How Does A Rotary Screw Compressor Work?

The rotary screw belongs to the positive displacement compressor family. Positive displacement pumps create flow by applying an expanding cavity on the suction side and a decreasing cavity on the discharge side. Gas that is trapped inside the positive displacement machine is a fixed volume which is then compressed or displaced into the discharge manifold.
The two most commonly used compressors today are the rotary screw (helical rotor) and the reciprocating piston. In comparison of the two, the rotary screw does not use valves, is lighter in weight than the reciprocating piston, is pulsation free making foundation requirements less extreme and maintains its design efficiency over operational time as the rotors never come in contact with each other. The screw compressor was originally designed in the mid 1950's and eventually developed to operate between the reciprocating piston and centrifugal machine capabilities for commercial, industrial and gas type applications.
The Rotary screw compressor is composed of two intermeshing helical rotors contained in a housing. Clearance between the rotors and between the housing and the rotors is typically.003" to.005". The male or drive rotor is connected through a shaft extension by an electric motor or engine. In the case of an oil injected machine, the female rotor is driven by the male rotor through a thin film of oil. A dry rotary screw compressor employs a set of timing gears to achieve proper rotation.
The diameter and length of the rotors regulate the final pressure and capacity the machine can produce. As the rotor diameter increases, so does the air pumps capacity; As the length of the rotor increases, so does the pumps final pressure.
As power is applied to the male rotor it begins to move out of mesh with the female rotor creating a void allowing gas to be drawn in through the inlet port. As the rotor maintains past the inlet port the intermesh space continues to expand until the gas completely fills the interlobe space. When the male rotor enters the interlobe space it begins to convey and compress the gas towards the discharge port. As the rotors turn the gas filled grooves are isolated by the housing walls, creating a compression chamber, where lubricant is then injected to provide cooling, sealing and lubrication.
Continued rotation causes the gas volume to reduce to the stated design pressure. The compressed gas and lubricant is finally sent through the discharge port, then into a two phase separator where the oil and gas are divided. The oil is filtered by a 10 micron automotive type spin on filter and then cooled via air or water before being re-injected into the compression chamber. The oil type used in these machines is a hydrocarbon synthetic of ISO 100, 150 or 220 viscosities and is selected based upon specific gravity of the gas. Proper gas analysis is critical in oil selection as during initial start up, gas will dilute the viscosity of the oil. In the case of an air compressor the gas is then directed to an air cooled after-cooler where up to 70% of the ingested water vapor is condensed out of the gas stream before entering the supply manifold.
The compression porting is located and cut to attain the application pressure ratio. To achieve the greatest efficiency, it's central the corresponding geometry match the application pressure requirements. Some rotary screw compressor designs employ a variable discharge valve that continuously seek maximum efficiency by opening and closing depending on system pressure conditions. When the compressor senses a decreased system air demand (rising pressure) the discharge valve allows air to circulate back to the inlet without being compressed to meet system demand. The net effect is a shorter length rotor resulting in variable displacement operation allowing power requirements to drop.
The displacement of the screw compressor is a function of the interlobe volume and speed. The interlobe volume is a function of rotor profile, length and diameter. The interlobe volume can be expressed by the equation;
Qr = d3 (L/d) / C
Where: Qr = Displacement/Revolution d = rotor diameter C = typical profile constant
Qd = Qr x N
Where: Qd = discharge volume N = compressor speed Qi = Qd x Ev
Where Qi = actual inlet volume Ev = volumetric efficiency
Volumetric Efficiency is a function of rotor slip which is the internal leakage created by gas expansion back to the low pressure side therefore reducing the potential volume capacity of the compressor.
The screw compressor discharge temperature can be evaluated assuming adiabatic compression, assumes no energy (heat) is transferred to or from the gas during the compression and all supplied work is added to the internal energy of the gas resulting in increase of temperature and pressure.
Discharge temperature can be calculated by taking the adiabatic temperature rise and dividing by the adiabatic efficiency, and then multiplying by the temperature rise efficiency to account for cooling. To find the final discharge temperature, add the inlet temperature to the temperature rise.
t2 = t1 + T1 (rpk-1/k-1) / na x nt
Where: rp = pressure ratio na = adiabatic efficiency nt = temperature rise efficiency

