there are a few reasons why we need to purge out the steam condensate from the system before operation. first,to prevent water hammering. water hammering is a phenomenon where there is a sudden surge of high velocity steam along the piping and causes the condensate to hit the pipe walls with great impact and this can lead to the damage of plant equipment such pumps and heat exchangers and rupture of piping. water hammering may also cause backpressure in the system,thus back flow may occur. secondly, to prevent piping corrosion. the steam condensate contains dissolved solids that will attack the metal piping in the system. the dissolved solids will also precipitate out and thus causes fouling problem. thirdly, to ensure that sufficient heat is transferred to the process fluid. steam condensate may absorb some of the heat from the inlet steam,so less heat is available for the process fluid and thus heat transfer efficicncy is reduced.
Depends what sort of heat exchanger you're talking about. In a beverage dispensing application where you might be referring to a glycol chiller for beer chilling, the heat exchange temp may be as low as -15 deg C. Obviously at this temperature condensation would freeze and block HX plate. There are other reasons as well, for instance condensation travelling back to the compressor could have disastrous effects
Because if you have condensed water present then it could freeze and cause damage.
Purging of condensate is always recommended to avoid hammering resulting in severe vibration and noise in the system when steam is charged.
