Table 2. Annual average plant performance improvement for NW- 16
tubes. Source: Dr. Ralph L. Webb
Item
Number of tubes
Qh (steam heat input)
HR correction factor
Heat rate (actual)
Qc (actual condenser load)
W (turbine output at Psat)
Wnet (less pump power)
ΔEg (increase net over plain tube)
Units
MW
%
kW/kW
MW
MW
MW
MW
Plain
23,150
2,940
2. 12
3.00
1,857
927
926
0
Tube-for-tube
replacement
23,150
2,940
1.06
2.97
1,847
936
935
9.87
New bundle with
NL = constant
28,575
2,940
0.54
2.96
1,842
941
9,409
14. 65
using the same total lineal tubing length as
is used in the plain tube design (Table 2).
However, the enhanced tube bundle will have
about 25% more tubes and will provide the
same condenser water flow rate as the plain
tube design. The full potential of enhanced
tubes can only be obtained with a new condenser bundle design. A tube-for-tube replacement results in reduced water flow rate,
which diminishes the performance potential.
This evaluation assumes that the only cost
affected is the cost of the tube bundle; the
cost of the condenser shell is not included
in the analysis. The capital benefits of the
enhanced tube condenser are described by a
simple payback analysis. This analysis calcu-
lates the simple payback based on:
The condenser heat rejection is given by
where Tw,in is known for each month of the
year. The calculation procedure requires iter-
ation of Qc and Tsat (which is a direct function
of Psat —found in the steam tables). To obtain
annual average values, this calculation is per-
formed for each month of the year using the
known Tw,in for each month of the year.
■ The capital value of the increased generation rate, assuming a value of $2,000/k W
(= 2 × 106 × ΔE).
■ The increased tube cost of the enhanced
tube design, relative to the plain tube design given by
Increased Net Generation. Using the
calculated values of W and Qc, the incremental generation rate ΔEg (MW) made
possible by the enhanced tube geometries
is determined using the calculated values of turbine output (W) and the pumping
power (Wp). The new generation output is
Wnet = W – Wp, assuming the main steam flow is
held constant (line 6).
The net increased generation rate provided
by the enhanced tube condenser (relative to
the plain tube condenser) is ΔEg. The ΔEg = We
– Wnet,p, where subscripts e and p refer to the
enhanced and plain tube respectively (line 7).
Calculated Results
Calculations were performed for two cases: a
tube-for-tube replacement with fixed pumping power and a bundle replacement having
NeLe/NpLp = 1 with fixed condenser flow rate
and pumping power, as discussed earlier.
Tube-for-Tube Replacement. The analy-
sis showed that the tube-for-tube replacement
will provide significantly lower ΔEg than the
bundle replacement. This is because the tube-
for-tube replacement results in a reduced
condenser water flow rate. For the Figure 2b
NW- 16 tube, the flow rate reduction is 19%.
This reduced flow rate causes the condenser
to operate at higher thermal effectiveness,
which degrades the condenser performance,
relative to the second case, which holds water
flow rate constant. For the NW- 16 tube, the
resulting annual average ΔEg is 9.87 MW.
Table 3 shows the results of the payback
analysis. Line 1 shows the increased generation capacity for the enhanced bundle design.
The second line shows the capital value of
the increased generation capacity (Ccap). Line
3 shows the total tube cost of the plain and
enhanced tube bundles (Ct). The cost of plain
and enhanced titanium tubes was based on
data provided by Plymouth Tube, a manufacturer of titanium tubes. The present analysis is
for 28.6-mm OD, 0.7-mm wall titanium tubes.
It was also assumed that two-thirds of the tube
cost is material and one-third is fabrication.
The material costs were then adjusted by multiplying by the diameter ratio ( 28.6/25.4).
Table 3 shows that the material content cost
is $10.35/m (or $36.85/kg) at the time this
analysis was completed in late 2009. The plain
tube fabrication cost was assumed to remain
constant. It is conservatively assumed that the
fabrication cost of the enhanced tubes is 75%
greater than for the plain tubes. This resulted
in tube costs of $14.93/m and $18.37/m for the
plain and enhanced tubes, respectively.
Using the values given above, the tube bundle cost increment for the enhanced condenser
tubes, relative to plain tube design (ΔCt), is
Economic Evaluation
The greatest economic benefit to the plant
will occur if the entire tube bundle is replaced
This value is shown on Line 4 of Table 3.
