Hydronic Flow Optimization

Design Evolution

Older constant volume system designs use considerable energy compared to more modern methods because pumps see a relatively constant head pressure having to continuously pump water throughout the entire piping network. In the example shown in Figure 1: Constant Volume, the pumps are either on or off, and the chillers supply a constant volume or flow of water via distribution piping to our consumption side of the loop.

On the consumption side, air-side loads are transferred to the heat transfer coil, and flow is varied through the coil via three-way valves bypass to maintain load requirements. The total flow through the distribution mains, however, remains constant. A balancing valve is required to balance out the circuits to ensure proper flows to each load.

A variable volume secondary design (see Figure 2) distributes chilled water to the consumption coils that are decoupled from the generation or primary loop. The advantage is that a secondary pump is used with a variable frequency drive (VFD) to control flow, which allows a two-way valve to be used instead of a three-way valve at the consumption coils. The benefit of this design is that the pumping energy is greatly reduced to just the flows needed to each load. However, a balancing valve is still required to balance the secondary circuits.

A variable primary volume design still maintains a generation or primary loop and consumption or secondary loop (see Figure 3). With this configuration, the chillers are selected so that they can unload with variable flow through the chillers. Each chiller gets its own VFD on its respective pump. The bypass now gets a control valve that effectively directs water into the consumption or secondary loop. The advantage of the variable primary volume design is that only what is needed is pumped through the chillers and the bypass control valve directs the flow to the consumption or secondary loop. The entire distribution piping network is seeing only the flow it needs when it needs it, effectively reducing any unnecessary head losses in the system. This allows a two-way valve to be used instead of a three-way valve at the consumption coils and a balancing valve will still be required to balance the secondary circuits.

Overflow Effect on Coil Efficiency

Coils can encounter overflow scenarios that lead to the erosion of system designed Delta T. The most common is overflowing the maximum limitation of the coil due to pressure fluctuations in the system or due to an incorrect understanding of those limitations, particularly in chilled water coils. As seen in (Figure 4) – a typical chilled water coil’s heat transfer curve plateaus out as it nears maximum flow. In general, if 200% flow is forced through the coil only 10% more heat transfer will take place. This can be a primary cause of delta T erosion as the extra flow will not pick up the additional heat due to coil design limitations. The impacts of delta T erosion at just a few of the heat transfer coils then can cause two negative systemic effects.

First, recall our waterside heat transfer equation stating that the heat transfer potential of our water is directly proportional to our flow rate (GPM) and our delta T (ReturnSupply temperature). As coils are overflowed in the system our delta T shrinks, effectively derating the rest of the coils within our system. As this starts to occur the control system may react by increasing flows across the system which will lead to further delta T erosion as the coils were selected at the initial delta T conditions. As more flow is required at this lower delta T to satisfy load conditions, our pumps are negatively impacted by the increase in energy used to circulate this additional flow. Second, in the case of chillers and heat pumps, overflow causes inefficiencies in the energy generators. Overflow of dedicated consumers can lead to a return temperature lower than the nominal design value in cooling mode and a return temperature higher than the nominal design value in heating mode. This decreases the energy efficiency of boilers and chillers by 2% and 3%, respectively, because a decrease of the evaporation temperature of a chiller below its design value by 1° decreases its performance by about 3%.

Dynamic-Balancing against Pressure Differences

One way to address the inevitable pressure fluctuations in hydronic systems is with pressure-independent control valves (PICVs). Within their range of operation, PICVs are not affected by pressure fluctuations in the building’s hydronic system. This is called dynamic-balancing or auto-balancing.

This basic functionality is achieved by an internal differential pressure regulator working in series with the main flow control valve and regulating the pressure differential of the flow control valve using a pressure inlet and membrane. Hence, the flow across the entire device is independent of the pressure changes in the system, and flow is determined only by the travel of the control valve. When used with a terminal unit, PICVs ensure proper flow, optimizing heat transfer, and system performance. In heating and cooling applications, the auto-balancing function generates energy savings by:

• Eliminating heat exchanger overflow at any time and under any operating condition;
• Improving control accuracy by eliminating hydraulic pressure dependency between neighboring control loops; and
• Enabling advanced energy distribution strategies by eliminating the risk of heat exchanger starvation.

Variable Frequency Drive Considerations

Reducing the speed of pump motors using variable frequency drives (VFDs) lowers energy costs by reducing the amount of energy it takes to operate the pumps. A VFD often is specified to save money by reducing energy consumption in pumps or other motor loads that may be found in a typical building.

The following best practices provide engineers with information on how to specify a VFD to meet load conditions while achieving efficiency. To effectively specify a VFD, engineers must understand these key points:

• Load (operating power, torque, and speed characteristics)
• Duty cycle (percentage of operation at 100% load, 50% load, etc.)
• Desired benefits by using a VFD (energy savings, soft start, controllability, etc.)

Understanding the load is the first step to determining what can be gained by deploying VFDs. In building hydronic systems, the most common type of motor providing power to a load is a three-phase AC induction motor. For a given power rating, the motor base speed is inversely proportional to the effective torque rating for that motor. For example, selecting a 1,800 rpm motor instead of a 1,200 rpm motor reduces torque by one-third. Once a load’s starting requirements are determined, the next step is to look at the load’s running requirements. In building systems, excluding constant horsepower and constant speed/ torque loads, typical loads that can take advantage of VFDs are:

• Variable speed, variable torque (fans, blowers, and centrifugal pumps)
• Variable speed, constant torque (positive displacement loads, such as screw compressors, reciprocating compressors, or elevators).

To support a characteristic load, select a motor to meet a specific starting requirement and running output power, torque, and speed.