Pumps are used in HVAC applications to circulate water for heating and cooling purposes. As long as theres a need for hot or chilled water to satisfy the interior environmental conditions, pumps are running; as such, pumps are large consumers of electrical energy in a heating and cooling plant. Thats why you should explore various flow pumping strategies that might be applied to new systems, as well as those that might be adaptable to existing hydronic systems. These strategies can save energy and operating costs while increasing the controllability of a plant.
The pumps role
To provide the design flow rate, which is a function of both the design load and desired temperature difference between the return line and the supply line, the pump must be capable of operating against the greatest resistance to flow (pressure drop) in the system.
The greatest resistance, also called the hydraulically most remote point, is a function of the piping network, pipe sizes and flow rate to and from the most remote point. Its not necessarily the geographically most remote point, though. As long as the pump can maintain the required flow at the hydraulically most remote point, it will maintain flow at areas of less resistance (lower pressure drop). The power required to push water throughout the piping network is proportional to flow rate and pressure difference. Some useful hydronic formulas are presented in the sidebar.
Constant volume primary systems
Many older or smaller-capacity hydronic systems, either chilled water or hot water, use a two-pipe (supply and return) primary pumping system similar to that shown in Figure 1. The pump maintains a constant flow rate throughout the system and three-way bypass valves allow flow to vary through the load while maintaining constant flow through the branch. The required, or peak design, flow rate is therefore pumped continuously during periods when heating or cooling are needed.
Figure 1. A typical two-pipe primary pumping system continuously circulates water at full flow. (Click to enlarge)
But, neither heating nor cooling loads are constant. They vary with, among other things, occupancy, equipment usage and changes in the external environment. Bypass valve modulation provides for proper response to the varying load. However, some of the supply water flows directly into the return line to accommodate the varying load. This results in the return water temperature being brought closer to the supply water temperature or a reduction in the temperature difference (delta-T).
As the temperature difference decreases, the load on the boiler (or chiller or heat exchanger) is reduced, which causes it either to unload or shut down. The design, or full load, temperature difference across the boiler (or chiller) must be equal to the design temperature difference across each load for this type of system.
Bell and Gossett pioneered the development of primary-secondary pumping in the mid-1950s. This arrangements original intent was to increase the allowable system design temperature difference, thereby reducing system design flow rates (Sidebar, Eqn 1) with the resulting decrease in pump horsepower without sacrificing system controllability. The basic operation of the primary-secondary arrangement is predicated on a fairly simple premise:
When two piping circuits are interconnected, flow in one causes flow in the other, to a degree depending upon the pressure drop (resistance to flow) in the piping common to both. (1)
This premise is illustrated by the mono-flow circuit shown in Figure 2. The valves provide a fixed resistance (pressure drop) in the main circuit, which induces flow into the load circuit. This equalizes the pressure drop between the two valves, regardless of whether the water flows through the load circuit or the common piping.
Figure 2. In the mono-flow circuit, the valve position induces flow in the load circuit. (Click to enlarge)
If the valves were removed and the pressure drop in the common piping reduced to practically zero by adjusting the locations of the load circuit connections on the main circuit, practically no flow would be induced into the load circuit (Figure 3). The arrangement wouldnt be very good at providing flow to a load, as the pressure difference in the common piping is negligible, the flow in the secondary loop also would be negligible.
Figure 3. This arrangement doesnt provide flow to a load. (Click to enlarge)
However, a pump installed in the secondary circuit establishes flow (Figure 4). This secondary pump is selected to provide the design flow rate to the load in the secondary loop at the pressure drop through the secondary loop. Because the pressure drop in the common piping is negligible, it wont affect the secondary pump. Therefore, the secondary pump can be selected independently of the primary loop. Similarly, the secondary loop will have a negligible affect on the primary loop.
Figure 4. A pump installed in the secondary circuit ensures flow to a load. (Click to enlarge)
Now, the two loops can be considered in isolation, and a large system could be designed as a series of smaller secondary systems connected to a primary loop (Figure 5). The primary loop provides heat (or cooling) while the secondary loops act as distributors.