Since water is compressible only to a small extent , pressure in a boiler has r be increased after it is sent to the shell. If water is up to the brim and then as more vapor is generated, the vapor molecules will have no place to escape and hence the pressure will increase and higher pressure of vapor in the top level will again increase the temperature of water in the bottom, this happens till all of the water gets converted into vapor; thus pressure keeps on increasing. But if the water in the shell of water tube boiler is just say 1/10 the of the total shell capacity , then as vapor is generated , there is enough space in the shell and there is enough freedom for the vapor molecules and therefore pressure may not increase. In such a case , how can we deliver super heated steam at higher pressure ?