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Chilled-Water System Design Decisions providing insights for today’s hvac system designer Engineers Newsletter volume 47–3 impacts of Chilled-Water System Design Decisions Primary-secondary system bypass This Engineers Newsletter walks through a number of design decisions, with Bypass line sizing sizing. The premise of a primary- secondary system is to hydraulically discussion and examples to explain how This seemingly simple decision can have and why those decisions are made. separate (decouple) the primary (chiller) significant consequences if not done flow from the secondary (system) flow. correctly. This decoupling prevents the operating • In a primary-secondary system, the While designing a chilled-water pressures of chiller pumps from impacting bypass pipe should be the same system, a myriad of decisions must the operating pressures of the system diameter as the pipe going into the be made. Experienced engineers pumps. This is accomplished by installing largest chiller. Its length should be often make these decisions a bypass line with a small pressure drop. about 8-10 pipe diameters long or "automatically" as they have in the The bypass allows water to flow in either have an equivalent pressure drop. past, based on what they have direction and is used to indicate chiller and learned from experience. Today, all • In a variable primary flow (VPF) primary pump sequencing in this system engineers likely use an internet system, the bypass line should be arrangement (Figure 1). search engine to get direction, but sized for the largest minimum flow Chillers and constant flow primary pumps when there are differing—or even rate and it will have a control valve. are enabled in pairs, making the primary conflicting—recommendations a The reasons behind this guidance follow. flow rate a step function. As system load decision must be made. and flow increase, the excess flow rate Common decisions regarding (from supply to return) in the bypass line chilled-water system designs decreases. At the point that system flow include: rate exceeds chiller flow rate, deficit flow • bypass line sizing in variable Figure 1. Primary-secondary system flow systems • dynamically varying condenser water flow primary pumps • number of chilled-water pumps to operate secondary pumps with • series chillers and power VSDs consumption chilled-water coling coils • whether to use pressure- with two-way independent control valves valves bypass line, free of restrictions, sized the same as the largest chiller’s pipe, aim for a pressure drop equivalent to a pipe that is 8-10 pipe diameters long. ©2018 Trane. All rights reserved. 1 (from return to supply) in the bypass Figure 2. Oversized bypass can allow simultaneous flow in both directions line occurs. This warm, deficit water flow increases the system supply- chiller return chiller supply water temperature and at some point water water an additional chiller and primary pump are enabled. short-circuited chiller A chiller and pump are disabled when supply water excess flow rate in the bypass line is high enough to still have excess flow after a primary pump is turned off. Therefore the maximum flow rate the bypass line ever experiences is a little short-circuited system higher than the design flow rate of the return water largest chiller. Often designers wait for 10-15 percent excess flow, to ensure system that chillers are not cycled on and off system return water supply water rapidly. Therefore the bypass pipe should be sized for 110 to115 percent • Much of that water continues to the of the largest chiller's flow rate—which Add pressure drop. A better option chillers but some of the warm water most designers simplify to the same may be to place an orifice in the line. This (due to low pressure drop in the as the pipe size going into the largest can simply be a plate with a hole in it. bypass) flows in the deficit direction chiller. This imposes a pressure drop, but allows in the bypass line (from return to the water to flow freely in either the surplus supply). or deficit direction. In a few extreme What happens if bypass sizing is cases it has been neccessary to not right? Too little or too much As a result there is simultaneous flow in completely block off the oversized/short pressure can cause issues... opposite directions because the bypass bypass pipe and install a properly sized line has become so large it functions like pipe of sufficient length to eliminate the Too small, pressure too high? An a tank! mixing condition. undersized bypass line can result in high fluid velocity, which in extreme The diluted (higher) system supply-water The best way to avoid issues in a conditions may cause pipe erosion, temperature