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Air Compressor

Air Compressor

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Air Compressors

What is air compressor?

Air compressors are used for compressing air to an air receiver, and they raise the pressure of air.

Why we need compressed air?

Simply, cars’s wheel can be an example for the need of compressed air. In order to inflate the wheel there must be an air compressor. Nonetheless, there are numerous of mechanical tools that need compressed air generally in industrial areas.

The concept of  “500 liters compressor”

Actually there is no concept of  “500 liter compressor”. There are a lot of compressors that is in different capacities and sizes. But, people genereally suppose that air receiver’s capacity is air compressor’s capacity. This is not true. There is no effect of air receiver for consumed air as flow. The capacity of air compressors can be determined by compressor’s pump and engine power.

Flow

Flow is the amount of air that sent to air receiver or system per unit. This determines the capacity of the compressor. In order to indicate the actual measurement of flow, there are some units such as m3/min, m3/hr, lt/min, lt/hr and CFM. These are the most used units to determine air volume. These units can be converted to each other. For example, 1000 lt/min can be said as 1 m3/min or 60 m3/hr. (1 CFM = 28,328 lt/min, 1m3/min = 1000 lt/min)

Pressure

Pressure is the amount of force per unit of area. Most commonly used units for pressure are bar, atm, psi, mpa and kg/cm2. These units also can be converted to each other. (1 bar = 14,5 PSI, 1 bar = 0,98 ATM)

As you see, there are 2 factors effecting the capacity of a compressors; flow and pressure.

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Air Tank Capacity

The size of an air tank refers to the volume of the tank, and is measured in liters or m3 (1m3 = 1000 liters). The air tanks on a compressor are used to store and supply air on a regular basis.
As mentioned in our previous articles, the air tank has no influence on the flow rate, although some users believe that the problem of insufficient air can be overcome by increasing the size of the air tank. This is completely false. If the air is insufficient in the plants, the first thing that should be checked is the capacity of the compressor.

Situations in which the size of the tank is important:

In addition to those mentioned above, there are other special cases in which a larger air tank would be required. For example, when using a machine that consumes air (pneumatic), or when the machine works once for a certain period and consumes large amounts of air. Let us try to explain this with an example:

The amount of air consumed is 1500 l/min in one second when operated once –this is consumed air (ca).
When your machine works once, the time of consumption is 5 seconds – this is consumption time (ct).
Machine requires a minimum pressure of 6 bar.

Calculation:

ca * ct= the total amount of air consumed in one work cycle
5 * 1500 = 7500 liters, the total amount of air consumed in the work cycle
We know that 7500 liters of air is consumed within 5 seconds, and so we need to choose the air tank accordingly.
This can be calculated using two different methods.

1st Method – increasing the pressure.

Let’s assume we have a 1000-liter air tank, and the pressure is 8 bar. In this case, when our air tank is full, 8 * 1000 = 8000 liters of air will be stored.
It would appear that the 8000 liters of air stored in our air tank would meet the 7500 liters of air required by the machine, but since the machine’s minimum operating pressure is 6 bar:
(1000 liters (nominal volume of air tank) * 6 bar) = 6000 liters is our lower limit.
8000 – 6000 = 2000 liters of air in the tank, as the available amount. But 7500-2000 = 5500 liters, so more air is needed.

Result: The air is insufficient.

If we think we were going to use 15 bar pressure with the same air tank (1000lt)
15 * 1000 = 15000lt, which is the total amount of air stored in the air tank.
Lower limit = 6000 liters.
15000 – 6000 = 9000 liters, which is the amount of air available in the tank
7500 liters is the amount our machine needs, at a pressure of least 6 bar at any one time.
9000 > 7500 – meaning that the available air quantity is greater than the amount of air needed by the machine.
9000 – 7500 = 1500 liters of surplus air, increasing the amount of air remaining after use.
6000 + 1500 = 7500 is the total amount of air in the tank after use.
7500/1000 = 7.5 bar total pressure remaining in the tank after use.

Result: The air is sufficient.

2nd Method – Increasing the size of the air tanks

Considering a 5000-liter compressor air tank, when the compressor comes to 8 bar, 8 * 5000 = 40,000 liters, which is the total amount of air that may be stored.
Because of our air tank volume has increased, 6 * 5000 = 30,000 liters, and we have the lower limit at a pressure of 6 bar.
40000 – 30000 = 10000 liters of storage volume of air that can be used.
10000 > 7500 – the available air quantity is greater than the amount of air needed by the machine.
10000 – 7500 = 2500 liters of surplus, increasing the amount of air after use.
30,000 + 2,500 = 32,500 liters, which is the total amount of air remaining in the tank after use.
32500/5000 = 6.5 bar, which is the pressure in the tank after use.

