Air compressor – types and characteristics

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In this Article: A History of Compressors types of compressors; how to make an air compressor with your own hands; compressor selection criteria; compressed air requirements table for some pneumatic tools; comparison of reciprocating or screw compressors; what is the difference between the passport capacity of domestic and imported compressors; what determines the compressor resource.

Air compressor

On modern construction sites, it is noisy – pneumatic tools are working: pneumatic perforators and jackhammers rumble, paint-and-paint pistols hiss. For all its noise, this construction equipment greatly facilitates the work, at the same time improving its quality – today it is impossible to do without a compressor at a construction site. The market offers air compressors in a wide variety and for orientation on it, for choosing the compressor of the desired characteristics and this article is intended.

Compressor types

The task of any compressor is to compress gas; ordinary air that we breathe can act in its role. Air compressors with a capacity of up to 100 cubic meters per minute are divided into two types of design – rotary (screw) and reciprocating.

The first reciprocating compressor was created about three centuries ago by the German Otto von Guericke – it was an experimental prototype driven by human power. The mechanical (steam) compressor was invented later, around 1800, by the Englishman George Medhurst and was used in coal mining. And the first pneumatic puncher was created by a resident of England, Thomas Cochran – it worked from compressed steam and often exploded, causing burns and injuries to workers.

Otto von Guericke. German physicist, engineer and philosopher.
Otto von Guericke. German physicist, engineer and philosopher.

Modern reciprocating compressors are much safer – there is no steam, which means there is no danger of getting burned. This type of compressor is widespread, it works practically on the same principle as the piston engine of a conventional car, with the difference that it does not have an ignition phase and a different gas distribution scheme – compressing air, the pistons force it into the receiver, the valves block the return path of the air.

The prototype of the screw compressor was developed in 1878, its author was Heinrich Krigar, an engineer from Germany, its modern counterpart – in 1932 by the Swede Lisholm. The principle of operation of this type of compressor is that air is pumped by two screws (worm screw pair). They are inserted into each other – the leading rotor-screw has four turns, the driven one has six. Air is compressed in the space between the turns of the worm pair and the walls of the outer casing, therefore, all internal elements of the screw compressor chamber must be made with maximum precision. The chamber itself is filled with oil, which: minimizes gaps, “transports” air, lubricates working elements and removes heat (the operating temperature of the oil is set by a specific manufacturer, usually its range is 80-900).

Both types of air compressors can be driven by an electric motor or an internal combustion engine. Depending on the size and purpose, the compressors can be equipped with a wheelbase.

DIY air compressor – why buy

It is quite possible to create it if there is a desire and a penchant for technology – the main element in it will be the compressor from the refrigerator. It should be noted right away – for something more than painting and pumping up tires of a car, a homemade compressor will not work, because the pressure created by it will not exceed 5-6 atmospheres. However, it will cost much less than factory counterparts..

The main element of a home-made compressor is the unit of the same name from any household refrigerator. It can be bought or removed from an old refrigeration device, in the second case you will need: side cutters or pliers, “plus” and “minus” screwdrivers, a pair of keys “12”.

How to change the oil in a homemade air compressor

The process of extracting the refrigerator compressor: after measuring the maximum length of the copper pipes going from the compressor to the radiator grill, bite them with side cutters (pliers); disconnect the starting relay (a plastic box next to the compressor, in relatively modern models the relay is fixed on the compressor itself – it does not need to be removed), disconnect the wire in it leading to the compressor. We cut two wires entering the relay with wire cutters, on the case or cover of the relay you need to mark where before dismantling was the top, and where was the bottom.

Attention:the copper pipes need to have a bite, without trying to saw them off – the slightest ingress of shavings into the compressor and its value will be zero, because metal grains of sand will cause serious internal damage during the operation of the piston, and they cannot be removed in any way!

Now you need to disconnect the compressor, for which we use the keys “12”. All fastening elements – bolts, nuts, rubber gaskets (if they are still suitable) – we take with us.

Before continuing to create a compressor with your own hands, you need to make sure that the compressor removed from the refrigerator is working. If the relay was not attached to the compressor housing, we connect its connectors to the compressor contacts in the same order as before dismantling. The relay case of such a relay must be temporarily fixed on any flat and horizontal surface using adhesive tape. We connect a wire with a plug to the inputs of the relay – this is a “temporary” wire, necessary only for testing the compressor’s performance. Carefully (this is important!) We twist the bare wires together and wrap them on top with electrical tape. Next, you need to unclench the ends of the copper tubes flattened when biting for free air flow.

Upon completion of all these operations, we turn on the compressor into the outlet – after a small pop, the unit should rattle, while one of its tubes will suck in air, and the other will spit it out. Mark with a marker which of the tubes is sucking in and which is giving out air. Let the compressor run for five to ten minutes – if it rumbles without failures and sudden shutdowns, then it is serviceable and suitable for creating a homemade air compressor.

