Quite often, the compressed air pipe connected to an air compressor is sized to the same diameter as the connection on the compressor outlet. But this can be a mistake because it may lead to excessive pressure loss that can affect the operational efficiency of the compressor.
For example, one manufacturer, to save costs, puts a 1-in. connection on their 40 hp (30 kW) compressor outlet. Then the compressor is tested, the pressure drop developed in the short length of pipe internal to the unit does not cause problems. However, if the same size is used for logger connections in a real industrial plant, it can cause excessive pressure loss. An additional problem develops if more than one compressor of this same size is combined to feed the compressed air system using the same sized pipe.
Some leading compressed air professionals have determined that if the maximum velocity of the compressed air inside the pipe is kept between 20 and 30 feet per second, then minimal pressure drop will develop across reasonably long compressed air piping and fittings (for very long runs the piping must be oversized). Thus, for the example 160 cfm, 40 hp compressor, piping size of 1.5 in. would be recommended at an operating pressure of 100 psi (see table). If two of these compressors are to operate at the same time, then the piping header in the compressor room should be sized at least 2 in.
Note that it is often understood that the use of smooth bore piping like stainless steel, copper or aluminum will reduce the pressure loss per unit length, so smaller equivalent piping can be used. This makes sense for straight runs of piping, but the higher velocities that develop in direction changes like 90° turns or piping tees will cause higher than desired pressure loss. For this reason downsizing the piping is not always desirable.
A table is presented for your reference at the beginning of this blog. Note that these calculations are for a system pressure of 100 psi, but as the pressure changes the velocity of the compressed air within the pipe will also change, so new calculations need to be done at worst cases conditions. The goal should be to provide enough piping capacity to ensure the pressure loss across the entire system is less than 2% of nominal pressure (excluding air dryer and filters).