift-off to prevent overstressing the vehicle during maximum dynamic pressure.

This paragraph discusses not only the fuel mixture but also the configuration of the channel drilled in the center of the fuel. An "11-point star-shaped perforation" might look like this:

The idea is to increase the surface area of the channel, thereby increasing the burn area and therefore the thrust. As the fuel burns, the shape evens out into a circle. In the case of the SRBs, it gives the engine high initial thrust and lower thrust in the middle of the flight.

Solid-fuel rocket engines have three important advantages:

• Simplicity
• Low cost
• Safety
They also have two disadvantages:
• Thrust cannot be controlled.
• Once ignited, the engine cannot be stopped or restarted.

The disadvantages mean that solid-fuel rockets are useful for short-lifetime tasks (like missiles), or for booster systems. When you need to be able to control the engine, you must use a liquid propellant system. We'll learn about those and other possibilities next.

Liquid-propellant Rockets
In 1926, Robert Goddard tested the first liquid-propellant rocket engine. His engine used gasoline and liquid oxygen. He also worked on and solved a number of fundamental problems in rocket engine design, including pumping mechanisms, cooling strategies and steering arrangements. These problems are what make liquid-propellant rockets so complicated.

Dr. Robert H. Goddard and his liquid oxygen-gasoline rocket in the frame from which it was fired on March 16, 1926, at Auburn, Mass. It flew for only 2.5 seconds, climbed 41 feet, and landed 184 feet away in a cabbage patch.

The basic idea is simple. In most liquid-propellant rocket engines, a fuel and an oxidizer (for example, gasoline and liquid oxygen) are pumped into a combustion chamber. There they burn to create a high-pressure and high-velocity stream of hot gases. These gases flow through a nozzle that accelerates them further (5,000 to 10,000 mph exit velocities being typical), and then they leave the engine. The following highly simplified diagram shows you the basic components.

This diagram does not show the actual complexities of a typical engine (see some of the links at the bottom of the page for good images and descriptions of real engines). For example, it is normal for either the fuel or the oxidizer to be a cold liquefied gas like liquid hydrogen or liquid oxygen. One of the big problems in a liquid-propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to cool them. The pumps have to generate extremely high pressures in order to overcome the pressure that the burning fuel creates in the combustion chamber. The main engines in the Space Shuttle actually use two pumping stages and burn fuel to drive the second stage pumps. All of this pumping and cooling makes a typical liquid propellant engine look more like a plumbing project gone haywire than anything else -- look at the engines on this page to see what I mean.

All kinds of fuel combinations get used in liquid propellant rocket engines. For example:

• Liquid hydrogen and liquid oxygen - used in the Space Shuttle main engines
• Gasoline and liquid oxygen - used in Goddard's early rockets
• Kerosene and liquid oxygen - used on the first stage of the large Saturn V boosters in the Apollo program
• Alcohol and liquid oxygen - used in the German V2 rockets
• Nitrogen tetroxide/monomethyl hydrazine - used in the Cassini engines

The Future of Rocket Engines
We are accustomed to seeing chemical rocket engines that burn their fuel to generate thrust. There are many other ways to generate thrust however. Any system that throws mass would do. If you could figure out a way to accelerate baseballs to extremely high speeds, you would have a viable rocket engine. The only problem with such an approach would be the baseball "exhaust" (high-speed baseballs at that) left streaming through space. This small problem causes rocket engine designers to favor gases for the exhaust product.
Many rocket engines are very small. For example, attitude thrusters on satellites don't need to produce much thrust. One common engine design found on satellites uses no "fuel" at all -- pressurized nitrogen thrusters simply blow nitrogen gas from a tank through a nozzle. Thrusters like these kept Skylab in orbit, and are also used on the shuttle's manned maneuvering system.

New engine designs are trying to find ways to accelerate ions or atomic particles to extremely high speeds to create thrust more efficiently. NASA's Deep Space-1 spacecraft was the first to use ion engines for propulsion.

Photo courtesy NASA
This image of a xenon ion engine, photographed through a
port of the vacuum chamber where it was being tested at NASA's Jet Propulsion
Laboratory, shows the faint blue glow of charged atoms being emitted from the
engine. The ion propulsion engine is the first non-chemical propulsion to be
used as the primary means of propelling a spacecraft.

Plasma Spray

PLASMA SPRAY PROCESS

Schematic Diagram of the Plasma Spray Process

The Plasma Spray Process is basically the spraying of molten or heat softened material onto a surface to provide a coating. Material in the form of powder is injected into a very high temperature plasma flame, where it is rapidly heated and accelerated to a high velocity. The hot material impacts on the substrate surface and rapidly cools forming a coating. This plasma spray process carried out correctly is called a "cold process" (relative to the substrate material being coated) as the substrate temperature can be kept low during processing avoiding damage, metallurgical changes and distortion to the substrate material.

