Evaluation of HVAC System Configurations for Multi-Tower High-Rise Project with VPF chiller system

[smali1]

I am currently conducting a review of the HVAC design for a project comprising three high-rise towers situated at a single location. These towers are air-conditioned by a singular variable primary flow chiller plant with a total capacity of 15,000 tons. My concern lies in understanding how a single variable primary flow chiller plant, equipped solely with a variable primary pumping system, can adequately fulfill the cooling requirements for all three towers using a single differential pressure transmitter control. This setup may pose challenges in identifying potential control issues, particularly in instances where certain sections of the chilled water piping might be experiencing a shortage of chilled water.

One approach to address this concern could involve the utilization of three differential pressure transmitters, with an average reading taken to effectively control the primary pumping system. However, I remain skeptical about the efficacy of this control method. Conversely, exploring the suitability of a constant primary and variable secondary system, complemented by an additional variable tertiary pumping system for each tower, along with independent differential pressure control for each tower interlocked with the tertiary pumping system, appears to be a more robust option.

While the variable primary flow system is recognized for its energy-saving features, considerations for pumping energy efficiency, the possibility of control issues, and the potential for system optimization warrant further research and discussion. I welcome insights and feedback from individuals who can shed light on this subject based on their experiences and lessons learned from previous projects.

 

[Az75]

From my understanding, the DPT (differential pressure transmitter) can be located at the furthest component of the critical loop (i.e loop with the highest differential pressure requirement for the necessary flow)to operate the entire system, it doesn't matter if its 3 buildings or 1 building it's hydraulically a single system. ensuring the furthest component of the critical loop receives the necessary flow at the required pressure would mean that all other components in the loop receive their required flows at pressures higher than this, so it's all good.

However I would definitely be concerned if all three high-rise buildings were connected to a single loop without hydraulic separation like heat exchangers separating the building height into secondary and tertiary loops, otherwise, the hydrostatic pressure due to gravity can cause the pipes to burst.

 

[SAK9]

VPF is not a panacea for all new buildings. Its use is limited to simple designs. A more complex campus like building layout still requires the conventional primary-secondary system. Functionality is more important than minor improvement in energy efficiency.

 

[HVAC-Novice]

You use valve feedback to reset the dP setpoint. Static pressure reset.

I don't know how large the towers are and how tall. Very tall towers usually have separation due to static pressure.

 

[moideen]

Despite the advantages of a primary-secondary system, problems with low delta T and water bypassing may arise in actual operation. The three structures may not be subject to the same load or will have different responsibilities from a practical perspective. It seems that the most efficient and controllable approach for this project would be to use individual heat exchangers for each tower with the main supply coming from a VPF.

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[HVAC-Novice]

" three differential pressure transmitters, with an average reading taken to effectively control the primary pumping system"

if you use an average, the line that is below average won't be satisfied. with static pressure reset you can just measure the pressure at the pump. there you have faster reaction and larger pressures that are easier to measure. You just need to make sure you have feedback from all valves. this may be an issue in an existing system.

what is your role? Are you re-designing?

 

[smali1]

@Moinbeen: If we opt to incorporate individual heat exchangers for each tower within the primary Variable Primary Flow (VPF) system, it is imperative to acknowledge that this modification would preclude the classification of the system as a VPF system. This deviation arises from the inclusion of new heat exchangers, each accompanied by distribution pumps functioning as decentralized secondary pumps. Consequently, the system would transition to a configuration akin to a primary-secondary system.

@HVAC-Novice: I am of the opinion that relying on averaged readings from three differential pressure transmitters for controlling the variable pumping system may not adequately address the challenge of achieving the design flow rate at the furthest index circuit. Therefore, I propose that we consider regulating the variable pumping based on the maximum demand for chilled water flow rate among the three differential pressure transmitters. I am currently reviewing the design of this project and trying to rectify any design flaw.

 

[smali1]

@SAK9: I think a properly designed control system of a VPF system can satisfy the cooling requirement of a large multi-tower project but we need to explore this opportunity.

 

[smali1]

I align to some extent with the perspective presented by Az75.

 

[HVAC-Novice]

Valve position indicates demand for chilled water. That will increase or decrease pressure setpoint.
This explains it for VAV, but is the same for water. You basically try to keep pressure as low as possible while still providing enough flow to the critical zone. Keeping all valves as open as possible to minimize throttle losses.

Average 3 pressure readings isn't great since that leaves at least one leg with too little pressure.

 

[Az75]

@smali1: Is there any reason you'd want to stick with a VPF system, if you are concerned about energy efficiency or low delta T mitigation those can be addressed with a variable primary-secondary system. basically with all the advantages of the VPF along with the reliable supply of a secondary loop.

I'm also interested to know how you are dealing with the hydrostatic pressure issue.