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Compressor

Compressor

The compressor is a machine that generates a “force” by compressing a gas or vapor, which can be exploited in different ways; so much so that the compressor is used in many professional and industrial sectors. The compressor is able to increase the pressure of an aeriform body thanks to an entirely mechanical procedure. So its main objective is to compress the air and produce a pressure necessary for multiple purposes.

Compressed Air

Is compressed atmospheric air; i.e. reduced in volume with a compressor and stored in a pressure-resistant tank or cylinders, or used immediately.  Compressed air is the fourth form of energy used in industry as it has significant strengths and unique characteristics. Unlike gas, water and electricity, supplied by external companies; compressed air is generated on site directly by the user; for this reason the quality and the relative production costs depend on the user and on the system inside. of the industrial complex.

How A Compressor Works

The principle of operation of an air compressor is relatively simple. Depending on the type of machine, the compression process requires a motor; an air inlet valve and an air outlet valve and, in most cases, a compressed air storage tank. The air is sucked into the machine; compressed by the internal components in different ways according to the available technologies, and pushed into the tank. The compression process causes a pressure increase; when the maximum pressure is reached inside the tank, the cycle is complete; and the compressor stops and then starts up again when the pressure drops again below a predefined threshold. The range of commercially available compressors is really wide. Each type of compressor has specific characteristics and purposes; so it is important to understand which one to choose in order to use it correctly. Air compressors are used in all production processes and industrial applications that require compressed air.

Industrial Air Compressors

Compressed air is “produced” by compressors that can have very different sizes and dimensions; ranging from a simple compressor for the workshop to very powerful and complex compressor stations.  To choose a compressor you must first of all determine how often you use it; and define what the compressed air requirement of the system will be and at what pressure the compressed air will have to be produced. The flow rate is the amount of air that the compressor is able to deliver to power several devices at the same time. This value can be indicated in liters per second (l / s) or cubic meters per hour (m3 / h). Usually a compressor is chosen that offers a safety margin of about 30% more than the estimated requirement. The flow rate in normal cubic meters per hour (Nmc / h) is a conventional way of expressing the flow rate under conditions that are standard for everyone. They are defined so that everyone can speak the "same language" avoiding misunderstandings.

Normal Cubic Meter

The normal cubic meter is a quantity that has been taken as a convention; precisely because we all have a common way of referring to this volume. This is why it was decided to introduce the normal cubic meter (Nm³), and consequently the normal flow rate in Nm³ / h. This volume refers to “normal” conditions, ie dry air at 0 ° C (therefore 273 K); barometric pressure equal to 101325 Pa (1.01325 bar) and altitude above sea level. However, it is a fictitious flow rate, not a real one; which must then be converted into what is the actual flow rate, before choosing the compressor to buy. The transformation of normal cubic meters into effective cubic meters is obtained from the ratio between the two different densities of the gas. Where QN is the normal flow rate, Q is the effective flow rate, ρN is the density in normal conditions; and ρ is that in the conditions in which the compressor is to be operated. The pressure will depend on the industrial equipment or pneumatic circuit that the compressor is intended to power. The pressure can be indicated in bar or in pascal (Pa). Depending on this factor, we will choose a single-stage compressor (max. 10 bar); or a multistage compressor, capable of significantly increasing the pressure (up to 400 bar). The power of the compressor depends on the desired air flow and the outlet pressure.  Compressors generally work with a compressed air reservoir; which allows you to start the engine according to your needs. It is necessary to size this tank correctly so as not to constantly stress the engine.  The machine can be cooled by air or by water.

Air-cooled Compressors

Most current compressor units are also available in an air-cooled version, where the forced ventilation inside the air compressor unit; contains almost 100% of the energy consumed by the electric motor.

Water Cooled Compressors

There are three water cooling methods. Let's look at the different cooling options to determine which one should be used in the compressed air network in question. The more the compressed air is cooled inside the intercooler and aftercooler of a compressor; the higher the efficiency of the compressor and the more water vapor condenses. An installation with water-cooled compressors places modest demands on the compressor room ventilation system, as the cooling water contains, in the form of heat; approximately 90% of the energy absorbed by the electric motors. Compressor water cooling systems can be based on the following three basic principles:
  • Open systems without water circulation (connected to an external water supply)
  • Open systems with water circulation (cooling tower)
  • Closed systems with circulation of water (including an external radiator / heat exchanger)

Open System Without Circulation Of Water

In an open system without circulation of water, the water is supplied from an external source, such as an urban water supply, a lake, a river or a well, and after passing through the compressor it is discharged as wastewater. The system must be controlled by a thermostat to maintain the desired air temperature and regulate water consumption.

