Waste Heat Boiler Problem

Figure 1: Process Diagram for Waste Heat Boiler Problem
The deaerator (feed) tank contains saturated liquid water at 105 ̊C and the water level in the tank is 10 ft
above the pump inlet nozzle. The suction line (i.e., feed line to pump) comprises of 55 ft of equivalent
length of NPS 6” Sch 40 carbon steel pipe. The total equivalent length of pipe on the discharge side of the
pump is 650 ft of Sch 40 NPS 4” pipe. The design flowrate of BFW is 82,000 kg/h. The waste heat boiler
will operate at 6.5 bar pressure and produce saturated steam at that temperature and the liquid level in
the WHB will be 9 ft above ground.
In analogy to the pumps and circuits flowsheet used in class, please set up the flow circuit in CHEMCAD
for the WHB problem (shown in Fig. 1) using the following design parameters:
• Use SRK for both the VLE and H thermo options
• The pump is operated at a single speed of 3600 RPM. The performance curve for this
operation is given as follows:
Flow
(m3
/h) Efficiency ∆Ppump
(bar)
0 0.61 9.68
21 0.66 9.68
43 0.71 9.67
64 0.76 9.57
85 0.81 8.80
105 0.75 5.37
116 0.71 1.18
119 0.61 0.02
• The pipe between pump and control valve is elevated to reach the height of WHB liquid level
2 ft
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• The control valve should be designed to be 60% open (valve position) at the normal operating
flow (82,000 kg/h of BFW)
• The pressure in the exchanger shell should be regulated at 6.5 bar to allow the generated
steam enough pressure to overcome frictional losses in the discharge piping leading to the lps
header (which operates at 6 bar)
• You should also model the pressure drop across the heat exchanger for the shell side as a
length of (NPS 4” Sch 40) pipe that gives a ∆P = 0.35 bar at design flow (82,000 kg/h).
(a) Report the flowsheet simulation set up in CHEMCAD.
(b) Report the length of NPS 4” Sch 40 pipe used to simulate the ∆P across the exchanger.
(c) Report the Cv value for the valve and the ∆PCV at the design flow for both (i) linear and (ii) equal %
valves.
(d) Plot the % opening of the CV (x-axis) vs feed flowrate (m3
/h) for the cases of (i) linear (ii) equal %
control valves. Use feed flowrate range from 2000 to 92000 kg/h.
(e) Plot the NPSHA (y-axis) as a function of flowrate (x-axis). Use feed flowrate range from 2000 to
92000 kg/h
(f) What is the minimum (maximum) NPSHR for the pump to avoid caviation when the CV is wide
open?
Hint: A new version of Topic 7 lecture slides has been upload to eCampus, with Sensitivity Study set
up details.