Without the "sieves" you can do:
if ( cel >= 29.0 && cel < 30.0)
The && means AND in the 'c' language. You can only compare a variable with a single value.
Your sketch with the sieves had the sieves upside down. You started with "if (cel <= 29){", but all the others are also below 29. Comparing it after that for 28 was not executed, because it already dropped out of the sieves at the first sieve.
Note that I use "29.0" and not "29". When using 'float' variables I like to use 'float' numbers. It is a reminder for myself, but also the compiler knows that it should be done with float numbers.