You can estimate the amount of water in the pipes by multiplying the total length of pipe in the system by the pipe's cross sectional area. If there are multiple diameters of pipe in the system, you can simply calculate each size separately (using the total length and area of each one), then add them together. There are multiple websites where you can look up the cross-sectional area for various types of pipe (including sch 40 steel). At the very least, you can look up the inner diameter, then calculate the area as 0.7854*ID^2 (That's \pi r^2, the area of a circle). You just need to make sure to keep your units straight. For example, if the diameter or area is given in inches (or square inches), use the total pipe length in inches as well. That way, your volume will be in cubic inches, which you can easily convert into cubic feet, cubic meters, liters, gallons, etc.
If you have an architectural drawing for the plumbing, that would make your life easier. Otherwise, you may have to do a lot of estimating for the pipe lengths.
As for the volume in the chillers themselves, this should be listed somewhere in the manufacturer's specifications. Can you look up the specs or call the manufacturer?