True Partners Consulting Engineers - CHILLED WATER BY-PASS

Understanding Bypass Line in Chilled Water Systems

As engineers, project managers or operators, you've likely encountered the term "bypass" in chiller contexts. But what does it mean? In chilled water systems, prevalent in Australia, a bypass ensures minimum water flow through chillers and pumps, preserving consistent rates even during low-demand periods. Insufficient flow can compromise efficiency and equipment integrity.

A bypass also enables operation control and damage prevention. It allows flow rate or temperature adjustments for flexible response to changing needs. Moreover, it safeguards equipment during startup or shutdown by diverting flow, preventing abrupt temperature shifts or water hammer effects.

Bypass lines are crucial in maintaining flow, controlling operations, and mitigating potential damage, thereby ensuring system efficiency and longevity. Different chilled water systems have corresponding bypass systems, each with distinct applications.

Bypass Use in Hydronic Systems: A Broad Perspective

The function of bypass in a chilled water system is influenced by the system type, bypass placement, and the intended design of the bypass system.

Herein, we delve into the specificities of bypass usage across varied applications:

1. Chiller Staging Control (On/Off) – Decoupler line

In primary-secondary chilled water systems, the primary and secondary circuits operate independently. The primary circuit generates chilled water, modulated by the number of operating chillers, while the secondary circuit distributes this water based on demand.

This configuration ensures that the minimum chiller flow is continuously maintained in the primary circuit and the demand for chilled water is met in the secondary circuit. However, to always meet the demand, the system ensures the chilled water flow rate in the primary circuit surpasses that in the secondary circuit (production > demand). This discrepancy necessitates a bypass line connecting the primary and secondary circuits, equilibrating their chilled water flow rate differences.

For instance, in the provided diagram below, the total chilled water flow rate demanded by the air handling units (AHUs) amounts to 150 L/s. Responding to this, the secondary chilled water pumps, with their variable speeds, adjust to match the demand. The primary circuit, however, cannot reduce its chilled water flow rate due to its fixed-speed pumps. For example, if one AHU demand drops to 50%, the overall flow rate for secondary loop will drop to 112.5L/s. If a primary pump and its associated chiller were deactivated, the chilled water flow rate would fall to an insufficient 75 L/s.

Hence, to maintain demand, both chillers must operate, resulting in a surplus 37.5 L/s of chilled water flowing back to the chillers via the bypass line.

Sizing criteria: The maximum pressure drop in the decoupler pipe should not exceed approximately 300 Pa/m as a rule of thumb of 5kPa overall header loss. By restricting the pressure drop to this maximum value, water flowing in the primary loop will not flow into the secondary circuit until its circulator is activated. This strategy achieves hydraulic isolation between both the primary and secondary circuits, providing the fundamental basis for primary-secondary pumping. Note that a higher friction loss in the common pipe tends to cause the primary and secondary pumps to act in series, leading to an induced flow in the system. Normally full flow through the decoupler line is about 115% of the flow rate associated with the largest chiller. Flow rates more than this value would indicate an operating chiller should be stopped.

True Partners Consulting Engineers pipe sizing tool can be used for copper or steel pipes to size up the required bypass line (decoupler pipe) size in the example above, simply enter the waterflow rate and maximum pressure drop of 300Pa/m and calculator will provide the pipe size required e.g. 115%x75L/s = 86.25L/s @ 300Pa/m >> 200 DN decoupler pipe size

2. Meeting Pump Minimum Flow in Primary-Secondary Chilled Water Systems

Within the same primary-secondary chilled water system, the secondary chilled water pumps are variable-speed, permitting the modification of their pumping speed. However, with this variability comes a minimum flow requirement. Each secondary chilled water pump has a design flow of 75L/s. Minimum recommended motor speed is 500RPM, therefore the minimum recommended flow is 500 / 1450 x 75 L/s = 25.8L/s. If the AHUs demand drops to 20 L/s, the secondary circuit's minimum possible circulation would still be 25.8 L/s. This circumstance results in a surplus of 5.8 L/s.

Therefore, a bypass line is essential at the air handling unit (AHU) to accommodate the excess chilled water through the circuit. Implementing this bypass can be achieved with a 3-way valve.

If the flow difference is higher and adding three way valves is not adequate, a bypass line will need to be added in the system.

