Designing for Multiple Operating Points in Power Plant HVAC Heating System
October 24, 2021 -- Colorado Springs, CO, USA -- Pump Engineer Magazine features an article highlighting a popular case study from Energoprojekt Katowice in Poland. The case study details how an engineering was tasked with modeling the hot water network of a power plant, in AFT Fathom software, to capture heat to be delivered to the surrounding buildings HVAC.
The assigned engineer was to consider the heat transfer requirements of the system, as well as what options where available when heat requirements shifted or were temporarily closed off completely.
As is typical in hydraulic system modeling, the designer began by making a precise model of the system infrastructure, relying on technical documentation for fittings and instruments and system data for piping dimensions. Complex components like the system’s two heat exchangers were modeled as resistance curves, capturing each exchanger’s pressure loss across a range of flowrates.
Additional considerations such as the temperature increase across the heat exchanger was similarly modeled in the initial system. Energoprojekt also color-coded the model, indicating the colder feed water in blue and hot delivery water in red, making the model immediately readable to other engineers and the client. An image of the model during normal operation is included in Figure 1, highlighting the control valve elements.
With an established model, the engineering team could then size the system’s pump according to the design required flow and corresponding pressure requirement. This single pump would provide the HVAC system’s wide range of required flows, anywhere from 20 m-tons/hour at minimum circulation flow, 67-174 m-tons/hour in emergency cases, and up to 241 m-tons/hour at normal operation.
The Process
The range of operating flowrates for this project were addressed in a few ways. First, a variable frequency drive (VFD) was used to change the speed of the pump. Increasing or decreasing the speed of a pump provides an engineer more flexibility to meet a desired operating point efficiently, rather than relying solely on control valves to create additional losses. The VFD was used in conjunction with a pair of control valves controlling flow to the heat exchangers, and to a bypass depending on the operating case.
Due to the wide range of potential flowrates to control, multiple components were tested across a range of diameters, relying on AFT Fathom to indicate the corresponding Cv and Open Percentage for each operating point. Properly sizing these control valves in conjunction with variable pump speed ensured the most optimal variant was selected.
The system required additional consideration for emergency and circulation cases. One potential emergency that can occur is if there is excessive consumer delivery temperature, where flow must bypass the heat exchanger and mix with the overheated stream to reduce the delivery temperature. In these cases, the bypass control valve setpoint could be determined according to the required delivery temperature and degree of overheating, to mitigate the risk of failure. It is also important to consider the heat transfer effects of slower flow through the heat exchangers, in these scenarios.
During normal operation, the system’s two heat exchangers provide for the system’s two consumers. The consumers have an unequal consumption distribution of 67 m-tons/hr for one consumer and 174 m-tons/hr for the other. If either heat consumer is taken offline, the offline consumer’s flow bypasses the exchanger via a control valve as in the overheated case. In cases where both consumers are closed off, the bypass control valve would instead replicate the pressure losses of the consumers as normal flow recirculated through the system.
Recirculation through heat exchangers can be concerning but is also valuable when preheating a system for operation. The preheating circulation avoids suddenly heat-shocking the system. During preheating circulation, the operating flowrate through the system dropped from 241 m-tons/hr to 20 m-tons/hr, one of the causes of the wide range of control valve setpoints. The minimum recirculation flowrate also does not meet the minimum flowrate requirement for the pump. This means there must be an additional recirculation loop for the pump itself, with an orifice sized to meet the desired loss requirement. This orifice could be sized via Goal Seeking methods, eliminating manual iterations otherwise performed by an engineer. Similarly, orifices replicating the loss of heat exchanger elements could be sized for use during HEX maintenance, again ensuring the pump has sufficient losses for its intended flowrate.
The Result
According to Energoprojekt, AFT Fathom provided immense value in this analysis by capturing the complications of a large, interacting heat transfer system across many operating conditions. Each case could be examined in detail without completely isolated models, ensuring that a single design could meet all the potential operating conditions efficiently from VFD and control valve considerations.
System design is a challenge, especially when considering multiple operating points and system requirements. Energoprojekt put it best that AFT Fathom provided “results in a short time, providing the possibility for checking other than typical solutions," revealing the most optimal solution instead of the most immediately apparent.
Capability Used In This Case
•Flow Coefficient (Cv) for the control valves allowed engineers to check the valve loading in every possible case.
•Use of the Goal Seek & Control add-on module to quickly determine the diameter of the orifices which caused the drop of pressure demanded in the by-pass pipes.
• Circulation pump selection was based on hydraulic loss of pressure.
• Heat Transfer with Energy Balance capability was used for water heated in heat exchangers.
• Scenario Manager was used to easily compare multiple operational cases and choose the optimal system solution.
• An external Pipe Material Database was imported for steel pipes with nominal diameter