Chilled-Water System Design Issues- Learning from Past Mistakes

providing insights for today’s hvac system designer

Engineers Newsletter volume 43 –2

Chilled-Water Systems Design Issues Learning from past mistakes

designed as variable primary flow (VPF). [9.0 kPa]. Even if the differential pressure As with anything in life, you can learn The bypass line (sized the same as the sensor is of high quality, it will be difficult to HVAC design the hard way, from supply manifold) and valve that allow select one that has the required precision to painful and costly mistakes, or the minimum chiller flow rate are located in accurately measure such a small difference easy way, from veteran engineers who can help you avoid those mistakes. the mechanical room, as are the air between design flow and minimum flow The purpose of this EN is to help new handlers. A differential pressure sensor pressure drops. engineers with the latter. across the evaporator is used to determine flow rate and to control the Solution 1: Select the proper flow bypass valve.(Figure 1). sensing technology. Flow meters are generally more expensive, require more Background See any red flags? Let’s take a look at the calibration, and must be installed per the problems that occurred on the job and manufacturer's requirements with a specific The following three cases are how to avoid making the same mistakes. amount of straight piping up- and down- composite scenarios of various chilled- stream. Despite these limitations, they're water system design issues we have Issue 1: Flow sensing device is not better suited to monitor flow for this witnessed over the years. In each accurate enough at the minimum flow system. scenario, the design issues are rate. As mentioned, the design highlighted and followed by possible evaporator pressure drop is 5 ft of head Issue 2: Loop volume does not meet the solutions. [15 kPa]. At the minimum flow rate of chiller unit control requirement. Chiller 1.2 gpm per ton [0.022 L/s per kWr], the unit controls often require a certain loop By sharing the hard-earned HVAC pressure drop will be about 3 ft of head design wisdom that comes from trial volume to maintain good chilled-water and error, we hope to shorten the learning curve for new engineers. Figure 1. Case 1: New variable primary flow (VPF)

DP 200-ton helical rotary chillers: • minimum flow rate: 56ºF (13.3ºC) 40ºF (4.4ºC) Case 1: New Variable 1.2 gpm/ton Primary Flow • design pressure drop: 5 ft of head variable-speed A new system has been designed drive (VSD) using an increased system chilled- water temperature difference as recommended in the ASHRAE GreenGuide1 and by Taylor2. The 6 in pipe chillers are selected at low design VSD evaporator pressure drops (5 ft of head [15 kPa]) per chiller to minimize pump modulating AHUs in same power and energy. To reduce installed control valve equipment for minimum cost and operating costs, the system is room chiller flow

temperature control and ensure that Many designers will use 4 in [10 cm] line The series chiller layout has two compressors are not cycled for this flow rate. Then select a valve to significant benefits: unnecessarily. The loop volume give proper bypass control. While not includes the fluid in the evaporator required, some consulting engineers • The upstream chiller operates at an bundle, pipes, and coils. In many close- specify a pressure independent valve for elevated temperature and increased coupled systems—for example where bypass. efficiency. Very often this the air handlers and chillers are in the compensates for any increase in pump energy. same equipment room—the loop Issue 4: Flow can't be significantly volume will not be adequate. The reduced. Chiller design flow rate is 1.5 • When chillers are piped in parallel in required loop volume varies by chiller gpm per ton [0.027 L/s per kWr] and the a VPF system, there is a significant manufacturer, chiller type, and unit minimum is 1.2 gpm per ton [0.022 L/s flow rate change in the operating control capabilities. Check with the per kWr]. This means the flow rate can chiller when the second chiller is manufacturer for each chiller specified. only be reduced 20 percent before added. When the chillers are piped in In this case, the required loop time reaching minimum. That's not much for series, there is no flow rate transition was two minutes. So the system water a system that is supposed to be variable when the second chiller is enabled. volume required is the loop time primary flow. This can greatly simplify system multiplied by the water flow: control. Solution 4: Put the chillers in series! 2 min. x 300 gpm = 600 gal It may be beneficial to pipe the chillers in [2 min. x 60 sec. per min. x 18.9 L/s = With chillers in series (Figure 2), the series if the system 2270 liters] design flow rate per chiller is about 3.0 • is designed for variable primary flow, gpm per ton [0.054 L/s per kWr], Solution 2: Increase the loop volume allowing significant turndown. Design • has a chilled-water design to the required level. If the loop evaporator pressure drop increases temperature difference of 14°F volume is close to that required, since water must flow through both [7.8°C] or larger, increasing the piping manifolds may be evaporator bundles. This increases • has two chillers, and adequate. Otherwise add a buffer tank, design pump power. However, in many preferably on the return side of the systems the flow rate is less than design • is not likely to require a future system, and on the chiller side of the flow much of the year, so the resulting expansion. bypass line. Ensure the system volume pump energy increase may be small. If the system ΔT is 10°F [5.6°C], consider with the tank meets the required loop increasing it to 14°F [7.8°C] or larger. volume. To attain smoother system To allow for servicing, engineers often control, consider making the tank a design manual bypass lines into the little larger than the absolute minimum system. requirement. The additional cost and space will likely be small.