We continue to create a homemade compressor

The next step is to purchase the necessary materials, for this you have to visit the car market. Required to purchase:

  1. Motor oil (10w40, mineral or semi-synthetic) – you need half a liter. However, you can use mineral water or semi-synthetics that remain when you change the oil in your car;
  2. Take with you a piece of copper tube from the compressor and buy under it a reinforced oil-and-petrol resistant rubber tube – inner diameter 4 mm, length 1 m;
  3. 10 metal clamps, clamping – with a diameter slightly larger than the diameter of the rubber tube (this number of clamps is more than necessary – with a margin);
  4. Automotive filters for fine cleaning of fuel – two gasoline and one diesel (in the first there is a paper accordion, in the second – a mesh made of synthetics);
  5. Oil and petrol resistant silicone sealant – one tube. Check its consistency – it should be gray and thick.

Next visit the hardware store, you will need:

  1. Household switch (the same one used for on / off lighting) – one button, outdoor (external) installation;
  2. Double electrical wire and a plug for powering the compressor into the network – it is better to buy a ready-made kit (double insulation), the length of the wire is about 2 meters;
  3. 2 meters of insulated wire for pressure switches;
  4. Air pressure switch (for example, RDM-5);
  5. A set of plumbing fittings, including adapters (almost all connections will go through them);
  6. Oxygen reducer (blue tap);
  7. Check valve with diameter and thread for cross fitting (optional, but desirable).

Now it is worth answering the question – why do you need 10w40 engine oil? At the manufacturing plant of compressors for domestic refrigerators, this unit is filled with mineral oil “spindle”, specially designed for it. There are no additives in such oil, since they are not needed, because the refrigerator system is closed – 100% isolated from the surrounding atmosphere. But for an air compressor such oil is completely unsuitable, because begins to react with oxygen from the air (oxidize), clog up with dust and absorb moisture. As a result, the compressor overheats, makes a strained noise, its pistons wear out quickly and soon they will inevitably jam. In addition, mineral oil evaporates during the operation of the air compressor, following the air flow, its level in the compressor decreases rapidly.

How to change the oil in a homemade air compressor

There are three tubes coming out of the compressor casing – an intake air that discharges it and a tube with a sealed hole through which oil was poured at the factory. It is this sealed tube that needs to be opened by making a circular cut with a file a little lower from the sealed end – file it slightly without sawing to the end! Then you need to swing and break the tube along the cut line, after the break, remove the sealed fragment of the tube and tap the break point with a small hammer (“nail down” the burrs). All that remains is to drain the oil from the compressor into a suitable container..

Attention:the compressor cannot be switched on without oil in the system!

Having completely removed the old oil from the unit, you need to fill in a new one (about 300 grams). The only way to do this without spilling oil is with a large syringe, fill it with oil and slowly pour it into the opened tube.

DIY compressor

After filling with oil, the hole in the tube must be closed, otherwise it will pull / dispense air. To do this, it is better to use a self-tapping screw of sufficient diameter, after putting a rubber gasket on it.

DIY compressor – preparing the receiver

You cannot do without a receiver; its functions are to equalize the air pressure supplied by the piston compressor in a stepwise manner. In addition, the reservoir bottle will trap oil droplets and vapors. For a home-made receiver, a cylinder (from 5 liters) from under a fire extinguisher with a capacity is ideal, better than a carbon dioxide one, because it has no welds. It is required to completely remove the paint from the balloon by doing it in one of these ways.

Assembling a homemade compressor

Look inside the cylinder with a flashlight – if there are traces of rust on the inside of the bottom, you should completely remove them. This is necessary, since during the operation of a home-made compressor, the pressure will greatly accelerate the rusting process – for removal we use chemical compositions with orthophosphoric acid or tannin.

After removing the paint, we carefully clean the body of the former fire extinguisher with sandpaper, then paint the tank-receiver with new paint.

Assembling a homemade compressor

An adapter must be screwed into the hole of the former fire extinguisher, having previously wrapped the thread with fum tape (all subsequent connections must be wound with fum tape), into it – a cross fitting. A pressure switch must be installed in its upper outlet (through an adapter), on the side (also through an adapter) – a fine diesel filter, then an oxygen reducer. In the free outlet of the cross, we put an adapter, a check valve and a fitting for entering an oil and petrol-resistant tube (tighten it with a clamp), its second end is fixed with a clamp on the copper air discharge pipe of the compressor. Installing a check valve will completely eliminate the return flow of compressed air to the compressor. For complete confidence in the absence of leaks at the joints, coat them with silicone sealant.

On the copper tube of the compressor, through which air is sucked into it, we install a gasoline filter for fine fuel purification, put the second gasoline filter at the outlet from the compressor (we cut it into the rubber hose between the compressor and the receiver), tighten with clamps. We connect the wires to the pressure switch, their ends are connected in series with the compressor power supply cable – after the switch, closing / opening only one wire. The task of the pressure switch is to open the compressor supply circuit if the pressure in the receiver exceeds the set one and close the circuit back if the pressure is below the set.

We attach a reinforced pvc hose of the required length to the outlet of the oxygen reducer, connect a gun to it for painting or inflating tires – in a word, the device whose functions are needed at the moment.

