A steam trap is a device which distinguishes between water and steam and automatically opens a valve to allow water to pass out but which closes to steam and traps it. Traps are of three broad kinds.
Saturated steam and the condensate it is forming have the same temperature. A steam trap which makes its choice by temperature must impose a delay on removing the condensate, because the condensate must cool to below steam temperature before the trap can make up its mind that it must open. On the face of it this seems to put the thermostatic traps at a great disadvantage. This is only true up to a point. Thermostatic traps have certain advantages for certain applications which will appear in subsequent sections.
Table of characteristics of the various kinds of steam traps