Issue 3: Oversized bypass line and Figure 2. New variable primary flow (VPF) series chillers valve do not provide adequate 56ºF (13.3ºC) 47.5ºF (8.6ºC) 40ºF (4.4ºC) control. The bypass line is the same size as the pipe manifold. Given this fact, if a large valve is installed in this line, control at the minimum flow rate Solution: may be a challenge, since initially • install flow meter opening the valve a little allows • increase loop volume by significant flow to be bypassed. VSD adding a buffer tank buffer manual bypass for servicing flow • size the bypass line for the tank meter Solution 3: Size the bypass line and largest minimum flow rate valve properly. In a variable primary flow system, size the bypass line for the largest minimum flow rate. In this case:

200 tons x 1.2 gpm per ton = 240 gpm [15 L/s]

2 Trane Engineers Newsletter volume 43–2 providing insights for today’s HVAC system designer Case 2: Conversion from Figure 3. Case 2: After value-engineered conversion to VPF Primary Secondary to DP VPF conversion plan: Variable Primary Flow 54ºF (12.2ºC) 44ºF (6.7ºC) • additional 30-tons of heat The system includes two 500-ton chillers recovery in a primary-secondary configuration. The Value engineering: owner wants to convert the system to • pumps variable primary flow (VPF) and add a small (30-ton) heat recovery chiller. - use secondary pumps, don’t move them This job is "value engineered" to: - remove primary pumps • $100 DP sensor rather than • use the existing secondary pumps magnetic flow meter (which can handle the flow and VSD VSD pressure drop requirements), control • switch out the high quality differential valve pressure sensor, which costs $1500, to a $100 sensor, and

• connect the heat recovery chiller in • Add a matched pair of temperature and repeatability required to operate a parallel with the existing chillers sensors to monitor the system return- variable primary flow system. (Figure 3). water temperature and the chiller Unfortunately this seems to occur often. return-water temperature. We suggest you resist the temptation to reduce costs when value will be lost. A Issue 1: The pumps are positioned • Control the primary pump VSDs to VPF system operates on flow rate, so incorrectly to allow bypass for maintain the chiller return-water accuracy is critical. minimum flow. Water cannot flow temperature a degree or two lower from right-to-left in the bypass line, so than the system return-water minimum flow cannot be maintained. temperature. This ensures there is Solution 2: Specify and install an always a little more chilled water being accurate, reliable and repeatable flow- Solution 1: Rather than converting to produced than demanded. sensing device. It's very important that full variable primary flow, consider the flow-sensing device is accurate, • If the temperature control results in converting to variable primary/variable reliable, and repeatable. Another benefit chiller flow dropping below its secondary. As its name implies, a of a higher quality sensor is that, in minimum flow rate, increase the variable primary/variable secondary general, it requires less calibration. The pump speed to maintain the required system (Figure 4) employs variable cost of a proper flow-sensing device will minimum chiller flow rate. primary (chiller) flow as well as variable often be closer to $1000 than $100. secondary (coil) flow. In retrofitting a Informative Appendix E of ASHRAE system, the use of the existing pumps Issue 2: It is likely the flow-sensing Guideline 22-20083 provides an example already installed is generally simple. In device is not accurate enough. A $100 of flow measurement accuracy and contrast, converting to primary secondary differential pressure sensor is very precision specification language. requires significant piping changes, and unlikely to provide the accuracy, reliability the present secondary pumps must be moved and possibly increased in size. Figure 4. Convert to variable primary-variable secondary