Result: The air is sufficient.

If you paid attention, you would have seen that no calculations were made related to the capacity of the compressor, as I assumed that the machine waited long and run the one time, and that the compressor fed only this machine. If this machine is run often, the capacity of the compressor will need to be increased.

Evaluation:

In the first method, a 1000-liter vessel must be resistant to high pressures. An air tank that is to be subjected to 15 bar is required to be tested and certified by authorized persons with water to at least 23 bar.

Advantages:

• Since the tank volume is small, there are advantages of space.
• Low cost.

Disadvantages

• The compressor providing the air must be designed to suit high-pressure compressors. In reciprocating compressors, high pressures can be reached in two steps, while in screw compressors, the recommended maximum pressure is 13 bar, and the speed should be reduced by acting on the belt-pulley system.

• Prices are high in comparison to a 1000-liter air tank to be subjected to 8 bar, as the sheet thickness of the tank is increased.

• The risk is greater.

The more the air is compressed, the higher the air pressure will be, and the greater the air pressure exiting the vessel. While this pressure can sometimes be an advantage, there are disadvantages too.
In the 2nd method, the pressure in a substantial volume 5000-liter tank is increased to 8 bar.

Advantages

• Any 8bar compressor can be used to fill the tank.
• There is no need for high pressures.

Disadvantages

• Volumetrically, it occupies a greater area.
• Transportation costs are high.

Note: the dimensions of a 1000-liter vertical air tank are approximately 200 cm high and 80 cm in diameter. In contrast, a 5000-liter air vessel is about 380 cm high by 130 cm diameter.
Different tanks to those given in the example can also be used, and using the same logic you can reach conclusions about the maximum pressure and capacity.

Text can be confusing, but I tried to explain as clearly as possible. 🙂

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Flow Rate (CFM, m3/min, lt/min)

The flow rate describes the intensity of fluid transition per unit of time from a specific area. Since air is a fluid, a flow-rate calculation is made.

In compressors, the flow rate is the factor that determines the size of the compressor, while pressure is the amount of force applied to a unit area. The two terms are often confused. The relationship between pressure and flow is explained in our “Relation Between Pressure and Flow Rate” article.

In piston compressors, to increase the flow rate, physical changes have to be made to the compressor that may increase the size, weight and the power consumption of the compressor. To increase or decrease the flow, the factors to be taken into account are: diameter of the piston and the cylinder, head count, stroke volume, crankshaft speed (RPM) and engine power. The flow rate can be varied by increasing or decreasing these factors.

Physical changes in the screw compressor are mandatory. To change the flow rate in a screw compressor, the factors to be taken into account include screw size, screw speed and engine power. It is important that all auxiliary devices used in the compressor are in compliance with these factors.

Flow rate can be expressed in the following terms: CFM, m3/min, m3, liters/min, l/h.

Note: The amount of air compressed in a piston compressor is less than the amount taken in. The reason for this is that the air filling the cylinder in the piston while it moves back passes through suction valves and leaks out from between the snap rings and getting between pistons, cylinder in advance, and escape back. As a result, the air pumped in the opposite direction enters into the casing, thus increasing the internal pressure in the crankcase. There are various methods used for the release of air. Some methods used are as oil fill cap, some of them create ways to suction valve by means of a hose, which is connected to the hole that opens into the crankcase.

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Electrical Board

Electrical Board

The electrical panels contain such components as protection relays, fuses and control elements. Generally, in compressors, electric motors which provide the star-delta starting are used to prevent current surges, and to control the semi-mechanical parts of the compressor.

Electricity Board of Reciprocating Compressors

In reciprocating compressors, while varying according to manufacturer’s preferences, generally for compressors with an electric power rating that is less than 7.5 kW, the inclusion of an electrical panel is rare, or elements such as fuses, contactors and running switches are used.

Since the amount of current drawn (inrush current) during startup is more than seven fold of normal running current, by the increasing KW value, the need of the electric motor to use star-delta increases.

In some compressors, the check valves in the system are used one-way. In this type of compressor, the compressed air between the head and valve is controlled using an external solenoid valve, which is opened or closed (varying according to the electrical panel circuits) from the electrical panel circuit, allowing the trapped air to be evacuated.

Electricity Board for Screw Compressors

In a screw compressor, the electrical panel controls the entire system. The control altering the direction of rotation of the electric motor affects many variables, such as the compressor temperature, pressure control, excessive flow and the solenoid valves. In the event of any risks, the compressor will shut down.

