Priming Pumping Possibilities

As long as HVAC systems use water as a means of heat transfer, engineers will continue to design variable flow systems to maximize efficiency.

There are numerous combinations currently in use. Most common are primary-secondary, variable primary and variable flow with distributed pumping . Each system provides variable flow and a degree of system efficiency. However, fine-tuning on any of these applications will provide even higher efficiencies, sometimes at reduced cost.

The most common variable flow system is primary-secondary. This type uses constant-flow primary pumps at the chilled or production side and secondary variable flow pumps on the distribution side. Secondary pumps are usually selected to operate in a parallel configuration, sequenced by wire-to-water efficiency control strategy, which evaluates the most efficient operating point running single or multiple pumps. Pump speed is controlled by differential pressure transmitters at coils or near ends of piping loops. This system works well with multiple-capacity chillers or for retrofit applications with piping or chiller control limitations. Distributed pumping has been around for more than 30 years and is most commonly used for campus-type systems. The concept is relatively simple: Rather than having the buildings closest to the production plant risk system overpressurization, at each zone (building) provide a pump that’s capable of pumping the required flow at a pressure drop equal to the zone pressure drop. Of course, the piping and equipment losses from the zone to the production plant must also be accounted for. A constant-flow primary pump and bypass piping are used on the production side only. There are no secondary pumps in the plant, as they are located at the zone or building. This system offers wide-range flexibility with respect to system diversity and future expansion capacities. In the case of expansion, the main pipe loop size must be determined based on an ultimate load. Additional chillers and zone pumps can be added over time, which minimizes first cost with the exception of some temporary oversized pipe.

A new path

Advances in chiller technology now allow direct variable flow through the evaporator. Computer software will operate the chiller through a proportional integral derivative control. The design engineer must know the evaporator’s minimum flow requirements as determined by the chiller manufacturer. Once again, multiple pumps configured in parallel are used. Speed is controlled by differential pressure, and pump sequencing is controlled by wire-to-water efficiency. Unlike the primary secondary piping configuration, the system bypass is on the discharge side of the pumps between the supply and return system header.

A two-way control valve is positioned in the bypass line and controlled by two flow meters. One is located in the main supply, and the second is located in the bypass piping. In some instances the bypass flow meter may be eliminated. The bypass pipe and control valve are used to maintain minimum evaporator flow rates at low load conditions.

Each system described is an effective means of controlling water flow. The final solution must be based on the application and limitations of the equipment. To accentuate the energy efficiency, consider variable flow condenser water and variable speed compressors on the chillers. With today’s technologies almost anything is possible.

Distributed pumping characteristics

Pumps located in each zone

No secondary pumps in the plant

Flexible to system diversity and expansion

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