Context :

Aims : Recently , the study of globular cluster ( GC ) color-magnitude diagrams ( CMDs ) has shown that some of them harbor multiple populations with different chemical compositions and/or ages .
In the first case , the most common candidate is a spread in the initial helium abundance , but this quantity is difficult to determine spectroscopically due to the fact that helium absorption lines are not present in cooler stars , whereas for hotter GC stars gravitational settling of helium becomes important .
As a consequence , indirect methods to determine the initial helium abundance among populations are necessary .
For that reason , in this series of papers , we investigate the effects of a helium enrichment in populations covering the range of GC metallicities .

Methods : In this first paper , we present the theoretical evolutionary tracks , isochrones , and zero-age horizontal branch ( ZAHB ) loci calculated with the Princeton-Goddard-PUC ( PGPUC ) stellar evolutionary code , which has been updated with the most recent input physics and compared with other theoretical databases .
The chemical composition grid covers 9 metallicities ranging from Z=1.60 \times 10 ^ { -4 } to 1.57 \times 10 ^ { -2 } ( -2.25 \lesssim [ Fe/H ] \lesssim -0.25 ) , 7 helium abundances from Y=0.230 to 0.370 , and an alpha-element enhancement of [ \alpha /Fe ] =0.3 .

Results : The effects of different helium abundances that can be observed in isochrones are : splits in the main sequence ( MS ) , differences in the luminosity ( L ) and effective temperature ( T _ { eff } ) of the turn off point , splits in the sub giant branch being more prominent for lower ages or higher metallicities , splits in the lower red giant branch ( RGB ) being more prominent for higher ages or higher metallicities , differences in L of the RGB bump ( with small changes in T _ { eff } ) , and differences in L at the RGB tip .
At the ZAHB , when Y is increased there is an increase ( decrease ) of L for low ( high ) T _ { eff } , which is affected in different degrees depending on the age of the GC being studied .
Finally , the ZAHB morphology distribution depending on the age explains how for higher GC metallicities a population with higher helium abundance could be hidden at the red ZAHB locus .

Conclusions :