When Should You Use the HSS vs the HX 2-Pipe Device?

Both of the new devices added in PIPE-FLO® Professional version 15 can model real-world processes that facilitate sensible heat transfer in flowing fluids. The heat source/sink device is used to model heat transferred into or out of a single fluid, whereas the heat exchanger 2-pipe device is used to model sensible heat transfer between two fluids. Both devices model the hydraulic performance of only one side of a heat exchanger.

It is up to the user to decide which to use based on the actual device being modeled, how much information is available and what needs to be evaluated in the system.

The Heat Source/Sink Device

The heat source/sink device is used to calculate one unknown thermal parameter of the heat transfer device being evaluated: the heat transfer rate, flow rate, inlet temperature or outlet temperature. The other parameters must be defined by creating two unique fluid zones and assigning them to the inlet and outlet pipes of the device. Hydraulic calculations can also be performed using the pressure drop vs. flow rate performance curve data.

The heat source/sink device must be used when the actual device contains only one flowing fluid. Such as with a solar panel, a nuclear reactor, an engine block, or some other source or sink of thermal energy. For example, the heat source/sink is used to model the reactor vessel in the primary coolant system of a pressurized water reactor used in the nuclear industry shown in Figure 1.

Figure 1. Primary coolant system of a 3-loop pressurized water reactor (PWR) modeled in PIPE-FLO® Professional. 

Even if the actual device contains two flowing fluids, the user may want to evaluate the effects of heat transfer on just one of the fluids. One reason to do this may be because a phase change occurs on the other side, which occurs in boilers, condensers and evaporators. The steam generators in Figure 1 are modeled as heat source/sinks because the primary coolant releases sensible heat through the tube bundle, which results in a phase change on the feed water/steam side of the generator.

Another reason to model a heat exchanger as a heat source/sink could be that the user is only responsible for the piping system on one side and they want to evaluate the hydraulic and thermal performance of the system. Figure 2 shows a model of two shell-and-tube heat exchangers used in the pulp and paper industry to cool weak acid from 107°F to 60°F prior to strengthening in the fortification tower. The design production rate of the plant determines the hydraulic requirements of the acid system, and the thermal requirements of the acid system determine the hydraulic requirements of the two cooling water systems.

The two real-world heat exchangers in Figure 2 are modeled as four heat source/sinks, one for the hot and cold side of each heat exchanger. A process engineer responsible for just the acid system may not even model the cooling water side, and vice-versa. If one engineer handles all the systems, they may want to model the entire process.

The two heat source/sinks on the acid system side are set to calculate the heat transfer rate in the thermal calculation dialog. That value is then entered as the input to the two heat source/sinks set to calculate the required flow rate of the cooling water systems. The control valves in the cooling water systems are set as the flow control devices for these heat source/sinks. This automatically sends the calculated mass flow rate to the control valves set to temperature control operating mode.

Figure 2. Modeling shell and tube heat exchangers as heat source/sinks.

The thermal capacity (UA), effectiveness, log mean temperature difference (LMTD) and other thermal parameters are not calculated for the heat source/sink since important heat exchanger design data and key fluid properties on the un-modeled side are not known. The heat exchanger 2-pipe device must be used to evaluate these thermal parameters.