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How Do Rotary Screw Aircompressors Operate

The most common type of compressors used in industrial settings are the rotary screw air compressors. There are many weird ideas surrounding these rotary screw compressors. Most of those ideas have to do with cold weather and winterization. Simple maintenance can solve many of the problems and failures that go along with winter weather.
Many people think that because their compressor is inside a heated building, it will not be affected by outside air temperatures. This is not true, because the oil coolers are often vented to the outside of the building. Oil that is trapped in a cooler that is exposed to cold air can become thick and will not circulate. Some rotary screw air compressors have thermal or bypass valves in the oil cooling system. This helps protect the compressor parts during startup in cold weather.
The valve must be checked regularly to ensure that it is operating properly. Synthetic oil can be a good help because it will circulate easier at low temperatures. However, the real problem is water build up in the oil sump because the water will not boil off and evaporate when the compressor is working under 140 degrees F. Problems caused by the water buildup can be avoided by occasionally draining a small amount of oil during cold weather before starting the compressor up.
Cold weather sometimes causes condensation to buildup in the air lines of the rotary screw air compressors. This can cause sensors to send the wrong signals to the screw compressors computer control systems. This will cause the compressor to run with a full load until someone notices the constant lifting of the pressure relief valves. Condensation is removed through drains that are placed strategically in the air system.
These discharge lines are usually taken to a drain point outside of the building. During cold weather, these lines can become clogged with ice, forcing condensation back into the air system. If not properly drained before the compressor is shut down, this can damage some of the compressor parts and expensive components.
It is common for businesses to take their rotary screw air compressors out of service during the winter months. This is good only if the proper steps are taken to winterize the screw compressors. This includes oil and filter changes.
Used oil generates an acid that can damage the bearings in the motor and the compressor parts. Manually rotating the motors and keeping the rotors oiled are also important to avoid seizing at startup. Proper maintenance of the screw compressors is important to avoid expensive service calls and to minimize downtime.

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Inroduction To - Rotary Screw Air Compressors

Rotary screw air compressors are a type of compressor. The rotary screw air-compressors are first of all known for their capacity to conserve energy. You may have heard of tips and tricks to save energy with other types of compressors. Well, this one has all of them in a single piece.
Rotary screw air compressors are even known for their durability. If they are well taken care of, these types of devices can last for decades even. You just need to make sure you consult the manual of instructions and know precisely what you are doing. The durability comes due to the contacts of the compressor. Basically, you can search forever, you will never find a metal-to-metal contact and this is the key to their durability.
Compared to other types of air compressors, this one is quite easy to install too. Years ago, compressed air was considered to be useless. Lately, it is known to be the fourth utility, call it innovation. So, the rotary screw air compressors which are improved compressor types are now considered to be the peak of technology in the domain.
The pressure on the air comes due to two rotors moving at the same time, which is obvious by the name. But these aren't the only superlatives of rotary screw air compressors. They also bring in a lot of power (HP), compared to other types. Capacity is also very important. As a matter of fact, although a lot of people fail to realize this, the capacity is actually the most important aspect when it comes to the whole operation.
Since these rotary screw air compressors are mostly used in large scale industries, they also come in larger sizes compared to the compressors you generally see around. Their most common use is in the construction industry, on building yards, to increase the power of air devices. The most appropriate criteria for classifying the rotary compressors is by their construction. You can find dynamic or positive displacement compressors.
Additionally, the rotary screw air compressors maybe those having one screw or auger or those with two or more augers. There is no difference in the way one-screw rotary air compressors function to that of many-screw rotary units, and the air gets compressed between the place where screw is housed and screw, than between many screws.
Each of these types of compressors comes with merits and demerits. However, the user has to best fit them for the situation they are purchased for. The design may also represent a good classifying criterion, but looks are way less insignificant and important than the actual use ain't.

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