temperature of raiser and downcomer in boiler system

Cannot understand what you are looking for? What do you mean by raiser temperature and down comer temperature?

In boilers on saturated steam, the steam pressure is interrelated with temperature, the higher the pressure the higher the temperature and vice-versa. To ensure a good quality steam you must have both a pressure gauge & temperature gauge installed together and refer to the steam tables on all reading for, the pressure gauge will mislead you but the temperature gauge won't.

Eg. You might be reading 10 Bar on the pressure gauge but if you have 5% wet steam & 5% air then it should actually be at 9 bar but the pressure gauge will indicate 10 bar and when you cross reference with the readings of the temperature gauge from the steam tables you will find the pressure indicated 10 bar is actually 9 bar.

Does this answer your riser temperature & down comer?

thanks for your replay,

i need to know why the temperature of the vapor phase equal to the temperature of the liquid phase inside the boiler .

Thats too long to detail. Say because it is pressurized. Read the Books please.

Well, that's what a boiler does. It changes the phase at a uniform temperature. Which is where latent heat comes in. Like when a kettle boils, steam is generated though the water (and the steam) remain at the same temperature. Nearly 100degC at sea level, if memory serves correctly...

Steam Tables?

Wikipedia?

The downcomers are larger tubes holding a larger volume and thus the furnace heat and gases do not have the same 'steam generation' effect as they do on the smaller diameter generating tubes. Also, downcomers are not located in the direct path of heat and gasses (at least not on marine boilers that I'm familiar with) specifically so they do not 'generate'. In the generating tubes, there is a higher rate of the transfer of heat (latent) where the steam is released up to the steam drum. This creates a difference in density between the risers and downcomers and you thus have circulation.