Water utilities for heating and cooling are critical to the safe and effective operation of many power plant systems. These utilities are also valuable for heating a plant’s surrounding infrastructure, providing HVAC heating to buildings from a plant’s existing heating infrastructure. The design of these infrastructure systems is subject to a range of operating conditions, such as variable seasonal heat demand, or changing demand from day to night for continuously operating power plants. These systems must also consider start-up and emergency conditions, requiring a robust system capable of a wide range of operating points.
Agnieszka Markwica with Energoprojekt Katowice was tasked
with modeling the hot water network of a power plant in AFT Fathom, capturing
heat to deliver to the surrounding buildings’ HVAC via heating water. Markwica
was to consider the system’s heat transfer requirements, as well as
alternatives when heat requirements shifted or consumers were temporarily
closed off.
Building the model Markwica 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
heat exchangers were modeled as resistance curves, capturing pressure loss data
across a range of flowrates. Heat exchanger temperature considerations were
similarly modeled via the Fixed Outlet Temperature model. Markwica also color coded
the model, indicating colder feed water in blue and hot delivery water in red.
Color coding made the model immediately readable to other engineers and the
client. Figure 1 shows a Workspace diagram of normal operation, highlighting
the control valve elements used for the alternative emergency and circulation
operating cases below.
With an established model, Markwica sized 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/hr (22 tons/hr) at minimum circulation flow, 67-174
m-tons/hr (74-192 tons/ hr) in emergency cases, and up to 241 m-tons/hr (265
tons/hr) at normal operation. The flow paths for these alternative cases are
shown in Figure 1.
Sizing Equipment
The wide range of operating flowrates was addressed with control valves and a variable frequency drive (VFD) to change the speed of the pump. Adjusting pump speed provides more flexibility to meet an intended operating point efficiently rather than relying on control valves to create additional losses. A pair of control valves controlled the flow distribution between the heat exchangers and a bypass depending on the operating case. Due to the wide range of flowrates to control, Markwica tested multiple components 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 Markwica selected the most optimal variant in each case.
Markwica faced other considerations for the system’s
emergency and circulation cases. One emergency case occurs from 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 was determined according to the
required delivery temperature and degree of overheating, also considering the
heat transfer effects of slower flow through the heat exchangers. During normal
operation, the system’s two heat exchangers provide for the system’s two
consumers. The consumers have an unequal consumption of 67 m-tons/ hr (74
tons/hr) and 174 m-tons/hr (192 tons/hr). If either heat consumer is taken
offline, the offline consumer’s flow bypasses the exchanger via a control valve
as in the emergency 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.
Sustained recirculation through heating systems can be
concerning, but it is also essential when preheating a system to avoid sudden
heat-shock. During preheating circulation, the operating flowrate through the system
dropped from 241 m-tons/hr (265 tons/hr) to 20 m-tons/hr (22 tons/hr), one
cause of the wide control valve setpoint range. The minimum recirculation
flowrate did not meet the minimum flowrate requirement for the pump, requiring
an additional recirculation loop for the pump itself. An orifice was sized to
meet the desired loss requirement for minimum pump flow via Goal Seeking
methods, eliminating manual iterations otherwise performed by an engineer. The
goals and variables used to size both control valves and orifices are found in
Figure 2.
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.
According to Markwica, 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. After the infrastructure of the system was built
out, it was trivial for Energoprojekt and their client to test alternative pipe
geometries or components within the system. System design is a challenge,
especially when considering multiple operating points and system requirements.
Markwica put it best that AFT Fathom provided “results in a short time,
providing the possibility tor checking other than typical solutions”, revealing
the most optimal solution instead of the most immediately apparent.