DP To convert to variable primary/variable 54ºF (12.2ºC) 44ºF (6.7ºC) Solution: secondary: • keep primary pumps and add VSDs Δ • Contact the chiller provider to ensure VSD • add DP and T sensors that chiller unit controls can tolerate • control primary pump VSD speed to variable evaporator water flow. If not, maintain ΔT of 1-2°F update the unit controls. VSD • if necessary, override temperature

T control to maintain minimum flow • Add variable-speed drives (VSDs) to Δ rate the primary pumps (make sure the pump motors are compatible).

• Add a high quality differential VSD VSD pressure sensor across each evaporator. control valve

providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 43–2 3

Issue 3: Poor heat recovery chiller Figure 5. Value-engineered conversion heat recovery location. Flow and control issues can DP Issues: result when a smaller chiller is piped in 54ºF (12.2ºC) 44ºF (6.7ºC) • flow balance between parallel with larger chillers. When dissimilar chillers evaporators are piped in parallel, the • inability to fully load the heat VSD chillers are loaded to equal recovery chiller due to mixed percentages. This does not allow the return water temperature heat recovery chiller to be preferentially VSD Solution:

loaded or to deliver the desired hot T

Δ • put heat recovery chiller in water temperature and quantity. "sidestream’ position • load using either condenser- Solution 3: Pipe the heat recovery leaving or chilled-water chiller in the sidestream position. temperatures VSD VSD The sidestream configuration (Figure 5)

-allows the heat recovery chiller to: control • Be preferentially loaded, since it valve receives the warmest return water temperature. • Operate as a “heater” to make the Misconception 1: Existing coils cannot • What is the coldest (or lowest) desired leaving condenser water be reselected at higher temperature chilled-water supply temperature temperature. The chilled water differences. possible before condensation produced reduces the load on the forms within the pipe insulation? central chillers. Solution 1: Coils are heat transfer media Often with existing pipe insulation, that can be selected at many supply water temperature reduced • Meet its needed evaporator flow conditions. Table 1 shows a coil to 40°F [4.4°C] can be successful, requirements independent of other originally selected for a 10°F [5.6°C] chilled but perhaps not 36°F [2.2°C]. chiller operation. Δ water T reselected to produce the same • If coil control valves were oversized cooling capacity. This can be done if the at a 10°F [5.6°C] ΔT, they are even entering water temperature is lower and Δ more oversized at a higher T and Case 3: Conversion of a results in a lower flow rate and higher lower flow rate. Valves may need to Δ Parallel-piped System to chilled-water T. This change has no effect be changed to allow good control. on the airside of the system, but a Series, and from substantial impact on the waterside. “Conventional” to Misconception 2: Use of three-way valves as the method of bypass can Increased ΔT. The advantages of series chillers (Figure 6) to produce the higher chilled-water ΔT was use more pump energy. This 400-ton system includes two discussed earlier in Case 1. water-cooled chillers piped in parallel. Solution 2: Yes, but this is mitigated The chillers and air-handling coils were When making supply water temperature by a number of factors. both selected with water temperatures and flow rate changes to existing systems, consider the following factors: • First, the system flow rate dropped of 54°–44°F. The system serves a by nearly 38 percent because of school, which does not have a trained • Many coils in air-handling units have the wider ΔT across the coils and plant operator on site—simplicity is adequate heat transfer area to perform reduced water flow rate. This lower beneficial. similarly to the data in Table 1. On the flow rate results in lower system other hand, if a coil has limited heat pressure drop and pump power. The school officials would prefer air- transfer capability (e.g., small, two-row cooled chillers to eliminate cooling fan-coils), reselecting at a • In many constant flow systems, it’s tower maintenance requirements. They higher ΔT is unlikely to work well. difficult to retrofit a separate have decided to use 50 percent glycol to keep the fluid from freezing in the Table 1. Air-handler coil reselection using low entering-water temperature winter. No changes are to be made to original selection reselection the airside of the system. In addition, capacity (MBh) [kW] 504 [148] 504 [148] energy usage reductions are desired. entering water temperature (°F) [°C] 44 [6.7] 41 [5.0] For this scenario, let’s examine some flow rate (gpm) [L/s] 101 [6.37] 63 [4.0] common misconceptions that might foil leaving water temperature (°F) [°C] 54 [12.2] 57 [13.9] a new engineer. water ΔT (°F) [°C] 10 [5.5] 16 [8.0]

