Which of the following is NOT a design factor affecting compressor capacity?

Compressor speed is indeed a key factor affecting the performance of a compressor, including its capacity. The capacity of a compressor can vary with the speed at which it operates; higher speeds generally increase the volume of gas being compressed and, therefore, increase capacity. However, compressor speed is not typically classified as a design factor in the same way that the other options are.

Design factors relate more directly to the physical characteristics and configuration of the compressor itself, influencing its operational capabilities. Clearance space, for instance, affects the efficiency and volume that can be compressed within the cylinder. Piston displacement directly determines the amount of fluid the compressor can move per cycle, while suction and discharge valve designs impact how well the compressor can intake and exhaust gas, affecting overall performance and efficiency.

Thus, while compressor speed influences capacity, it doesn't fall into the category of design factors like the other choices do, which are inherent design specifications affecting the compressor's functioning.