In a situation where you want to have several pumps for a particularly big multi block loop, and you are trying to work out the best combination. Multiple blocks in large loops where there is bigger volumes of water requires more head pressure to operate. Many people assume D5's to be the best choice and would put them in serial as parallel pumps/loops are no longer fashionable in the watercooling world!
Serial pumps add flow rates and keep the same head pressure, while parallel is visa versa (the numbers are a little less than the sum in the real world, but that irrelevant). D5's sport the highest flow with moderate head, while DDC's have it the other way round, with the DCP 4.0 pump in the middle.
So with two pumps:
For D5
Serial: same moderate head, huge amount of flow. In smaller loops and high flow loops, they will have the fastest flow rate resulting. In large loops with high restrictions, the pressure may drop far enough to prevent the water from moving much at all. This is due to pressure drop being directly proportional to the square of the flow. These super high flow set ups will find the pressure drop massively (not an issue for smaller loops as the resultant flow is enough)
Parallel: In small loops for D5's you will see no performance change going parallel over serial due to the pumps having a large enough flow by itself to keep the loop temps uniform. Larger loops benefit from parallel D5's as their moderate head almost doubles while flow rate is the same as a single pump, which is more than enough to keep temperatures uniform. The pressure drop would be almost four (remember that drop is proportional to square of the flow) times less compared with the serial looped pumps. The doubled head would easily handle larger external loops with restrictive blocks and large volumes of water, while the naturally high flow rate of a single D5 will be enough to keep temps fine.
For DDC's and DCP's its the reverse. I said that the 4.0 is in between the DDC and D5's in spec but the truth is that there is little benefit to having more flow rate than needed to keep temps uniform and that watercooling pumps actually have a fairly impressive amount of flow but a huge amount of head, so as a result the 4.0 shares similar advantages with the DDC when in series compare to parallel.
In series these pumps flow rate, which is lacking compared to the D5, almost doubles. The increased flow rate is no where near as high as series D5's and the head of a single one is enough to push large amounts water in big loops even with the new almost doubled flow rate.
In parallel these pumps tend to struggle in large loops or small loops with many blocks. The water will be pushed around even if you were to have 6 meters of tubing go vertically straight up in the air! Sounds impressive but the resultant flow of these pumps in parallel may end up being low enough that temperatures are not uniform. High head means little if you have low flow in the loop due to multiple blocks.
