Question

Chiller water out in 7 degree and return 12 degree

Answer 1

Yes you are correct the description can be confusing.
The majority of chillers operate on return water temperature control for stability of the chiller, easy control and for straight air conditioning this is ok – but it works backwards – this explanation is a brief overview.
The chiller is capable of a certain capacity in KW/BTU’s and this creates an ability for the chiller (due to is engineering such as the heat exchanger design and the water flow in liters/min or gal/min) to provide a temperature difference across the chiller (Evaporator Split or Delta T)
Eg;
If a chiller design is capable of 7 deg C split (Example only) at the specified water flow at 100% capacity. Therefore if the return water temperature is 14 degrees then the leaving should be 7 degrees.
The operation works thus; If the chiller is running at 50% load and the return water temperature is correct (14 Degrees) the chiller capacity will stay the same but the leaving water temperature will be approximately 10 degrees. This will remain the same while the Computer Room Air Conditioning Units (CRAC) are satisfying the room temperature, as the correct KW/BTU’s are being removed. The CRAC units will cope with the higher chilled water temperature if it is still with in the design range.
If the room temperature rises the CRAC units will require more chilled water and remove more heat, this raises the chilled water temperature returning to the Chiller. The chiller will then see the return water rising and the chiller will increase it’s capacity (EG 70%) and the chiller leaving water temperature will drop giving the CRAC units more capacity and a lower temperature. The reverse will occur is the load in the computer room drops.
I agree it is not the best way but providing the chilled water design and the computer load is balanced, it is sufficient. Remember you are trying to remove heat not control water temperature. (Provided the water temperatures are within the design range)
It you have water cooled servers or main frames then the above is not sufficient and you need leaving water control and most chillers have to be designed for this as the chiller needs to be considered as a supplier of process chilled water not air conditioning chilled water. A lot of modern chiller control systems are already set up for a duel type control (Even if the Technicians don’t understand it) where the control can be on entering with a reference to the leaving temperature to trim the controls or leaving control but it is a lot harder to obtain stability thus the technicians try to avoid it. Don’t let them bluff you.