4 Trane Engineers Newsletter volume 43–2 providing insights for today’s HVAC system designer bypass, control valve and controller Figure 6. Retrofit VPF series chillers Retrofit changes: into the system. Leaving enough three-way valves to allow the 57ºF (13.9ºC) 44ºF (6.7ºC) • new chillers in series required minimum chiller flow rate • send 41°F water to existing can be a simple, cost-effective way coils to increase ΔT to convert a system from constant • new pump with VFD to variable flow. • leave 3-way valves equal to chiller minimum flow rate • Minimum pump speed can be set 50% glycol to attain the minimum flow rate • add differential pressure sensor at remote AHU to with all converted two-way valves control pump speed closed. • add 50 percent glycol to the DP system for freeze protection Using three-way valves as a method of bypass is a good compromise between simplicity and performance.

Misconception 3: More glycol is better. When antifreeze is added to a system, fluid viscosity and pump power are often considered. However, Freeze vs. Burst Protection these impacts may be small compared to the effect on chiller and coil heat As the temperature drops below the glycol For a chilled-water VAV system, since the solution's freeze point, ice crystals begin to cooling coil is typically shut off during sub- transfer capability and capacity—which form. Because the water freezes first, the freezing weather, burst protection is usually can be degraded by 30 percent or remaining glycol solution is further sufficient. Freeze protection is mandatory in more. concentrated and remains a fluid. The those cases where no ice crystals can be resulting ice crystals and fluid combination permitted to form (such as a coil loop that make up a flowable slush. The fluid volume operates during very cold weather). Solution 3: Add only the level of increases as this slush forms and flows into antifreeze necessary. In general, use available expansion volume. When an air-cooled chiller is used, an the smallest amount of antifreeze to alternative approach is to use a packaged Freeze protection indicates the concentration condensing unit (condenser and compressor) adequately protect the system. This of antifreeze required to prevent ice crystals located outdoors, with a remote evaporator minimizes first cost of the antifreeze from forming at the given temperature. Burst barrel located in an indoor equipment room. itself, plus minimizes the first cost and protection indicates the concentration The two components are connected with operating cost impact on the heat required to prevent damage to equipment field-installed refrigerant piping. This transfer surfaces and pumps. When (e.g., coil tubes bursting). Burst protection configuration locates the part of the system requires a lower concentration of glycol, that is susceptible to freezing (evaporator) adding antifreeze to an existing which results in less degradation of heat indoors and still uses an outdoor air-cooled system, ensure the chiller and coil transfer. condenser. capacities are adequate given the level of antifreeze. Concentration required for freeze protection vs. burst protection

ethylene glycol propylene glycol concentration volume (% volume) concentration volume (% volume) temperature freeze burst freeze burst Final Thoughts ºF (°C) protection protection protection protection While it's often more memorable to 20 (-7) 16 11 18 12 experience issues in the field, it's 10 (12) 25 17 29 20 cheaper and much less painful to learn 0 (-18) 33 22 36 24 from others! -10 (-23) 39 26 42 28