The plasma spray gun comprises a copper anode and tungsten cathode, both of which are water cooled. Plasma gas (argon, nitrogen, hydrogen, helium) flows around the cathode and through the anode which is shaped as a constricting nozzle. The plasma is initiated by a high voltage discharge which causes localised ionisation and a conductive path for a DC arc to form between cathode and anode. The resistance heating from the arc causes the gas to reach extreme temperatures, dissociate and ionise to form a plasma. The plasma exits the anode nozzle as a free or neutral plasma flame (plasma which does not carry electric current) which is quite different to the Plasma Transferred Arc coating process where the arc extends to the surface to be coated. When the plasma is stabilised ready for spraying the electric arc extends down the nozzle, instead of shorting out to the nearest edge of the anode nozzle. This stretching of the arc is due to a thermal pinch effect. Cold gas around the surface of the water cooled anode nozzle being electrically non-conductive constricts the plasma arc, raising its temperature and velocity. Powder is fed into the plasma flame most commonly via an external powder port mounted near the anode nozzle exit. The powder is so rapidly heated and accelerated that spray distances can be in the order of 25 to 150 mm.

Plasma Spray Process

The plasma spray process is most commonly used in normal atmospheric conditions and referred as APS. Some plasma spraying is conducted in protective environments using vacuum chambers normally back filled with a protective gas at low pressure, this is referred as VPS or LPPS.

Plasma spraying has the advantage that it can spray very high melting point materials such as refractory metals like tungsten and ceramics like zirconia unlike combustion processes. Plasma sprayed coatings are generally much denser, stronger and cleaner than the other thermal spray processes with the exception of HVOF and detonation processes. Plasma spray coatings probably account for the widest range of thermal spray coatings and applications and makes this process the most versatile.

Disadvantages of the plasma spray process are relative high cost and complexity of process.

Plasma Flame Theory

Some argue that this page should be titled "Plasma Jet Theory" and not "Plasma Flame Theory". A plasma does not necessarily involve the process of combustion, burning or oxidation of material. The argument that the term "flame" can only be applied to a process of combustion, burning or oxidation is dependent on the definition of "flame". The title "Plasma Flame Theory" used here assumes this particular definition of "flame" to be "a stream of vapour or gas made luminous by heat" or "something resembling a flame in motion, brilliance, intensity, or shape".

Temperature as a Function of Gas Energy Content

A plasma is an electrically conductive gas containing charged particles. When atoms of a gas are excited to high energy levels, the atoms loose hold of some of their electrons and become ionised producing a plasma containing electrically charged particles - ions and electrons.

The plasma generated for plasma spraying usually incorporates one or a mixture of the following gases:

• Argon
• Helium
• Nitrogen
• Hydrogen

Plasma flames for thermal spraying can produce temperatures around 7,000 to 20,000K far above the melting temperature (and vapour temperature) of any known material. The extreme temperature of the plasma is not the only reason for the effective heating properties. If for example helium gas is heated to around 13,000K without a plasma forming, it would have insufficient energy for normal plasma spraying. Nitrogen on the other hand heated to 10,000K going through dissociation and ionisation forming a plasma is an effective heating media for thermal spraying, being able to supply about six times more energy than an equal volume of helium at 13,000K. The plasma is able to supply large amounts of energy due to the energy changes associated with dissociating molecular gases to atomic gases and ionisation which occur with little change in temperature.

• N₂ + E = 2N
• Diatomic molecule of nitrogen + energy gives 2 free atoms of nitrogen
• 2N + E = 2N⁺ + 2e^-
• 2 free atoms of nitrogen + energy gives 2 nitrogen ions and 2 electrons

The reverse process provides most of the energy for heating the spray material without a dramatic drop in temperature:

• 2N⁺ + 2e^- = 2N + E
• 2N = N₂ + E

Nitrogen and hydrogen are diatomic gases (two atoms to every molecule). These plasmas have higher energy contents for a given temperature than the atomic gases of argon and helium because of the energy associated with dissociation of molecules.

Argon and Helium are monatomic gases (the atoms don't combine to form molecules) These plasmas are relatively lower in energy content and higher in temperature than the plasmas from diatomic gases.

Nitrogen is a general purpose primary gas used alone or with hydrogen secondary gas.

Nitrogen also benefits from being the cheapest plasma gas. Nitrogen tends to be inert to most spray material except materials like titanium.

Argon is probably the most favoured primary plasma gas and is usually used with a secondary plasma gas (hydrogen, helium and nitrogen) to increase its energy. Argon is the easiest of these gases to form a plasma and tends to be less aggressive towards electrode and nozzle hardware. Most plasmas are started up using pure argon. Argon is a noble gas and is completely inert to all spray materials.