So, the compressor is created, as they say, by hand and is ready to go. For greater convenience, it is necessary to fix its elements on a wheelbase – for example, on a frame from a corner, connecting the refrigerator compressor, receiver and frame with two metal strips with rubber pads placed under them, tightening the strips with bolts.

Homemade compressor – operation and maintenance

Make sure that the compressor does not overheat – in half an hour it warms up to an average of 450C and this temperature harms it, so we take breaks from work.

Once a year, you should completely replace filters and oil, drain the oil from the receiver (it will be there, so drain it). Despite the reluctance to carry out partial disassembly of an externally properly working compressor, this must be done!

How to choose the compressor you really need

First of all, you need to assess the upcoming compressor load – will it work for a full 8-12 hours or the need for it arises periodically. The cost of compressors that are outwardly similar in a number of parameters (receiver volume, power, energy consumption) may differ many times, depending on the types of compared units.

When choosing a compressor model, you should pay attention to:

  • air pressure generated by it (indicated in bars, atmospheres or kg / cm2). The documentation contains the parameters of the working and maximum pressures, the first means the pressure at the maximum efficiency (it may not be indicated, i.e. the manufacturer guarantees the equality of the maximum and working pressures), the second – the highest pressure allowed for a given compressor. Nuance: most often the manufacturer does not indicate the time period during which the compressor can produce the maximum pressure, respectively, this figure is only partially correct. A short-term peak power for cheap compressors cannot put them on a par with expensive units that can hold maximum pressure for hours.
  • productivity per unit of time (indicated in m3 / hour, l / min or m3 / min). For foreign products, this indicator means the volume of air absorbed by the compressor from the atmosphere, for domestic products – the volume of air leaving the compressor (GOST 28567-90). Calculation of performance according to GOST will give lower data figures than suction performance – the reason is the loss of air volumes in the discharge (suction) valves of a piston compressor, the formation of “dead” air volumes in the working cavity of this unit. The actual performance of the compressor “at the outlet” differs from the volumes “at the suction” by 30-40% downward, i.e. the power declared in the passport of any imported compressor is lower than that of similar domestic models.

When choosing a compressor in terms of power, you need to summarize the compressed air requirements for all pneumatic devices connected to it at the same time and remember that it is impossible to load the unit by more than 70% of the power “according to the passport” – it will start to fail from overload. Add 150-200 l / min to your real compressed air needs – a compressor of this capacity would be the best choice.

Compressed air requirement of some pneumatic tools:

Name Pressure, P (bar) Average air consumption, G (l / min)
(for domestic compressors)
Average air consumption, G (l / min)
(for imported compressors)
Paint gun 4 240 400
Nailer 4 140 200
Screw gun 6 290 410
Pneumatic chisel 6 400 460
Cutting machine 6 1200 1700
Air drill 6 700 1000
Wrench 6 700 1000
Jackhammer 6 1300 1900
Puncher 6 3400 4800

Piston or screw compressor

Arguments in favor of a reciprocating compressor:

  • cheap, in comparison with a screw of similar power – 5, or even 10 times;
  • resistant to operation in dirty conditions (low probability of jamming from dust);
  • heats up less and emits heat;
  • Spare parts and maintenance are cheaper than those of a screw compressor;
  • ideal if the compressor is only needed intermittently. It is not afraid of frequent on / off cycles.

Arguments in favor of a screw compressor:

  • several times superior to reciprocating compressors in terms of service life, durability and reliability, weight and dimensions;
  • minimum noise level, almost complete absence of vibration – does not require a separate room and a special foundation;
  • ideal for high performance (load) conditions;
  • its efficiency is 25% higher than that of any piston;
  • contains a small number of quickly wearing parts in its design – it is economical to maintain;
  • allows remote control (via microprocessor) via any communication channels.

Compressor selection criteria – nuances

If we compare a compressor containing oil in the system and an oil-free one, the resource of the first type is higher, but filtration of vapors and oil droplets at the air outlet is required.

The capacity of the receiver is not as important as it looks at first glance – it is much more important to study the characteristics of the compressor, on which its actual performance will depend. By the way, some manufacturers offer compressors without receivers – make sure that a container for compressed air is included in the package of this model, otherwise you will have to buy a receiver cylinder separately (for operations with pneumatic tools that do not require high pressure, and with sufficient compressor performance, you can do without a receiver ).

Find out what type of cooling is used in this compressor model – if it is water cooling, you should find out the estimated water flow for a certain period of time.

Certain gases require certain compressor models – they are not universal!

The belt drive in the piston compressor design makes it more durable, allowing it to work at low speeds while maintaining full performance. Belt driven compressors are “semi-professional” and can produce 16 bar outlet pressure.

The resource is determined, first of all, by the operating temperature of the compressor head – these data are not “traditionally” indicated in the unit’s passport. However, the operating temperature can be judged by the crankshaft speed – the higher the speed, the higher the temperature of the compressor head. Accordingly, among the units of the same performance, it is worth choosing the model, in the passport of which a lower number of revolutions is declared – it will last longer.

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