Liquid can be boiled in closed container like vacuum chamber by controlling pressure but we can not boil liquid just by increasing temperature in a closed vessel for boiling, the vapour pressure of liquid must be equal to external pressure continously increases(volume is constant). There is not a particular temperature of which external pressure become equal to vapour pressure of liquid . Therefore in closed vessels, boiling cannot occurs. but in open vessels boiling occurs.
Hence, liquid in closed vessel take more times to boil.