Costs

An open system is typically cheap and easy to install; but, costly in terms of operating costs, especially if the cooling water comes from a municipal water supply. Water from lakes or rivers is usually free; but, must be filtered and purified to limit the risk of clogging the cooling system. Water rich in limescale can also give rise to boiler scale inside the chillers; causing a gradual deterioration of the cooling. The same goes for salt water, which can however be used if the system is properly designed and sized accordingly.

Open System With Water Circulation

In an open system with water circulation, the cooling water from the compressor is cooled again in an open cooling tower; by dropping it by gravity into a chamber through which the air is blown. In these conditions, part of the water evaporates; and the remainder is cooled down to 2 ° C below the ambient temperature (this value may vary depending on the temperature and relative humidity). Open systems with water circulation are mainly used when the availability of water from an external source is limited. The disadvantage of this system is that the water is gradually contaminated by the surrounding air. The system also requires constant dilution with external water due to evaporation. Soluble salts are deposited on hot metal surfaces which reduce the heat transfer capacity of the cooling tower. The water must be regularly analyzed and treated with chemicals to avoid the growth of algae inside. When the compressor is not running, the cooling tower must be emptied or the water heated to prevent it from freezing.

Closed System With Water Circulation

In a closed cooling system, the same water circulates continuously between the compressor and some form of external heat exchanger; which in turn is cooled by means of an external water circuit or by the surrounding air. When the water is cooled via another water circuit, a flat plate heat exchanger is used. When the water is cooled using the surrounding air, a cooling matrix consisting of tubes and cooling fins is used. The surrounding air is forced to circulate through the tubes and fins by one or more fans.

When this method is used:

This method is suitable if the availability of cooling water is limited. Open or closed circuits have comparable cooling capacity; in the sense that the compressor water is cooled to a temperature 5 ° C higher than that of the refrigerant. If the cooling water is cooled by the surrounding air, the addition of an antifreeze (e.g. glycol) is required. Closed cooling water systems are filled with softened pure water. When glycol is added, the water flow of the compression system must be recalculated; as the type and concentration of the glycol affect the heat capacity and viscosity of the water. It is also important that the entire system is thoroughly cleaned before being refilled for the first time. A properly constructed closed water system requires very limited supervision and has low maintenance costs. For installations where the available cooling water is potentially corrosive; the chiller design must be based on a corrosion resistant material.

What Type Of Compressor To Choose?

There are two types of compressors: volumetric or positive displacement compressors and centrifugal or dynamic compressors. Most compressors are volumetric machines, which means that compression is achieved by reducing the volume occupied by the air. Most volumetric compressors are equipped with an electric motor.

Piston

Compressors Piston compressors can be single-cylinder; in which case they can reach a pressure of 10 bar. The multistage models, on the other hand, are able to increase the pressure in successive stages up to 400 bar. Among the various types of compressors proposed by manufacturers; piston compressors are usually the cheapest and are used when you need a machine to be used on a non-continuous basis. Their operating cycle, in fact, does not exceed 60%, which means that they cannot be used for more than 35 minutes per hour. The compressed air stored, in any case, must be sufficient with respect to the flow rate in order to reduce the number of pauses during use. A potential defect of this type of compressor is that, together with the compressed air; it also expels some oil. Consequently, if you need outgoing air that is clean; you will have to equip the compressor with a filtration system or opt for an oil-free compressor. This is the case in clean rooms as well as in various sectors, such as the electronics, pharmaceutical and food industries. Also, being relatively noisy, piston compressors can cause annoyance to people working nearby. 

Screw Compressors

Screw Compressors use helical screws instead of pistons and are the most commonly used model in the industrial sector. The pressure that these models can reach can range from 5 bar for a single-stage model to 13 bar in the case of multistage models. An advantage of screw compressors is that, in general, they are able to offer a considerable flow rate and a high compression ratio even if they are single-stage. In the case of the piston models, however, only the multistage models, which have larger dimensions and guarantee equivalent performance. Another particularly interesting feature of screw compressors is that they can have a duty cycle of 100% and therefore operate continuously. Finally, there are variable speed screw compressors (equipped with inverters); ie able to adapt their rotation speed to the compressed air requirement to optimize their energy needs. 

Vane Compressors

Vanes are foils that slide in an eccentric rotor, causing air compression. The energy efficiency of these compressors is generally good. With the same pressure and flow rate, vane compressors have a lower rotation speed than screw compressors; which reduces wear on components and the need for maintenance. These compressors are used in various industries, such as the printing and wood and packaging industries. In sectors where the air used must be clean, such as the energy or medical sectors, oil-free models are used. 

