Author: Ray Hardee, P.E.
In past articles, we have focused on piping systems made of only one or two circuits. These types of system are often found in process systems where fluid is pumped from a supply tank acting as the inlet boundary, through the process elements of the system to make the product, and then through the control elements to adjust the flow rate of the system to maintain the desired operating condition.
This month we will look at multi-loop closed systems used to recirculate a heat transfer fluid to heat or cool loads in the system. These systems are often used in commercial buildings for HVAC chilled water cooling or hydronic heating, closed loop cooling systems to keep operating equipment cool, and hot oil systems to provide process heating.
Figure 1 shows a closed loop cooling system consisting of four circuits. As always, this system is made of three elements working together. The pump elements Pump P-1 and Pump P-2 provide all the hydraulic energy to the system. The process elements consists of a Tank providing a point of known pressure, the interconnecting pipelines, and the air handlers used of remove heat from the conditioned system. The control elements consist of control valves CV1 through CV4 which regulate the flow rate through each circuit to the specified value.
Each of the four circuits share a number of common elements, starting clockwise from the common Return Header (RH-1) through the Water Chiller and pipe back to Pump P-1 and out to the common Supply Header (SH-1). At this point each circuit has its own combination of process and control elements before rejoining at RH-1. Let’s take a look at how the fluid energy changes as we follow it through the first circuit going through Air Handler 1.
At the suction side of the pump (P-1 In) we see the energy grade is a little over 45 feet of head. Pump P-1 adds 227 feet of head to the fluid bringing the fluid energy at the discharge to 272 feet. The fluid travels through pipes P-1 Out and the common leg of the supply header SH-1, where the energy grade drops to a little under 231 feet due to friction losses and elevation change. Following the fluid through the first loop, we see at the outlet of pipe L1-1 the head drops to under 223 feet of fluid. The head loss through Air Handler 1 is 23 feet to a little over 199 feet of fluid. At the outlet of pipe L1-2 going into the control element CV1 the energy grade is 196 feet. Control valve CV1 has a head loss of almost 65 feet, resulting in fluid energy of over 131 feet at the inlet of pipe L1-3. The fluid flows through L1-3 and the common return header RH-1 resulting in energy grade of under 91 feet of fluid at the inlet to the water chiller. The energy level drops 16 feet through the chiller, then flows through three more pipe segments (C-Pipe 1, C-Pipe 2, and P-1 In) to return to the energy level we noted previously at the suction side of the pump.
If we work through all the rest of the circuits in the same manner, we get the results shown in Table 1.
Taking a look at the overall data we can make a few observations. First, since the same pump element is being used to drive the fluid through all four loops (i.e. Pump P-1), the energy added for each loop is the same – 226.7 feet of fluid. Second, note that while the head loss in the process elements is different for each loop as well as the head loss for the control elements in each loop, the total head loss for control and process elements is always equal to the pump head (the energy added to the fluid by the pump). Lastly, note that the control elements – the control valves – are consuming from around 10 to 30% of the energy being added to each loop by the pump; for installations where lowering operational costs is a priority, further system analysis and optimization is possible to reduce the amount of energy that is not being used directly for the process elements.
In summary, we can see that in a multiloop system typical of HVAC and other distributed service installations, each loop typically contains all three of the piping system elements: pump, process, and control, and that the head loss from process and control elements in each of these loops is equal to the pump head added by the pump elements.