results in more pumping primary-secondary system is to size the vibration and acoustic issues. The high energy. The short-circuited supply water bypass line properly during the design fluid velocity can also result in high mixing with return water results in process. Simply check the drawings and enough pressure drop so that flow is reduced chiller return-water temperature. ensure that the bypass line is smaller restricted through the bypass, and This can restrict the chillers’ ability to than the manifold, and the same size as may actually cause the primary and fully load. the pipe going into the largest chiller and secondary pump pressures and flow 8 to 10 equivalent pipe diameters long. If rates to affect each other. If the pipes are already installed how the pipe is less than 8 to10 pipe can the situation be improved? A diameters long, using elbows to form a Too large, pressure too low? A resolution for too low of a pressure drop "U" adds an appropriate pressure drop. benefit of an oversized bypass pipe is a is to impose a modest restriction to keep very low pressure drop, however if it's water from short-circuiting. Aim for the too low, the water may not flow as Variable-primary-flow system bypass equivalent pressure drop of a pipe that is intended. Figure 2 illustrates the issue: line sizing. Conceptually a variable- 8 to 10 pipe diameters long. primary-flow system is simpler to get • Cold water leaves the chillers and right. The valve in the bypass line only Often the first solution considered is to flows into the chiller supply opens when the system flow rate add a valve. Given that the bypass line is manifold. approaches the minimum flow rate of the oversized, it’s likely the additional valve operating chiller(s). So the bypass pipe • A portion of that cold water flows will be big and expensive. It’s also likely and valve only need to be sized for the toward the secondary pump. that at some point a well-meaning but largest minimum flow rate. Usually that's However due to the very low uninformed operator will close the valve the largest chiller's minimum flow rate. pressure drop in the bypass, some too much. Or, they might open it all the However, depending on chiller of the cold water seeks the path of way and defeat this solution. Recall that selections, the largest minimum flow least resistance and flows through the bypass line in a primary-secondary rate might not be for the largest chiller in the bypass line in the excess system should allow water to flow freely, the plant. Also consider the combined direction. in either direction, as needed. minimum flow required when two • Similarily in the return-water side, chillers are operating at part load, just warm return water flows in the before sequencing off one chiller. system return manifold. 2 Trane Engineers Newsletter volume 47-3 providing insights for today’s HVAC system designer Should we dynamically vary Table 1. Plant annualized kW/ton and percent savings compared to two-chiller base for three alternatives Condenser Condenser Tower Plant the condenser water flow? Chiller Cooling water flow Alternative water flow control annualized Savings type tower fan rate type method* kW/ton Yes, savings are available in existing (gpm/ton) systems designed between 2.5 to 3.2 gpm/ton (12 to 9.4ºF ΔT) condenser water Base VS VS 3 CF Opt 0.5462 NA flow rate. Controls complexity should be accounted for and the system needs to be Variable CW flow VS VS 3 VF Opt 0.5260 3.7% properly commissioned. Reduced design VS VS 2 CF Opt 0.5255 3.8% No, in new systems designed for 1.8 to 2.2 CW flow gpm/ton (16.6 to 13.6ºF ΔT) condenser water flow rate since they already achieve Reduced design VS VS 2 VF Opt 0.5252 3.8% almost all the savings and reduce system CW flow and variable CW flow complexity—a lot. Systems designed at these flow rates do not require varying the *Near optimal control (Opt) is minimum the sum of chiller + cooling tower fan kW at each operating point during condenser water flow rate. the year Why? Varying condenser water flow rate Example. Let’s compare the condenser • Pumps are smaller with significantly can be complicated. There are several limit water flow options. lower power conditions and setpoints to manage: It's imperative that the sum of chiller, • Chiller power rises marginally • The flow rate has to stay above the condenser water pump and cooling tower minimum flow rate as defined by the • Cooling towers become more effective fan power is considered. Less-than-optimal highest of minimum tower flow, as heat exchangers, since warmer results can happen if decisions are based minimum chiller flow, or to produce the water is sent to the cooling tower, on only one component.
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