Primary Secondary Chilled Water System

[Buildtech2]

Currently, I am assessing the old primary secondary chilled water system. The system has 2 duty and 1 standby chillers with constant primary and variable secondary pumping system with a decouple pipe. the secondary pumping system is serving chilled water to total 10 low rise and high rise towers and this system is dedicated for only IT rooms in all towers. I have noticed that secondary pumping system is being controlled by a common DP sensor and it is apparent that a single DP sensor cannot be used for supplying chilled water for all 10 towers because we cannot assure all the cooling coils can receive design chilled water flow. Therefore, I would like to recommend that each tower shall be provided with a separate tertiary pumping system with its own DP sensor for enhanced control and adequately supplying the chilled water. Appreciate if anybody can put their valuable thoughts.

 

[BronYrAur]

I would put a DP sensor across each riser and have the pump look at all setpoints. One setpoint will ultimately be "satisfied", and the others will all be "over-pressurized." Unless you have extreme differences, the end-use control valves should be able to overcome the additional DP.

If you have one or two risers that are always lacking for enough DP, then you can probably get away with just putting sensors there. The others should get enough if you maintain DP across the ones that are historically chronically low.

Pumps everywhere will be costly.

 

[GT-EGR]

Agree you can monitor DP in multiple locations and satisfy the worst case. If the pressure losses aren’t extremely different the valves will do the work

Having multiple tertiary pumping systems usually needs more of a good reason than just slightly different DP, it becomes worth it if you have buildings with different operating schedules or low flow requirements or sequences within the building.

 

[SAK9]

Tertiary pumping should be used if the head requirement on your secondary pump goes higher than say 3 bar. A very high pressure secondary pump say 4 or 5 bar creates balancing and over pressure issues for buildings that are very close to the pump. Control valves will be operating nearly closed resulting in poor control. Most district cooling plants end up using tertiary pumps for this reason

 

[LowDeltaTSolver]

A tertiary pump system is not required and makes the system pumping control more complex.

The current DP sensor can be effectively used to control the secondary pump VFD if the current DP sensors set point can be reset based on Chilled Water Control Valve positions.

At least one valve position should be greater than or equal to 90% open at all times.
The differential set point should be reset as required to maintain at least one control valve at 90% at all times.

It is assumed the secondary pump VFD speed is being modulated to maintain DP setpoint.
If that is the case, resetting the DP set point can be an issue.
It can lead to instability in the system if it is modulated like control valve.

It is best to modulate the VFD in % speed increments over time.
When this is implemented there should be an increase in secondary temperature differential and a decrease in secondary pumping energy required.

Instead of investing in tertiary pumping, you may want to consider replacing the existing constant speed primary pumps with variable speed primary pumps.

This type of approach (Variable Primary Flow - Variable Secondary Flow VPF-VSF) is covered in the attached October 2014 ASHRAE Journal Technical Feature "Simplified Chiller Sequencing for Primary/Secondary Chilled Water Flow System"

The article discusses how the primary pump VFD speed is controlled and how sequencing of chillers is accomplished with only four temperature sensors and a simple algorithm.

No expensive flow sensors, differential pressure sensors, expensive control valves or complex control algorithms as required by a Variable Primary Only System.

 

[Buildtech2]

Thanks everybody for your valuable advice. As advised by LowDeltaTsolver, variable primary flow would be a best option. I agree uptocertain extent because VPF system has few draw backs as it cannot be used for the system where high reliability is required because there may be possibility of sudden loss of flow to a fully loaded chiller when adding a chiller. Such sudden drop in flow may cause safety trip.
How about considering DPCV valves at the base of riser of chilled water riser for each tower (differential pressure control valve). Since demand for chilled water will be constant due to IT rooms, so it will try to maintain constant chilled water flow to each riser.
Issues with VPF system can be resolved by careful design of control system but we have to avoid using it for the system where high reliability is required.