We can all listen to those with -20 (-29) 44 30 46 30 experience, ask probing questions, and -30 (-34) 48 30 50 33 attempt to learn. By doing so the -40 (-40) 52 30 54 35 industry can be elevated, deliver higher -50 (-46) 56 30 57 35 performing systems that result in more -60 (-51) 60 30 60 35 satisfied building owners and Source: Dow Chemical Company. 2008. HVAC Application Guide: Heat Transfer Fluids for HVAC and Refrigeration Systems occupants. www.dow.com/heattrans

providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 43–2 5

This article is based on a seminar presented by www.Trane.com/bookstore Mick Schwedler, Trane and Jason Atkisson, Affiliated Engineers,during the 2013 ASHRAE Learn HVAC design strategies and earn credit annual meetings in Denver. The acknowledgment of Mr. Atkisson in this newsletter in no way implies the endorsement of a specific system provider or manufacturer by either Affiliated Engineers or Mr. Atkisson. You can find this and previous issues of the Engineers Newsletter at www.trane.com/engineersnewsletter. To comment, e-mail us at [email protected].

References

[1] American Society of Heating, Refrigerating and Air-Conditioning Engineers. 2010. ASHRAE Green Guide: The Design, Construction, and Operation of Sustainable Buildings, 3rd ed. Atlanta, GA: ASHRAE. Air conditioning clinics. A series of Application manuals. Comprehensive [2] Taylor, S. 2011. “Optimizing Design & Control educational presentations that teach HVAC reference guides that can increase your

of Chilled Water Plants; Part 3: Pipe Sizing fundamentals, equipment, and systems. The working knowledge of commercial HVAC and Optimizing ΔΤ.” ASHRAE Journal. series includes full-color student workbooks, systems. Topics range from component 53(12):22-34. which can be purchased individually. Approved combinations and innovative design concepts to system control strategies, [3] American Society of Heating, Refrigerating by the American Institute of Architects for 1.5 and Air-Conditioning Engineers. 2008. (Health, Safety and Welfare) learning units. industry issues, and fundamentals. The ASHRAE Guideline 22: Instrumentation for Contact your local Trane office to sign up for following are just a few examples. Please Monitoring Central Chilled Water Plant training in your area. visit www.trane.com/bookstore for a Efficiency, Appendix E. Atlanta, GA: ASHRAE. complete list of manuals available to order. Engineers Newsletter Live. A series of 90- Central Geothermal Systems discusses minute programs that provide technical and educational information on specific aspects of proper design and control of central HVAC design and control. Topics range from geothermal bidirectional cascade systems 2014 water- and airside system strategies to that use borefields. This manual covers Engineers ASHRAE standards and industry codes. central geothermal system piping, system Contact your local Trane office for a schedule design considerations, and airside Newsletter or view past programs by visiting considerations. (SYS-APM009-EN, February LIVE! www.trane.com/ENL. 2011) For event details and registration contact your local Trane office. On-demand continuing education credit Chilled-Water VAV Systems discusses the for LEED® and AIA. These 90-minute on- advantages and drawbacks of the system, May demand programs are available at free of reviews the various components that make charge. The list of HVAC topics includes many up the system, proposes solutions to Applying Variable LEED-specific courses. Check out the latest common design challenges, explores several Refrigerant Flow courses: Single-Zone VAV and All -Variable- system variations, and discusses system- Speed Chilled-Water Plants. All courses level control. (VRF) available at www.trane.com/ (SYS-APM008-EN, updated May 2012) continuingeducation. Water-Source and Ground-Source Heat Pump Systems examines chilled-water- October Engineers Newsletters. These quarterly system components, configurations, articles cover timely topics related to the Chilled-Water options, and control strategies. The goal is to design, application and/or operation of Terminal Systems provide system designers with options they commercial, applied HVAC systems. can use to satisfy the building owners’ Subscribe at www.trane.com/EN. desires. (SYS-APM010-EN, updated November 2013)

Trane, Trane believes the facts and suggestions presented here to be accurate. However, final design and A business of Ingersoll Rand application decisions are your responsibility. Trane disclaims any responsibility for actions taken on the material presented. For more information, contact your local Trane office or e-mail us at [email protected]

6 Trane Engineers Newsletter volume 43–2 ADM-APN051-EN (May 2014)