Calculating cooling channel diameter is a very much subjective to mould configuration & layout. However it is generic to estimate heat extraction with respect to injected volume (shot volume), depending on the plastic material’s solidification characteristics (PVT equation of state) difference between injected melt temperature and maximum solid state temperature of material, you can estimate total amount of heat to extracted within cooling phase, to achieve such an heat transfer rate your cooling ducts sizes and shapes have to be designed.
Remember cooling should neither be minimum (long cooling time) and maximum (long fill time), rather it should be optimum.
If anyone has designed injection mould cooling channel based on HVAC, that design will miserably fail without a pinch of doubt. Because that HVAC design are fundamentally based on Newtonian fluids and plastic melt heat transfers are non-newtonian type. So injection mould cooling design is very much unique to itself.
All plastic have a rate in which they transfer there heat, so in this case depending on the type of plastic will depend on the amount of time needed to allow the plastic to transfer the heat into the cooling channels of your injection mould. This is better known as the heat transfer coefficient and it’s different for all materials, plus all heat dissipates at 90 degree angles. Cooling design is very unique, there’s no 2 parts alike so your cooling designs are going to be different for every injection mould you build, but the guide lines you use should be the same, so many water lines for amount of surface area of the part, the lines need to be within a certain distance to each other, the lines must be within a certain distance to the part surface and so on.