Centrifugal Or Dynamic Compressors

Are the second type of compressors. These compressors suck in the air thanks to the movement of a paddle wheel; ie according to the same principle as turbochargers used in the automotive industry. Centrifugal air compressors are mainly used when there is a need to reach and maintain a high flow rate and pressure without interruption; such as in the energy sector and in the chemical industry. The regulation of the flow rate and the pressure is carried out thanks to an integrated reducer that allows the machine to operate at an optimal engine speed. These compressors can at the same time reach a flow rate of 500,000 m3 / h and a pressure of 200 bar. 

Air Compressors For Oil-free Air Clean

Oil-free air compressors are specifically developed for applications where air quality is essential for the final product and production processes The oil-free compressors for bottling, not they use oil inside the compression chamber; the air is then compressed without ever being contaminated and therefore without any need for special filtrations. These models provide clean air and are therefore more environmentally friendly. Oil-free compressors are mainly used in the electronics industry, especially in the production of semiconductors; in the pharmaceutical and chemical industries, in the medical sector; as well as in the processing of food products and in the assembly and finishing operations of motor vehicles. An oil-free compressor must satisfy the ISO 8573-1 Class 0 standard. Only these compressors, in fact, guarantee the total absence of oil. The classes above 0 define the maximum quantities of oil that can be found in the compressed air supplied by these compressors; which, not being oil-free, will need to be accompanied by an air dryer. These compressors do not offer very high pressures.

Compressor Room

The place where most of the compressed air network is located is called the compressor room. It can be a room designed and used for other purposes or built with the compressor itself in mind. In both cases, in order to get the most out of the selected compressor installation, the room must meet certain requirements. The main rule for an installation consists first and foremost in defining a plant with a separate central compressor. Experience indicates that centralization is preferable, regardless of the sector. Among other things, it ensures better operating economy; better design of the compressed air network, easy use and maintenance, protection against unauthorized access; correct noise control and simpler possibilities for controlled ventilation.

Versatility of the installation

If there are no facilities available for installing the compressor indoors; it can also be installed outdoors, under a canopy. In this case, however, some problems must be taken into account, such as the risk of freezing of the condensate pockets and drains; the protection from rain and snow of the air inlet opening, the suction inlet and the air inlet opening. ventilation, the need for a solid flat base (asphalt, concrete slab or leveled cobblestone bed), the risk of dusts and inflammable or aggressive substances and protection against unauthorized access. In the case of large installations with long pipes; the compressed air system must be installed in order to simplify the arrangement of the distribution system. Service and maintenance can be facilitated by installing the compressed air system near auxiliary devices such as pumps and fans; a position close to the boiler room can also be advantageous. The building must have lifting equipment sized to handle the heavier components of the compressor installation (usually the electric motor) and / or allow access to a forklift truck.

Space for the installation

The building must also have sufficient floor space for the installation of an additional compressor for possible future expansion. The clear height must also be sufficient to allow, if necessary, the lifting of an electric motor or the like. The compressed air system must have a floor drain or other condensate management facilities from the compressor, aftercooler, air reservoir, dryers, etc. The floor drain must be made in compliance with current legislation.  To install a compressor system, usually only a level floor with sufficient load-bearing capacity is required. In most cases, anti-vibration equipment is integrated into the system. For new installations, a plinth is usually used for each compressor group to allow for floor cleaning. Large piston and centrifugal compressors may require a concrete base anchored to rock or a solid earth foundation. In systems equipped with centrifugal compressors, it is possible that vibrations transmitted to the base of the compressor room may need to be attenuated with anti-vibration pads.  All the units in the compressor room produce heat, which is disposed of through the ventilation of the room itself. The amount of ventilation air required is determined by the size of the compressor and whether it is air-cooled or water-cooled. Almost 100% of the energy consumed by the electric motor is present in the ventilation air of air-cooled compressors in the form of heat. On the other hand, about 10% of the energy consumed by the electric motor is present in the ventilation air of water-cooled compressors. To keep the temperature inside the compressor room at an acceptable level, the heat must be extracted.

Ventilation

The compressor manufacturer must provide detailed information on the required ventilation flow. A better way to tackle the heat build-up problem is to recover waste heat energy and use it in the premises.  The ventilation air must be taken outside, preferably without using long ducts. The suction mouth must also be placed as low as possible; but without running the risk of being covered by snow during the winter. It is also necessary to take into account the risk that explosive or corrosive dust and substances can enter the compressor room. The ventilation fans must be placed high on one of the walls at the end of the compressor room, and the air inlet on the opposite wall. The air velocity at the ventilation inlet opening must not exceed 4 m / s. If it is problematic to ensure sufficient ventilation within the room; the possibility of using water-cooled compressors should be considered. 

