By Chris Edmondson
There’s more than one way to pump a chiller or boiler system. In fact, there are five common approaches, and all but one includes some variation of our topic of late -- primary secondary pumping.
Variable Primary Pumping
Contemporary systems that do not utilize primary secondary pumping are typically known as variable primary systems. In this simple design, there is only one set of pumps (chiller or boiler pumps) creating flow for the entire system. As you can see in Figure 1, there is still a common pipe between the chiller pumps and the system distribution, but there are no secondary pumps. A control valve is installed in the common pipe. By throttling this valve, we create a pressure drop through the common pipe, which forces more water into the distribution piping. The control valve is the only means of control in this type of system.
Pros/Cons: Generally speaking, variable primary systems can have lower first-cost, but pumping operating costs can increase because of the wasted horsepower that is associated with throttling a control valve. Variable primary on chilled water systems allows for delta-T compensation if you can over-pump a chiller. Control sequences can be difficult to master and such systems are not as easily expanded to meet growing or changing demands as systems with secondary pumps.
Primary Secondary Pumping
The next step up is a simple primary secondary system – something we’ve been talking about extensively over the last several weeks. In Figure 2 you can see all the basic building blocks of virtually any type of primary secondary configuration. Constant speed pumps drive the flow through the primary (chiller or boiler) loop. A separate secondary pump delivers variable flow to the secondary loop and two-way valves are applied to control flow through each zone.
Pros/Cons: This simple design typically has a low first cost and good flexibility. It also decreases the complexity of the chiller or boiler staging and control by eliminating on/off valves and min/max flow requirements. However, efficiency is limited because the zone nearest to the primary loop is susceptible to over-pressurization.
In a primary-secondary-tertiary system (Figure 3), things start to get a bit more complicated, but the principles are all the same. The common pipe (whether it is between the primary loop and the secondary loop, or the secondary loop and the tertiary loop) acts as the decoupler, so flows act independently of one another and there is extra degree of thermal isolation.
Pros/Cons: Primary-Secondary-Tertiary systems have long been applied to large campus-type installations with long pipe runs. They give the designer the flexibility to separate far-off zones so that the secondary pumps needn’t bear such high head loss burdens. They offer excellent design flexibility because flow to each zone is independent, thus primary-secondary-tertiary pumping is a good option for systems with diverse load patterns. First costs are higher given the extra pumps and two way valves, but when properly designed primary-secondary-tertiary pumping offers significantly lower operating cost. Such systems are also easily expandable because additions will not impact the flow or balance of existing pumps.
Primary-secondary-tertiary designs may also be hybridized, meaning you don’t have to put a tertiary loop on every zone. Rather, you can isolate a high head loss zone with a tertiary pump while continuing to efficiently serve nearby zones with the secondary pumps. Figure 4 shows a primary-secondary-tertiary hybrid design.
Pros/Cons: This approach offers all the efficiency and flexibility of a standard primary-secondary-tertiary, but does eliminate some of the extra equipment costs.
Primary-Secondary – Zone
Finally, there is the primary-secondary- zone approach. In a primary-secondary zone design, separate pumps serve each individual zone; there is not a dedicated secondary loop pump. As you can see in Figure 5, there is no common pipe between the individual zones, therefore a change to one zone will impact flow through the other zones. For that reason, this is not the best choice for a system that is likely to face future expansion or renovation, as changes in existing loads might require resizing of all pumps.
This type of design has the potential for high energy savings, but with certain caveats, including slow reaction time to changes in system demand. Because the pumps are in parallel, special attention must the paid to the selection and performance curves must be compatible. Also, under certain load conditions, the return pressure may be higher than the supply pressure, which could create problems with the primary loop equipment.
Pros/Cons: The main advantage to this piping arrangement is significantly lower system pressures, and reduced horsepower. Controllability, however, is a challenge because there is no decoupling between zones. Expanding the system could also be quite costly.