Portable-compressors; the local tire big-box store had a special on recently. If you went in for routine service, they would give you an air compressor, free of charge! I took advantage of that offer as I do most times when something is free.
My free air compressor plugs into my cars 12 volt power outlet. When switched on, it will discharge compressed air at up to 125 PSI.
Of course the flow rate of air from this air compressor is very low. So low that it might take what seems like forever to fill up a tire or air mattress. But...it is a portable air compressor, for sure!
So, when looking for portable-compressors, you have to first decide what are the applications you need compressed air for?
If you are not sure about your air tool consumption here is additional information of air tool compressed air demand.
The little compressor shown below is just that, a little air compressor.
Yet, if we had a low tire and were stuck somewhere in the wilderness, and we could plug this air compressor into the vehicle power supply, we could sit in the relative comfort of the vehicle while it slowly...oh yes, oh so slowly, brought the tire up to pressure.
You do need to be a bit careful when using these little air compressors, too. Some of these low flow, compact air compressors can put out 250 PSI, and with enough time might blow the tire, or an air mattress or a beach ball to smithereens!
While the air compressor shown in the first photo will need a special power receptacle to plug it into the car, the one to the right is a portable air compressor that can be carried from place to place on the job site, and plugged into a standard 120 volt outlet wherever compressed air is needed.
If the portable air compressor you choose does not come equipped with an in-line compressed air filter you will want to add one if your air use is fairly constant.
The majority of portable air compressors I have seen are factory equipped with a compressed air regulator. Air regulators offer benefits such as reduced air/energy use and reduced air equipment wear. You do not drive your car with the foot to the floor on the gas pedal all the time, why operate your air tools with the air pressure equivalent? Run your tools at the lowest working air pressure for longer tool life.
Too, an air regulator, set 10-15 PSI below the compressors low pressure set point (also known as cut in pressure point) will help ensure that your air-using application will see a steady stream of compressed air at a non-varying pressure. As the pressure in the receiver peaks and wanes due to compressed air use, with the regulator set below the compressors lowest pressure setting, your application should never experience a change in that constant, lower pressure flow.
That is, unless the air tools demand for compressed air outstrips the ability of the compressor to keep up. In this case, the pressure coming from the receiver will continue to decay down past the regulator setting, and it will keep dropping as long as more compressed air is being used by your air tool than your compressor can generate.
On the other hand, if your application is, for example, air brushing, then you do not want oil spraying out with the paint, and no lubricator would be installed in an air line to a paint spray gun.Here is more information about portable air compressors.