Hydrogen is mainly used as a secondary gas, it dramatically effects heat transfer properties and acts as anti-oxidant. Small amounts of hydrogen added to the other plasma gases dramatically alters the plasma characteristics and energy levels and is thus used as one control for setting plasma voltage and energy.

Helium is mainly used as a secondary gas with argon. Helium is a noble gas and is completely inert to all spray materials and is used when hydrogen or nitrogen secondary gases have deleterious effects. Helium imparts good heat transfer properties and gives high sensitivity for control of plasma energy. It is commonly used for high velocity plasma spraying of high quality carbide coatings where process conditions are critical.

HVOF

High Velocity Oxygen Fuel Thermal Spray Process

Schematic Diagram of the HVOF Process

The HVOF (High Velocity Oxygen Fuel) Thermal Spray Process is basically the same as the combustion powder spray process (LVOF) except that this process has been developed to produce extremely high spray velocity. There are a number of HVOF guns which use different methods to achieve high velocity spraying. One method is basically a high pressure water cooled HVOF combustion chamber and long nozzle. Fuel (kerosene, acetylene, propylene and hydrogen) and oxygen are fed into the chamber, combustion produces a hot high pressure flame which is forced down a nozzle increasing its velocity. Powder may be fed axially into the HVOF combustion chamber under high pressure or fed through the side of laval type nozzle where the pressure is lower. Another method uses a simpler system of a high pressure combustion nozzle and air cap. Fuel gas (propane, propylene or hydrogen) and oxygen are supplied at high pressure, combustion occurs outside the nozzle but within an air cap supplied with compressed air. The compressed air pinches and accelerates the flame and acts as a coolant for the HVOF gun. Powder is fed at high pressure axially from the centre of the nozzle.

HVOF PROCESS

The coatings produced by HVOF are similar to those produce by the detonation process. HVOF coatings are very dense, strong and show low residual tensile stress or in some cases compressive stress, which enable very much thicker coatings to be applied than previously possible with the other processes.

The very high kinetic energy of particles striking the substrate surface do not require the particles to be fully molten to form high quality HVOF coatings. This is certainly an advantage for the carbide cermet type coatings and is where this process really excels.

HVOF coatings are used in applications requiring the highest density and strength not found in most other thermal spray processes. New applications, previously not suitable for thermal spray coatings are becoming viable.

Scuba Diving

PADI OPEN WATER DIVER

Now is the time to dive into the PADI Open Water Diver course, the most popular dive programme in the world! This is your ticket to a lifetime of intense adventure with PADI, the dive association that sets the standards in the global diving community. Why not learn to dive in Malaysia!


Throughout the course, you'll learn fundamentals of scuba diving, including dive equipment and techniques. You will also earn a PADI Open Water Diver certification that is recognised worldwide. You earn this diving license by completing five sessions in a diving pool, five knowledge development sessions and by making four dives (plus one extra dive, free of charge, just for fun) at some of the best Malaysia diving sites.

As a certified PADI Open Water Diver you have the freedom to dive with a buddy independent of a professional. If you already tried a PADI Discover Scuba Diving experience or are PADI Scuba Diver certified, you may get credit from these courses to the open water programme. Your underwater adventure can begin as soon as today with the PADI Open Water Diver video! You can view this at your own pace and then meet with one of our instructors for further assistance.

Virtually anyone who is in good health, reasonably fit, and comfortable in the water can earn a PADI Open Water Diver certification. At some point during your Malaysia scuba diving course, your PADI Instructor will ask you to demonstrate the ability to swim 200 metres and, on the first morning, complete a bit of paperwork to get you on your way.

The PADI Open Water Diver license is a permanent scuba qualification. However, if you do not dive for over 12 months and you are a relatively an inexperienced diver, then we recommend you complete a refresher programme, such as the PADI Scuba Review. This can take as little as one hour of your time in the dive pool and gives you the chance to refresh those diving skills you may have forgotten. That way, when you go scuba diving again you can relax and enjoy your dives, and be sure that you won't have any nasty surprises in store.

PADI ADVANCED OPEN WATER DIVER

Looking for the ultimate adventure? You found it! PADI's Adventures In Diving programme fine-tunes your dive skills and allows you to explore all that Malaysia diving has to offer. It's your dive - go for it!

PADI's Adventures In Diving programme has something for everyone. This in-water, performance-based programme includes the following choices:

The PADI Adventures In Diving programme offers two certification options. Complete any three Adventure Dives to earn the PADI Adventure Diver rating. Complete the Deep and Underwater Navigation Adventure Dives and three additional Adventure Dives and earn a PADI Advanced Open Water Diver certification.

The Adventures In Diving programme offers you a structured programme where you gain additional experience and skills under the guidance of a PADI Professional. If you're an Open Water Diver, then you're ready for the PADI Adventures In Diving programme. What's more, PADI Adventure Dives also count towards PADI Specialty Diver certifications.

