Its fittings of piping, pipeline.

Used for change direction

Example 45deg,90deg,

Elbow center

4" 45 deg - 64mm(4×16.1)

4" 90 deg- 152mm(4×38.1)

Elbow mainly used change the direction purpose

Three types of elbow

Long radius

Short radius

Standard radius

For STD 1.5 dia inch

I KNOW .ELBOW CENTER 1.5D

Elbow is that rotate direction 

Formula of elbow center :

Tan(degree/2)×25.4×1.5×size of pipe

elbow , there is many type 1d, 2d, 3d, 4d, 5d,ect

mostly cuclution as , 90*- 38.1xinch, 45*-15.8xinch