Primary- Secondary Pumping Revisited
During AHR Expo a couple weeks ago, I overheard a discussion among engineers about the benefits of primary-secondary pumping. It got me thinking about how valuable primary-secondary pumping can be for the right application.
As you know, primary-secondary pumping is simple in theory as well as in operation. It is based on a simple fact: when two circuits are interconnected, flow in one will not cause flow in the other if the pressure drop in the piping common to both is eliminated.
Each type of pumping system configuration has distinct advantages for the user or operator. Fundamental to all of these configurations is sound design principles. Without the proper application of the basics, the most sophisticated systems will not operate effectively. To optimize system performance, the designer has a number of variations of basic primary-secondary pumping to explore. Standard rules of thumb are:
- THE COMMON PIPE: The key to all primary-secondary applications is the use of a common pipe, which interconnects the primary and secondary circuits. The length of this pipe should be kept very short in order to keep the pressure drop very low, and the supply and return tees to the secondary circuit should be a maximum of four pipe diameters apart. By keeping the pressure drop very low, water that is flowing in the primary loop will not flow into the secondary circuit until its circulator turns on.
- THE SECONDARY CIRCULATOR: A separate circulator is installed in the secondary circuit to establish flow. This circulator is sized to move the flow rate and to overcome the pressure drop of its circuit only. The circulator should be located so it is pumping away from the “common piping” and discharging into the secondary circuit. This causes an increase in pressure in the secondary circuit rather than a reduction in pressure, which would occur if the pump were located on the return pumping towards the common pipe.
- THE LAW OF THE TEE: This rule determines the flow rate and direction of flow that occurs in common piping. It is based on the relationship of the primary and secondary flow rates, and there are three possibilities to evaluate:
- Primary flow more than secondary
- Primary flow equal to secondary
- Primary flow less than secondary
This rule of thumb is best described by a simple statement: flow into a tee must equal flow away from the tee.
- FLO-CONTROL VALVES: Flo-Control valves are recommended to prevent any flow into the secondary circuit induced by either the slightest pressure drop that may exist on the common pipe or by gravity heads. Because gravity flow can occur within a single pipe, two Flo-Control valves are best, one on the supply and one on the return. However, if the secondary circuit’s return is under-slung, only one valve is needed.
Finally, there are two important advantages to primary-secondary pumping:
- Less energy is required to move water through the entire system (rather than one large circulator, small energy efficient circulators can be used to overcome the friction and inertia (“pressure drop”) of their respective loops).
- More control can be taken over zones (and each zone can operate at its own optimum temperature).
There are many ways modern HVAC systems can be pumped. Each method has its own advantages and disadvantages to the user or operator. The designer must consider present system use, plans for the future, cost considerations, and the ability level of the operator. With more tools in the toolbox, the engineer is better equipped to make the decision, “Which system is the best for my client?”