So what does Adventures In Diving offer you? New experiences, new skills, lots of diving and thrilling adventures.


PADI EMERGENCY FIRST RESPONSE

A family member cuts a finger in the kitchen. Someone collapses from a heart attack on your dive boat. Two cars collide, seriously injuring the occupants, whilst on your way to the scuba club. How can you help? Then this programme is for you!


Accidents and illnesses happen every day. Some people just need a helpful hand, but others will suffer serious injury without assistance. Whether you're diving or a non-diver, Emergency First Response prepares you to properly handle potentially life-threatening situations.

This comprehensive program is composed of two core modules that can be taught in tandem or as stand-alone courses: Primary Care (Cardio Pulmonary Resuscitation - CPR) and Secondary Care (first aid). Together, these courses prove extensive instruction in CPR and first aid, as well as providing optional (yet recommended) Automated External Defibrillator (AED) and emergency oxygen sections.

The programme is designed meet the CPR and first aid pre-requisite training requirements for earning your PADI Rescue Diver rating.

Diving Equipment

SS Travel Backplate

Koplin Ergonomic Stainless Steel Travel Backplate

>Beautifully machined series 316 / Laser cut
>Designed for travel and warm water
>Fits most wings with holes on an 11 inch center
>15 inches (38cm) long
>2.2 Pounds (1kg)
>Cam Band Slots for single tank divers


OxyCheq Standard Single Cylinder Wing

This is exactly the same as the OxyCheq Single Cylinder Signature Series except this wing does not have the reinforced inner bladder, the rest remains the same.

The use of a soft expandable gussett allows the wings to be as flat as possible when deflated. The two pair of grommets, spaced 11" apart, allows the diver to fine tune their trim. The large CAM strap slots allows you to use our wings with a variety of other manufactures harness systems. The Standard Series wings can be used with standard back plates with a single tank adapter or with the O-Pac (no single tank adapater required) or the A-Pac (no single tank adapter required).

Our "water drains" efficiently dump water. The stitching used is twice as thick as the normal stitching used in the manufacture of wings.

Suunto Stinger Dive Computer It's a lifestyle!

The Suunto Stinger Dive Computer, thirty years ago, who would have thought that a diver could wear a full decompression computer on their wrist? Leave it to Suunto to give us that kind of small electronic miracle.

It’s hard to believe that a company that got its start by making compasses in the 1930s makes the most advanced dive computers available today. But the Suunto Stinger dive computer is proof that no one does dive computers better than Suunto. The Suunto Stinger dive computer is about twice the size of the average wrist watch, but it offers an unbelievable array of features.

Too many dive computers simply track your depths and time and leave it at that. When you re-enter the water, you’ll see your new maximum dive time change depending on your depth.

In my opinion, these dive computers encourage lazy divers to neglect proper dive planning. The Sunnto Stinger dive computer doesn’t have this weakness. This dive computer has a built-in dive planning function to help you plan repetitive dives.

Just because your dive computer has decompression dive capabilities doesn’t mean you should use them if you aren’t properly trained.
Suunto Stinger Dive Computer Features
Built-in dive planning functions aren’t the only feature the Suunto Stinger has. Are you an air diver? The Suunto Stinger will track your downtime and let you know if you go into decompression.

It will even tell you what depth you need to stop at and how long you need to stay there.

Are you a Nitrox diver? No problem. The Suunto Stinger will track Nitrox mixes from 21 to 50%. You can even program your maximum PO2 (from 1.2 to 1.6 bar) and the Stinger will signal you with visual and audible alarms if you reach your maximum PO2 level.

This dive computer even has features to help high altitude divers. You can set the Suunto Stinger to track dives done at up to 10,000 feet (3,000m) above sea level.

This dive computer comes in the standard stainless steel model or the high-end titanium model. Not only is titanium incredibly corrosion resistant, it’s also a lot lighter than steel. The difference in weight is huge. When you dive with the titanium Suunto Stinger it feels like you’re wearing an ordinary wristwatch.

Of course, Suunto hasn’t forgotten the basics when they made this sophisticated piece of equipment. This dive computer can also act as a log book by storing your dives until you can download them to your computer. Once on your computer, you can post (brag) about your dives on the Suunto forums. It’s not just a dive computer, it’s a lifestyle!

Suunto has long been known for making incredible diving electronics, but they’ve really outdone themselves with the Suunto Stinger. Its list of features is impressive and it can adapt to any diving situation. Whether you’re diving air, Nitrox or you’re freediving; the Suunto Stinger will keep track of all of your important statistics.

I’ve barely scratched the surface of what this computer can do. Head down to your local dive shop and check one out.

This could be the last dive computer you ever buy.