As an example, in the event of malfunction of the static switch that sets the working pressure of the compressor, the compressor’s pressure will begin to rise, exceeding the values set. In this case, the second static switch, which is provided with safety in mind, sends information to the pressure value control cabinet, and the compressor shut off by the electric panel. In addition, as long as high-pressure data continues to come from the second static switch, the high-pressure warning light on the control panel will flash.

In addition, in some cases, a soft-starter unit can replace contactors on electricity boards, allowing the electric motor to run slowly without taking over-current, subsequently allowing it to reach full power. The use of such units is rare, as they are expensive.

Electrical panels are made according to the values determined by the manufacturers, and there is no limit on what can be done using the electrical panel.

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Compressor Cooling Process

When functioning, air compressors convert the electrical energy they obtain from the power source into kinetic energy through the use of a motor. Then, due to the structure of the compressor, being divided, a large proportion of this kinetic energy is converted into heat energy.

The Reason of Compressor Warm up

During compression, heat is created due to the friction on the surface of the line in which the air is located. Additionally, the cylinder and piston friction in reciprocating compressors and friction between male and female rotor which are located in screw rotor in screw compressors are important factors in the release of heat.

Cooling

In piston compressors, the cooling is generally is done by the pulleys of the compressor. On the large diameter pulley of the compressor, the supports, fixed at an angle, act as a fan, allowing air to be blown over the compressor. The compressor cylinders and the outer surfaces of the cylinder head contain channels, along which the blown air passes and is cooled. The structure of the system may vary between different manufacturers and different models, although coil pipes for cooling and radiators are frequently encountered.

Rotary screw air compressors operate in a slightly different way. For cooling, there is a need for a radiator, which generally has two compartments: oil is cooled in one, while air is cooled in the other. The oil recirculates as the screw rotates in the system and is cooled in the radiator. The oil transmits all of its retained heat in the compressor, thus lowering the temperature of the compressor.

In screw compressors, the oil cannot be cooled only by passing through the radiator. A major feature of the radiator is its ability to facilitate the rapid exchange of heat. In order for the radiator to cool the oil as it passes through, it should first be cooled. The air passes between the radiator cores, which in turn cools the radiator.

In order to cool the radiators, an external motor fan is placed just in front of the radiator in some brands and models, although in some brands and models the main motor is placed without an external motor fan is being placed. In models in which the main engine has a fan, ambient air is sucked into the compressor cabinet and is forced through the radiator cores. In some models which suck air into the cabinet, if the cabinet door is left open, the compressor temperature rises.

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Dryer Bypass System

Compressor SetIn the compressor setup depicted above, it can be seen to comprise the compressor, air tank, dryer and filter. Through the valves, shown in red a bypass system has been created that allows for the deactivation of the dryer at any time.
In order for the dryer to operate, no: 2 valve must be closed, while valves 3 and 4 are required to be opened, while to disable the dryer, no: 2 valve must be opened and valve nos. 3 and 4 must be closed. This is referred to as bypass system.
When the no: 1 valve is closed, all of the air from the tank to the installation is cut off.

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Dryer Working Principle

In an environment, under normal conditions, invisible water molecules (humidity) are always present in the air. When running, compressors compress the air that they suck from the environment. Once compressed, the moisture in the air is converted into water, part of which is deposited into the air tank, with the remainder carried to the installation. The amount of liquid water that is created depends on several factors: weather conditions, geographical location of the compressor, climate and the capacity of the compressor. For example, the amount of water formed by a compressor running near the sea is different to the amount of water accumulated by a compressor with the same capacity far from the sea. The moisture amount near the sea is more. For this reason, more water is occured near the sea.

Note: since the air coming from a compressor condenses more on metal, the materials used in the structure of the installation are a factor in the formation of water. In other words, there may be a difference in the amount of water accumulating on metal pipes in an installation than on plastic pipes.

It is well known that water damages metal and causes corrosion, but the level of resulting damage varies from sector to sector. For example, in the health sector and in hatch paintwork services, great care is taken to keep water away from the installation, while in workshops where compressors are used for cleaning, the water produced by an installation cannot be a concern for the user.

Here, the intended use of the dryer is to separate and eliminate water from the installations. Dryers provide cold structures by recirculating of a special gas, and the air coming from the compressor is cooled when passing through these structures. The cooled air leaves the water molecules into drier. This water is then expelled from the dryer.

For most types of coolers, such as air conditioners, dryers, refrigerators and the like, a particular gas is used for recirculation within the system using an ecowatt compressor.

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Troubles, Indicators in Rotary Screw Compressors

Screw compressors have a control panel and are capable of protecting themselves. If a failure occurs, the screw compressor stops running without causing major problems and gives a breakdown warning.