Dryers

The atmospheric air sucked in by a compressor, is a mixture of gas and water vapor and the capacity of the air to contain water in the form of water vapor increases considerably with the increase in air temperature caused during the compression phase. Subsequently, when the compressed air leaving the compressor begins to expand and consequently to cool, the water vapor it is saturated with is transformed into condensate; which must be separated inside the tank or in the cyclone separator. But to effectively eliminate all the water vapor of which the compressed air continues to be saturated and which would compromise the correct functioning of the equipment, the air is treated through suitable dryers. The most used air drying process is that of refrigeration dryers, which exploit the property that links the lowering of the temperature to the condensation of humidity in the air. The efficiency of water vapor removal by refrigeration cycle dryers is expressed with the value of the pressure dew point (PDP, or Pression Dew Point); i.e. the condensation temperature of the air with pressure higher than the atmospheric one.

Refrigeration

The limitation of refrigeration dryers is that they can only work with PDP above 0 ° C; otherwise the condensed liquid would freeze. They are therefore not suitable for systems with external piping or in environments with temperatures below the dew point of the dryer. In critical environmental conditions and in particular areas (for example in the food or pharmaceutical sector) the so-called "adsorption" dryers are preferable; which - by exploiting the chemical property of some materials to absorb (or release) moisture - are able to guarantee dew points down to -70 ° C. Cold regenerated HDT adsorption dryers; for example, consist of two chambers containing molecular sieves with hygroscopic capacity: while in the first the humidity is absorbed by the compressed air; in the second the saturated desiccant material is regenerated.

Condensate Drains

In the production of compressed air, condensate inevitably forms, which also contains emulsified oil particles, dust and impurities. To safeguard the correct functioning of the compressed air production and distribution system, it is advisable to drain the condensate in collection points and expel it through different types of drains; usually positioned on the bottom of the compressed air tank. In a simple float drain, when the level of the accumulated liquid exceeds a certain threshold; the drain valve located on the bottom of the container opens allowing the condensate to be discharged outside; closing automatically before the compressed air escapes. The electronic dischargers also work in a similar way; in which the valve is however controlled by an electronic level sensor instead of by the float. Finally, the timed drains are equipped with a solenoid valve that opens the drain located on the bottom of the tank at pre-established time intervals; with the possibility of adjusting the drain intervals and times according to the needs and environmental conditions.  Compressed air users risk hefty fines for discharging condensate into sewers as condensate; as a waste product of compressed air, is a noxious mixture. Due to increasing environmental pollution, in addition to solid particles, the condensate contains more and more quantities of hydrocarbons, sulfur dioxide, copper, lead, iron and other harmful substances.

Filtration

The air coming from the atmosphere that enters the compressor naturally contains fine dust and impurities, to which are also added particles of oil coming from the lubrication system of the compressor itself, in addition to the presence of water in the form of aerosols. On the other hand, it is important that the compressed air is dry and clean, both in order not to compromise the correct functioning of the machinery and preserve the sophisticated instrumentation, and to guarantee the necessary hygiene in certain areas of use of compressed air. The different air purity classes are defined by the ISO 8573-1: 2010 standard, which, depending on the type of use, determines the required air quality, specifying the maximum content of contaminants for each class. As a result, different filter elements have been developed, each of them specific to effectively remove solid particles or water and oil mists. In the same filtering system there are usually several filter elements gathered, in order to guarantee the correct removal of both the particulate and the water aerosol and oily vapors. These items may include particle filters, air de-oiling filters, coalescing filters, activated carbon filters. The food, pharmaceutical and cosmetic industries must use sterile filtration because the presence of microorganisms in the compressed air would compromise the quality and safety of the products. In these sectors, special filters will be used to deliver sterilized compressed air.

Certified Safety Tanks And Valves

The compressed air tanks, generally made of carbon steel painted externally with epoxy powders to ensure greater resistance to corrosion, can be vertical (more common) or horizontal and must be sized according to the compressor capacity , the regulation system and the expected consumption of compressed air. In addition to accumulating compressed air, they are used to keep the system pressure constant, compensating for any consumption peaks higher than the compressor capacity. It is advisable to install the tank near the compressor, preferably in a cool environment and in such a way as to allow both access to the condensate drain device and to allow control of the safety valve. The safety valves are used to prevent the maximum permissible pressure increase from being exceeded, even if the other autonomous safety devices installed upstream are blocked. They can be free exhaust, conveyed exhaust (i.e. with the bleed hole connectable to a pipe), or specific for high pressure. Among the fundamental parameters for sizing the safety valve are the calibration pressure (which indicates the pressure at which the safety valve begins to open in operating conditions), the discharge flow rate and the operating temperature. The relative declaration of conformity is always supplied with the safety valve.