Splitting of data based on condition

Dear Expert,

I have 2 columns and few hundred thousand row of data. My first column has got the time and my second column has values. I have to split the data based on different timings that I have noted manually.

Eg)

Field1 Field2
4 0.05484
8 0.04985
12 0.04985
16 0.05484
20 0.04985
24 0.04985

I want to save data, based on the conditions say 12-20, 40-60, 1000-1020 onto 3 separate text file.

Thanks
Arun