Needle Valve - A valve having a slender point fitting into a conical seat, used to regulate accurately the flow of a liquid or gas.

Nipple - A pipe coupling threaded on both ends.

Noncondensables - Gases mixed with refrigerants that cannot be condensed at the same temperature and pressures as refrigerants.

Nonconductors (Insulators) - Nonconductors resist electron flow. The atoms have almost no free electrons. Examples would be: A perfect vacuum, quartz, glass, ceramics, rubber, wood, plastics.