Selecting chillers, chilled water systems

09/16/2013


Chilled water pumping arrangements

A chiller plant is not complete without the pumping arrangement to distribute chilled water throughout the building or campus to the chilled water coils. There are multiple arrangements for distributing chilled water. This article will describe the two most commonly used methods of chilled water pumping, variable primary flow (VPF) and primary-secondary, and how they relate to initial investment costs and operating costs for the chiller plant. 

In both pumping schemes, the flow to chilled water coils in the system is variable flow as the cooling coils are equipped with two-way temperature control valves. The difference exists in the way the pumping is handled through the chillers. In a VPF system, the flow varies in the entire system, including through the chillers, which is a shift from the previous mind-set that chillers always require a constant flow through the evaporator. However, recent advances in control technology have allowed the ability to vary the flow through the evaporators within the manufacturer’s recommended ranges and can provide substantial energy savings. 

Because of the ability to vary flow, these systems eliminate the need for secondary distribution pumps and use multiple pumps in parallel to serve the entire chilled water system, including the chillers and any coils in the system. These pumps handle the entire system head at the required system flow rate. VPF requires a minimum flow bypass pipe, which includes a control valve to protect the chiller from falling below the minimum flow rate and temperature control isolation valves at each chiller to isolate flow through the chillers when they are not in operation. Some designers prefer three-way valves at a select number of coils to provide minimum system flow, but this will increase the pumping energy required in the system by increasing flow in all conditions except when the system is at minimum flow. The system minimum flow is maintained by using a flow meter or measuring the pressure drop across the chillers to determine system flow to control the bypass valve.

In a primary-secondary system, two sets of pumps are used, each with a dedicated function to the chilled water system. The primary pumps serve as production pumps and serve only the chillers in the system. These pumps are typically high-flow, low-head constant-flow pumps staged on with a chiller to provide available chilled water to the secondary pumps. The secondary pumps serve only the chilled water coils in the system, which are equipped with two-way control valves as explained above and are high-flow, high-head variable speed pumps that vary the system flow in response to the load. The primary and secondary loops are connected via a common pipe or decoupler that is a shared portion of the piping circuit in each loop that hydraulically separates the two loops so flow in one loop does not affect the flow in the other. This will allow the water to flow through the common pipe in either direction to hydraulically balance the system. These systems are typically older systems that require a constant flow through the chiller, but can still be used as a fail-safe solution due to their simplicity.

The advantages of VPF systems are directly related to the pumping concepts described above. VPF systems will always have fewer pumps compared to a primary-secondary system, which will save initial costs from less piping and valves, fewer electrical connections, less controls work, and no vibration isolation at the additional pumps.  Similarly, one set of pumps will save on mechanical room space requirements, which can lead to substantial initial cost savings in building footprint or increasing useable building square footage. The one thing that must be addressed is the more complex staging controls and ensuring minimum flow in the system at all times, but this will still maintain an overall net savings in initial cost. 

For operating costs, VPF systems will always have lower operating costs as there is less pressure drop in the system due to fewer pumps and accessories at the pumps, but also because the pumps will be more efficient. The VPF pumps allow the designer to select larger, more efficient horsepower pumps instead of smaller, constant volume, low efficiency circulator pumps typically found on the primary side of the primary-secondary system. Also, the use of variable speed in the entire system will allow the entire system flow rate to vary, which will provide operating cost savings as energy will vary approximately with the cubed power of the flow rate. This is in contrast to constant volume pumping and energy consumption at the primary pumps and variable flow at the secondary pumps in a typical primary-secondary system.

Chilled water plants and chillers are dynamic systems that rely on careful analysis to determine the right chiller for a building. There is no simple answer based on the type or size of the building; however, understanding the options based on building load and the compression methods can help narrow down the alternatives for further evaluation. One can select chiller variations that can be analyzed based on initial cost and overall system operating costs. A chiller with a more expensive initial investment may prove to have the best overall operating costs by using an energy simulation program that can evaluate the building load profile over time with respect to corresponding weather data. Clearly, there are multiple different chiller options, and careful analysis, rather than a blended kW/ton value, is required to make the best selection for your situation. 


David Grassl is a mechanical engineer at Ring & DuChateau, and an adjunct professor in the Civil & Architectural Engineering & Construction Management Department at the Milwaukee School of Engineering. He has analyzed and designed approximately 10,000 tons of chilled water systems for plants ranging from small, individual systems for office buildings to large, complex central plants for universities.


References

  1. Bhatia, A. 2012. Overview of Chiller Compressors. PDHengineer Course HV-4004. Houston, TX: PDHengineer.
  2. ASHRAE. 2011. Handbook of HVAC Applications. Atlanta, GA: American Society of Heating Refrigeration and Air Conditioning Engineers, Inc.
  3. AHRI. 2003. AHRI 550/590, Standard for Performance Rating of Water-Chilling Packages Using the Vapor Compression Cycle. Arlington, VA: Air Conditioning, Heating, and Refrigeration Institute.
  4. Geister, W. Ryan and Thompson, Mike. 2009. A Closer Look At Chiller Ratings. ASHRAE Journal, Vol. 51, No. 12, p. 22-32. Atlanta, GA: American Society of Heating, Refrigeration, and Air-Conditioning Engineers Inc.
  5. Taylor, Steven T. 2002. Primary-Only vs. Primary Secondary Variable Flow Systems. ASHRAE Journal, Vol. 44, No. 2, p. 25-29. Atlanta, GA: American Society of Heating, Refrigeration, and Air-Conditioning Engineers Inc.

 


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