The primary warnings in case of failure may include:

  1. Main Engine High Amp Alert
  2. If any, Fan Engine High Amp Alert
  3. High Temperature Alert
  4. High Pressure Alert

1. Main Engine High Amp Alert

Ampere: Known as current intensity. It is a measure of the amount of electric charge passing a point in an electric circuit per unit time and is indicated by “A”.

Electric engines are manufactured at the factory by KW in accordance with the amount ampere driven. You can see the amount of ampere on the metal label on the electric engine.

Electric engines can be forced to run up to the amount of amps on the label. As the engine is forced, the amount of power amps increases at the same rate. If the upper levels of the engine are forced, it may cause the engine to burn or other failures to occur.

Screw compressors have relay measuring the current placed in electrical panel in order to ensure the protection of the electric engine. This relay typically has two buttons, blue and red.

. The operation of the electric engine is dependent on this relay. Relays have the ability to stop the system if the current is overloaded.

In the case of any current overload, the relay will be activated and stop the main engine.

What are the situations that may cause the electric engine to draw too much current?

•High Pressure: a) When set at a higher pressure than factory setting, the ampere level of the compressor will increase because the compressor will be overloaded by pressure. This may cause the ampere level to exceed the amount allowed by the relay, which is compatible with the engine, and the system may stop working; b) If the screw compressor starts running before discharging the air when it stops after filling the air, the engine would overload upon starting. This would cause too much inrush current. In this case, electric engine is again disabled by the system and will stop working.

•• Any strain occurring in the screw unit (e.g., rotary screw lock): The screw unit is an element directly connected to the electrical engine. Any failures or strain that may occur here may cause the electric engine draw more amps, causing the system to stop.

  1. Fan Engine High Amp Alert

If the compressor has an external fan engine, this is the warning that there is a problem on this engine.

  1. High-Temperature Alert

The temperature level is measured with the help of a gauge connected to the screw unit of the compressor.  Screw compressors are usually designed to run between 0°C and 115 °C. If the compressor heat level rises above 115 °C the compressor will automatically stop running and continue to give an overheat alert as long as the temperature is high. Compressors with a manual control panel have a gauge indicating the temperature. This gauge has an adjustable or fixed temperature alarm point. These are contact temperature gauges and when the set value is exceeded, the system will produce a failure warning, thanks to the contact connected to the electrical panel.

Note: Breakdowns and danger are a potential if the heat value of the compressor exceeds 115 oC.

What situations may cause the compressor to overheat?

Please refer to the article: Compressor overheating .

  1. High-Pressure Alert

In cases where the compressor pressure excessively increases, the safety switch is activated and causes the electric system to halt.

What could cause an increase in pressure?

  • The safety switch is activated in the event values of compressor pressure increase.

•Failure of the static switch (or pressurestat) can cause the pressure rise.

•A malfunction of the suction valve may cause an increase in pressure. Although it must normally be closed in the idle mode, the sucking valve may remain open due to failure in the cover part, causing the pressure rise, although the compressor is idle. In this case, the safety switch is activated and the system is halted.

•Internal pressure may rapidly rise with sudden loadings of the compressor, resulting in a safety warning. This may also result in the explosion of the separator. Such a situation may occur when a valve is connected to the outlet of the compressor (before the tank). When high-flow compressor is rapidly loaded on the valve, the pressure suddenly rises. As the compressor cannot halt at a sudden rise of high pressure, the high pressure safety switch is activated.

This may not precisely apply to every type of compressor. Different instructions and safety systems may be used by manufacturers.

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Belt

kayis

In belt and pulley driven compressor systems, the engine movement, as the name suggests, is transmitted to

  the compressor crankshaft or the screw unit through a belt. Belts are produced, as many products, in many different varieties. The manufacturer determines the number and types of belts to be used in the compressors.

If you should need to replace compressor belt, you need to consider the following;

•The pulleys and belts must be compatible with each other. The belt type and channel structure are important. Compressors often use V-belts.

•The belt purchased in accordance with pulley dimensions should fit on the pulley so as to allow tensioning.

•To avoid strain when you install the belt, pulleys should be set in the axis and exactly parallel to each other. Otherwise, a high-speed rotating belt may whip or cause other malfunctions.

,•In the event of very tense belts, electric engines may be burned, electric engine bearings may break down, or the compressor crankshaft may be damaged. For this reason, particular attention should be paid to belt tension in reciprocating compressors. If the belt is set loose, the belt may increasingly whip as pressure increases resulting in the breaking of belt due to spinning.

