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Air compressors, including portable air compressors, arrive in at least three varieties:

  1. a handheld air compressor, for car tires,
  2. a block air compressor, without a tank, still for car tires, but perhaps suitable for continuous usage, and
  3. an air compressor with an adjacent air tank, with varying capacities (1 to 5 gallons).

The first two are easy enough to understand. Air comes in from one side, and is compressed in the other.

The practical difference in usage is less clear.

What is the use of the tank in an air compressor fitted with one? If the idea was to fill the tank with compressed air (say up to 120 psi), disconnect the power supply, and take the compressor around to the usage site (whether it's just for tires at 30 psi, or for power tools), then that appears to be a moot point now that air compressors with Lithium-Ion batteries are available. The tank must be needed for something else. Is it just cfm (cubic feet per minute) of air? But the first two kinds also reach 100+ psi, and so they could conceivably be used for the same applications, just not on a continuous basis, and/or with a cooling down period, which is alright for once-in-a-long-while usage.

How does the air circulate? Is it necessary to first fill the tank, or can an air compressor be used directly?

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  • A tank usually for larger jobs so compressor does need to be working all the time, it can cycle on and off(cool down). A non tank compressor good for bike tires, will get quite warm filling a car tire, and maybe get too hot for a big truck tire(and die). A compressor will send air to tank first and then out the hose, so blowing dust bunnies can start right away, but adding pressure takes some time.
    – crip659
    Commented Nov 10, 2021 at 23:39
  • You're adding an important detail (this is probably obvious to anyone who used one). It is necessary to first fill the tank before using the air compressor. In other words, if the air compressor is in "parked mode" (with the tank empty), then one must first fill the tank and then use the compressor. In a sense this is a small delay in usage. An ordinary tire inflator can be used immediately after it's switched on.
    – Sam7919
    Commented Nov 11, 2021 at 13:18
  • A tank is never empty, it can have not enough pressure to pump air out of it(equal pressure to atmosphere). It is picky.
    – crip659
    Commented Nov 11, 2021 at 13:32
  • @crip659 Understood that the pressure in the tank is never less than 1 bar. My point was that after you switch on a tire inflator, you can at the same moment start using it, whereas after you switch on an air compressor, you must wait for a few seconds (or tens of seconds for a larger tank/weaker inflator?) before you can use it. There is no alternative direct connection from the motor. The only connection goes through the tank.
    – Sam7919
    Commented Nov 11, 2021 at 14:42

2 Answers 2

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Tanks maintain a pressurized reservoir to allow for spiky usage (refilling a pneumatic tool) without having to cycle the compressor pump/motor/engine every time. This prevents short cycling for energy-efficiency and uses fluid pressure, which tends to be very quickly responsive compared to pumping to equalize the pressure during usage. This is the same idea behind well pressure tanks or municipal water towers.

Even in applications where the compressor is running continuously and the tank is filling as fast as it's draining, for example when you're close to the "maximum" rating of the compressor (e.g. high intensity framing nailer applications), the tank is critical to smooth out the spikes in usage. The tankless alternative, to short cycle every time you fire your framing nailer, is sure to be much slower.

The tank adds no utility for sustained throughput applications like tire inflation, so there's no need for one in tire inflators, portable or otherwise.

How does the air circulate? Is it necessary to first fill the tank, or can an air compressor be used directly?

I'm not aware of any common setup where a pneumatic tool connects to a compressor pump directly, but others may know better. Typical compressors have the pump push air into the tank and then use a regulator to send air to your tool. You could open the regulator to the desired pressure and the tool becomes operable once the system reaches that pressure. You can operate your tool while the tank is filling, so it's not technically necessary to "fill" the tank to full in order to start working. But I don't know of any way to bypass the tank entirely.

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    You've probably seen one and not known it, but large tow-behind air compressors that are made for industrial jack hammers and sand blasters do not have a tank. These are made for high capacity air tools that require over 100 CFM at 100PSI constantly. They are screw-type compressors similar to a supercharger on a car engine.
    – JPhi1618
    Commented Nov 11, 2021 at 16:42
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The tank is a buffer. Some benefits come to mind:

  1. With a tank, it becomes possible to use tools which require shorts bursts of air at higher flow volumes than the compressor/pump itself is capable of delivering.
  2. Several compressor designs produce a pulsating flow of air: when the pressure inside the pump exceeds the opening pressure of a check valve, the valve pops open and a burst of compressed air passes. Without the buffer of a tank this bursty flow of air would cause vibrations and unsteady operation in some tools (die grinder, drill, etc)
  3. Cost of the mechanical parts may be reduced. Consider a compressor/pump whose output goes directly to a valve. What happens if the valve is open, the pump is running, and suddenly the valve closes? Pressure spikes inside the closed space. To prevent rupture of piping or gaskets, breakage of pistons, valves, connecting rods, and so on, these would all have to be strengthened significantly to endure the stress. A tank connected to the pump output ensures there's always some place for a little more air to go without a dramatic increase of pressure.
  4. The control system is simpler and/or safer. Pressure control without a tank could involve variable speed or torque requiring a more costly motor design, more complicated motor driver circuitry, etc. It's much harder to start a loaded compressor (ie head is under pressure) as well. The simple tank-mounted pressure switch often provides an unloader valve to bleed away pressure in the compressor head so the subsequent re-start will be easier.
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    you forgot one: not having to listen to the d#@$ thing the whole time you're working...
    – dandavis
    Commented Nov 17, 2021 at 5:07

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