Criteria for sizing of the bypass line is: flow = (minimum flow of one pump – smallest zone load flow) and 300 Pa/m pressure loss.

smallest zone load: This refers to the smallest zone load at which the system will be running before shutting down, this flow can be impacted by the type of chiller (e.g. fixed or variable, type of compressor and capacity control, etc) and weather or not a storage/buffer tank is implemented in the system

Crucially, the sizing of secondary chilled water pumps should align with the building's load profile to meet chilled water demand during low-load conditions. Simultaneously, it's imperative to provide an adequate bypass line to maintain the minimum flow of the variable-speed pumps.

3. Preserving Chiller Minimum Flow in Primary-Variable Chilled Water Systems

In primary-variable chilled water systems, the chilled water pumps operate at variable speeds, adjusting their pumping rates based on the chilled water demand. Unlike the primary-secondary systems, these setups do not incorporate secondary chilled water pumps.

Variable-speed pumps come with a minimum flow requirement, and the chillers, crucially, have their minimum flow requirement as well. If the chilled water flow rate falls too low, the chiller's evaporator risks freezing, jeopardizing the system's functionality.

To circumvent such risks, implementing a chiller minimum flow bypass is indispensable. If the demand declines beyond the chiller's minimum flow, a bypass valve activates to redirect excess chilled water back to the chiller. This proactive step ensures the flow rates stay within safe, operational parameters, thereby preserving the chiller's integrity and the system's overall efficiency. Bypass line sizing in this case will be similar to item 3 described above.

4. Regulating Condenser Water Temperature in Water-Cooled Chillers

While water-cooled chillers exhibit enhanced efficiency under low condenser water temperatures, there's usually a lower limit, often around 15-18°C .

If the supply temperature of the condenser water drops excessively, a cooling tower bypass becomes necessary. This bypass is essentially a line installed at the chiller's condenser water outlet, allowing the condenser water to circulate back to the chiller, thus rapidly elevating its temperature.

Such a bypass is particularly needed in colder climates, where environmental conditions can naturally reduce the condenser water temperature at the cooling tower's basin beyond the chiller's operational threshold. Consequently, the bypass ensures temperature regulation within the chiller's optimum range, maintaining system performance and efficiency.

5. The Necessity of Bypasses in Chilled Water System Commissioning

Once the chilled water piping is installed and before the system is commissioned, it's critical to flush the pipes to eliminate any dirt and debris within. This debris can cause potential blockages, hampering the system's performance and reducing its lifespan.

Conversely, cooling coils are typically constructed of thin, delicate copper that is highly susceptible to damage if dirt and debris are allowed to pass through. Such damage can lead to leaks and inefficiency in the cooling process, impairing the system's functionality and escalating operational costs.

To circumvent this issue, a bypass should be incorporated at each air handling unit (AHU) and fan coil unit (FCU) in the system. This bypass ensures that cooling coils are excluded from the pipe flushing process, shielding them from potential harm. By doing so, the system integrity is preserved, guaranteeing optimal efficiency and longevity.

The Art and Science of Bypass Line Design in Chilled Water Systems

The bypass line, despite its simplicity, is a fundamental cog in the well-oiled machine that is any open or closed hydronic system. Ensuring its proper design and sizing is pivotal as many performance issues within chilled water systems can be traced back to inappropriately sized bypasses.

What makes these issues particularly challenging is their elusive nature. Problems resulting from bypass size discrepancies are often difficult to diagnose and, consequently, easy to overlook. As such, an inadequate bypass can silently impair system efficiency, posing continuous, unseen threats to performance and equipment longevity.

In essence, bypass design is not just a technical requirement, but an art that demands meticulous attention to detail. It stands as a testament to the profound impact that even seemingly minor components can have on the overall functionality and efficiency of a system. Therefore, a deep understanding of bypass sizing and operation is integral to the successful design, operation, and maintenance of chilled water systems.

Could you identify additional types of bypass lines utilized in a hydronic system? If so, leave a comment in the section below!

At True Partners Consulting Engineers, our seasoned team brings over five decades of collective expertise in elevating the efficiency of hydronic central mechanical systems for mid to high-rise commercial buildings. We invite you to engage with us for proficient guidance tailored to your specific project requirements.

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