•Belt quality is also an important element; ensure that the belts are of high quality, which will extend the life of the belt.

It is essential that mechanical work regarding compressors, such as replacing the belt, be carried out by competent and experienced persons.

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Relation Between Pressure and Flow Rate

The first thing to mention is that pressure and flow rate are inversely proportional; that is, as pressure increases, compressor flow rate decreases.

Compressors have been designed to have a structure that is capable of maintaining a pressure of 7-8 bar,  using the most appropriate electric engine and materials.

In general, air consuming devices in the industry are compatible with compressors’ pressure. For example, many air consuming machines such as CNC plasma cutting lathe produce an alarm if the pressure is lower than 5 – 6 bar.

There are some special cases where high-pressure air is needed. As was stated in the article “How to select a compressor” when you first decide to buy a compressor, you will need to determine the minimum pressure that is required for the area of utilization. The high pressure compressor can meet your need of low-pressure air, while a low-pressure compressor cannot meet high pressure needs.

Why should pressure requirements be determined first?

The structure of the compressor system, resistance, the testing of the air tank pressure, and other components are established at the factory according to the desired pressure value. Therefore, if you purchase a compressor with maximum 8 bar, and later you want to increase it to 13 bar, then you would likely need to replace the entire compressor and other products included with the compressor.

What happens if we increase the pressure of an 8 bar compressor in an uncontrolled manner??

  • You will increase the risk of explosion of your air tank. This is critical. You should never exceed the working pressure value on the label on your air tank. Increasing the compressor’s pressure without being cognizant of the actual pressure creates a dangerous situation in which it could explode like a bomb at any moment. In addition, even if the air pressure value of the compressor is not increased, the pressure should be tested annually. This is also controlled by the Ministry of Labor.

•Fuses and circuit breakers on the compressor, if any, switch to protection mode. •Safety valves on the tank and air compressor are activated and discharge the excess air from the air tank.

•The 2nd static switch for safety, if available, is activated and stops the electrical system. If the protection units on the compressor do not activate, the air tank may explode.

•The electric engine may catch fire.

•The structure of the compressor may be damaged. For example, the crankshaft in the reciprocating compressors may break.

Thus far, you have read about the pressure portion of this product’s operation. If you want to increase pressure but desire to avoid problems of this kind, you must decrease flow while increasing pressure and this should be determined when deciding to purchase the compressor.

Compressed air and flow rate are inversely proportional. As pressure increases, flow decreases.

In reciprocating compressors

Pressure should be increased gradually in reciprocating compressors. Eight bar compressors do not have stages and the intake air will be sent directly to the air tank through heads. However, with 2-stage compressors, the intake air is not directly sent to the air tank, but it is sent to the pumping head of a smaller structure. It is compressed here once again and then pumped to the air tank. In this manner, flow rate is reduced and the pressure of the compressor is increased.

Thus, the structure of your compressor would become compliant for high pressure. Therefore, a reciprocating compressor manufactured in this way at the factory will have an air tank compatible with high pressure.

In screw compressors

The pressure of screw compressors can be increased by changing the belt and pulley dimensions of the system. With screw compressors, again, as pressure increases flow rate decreases. Although varying by manufacturer, screw compressors can be manufactured with a 13 – 15 bar maximum working pressure. Belt and pulley changes should be carried out by authorized personnel.

Warning

•Do not allow anyone other than authorized personnel to change the pressure setting of the compressor.

•Have the air tank certainly tested annually.

•If you buy a high pressure air compressor tank, be sure to check how many bars of pressure your air tank is compatible with.

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Suction Valve

hak_04-250x250In screw air compressors, the suction valve is the component fixed to the body of the screw block, which controls the air intake. It is a valve controlled by solenoid valves, whose size may vary by compressor capacity.

There are different types of suction valves, as compressor manufacturers use suction valves appropriate for their models. Some suction valves operate in a balanced weight system, while others have a spring system.

Function of the Suction Valve

Suction valves are an integral part of the screw unit. They are placed at the upper inlet of the screw block and are fixed by bolts. In order to prevent leakage, the surfaces of the suction valve and screw unit are sealed. When the compressor runs, the air in the system is sucked through the air supply inlet in the sucking valve structure. The intake of air from here raises the internal pressure of the compressor 1-2 bar. After that, the compressor will be fully loaded by the opening of the pump through the solenoid valve and continue to run at full capacity.

After the air fills to a certain pressure, the compressor sends information to the solenoid valve, ensuring the suction valve is closed, and thus the operation is complete.

In addition, when the compressor is in idle mode, as pressure increases, the flow rate of the compressor decreases.